Zero-backlash class Harmonic driveFrame 25
THD-25 Hollow-Shaft Harmonic Drive, Size 25
- Rated torque
- 39 N·m
- Backlash
- ≤ 20 arcsec
- Ratios
- 30–160:1
from ₹56,000 + GST
In stockHarmonic drive gearboxes (strain wave gears) for cobots and humanoids
Zero-backlash class strain wave gear units, ratios 30:1 to 160:1 in a single stage, in cup and hat styles with hollow-shaft options for compact robot joints.
- Ratios
- 30:1 to 160:1 (single stage)
- Backlash
- ≤ 20 arcsec (near zero)
- Efficiency
- 70–85%
- Best for
- Cobot wrists, humanoid joints, hollow-shaft actuators, precision optics
1 gearbox
Zero-backlash class No gearbox matches these filters. Loosen the torque or backlash limit, or send us your requirement and we will build one.
Request a custom gearboxHow a harmonic drive gets its ratio
Change the flexspline tooth count. The circular spline always has two more teeth, and each input turn shifts the flexspline back by those two teeth, so the ratio is flexspline teeth ÷ 2.
Turn on JavaScript to change tooth counts and see the ratio, output speed and torque update live.
Shaft sizes do not match? We make the coupling
Every harmonic drive can be joined to any motor, robot joint or housing: we machine a custom coupling bored to both shafts, plus an adapter flange if the mounting faces differ. Send both shaft sizes, a drawing or just the motor model number.
Get a custom coupling- Send your requirementMotor model, shaft and pilot sizes, or a STEP/PDF drawing.
- We confirm the designA 2D drawing and quote from our engineers within 48 working hours.
- We machine and assembleCoupling, flange or housing is fitted and tested with the gearbox before dispatch.
How a harmonic drive works
A harmonic drive, also called a strain wave gear, has only three parts. The wave generator is an elliptical cam with a thin ball bearing, mounted on the input shaft. It sits inside the flexspline, a thin-walled steel cup with external teeth. Around the flexspline is the circular spline, a rigid ring with internal teeth. The circular spline has two more teeth than the flexspline.
The elliptical wave generator pushes the flexspline into the circular spline at two opposite points. As the input turns, these engagement points travel around, and each full input revolution makes the flexspline shift back by two teeth relative to the circular spline. With 200 flexspline teeth and 202 circular spline teeth, that is a 100:1 reduction in a single stage with no gear train.
Why harmonic drives suit robot joints
About 30% of the teeth are in contact at once, so torque capacity is high for the size and weight, and the preloaded engagement gives near-zero backlash. Harmonic drives are the standard reducer for collaborative robot joints, SCARA arms, humanoid and legged robots, robot wrists, satellite and telescope mechanisms, and precision rotary stages. The hollow-shaft hat style lets cables and pneumatic lines pass through the centre of the joint.
Things to check before you choose
- Repeated peak torque during acceleration and deceleration, which usually decides the size in a robot joint.
- Average input speed, which sets the wave generator bearing life.
- Torsional stiffness, which is lower than an RV reducer. For the heaviest base joints of large robots an RV reducer is often a better fit.
- Housing design. Component sets need an accurate housing; unit types come with their own output bearing and housing.
Component sets and units
We supply harmonic gearing as component sets (wave generator, flexspline, circular spline) for builders who design their own joint housing, and as sealed units with a cross-roller output bearing that bolt directly into your robot. Custom housings and input adapters for frameless motors are available on order.
Further reading on harmonic drives
All harmonic drive articlesGearbox backlash explained: arcmin, lost motion, hysteresis and how much you need
What backlash means on a datasheet, how it is measured, how it differs from lost motion and hysteresis, and a table converting arcmin to millimetres…
Gearboxes for CNC rotary and tilting axes: accuracy budget and clamping
How much angular error your part can tolerate, which reducer meets it, and why rotary axes should clamp during heavy cuts. With an accuracy table by…
Gearboxes for SCARA and delta robots: fast cycles, low backlash
Fast pick-and-place robots need low inertia, near-zero backlash and high repeatability. How to choose and size the reducer, with a SCARA joint 1 exam…
Harmonic drive questions
Is a harmonic drive really zero backlash?
The tooth engagement has practically no backlash because the flexspline is preloaded against the circular spline. What remains is a small hysteresis loss, typically under 1 arcmin, and the published positioning accuracy.
What is the difference between cup and hat type?
The flexspline of a cup type is shaped like a cup with the output on the closed end. A hat type has an outward flange, which leaves a large hollow bore through the centre for cables and shafts.
What limits the life of a harmonic drive?
The wave generator bearing and the fatigue of the flexspline. Stay within the rated torque and average input speed, and the rated life is typically 7,000–10,000 hours at rated conditions, longer at lighter loads.
Why is efficiency lower than a planetary gearbox?
Flexing the thin flexspline and the high tooth-contact count cost some energy, especially at high input speed and low temperature. Efficiency improves near rated torque.