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Purvis Industries - Proud Fenner Dunlop Distributor

2021-05-10T18:48:17-05:00

Fenner Dunlop

The backbone of every Fenner Dulop belt is made right here in the USA! Purvis Industries are proud to be a Fenner Dunlop Distributor! Check out some of our available screw conveyors online, click here for more.

Screw Conveyor Alignment

2021-04-13T19:48:35-05:00

Screw Conveyor Alignment

Article By: Danny Clark

KMEC Project Sales

Screw conveyors like most other types of rotating equipment require proper alignment to ensure efficient long-term service. The full length of a screw conveyor should be aligned in both the horizontal and vertical directions when initially installed, and anytime a user of a screw conveyor is experiencing unexplained vibrations, noise, premature wear of hanger bearings and coupling shafts etc.… The first thing that should be checked is the alignment.

The simplest way to check alignment is with the string line method. Simply secure the string line to one end of the conveyor at the trough end. The string should be secured at the side of the trough end at the centerline of the screw or fixed distance from the top of the trough. Pull the string tight to the opposite end of the conveyor and secure it there. Now you can visually see the alignment of the trough sections.

Now check the alignment every 5 feet with a measuring tape by measuring the distance from the string line to the edge of the trough horizontally then repeat the process vertically. The differences in the measurements will show you the alignment of the screw conveyor. Each measurement should be documented for both the horizontal and vertical direction.

The maximum allowable misalignment over the full length of the screw conveyor is 1/8” in either the horizontal or vertical direction. Excessively worn hanger bearings or coupling shafts can also cause misalignment, and failure at the hanger bearing locations. When a hanger bearing, or shaft has worn more than 1/8” it must be replaced.

Check out some of our available screw conveyors online, click here for more.

Brennan Inc. presents 'How to Measure Threads in Three Steps'

2020-09-10T19:31:21-05:00

Brennan is a tier 1 supplier to Purvis Industries offering hydraulic and pneumatic fittings and adapters for industrial and mobile applications. Here is a useful article on how to measure threads to get the correct fitting or adapter for your project.

For any questions please feel to contact Steve McKown at steve.mckown@purvisindustries.com.

Click here to learn more about our HydraAir team!

Softstarters

2020-07-09T16:07:44-05:00

Softstarters

Softstarters are another way for customers to start an electric motor, they are often the most economical solution for applications that only require controlling torque and speed during the motor start up. They perform this task by supplying the motor a reduced voltage during start up. They then slowly increase the voltage supply until the motor is running at full speed with full voltage. By starting a motor in this fashion, it not only reduces the mechanical stress in the system it also reduces the high in-rush current required to start the motor. This is done using switching devices inside a Softstarter such as thyristors or silicon-controlled rectifiers (SCRs). These switching devices turn off and on quickly adjusting the voltage output to the motor.

Once the Softstarter has completed the initial ramp up to full speed, they switch the load to an integral or external bypass contactor. Removing the switching devices from the electrical circuit helps to improve overall system efficiency and reduce the heat generated inside an electrical enclosure. This feature can reduce or eliminate the need for any thermal management for an electrical enclosure.

Softstarters are ideal for applications where space is a concern since they are usually smaller than a comparable variable frequency drive (VFD). Softstarters are unlike a VFD as they can only control the speed of which a motor starts not the running speed of a motor.

For any questions please feel to contact one of our Triad Regional Application Engineer at triadsupport@purvisindustries.com and we would be happy to assist you!

Click here to learn more about Triad Industrial Automation.

Self-locking of a Worm Gearbox

2020-06-09T17:29:50-05:00

Self-Locking Of A Worm Gearbox

Article By: Joe Hardwick

Business Unit Manager

Self-Locking is the inability of a gearbox to be driven backward by it’s load. In other words, a self-locking gearbox cannot be driven from the output shaft, either through a source of power or an external torque load.

The ability for a gearbox to lock is related to its efficiency. The higher the efficiency, the less likely it is to lock. Because wormgear reducers are generally less efficient than other gear types, they are often considered for use in applications where locking is desirable.

A rule of thumb is that the higher the ratio, the more apt it is to lock. There are too many factors that play into a reducers ability to self-lock for that rule of thumb to be considered effective. Things that influence the gearset locking include ratio, lubrication used, amount of load, service time, input speed, altitude and many others.

When it comes to utilizing a gearbox as a holding or braking device, there is more gray area than black and white. Application concerns have been witnessed where a reducer with a ratio of 60:1 or80:1 single reduction reducers that where utilized in lifting applications without a brake. After several load cycles, the load can start to creep backwards.

Because of the uncertainties with locking gear sets: NO GEARBOX SHOULD BE CONSIDERED TO BE SELF LOCKING. If a load needs to be stopped or held, use a suitable sized brake device.

MOUNTING A TAPERED BORE SPHERICAL ROLLER BEARING

2020-04-28T18:32:31-05:00

Mounting a Tapered Bore Spherical Roller Bearing on an Adapter Sleeve

Article By: Dan Whitehouse

Business Unit Manager

The install of a tapered bore spherical on an adapter sleeve can be a challenging task due to all the steps. The confusion comes when reading through the steps, checking the clearance gap before and during install and finally knowing how to read the manufacturer’s clearance chart. To the right are the listed steps for the install procedure so that we can clear up misunderstandings and confusion that goes with the bearing’s install. We recommend following the steps below and keeping the appropriate tools within arm’s reach to help streamline the install.

Image courtesy of ABB.

1.) Clean the shaft with an emery cloth or file to remove any rust or burrs from the shaft and then wipe the shaft down to remove the filings.

2.) Measure the shaft size to confirm that it is in spec based off the manufacturer’s suggestion for that sized bearing.

3.) Remove the adapter sleeve assembly from its packaging and disassemble.

4.) Position the adapter sleeve on the shaft in the approximate location of where the bearing will be operating when installed.

5.) Remove the bearing from its packaging and measure the unmounted radial internal clearance of the bearing to be certain that the bearing is in spec before install. Measure the clearance at the 12 o’clock position between the outer race and the top of the roller. When checking the gap in the bearing with the feeler blades, the blade should slide though the gap with some resistance; if the blade becomes pinched, pick a smaller sized blade and try again. When measuring with feeler blades use a sawing motion from the face of the roller to the middle of the bearing. When reading the clearance chart from the manufacturer make sure you are on the correct line for your bearing’s bore with the correct clearance columns. The nominal bearing bore column in the chart has two bore sizes listed. The left bore value is not included but everything over that bore is included up to and including the right bore value.

