Strain wave gears run 75–90% efficient at rated load, genuinely good for a 100:1 single stage. But engineers get burned by that number, because unlike a ratio or a tooth count, efficiency is not a property of the gearbox. It's a property of the gearbox at an operating point, and the operating points where machines actually live — light load, low speed, cold mornings — are where the datasheet number quietly falls apart.
Where the power goes
Three loss mechanisms, all speed- and temperature-dependent:
- Tooth friction. Engagement is spread over dozens of teeth, and each one slides slightly as it enters and leaves mesh. This scales with load: the "honest" loss.
- Flexspline hysteresis. The cup flexes twice per input revolution and the steel dissipates a little energy every cycle. This scales with input speed, load or no load, and it comes back as heat.
- Lubricant drag. The wave-generator bearing churns grease continuously. This is the term that explodes when the grease is cold.
The shape of the map
Because the last two losses are roughly fixed at a given speed, efficiency depends heavily on how much useful torque you're pushing through against them:
| Operating point | What happens | Consequence |
|---|---|---|
| At rated torque, warm | Fixed losses are small next to the load | The catalogue figure: 75–90% |
| At 10–20% of rated torque | Fixed losses dominate | Efficiency can fall below 50% |
| Cold start (0 °C and below) | Grease viscosity multiplies drag | Starting torque several times the warm value |
| High input speed, sustained | Hysteresis + churn heat the flexspline | Thermal limit before torque limit |
None of this is a defect; it's the physics of any preloaded, grease-filled precision mesh. The mistake is budgeting motor torque, battery capacity, or thermal headroom from the single rated-point number.
The three torque numbers nobody reads
Buried below the headline specs, every honest strain wave datasheet carries three small torque values that answer real design questions:
- No-load starting torque. What the motor must produce at the input just to begin turning an unloaded gear. This is the number that decides whether your axis moves on a cold morning. Size your motor's torque margin against it at your minimum operating temperature. Our joints are specified down to −20 °C, and starting torque at −20 °C is a very different number than at 40 °C.
- No-load running torque. The continuous input torque eaten by the gear at speed. Multiply by duty hours and this is the efficiency floor your energy budget actually pays, every hour, load or no load. On battery machines with many joints, it adds up faster than the motion itself.
- Back-driving torque. The output torque at which the gear can be driven backwards. Read it and note that it is a finite number: a strain wave gear is not self-locking. High ratio makes it reluctant, not safe.
The thermal loop
Efficiency and temperature chase each other: losses heat the flexspline, heat thins the grease (briefly helping), then sustained heat ages the grease and raises friction (permanently hurting: the wear clock from the failure-modes article ticks faster). For high-duty-cycle axes, check the steady-state housing temperature against the operating window under the real ambient, enclosure, and mounting conduction. A joint bolted to a big aluminium arm sheds heat; the same joint in an insulated shroud doesn't.
The checklist
Before you commit a motor and battery to a strain wave axis:
- Motor torque margin covers no-load starting torque at minimum temperature, plus acceleration load.
- Energy budget uses efficiency at your actual typical load fraction, not the rated-point figure, and adds no-load running torque × duty hours.
- Axes that spend their life below ~20% of rated torque are candidates for a smaller frame, or a planetary, if the precision spec allows it. An oversized harmonic drive is an inefficient one.
- Gravity axes get brakes. No exceptions worth writing down.
- High-speed, high-duty axes get a thermal check at steady state, not just a torque check.
If you'd rather hand us the duty cycle and ambient conditions and get back a sized, quoted part number, that's the same-working-day service; it's what the RFQ form is for.
Related: How to size a harmonic drive · Reading the datasheet: backlash, lost motion, stiffness.