MEC2048S Mechanical Engineering Design I Bearings – Types and Operation Dr Reuben Govender September 2020 Module Aims • Explain operation of basic bearing types • Understand bearing constraint from geometry • Use load path diagrams to determine requirement for bearing mounting • Select ball bearing size based on load, lifetime and reliability requirements MEC2048S Bearings 2 Bibliography • “Bearings – A Tribology Handbook”, MJ Neale, Butterworth Heinemann • “Machine Component Design”, RC Juvinall & KM Marshek • SKF Rolling Element Bearing Manual • NSK Rolling Element Bearing Manual MEC2048S Bearings 3 Basic bearing types • Bearing restricts motion in one (or more) direction (s) / degree of freedom while permitting free motion in another • Bearings support forces / moments in some directions while offering no resistance in others • Rotary (angular) bearings or linear bearings MEC2048S Bearings 4 Bearing types Plain Bearings Rolling element bearings • Relative motion permitting by sliding of surfaces • Sliding friction determines drag • Relative motions permitted by cylinders or spheres rolling over surfaces • Drag determined by rolling resistance • Rollers don’t slip MEC2048S Bearings 5 Tribology • Study of interacting surfaces in relative motion • Friction, Lubrication & Wear • Bearings intended for multiple cycles of use • How useful is a door hinge which fails after opening & closing once? • Bearings designed with some finite lifetime • Lifetime limited by wear • Wear rate proportional to pressure & velocity MEC2048S Bearings 6 Bearing type selection – 10 000 hr life MEC2048S Bearings 7 Bushings • Can operate with minimal lubricant (normally greased at installation) • Limited to lower speeds due to sliding friction • Typically constrain radial motion • Constrain axial motion (with shoulders on shaft) • Only constrains rotation if non-circular geometry is used • Bushing should be a softer material than shaft / housing (preferential wear) MEC2048S Bearings Polymer bushings Bronze bushing 8 Bushings • Must have clearance between sliding surfaces! • Materials: • Polymers – Nylon (often oil impregnated), Teflon (PTFE) • Copper based alloys (brass, bronze) • Bronze – porous metal often impregnated with lubricant • White metal bearings (tin / lead alloy) – very soft coating applied to a steel shell • Friction depends on interaction between bushing material and shaft • Surface finish is critical! MEC2048S Bearings 9 Split bushings • Bushings or shell of hydrodynamic bearings often split to accommodate crankshafts MEC2048S Bearings 10 Hydrodynamic bearings • MUST have lubricant supply • Rotation of shaft creates “pumping action” which maintains fluid film between surfaces • Only works above a certain speed! • Shaft will sit slightly eccentric in bore • More details in 3rd year Design! • Typically used for very large shafts, at high speeds: • Turbines • Propellor shafts MEC2048S Bearings 11 Lubrication – oil / grease supply • Lubricant reduces friction • Flowing lubricant removes heat by convection MEC2048S Bearings 12 Lubrication – oil / grease supply • Various ways to ensure continuous oil supply • Sealing • Keeps lubricant in! • Keeps dirt / contaminants out! • Deep groove ball bearings might have integral seals • Other bearings – need external shaft / housing seals MEC2048S Bearings 13 Linear plain bearings • Allow linear motion along one axis but not perpendicular • Constrain rotation with shape of rail or using pairs of rails MEC2048S Bearings 14