Support - All about bearings

Support - All about bearings

By Grant Laidlaw

A look into the basic procedures applicable to bearings to see how failure can be prevented. 

Moses asks: Grant, I work in the industrial air-conditioning industry. We deal with bearings on our pumps, cooling towers, electric motors, and air handlers, but we don’t have procedures for this and when we replace the bearings, the new ones fail quickly. Can you help with a solution?

Hi Moses, yes I can help. I have previously discussed the types of bearings used, but let us look into basic procedures applicable to bearings. Of course, as a basic principle, always try to determine the cause of the failure in the first place.

Let us begin with seals.

Whenever you apply force, always apply it through a sleeve or similar means to the ring that is being moved. Never transmit force through the balls.

All installed bearings are sealed from the atmosphere in one way or another. If they are carried in housings such as a plummer/pillow block or a flange unit, the housing will be provided with the seals. In some cases, the bearing itself is provided with an integral seal or shield on one side, or seals/shields on both sides. In the latter, the bearing is factory lubricated for the life of the bearing. Often, you will find the bearing size followed by a ‘z’ or ‘zz’; alternatively, the size is followed by ‘rs’ or ‘2rs’. This simply means the following:

6202:               No shields or seals
6202 z:            Shield on one side
6202 zz:          Shield on both sides
6202 rs:           Seal on one side
6202 2rs:         Seal on both sides.

Bearing with shieldBearing with shield (z and zz).

bearing with rubber sealBearing with rubber seal (rs and 2rs).

Practices to be followed

Always follow these practices with bearings, new or old:

  • Never remove bearings from their packaging until the very last minute (immediately before mounting).
  • Inspect housing seals and replace if necessary. Always replace a rubber seal.
  • Leave rust-inhibiting coatings on new bearings intact, except for wiping this away from the bore and from the outside of the outer ring.
  • Work only in a clean environment; do the job away from any other operation (grinding, drilling, and so on) that is contaminant producing.
  • Cover and protect all bearings during standing time or breaks.
  • Thoroughly inspect used bearings that are being reused, as well as new bearings which are being taken from previously opened packages or from damaged packaging.
  • Clean and dry all components.
  • Do not spin bearings with compressed air. Although entertaining, this is an extremely dangerous practice, as the bearing may disintegrate or be ricocheted at high speed, creating a high likelihood for injuries.

Cleaning bearings

From time to time, you may wish to clean bearings for further use.

Except:

  • Do not clean new bearings that you have taken from unopened and undamaged factory packaging.
  • Do not clean (except by surface wiping) any bearing with integral shields or seals.

Cold cleaning: Wash in clean, white spirits or paraffin. Dry the bearing and protect it against rust by applying oil or grease immediately after drying.

Hot cleaning: Heat some thin, clean oil in a clean container over a hotplate to 120°C maximum. The oil selected must have a flashpoint of at least 250°C. Use a thermometer to determine the temperature of the oil. Use clean, long-nose pliers to hold the bearing in the oil. (This method is very effective.)

Dry immediately after cleaning, using a clean and lint-free cloth. Do not use cotton wool. Apply grease only at the last minute, immediately before you close up the bearing housing.

Fitting of bearings – tips

Never transmit force through the rolling elements. Whenever you apply force, always apply it through a sleeve or similar means to the ring that is being moved. Never transmit force through the balls. In other words, never apply pressure to one ring in order to move the other. This is true whether a press, pulley, or hammer and sleeve arrangement is being used.

The solution is not always as easy as is shown here. Sometimes special tools or spacers are required, and often you will have to do some thinking.

Never directly strike a bearing’s rings, cage, or rolling elements. Apart from direct damage to the bearing, metal chips could fly off and remain in the bearing. These will do great harm.

Before starting to fit any bearing, confirm that it is the correct size for the job. That includes any prefix or suffix that appears with the basic number.

Ensure that the bearing is entering squarely onto the shaft as you start to fit it.

Cold mounting

Bearings of up to 55mm may be cold mounted onto a shaft. A one-piece capped tool (sleeve or drift) should be used between the hammer and the bearing. Check carefully that there is no loose dirt or half-chipped pieces of metal on this tool. Use a standard steel hammer. Soft-headed hammers tend to have breaking fragments fly off. These could enter the bearing, which is to be avoided. Do not hit a bearing directly with a hammer, as this will cause damage to the bearing and may cause splinters of metal to fly off at high speed, causing damage and possible injury.