6.) After determining the bearing is in spec (write down your radial clearance you will need it later), place the bearing on the adapter sleeve and push the bearing up the adapter by hand until you cannot move it further, making sure the adapter has not moved on the shaft. Screw the locknut onto the threads of the adapter. Some people install the lockwasher at this point as well and some do not due to the fear of damage to it during the rest of the install.

7.) Refer to the manufacturer’s radial internal clearance chart for the correct reduction range (use your radial clearance before mounting value to determine the final mounted clearance). Using a spanner wrench or hammer and punch begin to tighten the locknut to reduce clearance in the bearing, while checking that the adapter hasn’t moved on the shaft. Be sure to check the reduction in clearance using feeler blades (the blade should slide though the gap with some resistance; if the blade becomes pinched, pick a smaller sized blade and try again) in-between rotations of the locknut to ensure that too much clearance is not taken out based off your mounted clearance value.

8.) If you didn’t install the lockwasher before, remove the locknut and install the lockwasher so that is sits flat against the bearing. Re-install the locknut until tight.

9.) Locate the lockwasher tang that is closest to the locknut slot. If slot doesn’t match up with a tang on the lockwasher, tighten the nut to match with the next closest lockwasher tang. Bend the tang into the locknut slot.

10.) Finally, ensure that the shaft and outer ring can be rotated

Take-ups and Allowing for Belt Stretch

2020-02-05T07:32:22-06:00

TAKE-UPS AND ALLOWING FOR BELT STRETCH

Article By: Billy Witherspoon

Business Unit Manager - Heavyweight Belting

All conveyor systems require a type of belt take-up that keeps the belt tight enough to run on a system without slipping and is adjustable to allow for belt stretch. There are multiple types of belt take-ups. However, they generally fall into two major categories, belt take-ups that are adjusted manually or belt take-ups that are self-adjusting. The self-adjusting belt take-ups use gravity as a counterweight or some type of stored energy such as like Pneumatic/Hydraulic pressure. The longer the belt, the more take-up travel will be needed to allow for stretching. A key rule is that for every foot of belt stretch the take-up system will require at least 6 inches of take up travel.

Fenner Dunlop, RockMaster belts, are designed to have 1.5% stretch at max PIW rated Operating Tension. All belts are not rated the same, so we highly recommend our customers to consult the manufacturer’s spec sheets for their specific belt). A 400 foot Rockmaster belt running at MAX rated operating tension would be expected to elongate or stretch to 406 feet (i.e. 400ft x 1.015% = 406ft) and the extra 6 feet would require at least 3 ft of take-up travel. On average, most belts are not running at or near the Max Tension rating and will stretch less. Operating conditions like temperature, moisture, oil or belt overloading can cause the belt to stretch or shrink more than usual. On systems with short take-ups you must make the spliced length as short as possible to allow for lack of adjustment. On longer systems, it is best to run the belt for a period of time to break in and generate the stretching of the belt, then return and shorten or resplice. Repeat this task if needed, as it could run out of take-up over time. A 2,000 foot Rockmaster belt running at MAX rated Operating Tension would be expected to elongate or stretch to 2,030ft (i.e. 2,000ft x 1.015% = 2,030ft) and the extra 30 foot would require at least 15 foot of take-up travel.

The MORE Tension you put on a belt the MORE Stretch a belt will experience!

Belt manufacturers recommend that a belt is run with as little tension as possible. There are calculations and programs that will tell you the correct counterweight that a system should have based on the normal operating data. Ideally, the belt should be adjusted just tight enough so that the material/load is conveyed, without the belt slipping on the drive pulley. This puts less stress on the whole system resulting in increased belt and component life!

For more information on Take-up and Belt Stretch, contact one of your Purvis Industries Sales Representative or connect with our Heavyweight Belting team.

How hot is too hot?

2019-12-26T22:01:01-06:00

HOW HOT IS TOO HOT?

Article By: Joe Hardwick

One of the most common concerns from a customer is one stating “My gearbox is running hot, what should I do”?

Most of the time the concern is based on a personal feeling that the housing feels hot to the touch. Generally, if a person can hold their hand on the housing for a few seconds, the temperature is not too high.

Temperature limits of industrial gearboxes are much higher than a person’s tolerance to heat. Every person’s tolerance to heat is different, which makes this “test” of determining temperature purely subjective. The limit of human touch to heat is generally 130 deg F-140 deg F. To compare, water is said to cause 3^rd degree burns to adult skin at 140 deg F in 5 seconds.

General temperature limits of gearbox components are as follows:

Standard Rubber oil seal lips 212 deg F

Mineral Oils 212 deg F

Fluorinated Rubber Oil Seal Lips 300 deg F

Synthetic Oils 300 deg F

Roller bearings, bronze gears and steel gears have temperature limits that exceed 250-300 deg F and usually do not limit the thermal capacity of most general purpose industrial gearboxes.

Temperature is a result of load. As load increases, temperature increases and as load decreases, temperature decreases. If the load applied to a gearbox is cyclic in nature the temperature should reach a medium and hold steady until the load cycle changes.

There are significant benefits to reducing the operating temperature of a gearbox. Cooler temperatures can help extend the life of oil seals. In fact, for nitrile lip seals, every 25 deg F change in temperature can either double the seal life or cut it in half. For example, a gearbox may operate for 10,000 hours at 125 deg F, but another one may only operate 5000 hours at 150 deg F before seal failure occurs, in the same application. Temperature has similar effects on Viton and other gearbox components.

If a gearbox is allowed to run at excessive temperatures other problems can arise such as burnt lubricant, galled bearings, damage to seals and gears which result in damage to other system components.

The maximum housing limit on most gearboxes is from 180 deg F-200 deg F. For quick, accurate temperature checks, use of infrared thermometers, thermocouples, or temperature sensitive crayons or marking devices such as those available from a Welding Supply, is suggested.

If the temperature of the gearbox exceeds the manufacturers limit, there are a few options to consider. If the physical space will allow a larger reducer should be considered or an auxiliary cooling device can be implemented. Auxiliary cooling devices may include a shaft mounted fan, an electric fan or a radiant cooling system.

Pulley Lagging

2019-11-06T15:09:17-06:00

Article By: Billy Witherspoon

Strategic Business Manager

One of the most often overlooked/neglected components of a conveyor system is Pulley Lagging.

