Harmonic Drive FAQ: Answers from Someone Who Tests Them Every Day

These are the questions I hear most often—from engineers, from procurement, from people starting their first automation project. I've been reviewing harmonic drive units before they ship for four years now, roughly 2,000 units a year. In our Q1 2024 quality audit, I rejected about 6% of first-pass assemblies for backlash values that didn't meet spec. So what follows comes from the test bench, not the brochure.

What exactly is a harmonic drive?

A harmonic drive—more accurately, a strain wave gear—is a gearbox that uses elastic deformation to achieve speed reduction. Three components matter: the wave generator, the flexspline, and the circular spline.

The wave generator is basically an elliptical cam wrapped in a thin-section ball bearing. It pushes the flexspline (a thin, flexible steel cup) outward so its teeth engage the rigid circular spline in two opposite zones. Since the flexspline has two fewer teeth than the circular spline, each full rotation of the wave generator advances the output by just two teeth. That's how you get 30:1 to 160:1 reduction in a single stage.

The result is near-zero backlash and quiet operation, which is why harmonic drives are standard in robotics, semiconductor equipment, and precision telescopes.

Is harmonic drive efficiency really about 80%?

Short answer: yes, around 80% at rated torque—but that number comes with conditions.

When I first started inspecting drives, I assumed the efficiency on the datasheet applied across the whole operating range. That's wrong. The 80% figure assumes near-rated torque and speed. In dynamometer tests, most 50:1 to 100:1 units measure between 75% and 85% under those conditions (this matches published curves from major strain wave gear manufacturers, as of early 2025). At higher input speeds, efficiency can push above 85%. At low load—say 10% of rated torque—it can drop below 50%. The flexspline still has to flex on every rotation, and that elastic deformation consumes energy whether the output is loaded or not.

Lubricant matters too. Grease behaves differently than oil, especially below 10°C, so expect lower efficiency until the drive warms up.

From my perspective, the number you actually need is efficiency at your torque, your speed, your temperature. Ask the supplier for an efficiency curve rather than a datasheet figure. If they provide a curve for a specific ratio and lubricant, that's a good sign they've actually tested their product.

How does a harmonic drive compare to a cycloidal drive?

Cycloidal drives are a legitimately good technology. They also offer high reduction in a single stage and near-zero backlash. The difference comes down to how they achieve that reduction, and what it means for your specific application.

Harmonic drives tend to peak around 80% efficiency at rated torque, while cycloidal drives are often somewhat higher, especially at partial load. Where strain wave gears tend to stand out is smoothness. The tooth engagement in a harmonic drive rolls continuously around the circumference, which produces less torsional vibration than the lobed motion of a cycloidal disc. On a telescope mount or a semiconductor wafer stage, that smoothness shows up in the final image or overlay tolerance.

To be fair, the gap narrows as both technologies mature. And in lower-precision applications, cycloidal drives have strong torque density at a competitive price point.

From my inspection standpoint, the right question isn't "which gearbox is better" but "which gearbox fits your requirements." If you need the smoothest rotation, a harmonic drive is a strong choice. If you need maximum efficiency at partial loads and vibration isn't your primary concern, cycloidal deserves serious consideration.

I'm a small company. Will a harmonic drive supplier take me seriously?

This one comes up constantly, and honestly, it bothers me that engineers have to ask it. When I was on the buying side, a supplier once ignored my email for two weeks because I was asking about a two-unit evaluation order. That left a lasting impression.

Here's the thing: a good supplier understands that small orders are usually the first step in a bigger relationship. Customers who started with a single evaluation unit have come back a year later with 200- or 500-unit orders. It took me a while to realize that the suppliers who treated small orders well weren't just being nice—they were building a pipeline.

That said, don't expect small-order pricing to match volume pricing. Manufacturing economics don't work that way. What you should expect is a straight answer on lead time, a willingness to answer technical questions, and no pressure to buy more than you need.

One practical tip: ask for the inspection sheet. Any reputable supplier can provide backlash and runout measurements for each unit. If they can't or won't, consider that a red flag. If you ask me, that's the single best test of a supplier's attitude toward small customers. And we apply the same test to our own suppliers—anyone who can't document their quality gets cut, regardless of price.

Should I use a harmonic drive or a hydraulic linear actuator?

To be fair, these are different tools, and the right answer depends on the application. A hydraulic linear actuator uses fluid pressure to drive a piston in a straight line. A harmonic drive is a rotary gearbox—it doesn't produce linear motion unless you pair it with a screw.

If you need high force in a rough environment, like a press or construction equipment, a hydraulic linear actuator is a proven option. Hydraulics are rarely the first recommendation for precision applications, but that doesn't mean they're never the right choice.

Where a harmonic drive shines is precise rotary motion, or linear motion with micron-level repeatability when paired with a ballscrew. A servo motor, harmonic drive, and screw assembly will cost more than a cylinder and pump, but you get smooth positioning that hydraulics basically can't match, especially at low speeds. You also avoid hydraulic oil, leaks, and filter maintenance.

That's not a knock on hydraulics. In high-force, low-precision applications, a well-designed hydraulic linear actuator is hard to beat. The key is being honest about what precision you actually need. Looking back at projects we've supported, I wish we'd pushed more customers to compare total system costs up front—not just the gearbox price. If you're on the fence, ask the supplier to break down the complete motion system: motor, drive, coupling, screw, and controls.

What role do ball bearings play in a harmonic drive?

Ball bearings are doing serious work in every harmonic drive, and most people don't realize it. The wave generator is essentially an elliptical cam with a thin-section ball bearing on its outer race. That bearing allows the flexspline to deform elastically as the cam rotates. Without it, you'd have metal sliding on metal, and the unit would fail quickly.

If you've worked with ball bearing wheels on linear carriages or conveyor systems, the concept is similar—races, balls, cage—but the duty is harsher. The wave generator bearing operates under continuous elliptical strain, not just rotation.

In every unit that comes through my bay, the wave generator bearing is one of the first things I check for play. If I can feel radial play before installation, I reject the unit. It sounds picky, but bearing play shows up directly as positioning error at the output flange. ISO 492 defines the dimensional classes for these bearings, and quality units typically use a P5-class or better wave generator bearing.

Honestly, I've never fully understood why some manufacturers spec lower-grade bearings in otherwise high-end gearboxes. My best guess is that it's a cost decision that later turns into warranty claims. If someone has insight, I'd like to hear it.

What motors are compatible with VFDs—and which pair well with harmonic drives?

I'm not an electrical engineer, so I'll keep the VFD guidance practical. Most three-phase induction motors can work with a variable frequency drive, but inverter duty rating matters. At low speeds, the motor's internal fan slows down, and on a constant-torque load, the motor can overheat. Inverter-rated motors use Class F or H insulation and handle the PWM waveform better than standard induction motors.

Always verify the motor nameplate for inverter duty rating before connecting it to a VFD. Some synchronous motors and older induction motors may not tolerate the voltage spikes from modern VFDs.

For harmonic drives, the most common partner is an AC servo motor, typically a 17-bit or 23-bit encoder servo that matches the gearbox's positioning accuracy. Stepper motors work for simpler index-and-hold tasks, and induction motors with VFDs are fine for speed control.

One detail I scrutinize: the motor-to-gearbox connection. A number of field returns have turned out to be coupling issues, not gearbox issues. Keyed connections are fine, but the key fit has to be tight. If you need repeatability in the arc-minute range, consider a flange-mounted servo with a direct shrink disk rather than a keyed shaft.