Mechanical Design 1: Bearings Part 1 : Plain Bearings
A short lecture on Plain Bearings as Part of MECH2400 5400: Mechanical Design 1 By Paul Briozzo
Reference “Mechanical Power Transmission” By William J Patton 1980 and Applied Design by A.K. Hoskings and M. R. Harris 1st Edition Plain Bearings – An Overview
For slow moving contact velocities bearing manufacturers typically make a range of
plain slide bushings that range from using steel on steel (for very high contact loads)
to steel on bronze for lesser loads.
Plain Bearings – Loading
Radial Loads:
This type of simple designed bearing is intended to resist radial loads. Radial loads are loads which are perpendicular to the axis of the shaft. The word journal refers to the supporting part of the shaft. Plain Bearings – Loading
Thrust Bearings:
This type of bearing is intended to resist thrust or axial loading. Loading which is the along the axis of the shaft. Plain Bearings – Loading
Flanged Bearings:
This type of simple bearing is used when there is a combination of both radial and thrust loading. It is a combination of both the journal and thrust bearings and as its name suggests, resembles a bush with a flange attached. Plain Bearings – Practical Sizes
For practical reasons, plain bearings fit in to a particular size range.
• The length of the bearing should be between one and two times the shaft diameter.
• The outside diameter of the bearing should be approximately 25% larger than the shaft diameter. These two key points should be closely noted to the designer for reasons of packaging and weight. Plain Bearings – Calculations
For plain bearings the most important operating conditions are:
• Bearing pressure (P) in pounds per square inch
• Surface velocity (V) of the rotating shaft in feet per minute
• Product of P and V (PV)
The product of these two values, called the PV value is a measure of
the frictional heat generated on a unit area of the bearing surface.
The severity of bearing service increases with PV, i.e. with an
increase in load, shaft diameter or RPM.
Greater loads must be supported by larger shafts and bearings of
larger diameter.
Maximum Pressure and Velocity for some Bearing Materials
Material
P
V
PV
Sintered Leaded Bronze
Cast Bronze
Tin Babbitt
Lead Babbitt
Nylon
Acetal (Delrin)
Teflon
800
3000
1500
1300
2000
2000
500
1500
750
1200
1400
600
600
50
60,000
75,000
30,000
18,000
3000
3000
1000
Example
Consider the case of a ¾ inch shaft rotating at 500 RPM, with a load on each bearings of 400 lb. Assume a bearing length of the same dimension as the bore i.e. ¾ inch. SOLUTION:
1 inch = 25.4mm
1 pound force = 4.448 Newtons
Bearing Area = 0.75 x 0.75 = 0.56in2
Bearing Pressure (P) = 400 / 0.75 = 700 psi
Surface Velocity (V) = ¾  x 500 = 1185 ipm = 100 fpm
PV = 700 x 100 = 70,000
Recognise that this is a high value. Of the materials tabulated previously, only cast bronze is rated for a PV of 70,000
What other significant implications in terms of size does this result present to the designer ?
Plain Bearings ‐ Lubrication
The three bearing variables viscosity, pressure and speed are sometimes combined as ZN/P Where;
Z = Viscosity
P = Pressure
N = R.P.M.
Note the conditions when;
1. The shaft is at rest (N=0). 2. The optimum point where friction is at its lowest.
3. The increase in friction as N increases and the oil is being churned in the bearing.
Plain Bearings ‐ Lubrication
When the shaft is operating at full speed, the fully lubricated condition prevails. Note the location of the oil input hole where pressure is at a minimum.
Theoretically a shaft that it is never stopped should never wear however either;
1. Corrosion
2. Contamination
3. Shaft deflection
will eventually cause some wear.
Hydrodynamic Lubrication
in a Plain Bearing
The shaft, due to its eccentricity within the bearing acts as a pump and circulates the oil within the bearing. Plain Bearings ‐ Lubrication
Self ‐ Lubrication:
The majority of the plain bearings used today are the self‐lubricating type.
