ESDU Engineer
Issue 12
Selection of Lubricant Class
The majority of tribological contacts are designed to be used with the addition of a lubricant. A lubricant is defined as a substance introduced into a loaded moving contact in order to control, and usually reduce, friction and will normally also reduce wear. Lubricants can be divided into four broad classes, namely oils, greases, solids and gases.
The use of an additional lubricating substance can, however, bring its own problems such as contamination of the product and environmental impacts, chemical incompatibility with the machine components, and extra maintenance. In many cases the use of a separate lubricant can be avoided, for example by the use of appropriate contact materials or by using a process fluid as a lubricant. Avoiding the use of a separate lubricant can often provide a simple and effective solution and should always be considered as a first option.
During an early stage in the design process, decisions are made on the types of tribological device used in the machine or mechanism and the choices made often imply that a certain class of lubricant be used. A traditional hinge, for example, typically consists of two separate metallic plates with their edges wrapped around a common pivot pin. The choice of such a device, however, virtually presupposes that oil or grease lubrication will be required or, at least, be beneficial. Alternative devices for providing a hinge motion do exist, such as non-metallic hinges and elastomeric bushes, which do not require any additional lubrication and can often provide a better solution than the conventional choice.
There are several possible strategies for selecting a lubricant class for those applications in which the use of an added lubricant is still required and the correct strategy to apply depends on the particular circumstances. If the mechanical aspects of the tribological system are already defined, a relatively straightforward process, albeit one requiring considerable knowledge, can be applied to select a lubricant for that system. Such an approach involves identifying the type of motion, the speed, the temperature, the load, and the environment as a necessary preliminary to selecting the lubricant. Various types of analysis, such as elastohydrodynamic lubrication analysis, can then be applied to guide the selection. Knowledge of normal lubrication practice with similar devices can also be beneficially applied.
The newly issued Item No. 03016 provides guidance on the selection of the appropriate lubricant class for tribological applications. For this purpose lubricants are divided into the aforementioned four classes of oils (liquids), greases (semi-solids), solids and gases. As a preliminary to making a selection, however, guidance is also given on identifying those applications in which the use of a separate lubricant can be avoided.
The first part of the Item provides a strategy for the preliminary selection of tribological devices, such as bearings and drives, aimed at avoiding or, at least minimising the need for additional lubrication and associated support equipment and maintenance. This selection procedure is discussed with particular reference to rotary bearing systems but similar considerations apply to other tribological devices, such as drives and dynamic seals.
Later sections of the Item present further strategies applicable when additional lubrication cannot be avoided. These strategies allow the lubricant class to be chosen based on analysis of the tribological system and on knowledge of normal practice with similar or related devices. The broad characteristics of the various lubricant classes are discussed as well as some of the systems that might be used to supply them. The primary requirements of the lubrication system and the usual choice of system for some specific components are also discussed.
RELATED DATA ITEMS
Data Item No. 68039
Guide to the design of tanks for forced-circulation oil-lubrication systems.
Data Item No. 83030
Selection of filter rating for lubrication systems.
Data Item No. 85027
Film thicknesses in lubricated Hertzian contacts (EHL).
Part 1: two-dimensional contacts (line contacts).
Data Item No. 89045
Film thickness in lubricated Hertzian contacts.
Part 2: point contacts.
Data Item No. 91037
Film thicknesses in lubricated Hertzian line contact.
(Use of computer program A9137).
Data Item No. 91038
Film thicknesses in lubricated Hertzian point contact.
(Use of computer program A9138).
Data Item No. 94020
Selection of synthetic oils.
Keith Waters is the Senior Engineer within the Mechanical Motion and System Dynamics Group and he can be contacted at kwat@esdu.com.