Bearings support rotating or sliding parts while reducing friction, wear, heat, and vibration. This cheat sheet helps students compare common bearing types, estimate basic bearing loads, and understand why lubrication is essential in machine design. It is useful for engineering drawing, robotics, automotive systems, manufacturing, and mechanical design projects.
The most important ideas are load direction, bearing life, friction, viscosity, and lubrication film formation. Rolling-element bearings are often selected using equivalent load and L10 life, while plain bearings depend strongly on speed, pressure, clearance, and lubricant behavior. Good lubrication separates surfaces, removes heat, prevents corrosion, and reduces energy loss.
Key Facts
- Radial load acts perpendicular to a shaft axis, while axial or thrust load acts parallel to the shaft axis.
- Basic bearing life is often written as L10 = (C / P)^p million revolutions, where C is dynamic load rating, P is equivalent dynamic load, and p = 3 for ball bearings.
- For roller bearings, the basic life formula uses L10 = (C / P)^(10/3) million revolutions.
- Equivalent dynamic load for many rolling bearings is estimated by P = XFr + YFa, where Fr is radial load, Fa is axial load, and X and Y are bearing factors.
- Bearing speed life in hours can be found from L10h = (10^6 L10) / (60n), where n is rotational speed in rpm.
- Dynamic viscosity relates shear stress to velocity gradient using tau = mu(dv/dy), where mu is dynamic viscosity.
- Kinematic viscosity is nu = mu / rho, where mu is dynamic viscosity and rho is fluid density.
- Hydrodynamic lubrication occurs when a full lubricant film separates moving surfaces, greatly reducing direct metal-to-metal contact.
Vocabulary
- Bearing
- A machine element that supports motion between parts while reducing friction and controlling load direction.
- Radial Load
- A load that acts perpendicular to the axis of a shaft or rotating component.
- Axial Load
- A load that acts along the axis of a shaft and is also called a thrust load.
- L10 Life
- The calculated bearing life at which 90 percent of identical bearings are expected to survive under stated conditions.
- Viscosity
- A measure of a fluid's resistance to flow and its ability to maintain a lubricating film.
- Hydrodynamic Lubrication
- A lubrication condition where motion creates enough pressure in the lubricant to fully separate the surfaces.
Common Mistakes to Avoid
- Confusing radial load with axial load is wrong because bearing selection depends on the direction of the force relative to the shaft.
- Using the ball bearing life exponent for roller bearings is wrong because ball bearings use p = 3, while roller bearings use p = 10/3.
- Ignoring speed when converting bearing life to hours is wrong because the same revolution life lasts fewer hours at higher rpm.
- Choosing lubricant only by thickness is wrong because viscosity must match speed, load, temperature, clearance, and lubrication method.
- Assuming more grease is always better is wrong because over-greasing can increase heat, churning losses, seal damage, and bearing failure.
Practice Questions
- 1 A ball bearing has C = 12000 N and P = 3000 N. Calculate L10 in million revolutions using L10 = (C / P)^3.
- 2 A bearing has L10 = 64 million revolutions and rotates at 800 rpm. Calculate L10h using L10h = (10^6 L10) / (60n).
- 3 A lubricant has dynamic viscosity mu = 0.18 Pa s and density rho = 900 kg/m^3. Calculate kinematic viscosity nu = mu / rho.
- 4 Explain why a lightly loaded high-speed bearing may need a different lubricant than a heavily loaded slow-speed bearing.