If you’re working with harmonic drives—you know, those compact, high-precision gear systems that replace bulky old planetary gears or worm drives for stuff like robotic arms, medical devices, or CNC machines—you’ve probably stared at a motor datasheet and thought, “Wait, can this motor actually work with my harmonic drive?” I get it. As someone who’s been selling harmonic drives for almost a decade, I’ve had engineers call me panicking because they paired a 100W servo with a harmonic drive only to find it couldn’t hold a load at the end of a robotic arm, or they overspent on a motor that was way too powerful for their application. Today, let’s break down how to pick the right motor for your harmonic drive—no overly jargon-heavy stuff, just real-world tips I’ve picked up fixing or helping troubleshoot hundreds of projects. Harmonic Drives
First, let’s get one key thing straight: harmonic drives aren’t plug-and-play with every motor. Their unique design—flexsplines, wave generators, circular splines—creates specific torque, speed, and load requirements that your motor has to match. Skip this step, and you’re either dealing with constant slipping, overheating, or a system that dies in 6 months. No one wants that, especially when you’re building something that needs to be reliable, like a surgical robot joint or a collaborative robot (cobot) that works next to humans.
Let’s start with the most important number: peak torque. Wait, not just any torque—peak torque at the output of the harmonic drive. I can’t tell you how many times an engineer will give me the motor’s peak torque and assume that’s the same as the harmonic drive’s output. Big mistake. Harmonic drives have a gear ratio, right? So the motor’s torque gets multiplied by that ratio, but only if the motor is strong enough to push it. But wait—there’s a catch: harmonic drives also have something called “efficiency,” which varies by model, gear ratio, and application. A typical harmonic drive is around 85-92% efficient, but if you’re running it at a very low ratio (like 50:1) or under heavy load, that efficiency can drop to 75% or lower.
Here’s a real example: last year, a cobot designer came to me saying they had a 200W servo motor with a peak torque of 0.6 Nm. They had a harmonic drive with a 100:1 ratio. They calculated 0.6 Nm x 100 = 60 Nm of output torque, which they thought was perfect for their joint. But when they tested it, the arm couldn’t lift a 12kg payload at full extension. Why? Because the harmonic drive’s efficiency at that ratio was only 78%, so actual output torque was 0.6 x 100 x 0.78 = 46.8 Nm. Their arm needed 52 Nm to lift that payload, so they were 5 Nm short. The fix? We bumped them up to a 300W servo with a peak torque of 0.85 Nm—0.85 x 100 x 0.78 = 66.3 Nm, which was more than enough. They saved themselves months of rework by not guessing this number.
Another point about peak torque: your motor also has to handle the “breakaway torque” of the harmonic drive. That’s the torque needed to get the flexspline moving from a standstill—think of it like the torque needed to start turning a stiff door hinge. Harmonic drive breakaway torque is usually 5-10% of their rated output torque, so you need your motor’s peak torque to cover that too. If you pair a motor that’s exactly at the continuous torque requirement, it will stall every time the system needs to accelerate or overcome a small amount of friction.
Next up: continuous torque. This is the steady-state torque the motor needs to provide, both at the motor shaft and factoring in the harmonic drive’s efficiency. Why is this important? Because if your motor is working at its absolute limit 24/7, it will overheat, burn out, or have a drastically shorter lifespan. For most applications—like a conveyor belt with a constant load, or a robotic arm moving a steady payload—you want the motor’s continuous torque to be at least 1.2 times the required torque at the harmonic drive output (after efficiency). That safety margin might seem small, but it makes a huge difference in longevity.
I remember a medical device client who skipped this step for a diagnostic analyzer that moves a test tray. They used a 50W stepper motor with a continuous torque of 0.15 Nm, paired with a 50:1 harmonic drive. Calculating output torque: 0.15 x 50 x 0.85 (efficiency) = 6.375 Nm. The application only needed 5 Nm, so they thought it was fine. But because the analyzer runs 24/7, the motor was operating at 100% continuous torque, and within 3 months, the motor windings overheated and failed. We swapped them out for a 75W stepper with 0.22 Nm continuous torque—0.22 x 50 x 0.85 = 9.35 Nm, which is way over the needed 5 Nm, and that unit has been running for 2 years with no issues.
Now, speed. This is where a lot of people overcomplicate things. The rule of thumb here is: calculate the required output speed of your harmonic drive, then reverse-engineer the motor speed based on the gear ratio. Let’s say you need the harmonic drive output to spin at 10 rpm. If your gear ratio is 100:1, the motor needs to spin at 10 rpm x 100 = 1000 rpm. If you pick a motor that only goes up to 800 rpm, your application is too slow. If you pick a motor that goes to 3000 rpm, you’re wasting energy and might even have to add an extra brake or speed control, which adds cost and bulk.
