Essential Design Practices for Composite Bearings and Bushings
Composite materials have been the go-to choice for bearing and bushing applications for the better part of the last century. This is due to their self-lubricating wear properties and also because they are much lighter and easier to handle than their metal counterparts. When using composites for these applications, however, there are some essential practices that one must consider. These include: running clearances, press fit interference, ID close-in, post curing effects, wall thickness, surface finish and lubrication. To give a better understanding of what these are and how to apply them to your application, we described them in this paper.
Clearances for Lamitex composites will be greater than for metallic bearings. This is one of the more overlooked considerations in material selections and the most frequent design issue for composite bearings and bushings. Plastics are thermal insulators so consideration must be given also to frictional heat and coefficient of thermal expansion.
Composite bearing materials have approximately 100% greater thermal expansion values than sintered bronze and must be considered to ensure adequate running clearance similar to bronze.
Press Fit Interference
Press Fit interference is the allowance that gives a journal bearing a tight fit. Phenolic composites require greater interference than metallic bearings because of their lower modulus of elasticity.
Thermal expansion should be a consideration of Composite Bushings’ bore size which should be larger their metal counterparts. This is in addition to the running clearance.
Less than normal press fit is satisfactory if the bearing is long and the wall is not exceptionally thin and if the bearing is carrying a moderate load in one direction.
ID Close In
This is the amount the inside diameter will close in after press fitting. One can expect an ID to close in by as much as up to 100% of the press fit interference.
Post Curing Effects
In order to stabilize dimensions of composite materials for use in continuous operating conditions above 200°F a post bake should be performed. Bearings given the recommended post bake will remain secure up to 300°F.
A recommended treatment is either a post bake in air at 285°F/ 300°F for 4 hours or annealing in SAE 20 oil at 225°F to 265°F depending on the components mass. High pressure cotton and Kevlar phenolic laminates require 24 hour hot oil annealing @ 225°F to 245°F.
Annealing in hot oil is recommended if the oil can be used in the bearing application mechanism because the additional oil will act as an initial lubricant that enhances the PV capabilities of the plastic material. (Figure 3 exhibits the effect of a post cure cycle)
Bushing Wall Thickness
In standard practice bearing walls should be at least 1/5 the ID or over. Thickness of plastic bushings can be made thinner than metallic bushings because of the ease of fabrication, no critical pressing and sintering operations are needed.
Thicker walls are used when there is a need for increased stiffness when pressing in abnormally long bearings and for a replacement of ball or roller bearings by sleeve bearings.
Flange and Washer Thickness
The thickness of a flange bushing is normally the same as the wall thickness of the bushings. Recommended thicknesses for washers range from .125” to .500” or more, depending on the diameter. A formula that may be used is: Washer = (OD + 4) / 32
Bearing length is determined by the load the bearing is required to carry. The greater the length, the greater the permitted load.
Ideally, the bearing length should be approximately as long as the diameter of the shaft. For best design, the length to diameter ratio should not exceed L/D = 3.5 because deflection of the shaft may cause edge concentration of loading.
The smoother the surface of mating surfaces, the harder the mating material, the longer the bearing life. A shaft finish of 16RMS or better is recommended. The harder materials should be at least five times as hard as the softer material.
Surface Finish and Preparation
Whenever possible use a molded surface instead of a machined or lapped surface for best wear qualities and lowest coefficient of friction. On a machined surface there are hills and valleys and asperities that have to be broken off before a smooth surface will appear. A molded surface also contains these irregularities but wear resisting additives are readily available in the resin rich surface, promote an efficient wear in period.
It is good practice to use some lubricant initially at bearing installation to help the startup period for bearings and extend wear life.