A facility replaced two worm gearmotors with planetary ones on a continuous mixer line and expected the energy bill to drop. It did, but not by the amount the sales sheet promised. The sheet said “up to 97% efficiency.” The actual saving was about half the projection. The gearboxes weren’t wrong. The efficiency number was, because it was quoted for a narrow condition that the application didn’t match. Efficiency in a planetary gearbox is not a single number. It moves with ratio, speed, load, and temperature.
Where the losses actually are
A single planetary stage has three loss sources: gear mesh friction, bearing friction, and churning losses from the oil. At full load and moderate speed, a well-made single stage runs at about 97-98% efficiency. The mesh loss per planetary stage is small — roughly 1% per stage at full load. But that’s the key phrase: per stage. A planetary gearmotor with a 50:1 ratio usually has three stages, not one. Three stages at 97% each gives a total of 91%. The catalog number of 97% was for a single stage, or for the gearbox at the ratio where only two stages are engaged. The user bought the gearbox at 50:1, which is a three-stage configuration, and the real number was closer to 90-92%.
The second loss is load-dependent. At 25% load, the mesh losses don’t drop to a quarter. They drop by less, because the bearings and seals still have their base friction, and the oil is still being churned. The efficiency curve of a planetary gearbox peaks around 80-100% load. At 50% load, a 91% gearbox runs at maybe 88%. The mixer line ran the gearmotors at 60-70% load most of the time. Every point on the efficiency curve moved the saving in the wrong direction.
The comparison that was actually fair
The worm gearmotors were 30:1, which at full load runs at about 75% efficiency (worm gears are always worse, especially at higher ratios, because the sliding contact at the worm mesh dominates). The planetary units at the same ratio and full load were at roughly 90%. That’s a genuine 15-point improvement, and it’s why the plant switched. The projected saving was based on the 97% number, which overstated the improvement by 7 points. The real saving was still there. It just wasn’t as big as the brochure.
There’s a second advantage that showed up later. Worm gearboxes get hot — the sliding mesh dumps heat into the housing, and the oil degrades. Planetary units run cooler because the losses are smaller and the housing has more surface area per unit of torque. The oil change interval on the planetary units was 3 years instead of 1. The maintenance saving was small but real.
How to get the real efficiency number
Ask for the efficiency curve, not the headline number. The curve shows efficiency versus load at the gearbox’s actual ratio and input speed. Any serious gearbox vendor has it. The curve for a three-stage unit will show 90% at full load, dipping to 85% at 30% load. If the vendor can’t produce it, that’s a sign the number in the brochure is the best-case single-stage figure. Also check whether the number includes the motor. A gearmotor’s overall efficiency is gearbox efficiency times motor efficiency (typically 88-92%). The gearbox alone at 91% and the motor at 90% gives a combined 82%. The energy bill sees the combined number.
The speed effect
Speed matters less than load but it matters. At very low input speeds (below 100 rpm), the oil film in the bearings gets thinner and the mesh losses rise slightly. At very high speeds (above 1500 rpm input), churning losses grow. The efficiency peak for a planetary stage is usually between 500 and 1200 rpm input. Gearmotors on mixers often run at 1450 rpm input, which is fine. But a slow-speed application (say, 50 rpm output with a 30:1 ratio, so 1500 input) is fine too. The trouble starts when someone uses a high-ratio planetary unit at low input speed, where the bearing losses get relatively bigger.
The practical takeaway for specifiers: compare like for like, at the actual ratio and load, with the motor included. A planetary gearmotor is a genuinely efficient drive. It’s just not 97% efficient everywhere. Knowing where the losses hide makes the energy calculation honest, and honest is what the plant manager’s spreadsheet needs.
The energy saving was real but half the brochure number. Three stages at 97% each is 91%, and running at 60% load drops it further. Ask for the efficiency curve at your ratio and load, include the motor, and the projection will be right. Planetary beats worm on efficiency — just not by the margin the catalog suggests.
