A question we hear often from customers comparing our MG Series (Standard) and EG Series (High Precision) planetary gearboxes: "Why does the high-precision gearbox feel harder to turn by hand? Isn't it supposed to be more efficient?"
It's a fair question — and the answer reveals a lot about how gearboxes actually behave under real operating conditions. This post covers manual rotation, internal friction, lubrication, and how efficiency changes with speed and load.
The "Hand Test" Paradox
Spin both gearboxes by hand and the MG Series usually feels easier to turn. This says nothing about overall efficiency — it's a direct result of different design priorities.
Static friction (breakaway torque)
- EG Series: Designed for low backlash. Internal tolerances are tighter and components may be preloaded, which raises static friction (stiction) and makes it harder to "break" the gears into motion by hand.
- MG Series: Uses standard tolerances. The gears have more room to move — higher backlash — which lowers the initial resistance to movement.
There's one more thing working against you in a hand test: turning the output shaft means backdriving the gearbox, which multiplies internal friction by the gear ratio. You're testing the worst-case friction scenario, not normal operation.
The Role of Lubrication
The internal grease is a major contributor to the "stiff" feeling of the EG Series.
- Viscosity and drag: The EG Series uses a high-performance synthetic grease that is more viscous at room temperature. When the gearbox is cold or turned by hand, this creates noticeable fluid drag.
- Hydrodynamic lubrication: At operating speed, the grease warms up and forms a thin film between gear teeth (a hydrodynamic wedge). Friction drops significantly and the gearbox reaches peak efficiency.
- Damping: The EG grease is also chosen for its damping properties. In a robot arm, that damping settles the arm quickly and prevents "ringing" after a sudden stop.
Efficiency Depends on Speed
Efficiency is a dynamic measurement — a "high efficiency" gearbox can perform very differently at low versus high speed.
| Series | Low Speed | High Speed |
|---|---|---|
| MG Series | Higher efficiency: less internal "squeeze" and lower grease drag. | Lower efficiency: standard tooth geometry produces more heat loss at speed. |
| EG Series | Lower efficiency: tight fits and viscous grease act as a "torque tax" at low RPM. | Higher efficiency: optimized geometry and thinned-out grease deliver superior power transfer. |
Efficiency Also Depends on Load
Planetary gearboxes are generally most efficient at medium-to-high speeds under meaningful torque load — not in light, low-speed conditions. Two effects drive this:
- Fixed losses shrink in proportion: Much of a gearbox's friction — seals, bearings, stiction — is roughly constant regardless of load. As load increases, these fixed losses become a smaller and smaller percentage of the input energy. (Viscous drag from the lubricant rises with speed, and aerodynamic losses with the square of speed, but both are small compared to coulomb friction.)
- Better tooth contact: As load increases, contact spreads more fully across the gear teeth, improving power transfer.
There is an upper limit. Push far enough and friction drag outweighs the contact gain, or shaft deflection starts degrading tooth contact, and efficiency drops again. But within the rated operating range, the rule of thumb holds: efficiency improves with load, up to a point. Well-designed planetary stages routinely exceed 95% efficiency per stage when fully loaded at speed.
What This Means for Your Robot Arm
- Don't judge by hand. Manual rotation tests backdriving at zero speed with cold grease — the worst possible case for friction and unrepresentative of real operation.
- Stiffness is the point. The EG Series is harder to turn because it's tighter — and that tightness is exactly what delivers the low backlash and path accuracy robotic applications demand.
- Trust operational numbers. At operating speed and load, the EG Series runs more efficiently, runs cooler, and holds position far better than the MG Series.
Choosing between the MG and EG Series for your application? Get in touch — we're happy to help you match a gearbox to your speed, load, and precision requirements.