Guidelines for Selection and Integration of Strain Wave Gearing with a Motor

Most commonly used in robotics, automation, and other applications, strain wave gears are used when a precision, high torque, compact package is required. Strain wave gearing consists of three components: a wave generator, a flexspline, and a circular spline. The wave generator is supplied loose, while the flexspline and circular spline are assembled units. This blog post discusses the selection of a strain wave gearing unit and includes the key considerations for assembly with a motor.

Strain Wave Gear Unit Selection

The first step in the selection of a strain wave unit involves a calculation of the required torque and speed of the mechanism. For applications with varied load points, the average (or RMS torque and speed) should be calculated. These values can then be compared to motors with similar power capabilities.  The comparison of the maximum application speed requirement (output) to the maximum motor speed (input) can determine the required gear ratio to select the appropriate strain wave model and closest gear ratio.

The next step is to review the check points below to confirm the preliminary selection. If any specifications are not met by the preliminary strain wave selection, an alternate model should be chosen and the confirmation steps repeated.

1. The calculated maximum input speed (motor) should be less than or equal to the maximum input speed rating of the selected strain wave model.
2. The calculated average input speed (motor) should be less than or equal to the average input speed of the selected strain wave model.
3. The maximum repeated peak torque required in the application should be less than or equal to the maximum repeated peak torque rating for the selected strain wave model.
4. The required emergency stop torque in the application should be less than or equal to the momentary peak torque for selected strain wave model.
5. The number of rotations during emergency stop torque (from equation given in strain wave drive supplier catalog) should be less than the allowable number of rotations in an emergency stop condition for the selected strain wave model.

Strain wave manufacturers also provide equations to calculate the expected life of the strain wave generator, which will dictate the expected life of the strain wave solution.

Design Considerations

Several key design elements require review when integrating a strain wave gearbox with a motor:

Axial Loads on Wave Generator – The flexspline undergoes elastic deformation when transmitting torque. This deformation generates axial forces (Fx) on the wave generator that must be supported by the bearing supporting the motor shaft. Therefore, the wave generator should be fixed on the input shaft in the axial direction.

Axial Position of Wave Generator – To get proper power transmission, the wave generator must be mounted at a defined axial position relative to the flexspline. This position can be defined by the step/shoulder on the gearbox input shaft. If a step is not provided on the input shaft, a clamping set provided by the strain wave drive supplier can fix the wave generator on the shaft. It is important to carefully maintain the axial position of the wave generator to the flexspline.

Screw Connections – High torque capacity strain wave gear drives require mounting screws that are tightened to the proper torque with a calibrated torque wrench. A thread locker can be used to ensure that the screw connections will not loosen during operation.

Housing Flanges and Motor Flask Tolerances – Concentric alignment of components, including the input shaft, wave generator, circular spline, and flexspline, is critical to achieve smooth and efficient power transmission. The design engineer should incorporate the assembly tolerances (specified in the manufacturer’s catalog) for the strain wave gearbox models selected in the design process.

Sealing – The inclusion of seals is recommended to prevent grease leakage from the gearbox to the motor. Generally, O-rings and radial shaft seals are provided by strain wave suppliers.

Environmental Conditions – Select the grease for the strain wave gearbox per the working temperature of the application. To eliminate rusting, use a rust proof coating or an alternative material (like stainless steel) for gearbox housings.

Portescap has recently integrated a strain wave gear drive with the newly launched 90ECF outer rotor BLDC motor. This prototype was successfully tested in a rehabilitation robot. For specific high torque applications that require strain wave drives, Portescap can provide an integrated motion solution of select Portescap motors integrated with strain wave drives (supplied by external strain wave drive suppliers). Reach out to us here to discuss!

For further information on construction, working principle, advantages, and applications of strain wave drives, we recommend readingThe Advantages and Applications of Strain Wave Generator Gearings (portescap.com).