Pulley lagging is a covered/coating over the surface of either a driven or non-driven pulley used as a wear surface and/or friction layer. Typical conveyor pulleys are made of a steel outer shell with end discs welded in each end to allow for mounting to a conveyor shaft, with numerous choices on mounting options. Lagging is added to the steel outer shell of the pulley, providing a replaceable wearing surface for non-drive pulleys. For driven pulleys lagging is typically made to increase the friction between the pulley and the belt being driven, with the added benefit of being a replaceable surface. Lagging comes in many rubber and non-rubber compounds, multiple surface patterns for varied applications and needs to be chosen based on the application for best results.

Increasing the friction at the drive pulley allows your conveyor to run at less overall tension, increasing belt life, as well as, bearing and component life. Proper lagging can eliminate belt slippage in wet and frigid conditions preventing “Burn Throughs” (Pulley spinning due to loss of friction will burn through the bottom of the belt).

Pulley Lagging can be purchased already applied to a new pulley direct form the manufacturer or there are numerous aftermarket options. We stock and can provide installation on multiple aftermarket lagging products. High Tensile strength rubber with Diamond pattern or Flat surface can be bonded to your existing pulley either in our shop or on your conveyor. In areas where you need more friction we can provide Rubber lagging with Ceramic inserts that provide more friction and wear resistance. Weld-on “Slide Lag” is also available. Whatever your application requires, we have you and your pulley Covered!

Backdriving A Gearbox

2019-10-28T20:29:46-05:00

Where Self Locking is the inability to drive the output shaft of a gearbox, backdriving is the ability to drive the output shaft.

This essentially makes the gearbox become a speed increasing device. This can be done intentionally or unintentionally and can occur in various ways. Backdriving can occur when the applied load overcomes the frictional resistance or holding power of the rotating elements in the gearbox, causing the output shaft to speed up. This is more common in lifting and inclined applications because gravity assists the force pulling the load down.

A gearbox that is backdriving may cause the motor to overspeed, which may lead to damage to the gearbox, motor or other system components.

Intentionally using the gearbox as a speed increaser will also cause backdriving. Not all worm gears can be backdriven because of their tendency to lock when driven in reverse rotation. Worm gear sets generate more heat than other gear types due to the amount of sliding friction produced while rotating. Spur and helical gears are commonly used as speed increasers. Under a backdriving or speed increasing operation, increased noise and temperature levels may become undesirable.

If an application has a sufficient amount of reverse momentum that may cause the load to overhaul the frictional resistance of the gear set in a reducer, a “backstopping” or “one direction” mechanical clutch should be used to prevent backdriving. If the application requires the speed to be increased, carefully review and select a gearbox suitable for that type service.

Screw Conveyor vs. Screw Feeder

2019-07-31T04:03:40-05:00

Article By: Danny Clark

Business Unit Manager

"When buying a screw conveyor, how do you know if you need a standard screw conveyor or a screw feeder instead?"

This is a very common question asked by many folks within the industry. Many users ranging from engineers, buyers, and other end users are often unaware there is a difference between the two. You can avoid one of the most common mistakes made in designing a screw conveyor system by understanding the difference between a screw conveyor and a screw feeder. In this article we will briefly compare the two and the role they play in a screw conveyor system.

Screw conveyor systems: Frequently used for unloading bulk materials from rail cars, bins or piles. They are commonly found in cement and lime processing plants, aggregate plants, mining, grain storage plants, feed mills, chemical plants, food processing plants, and many more.

Screw conveyors: Simply move the material from point A to Point B. The inlet of a screw conveyor will always be control fed by another device such as: another screw conveyor, screw feeder, belt conveyor, bucket elevator, rotary airlock or a volumetric feeder. They can be used up and down inclines. Mixing, cooling, and heating materials can also be done with a standard screw conveyor. These can also be referred to as a “transfer conveyor” in the industry. They are typically designed to convey bulk materials at 15, 30 or 45 percent trough loading, depending on the material characteristics. They are available in a wide range of sizes, lengths, configurations and material of construction.

Screw Feeders: Designed to meter bulk materials and are typically located at the beginning of a process. The capacity or feed rate of the conveyor system can be accurately controlled with screw feeders. The inlet of a screw feeder is always flood loaded (100-percent full) and typically it will be mounted directly under a hopper, a bin or a silo. With the inlet being flood loaded, the design of the screw in the inlet area and screw speed determine the capacity or feed rate. They are also available in a variety of sizes, lengths, configurations and material of construction. However, typically most screw feeders are less than 20 feet in length because the use of hanger bearings are not recommended. So, in very simple terms: The inlet of a screw conveyor is always control loaded and the inlet of a screw feeder will always be flood loaded.

Example of a Screw Conveyor

Example of a Screw Feeder
If you are interested in receiving a quote on your latest screw conveyor needs, contact one of our local sales representatives and provide us the answers to screw conveyor survey below. These six questions must be answered in order to accurately size out a screw conveyor and make sure your system is getting properly designed to fit your needs.

1.) What is the Capacity?
1. (in cubic feet per hour, tons per hour or bushels per hour)
2.)What is the Length?
1. (Inlet-to-discharge or overall length)
3.) Is it Horizontal or Inclined?
1. (If inclined, what is the degree of angle?)
4.) How will it be loaded?
1. (If flood fed, what is the size of the inlet?)
5.) What Material is to be conveyed?
1. (Material type, weight, maximum lump size, and temperature)
6.) What is the Material of Construction?
1. (Mild steel, stainless steel, special paint requirements, & etc.)

Bulk Material Flow Control Gates (Part 2)

2019-04-11T16:46:17-05:00

Mass Flow Gates

There are many reasons to store some bulk materials in storage silos that promote mass flow. If the discharge gate of a mass flow silo does not promote mass flow, mass flow will not occur in the storage silo. One of the most reliable methods of controlling the flow from a mass flow storage silo without disturbing the flow is a properly designed mass flow gate. Purvis Industries provides two types of standard mass flow gates.
Single Blade Mass Flow Gate
The single blade mass flow gate has one fixed inclined side that is not less than the calculated slope to promote mass flow for the product stored. The moveable blade when fully closed is the same slope as the fixed side. The other two sides are vertical. As the moveable blade is opened and closed the flow rate is changed but mass flow is always maintained. The center of discharge of the single blade mass flow gate changes by half the opening size but in most applications that produces no significant design challenges.
Purvis Industries’ standard single blade mass flow gate is produced in 48” square and 60” square sizes and fabricated of ¼” mild steel with 3/8” 400 BHN steel liner in wear areas. The movable blade is actuated by a hydraulic cylinder and mounted to a round shaft supported by flange bearings. The entire gate, including the moveable blade, is structurally reinforced and the entire unit is totally sealed.
Double Blade Mass Flow Gate
Two opposing sides of a double blade mass flow gate are moveable while the two other sides are vertical. When closed, the slope of the moveable blades is not less than the calculated mass flow angle of the material in storage. When discharging material, both blades move in unison to control the flow rate. Mass flow is therefore always assured while discharging on the gate center.
Purvis Industries’ standard double blade mass flow gate is produced in 48” square and 60” square sizes and fabricated of ¼” mild steel with 3/8” 400 BHN steel liner in wear areas. The blades are actuated by hydraulic cylinders and mounted to round shafts supported by flange bearings. The entire gate, including the moveable blades, is structurally reinforced and the entire unit is totally sealed.

