An industrial robot is a chain of rigid links connected by powered joints, and its usefulness depends on how precisely and repeatably those joints move. Behind almost every joint sits a reducer and a set of bearings, and the stiffness, accuracy and life of those bearings set the limits on what the robot can do. The same is true of collaborative robots, or cobots, which add the requirement that the arm be safe to work beside a person, changing the bearing demands once again. Understanding the bearing content of a robot joint is therefore essential to understanding robot performance and to specifying replacements correctly. This article follows the load path from the base to the wrist.
Following the load path from base to wrist
A typical 6-axis industrial robot has a base, a rotating waist or axis 1, a shoulder, an elbow and a three-axis wrist. Each joint contains an actuator, usually an electric servo motor, and a speed reducer that converts the motor's fast rotation, often near 3,000 rpm, into slow, high-torque motion. The joint also contains bearings that support the moving link and resist the loads the arm carries. The loads are not trivial. A robot holding a 10 to 200 kg payload at full extension subjects its joints to large bending moments, and those moments change constantly as the arm moves. The bearings must carry these varying loads without deflecting enough to compromise positioning, and they must do so for 1 to 5 million cycles in a busy cell.
How cycloidal and harmonic reducers differ
Two reducer types dominate robot joints, and the bearings inside them are as different as the reducers themselves.
Cycloidal (RV) internals
Cycloidal reducers, often called RV reducers, use an eccentric cam and a set of cycloidally shaped discs to produce high reduction ratios, typically about 30:1 to 200:1, with high stiffness and high torque capacity. They typically contain cylindrical roller bearings supporting the eccentric cam, needle or cylindrical rollers at the disc contacts, and large-diameter output bearings that support the joint load directly. These positions lean on the needle and cylindrical roller families for their compact, high-capacity contacts.
Harmonic strain-wave internals
Harmonic reducers, also called strain wave gears, use an elliptical wave generator that flexes a thin-walled cup to engage a rigid ring gear. They offer very high ratios, commonly 50:1 to 160:1, in a compact, lightweight package with low backlash, which suits the smaller joints of a robot arm and many cobots. Their internal bearings are thin-section and precision-ground, and their smooth action is a major reason cobots can be backdriven by hand; the thin-section bearing guide covers the families used.
The joint output bearing and its moment load
The bearing that carries the joint's external load is often the most critical component. Where a joint must resist a large overturning moment in a compact space, a crossed roller bearing is a common answer: two rows of rollers arranged at right angles to one another, running on a single ring, allowing one bearing to carry radial, axial and moment loads simultaneously. Crossed roller bearings give high stiffness and accuracy with a bearing height one roller diameter thick.
Alternatively, a precision angular contact pair or a large-diameter thin-section bearing may be used, depending on the load case and the space available. In all cases, the bearing's runout and the joint's preload determine the arm's positioning accuracy; a bearing that is accurate on the bench but installed with the wrong preload will degrade the whole joint. Precision angular contact families are discussed in the angular contact range, and their preload in the preload guide.
Mapping each axis to its bearing role
The table below maps the principal joints of an articulated robot to the reducers and bearings commonly used. The general pattern is that heavy, slow joints need high-capacity contacts while small, fast joints need compact, low-friction bearings.
| Joint | Reducer type | Bearing role |
|---|---|---|
| Base (axis 1) | Servo with high-ratio gearbox | Tapered or crossed roller carries the overturning moment |
| Shoulder (axis 2) | Cycloidal (RV) | Cylindrical roller in the reducer; crossed roller at the joint |
| Elbow (axis 3) | Cycloidal or harmonic | Thin-section and angular contact for stiffness |
| Wrist (axes 4-6) | Harmonic (strain wave) | Miniature crossed roller and thin-section ball |
| Linear or gantry axis | Ball screw or rack and pinion | Linear guides and precision angular contact |
Cobot safety rewrites the friction budget
A cobot must be safe to work alongside a human, which means it must be able to detect a collision and stop quickly, and it must present no sharp or pinching hazards. A practical consequence is that cobots are often designed to be backdrivable, so that a person can push the arm aside with modest force.
Backdrivability and safety
Backdrivability requires low friction and low inertia in the drivetrain, which pushes designers toward harmonic reducers and toward bearings with low starting and running torque.
Low-torque and low-noise needs
Low friction and low noise also improve the control loop, because the motor sees less resistance and the controller can command finer motions. For bearings, this means smooth, precisely finished raceways and careful grease selection, because a grease with high drag will spoil the backdrivability the design depends on. The cobot friction budget is spent on a short list of choices:
- Reducer type - a harmonic drive for low backlash and light inertia, an RV unit for heavy joints.
- Bearing starting torque - the torque needed to break away from rest sets how light the arm feels.
- Raceway finish and grade - a smoother, more precise bearing runs with less drag and less noise.
- Grease choice - a stiff, high-drag grease quietly defeats the backdrivability the design needs.
- Seal drag - a contact seal that protects the joint also adds friction the controller must see.
Stiffness, preload and lost motion
The accuracy of a robot at the tool centre point depends on the accumulated deflection of every link and joint under load.
