Composite dry sliding bearings – maintenance-free and space-saving

Maintenance-free and space-savingMachinability ofcomposite materialsSKF composite dry sliding bearingmaterials – with the exception of thesliding layer – can be machined usingconventional methods.If bushings are required to have asmaller width than the standard size,this can easily be achieved by turningor parting-off. It is also possible to drilllubrication holes. Care must be takento see that any burrs are removed,particularly from the sliding surface.Strip can be bent, coined, pressed,cut, bored or drilled to shape to fit theindividual application. When cutting ordrilling it is advisable to work from thesliding surface side to avoid creatingburrs in the sliding layer.Any bright metal surfaces producedas a result of machining should begiven protection against corrosion.PTFE composite bushingsThese bushings are supplied in aready-to-mount condition. Any subsequentmachining of the bore, i.e. thesliding surface, should only be undertakenin exceptional cases as it willreduce the service life. If necessary,however, the bore should be calibratedusing a mandrel (➔ fig 3 ). If a PTFEFig3composite bushing having an outsidediameter D is to be mounted in a steelhousing having an outside diameterD G , guideline values for the requisitediameter d K of the calibrating mandrel(= d + ∆d K ) will be found in Diagram 1 .This diagram gives the requisite interference∆d K of the calibrating mandrelas a function of the desiredexpansion ∆d of the bushing bore forvarious nominal bushing bore diametersd. The diagram is valid for theratio D G /D = 2. No values are given forbushings installed in light alloy housingsas the influence of design parametersand the housing material is toogreat. In such cases, the requisitediameter of the calibrating mandrelmust be established by trials.POM composite bushingsBushings made of the POM compositematerial are supplied in a ready-tomountcondition but can be machinedin the bore, for example, by turning.When machining the bore an R a valueof 2,5 µm should be aimed at for thesliding surface. The following machiningrecommendations have beenfound to give good results:• a cutting speed > 150 m/min,• a slow feed rate (0,05 to0,1 mm/revolution),• a cutting depth ≤ 0,1 mm, and• a cutting tool of poly-crystallinediamond.In addition, cooling must be efficientto prevent excessive heating of theplastic with an attendant risk of smearing.The swarf produced must be removedduring machining. Both coolingand swarf removal can usually beachieved using compressed air.FrictionThe friction in composite dry slidingbearings is primarily determined by theload, the sliding velocity and the operatingtemperature. It is also influencedby the roughness of the surface onwhich the bearing runs, the degree ofcontamination and the lubrication conditions.For PTFE composite bearings, thevalue of the coefficient of friction µ liesbetween 0,03 and 0,25 depending onthe operating conditions. Similar valuesare found for the POM composite material,but the influence of lubrication isstronger. The lowest values are normallyobtained under high specificloads and low sliding velocities, seeguideline values for µ in Diagram 2 .Under particularly unfavourable operatingconditions as well as under lightloads, the maximum guideline valuesmay even be exceeded. The friction ofPTFE composite bearings is increasedat temperatures above +100 °C.Stick-slip effects are negligibly smallin bearings of both materials.Mandreld K = diameter of calibrating mandrelB = bushing widthMinimum hardness: 50 HRCSurface roughness: R z ≈ 1 µm8