Since mass flow gates are intended only to insure mass flow while controlling the material flow rate, it is necessary to install a shut off gate ahead of the mass flow gate. Purvis Industries recommends the double blade horizontal slide gate be installed on the inlet flange of a mass flow gate.

Custom Gates

It is not possible to anticipate all types or sizes of gates that will be required. Some of the factors that determine gate sizes and arrangements are:

The type and size of the material to be handled. If the flow of large material (+6” for example) is anticipated, it is important gates be provided that gate blades be designed in a manner that prevents the material from interfering with the closing of gate.
If the material’s surface moisture is high, water can be expected to drain during storage. If the flow control gate is not designed to drain that water to a sump, a very dangerous situation can develop. Fluidized bulk material flowing freely in a confined space can endanger life and property.
Should a flow control gate be required to be retrofitted in a confined space, the designer may be called on to use ingenuity to produce a design that will both perform the desired function and fit into the available space.
Many other special conditions are often encountered to challenge the design capability of the gate designer.

Purvis Industries has the design and fabrication facilities required to produce the gate you need.

Gate Power and Control Systems

Purvis Industries is also uniquely capable of designing and supplying the actuating power units and complex control systems for all applications. Some operations depend on the flow control gates operating in a very complex and accurate manner. For example, loading railcars to gross weight without overloads at loading rates in excess of 16,000 TPH requires very exact blade positioning, timing, gate, acceleration, and velocity to meet governmental metrological requirements. The control system required to provide that operation includes computer and PLC programming that not only allows the goal to be achieved but allows it to be achieved in a manner that minimizes maintenance required by the operating equipment.

Bulk Material Flow Control Gates (Part 1)

2019-04-04T17:25:46-05:00

Producing, processing, transporting, and using bulk materials requires gates to control the volume and direction of material flow. Purvis Industries can provide state of the art flow control gates of all different types. Purvis Industries’ standard gates are designed for bulk material of highly abrasive material with bulk densities of up to 120 PCF. This blog will describe several of the standard gate types we offer and briefly describe their operation and use.

Horizontal Slide Gates

These gates may be single or double blade gates of standard or custom sizes. When closed these gates stop the flow of material and when open allow the material to flow freely. When required the gate position can be modulated to control the rate of material flow.
Horizontal slide gates consist of a gate frame, gate blade supports, blade support protection liners, blade support quick inspection and replacement equipment, gate blades, gate blade actuators, gate blade position indicators, one touch doors to facilitate access to the actuator to blade attachments, and unique hermetic seals. The standard gate actuators are hydraulic cylinders. Options are available for electrical, pneumatic, or manual actuation. Standard gate blades are made of 360 BHN steel plate lined with 7 ga. stainless steel. Gate blade supports are special UHMW blocks designed to minimize gate blade wear and gate maintenance.
Several options can be ordered with these gates. Blade position indicators, a very wide range of operational controls, dust tight seals of proprietary designs, high gate speeds, and precision gate blade position control are a few of the most requested options.
Standard sizes of single blade horizontal slide gates are 24” to 60” width and 24” to 60” stroke. Standard sizes of double blade horizontal slide gates are 24’ to 60” width and 48” to 60” stroke.

Rack and Pinion Gates

These gates may be single or double blade gates of standard or custom sizes. When shut these gates stop the flow of material and when open allow the material to flow freely. The gate position can be adjusted to set the rate of material flow. The gate blades are made of 360 BHN steel plate lined with 7 ga. stainless steel plate. The blade supports may be specially designed UHMW blocks or cam followers.
Rack and pinion gates are generally operated by a hand wheel or chain wheel attached to a gear reducer. The force required to open a gate is limited to not more than 50 lb.

Rod Gates

- These gates consist of steel frames with perimeter openings into which round rods may be manually inserted. When all rods are inserted, material flow is shut off. This is sometimes done to facilitate maintenance downstream of the gate. Rods can also be inserted to control the flow of material through the gate.
- Standard rod gate sizes are 24” to 60” wide and 24” to 60” long.

Diverter Gates

Diverter gates are used to direct material in the desired direction. Purvis Industries’ standard diverter gates are built of ¼” mild steel plate with 3/8” 400 BHN liners in wear areas. The gate blades are 3/8” mild steel plate welded to a round shaft supported as required by internal stiffeners. Blade liners are 3/8” 400 BHN steel plate. Standard outlets for the diverter gates will be 30 degrees or 0 degrees from vertical.
Each gate blade will be equipped with a removable panel to facilitate blade removal for maintenance without having to remove the entire gate.
The gates will be 18” square inlet with two or three 18” square outlets. Each blade shaft will be supported by two flange bearings bolted to the gate body. A torque arm will be attached to each shaft to transfer the torque to rotate the blade. The standard torque arm actuator will be a hydraulic cylinder. Alternate actuators may be electric, pneumatic, or manual.

Bucket Elevators

2019-03-13T04:19:59-05:00

Bucket Elevator Selection

The following factors should be considered when selecting a bucket elevator.

1.) Type and characteristics of material being handled:

2.) Abrasive, free-flowing, sluggish, temperature, fluffy, friable, subject to degradation etc.

3.) Weight of material in pounds per cubic foot.

4.) Maximum rate in tons, bushels or cubic feet per hour.

5.) Maximum size and percentage of lumps, along with average size of material.

6.) Operating conditions: indoors, outdoors, corrosive, contamination, etc.

7.) Service required: continuous or intermittent.

