Floating sun gear benefits planetary reducers

Groschopp’s new planetary reducers employ a floating sun gear rather than a fixed position one.

The saying ”There’s nothing new under the sun’ certainly applies to planetary reducers. And, while floating sun gears have been around a long time, some engineers may not be aware of the benefits this unusual gear design can offer.

Traditionally, planetary reducers have used a fixed sun gear, where the centre gear is attached to or machined into the shaft. When this fixed sun gear revolves, it turns the planet gears to create motion and/or power. Groschopp’s new planetary reducers, however, are employing a floating sun gear rather than a fixed position sun gear.

Why a floating sun gear? ‘In the planetary concept, the sun is the driver, or pinion, in the gear set,’ Groschopp design engineer Scott Hulstein said. ‘Because the sun gear is in constant contact with the planets, it’s important that it is perfectly centred among the three planets in order to provide equal load sharing among itself and all three planets.’

‘Due to normal manufacturing tolerances however, a sun gear which is securely fixed on a shaft will intermittently have more load on one planet gear than on another gear,’ Hulstein explained. ‘By allowing the sun gear to float, it centres itself among the three planets and produces constant, equal load sharing.’

Equal load sharing is just one of the benefits of this design. The floating sun gear provides ‘true involute action,’ according to Hulstein. True involute action occurs when the rolling motion between the mating gears is as complete as possible. The benefit of this complete meshing of gears is longer reducer life, since less internal gear slippage means fewer broken gear teeth.

That also means lower noise levels. When the sun gear is allowed to completely roll into the planet gears, there’s less ‘rattling’ as the teeth mesh. In effect, the Groschopp product has ‘designed out’ the gear mesh noise by allowing the sun gear to float into place.

So why use a fixed sun gear at all? ‘Fixed sun gears are often used in true servo applications,’ Greg Pennings, Groschopp Customer Advocate, explained. ‘A fixed sun gear is necessary when exact positioning and low backlash are an integral part of the application.’ Groschopp’s engineers, however, were less concerned with low backlash and more interested with higher torque and/or lower noise applications.

‘Our planetary reducers with floating sun gears were designed to compete with parallel shaft reducers, where backlash was less critical,’ Pennings said.

By using the floating sun gear concept, the Groschopp planetary reducers are able to exceed the torque ratings of similar sized and larger sized parallel shaft reducers, and yet maintain a lower noise levels.

‘If I were designing a conveyor system, I’d be interested in it,’ Hulstein said. ‘Many companies are now using parallel shaft units, or even true servo planetary units, on PseudoServo applications where they’re not necessary.’

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