Plain Bearings

Plain (journal) bearings are used in many applications and have the following advantages :-

1. Small outside diameter as compared to rolling element bearings.
2. Quietness of operation.
3. Good capacity to absorb shock loading.
4. Will take oscillating motion.
5. Low cost.

Types of plain bearing

Hydrostatic.- In a hydrostatic bearing the load is carried by fluid pressure generated outside the bearing. Fluid is pumped under pressure into the bearing, therefore the bearing will operate whether static or rotating.

Hydrodynamic.- In a hydrodynamic bearing a fluid is drawn into the region between the moving parts of the bearing by the virtue of its adhesion to the surfaces of the bearing and its viscosity and due to the shape of the bearing surfaces a pressure is generated within the fluid which keeps the bearing surfaces separated. The separating film is only generated when the there is relative motion in the bearing, i.e. when the shaft is rotating.

Boundary Lubrication.- If the working fluid in a bearing adheres to or "wets" the bearing surfaces then some load may be carried. Boundary lubrication will apply at low speeds but as the speed increases some degree of hydrodynamic lubrication will be needed.

Dry Bearings.- Dry bearings operate without a significant fluid film to separate the moving surfaces therefore low friction materials, or materials impregnated with a lubricant, must be used.

Materials - Lubricated bearings

• Whitemetal - usually used as a lining bonded to bronze, steel or cast iron.
• Copper Based alloys - most common alloys are copper tin, copper lead, phosphor bronze: harder and stronger than whitemetal: can be used un-backed as a solid bearing.
• Aluminum based alloys - running properties not as good as copper based alloys but cheaper.

Materials - Dry bearings

• Sintered bronze - Sintered bronze is a porous material which can be impregnated with oil, graphite or Ptfe. Not suitable for heavily loaded applications but useful where lubrication is inconvenient.
• Ptfe - suitable only in very light applications. Mechanically weak material which has a tendency to flow and is seriously affected by high temperatures.
• Nylon - similar to Ptfe but slightly harder: used only in very light applications.

Plain Bearing Guidelines

In general, plain bearings should be kept as short as possible in order to avoid long controlled diameters and highly finished portions on the shaft. Length/diameter ratio of 0.75 - 1.5

Two short bearings well spaced are better than one long one.

Dry bearings of the impregnated sintered bronze type are available in stock sizes and these should be used if possible.

Mechanical Engineering Design Topics Page - Bearing Selection Page

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Pages created by David Price - 9/08/95
This Information is an educational resource and is not to be reproduced (other than for private study) without written consent of the University of Southampton. copyright © Dave Whatley 1995
GlobalSpec.com is presenting this material with the consent of Dave Whatley.

Disclaimer Whilst every care has been taken to ensure the accuracy and completeness of this information, neither the author nor the University of Southampton will accept responsibility for loss, damage etc. however arising occasioned by the use of this information.