Common Types of Bucket Elevators

Centrifugal Discharge Elevators

Elevators of this design are predominate in the bulk handling of free-flowing, fine, and loose materials with small to medium size lumps. They are generally selected when there is a need to move large amounts of material quickly. Instead of direct loading, the buckets serve as the loading apparatus, scooping material up from the boot/inlet section. For this reason, durable buckets should be selected with this design. Centrifugal force at the head pulley “throws” material into the discharge chute. The buckets are spaced in wider intervals to prevent discharge interference from the preceding bucket and to assure maximum fill of the buckets at the boot end (inlet) while moving at a higher speed. The design of this style yields optimized material fill and reduced interference between buckets. They can be supplied with belt or chain depending on the material be handled.

Continuous Discharge Elevators

Continuous elevators are designed to handle friable, fragile materials to minimize product degradation, or damage. They are also ideal for handling sluggish or abrasive materials. They are used to convey light, free-flowing material where aeration of the material must be avoided. Material is fed directly into the bucket from the inlet chute. The buckets are designed for gentle discharge, the buckets are closely spaced on the belt or chain to allow the material to flow over the backside of the preceding bucket, whose extended sides form a chute to guide the material into the discharge spout. Direct loading of the material combined with the slow speed of this elevator avoids the “throwing” action associated with centrifugal style elevators. Making it ideal for use with fragile materials. This style elevator can also be supplied with belt or chain.

Positive Discharge

Positive discharge elevators operate efficiently at low bucket speeds and are suitable for handling light, fluffy and fragile materials and those having a tendency to stick in the buckets. The buckets are mounted at spaced intervals, are loaded by scooping up material from the boot or by feeding the material directly into them. After passing over the head wheel, the buckets are inverted over the discharge spout, thus providing a positive discharge of material. Normally, the buckets are malleable iron mounted at intervals on double strands of chain.

BEARING REPAIR: THE BASICS

2019-03-01T06:01:53-06:00

Bearing repair is a way for our customers to gain more service life out of their bearing. Once a bearing is damaged in service, your entire operation could be compromised. The damage can lead to additional costs, such as longer maintenance schedules, unscheduled downtime and possibly missing on-time delivery dates to your clients. Our experts will review the total cost to own and operate the bearing in the operation. Bearing repair can be effective approach to extended life and can help save our customers money by not having to purchasing a new replacement bearing. Bearings that are candidates for repair should be removed before they are able to reach their full usefulness.

A repair facility is able to frequently return a used bearing back to like-new specifications in about one-third of the time it would take to source a new bearing. Repairs can even be completed in as little to two to four weeks depending on the type of repair that is required. You may find that bearing repair facilities may be able to provide a warranty on the services that they perform.

Bearing repair when compared to the manufacturing of a new bearing is also a much more environmentally friendly procedure. Our repair team is able to return the bearing to like-new specifications with less energy input, while

simultaneously reducing the raw materials and waste production.

Bearing repair facilities offer various categories or types of “repair”. Normally, repairs will falls into one of three categories.

Level 1: Generally involves the recertification of the bearing (i.e. the cleaning and full inspection of each of the bearing’s components.)

Level 2: Consists of reconditioning a bearing where it is cleaned, inspected and then polished to remove minor imperfections.

Level 3: The most extensive and can be referred to as remanufacturing. Raceways could be reground, rollers and cages replaced with a new set and even a full replacement of a bearing raceway. The remanufacturing process we can also modify a bearing like changing the clearance on a spherical roller bearing.

These three levels of service are best suited for bearings with a 12” inner diameter or larger. For bearings smaller than 12" inner diameter or larger than a 3" inner diameter, are able to be reclaimed. The smaller bearings are usually thrown away, but they can be reclaimed if an economical lot size can be gained. In the reclaiming process, the bearings are cleaned, inspected and polished.

FAQS

Q:What types of bearings can be repaired and what is the max size?

A: Options: Ball, Cylindrical, Spherical, Taper and Slewing Rings can be repaired. The max size is usually around 240” on the outer diameter.

Can a bearing be repaired more than once? Bearings can be repaired more than once. Depending on repair facility most use a rule of thumb of no more than three level three repairs on one bearing.

Q:Will the repair shops repair bearings that they didn’t manufacture?

Most s,facilities will repair bearings and they don’t manufacture.

VFD Stopping Techniques

2019-01-29T18:12:23-06:00

We have a history of receiving inquiries on how to add braking to VFD applications. The terminology can often be challenging and confusing. There is always a possibility that the application is not a good fit. Below are three different types of braking techniques often used with a VFD.

What is Regenerative Braking?

Regeneration Braking occurs when a coasting or decelerating electric motor is driven by the load causing the motor to generate electricity. This is very similar to a generator. This can be best described using is a four-quadrant graph with Speed and Torque are plotted on X-Y Axis.

The motor is in “Regenerative Braking (REGEN)” when the following two combinations are met.

Combination 1:

Speed is Positive (+) and Torque is Negative (-)

Combination 2:

Speed is Negative (-) and Torque is Positive (+)

What is Regenerative Braking used for?

Regenerative Braking is used for applications that require continuous braking or have high inertia loads. This is the most expensive solution, but some of the cost can often be recovered by feeding the generated electricity back into a power grid. For example, this is similar to a conveyor that is running downhill where the weight on the belt causes the belt to move without a power driven motor.

What is Flux Braking?

Flux braking is a controlled method used to increase motor loss (slip). Motor loss, also known as slip, is the total difference in speed between the rotor speed and the rotating electrical field of the stator. When braking is required, the flux (current) in the motor is increased, which in turn increases the motor’s capability to brake. By raising the level of magnetization (current) in the motor, the load can be quickly decelerated. By increasing the flux in the motor, the energy of the mechanical system changes to thermal energy in the motor.

What is Dynamic Braking?

Dynamic Braking is where a braking chopper is used with a braking resistor. As the demand for braking increases, a switch turns on and bleeds the excess energy off to a resistor. Basically, the braking chopper serves as an electrical switch that connects the VFDs DC bus voltage and to a resistor, where the braking energy is converted into heat. During the deceleration period, the motor switches to a generator operation and supplies energy back through the inverter.

Summary:

Regenerative Braking – Breaking energy is converted to electricity and returned to the grid
Usually the most expensive solution
Often requires a Regenerative Drive
Can be used without limit

Flux Braking – Uses current to create magnetic flux to stop a motor
Good for lower horsepower(HP)
Over fluxing reduces slip, which reduces the braking capability
Usually the most cost efficient solution
Can only be used intermittently due to the heat generated in the motor

Dynamic Braking – Requires a brake chopper and brake resistor
Power is dissipated by heat
Brake Resistor must be sized by load and duty cycle
Can only be used intermittently from heat generated from the resistor

Let's Start Something

2019-01-04T14:54:15-06:00

Consider this; every motor needs a way to be started and stopped, and in some applications, reversed. The 3 most common will be discussed below, including the pros and cons of each method.