Press mounting

When cold mounting is being performed, it is always better to mount a bearing using a hydraulic press, if one is available. Bearings for a shaft size of up to 100mm may be mounted in this way. Follow safe working practices/procedures when using a press.

Hot mounting (oil bath)

Cold mounting may only be performed on bearings of up to 100mm in diameter. Hot mounting is necessary for larger sizes.

Hot mounting entails heating the bearing, which expands, allowing the bearing to simply be slid into the correct position on a shaft. As it cools, the bearing will shrink and tighten on the shaft. Sealed bearings, which are factory lubricated for life, may not be fitted by using any heating technique.

The oil bath must have a thermostat controlling the maximum temperature.

Procedure:

  • Heat the bearing up to 110°C in order to fit it to the shaft. Never heat any bearing to a temperature over 125°C. The oil bath may contain an immersion heater, or it may be heated over a hot plate.
  • The oil bath must have a wire mesh to support the bearing clear of the bottom and sides, and well clear of the immersion element.
  • The heated oil must freely flow around all parts of the bearing to ensure uniform heating. Under no circumstances allow the bearing to touch the heat source.
  • Never be tempted to heat a bearing that is to be installed with a flame. This is totally unacceptable.
  • Do not leave a bearing unattended while it is being heated in an oil bath.

Heating using an induction heater

An induction bearing heaterAn induction bearing heater.

An alternative heating method that does not involve hot oil, is an induction heater. This uses the principles of a transformer to induce a group of eddy currents within the metal of the bearing. This process will heat the bearing uniformly from within. The bearing is placed over a bar of laminations, which is then positioned correctly over other laminations of the induction heater unit. The unit will have electronic control of induced temperature.

A contact-type electronic thermometer may be used to read the temperature of the bearing inner ring (or outer ring, in the case of an interference fit to the housing). Disconnect the inductive heating field before taking a reading.

When the bearing is up to the fitting temperature, lift it, using clean, heat-resistant gloves or a clean, lint-free cloth to hold the bearing. Slide the bearing into place on the shaft. Hold it in position until it cools sufficiently to take a grip on the shaft.

Note: Always use a thermometer; never take a guess at the temperature. Permanent damage will be caused to the bearing if it is overheated to beyond the 125°C limit.

Lubrication

If bearings are fitted to applications such as many pumps that run in an oil bath, then naturally this lubrication method will stay in operation.

Grease lubrication is by far the most common lubrication system for rolling element bearings. Bearing greases employ thickeners to ensure good adhesion to the important surfaces. You could find bearing grease in use with calcium, sodium, or lithium thickener. Lithium has all the advantages of the other two without their disadvantages, such as limitations of temperature operating range. Use a lithium grease as standard, unless you are re-lubricating bearings you know to be lubricated with another grease. In that case, re-lubricate with the grease of the type already in the bearing.

Do not mix greases. When cross-lubrication takes place, the mixed grease is softer than either of the original greases.

Never leave an opened can of grease lying about. This is certain to collect harmful dirt. If you are filling a grease gun, do so only from a can of grease you know to be completely clean. Do this task in clean surroundings. However, when you are filling a bearing with grease, do so from a tube. Provided you first clean the tube end, you are sure in this way of having contaminant-free grease.

If you are handling grease from a drum, do not use wooden paddles or spatulas to help with the job. They are sure to carry dirt. Only use metal items, which have been completely wiped clean.

Only just before you fit the shaft and bearing into the housing, completely pack the bearing with grease. Fit the greased bearing on its shaft into the housing and add grease to the housing until the bearing is between 30% and 50% filled with grease.

Note that petroleum products can cause skin allergies. Use disposable gloves or other protection to keep your hands from becoming covered with grease.

Do not mix greases. When cross-lubrication takes place, the mixed grease is softer than either of the original greases.

If in doubt when re-lubricating, open the bearing housing and remove as much of the original grease as possible. Then re-lubricate with lithium grease or the manufacturer’s recommend grease.

Moses, I hope this assists you with the basic procedures for bearings and that this helps you to minimise repeat failures.

Thank you for all your questions. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with “Solutions Page” in the subject line. You may include pictures.

References

  • ACRA
  • FAG Bearings
  • SKF Bearings.


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