Robot joint failure modes: causes, symptoms and prevention

By Tolerance Labs engineering team · Updated 18 October 2026

A joint can fail in its gears, its electronics, its sensors or its firmware, and each kind of failure gives different warning signs. If you know the common modes, you can choose a joint, design around its weak points and find the cause faster when a robot stops.

How to use these tables

Each table lists a failure mode, its usual cause, the symptom you see, how to detect it before it stops the robot, and how to prevent it. It is a starting list for a failure mode and effects analysis (FMEA) of your own robot, not a complete one. Rank each mode by how likely it is, how bad its effect is and how hard it is to detect, and act first on the modes that score high on all three.

Mechanical failures

Ratcheting, where flexspline teeth skip over the circular spline, also shifts the output zero, and only an output encoder or a recalibration finds it. Backlash and torsional stiffness shows how wear shows up as position error. Brinelling often starts with a single fall. The impact dents the races in an instant, and the damage shows weeks later as noise. Log falls and impacts, and inspect the joints that took them.

Electrical failures

Regenerated energy is a common cause of transistor and capacitor damage; regenerative braking and bus overvoltage shows how to size a clamp. Connector fretting is easy to mistake for a firmware fault, because its resets look random until you compare them with motion.

Worked example: heat and insulation life

Winding insulation ages by chemical reactions that speed up with temperature. A common rule of thumb, based on Arrhenius behavior, is that insulation life roughly halves for each 10 °C rise in winding temperature. The rule is approximate and holds only inside the material’s normal range, but it shows how strongly temperature matters.

• Life factor for a rise of ΔT: 2^{−ΔT ÷ 10}.
• A joint whose winding ran at 90 °C moves to a harder task and now runs at 110 °C. ΔT = 110 − 90 = 20 °C, so life is multiplied by 2^{−20 ÷ 10} = 2⁻² = 1/4.
• Better mounting brings it down 15 °C, to 95 °C. Compared with 110 °C, life rises by 2^{15 ÷ 10} = 2^1.5 ≈ 2.83.
• Running at 120 °C instead of 90 °C, a rise of 30 °C, divides life by 2³ = 8.

The same reasoning applies, with different numbers, to electrolytic capacitors. Their rated life is commonly given at a maximum temperature such as 105 °C and is often estimated to double for each 10 °C below it. Thermal management of robot joint actuators shows how to estimate winding temperature and lower it.

Sensing failures

Encoders for robot joints covers encoder types and their weak points.

Firmware and communication failures

Decide what each joint does on a communication fault: hold position, engage the brake or go limp. For a legged robot, going limp can mean a fall. For an arm near people, holding can be the wrong choice. Wiring a CAN bus for robot joints covers the physical layer.

Finding the cause when a joint stops

When a joint faults in the field, collect the evidence before you cycle the power, because a restart often clears the fault record. Then work from the quickest checks to the most invasive ones.

• Read the driver’s fault code and the log for the minutes before it: temperature, bus voltage, current and bus errors.
• Check whether the fault follows motion, pose or temperature. Faults that follow motion point to cables and connectors. Faults that follow temperature point to heat or a marginal part.
• Swap the suspect joint with a spare of the same type. If the fault moves with the joint, the joint is the cause. If it stays, look at the wiring, the power supply or the host.
• With the power off, turn the output by hand and feel for roughness, play or notches. Compare with a good joint.
• Measure the resistance between each pair of phases; the three values should match closely. Measure insulation resistance from each phase to the housing only with the driver disconnected and at a test voltage the maker allows.
• Check the output zero against a reference mark to find slip or ratcheting.

Record each failure with the joint serial number, running hours, task and the cause you found. After a few failures, the record shows which modes dominate in your robot. That tells you which spares to stock and which prevention step to add first; see spare parts strategy for robots.

Checklist

• Build an FMEA for your robot from these tables, and add modes specific to your design.
• Rank each mode by likelihood, effect and detectability.
• Keep winding temperature well below the thermal class; each 10 °C costs about half the insulation life.
• Use an output encoder or a periodic zero check to catch ratcheting and slip.
• Log temperatures, bus voltage, encoder warnings and bus errors with time stamps.
• Define the safe action for every fault, including loss of communication.
• Review the log after every fall or crash before the next run.

Browse robot joint modules and motor drivers, and ask for protection and diagnostics details when you request a quote.