A frameless torque motor can make a robot joint smaller and more integrated.
It can also create problems if the motor is selected from peak torque alone.
Robot joint sizing requires a complete view of torque, speed, thermal load, mechanical integration and feedback.
The motor is only one part of the actuator.
Start With the Joint Torque Profile
The first question should not be the motor diameter.
It should be how much torque the joint actually needs during operation.
A robot joint rarely operates at one fixed torque.
Torque changes as the arm moves, accelerates, carries a payload and responds to gravity.
The designer therefore needs both peak torque and continuous torque requirements.
Peak torque may occur during rapid acceleration or an unusual operating condition.
Continuous torque is related to the average thermal demand over the operating cycle.
A motor that can produce the required peak torque for a few seconds may still overheat if its continuous capacity is too low.
Consider the Complete Motion Cycle
The most useful motor sizing information comes from the actual motion profile.
This includes expected joint speed, acceleration, load inertia, duty cycle and dwell periods.
Gravity also matters for joints that support a load vertically.
For some axes the motor spends significant time holding torque even when the robot is not moving.
That holding condition generates heat and must be included in thermal calculations.
Ignoring it can lead to a motor that performs well during a short demonstration but becomes thermally limited during continuous production.
Decide Whether the Joint Uses a Gear
Frameless motors are often associated with direct drive robotics but many robot joints still use a reduction gear.
A gear can multiply output torque and allow the motor to operate at a more favorable speed.
It also adds friction, compliance and mechanical complexity.
The selected gear ratio changes the torque and speed required from the motor.
It also changes how reflected inertia appears to the motor.
Motor sizing should therefore be performed together with gearbox selection rather than treating the two components independently.
For joints that truly use direct drive the motor must produce the full joint torque without mechanical multiplication.
That normally requires greater motor torque but removes the backlash and compliance associated with a gearbox.
Diameter and Length Affect Integration
Frameless motors consist primarily of a rotor and stator.
This gives the mechanical designer freedom to integrate the motor directly into the joint housing.
However motor dimensions still influence the complete actuator.
Torque is strongly related to motor geometry.
A larger motor diameter can often provide more torque without requiring a proportionate increase in axial length.
This can be useful in robot joints where keeping the actuator short is important.
The available bore diameter also matters.
Modern robotic joints often need space through the center for cables, hoses, brakes or mechanical shafts.
The motor should therefore be evaluated as part of the joint packaging rather than as an isolated component.
Thermal Design Can Limit Continuous Torque
Compact robot joints are difficult thermal environments.
The motor may be surrounded by gears, bearings, electronics and structural components.
There may be very little natural airflow.
The stator housing often becomes the main thermal path.
Good contact between the stator and the surrounding structure can therefore have a significant effect on continuous torque capability.
High duty applications may require additional cooling.
The designer should also consider how motor heat affects nearby encoders and lubrication.
A joint can remain below the winding temperature limit while still becoming too hot for another component.
Bearings and Encoders Are Part of the Design
A frameless motor normally does not provide its own bearing system.
The joint designer must select bearings capable of supporting radial loads, axial loads and moments generated by the robot structure.
Bearing stiffness can influence positioning accuracy and dynamic response.
Feedback is equally important.
Encoder resolution and accuracy should match the real performance requirement of the joint.
Mounting errors can reduce the benefit of an expensive encoder.
Mechanical alignment should therefore be considered during the earliest stages of joint design.
A frameless torque motor supplier such as HansMotor can be most useful when torque requirements are provided together with the available joint envelope, bore requirement, cooling conditions, speed range and feedback concept.
Validate More Than One Operating Point
A good motor selection should be checked at several conditions.
Maximum acceleration
Maximum operating speed
Continuous production cycle
Static holding condition
Highest expected ambient temperature
Maximum payload
Emergency or abnormal loading where relevant
A motor that passes only the maximum torque calculation is not necessarily suitable for the application.
Final Thoughts
Sizing a frameless torque motor is an actuator design problem rather than a catalog selection exercise.
Peak torque determines whether the joint can handle demanding movements.
Continuous torque determines whether it can operate repeatedly without overheating.
Motor geometry affects the mechanical envelope.Cooling affects usable output.
Bearings and feedback affect the final precision of the joint.
Treating these elements as one system usually produces a better robot joint than selecting each component independently.
