Running-in of grease lubricated bearings
Grease lubricated super-precision bearings initially run with a relatively high frictional moment. If they are run at high speeds without a running-in period, the temperature rise can be considerable. The high frictional moment is due to the churning of excess grease, which takes time to work its way out of the contact zone. For open bearings, this time period can be minimized by applying the required quantity of grease distributed evenly on both sides of the bearing during assembly. Spacers between adjacent bearings can also reduce the running-in period.
The time required to stabilize the operating temperature depends on the following factors:
- the type of grease
- the initial grease fill
- how the grease is applied to the bearings
- the number and arrangement of bearings in a set
- the available space for excess grease to accumulate on either side of the bearing
- the running-in procedure
Super-precision bearings can typically operate with a minimum quantity of lubricant when properly run-in, enabling the lowest frictional moment and operating temperature to be achieved. Grease that collects on each side of the bearing acts as a reservoir, enabling oil to bleed into the raceway to provide effective lubrication for a long time.
Running-in can be done in several ways. Wherever possible and regardless of the procedure chosen, running-in should involve operating the bearing in both a clockwise and counter-clockwise direction.
Standard running-in procedure
The most common running-in procedure can be summarized as follows:
- Select a low start-up speed and a relatively small speed increment.
Decide on an absolute temperature limit, usually 60 to 65 °C
(140 to 150 °F). SKF recommends setting the machine with limit switches that stop the spindle if the temperature rise exceeds the set limit.
- Start operation at the selected start-up speed.
- Monitor the temperature rise by taking measurements at the bearing outer ring position, and wait for the temperature to stabilize. If the temperature reaches the limit, stop the spindle and allow the bearing to cool. Repeat the process at the same speed and run the spindle until the temperature stabilizes below the limit.
- Once the bearing temperature has stabilized, continue to run the spindle for an additional 10 to 15 minutes. Then, increase the speed by one increment and repeat step 4.
- Continue increasing the speed incrementally, allowing the temperature to stabilize at each stage, until the spindle reaches one speed interval above the required operating speed. This results in a lower temperature rise during normal operation. The bearing is now properly run-in.
This standard running-in procedure is time-consuming. For a medium- to high-speed spindle, each stage can take anywhere from 30 minutes to 2 hours before the temperature stabilizes. The total time for the running-in procedure can be 8 to 10 hours (diagram 1).
Short running-in procedure
An alternative to the standard running-in procedure reduces the number of stages and shortens the overall running-in time. The main steps can be summarized as follows:
- Select a starting speed approximately 20 to 25% of the attainable speed for grease lubrication (→ Product tables) and choose a relatively large speed increment
- Decide on an absolute temperature limit, usually 60 to 65 °C (140 to 150 °F). It is advisable to set the machine with limit switches that stop the spindle if the temperature rise exceeds the limits set.
- Start operation at the chosen start-up speed.
- Monitor the temperature by taking measurements at the bearing outer ring position until the temperature reaches the limit. Care should be taken as the temperature increase may be very rapid.
- Stop operation and let the outer ring of the bearing cool down by 5 to 10 °C (10 to 20 °F).
- Start operation at the same speed a second time and monitor the temperature until the limit is reached again.
- Repeat steps 5 and 6 until the temperature stabilizes for 10 to 15 minutes below the limit. The bearing is run-in at that particular speed.
- Increase the speed by one increment and repeat steps 4 to 7.
- Proceed until the bearing is running at one speed increment above the required operating speed. This results in a lower temperature rise during normal operation. The bearing is now properly run-in.
Although each stage may have to be repeated several times, each cycle is just a few minutes long. The total time for this running-in procedure is substantially less than for the standard procedure.