Harmonic Drive Gearbox

Strain Wave Harmonic Drive Gearbox Working Principle

Assunmotor's harmonic drive gearbox is engineered for precision, efficiency, and compact performance. Unlike conventional gear systems, it uses a flexible strain wave mechanism that enables high torque transmission, zero backlash, and exceptional positioning accuracy. This technology ensures smooth motion control, making it ideal for robotics, automation, and semiconductor industries.

The gearbox consists of three core components: the wave generator, flex spline, and circular spline, working together to achieve high reduction ratios in a lightweight and space-saving design. With its innovative engineering and durable construction, Assunmotor’s harmonic drive solutions provide reliable and precise motion control for demanding applications.

How Does a Harmonic Drive Work?

The strain wave gear mechanism depends on the interaction of three precision-machined components. Understanding each one makes the selection and integration process more straightforward.

Wave Generator

The wave generator is the input element it connects directly to the motor shaft. It consists of an elliptical cam with a precision ball bearing fitted around its perimeter. When the motor turns the cam, the bearing transfers that elliptical rotation to the flexspline. The efficiency of the wave generator bearing is a primary factor in the overall gearbox efficiency, so Assun specifies precision bearings rated for the expected speed and load.

Flexspline

The flexspline is a thin-walled cylindrical cup made from high-strength alloy steel. External gear teeth are machined into its open end. The cup wall is thin enough to flex radially under the influence of the rotating wave generator, but the material and geometry are chosen for torsional stiffness the flexspline resists twisting even while it deforms laterally. As the wave generator's elliptical cam rotates inside, two points on the flexspline's perimeter are continuously forced outward to engage with the circular spline, while the remaining teeth are disengaged. This selective engagement is what creates the torque multiplication and the very high reduction ratio.

Circular Spline

The circular spline is a rigid, internally toothed ring that mounts to the gearbox housing. It has two more teeth than the flexspline. As the wave generator rotates and the engagement points travel around the flexspline, the flexspline advances by two teeth per full input rotation relative to the fixed circular spline producing the output displacement. This two-tooth-per-revolution advancement is the source of the high reduction ratio.

When the wave generator rotates, it deforms the flexspline, causing it to mesh with the circular spline in a specific pattern, achieving precise motion control with a high reduction ratio in a single stage.

Harmonic Drive vs Planetary Gearbox

Strain wave gearboxes (harmonic drive type) and planetary gearboxes serve overlapping but distinct application sets. Choosing between them involves a few clear trade-offs.

Choose a strain wave gearbox when:

• Zero backlash is a hard requirement robot joints where any mechanical play in the drive chain causes positioning error at the tool tip. The strain wave mechanism has no gear tooth clearance, so backlash is inherently zero rather than 'minimised'.
• High reduction ratio in a single stage is needed strain wave gearboxes achieve 30:1 to 160:1 in one stage. A planetary gearbox needs two or three stages to reach equivalent ratios, which adds length and mass.
• Compact coaxial design is essential the strain wave design's input and output share the same centreline, and the assembly length is remarkably short for its torque capacity.

Choose a planetary gearbox when:

• Higher continuous torque is required planetary gearboxes distribute load across multiple gear meshes and generally handle higher continuous torque for the same diameter than a strain wave equivalent.
• Cost is a primary constraint planetary gearboxes are significantly less expensive to manufacture than strain wave designs at equivalent sizes.
• Moderate backlash is acceptable for applications using closed-loop position control with encoder feedback, a few arcminutes of backlash is corrected by the control system and does not degrade system accuracy.

Assun supplies both see the planetary gearbox range for cost-effective precision reduction, and this CSF series for zero-backlash requirements.

Where Assun's Strain Wave Gearboxes Are Specified?

Assun's harmonic drive gearbox range is specified across several demanding US market segments:

• Collaborative Robots (Cobots): Each robot joint axis in a collaborative arm needs zero-backlash reduction to achieve the sub-millimetre repeatability specifications these systems carry. The AM-20XB (CSF-05) and AM-30XB (CSF-08) are commonly specified for wrist and elbow joints; the AM-40XB (CSF-11) handles higher-torque shoulder and waist joints.
• Semiconductor Equipment: Wafer handling stages, inspection optics positioning, and lithography systems operate at the nanometre level of positional accuracy. The strain wave mechanism's zero backlash and coaxial geometry make it the preferred gearbox type in this industry. The miniature CSF-2.5 and CSF-03 sizes serve the compact multi-axis stages in optical metrology equipment.
• Surgical and Medical Robots: Surgical robot joints carry human-level safety requirements. The absence of backlash ensures that commanded position and actual position remain aligned throughout a procedure there is no mechanical hysteresis for the control system to manage.
• Aerospace and Satellite Systems: Satellite antenna positioning and gimbal drives operate in vacuum and extreme temperature ranges. The compact, lightweight construction of the strain wave mechanism reduces launch mass while meeting the precision requirements of antenna pointing and instrument drives.

