We are going to continue with our chapter, but first a little announcement...
And now onto...Bearing Design Guide: Chapter Twenty: Thrust Bearings or Washers
There are three basic types of thrust washers which are defined by their mode of operation. Thrust bearings designed correctly to operate under either one of the following modes have theoretical justification for their load capacities.
1. Flat boundary lubricated 100 PSI.
2. Flat thermal wedge, hydrodynamically lubricated 1200 PSI.
3. Contoured wedge, hydrodynamically lubricated 5000 PSI.
However, in actual practice, these values are not achieved other than in theoretical design. In practice, the load capacity of each reduce to about 60% of theoretical values of 60 PSI, 700 PSI and 3000 PSI.
For the greater majority of flat thrust washers, it is impossible to prevent some degree of hydrodynamic lubrication so that even in the worst design, the allowable loading will be greater than 60 PSI.
In conditions of a sparse oil supply mated with steel, the high leaded tin bronze alloys (20% or greater lead content) are much more capable of satisfactory service providing their is no large amount of dirt or debris.
If there is a fair amount of dirt present which is fairly coarse in particle size and movement or motion is infrequent or slow, the lower content leaded tin bronze alloys (20% or less) are preferred.
For un-lubricated applications against steel, the preferred thrust washer materials are the sintered powdered bronze oil impregnated or Teflon-coated and plastic materials.
For applications in which the non-lubricating film such as water or silicone fluids are present, the preference should be for Teflon-coated or impregnated bronzes or plastics.
Oil Distribution Grooves: These are grooves that separate each sector of the thrust washers. The grooves must not go completely across the thrust face of the washer unless the thrust washer is completely immersed in the lubricant.
If the lubricant is supplied at the ID of the washer, then the grooves should extend from the ID going outward covering about 80% of the distance to the outer edge.
If the direction of rotation can be in either direction, the grooves should be radial; but if the direction of the rotation is fixed, the grooves should be slanted so that the viscous drag of the rotation will pull the lubricant into the groove in the direction of movement. The slanted angle should be between 10 to 40 degrees to the diameter.
If the lubricant is supplied from the outer circumference (which is an undesirable condition) then the grooves must be slanted 20 to 60 degrees; but if the grooves are in the stationery member, they should be pointed in the direction of the rotation; if the grooves are on the moving member, they should be pointed against the direction of the rotation.
Oil Collection Grooves: These grooves, like spreading grooves, should only go part way across the surface from 50 to 80% of the distance is suitable. They also should be slanted in the same direction as the oil-spreading grooves. These grooves are positioned just before the oil distribution grooves if the washer is supplied with oil from the ID.
Oil Groove Dimension: The length of the groove is controlled by the degree of slant and should go about 80 to 90 % of the way across the annular surface.
The groove cross-section should be in the form of a wide "V" to promote the formation of an oil film or a tear-drop design which blends into the surface.
The thrust washer with grooves such as through grooves, tear-drop (or stopped oft) and tapered land grooves are superior to a plain washer. By adding four through grooves to the plain washer, the oil flow increases across the thrust surface and the load-carrying capacity increases by 30%.
While restricting total oil flow through the washer with tear-drop grooves, an 85% increase can be obtained.
With the tapered land groove which promotes an oil wedge, the increase in load-carrying capacity increases to 3 00% of a plain washer.
According to theory, if the plain washer is perfectly flat, it would have no load capacity so surface waviness is not undesirable if it is fitted into a rigid aligned housing.
The speed of operation of a thrust washer is not generally a problem if the surface speed is at least 25 fpm. At very high speeds - above 2000 to 4000 fpm - it may become a problem to supply ample lubricant although there is a compensating advantage of realizing a higher unit load. There also is the possibility of the oil carbonizing at this high speed, depending upon the load.
Lubricant is important since the higher viscosity lubes such as SAE 50 offer greater chances that hydrodynamic film formation will be realized.
The normal hydrocarbon oil lubricants are suitable for washers but the non-polymer modified oils are preferred for their higher viscosity.
We are coming down to the home stretch, only three more chapters to go. I know its a lot of information and a lot to take in, but if I have helped one person with this, I will be happy.
That's it for today. Until next time my metal loving friends...
Next Up: Chapter 21: Corrosion Resistance of Some Bronzes