Humanoid Robot Motor Adhesives: The Role of Encapsulating Resin

Humanoid and collaborative robots increasingly rely on compact, integrated joint actuators to achieve precise and responsive movement. Frameless torque motors are often used as the core drive components within these actuator systems, helping reduce overall size and weight while enabling more compact joint designs.

However, the compact and highly integrated structure also leaves limited tolerance for assembly errors, heat buildup and component movement.

Permanent Magnet Bonding: Preventing Movement and Adhesive Overflow

Permanent magnets are repeatedly exposed to acceleration, deceleration and vibration. Over time, these loads can create cracks in the bonding layer. Once the crack expands, the magnet may gradually move or detach from the rotor.

A toughened structural adhesive combines bonding strength with resistance to crack propagation, helping keep the magnet securely fixed during repeated mechanical loading.

Another challenge is adhesive flow. The gap between the rotor and stator is extremely narrow. If excess adhesive flows into this area during assembly, it can interfere with rotor movement. A controlled viscosity and suitable thixotropy help keep the adhesive in the bonding area and reduce the risk of overflow.

Stator Potting: Filling Narrow Gaps Without Trapping Air

Heat generated by the windings must be transferred out of the motor. However, narrow spaces between winding components can trap air, and air creates a barrier to heat transfer.

The challenge is that highly thermally conductive potting materials often contain large amounts of filler. More filler can increase thermal conductivity, but it can also make the material more viscous. In narrow winding structures, a highly viscous material may not flow into every gap, leaving air pockets inside.

A balanced combination of thermal conductivity and flowability helps the encapsulating resin penetrate narrow spaces and reduce trapped air. For higher power density motors, a thermally conductive encapsulant is required. This creates a more continuous heat-transfer path while also providing electrical insulation and mechanical protection for the windings.

Sensor Bonding: Preventing Position Shift During Curing

Hall sensors, temperature sensors and encoder components must remain in a precise position after assembly. Even small movement can affect the accuracy of motor feedback.

One source of movement is adhesive shrinkage during curing. If the adhesive contracts, it can pull a small component away from its original position. Low-shrinkage materials help reduce this movement and maintain more stable positioning.

Another challenge is curing speed. Components may move before the adhesive reaches sufficient strength. Rapid initial fixation can shorten this vulnerable period and improve assembly efficiency.

For UV curing, metal brackets or surrounding components can block the light and create shadowed areas. A secondary curing mechanism can help ensure that areas with limited UV exposure are also fully cured.

Wire and Terminal Sealing: Reducing Stress at the Wire Exit

The motor housing is rigid, but the external cable moves with vibration and joint motion. This difference creates stress at the point where the wire exits the motor.

If the sealing material is too rigid, repeated cable movement can concentrate stress at the edge of the adhesive. Over time, this may contribute to wire fatigue or damage near the exit.

A flexible sealing material can deform with the cable and provide strain relief at this transition point. At the same time, it can help prevent moisture, dust or other contaminants from entering the motor through the wire exit.

The goal is not to make this area as rigid as possible. It is to create a softer transition between the fixed motor and the moving cable.

Structural Bonding: Solving the Problem of Small Assembly Gaps

The stator and housing are manufactured separately, and small dimensional differences are unavoidable. If the fit is too loose, gaps can leave the stator unevenly supported. If the interference fit is too tight, assembly becomes more difficult and excessive mechanical stress may be introduced into the components.

Structural adhesive fills small gaps between the parts and distributes the load across the bonding area. This helps maintain stable fixation without relying entirely on an extremely tight mechanical fit.

Different materials can also expand at different rates when the motor heats up. An adhesive layer can act as a transition between these materials and help reduce stress caused by repeated thermal expansion and contraction.

One Motor, Five Different Problems

A frameless torque motor does not need one “best adhesive.” It needs different material properties at different locations.

These applications also need to work together. A high-temperature curing process used for one adhesive should not damage nearby sensors or previously assembled components. Different materials used in the same motor should also be checked for process and chemical compatibility.

Adhesive Selection Starts With the Actual Problem

For robot motor applications, selecting an adhesive based on a single parameter is often not enough.

A material with higher strength may be too rigid for a moving cable. A material with higher thermal conductivity may be too viscous to fill narrow winding gaps. A fast UV adhesive may not cure completely in shadowed areas.

The selection process should therefore start with a simple question:

What can go wrong at this location?

Once the failure risk is clear, the required material properties become easier to define.

For frameless torque motors, adhesive design is not only about bonding components together. It is also about controlling heat, preventing movement, reducing stress and protecting sensitive components inside an increasingly compact structure.

The most suitable adhesive is the one that solves the actual problem at the actual application point.

At our facility, we formulate and manufacture encapsulating resins and thermally conductive encapsulants specifically for motor and robotics applications. From prototype sampling to production-scale supply, we support customers with technical guidance and consistent quality.