Direct On Line (DOL), also referred to as across the line starting, is the most common with the lowest initial cost, but with the most drawbacks. This is accomplished with a motor starter, which consists of a contactor and a motor overload. This method delivers high torque at startup, but puts the most stress on the system, mechanically and electrically. When using this type of starting, the starter immediately delivers full voltage to the motor, resulting in a temporary inrush current resulting in current draws up to 6-8 times the Full Load Amp (FLA) rating of the motor. An example of this is a 30HP motor with a nominal FLA of 40 Amps; starting DOL, the motor could see 320 Amps until the load gets up to speed. It stresses the motor windings, reducing motor life and can cause belts to squeal, reducing their life, and cause severe stress to couplings, gearboxes and whatever else is connected to the motor shaft, reducing their life as well. For reversing applications, the correct starter must be utilized. It consists of 2 contactors (to switch 2 of the 3 leads) to achieve reverse, and an overload to protect the motor. The overloads react relatively slowly versus the next 2 options. Note: The motor always goes to full speed.

Soft starters are another method of starting a motor. It ramps up the voltage being delivered to the motor, resulting in a “soft” start. It reduces the inrush current and can prevent belt slippage and is easier on couplings and the other components connected to the motor. There is a reduction in the startup torque with this method. It is easier on the motor windings because the inrush is typically half of the DOL method above. Most soft starters can be reversed with a switch closure to an input on the device. This is slightly higher in cost vs DOL. Again, the motor always goes to full speed but there is more control of the start and stop.

Variable Frequency Drives, also referred to as VFDs, can deliver full torque at start up and there is no inrush current with this method. Ramp up (acceleration) and ramp down (deceleration) times can be set for smooth starts and stops. Adjustable speed is another benefit of this type of motor control. It is by far the easiest on your motor and system, if the motor is suitable for use on a VFD. Minimum and maximum speeds need to be considered to ensure the correct motor is being used. This method comes at a higher cost than the previous 2 types, but in the right application, it can reduce energy consumption and there are incentives available from the utility companies to offset the initial cost, but the energy savings with also help pay for the VFD. To learn more about the incentives available, you can go to www.DSIREUSA.ORG . All that is required is the Zip Code of the customer location for a list of Policies and Incentives.

Feel free to contact your Purvis Industries sales representative to get connected with one of our Triad RAE for assistance in locating the best solution for you!

CHOOSING BETWEEN BALL BEARING & TAPERED ROLLER BEARING CONVEYOR IDLERS

2018-12-04T15:38:27-06:00

Article By: Dave Corson, Business Development Manager, IMSCO - Mining Division

In today’s increasing demands put on belt conveyors to transport material from point A to point B, the proper selection of the components used on your belt conveyors becomes even more critical. This includes, but is not limited to higher tonnage demands, wider belts to meet these demands, and the ever increasing belt speeds (FPM) that must be achieved for these increasing tonnages.

Over my 30+ years of experience working around these conveyors in the mining industry, I have had multiple discussions with mine managers, engineers, maintenance superintendants, etc. regarding the “myths” and “truths” around selecting conveyor idlers for their projects. There are a number of “quality” manufacturers in the USA, some of which manufacture BOTH, tapered roller and ball bearing idlers.

Let’s look at some of the “myths” surrounding the ball bearing -vs- tapered roller bearing controversy.

Ball Bearing idlers have lower rotational resistance.
- This is FALSE. The rotational resistance of the conveyor idler roll, depends on the grease and seal arrangement of the idler. NOT the bearing.
Tapered Roller bearings MUST BE REGREASED.
- This is FALSE. Although regreasing any bearing is always recommended to extend the life of the bearing and equipment, “Sealed for Life” tapered roller bearing idlers have been manufactured in the USA for over 40 years, and have proven themselves in the most demanding mining applications.

Tapered Roller bearings MUST BE PRELOADED.
- This is FALSE. Quality manufacturers of TRB idlers, assemble their roll heads with strictly controlled, proprietary end play tolerances. NOT PRELOADED!
Tapered Roller bearing idlers allow only 3 minutes of angular misalignment.
- This is FALSE. Quality manufacturers of TRB conveyor idlers, use “Modified Geometry” tapered roller bearings, and are tested to withstand up to 16 minutes of angular deflection.

We offer our customers many options when “selecting” the BEST “solution” for maximizing the life and efficiencies of the components on their belt conveyors. Choosing the right component for their application, is key to providing long term solutions for them.

In upcoming discussions, we will look at the different bearings and calculated L10 life associated with the different bearings used today in conveyor idlers.

WHY VARIABLE FREQUENCY DRIVES?

2018-12-04T04:13:30-06:00

Article By: Don Lovella, Triad Automation Business Development Manager

There are many reasons our customers use variable frequency drives, but probably the most popular reason is to control the speed of their electric motor. While other customers might want to use a VDF to start and stop their machine in a specific location or time. There are other customers who might want to use a VFD to minimize the impact on the mechanical system. VFD's accomplish all these tasks by controlling the speed, position or acceleration of an electric motor. While this all sounds great, there is still more a VFD can do for you and that is save energy! Saving energy is probably the most overlooked benefit of using a VFD.

For example, a typical variable torque application such as a fan or a centrical pumps energy consumption can be reduced approximately 50% by slowing down the motor just 25%!

Here are some interesting facts about electric motors.

Electric Motors consume approximately 28% of the world's total electrical energy production.
The cost of an electric motor is approximately 1% of the total cost of ownership of an an electric motor while the other 99% is the cost of the electricity to operate that motor.
Approximately half of all electric motors installed today could reduce energy consumption with the addition of a VFD.

This means there are literally hundreds of thousands, if not million, application within our industry that could benefit by the addition of a VFD! I'm sure by now you're thinking to yourself this all sounds great, but the I'm not sure we can afford the expense of adding VFD's. What if I were to tell you that electrical utility providers offer rebates to their customers to install VFD's and save energy? This sounds crazy, but on average the rebate from an electrical utility provider for a VFD package install is $125 a horse power. I know you're thinking I don't have the time to search through my utility providers website to find the rebates. I have an answer for that as well. There is a website 'www.dsireusa.org' that we have used for many years to assist our customers in locating rebates provided by their utility provider. All you have to do is enter your zip code in the search field and ti will take them to a page that shows rebates provided by their utility providers.

