Selecting a suitable oil

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

Oil selection criteria

When you select a lubricating oil, the most important parameters are the viscosity and viscosity index, the thermal stability (which influences the choice of oil type) and the additive package (EP/AW and corrosion protection) that fits the operating conditions for the application.
Viscosity and viscosity index
The required viscosity is primarily given by the lubrication condition κ, at the expected operating temperature, evaluated as described in Lubrication condition – the viscosity ratio, κ. The viscosity index, VI, is the measure of how the oil viscosity changes with temperature. VI is a part of the selection process, in particular for applications that operate in a large temperature range. Oils with a VI of at least 95 are recommended.

Oil type
There are two broad categories of oil types – mineral and synthetic – with the following types of synthetic oils available:
  • polyalphaolefins (PAO)
  • esters
  • polyglycols (PAG)
Choice of oil type is mainly determined by the temperature range in which the application is expected to operate.
  • Mineral oils are generally favoured as the lubricant for rolling bearings.
  • Synthetic oils should be considered for operational temperatures above 90 °C (195 °F) because of their improved thermal and oxidation resistance, or below -40 °C (-40 °F) because of their better properties at low temperatures.
The pour point of an oil is defined as the lowest temperature at which a lubricant will flow, but it must not be used as a functional limit when selecting oil type. If the temperature is above but near the pour point, the viscosity is still very high, which may impair pumping, filtering, and other characteristics.

The thickness of the hydrodynamic film is determined, in part, by the viscosity index (VI) and the pressure-viscosity coefficient. For most mineral oil based lubricants, the pressure-viscosity coefficient is similar, and you can use the generic values obtained from literature. However, for synthetic oils, the effect on viscosity to increasing pressure is determined by the chemical structure of its base stock. As a result, there is considerable variation in pressure-viscosity coefficients for different types of synthetic base stocks.

Because of the differences in the viscosity index and pressure-viscosity coefficient, the formation of a hydrodynamic lubricant film, when using a synthetic oil, may differ from that of a mineral oil with the same viscosity.

Regarding the lubrication condition for mineral and synthetic oils, the combined effect of the viscosity index and the pressure-viscosity coefficient normally cancel each other out.

Table 1 summarizes the properties of the different oil types. For additional information about synthetic oils, contact the lubricant supplier.

Oils, and in particular synthetic oils, may interact with such things as seals, paint or water in a different way than mineral oils, so such effects, as well as miscibility, must be investigated.

Lubricating oils usually contain additives of various kinds. The most important ones are antioxidants, corrosion protection agents, anti-foaming additives, and EP/AW additives. In the lubrication condition domain defined by κ < 1, EP/AW additives are recommended, but for temperatures above 80 °C (175 °F), a lubricant with EP/AW additives should only be used after careful testing.

Oil change interval

The oil change interval depends on the operating conditions and the oil type. With oil-bath lubrication, it is generally sufficient to change the oil once a year, provided the operating temperature does not exceed 50 °C (120 °F). Typically, at higher temperatures or with heavy contamination, the oil must be changed more often.
With oil circulation, the interval after which the oil needs to be changed is determined by an inspection of the oil quality, taking into account oxidation and the presence of water and abrasive particles. Oil life in circulation systems can be extended by removing particles and water from the oil.
Oil change intervals for mineral oils are shown in table 2.

Overview of main oil lubrication methods

The oil lubrication methods are:
  • oil bath without circulating oil
  • oil bath with self-circulating oil through bearing pumping action
  • circulating oil with external pump
  • oil jet method
  • oil air method
The choice of the oil lubrication method depends mainly on:
  • the bearing speed
  • the need to remove heat
  • the need to remove contaminants (solid particles or liquid)
SKF offers a wide range of products for oil lubrication that are not covered here. For additional information about SKF lubrication systems and related products → Lubrication solutions.
Oil bath without circulating oil
The simplest method of oil lubrication is the oil bath. The oil, which is picked up by the rotating components of the bearing, is distributed within the bearing and then flows back to the oil bath in the housing. Ideally, the oil level should reach the centre of the lowest rolling element (fig. 1) when the bearing is stationary. Oil levels higher than recommended will increase bearing temperature because of churning (→ Bearing friction, power loss and starting torque).
Oil bath with self-circulating oil
Oil from a bath is forced to circulate by different methods. Here are some examples:
  • Oil is salvaged and directed to the bearings by means of drain and ducts (fig. 2).
  • A dedicated component (ring, disc, etc.) picks up oil from an oil bath and transports it (fig. 3).
  • The pumping effect of some bearing types can be used to circulate the oil. In fig. 4, the spherical thrust roller bearing pumps oil which returns to the thrust bearing by connecting ducts located under it.
All designs of such lubricating methods should be validated individually by tests.
Circulating oil without a bath
Circulating oil by means of an external oil pump, instead of an oil bath, is mainly used when it is needed to remove heat generated by the bearing and/or other sources. Oil circulation is also a good lubricating method for evacuating solid or liquid contaminants from the bearing to filters and/or oil/liquid separators. The design and layout of the oil drainage must ensure that there is no build-up of oil level. → Heat flow from adjacent parts or process

A basic circulating oil system (fig. 5) includes:
  • oil pump
  • filter
  • oil reservoir
  • oil cooling and/or heating system
Oil jet
The oil jet lubricating method (fig. 6) is an extension of circulating oil systems, and is used for bearings operating at very high speeds. The dimensioning of oil flow and corresponding jet size is selected so that the oil jet speed reaches at least 15 m/s.

Oil injectors must be positioned so that the oil jet penetrates the bearing between one of the rings and the cage. To prevent churning that can cause increased friction and temperature, the design and layout of the oil drainage must ensure that there is no oil level build-up.
The oil-air lubrication method (fig. 7), also called the oil-spot lubrication method, uses compressed air to transport small, accurately-metered quantities of oil as small droplets along the inside of the feed lines to an injector nozzle, where it is delivered to a bearing. This minimum-quantity lubrication method enables the bearings to operate at very high speeds at a relatively low operating temperature. The compressed air also cools the bearing and prevents dust or aggressive gases from entering. For further information, refer to Super-precision bearings.
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