Grease service life and relubrication intervals

There are several interactive factors influencing grease service life, the effects of which are extremely complex to calculate for any particular application. It is, therefore, standard practice to use estimated grease service life based on empirical data.
The estimated relubrication interval for grease lubricated bearings is based on the estimated grease service life. Various methods can be used, however, SKF recommends the following to assist in making the best estimate for super-precision bearings.
Diagram 1 shows the relubrication interval tf for super-precision bearings in various executions. The diagram is valid under the following conditions:
  • bearing with steel rolling elements
  • horizontal shaft
  • operating temperature ≤ 70 °C (160 °F)
  • high-quality grease with a lithium thickener
  • relubrication interval at the end of which 90% of the bearings are still reliably lubricated (L10 life)
If necessary, the relubrication interval obtained from diagram 1 should be adjusted by correction factors depending on the bearing type, variant and operating conditions. The relubrication interval can be estimated using

Trelub = tf C1 C2 … C8
The curves for angular contact ball and thrust ball bearings are for single bearings only. Values for matched sets should be adjusted according to the arrangement, number of bearings in the set and preload, by multiplying the relubrication interval by factor C1 (table 1). When sets comprising more than four bearings are used, contact the SKF application engineering service.
For hybrid bearings, the estimated grease service life can be revised by multiplying the calculated value for a bearing with steel rolling elements by the correction factor C2 (table 2).
Depending on the operating conditions, the relubrication interval should be multiplied by each of the relevant correction factors from C3 to C8 (table 3).
Other conditions, not included here, such as the presence of water, cutting fluids and vibration may also affect grease service life.
Machine tool spindles often operate under conditions of varying speed, load and operating temperature. If the speed/load spectrum is known and is sufficiently cyclic, the relubrication interval for each speed/load interval can be estimated as above. A relubrication interval for the total duty cycle can then be calculated from

tf tot = 100 / Σ(ai/tfi)

tf tot=total relubrication interval [hours]
ai=part of the total cycle time at speed ni [%]
tfi=relubrication interval at speed ni [hours]
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