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.
Understanding Robotics: Bearing Preload and Mounting
Most rolling bearings contain a small internal gap when they are made. This gap allows the rings and rolling elements to move freely before installation. In a robot joint, reversing motion can make that tiny gap show up as a click, a wobble, or a delayed response at the tool.
A pair of angular contact bearings or tapered roller bearings can be arranged so that each bearing supports thrust in an opposite direction. When the pair is compressed, the rolling elements contact their raceways more firmly.
The shaft then resists movement in either axial direction. This is especially useful when a robot arm changes direction many times during a task.
The force path through the mounting parts matters as much as the chosen bearing. A nut should push on the correct bearing ring, usually the inner ring when the shaft is being clamped. The housing shoulder should support the outer ring squarely.
If a tightening force passes through the wrong ring or through the rolling elements, it can damage the bearing surfaces. Spacers are often used between rings to control the distance between the bearings.
Their faces must be flat, parallel, and clean. A tilted spacer can load one side of a bearing more than the other, creating rough motion even when the applied clamping force seems correct.
Preload changes how a joint behaves under real loads. With low preload, a wheel may shift slightly when it hits a floor seam or when the drive motor changes from acceleration to braking. That shift can alter gear tooth contact and encoder readings.
With excessive preload, the motor must spend more power overcoming bearing friction. Friction torque multiplied by rotational speed equals power turned into heat. High speed wheel hubs and spinning sensor units are therefore more sensitive than slow joints.
Heat causes metal parts to expand. If the shaft, housing, and spacers expand by different amounts, a preload set correctly at room temperature can become too high or too low during operation.
Assembly needs a repeatable method rather than a guess based only on how tight a nut feels. Builders commonly clean the seats, press bearings using a tool that contacts only the ring being fitted, and check that every ring reaches its shoulder. They may measure turning torque, axial movement, or the force needed to rotate the shaft after tightening.
A dial indicator can reveal remaining shaft motion when a controlled push and pull load is applied. Locking methods such as tab washers, thread locking compound, or clamp screws prevent the setting from changing through vibration.
Students should watch for the tradeoff between stiffness and smooth rotation. A joint that feels rigid but becomes hot, noisy, or hard to turn is not properly adjusted.
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.