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

Application of bearingsDesign of associatedcomponentsBushingsThe surface of the shaft on which thebushing runs, i.e. the counterface,should always be wider than the actualbushing – particularly where axialdisplacement of the shaft relative tothe housing may occur as a result ofchanges in shaft length – in order toprevent step formation in the slidingsurface.To ease mounting, shaft ends andhousing bores should have a lead-inchamfer with an angle of 10 to 15°(➔ fig 1 ). It is then easier to press thebushings into the housing bore and toinsert the shaft into the bushing borewithout the risk of damaging the slidingsurface.The housing shoulders intended foraxial location of the bushing shouldhave a diameter which is equal to orgreater than d + 0,8 mm.Where PTFE composite bushingsoperate without lubricant it is especiallyimportant to accurately align bearingpositions. If misalignment betweenthe positions cannot be avoided, it isnecessary to take steps at the designstage to prevent inadmissibly highedge stresses from occurring. Forexample, the housing bore seatingshould be relieved at both sides, or awider bushing should be used so thatit extends beyond the housing boreseating at both sides (➔ fig 2 ).If errors of alignment have to becompensated for and the operatingconditions permit the use of POM composite,then bushings of this materialshould be chosen. The covering layerof this material can be machined to aminimum degree after the bushing hasbeen mounted in a housing bore.Flanged bushings and thrustwashersFor shafts which not only need radialsupport, but also require axial location,flanged bushings or a combinationof bushing and thrust washer(➔ fig 3 ) can be used, depending onthe magnitude of the axial load. Theuse of flanged bushings or thrustwashers is advantageous even whereaxial loads are small, particularlywhere suitable surfaces are not availableto take the thrust, either becausethe material or its finish is unsuitable.It should be remembered that thecounterface should completely coverthe sliding surface of the thrust washerand the flange of a flanged bushing(➔ fig 4 ). For bearing arrangementswhere flanged bushings are used thetransition from housing bore to abutmentshould be chamfered so that itdoes not contact the bushing at thetransition to the flange (➔ fig 5 ).Thrust washers are generally locatedradially in a turned recess in thehousing (➔ fig 3 ) and secured by adowel pin or grub screw to preventthem from rotating. The appropriatedimensions for this type of location aregiven in the product tables. If a recesscannot be provided in the housing forsome reason, the thrust washer canbe attached to the housing by twopins or screws (➔ fig 6 ) or by glueing.The heads of the pins or screwsmust be recessed to at least 0,3 mmbelow the sliding surface and the entiresurface of the thrust washer mustbe supported.Lead-in chamfers for housing bores andshaft endsBushing extending beyond the bearingseating in the housing at both sides toprevent inadmissibly high edge stressesCombination of bushing and thrustwasherFig 1Fig 2Fig322