ACCUMULATORS IN A HYDRAULIC SYSTEM

2018-10-05T19:54:07-05:00

By: Ron Polvado

To start, there are three basic types of accumulators:

1) Bladder

2) Piston

3) Diaphragm

What are Accumulators?

Accumulators are gas charged vessels that can reduce pump capacity, provide auxiliary hydraulic power in case of an emergency, limit pressure fluctuation during temperature changes in a closed loop, compensate for leakage, minimize pump pulsations and acts as a system shock absorber.

When are Accumulators used?

It's used to increase system performance and efficiency, lower operating and maintenance cost, and provide fail safe conditions.

Comparison of Standard Accumulators

BENEFITS FROM ELECTRIC MOTOR BRAKING

2018-07-26T14:39:16-05:00

THE BENEFITS FROM ELECTRIC MOTOR BRAKING

Many of our customers have the need for electric motor braking whether they know it or not. Most of them are unaware of the potential time and energy savings they could receive from electric motor braking.

Once it has been determined braking is required there are several things that need to be known. The first thing that must be known is the weight and speed of the rotating load, next is how fast that load needs to be stopped and lastly how frequently this load needs to be stopped. Once these factors are known it can be determined if the VFD drive can stop the load on its own or if additional items are required. In the event the VFD drive needs additional items to stop the load you might need to add a braking resistor and or the brake chopper option to the VFD drives order code. On some occasions you will need to add an external brake module to a VFD drive that doesn’t have the option for a brake chopper. These items are used together to dissipate the excess energy that is created when stopping a rotating load.

The most common solution is to dissipate this energy by turning it into heat with a braking resistor. Braking resistors have a duty cycle and come in varying wattages and resistances. They can only be on for a certain amount of time and will require more time off to cool down.

On applications that stop frequently or that create an excessive amount of power it is recommended to use a regenerative unit or a regenerative VFD drive. This solution turns this excess energy into electricity that can be used elsewhere in the customers plant. While this solution is more expensive to purchase initially it’s cost can be recovered by the energy savings it creates for the customer.

Your Regional Triad Application Engineer can assist you and your customer in the decision of a braking resistor or a regen unit for their application.

ABB - PSTX SOFT STARTERS

2018-06-14T13:19:29-05:00

Article Submitted By: Robert Witte

When ABB first released their new PSTX Soft Starter, they were only available up to 370 amps (300 Hp @ 480v) but they have now released the full product line up to 1250 amps (1000 Hp @ 480v). Previously, all ABB Soft Starters were a part of the ABB Low Voltage Products Group, along with products like Circuit Breakers, Across the Line Starters, Disconnect Switches, Pilot Devices, etc. Recently the Soft Starter product line has been re-aligned into the ABB Low Voltage Drives Group.

The PSTX Soft Starters are the most feature rich Soft Starters on the market today. The detachable keypad is modeled after the current ABB Drives keypad, such as the ACS880, ACS580 and soon to be released ACS480. Navigating through the keypad to program the Soft Starter parameters has the same flow as the current ABB drives, making programming very easy, even without using the manual for assistance. This user-friendly keypad is standard on all PSTX Softs Starters.

The PSTX Soft Starters offer a Slow Speed Jog Function, in both Forward and Reverse Directions. This feature allows for greater flexibility when operating equipment such as Conveyor Belts and Cranes. This feature provides positioning capabilities, allowing the operator to take greater control of their process.

The PSTX Soft Starters also includes a Built-in Bypass which saves time and energy. When reaching full speed, the PSTX will activate its bypass. This saves energy while reducing the soft starter’s heat generation. This Built-in Bypass saves installation time and saves valuable panel space.

The PSTX Soft Starters includes many application enhancing features, such as Torque Control – the most efficient way to start and stop pumps. The Pump Cleaning feature can reverse pump flow and clean out pipes, securing uptime of pump systems. These features provide more complete control of pump systems.

The PSTX Soft Starter offers complete motor protection in a single unit. They can handle both load and network irregularities. The Built-in Electronic Overload Relay, available RTD inputs, Earth Fault Protection and Over/Under Voltage Protection along with many other functions designed to help keep motors safer than ever before. The PSTX also offers three types of current limits: Standard, Dual and Ramp, providing full control of motors during start. This also allows motors to be used in weaker networks.

BEARING LUBRICATION FAILURE

2018-06-04T16:25:43-05:00

Article by: Dan Whitehouse

Bearing failure due to lubrication is a very common occurrence. About 50% of bearing failure is related to lubrication. Below is a section of an article from www.machinerylubrication.com that details eight failure mechanisms. (Full article can be read here.)

When in doubt, it does not hurt to ask when lubrication comes into play.

1. Unsuitable Lubricant - First, you must choose the correct lubrication for the application. Fundamental properties, such as the viscosity, additive package and consistency (for grease), should be carefully selected based on the bearing type, speed factor and operating conditions. If these factors are not thoroughly considered and an unsuitable lubricant is applied, the lubricant may become overly stressed or be insufficient for the machine's lubrication needs. In either situation, the bearing will likely undergo premature wear and failure.

2. Lack of Lubricant - For greased bearing applications, the correct regreasing volume and frequency must be established to ensure the bearing load zones are lubricated properly. Too much time between regreasing intervals or applying too little grease will cause excessive boundary and bearing wear.

3. Excess Lubricant - More grease is not always better. When too much grease is added to a bearing in a medium to high-speed applications, the temperature will rise from the churning, and the machine must work harder to overcome the fluid friction. As the temperature rises for the excessive grease charge, the viscosity will drop and other adverse effects will ensue.

4. Hot Running Conditions - A bearing running at a higher than expected temperature can be either a root cause or a symptom. If the bearing is exposed to an external environment that is exceptionally hot, this would indicate a root cause.

5. Solid Contamination - Solid contaminants can enter a system in a number of ways, including through a new lubricant, ingested from a headspace port or hatch, via defective seals, etc. The type of solid contaminants can vary depending on the source, but typical airborne dust/dirt will consist primarily of sillica and alumina.

6. Moisture Contamination -Similar to solid contaminants, moisture can enter a system in many different ways, including through headspace entry point, seals, or new oil. WHen the headspace is humid, thermal cycles can cause moisture to escape the air, sweat onto surfaces, and find its way into the oil through gravity. Moisture may exist in a lubricant as dissolved, emulsified or free water. Emulsified water has the most destructive potential in oil.

7. Mixed Lubricant -Topping up (if oil) or regreasing (if grease) a bearing with the wrong lubricant can drastically change the physical and chemical properties of the resulting lubricant mixture. Not only can factors like the wrong viscosity impact lubrication, but additives can also react negatively with each other, impeding their functionality.