The bearing is made of a porous or sintered material and is impregnated with oil or some other suitable lubricant which automatically lubricates the bearing surfaces during operation. This type of bearing is a great asset where regular lubrication is difficult or sometimes impossible to supply. Plain Bearings – Lubrication
External Lubrication:
An external supply of lubricant is fed to the bearing by various means;
• For bearings which are not self lubricating e.g. phosphor bronze, brass or gunmetal ‐ oil or grease can be used.
• As a supplementary lubrication for self‐lubricating bearings in order to give longer life and better performance – oil only should be used. Plain Bearings – Lubrication
Lubrication Methods:
When lubrication is supplied to a bush it is desirable to have either:
• A flat on the axle (axles are stationary).
• The flat (say 1.5mm deep) should be slightly shorter in length than the bush and on the non‐
pressure side.
• A network of suitable grooves in the bush. Longitudinal grooves generally stop short of the ends of the bush.
• In the case of reciprocating motion there should be sufficient oil or grease grooves to ensure lubrication to all parts of the journal.
Plain Bearings – Lubrication Arrangements
Plain Bearing Materials
Plain bearings are generally made from a metal that has good wearing qualities.
Metals that are most commonly used for this purpose are:
Phosphor–Bronze
Brass
Gunmetal
Teflon
Cast Iron
Nylon
Plain Bearing Material Qualities
Hardness:
Greater hardness is accompanied by;
1. Greater strength
2. Provides good resistance to fatigue failure
3. Capacity to carry heavy loads.
• Harder bearing materials require harder shaft materials.
• Soft bearing materials tend to seize to the shaft more than harder materials.
• Harder bearings lack conformability and embedability. Plain Bearing Material Qualities
Compatibility
An ideal bearing material should not weld or seize to its shaft when metal to metal contact occurs. Compatibility is a measure of the anti seizing characteristics of the bearing material when operated against a given shaft material. A hard shaft does not tend to weld to a grey iron bearing.
An aluminium shaft in an aluminium bearing material would be prone to seize or gall.
Plain Bearing Material Qualities
Conformability:
The ability to :
•
•
•
•
Deform to a shape
Compensate for slight shaft misalignment
Shaft deflection
Geometric Tolerance errors within the shaft
Plain Bearing Material Qualities
Enbeddability:
This characteristic permits abrasive particles to embed or sink into the bearing material so that they do not abrade the journal area of the shaft. A bearing material sufficiently soft for embedment will also provide conformability
Plain Bearing Material Qualities
Corrosion Resistance:
• The bearing material must not be corroded by additives or other agents in the lubricant. • The lubricant must be compatible with the materials that it is exposed to.
• Oxidation by products are developed as the oil ages, as it is exposed to moisture or by corodents in the atmosphere.
Plain Bearing Advantages / Disadvantages
Advantages
•
Initial cost is lower in most cases.
•
Less radial space required than for rolling contact bearings.
•
Better suited to overload or shock conditions.
•
Quieter operation than rolling contact bearings (more noticeable after wear has taken place).
•
Less difficulty with fatigue related issues.
•
Less easily damaged by foreign matter e.g. grime, dirt etc. Disadvantages
•
Limited to relatively low speed (metallic types).
•
Require constant supervision for lubrication (except for self lubricating types).
•
Relatively high rate of wear.
Michell Bearing ‐ History
Australian engineer, George Michell
(pronounced “Mitchell”). In 1905 Australian engineer George Michell
developed a tilting pad bearing to overcome the limitations of the conventional thrust block design.
Up until this point in time the chief limitation on the size, speed and load carrying ability of a ship was governed by the available bearings of the day that were able to take the thrust load of the propeller. Michell Bearing ‐ Overview
The Michell bearing is constructed using a number of sector‐shaped pads that are able to tilt to allow an oil wedge to form. The pads are arranged in a circular pattern around the power transmitting shaft. The immediate application of Michellʹs
bearing was applied to replace the thrust block of propeller driven ships. The Michell bearing’s smaller size (roughly 1/10th that of the older design), low friction and longer life allowed larger, faster ships that could carry greater loads.