40% of joint motor failures are attributable to the selection of potting compound

The joint motor has failed; 40% of the blame lies with the bearing.

On the repair bench sits a frameless torque motor removed from a joint module; its outer diameter is 85 mm, barely thicker than a beverage can.

Disassemble the rotor, place your finger on the bearing, and rotate it once.

Smooth.

Look down at the stator. Near the slot opening, there is a small blackened area; the enameled wire has changed color from amber to dark brown, and the surrounding potting compound has developed several fine cracks. Bringing it closer to the nose, a sour odor is detected. This is the odor produced when insulating varnish decomposes under heat; once you smell it, you can’t forget it.

The bearing of this motor is not damaged; the winding has burned out.

Anyone who has worked on motor repairs is familiar with the standard procedure: when a motor stops rotating, the first suspicion is the bearing; if the problem persists after replacement, the electrical system is examined next. This approach is not based on intuition. In its own motor failure manual, ABB cites data from the IEA stating that approximately 51% of motor failures are related to bearings. Another industry-wide failure investigation, frequently cited, attributes 40% of failures to bearings, 38% to the stator, and 10% to the rotor.

The absolute values of the two sets of numbers do not match, but their directions are consistent. The bearing ranks first; there is no dispute about this.

However, after excluding the bearing component, winding and the outer rubber layer account for at least 40% of all faults.

Robot joint motors face even more extreme conditions. One industry analysis broke down and tallied after-sales failures involving this type of motor; the combined proportion of adhesive delamination, failed thermal insulation in the potting compound, and seal leakage ranked second only to bearings. I have not found any official standard to endorse this metric, so I will leave it at this point for now.

The winding has suffered thermal failure, not electrical failure.

The stator is the only source of heat in the motor. The heat generated by copper losses must be conducted outward through the turns, pass through the potting compound and the iron core, and finally reach the housing.

In this context, potting compound has only one role: serving as a thermal conductive pathway. Once it fails, heat becomes trapped within the windings and cannot escape.

The original gaps in the windings are filled with air, which has a thermal conductivity of only 0.026 W/(m·K). This constitutes a natural thermal insulation layer. Filling these gaps with potting compound effectively increases thermal conductivity by two orders of magnitude—provided that the material is filled completely.

Atmospheric-pressure hermetic sealing is incomplete. The stator slot openings are narrow and the windings are dense; when the adhesive has high viscosity, the minute gaps between turns cannot be filled, leaving air bubbles behind. The thermal conductivity of these bubbles is on the same order of magnitude as that of air, causing heat to accumulate and create localized hot spots.

Once a hot spot appears, the insulation layer of the enameled wire begins to age rapidly. In the field of insulation, there is an empirical rule that has been used for many years: for every 10°C increase in temperature, the thermal aging life is roughly halved. When the material reaches a certain critical point, the inter-turn insulation breaks down, causing a short circuit. At the moment of the short circuit, the temperature can reach several thousand degrees Celsius, causing the copper wire to melt directly.

From encapsulation bubbles to motor burnout, this chain is fully connected.

Start it once at minus 30 degrees Celsius, and the interface will feel a bit looser.

The linear coefficient of thermal expansion of epoxy potting compounds generally ranges from 50 to 80 ppm/℃.

Copper: 17; silicon steel sheets: 12.

The difference is three to five times greater. When the motor exits the cold storage unit, the temperature is around minus 30 to 40 °C; once it is running, the winding temperature can rise to over 100 °C. With each temperature cycle, the adhesive layer and the metal interface are subjected to repeated tensile stress: if the bond remains intact, the components remain attached; if it fails, they separate.

The consequences of delamination are not merely a loss of tightness. Once the interface cracks, the thermal conduction path is severed, and moisture can also enter the crack. Some motors do not fail suddenly; after operating for several thousand hours, they may suddenly trigger an insulation alarm on a winter morning.

Thermal conductivity of adhesives is not necessarily better the higher it is.

The more high thermal-conductivity filler is added, the higher the thermal conductivity becomes; simultaneously, the colloidal viscosity also increases. This is a physical trade-off that cannot be avoided.

Joint motors are becoming smaller. In the miniature frameless motors used in smartphones, the outer diameter has already shrunk from 16 mm to 14.5 mm; it is said that the next generation will be around 6–10 mm. With motors of this size, the slots are so narrow that they are barely visible to the naked eye. If you pour a high-fill, high thermal-conductivity adhesive into them, the fluid cannot reach its intended destination.

The selection process should be reversed: first consider the motor’s power rating, the width of its slots, and the density of its windings; then determine the required heat dissipation capacity.

For low-power exoskeleton motors, ordinary epoxy with a thermal conductivity of 0.5 W/(m·K) is sufficient; for conventional humanoid joint motors, a range of 1.0 to 1.5 is reasonable; only for joints that need to deliver high burst torque should the value be pushed above 1.8.

Applying a high thermal-conductivity solution to a small motor only results in paying more for added hassle.

