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

Application of bearingsMaterial and surface finish ofcounterfacesThe factors which are most importantto consider when selecting the materialand surface finish of the counterface(the surface on which the bearingslides) are the loading conditions (load,angle of oscillation, type of movementetc.) and the environmental influences.Where there is a risk of corrosion,the counterface must be sufficientlyresistant. Corrosion scars in the counterfaceand the products of corrosion(particulate contamination) increasethe surface roughness or are abrasive,thus increasing wear. In such casesthe use of stainless steel or a surfacetreatment such as hard chromium ornickel plating or electrolytic oxidationshould be considered.For PTFE composite and POM compositedry sliding bearing arrangementssoft carbon steels having aground surface are usually adequatefor the counterface. The surface roughnessR a (to DIN 4768:1990) should notexceed 0,4 µm for PTFE compositebearings and 0,8 µm for POM compositebearings. The corresponding R zvalues are 3 and 6 µm, respectively. Formore demanding applications the useof hardened shafts is recommended.These should have a surface hardnessof at least 50 HRC. Alternatively, hardchromium or nickel plating or someother form of surface treatment shouldbe considered. In all cases R a shouldnot be greater than 0,3 µm (R z ≤ 2 µm).The better the surface finish, the betterthe running properties and the less thewear (➔ Surface roughness factorc 4 , Diagram 6 , page 12).14Shaft and housing tolerancesIt is recommended that the seating inthe housing bore is machined to toleranceH6 for PTFE composite andPOM composite dry sliding bushings(including flanged bushings) having abore diameter up to and including4 mm, and to tolerance H7 for largerbearings. If this is done, after mounting,the bore diameter of the bushingand the clearance in the bearingarrangement will lie within the limitsquoted in Tables 1 and 3 for metricsizes of PTFE composite and POMcomposite dry sliding bushings, respectively,provided the shaft seatingalso has the recommended tolerance.The shaft and housing limits for inchsizedbushings are given in Tables 2and 4 , respectively, together with thecorresponding limits for the bushingbore diameter after mounting and theoperating clearance.The values quoted are for the operatingclearance at room temperature.If the operating temperature is higherthan this it is expected that the operatingclearance of• PTFE composite bushings will bereduced by 0,0016 mm and• POM composite bushings will bereduced by 0,005 mmfor every 20 °C temperature increase.The actual operating clearance can beincreased or decreased within the recommendedlimits by matching shaftand housing bores having appropriatevalues within the specified limits.If in certain applications very easyrunning is required, for example, or ifthe bearing is only lightly loaded, it isrecommended that maximum valuesfor the operating clearance should beaimed for.The recommended tolerances andguideline limits quoted in the tables arevalid for steel and cast iron housings.Where light alloy housings are used, agreater degree of interference may berequired because of the different thermalexpansion characteristics. There isotherwise a risk that the greater thermalexpansion of the housing would nolonger provide radial location for thebushing and that the operating clearancewould be too large.If it is not possible to adopt agreater interference for mounting reasons,or because of the force requiredto press the bushing into the housingbore, it is possible to use an adhesiveto retain the bushing in position. Inspecial cases it may be necessary, byselecting a suitable tolerance for theshaft, to prevent an inadmissibleincrease in operating clearance.