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

Calculation of servicelifeMany factors influence the life of a drysliding bearing, e.g. load, sliding velocity,operating temperature, surfaceroughness of the surface on which thedry sliding layer runs etc. Any calculationis therefore only approximate.The values obtained using the equationsgiven below for the basic ratingservice life are attained by the majorityof bearings and are often exceeded.This has been confirmed by rig testsand field experience.The basic rating service life for SKFPTFE composite and POM compositedry sliding bearings can be calculatedfromK MG h = c 1 c 2 c 3 c 4 c 5 ––––(pv) nwhereG h = basic rating service life, operatinghoursc 1 = load factor (➔ Diagram 3 )c 2 = speed factor (➔ Diagram 4 )c 3 = temperature factor(➔ Diagram 5 )c 4 = surface roughness factor(➔ Diagram 6 )c 5= factor for the type of load= 1 for point load (i.e. the loadedzone is always at the same positionon the bearing circumference)= 1,5 for rotating load (i.e. theloaded zone moves round thecircumference of the bearing)K M = factor depending on materialand bearing type= 480 for PTFE composite bushings= 300 for PTFE compositethrust washers= 1 900 for POM compositebushings and thrust washersp = specific bearing load, N/mm 2vn= sliding velocity, m/s= an exponent= 1 for PTFE composite bushingsand thrust washers= 1 for pv ≤ 1 for POM compositebushings and thrust washers= 3 for pv > 1 for POM compositebushings and thrust washersIf loads are very light and/or slidingvelocities very low and the value of theproduct pv• for PTFE composite bearings is lessthan the limiting value of 0,025 thenthe limiting value pv = 0,025 shouldbe used for the life calculations.• for POM composite bearings is lessthan the limiting value of 0,1 thenthe limiting value pv = 0,1 should beused for the life calculations.Calculation exampleThe suspension of a rail vehicle is tobe equipped with composite dry slidingbearings at the linkage position ofthe springs; in this case bushingsarranged in pairs are to be used.Design data:Pin diameter: d = 30 mmSurface roughness of pin: R a = 0,4 µmOperating data:Radial load at the linkage point:F r = 18 750 NHalf angle of oscillation: β = 1°(➔ fig 1 )Frequency of oscillation: f = 180 min –1Operating temperature: t = 30 °CBased on the design characteristics,PTFE composite bushing PCM 303420B having a basic dynamic load ratingC = 46 500 N is chosen. It is necessaryto check that the bushing can beused under the given operating conditionsand then to calculate the basicrating service life.As a first check that the bearing sizeis suitable (➔ Diagram 1 ) the specificbearing load p is calculated usingF 18 750p = K –– = 80 × ––––––––– ≈ 16 N/mm 2C 2× 46 500(with the specific load factor K = 80 forPTFE composite).The sliding velocity is calculated usingv = 5,82 × 10 –7 d β f= 5,82 × 10 –7 × 30 × 1 × 180= 0,0031 m/sAngle of oscillationϕ = angle of oscillation = 2 βA complete oscillation = 4 β(from point 0 to point 4)Fig1These values are within the permissiblerange I of the pv diagram for PTFEcomposite bearings. Furthermorethe load factor c 1 = 1 (➔ Diagram 3 ),the speed factor c 2 = 1(➔ Diagram 4 ),the temperature factor c 3 = 1(➔ Diagram 5 ),the surface roughness factor c 4 = 0,6(➔ Diagram 6 ), andthe factor for the type of load c 5 = 1,5as the load is rotating.The value of K M for PTFE compositebushings = 480. Thus using thebasic rating service life equation forSKF composite dry sliding bearingsK MG h = c 1 c 2 c 3 c 4 c 5 ––––(pv) n 480G h = 1 × 1 × 1 × 0,6 ×1,5 × ––––––––––16 × 0,0031G h = 8 700 operating hours13