Pairing With Assun's DC Motor Range

A strain wave gearbox requires a motor to drive the wave generator input. Assun's harmonic drive gearbox range is designed to work with Assun's brushless coreless DC motors (BLDC) and brushed coreless DC motors the motor shaft connects directly to the wave generator input, and the combined assembly forms a compact, high-precision servo actuator.

For engineers who want a fully integrated solution, Assun's servo actuator combines motor, strain wave gearbox, and drive electronics in a single matched unit eliminating the interface design and validation work of a separately sourced motor-gearbox combination.

For multi-axis robotic systems requiring a servo controller, Assun's servo motor controller and encoder range complete the drive chain. All products are available through the Assun online shop.

FAQs

Q1. What is a harmonic drive gearbox?

A harmonic drive gearbox is a type of precision gear mechanism more accurately called a strain wave gearbox that achieves very high reduction ratios in a single stage by using the controlled elastic deformation of a thin steel cup (the flexspline). As the input elliptical cam (wave generator) rotates, it continuously flexes the flexspline against the fixed outer ring (circular spline). Because the flexspline has two fewer teeth than the circular spline, each full input rotation produces a very small output displacement creating a large reduction ratio with zero backlash. The term 'harmonic drive' is used generically in the industry to describe this type of strain wave gear mechanism.

Q2. What is the difference between a harmonic drive and a planetary gearbox?

Harmonic drive gearboxes (strain wave type) and planetary gearboxes are both precision gear reducers, but they differ in key characteristics. Strain wave gearboxes achieve zero backlash through their elastic engagement mechanism, while planetary gearboxes have some residual backlash even in precision versions. Strain wave gearboxes achieve high reduction ratios (30:1 to 160:1) in a single compact stage; planetary gearboxes typically need two or three stages for equivalent ratios. Planetary gearboxes generally handle higher continuous torque for the same diameter and are significantly less expensive. The choice depends on whether zero backlash is a hard requirement or whether moderate backlash compensated by encoder feedback is acceptable.

Q3. What does 'zero backlash' mean in a gearbox?

Backlash is the angular movement of the output shaft when the drive direction reverses, without the input shaft moving essentially the mechanical play between gear teeth. In a positioning system, backlash creates a dead zone: when the motor changes direction, the gearbox output does not immediately respond. In robot joints and precision stages, this causes positioning errors that degrade system accuracy. Zero backlash means the output follows the input exactly in both directions with no dead zone the defining characteristic of strain wave gearboxes, achieved through the continuous elastic engagement of the flexspline against the circular spline.

Q4. What gear ratios are available for Assun's harmonic drive gearboxes?

Assun's harmonic drive gearbox range (CSF series) achieves high reduction ratios in a single stage. Specific ratios are available per CSF series size consult the datasheet for each AM-XB series for the available ratio options. Common single-stage ratios in strain wave gearboxes range from approximately 30:1 to 160:1 depending on the tooth count difference between the flexspline and circular spline. Contact Assun's US engineering team to confirm available ratios for your target size.

Q5. What size harmonic drive gearboxes does Assun supply?

Assun's harmonic drive gearbox range covers six CSF series sizes: AM-11XB (CSF-2.5, 11mm flange), AM-13XB (CSF-03, 13mm), AM-20XB (CSF-05, 20mm), AM-30XB (CSF-08, 30mm), AM-40XB (CSF-11, 40mm), and AM-73XB (CSF-14, 73mm). The 11mm size is among the smallest commercially available strain wave gearboxes, making it suitable for robotic fingers, miniature instruments, and compact servo actuators. All sizes are available for direct purchase through the Assun online shop.

Q6. Can a harmonic drive gearbox be back-driven?

Strain wave gearboxes are generally not backdriveable applying torque to the output shaft does not cause the input to rotate. This is a consequence of the gear geometry and the friction in the flexspline-circular spline engagement. For robotic applications, this has both advantages and disadvantages: it means the joint holds position when the motor is de-energised (useful for safety), but it also means the robot cannot be manually repositioned by pushing the arm (relevant for collaborative robot safety standards). If backdrivability is a requirement, discuss with Assun's engineering team it may influence the choice between strain wave and planetary gearbox architectures.

Q7. What is the CSF series in Assun's harmonic drive gearboxes?

CSF refers to the cup-type strain wave gearbox series configuration used in Assun's harmonic drive gearbox range. The cup-type flexspline (shaped like a cup or cylinder) is the most widely used configuration for compact, coaxial robotic joint applications. The CSF number (CSF-2.5, CSF-03, CSF-05, etc.) indicates the gearbox size in Assun's range, corresponding to the flange diameter of the unit. The model code AM-20XB(CSF-05) identifies a 20mm flange size cup-type unit.

Q8. What motors are compatible with Assun's harmonic drive gearboxes?

Assun's harmonic drive gearboxes are designed to work with Assun's brushless coreless DC motors (BLDC) and brushed coreless DC motors. The motor output shaft connects to the wave generator input. For a fully integrated solution, Assun's servo actuator combines motor and strain wave gearbox in a single pre-assembled unit. If you are integrating Assun's gearbox with a third-party motor, contact our US engineering team with the motor shaft diameter and flange specification for compatibility guidance.