Bearing preload is a controlled axial force applied to a bearing pair so the shaft and wheel hub have little or no looseness. In robotics, this matters because wheel modules, arms, and sensor shafts must hold position accurately while loads change direction. Proper preload improves stiffness, reduces wobble, and helps gears, encoders, and wheels stay aligned.
Too much preload, however, creates extra friction and heat that can shorten bearing life.
A typical robot hub uses paired bearings, spacers, a housing shoulder, and a retaining nut or clamp to set the axial load. Tightening the nut pushes the bearing rings and spacers together until internal clearance is removed and a desired preload is reached. The design must make sure each bearing is fully seated against its shoulder, because a poorly seated bearing can loosen later and change the preload.
Engineers choose a preload value by balancing stiffness, smooth rotation, temperature rise, and expected service life.
Key Facts
- Bearing preload is an intentional axial load applied before external working loads act on the bearing.
- Axial preload removes internal clearance, reducing play and improving shaft position repeatability.
- Higher preload increases stiffness but also increases friction, heat, and contact stress.
- Too little preload can allow vibration, fretting, noise, and inaccurate wheel or shaft motion.
- Approximate axial stiffness relation: k = F / delta, where F is axial force and delta is axial deflection.
- Bearing torque loss can be estimated by P = T omega, where P is power loss, T is friction torque, and omega is angular speed.
Vocabulary
- Preload
- Preload is a controlled force applied to a bearing assembly to remove internal clearance before normal operation.
- Axial load
- An axial load is a force that acts along the axis of a shaft or bearing.
- Radial load
- A radial load is a force that acts perpendicular to the axis of a shaft or bearing.
- Bearing seating
- Bearing seating is the condition in which a bearing ring is fully pressed against its shaft shoulder, housing shoulder, or spacer face.
- Spacer
- A spacer is a precision part placed between bearing rings or mounting surfaces to control distance, alignment, and preload.
Common Mistakes to Avoid
- Overtightening the retaining nut is wrong because it can create excessive preload, high friction, heat, and early bearing damage.
- Assuming zero play always means correct preload is wrong because a bearing can feel tight even when it is overloaded or misaligned.
- Ignoring clean seating surfaces is wrong because dirt, burrs, or uneven shoulders can prevent full seating and cause preload to change during use.
- Mixing up axial and radial loads is wrong because preload is mainly an axial setup force, while many robot wheel loads act radially through the hub.
Practice Questions
- 1 A bearing pair has an axial stiffness of 80,000 N/m. If the desired axial deflection from preload is 0.025 mm, what preload force is required?
- 2 A preloaded hub has a friction torque of 0.08 N m and spins at 120 rad/s. What power is lost to bearing friction?
- 3 A robot wheel hub develops wobble after 30 minutes of driving, even though the retaining nut was tight during assembly. Explain two possible mounting or preload causes.