Planetary reduction gets simpler

Written by: Tom Shelley | Published:

Tom Shelley reports on an even simpler planetary reduction gearbox

Following publication of the Nasa-designed planetary reduction gearbox in our February issue, it seems that an even simpler design may be available. What’s more, it is British and more amenable to low-cost applications – such as car seats and toys.
Tim Tod, former managing director of a South Wales development company and now retired, has – in partnership with his son – come up with a design developed and patented in 2001 (British patent GB2354049).
“The breakthrough we made was that the two annular rings have the same pitch circle diameter,” says Tod.
The patent also reveals that they have sun and planet gears made with straight-cut teeth, which were the same along their length – engaging with two internal ring gears, which he calls annulus gears with slightly different numbers of teeth.
One annulus gear is fixed. The input shaft is either connected to the sun gear or the carrier for the planet gears. Either way, the planet gears roll around the inside of the fixed annulus gear and also engage with the inside of the movable annulus gear. If this has slightly more teeth than the fixed teeth, it will rotate slightly backwards, with respect to the rotation of the input shaft. If it has slightly fewer teeth, it will rotate in the same sense, but by a greatly reduced amount.
One would normally drive the sun gear and allow the planet gears to move freely. But it is also possible to drive the planet gear carrier and allow the sun gear to rotate freely. Output can either be via a toothed belt attached to the moving annulus gear, teeth machined on to the outside of this gear or by a shaft attached to it.
It is also possible to turn this into a two-stage reduction gearbox fairly easily, achieving very large speed reductions and torque multiplications in a single compact unit. In this design, the second, rotatable, ring gear is externally toothed and serves as a sun gear for a number of outer planet gears, which mesh with third and fourth ring gears, coaxial with the first and second ring gears. The first ring gear, and one of the third and fourth ring gears, is held stationary, with the final drive taken from the other of the third and fourth ring gears.
While Nasa applications may well require machining different tooth sizes on the various gears (according to what they are meshing with) Tod says that – within normal ranges of tooth form tolerances – it is possible to have a single-sized tooth form on one gear meshing quite satisfactorily with annulus gears, with slightly different numbers of teeth on the same pitch circles. This enormously reduces manufacturing costs and allows the use of simply cut metal or plastic injection moulded gears.
The design, says Tod, “lends itself to being fitted to every seat in every car”. It would also appear suitable for use in toys and domestic appliances, and possible more mainstream engineering applications – although not to machine tools, because of inevitable backlash.

Tim Tod Tel: 0187 473 1092


* Planetary high ratio reduction gearbox has only five moving parts, annulus gears with the same pitch circle diameters and planet gears that are simply cut, spur gears – with the same tooth forms along their lengths

* The gearboxes can be made in various versions; and it is possible to devise a two-stage version that fits into a very compact space and could achieve reduction ratios of thousands to one

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