8. Other Contaminants - Depending on the machine type, bearings maybe introduced to other processed chemicals, blow-by contaminants, glycol, etc. Based on the type of contaminant, the lubricant can change chemically or physically, resulting in lubrication failure.

HYDRAULIC FILTRATION

2018-06-04T16:22:49-05:00

By: Steve McKown

The life blood of the hydraulic system is the fluid, and it is usually the last thing customers think about. Contamination in the fluid causes wear and damage to the moving components inside a system and can cause it to fail prematurely. Here are some key points on filtration.

New hydraulic fluid: New fluid should be filtered before it is used. Contamination gets into the fluid during the processing and packaging phases. Filter carts are available to clean this fluid before use.

Cleaning the tank: If the filters in the system clog they will bypass and return unfiltered fluid to the tank. Particles settle out of the fluid over time leaving a layer of gunk at the bottom of the tank. Most tanks have a panel that can be removed to clean this out. Keeping the tank clean will increase the life of the filters.

Suction filters in the tank: In the past most tanks were equipped with a screen suction filter on the suction tube. The tank manufacturers are going away from this because if the screen clogs it can pull a vacuum on the pump causing a catastrophic failure. They now rely on the high and low-pressure filters to catch debris in the system. This makes the high-pressure filtration even more important.

Desiccant Filler Breathers: Moisture acts like an abrasive in the hydraulic systems and can cause severe damage. Using a desiccant breather cap removes the water vapor from the air.

Return Filter: this is the filter you will see most often on the power units. This cleans the fluid as it returns from the system and removes the particulates introduced into the fluid by the seals, hoses, and moving parts in the system before returning to the tank. If you have a worn rod seal on a cylinder it will pull dirt into the cylinder contaminating the fluid. If you have older hoses they deteriorate and dirty the fluid.

High-pressure filter: This filter cleans the fluid coming out of the pump before it goes into the system. The main purpose of this is to remove any of the wear particles that the pump produces. It also prevents damage to the system if the pump fails for any number of reasons.

Testing your fluid: Donaldson has fluid testing available and we stock the test kits. Testing the fluid does more than just tell you the oils dirty. They examine the particulates in the oil and can report on the health of your system components since each item (pump, hoses, cylinders, valves, etc.) produces a specific type of contamination.

Reach out to one of our sales representatives to learn more on how we can assist you with your filtration needs.

GATES MECTROL: PC-10 AND PC-20

2018-05-11T07:41:49-05:00

By: Ken Harville

Looking for a better posi-drive belt option? Do you have complaints about excessive stretching, wavy belt edges, and sprocket disengagement? Do you need to replace plastic modular with fabric belting? Well then here is the answer, Gates Mectrol's Posiclean PC-10 and PC-20 Kevlar reinforced, posi-drive belting.

PC-10 and PC-20 are 1" and 2" pitch respectively, and their Kevlar reinforcement eliminates belt stretch, wavy belt edges and sprocket disengagement. Further, these 2 belts allow customers the ability to drive these belts without the aid of UHMW "shoes". Shoes increase belt wrap at the drive sprockets, but they also cause wear and tear on the belt’s surface and edges.

PC-10 and PC-20 can effectively and affordably replace many plastic modular belts. They can be perforated for vacuum or drainage, and are available with various heights T and Scooped flights, corrugated sidewalls, metal and plastic lace, and vulcanized endless.

CapCorp branches Fort Worth, Houston and San Antonio, as well as Gates Mectrol, are fully equipped to service your endless field splicing needs. Gates has been working with Purvis Industries in the Houston and San Antonio regions, as well as expanding to Arkansas region. Please contact one of your local sales representatives to learn more!

BEARING CURRENTS

2018-05-11T07:29:22-05:00

By: Blake Timmons

With the increased use of Variable Frequency Drives (VFDs) in industrial and commercial electric motors, there becomes a source of current flow through the bearing. Note that inverter-induced bearing currents and premature bearing failures occur in a relatively small percentage of installations and applications. Nevertheless, it's best to understand the topic when you run across the problem.

The damage to the outer or inner race of a motor bearing will look something like the pictures below.

The damage to the outer or inner race of a motor bearing will look something like the pictures below. In these photos. Notice that the “fluting” is seen as symmetrical damage which is a common sign of a bearing current issue. Also notice that damage can also occur that is not symmetrical, shown by individual random spots on metal surfaces. With motors using with an inverter, you need to be aware of the high-frequency current paths from the motor back to the inverter and to ground. This will help in understanding potential bearing current problems and remedies. High frequency motor bearing currents can occur in any motor driven by a drive using Pulse Width Modulation (PWM). This is a switching technique that switches high poser transistors to recreate the AC wave form to the desired frequency. The currents that are created are, induced into the motor bearing creating a current path and can damage the motor bearings if they reach a sufficient magnitude. Note that the, motor bearing damage typically doesn’t occur rapidly.To help in our understanding of this issue, realize that High Frequency motor bearing currents can be generated in three different manners. These can be present individually or in combination.

The first mode is the Circulating type.

This type normally only occurs in larger motors greater than 100 Hp. Essentially non-symmetrical or uneven currents in the motor’s magnetic paths cause a magnetic coupling to occur that induces a high frequency voltage into the rotor along its axial length. If sufficiently high, this induced voltage causes a current to flow in the circular path created by the rotor shaft, the two bearings, and the frame of the motor.

The second mode is from Capacitive Coupling.

This type occurs in all motor sizes but is normally only a problem in smaller motors. Current is coupled from the stator windings to the rotor. This tends to raise the voltage of the rotor above that of the stator frame. If the voltage reaches a sufficient level, one of more of the motor bearings will experience a voltage breakdown and discharge back to the stator frame. This discharge current tends to damage the motor bearings – most noticeably the inner race.

The last mode is from the Frame Voltage or Shaft Grounding.

This type of bearing current can occur in any size motor; however, it is limited to those cases where the motor’s physical load provides an electrical grounding path. Current is coupled from the stator windings to the stator frame, and tends to raise the voltage of the frame above that of the drive’s equipment ground. This type of high frequency bearing current can also damage mechanical load bearings such as gearbox or pump bearings.

A Grounding Brush from Helwig Carbon can be used to reduce the shaft voltages.

Shown below is a recommended shaft grounding device. As can be seen by the two oscilloscope traces, the shaft grounding device dramatically reduces rotor
shaft voltage to a safe level that avoids dangerous bearing currents.