Lubrication, Bearings and Power Transmission
What this domain tests
Lubrication, Bearings and Power Transmission tests lubricant selection, bearing types and fitting, and the transmission elements that connect machines: belts, chains, gears, couplings and shafts. Expect viscosity grades, grease properties, bearing clearance, belt tension, gear ratio and alignment.
Core rules
- Viscosity is the key oil property; ISO VG numbers refer to kinematic viscosity at 40 °C in cSt. Higher speed usually needs lower viscosity; higher load and temperature need higher viscosity.
- Grease = base oil + thickener + additives. Do not mix incompatible thickeners without checking; over-greasing can damage seals and churn.
- Bearings: deep-groove ball bearings carry radial and some axial load; cylindrical roller bearings carry heavy radial load; tapered roller bearings take combined loads and need correct preload or end play.
- Belt drives: match belt section to pulley groove; tension to manufacturer's deflection, not as tight as possible. Misalignment causes edge wear.
- Gear ratio i = N1/N2 = z2/z1. For a simple train, output speed = input speed × (driver teeth/driven teeth).
Worked example
A 20-tooth driver runs at 1440 rev/min and drives a 60-tooth gear. Ratio = 60/20 = 3:1, so output speed = 1440/3 = 480 rev/min. Torque rises by the same ratio, ignoring losses. If the drive uses a V-belt between the motor and gearbox, check pulley alignment with a straight edge and set tension so the belt deflects about 10–15 mm per metre of span, following the supplier's figure.
Traps
Mixing SAE and ISO viscosity numbers; adding incompatible grease; over-tensioning a belt and loading the bearings; fitting a bearing by pressing on the outer race when the load is on the inner race; ignoring coupling misalignment.