But wait—there’s another factor here: backdriving. Harmonic drives are great because they have very low backlash, but that also means if your application needs to be backdriven (like a robotic arm that can be moved manually when powered off), you need to pick a motor that’s compatible. Brushless DC (BLDC) motors are the most common for harmonic drive setups, but if you need very precise speed control, a servo motor with an encoder is a better pick than a stepper motor. Steppers can lose steps under load, which is bad for harmonic drive applications that need high accuracy.
Another thing I see all the time: motor size and mounting. Harmonic drives come in different frame sizes, right? A harmonic drive for a small watch joint is way smaller than one for a large industrial robot. Your motor has to fit the frame of your harmonic drive, not just match torque and speed. Some motors are designed to mount directly to harmonic drives with a custom flange, which saves space and eliminates extra couplings (which can add backlash and failure points). If you use a motor that’s too big, you’ll have to mount it separately with a coupling, which is more expensive and can throw off your accuracy.
Thermal management is another big one, even if it’s not the first thing you think of. Harmonic drives generate a little heat from friction, and motors generate heat from current. If you’re pairing a high-power motor with a harmonic drive in a tight, enclosed space (like inside a robotic arm joint), that combined heat can cause the flexspline to expand, which ruins the precision and can even damage the drive. So if your application is in a confined area, you might want to pick a motor with a built-in heat sink, or derate the motor’s power slightly to reduce heat output.
Let’s talk about a common mistake with linear actuators too—wait, a lot of harmonic drives are used in linear applications when paired with a ball screw or lead screw. If you’re doing that, you have to factor in the linear force, not just torque. Let’s say you have a harmonic drive with a 100:1 ratio, driving a 10mm pitch screw (meaning one full turn of the screw moves it 10mm). The linear force you need is, say, 1000N. The formula here is: Torque (motor) = (Linear Force x Screw Pitch) / (2π x Gear Ratio x Efficiency). Plugging in the numbers: (1000N x 0.01m) / (2π x 100 x 0.85) = 0.01 / 534.07 = ~0.0187 Nm motor torque. That seems small, but it’s correct. I’ve had a lot of engineers mess up this calculation by forgetting the screw pitch and just using linear force, so their motor was way too powerful.
Now, let’s narrow this down to a step-by-step checklist you can follow, no matter what application you’re working on:
- Calculate your required output torque at the harmonic drive (both peak and continuous).
- Account for the harmonic drive’s efficiency (usually 80-90%, check the datasheet for exact numbers for your model).
- Pick a motor that has a peak torque at least 1.1-1.2 times the required peak output torque (to cover breakaway and acceleration).
- Make sure the motor’s continuous torque is at least 1.2 times the required continuous output torque (for reliability).
- Calculate the required motor speed based on your desired harmonic drive output speed and gear ratio.
- Match the motor’s mounting and frame size to your harmonic drive to avoid extra couplings or space issues.
- Factor in linear force if you’re using the harmonic drive in a linear actuator setup.
- Check for backdriving needs and speed control accuracy (servo vs stepper vs BLDC).
Wait, one last thing: don’t ignore harmonic drive-specific specs like backlash or inertia matching. Inertia is the motor’s ability to accelerate, and you want the motor’s rotor inertia to be similar to the harmonic drive’s load inertia. If the motor is way too small and has low inertia, it will struggle to accelerate heavy loads, even if the torque is there. Most motor datasheets will list rotor inertia, and harmonic drive datasheets have load inertia specs—try to keep the ratio of motor inertia to load inertia between 1:1 and 1:10. That’s a sweet spot for most applications.
I’ve seen too many projects get held up because they skipped this step and paired a random motor with a harmonic drive. It’s not that hard, but it takes paying attention to those real-world numbers, not just the datasheet’s front-page torque rating.
If you’re currently working on a project and not sure if your motor is the right fit, or if you’re just starting out and need help picking both a harmonic drive and a motor, feel free to reach out for a consultation. We’ve helped everyone from small startup cobot teams to large industrial manufacturers get this right, and we can walk you through all the calculations, efficiency numbers, and mounting options so you don’t waste time or money on a bad match.
Harmonic Drives REFERENCES
Harmonic Drive Technical Guide for Motor Compatibility, Harmonic Drive Systems Inc., 2022.
Industrial Motor and Gear Drive Selection Guide, Motion Control Association, 2021.
Collaborative Robot Joint Design: Torque and Speed Calibration, International Journal of Robotics Manufacturing, 2023.
Sango Automation Limited
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