Preload sets joint stiffness
Bearing preload contributes directly to joint stiffness: too little preload and the joint deflects under load, degrading accuracy; too much and friction and heat rise, reducing life and increasing motor load. Selecting and setting preload correctly is one of the most consequential decisions in robot design: a working preload of only 2 to 10 micrometres can separate a tight, accurate joint from a sluggish or a loose one. The tolerance classes that support it are described in the P4 and P5 class guide.
Backlash and matched sets
Backlash, the lost motion when direction reverses, is equally important. Precision reducers minimise backlash by design, and the bearings within them must be matched to that goal. In high-accuracy applications, bearings are graded and matched in sets so that the assembly meets a backlash specification that no single part could guarantee alone, following the tolerance language of ISO 492.
Sealed for life: lubrication and repair
Robot joints are usually lubricated for life, because relubricating a sealed reducer in the field is impractical. That places a premium on grease quality and on contamination control during assembly, since a robot may be expected to run for years with no attention to its bearings. Life is estimated using the bearing's basic dynamic load rating and the calculated equivalent load over the duty cycle, following the ISO 281 approach, with an adjustment for lubrication and contamination. A joint that runs 16 hours a day for 250 days a year therefore accumulates roughly 4,000 hours of motion a year, and the grease has to last that duty without attention. When a joint does fail, the symptoms are characteristic.
| Fault symptom | Likely cause | Repair scope |
|---|---|---|
| Increased backlash or lost accuracy | Wear or reduced preload | Re-shim or replace the joint bearing set |
| Rising temperature or motor current | Excessive friction or failing grease | Inspect the seal and lubrication before condemning parts |
| Noise or vibration | Surface damage on a raceway | Replace the affected bearing or reducer assembly |
| Sudden position error on one axis | Reducer or encoder fault | Service the reducer unit; check the encoder as well |
Specifying a replacement joint bearing
Replacement bearings for robot joints are precision items, and successful substitution requires matching not only size but tolerance class, preload, clearance, cage and lubrication. A bearing that is dimensionally identical but a grade lower in precision will compromise the arm's accuracy, and a bearing with the wrong preload will change the joint's dynamics.
- Confirm the axis. Identify which joint and which reducer type the bearing serves.
- Match the class. Record the precision class, since a lower grade changes accuracy.
- Match the preload. Verify how preload is set and how it is checked on assembly.
- Match the closure. Confirm sealed or shielded construction and the grease family.
- Verify dimensions. Check bore, outside diameter and width against the drawing, not the old part.
For critical joints, using the original specification or an approved equivalent is the sound practice. A distributor serving robotics customers benefits from carrying precision angular contact, crossed roller and thin-section families with full specification data. A selector can narrow the family from an application description, the linear motion guide covers the gantry case, and the technical section explains the precision classes that robot joints depend on. Where the duty cycle is unusual, the comparison against a needle and thin-section specialist range or the general tapered roller family for the heavy base joint helps check the load case and the life estimate before an order is placed.
- Accuracy lives in the bearings. Joint stiffness and runout, not the motor, set a robot's positioning accuracy.
- RV for heavy joints, harmonic for compact ones. The reducer type dictates which bearing families appear inside the joint.
- One bearing can carry everything. Crossed roller designs handle radial, axial and moment load in a single thin unit.
- Cobots spend a friction budget. Backdrivability demands low-torque, low-noise bearings and careful grease choice.
- Replace like for like, in class and preload. A cheaper tolerance grade silently degrades the whole arm.
Frequently asked questions
Which bearing carries the load in a robot joint?
Usually the joint output bearing, not the small bearings inside the reducer. It must resist the overturning moment of the payload and the arm itself, so crossed roller or large-diameter thin-section designs are common. The internal reducer bearings locate the cam and discs; the output bearing ties the moving link to the joint.
Why do cobots use harmonic rather than cycloidal reducers?
Backdrivability and low inertia. A harmonic strain-wave reducer is compact, light and low-backlash, and it lets a person push the arm aside with modest force, which is a safety requirement for a robot working beside a human. Cycloidal reducers suit the heavier, slower joints where stiffness and torque matter more than light weight.
How does preload change a robot's positioning accuracy?
Preload removes internal clearance and stiffens the joint, so deflection under payload is smaller and the tool centre point stays where the controller thinks it is. Too little preload lets the joint deflect; too much raises friction, heat and motor load. Setting it correctly is one of the most consequential choices in the design.
What is a crossed roller bearing and why is it used at a joint?
It is a single bearing with two rows of rollers set at right angles on one ring, so one compact unit carries radial, axial and moment loads at once. That lets a joint resist a large overturning moment in a very thin radial space, which is exactly what a compact robot wrist or shoulder needs.
Can a lower tolerance grade replace the original robot joint bearing?
It can fit, but not honestly substitute. A dimensionally identical bearing a grade lower in precision adds runout and changes the joint stiffness, so the arm loses accuracy even though the part bolts in. For critical joints, the original specification or an approved equivalent is the sound choice.
Why are robot joint bearings lubricated for life?
Because relubricating a sealed reducer in the field is impractical, so the grease has to last the service interval without attention. That places a premium on grease quality and on clean assembly, and it means life is estimated from the duty cycle and the ISO 281 calculation with lubrication and contamination adjustments applied.