Page 329 - The Tribology Handbook

Plain bearing lubrication e7

Mineral oils and greases are the most suitable lubricants for plain bearings in most applications. Synthetic oils may be
required ifsystem temperatures are very high. Water and process fluids can also be used as lubricants in certain applications.
The general characteristics of these main classes of lubricants are summarised in Table 7.1.

Table 7.7 Choice of lubricant Table 7.2 Methods of liquid lubricant supply

~~
Lubricant Operating range Remarks
Method .f Main characteristics Exampies
WblY
__ -
Mineral All conditions of Wide range of viscosities
oils load and available. Potential Hand Non automatic, irregular. Low-speed,
speed corrosion problems oiling. Low initial cost. cheap journal
with certain additive High maintenance cost bearings
oils (e.g. extreme
pressure) (see Table 7.9) Drip and Non automatic, Journals in
wick adjustable. some machine
Synthetic All conditions if Good high and low feed Moderately efficient. tools, axles
oils suitable temperature properties. Cheap
viscosity Costly
available Ring and Automatic, reliable. Journals in
collar Efficient, fairly cheap. pumps,
Greases Use restricted to Good where sealing feed Mainly horizontal blowers, large
operating against dirt and bearings electric
speeds below moisture necessary motors
1 to 2 m/s and where motion is
intermittent Bath and Automatic, reliable, Thrust bearings,
splash efficient. bath only.
Process Depends on May be necessary to lubri- Oil-tight housing Engines,
fluids properties of avoid contamination of cation required process
fluid food products, High initial cost machinery,
chemicals, etc. general
Special attention to
design and selection of Pressure Automatic. High-speed and
bearing materials feed Positive and adjustable. heavily
Reliable and efficient. loaded
High initial cost journal and
thrust
bearings in
machine
tools, engines
The most important property of a lubricant for plain and
bearings is its viscosity. If the viscosity is too low the bearing compressors
will have inadequate load-carrying capacity, whilst if
the viscosity is too high the power loss and the operating Notes
temperature will be unnecessarily high. Figure 7.1 gives a
guide to the value of the minimum allowable viscosity for a Pressure oil feed : This is usually necessary when the heat
dissipation of the bearing housing and its surroundings is
range of speeds and loads. It should be noted that these not sufficient to restrict its temperature rise to 20°C or less
values apply for a fluid at the mean bearing temperature. Journal bearings: Oil must be introduced by means of oil
The viscosity alf mineral oils falls with increasing tempera- grooves in the bearing housing. Some common arrangements
ture. The viscosity/temperature characteristics of typical are shown in Figure 7.3
mineral oils are shown in Figure 7.2. The most widely used Thrust bearings: These must be lubricated by oil bath or by
methods of supplying lubricating oils to plain bearings are pressure feed from the centre of the bearing
listed in Table 7.2 Cleanliness: Cleanliness of the oil supply is essential for
The tubricatinq properties of greases are determined to satisfactory performance and long life
a large extent by the viscosity of the base oil and the type of
thickener used in their manufacture. The section of this
handbook on greases summarises the properties of the
various types.
Additive oils are not required for plain bearing lubrica-
tion but other requirements of the system may demand
their use. Additives and certain contaminants may create
potential corrosion problems. Tables 7.3 and 7.4 give a
guide to additive and bearing material requirements, with
examples of situations in which problems can arise.
C7.1