SEPTEMBER 1998

Are your bearings bearing up?

Problem: We're greasing our bearings regularly, but they run hot and wear out more quickly than expected. What's wrong?

Solution: Too much lubrication can be almost as harmful as too little. Follow these tips.

Greasing bearings: When installing an open-type bearing, the free space is packed about one-third full. Ensure that the grease is forced directly into the raceway and turn the bearing after packing for even distribution.

At first glance, this amount will appear to be insufficient, as you probably were taught that in most cases, "more is better." Not here. The bearing will only use so much grease, and will expel the rest. Adding more than this will cause overheating due to fluid friction.

If it is overfilled, the increase in temperature may be enough to cause bearing failure.

After packing the bearing, fill the housing cavity about one-quarter full. This grease is mainly to act as an additional barrier against contamination.

If the end bell is designed with a grease escape valve, any excess grease will exit through it before excessive heat can build up. The escape valve is an excellent way of introducing fresh grease. When adding grease, the shaft should be rotating, not stationary.

Next, run-in the bearing to expel excess grease. Check the bearing temperature to be sure it doesn't exceed 70°C (158°F)--the housing will be about 60°C (140°F). (Editor's Note: The average person can hold a hand on a 60°C part for five seconds before the brain instinctively tells you to remove it.)

If the temperature gets too hot, shut down the equipment and allow the bearing to cool before restarting the process.

Quicktips

Transporting bearings: Bearings with a mass of 5 kg (11 lb) or more should be transported on their sides to prevent vibration damage known as false brinelling.

Hot installation of bearings: To overcome the interference fit between the inner ring and the shaft, it is necessary to heat the inner ring 70-80°C (158-176°F) above the temperature of the shaft. The maximum temperature should not exceed 120°C (248°F). The expense of purchasing a digital thermometer will pay off.

These and other bearing tips are included in the 34-page booklet, Get a Bigger Hammer, (EMH97) available from NSK-RHP Canada Inc. It was written to dispel some of the old rule-of-thumb practices of bearing maintenance and includes advice on storage, handling, installation, lubrication and bearing failures. It also has tables for shaft and housing fits.