planetary gear geometry query!

[simplystupid]

If someone has any insight on this situation and can help out, I would be very grateful.

I have tried to put together a planetary gearbox using a spare parts set for a big Bosch drill, just the ring and 4 planet spur gears.
I calculated the PCD and 0.75 mod needed for the sun gear on the motor input.
Even though there is a few rizlas worth of backlash between the meshed gears - it doesn't work, the whole gearbox casing is forced to oscillate a couple of mm.
Ring gear has 47 teeth (why so damn awkward!) - planet spurs have 16 teeth, my homemade pinion sun has 13 teeth.
Therefore the rotating planet holder has the four gear shafts on a circle, but not opposite each other. At the corners of an oblong.
Images of a Bosch starter gearbox show the same kind of geometry, so I am a bit baffled.
It doesn't jam up when turned by hand, the 13 teeth are nicely cut.
But it obviously can't be bolted to the motor as it is now.
I can remove and drill the planet shaft holes again, but any idea where?

Many thanks.
Mark

 

[Miles]

Hi Mark,

I think you need a 15t sun gear.

Ns = Nr - (Np x 2)

The planets won't be evenly spaced for this combination, either, but it should work......

 

[simplystupid]

Thanks Miles,
I didn't see how the formula could apply if the planets were in two pairs on lines which did not pass through the axis (of the sun).
If that's what it takes to use the box, I'll pull off the sun using a blowtorch (threadlocked) and cut another with 15 teeth.

Mark

 

[Miles]

Unless Ns = Nr - (Np x 2), you don't get proper meshing, which I guess is what caused the oscillation.

To get uniform spacing, Nr + Ns has to be divisible by Np

(Nr - Ns)/2 must be integral.
(47-15)/2 = 16 so that's ok.

So, 47r, 16p, 15s should be fine......