Archive for the ‘Grease’ Category

Greasing Electric Motors (2)

December 26, 2008
Greasing Procedure
Following is the standard procedure for greasing ball bearings:

  1. Wipe grease from the pressure fitting, clean dirt, debris and paint around the grease relief plug. This prevents foreign objects from entering the grease cavity.
  2. Remove the grease relief plug and insert a brush into the grease relief as possible. This will remove any hardened grease. Remove the brush and wipe off any grease.
  3. Add grease per Table 2.
  4. Allow the motor to operate for approximately 30 to 40 minutes before replacing the grease relief plug. This reduces the chance that bearing housing pressure will develop.

How Often Should Bearings Be Greased?
Bearings should be lubricated at an average frequency as found in Table 3. Operational environment and type of grease may require more frequent lubrication.

Conclusion
It is recommended that the type of grease used on each motor is recorded in order to avoid premature bearing failure. In many cases, you may be able to standardize the type of grease used in a majority of your motors. It is also good practice to let your motor repair center know the type of grease in case the standard grease used by the repair center conflicts with your standard grease.
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Greasing Electric Motors (1)

December 26, 2008
The lubrication of electric motors can be a critical maintenance practice for improved motor system reliability. Unfortunately, a significant amount of misinformation is provided within industry. For instance, those with a lack of knowledge of how motors and bearing work will often promote the ‘purging’ of motor grease, frequently. This practice puts undue stress on the motor, reducing its life and increasing the chance for both bearing failure and winding contamination.

Grease purging is the practice of forcing grease completely through the bearing housing and bearing until old grease is removed and new grease shows at the grease relief plug. While this practice is performed in highly contaminated environments, it provides many dangers to the reliability of the motor. There was many a time, as a motor repair journeyman, that I would disassemble or troubleshoot a motor winding or bearing failure in which the motor was full of grease. Some grease additives will react with winding insulation or will just provide a thermal blanket, reducing the life of the winding, when purged grease leaks through the bearing or bearing cap and onto the winding, even when you have the grease relief open.

Bearing lubrication, on the other hand, is the practice of adding enough grease to allow for the lubrication of the bearing friction surfaces and the eventual removal of contaminants from the grease housing. The purpose of this paper is to provide an overview and direction for the proper lubrication of bearings.

How a Bearing Works

The most common type of bearing is the AFBMA-7 C-3 rated bearing. C-3 relates to the internal clearances of the surfaces of the bearing. In most motor rated bearings, there is a clearance of between 3-5 mils (thousandths of an inch) in which lubrication flows to reduce friction and wear of the machined surfaces. The bearing, itself, consists of an inner race, an outer race, balls and a cage which evenly distributes the balls. Common bearings are designed to allow for a radial load with some limited axial loading. ALL BEARINGS ARE LUBRICATED WITH OIL.
Grease, itself, is an oil sponge. The base (spongy) part of the grease varies depending on the manufacturer, temperature, environment and user preference. The grease holds the oil in suspension and allows the oil to flow during operation. The oil compresses between the bearing balls, inner and outer races and the cage, reducing friction. Ball bearings have small, microscopically rough surfaces on the balls, these surfaces move the oil, holding it to the ball during operation.
When too much grease is added, the grease is compressed between the bearing surfaces, increasing pressure and resulting with heat. Too little grease causes the surface friction to increase, resulting with heat. In any case, once bearing noise is audible, it has failed. Reducing noise by lubrication requires excessive grease, endangering the motor, and giving the technician the false security of extending the motor life when, in reality, additional damage is occurring to machined surfaces.
Bearings may also have shields or seals mounted on them. Bearing shields are metal fittings that have small clearances between the inner race of the bearing and contact the outer race on either side of the balls and cage. The small clearances near the inner race allows some oil and grease to move into the moving parts of the bearing, but prevents particles of large size from passing into the bearing potentially damaging machined
surfaces. Sealed bearings have seal surfaces touching the inner race, while ‘non-contact’ sealed bearings have extremely close tolerances between the seal surface and the inner race preventing particles under several thousandths of an inch. Sealed, and some shielded, bearings are referred to as non-grease able bearings.