There is another common pitfall: the thermal conductivity values listed in quotation sheets may have been measured using different methods. When the same adhesive sample is tested by both the steady-state heat flux method and the laser flash method, the resulting values may not match. Before comparing values, ask first which method was used for the measurement.

More specific on the production line

Over the past two years, manufacturers of joint motors have increasingly adopted vacuum potting. The fourth-generation FMK series from Buke and the FM series from Leise both specify vacuum potting in their product documentation. When the adhesive is applied under vacuum conditions, air bubbles in the adhesive and air trapped in the windings are removed, resulting in a significantly higher fill factor than under atmospheric pressure.

Having the equipment does not guarantee that the filling process will be performed correctly. Parameters such as vacuum level, evacuation time, adhesive injection rate, and the viscosity–temperature curve of the adhesive must be matched to the specific stator model. If the same tank is used but a motor of a different size is installed, the parameters must be readjusted accordingly.

There is another more subtle aspect: preheating before injection molding. When the temperature difference between the stator and the rubber compound is too large, the moment the rubber comes into contact with the cold iron core, its viscosity rises sharply and its flowability drops immediately. This step is often regarded as optional on many production lines.

Downstream volumes are on the rise. UBTECH’s factory in Liuzhou began operations in September this year, with a designed production rate of one complete unit per 10 minutes and an annual planned capacity exceeding 10,000 units. At this scale, a sealing defect rate of just a few percentage points per unit can accumulate into several thousand units of after-sales service requirements.

When you disassemble the unit next time, start by analyzing the potting compound.

There is a way of assessment that is more useful than simply examining the bearing.

When disassembling a burnt-out motor, do not touch the bearings at this stage. Use a slanting pair of pliers to cut a small piece of potting compound from the end of the winding, then examine the cross-section.

If there are strings of fine pores visible in a cross-section, this indicates that the material was not completely evacuated during encapsulation. If the adhesive layer can be peeled off the core as a continuous sheet with a clean interface, this indicates that delamination has already occurred at that location. If the adhesive flakes off easily when bent and becomes brittle, either the curing conditions were improper, or the material has been exposed to temperatures exceeding its Tg for an extended period.

Three types of cross-sections correspond to three root causes. If you look only at the bearing, none of them can be identified.

If a bearing is damaged, replace it—that’s the right approach. After replacing it, take a quick look at the rubber seal; otherwise, the same bearing will likely fail again next time.

Verified JOME Engineering Grades & Specification Cross-Reference

To qualify an industrial-grade adhesive, potting compound, or thermal interface material for this application, compare verified Shanghai Jome Industrial Co., Ltd. (JOME) formulations below or explore our primary category hubs: Anaerobic Threadlockers & Retaining Adhesives and Industrial Structural Adhesives.

JOME Grade / SKU	Polymer Chemistry & Cure	Key Verified Specification	Primary Industrial Application
JOM 3107	Anaerobic Magnet Structural Bonding Adhesive	High impact & centrifugal shear (>20 MPa) \| +150°C rated	Servo motor, traction motor & humanoid robot rotor magnet bonding
JOME 1767 / JOME 2029A/B	1K Heat-Cure & 2K High-Temp Motor Epoxy	Continuous service up to +180–200°C \| High Tg & chemical resistance	High-speed turbine rotors, rare-earth & ferrite magnet bonding
JOM 638	High-Strength Cylindrical Bearing Retaining Compound	Fills 0.05–0.25 mm radial clearances \| Prevents fretting wear	Motor shafts, harmonic drive bearings, gears, bushings & sleeves
JOM 263/277 & JOM 204/93	Liquid Anaerobic & Microcapsule Pre-Coated Threadlockers	Breakaway torque 20–42 N·m \| Oil-tolerant & dry-to-touch film	Chassis bolts, robot joint fasteners, mass pre-coated screws

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Matched JOME Industrial Formulations: For production bonding, potting, and sealing qualification in this application, evaluate Clear self-repairing encapsulating silicone gel, Vehicle-grade transparent self-healing encapsulated silicone gel, General-purpose epoxy potting compound (black, white, red, clear), and 10:1 Condensation type high bonding strength silicone potting compound (ISO 9001:2015 and CTI RoHS 2.0 verified; free engineering samples shipped via UPS freight collect).

Frequently Asked Engineering Questions

Q: Which JOME adhesive or potting grades are recommended for 40% of joint motor failures are attributable to the selection of ?
Depending on substrate pair, continuous operating temperature, and curing line constraints, Shanghai Jome Industrial Co., Ltd. (JOME) recommends evaluating JOM 3107, JOME 1767 / JOME 2029A/B, and JOM 638. Review our full specification table on this page or request a customized formulation sample.

Q: Can JOME customize viscosity, cure speed, or thermal conductivity for our automated production line?
Yes. JOME's Shanghai RandD laboratory customizes A/B mix viscosity, thixotropic index, pot life, Shore hardness, and thermal filler loading (0.75 to 25.0 W/m·K) to match your dispensing equipment and environmental test standards.