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Replenishment

As mentioned in the introduction of Lubrication, the free volume in the bearing should be completely filled during mounting, while the free volume in the housing generally should be partly filled. Depending on the intended method of replenishment, SKF recommends the following grease fill percentages for this free volume in the housing:
  • 40% when replenishing from the side of the bearing (fig. 1).
  • 20% when replenishing through the lubrication holes in the bearing inner or outer ring (fig. 2).
Suitable quantities for replenishment from the side of a bearing can be obtained from

Gp = 0,005 D B

and for replenishment through the bearing outer or inner ring from

Gp = 0,002 D B

where
Gp
=grease quantity to be added when replenishing [g]
=total bearing width [mm] (for tapered roller bearings use T, for thrust bearings use height H)
D=bearing outside diameter [mm]
To apply grease with a grease gun, a grease fitting is needed on the housing. If contact seals are used, an escape hole in the housing should also be provided so that excess grease does not build up in the space surrounding the bearing (fig. 1 and fig. 2). Otherwise, this can cause a permanent increase in bearing temperature. The escape hole should be plugged when high-pressure water is used for cleaning.
Excess grease collecting in the space surrounding the bearing can cause temperature peaks, which can have a detrimental effect on the grease as well as the bearing. It is more pronounced when bearings operate at high speeds. In these cases, SKF recommends using a grease flinger in combination with an escape hole. This prevents over-lubrication and enables relubrication to be performed while the machine is in operation. A grease flinger is basically a disc that rotates with the shaft and forms a narrow gap together with the housing end cover (fig. 3). Excess and used grease is flung into an annular cavity and leaves the housing through an opening on the underside of the end cover.
For additional information about the design and dimensioning of grease flingers, contact the SKF application engineering service.
To be sure that fresh grease actually reaches the bearing and replaces the old grease, the lubrication duct in the housing should either feed the grease adjacent to the outer ring side face (fig. 1 and fig. 4) or, preferably, into the bearing. To facilitate efficient lubrication, some bearing types, e.g. spherical roller bearings, are provided with an annular groove and/or one or more lubrication holes in the inner or outer ring (fig. 2 and fig. 5).
To effectively replace used grease, it is important to relubricate the bearing while the machine is in operation. In cases where the machine is not in operation, the bearing should be rotated during replenishment. When lubricating the bearing directly through the inner or outer ring, the fresh grease is applied directly to the free volume in the bearing. Therefore, the amount of grease needed is reduced, when compared with relubricating from the side. It is assumed that the lubrication ducts were filled with grease during the mounting process. If not, a larger quantity of grease is needed to compensate for the empty ducts during the first replenishment.
Where long lubrication ducts are used, check whether the grease can be adequately pumped at the prevailing ambient temperature.
The grease in the housing should be replaced when the free volume in the housing is approximately 75% full. When relubricating from the side and starting with 40% initial fill of the housing, the complete grease fill should be replaced after approximately five replenishments. Due to the lower initial fill of the housing and the reduced topping-up quantity during replenishment when relubricating the bearing directly through the inner or outer ring, renewal is only required in exceptional cases.
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