Precautions In Motor Greasing

When greasing electric motors, there are a number of precautions that must be considered:

  • When electric motors are manufactured, or repaired, grease fittings may be put in place on motors that are not grease able. Your supplier should be able to provide confirmation that the motor may be greased.
  • Electric motors must be de-energized and locked/tagged out (LOTO) before greasing.
  • There should be no paint on grease fittings.
  • The average grease gun will introduce 1 ounce for every 23 strokes.
  • Grease compatibility (See Table 1). The additives in some greases do not mix well and can cause the grease to solidify or liquefy.

How Much Grease?

December 17, 2008

Bearing: Initial Fill

1. Completely fill interior bearing spaces

2. 30-50% housing packed with grease

  • Ensure that the seals are well lubricated
  • May be > 50% full if operating at low speed

3. Sealed for life / high speed bearings

  • 30% of bearing free space grease (sealed bearing)

Re-greasing Practices

1. Duct should direct grease into bearing rolling element contacts.

2. Bearing housing should have a relief which allows excess grease to escape.

3. Clean grease fitting before adding grease.

4. If possible bearing should be rotating (not true for some electric motors).

5. Bearing and housing should be cleaned and repacked after several relubrication cycles.

Limitations & Advantages of Greases

December 17, 2008

Limitations of Greases

1. Heat removal is not efficient

2. Dirt removal is difficult

  • Grease is not re-circulated and filtered
  • Purging may be helpful
  • Cleanliness/lubrication frequency are key

3. Requires special equipment to apply

  • More powerful pumps/largerlines
  • Grease guns
  • Automatic lubricators

Advantages of Greases

1. Provide good sealing

  • Keeps dirt and water out
  • Less dripping and splattering

2. Excellent for many high temperature operations

  • Does not “thin out”

3. Ideal for intermittent operations

  • Will not drain away when operation stops
  • Speed changes, reverse direction

4. Load carrying ability

  • High pressure / low speeds / shock loading
  • Solid lubricant additives: moly, graphite

5. Aid to mechanical operation

6. Can be “fine-tuned” to an application

Grease Compatibility

December 17, 2008

1. Unlike grease may be incompatible

  • Chemical interaction between thickeners/additives
  • Mixture breaks down in service
  • More severe at high temperatures and high rpm

2. Highest risk when greases are composed of different thickener types

3. Incompatibility can result in bearing failure

4. Don’t mix incompatible greases in application

How to Avoid Grease Compatibility Issues

1. Lab test give a good indication of potential compatibility problems

2. When changing greases in the application: Purge the old grease out the degree possible

  • Decrease re-lubrication interval to complete purge
  • Watch carefully during transition period (monitor temperature, look for excessive leakage, look for excessive grease softening or hardening)

Types of Greases

  • Lithium (70%)
  • Calcium (9%)
  • Polyurea (7%)
  • Clay (5%)
  • Aluminium (5%)
  • Sodium (3%)
  • Other (1%)

Introduction to Grease

December 17, 2008

What is Grease?

Base Stock + Additives + Thickener = Grease

  • A solid to semi solid product of dispersion of a thickening agent in a liquid lubricant. Other ingridients imparting special properties may be included.
  • To dicrease dripping, spattering, and leakage of lubricants.
  • To seal out contaminants.
  • When oil circulation is impractical.
  • To suspend oil additives.
  • For intermittent, extreme, and special operation conditions.
  • Sealed for life operations.

What makes up a grease?

  • Thickener may be soap, polyurea, or clay
  • Base fluid may be mineral or synthetic
  • Many different additives may be used

Types of Grease Thickeners

1. Complex Soap Thickeners Soap Based Thickeners

  • Fatty Acid + Metal Hydroxide = Soap
  • Soap named according to metal hydroxide (i.e. Lithium)
  • Fatty Acid + Di-Acid + Metal Hydroxide = Complex Soap
  • Di-Acid increases melting point and shear stability of soap

2. Polyurea Thickeners

  • Non-soap thickeners
  • Inherent anti-oxidant properties

3. Clay (Bentonite) Thickeners

  • Do not melt at high temperatures