Robot joint lifetime and maintenance

By Tolerance Labs engineering team · Updated 18 October 2026

A robot joint is a set of parts that wear at different rates. If you know which part limits life, how that life changes with load, and which driver data shows wear early, you can plan service and spares instead of waiting for a failure.

What limits a joint's life

No single number describes the life of a joint. Each part ages by its own mechanism, and the first one to fail sets the service interval. The table lists the usual candidates.

Reducers and bearings have published life formulas. The other parts need a margin, an inspection or a logged trend. Some parts age with running time, such as bearings and gears. Others also age with calendar time, such as grease, batteries and capacitors, so a joint on a shelf still ages.

Rated life and how it scales with load

Bearings. Rolling bearing life is given as L₁₀, the number of revolutions that 90% of a large group of identical bearings complete before the first sign of fatigue. The basic formula in ISO 281 is L₁₀ = (C ÷ P)^p million revolutions. C is the basic dynamic load rating, P is the equivalent load, and the exponent p is 3 for ball bearings and 10/3 for roller bearings. Double the load on a ball bearing and its life falls to (1/2)³ = 1/8. Remember that 10% of bearings fail before L₁₀. Work from the current edition of the standard and the maker’s catalog, which add factors for lubrication and contamination.

Reducers. Makers publish a rated life at rated torque and rated input speed, and a formula to scale it. Many harmonic reducer makers use the cube of the torque ratio and a linear speed ratio: L = L_n × (T_r ÷ T_av)³ × (n_r ÷ n_av). Some cycloidal makers use an exponent of 10/3. Use the formula and the averaging method from the maker of the reducer you buy, because exponents and rated conditions differ.

These formulas assume that every peak stays inside the repeated and momentary peak torque ratings. One overload can end a reducer’s life, whatever the formula says. The output bearing has its own life, set by the tilting moment and the radial and axial loads on it; see bearings for robot joints.

Worked example: from a duty cycle to hours

A harmonic joint has an assumed rated torque of 50 N·m at a rated input speed of 2,000 rpm and an assumed rated life of 10,000 h. Its task repeats every 2.0 s:

The average torque is weighted by speed and time with the cube: T_av = (sum of n × t × T³ ÷ sum of n × t)^1/3. The dwell adds nothing, because n = 0.

• Accelerate: 1,000 × 0.2 × 80³ = 200 × 512,000 = 102.4 × 10⁶.
• Move: 2,000 × 1.0 × 30³ = 2,000 × 27,000 = 54.0 × 10⁶.
• Decelerate: 1,000 × 0.2 × 60³ = 200 × 216,000 = 43.2 × 10⁶.
• Sum: 199.6 × 10⁶. Sum of n × t: 200 + 2,000 + 200 = 2,400.
• T_av = (199.6 × 10⁶ ÷ 2,400)^1/3 = 83,167^1/3 ≈ 43.7 N·m.
• Average input speed over the whole cycle: n_av = 2,400 ÷ 2.0 = 1,200 rpm.
• L = 10,000 × (50 ÷ 43.7)³ × (2,000 ÷ 1,200) = 10,000 × 1.50 × 1.67 ≈ 25,000 h.

At 16 h a day for 250 days a year, which is 4,000 h a year, that is about 6 years. A heavier payload that raises every torque by 20% divides life by 1.2³ ≈ 1.73, to about 25,000 ÷ 1.73 ≈ 14,500 h. The accelerate segment is 10% of the cycle time but gives about half of the sum, so smoother motion profiles extend life more than any change to the dwell. Repeat the calculation for the output bearing and use the shorter life.

Maintenance schedule

Intervals depend on duty, temperature and environment. The table is a typical starting point for joints in daily lab or light industrial use. Shorten it for outdoor robots and for legged robots that take impacts, and adjust it from your own records.

Lubricating robot joint reducers covers grease service, and multi-turn encoder battery backup covers encoder batteries. Cable life depends mostly on routing; see hollow-shaft joints and cable routing.

Logging driver data to detect wear

The driver already measures most of what changes as a joint wears. Record a baseline when the joint is new, repeat the same test at a fixed interval, and compare the trend.

• Current at a fixed pose. Hold the same pose with the same payload and record the current. Then run a slow constant-speed move in each direction. Half the difference in current between the two directions, times the torque constant and ratio, is the friction torque. Rising friction points to grease breakdown, bearing wear or a dragging seal.
• Temperature on a fixed task. Run a standard cycle from the same starting temperature and record winding and driver temperatures after a set time. More heat for the same task means more loss inside the joint.
• Following error. Log peak and RMS following error on a standard motion. Growth suggests friction, backlash or a loose mount. A spike at the same angle on every revolution points to a damaged tooth or bearing race.
• Fault counters. Log encoder warnings, bus errors, overcurrent and undervoltage events with time stamps.

Compare like with like: the same pose, speed, payload and starting temperature, because cold grease alone can double friction. A change of 20 to 30% against baseline is a common trigger for an inspection. A spectrum of current during a constant-speed move can show peaks at gear mesh or bearing defect frequencies before the averages change. Robot joint failure modes lists what each symptom can mean.

Checklist

• List the parts in each joint that wear, and decide which one sets the service interval.
• Get the reducer’s rated life, rated torque, rated speed and life formula from its maker.
• Convert your duty cycle into average torque and speed, then into hours.
• Check every peak against the repeated and momentary peak ratings.
• Compute output bearing life separately and use the shorter result.
• Set a maintenance schedule and adjust it from your own records.
• Record a baseline of current, temperature and following error, and trend it.

Browse harmonic joint modules and robot joint modules, and ask for rated life and service data when you request a quote.