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Selecting grease or oil

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing specificationBearing executionSealing, mounting and dismounting

The first step in the lubrication selection process is to decide whether to use grease or oil. In most cases, grease is the appropriate choice for open bearings.

Lubrication selection flowchart and criteria
A flowchart to help select the correct lubrication method is shown in diagram 1.

The main reasons to choose grease are:
  • cost effectiveness
  • simplicity - grease is easily retained in the bearing and housing, thus requiring less complicated sealing arrangements compared with those for oil lubrication
The main exceptions to choosing grease are in applications where:
  • operating conditions require a grease relubrication interval that is unacceptably short
  • lubricating oil must be used for other purposes (such as in gearboxes)
  • heat removal via circulating oil is required
  • purging or removing used grease becomes cumbersome or expensive to handle
Estimating the relubrication interval for grease
Lubricating grease slowly degrades and therefore has a limited life. Grease life depends on the operating conditions of the bearing and the grease type. Rolling bearings therefore have to be relubricated if:
  • the grease life is shorter than the specified bearing life
  • the grease becomes contaminated
It is important to calculate the grease relubrication interval and if it is unacceptably short then, unless you use automatic (centralized) greasing (→ Selecting a suitable grease, Lubrication systems), you should choose oil instead.

Relubrication should occur frequently enough to avoid grease deterioration having an adverse effect on the bearing life. Therefore, the SKF relubrication interval, tf, is defined as the time period at the end of which there is only a 1% probability that the bearing will fail because of grease degradation. This represents the L1 grease life. L10 grease life represents a 10% probability failure due to grease degradation. Grease life depends mainly on:
  • bearing type and size
  • speed
  • load ratio C/P
  • operating temperature
  • grease type
As a rule, standard greases have a practical upper temperature limit of 100 °C (210 °F) on the ring with the highest temperature. Above this temperature, special greases or automatic (centralized) greasing systems should be used – otherwise, commonly the grease life would be too short.

Relubrication intervals

Use diagram 2 to estimate the relubrication intervals tf. The diagram is valid for bearings with a rotating inner ring on horizontal shafts under normal and clean operating conditions, using:
  • the ndm factor multiplied by the relevant bearing factor bf where
    • n = rotational speed [r/min]
    • dm = bearing mean diameter [mm] = 0,5 (d + D)
    • bf = bearing factor dependent on bearing type and load conditions (table 1)
  • the load ratio C/P
The relubrication interval tf is the estimated number of operating hours that a good quality lithium soap grease with a mineral base oil can perform adequately when the operating temperature is 70 °C (160 °F). High performance greases can extend relubrication intervals and grease life.

The relubrication intervals given in diagram 2 must be adjusted according to table 2.

If the speed factor ndm exceeds 70% of the recommended limits (table 1), check the influence of the selected lubricant on the operating temperature and speed.

In practice, relubrication intervals above 30 000 hours are not reliable, because intervals of that length exceed the predictable performance life (due to lubricant ageing) of most greases.

Adjustments for relubrication intervals

Table 2 describes various adjustments for relubrication intervals under various operating conditions. You may also calculate lubrication intervals using the SKF Bearing Calculator.

Determining grease quantity for initial fill and relubrication
Commonly, the free volume in bearings is completely filled during installation and the free volume in SKF plummer block housings is partly filled. SKF recommends that the free volume on each side of the bearing in a customer-designed housing is equal to the free volume of the bearing. For bearings with a metallic cage, the free volume in the bearing is approximately



where
V free volume in the bearing [cm3] (for standard grease, mass in grams multiplied by 0,9; for flourinated grease, mass in grams multiplied by approximately 2) 
B 
bearing width [mm]
D outer diameter [mm]
d bore diameter [mm]
M bearing mass [kg]


For bearings with non-metallic cages, the formula gives a slight over-estimation.

Depending on the intended method of relubrication, SKF recommends:
  • relubrication from the side of the bearing (fig. 1)
    • initial fill: 40% of the free volume in the housing
    • replenishment quantity: Gp = 0,005 D B
  • relubrication through holes in the center of the inner or outer ring (fig. 2)
    • initial fill: 20% of the free volume in the housing
    • replenishment quantity: Gp = 0,002 D B
where
Gp grease quantity to be added when replenishing [g]
D bearing outside diameter [mm]
B 
total bearing width [mm] (for tapered roller bearings use T, for thrust bearings use height H)


During a running-in period, excess grease in the bearing distributes or escapes. At the end of the running-in period, the operating temperature drops, indicating that the grease has been distributed.

In applications where bearings operate at very low speeds and good protection against contaminants and corrosion is required, SKF recommends filling 70% to 100% of the housing with grease.

Relubrication procedures
Select a relubrication procedure that suits the application and the relubrication interval tf. SKF recommends one of the following procedures:
  • Manual relubrication by replenishment is a convenient procedure. It enables uninterrupted operation and provides, when compared with continuous relubrication, a lower steady-state temperature.

  • Automatic (centralized) relubrication avoids performance issues related to over- or under-greasing. This is also commonly used where there are multiple points to lubricate, or where access to positions is difficult, or where equipment is operated remotely with no local maintenance staff (diagram 3).

  • Continuous lubrication is used when the estimated relubrication intervals are short due to the adverse effects of very severe contamination. Continuous lubrication of applications is recommended typically with ndm values < 150 000 for ball bearings and < 75 000 for roller bearings. In these cases, the initial grease fill for the housing can be from 70% to 100 % (depending on the operation condition and housing seal), and the quantity for relubrication per unit of time is derived from the equations for Gp (→ Determining grease quantity for initial and relubrication) by spreading the required quantity over the relubrication interval.

There must be provision for the used grease to be purged from the housing. If an excess of used grease needs to be purged from the housing, contacting seals must allow for this (consider seal type and seal orientation). Otherwise, an escape hole should be provided in the housing – tubing is not allowed, because it can restrict grease escape. The escape hole should be plugged during high­-pressure cleaning.

Where a variety of bearing types is used in a bearing arrangement, it is common practice to apply the shortest estimated relubrication interval from the bearings in the arrangement.

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