Differential Planet Gears - Printable Version

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Differential Planet Gears - Chris KC - 14-08-2019

Can someone with a slightly bigger brain than I tell me how the 6 pins the planet gears rotate on are secured into the halves of the differential casings? They appear to be staked with some kind of pin but I can't quite figure it out.

My 'best' diff assembly for crown wheel run out has slightly loose staking on these pins, I figure they can't really go anywhere and will be stabilised by assembly to the other half of the casing; but I'd be interested to hear if there is there a practical way to tighten them up.

RE: Differential Planet Gears - Ian Williams - 14-08-2019

I don't have a definitive answer for you Chris as I have never had to correct a carrier with excessively loose or worn pins. Agree that they are secured by a pin through the casting and as a result it would be possible to remove and replace them, I also agree that the other end of the pin is supported by the opposite side of the carrier. You might be able to replace the loose pins with some from a better carrier but I anticipate that the hole on the casting will have wear as well so it would likely entail making completely new pins oversize where they fit the casting!... Not sure I would go to the trouble. When you assemble the two halves of the carrier are the pins still noticeably loose or is it only apparent when the carrier is split, if the latter I would not worry about it, if the former, well personally I would rummage in the spares shed.

RE: Differential Planet Gears - Chris KC - 14-08-2019

Thanks Ian. I think I have a scrap carrier so I may try heaving one out. It looks like some kind of split pin? It might be possible to disassemble and replace with a new fastener, but like many of these jobs it may be better left alone unless one has confidence one can improve it.

The pins wobble slightly when the carrier is apart, but not noticeably when the thing is assembled; much as I'd love to put it right I'm inclined to agree it will be more work than it's worth...

RE: Differential Planet Gears - Bob Culver - 15-08-2019

The resulting radial displaceemnt of the planet would be tiny. Old diffs from other cars with mitre gears develop gears miles out of position and with notable wear; leads to backlash but effect in Seven would be negligible. Can you punch around with a ball end punch? The style of diff gear is noted for strength and was more commonly used for trucks. Try expalining how it works to some stranger!

RE: Differential Planet Gears - Robert Leigh - 15-08-2019

I have never investigated a problem with the planet gear system. It seems to me that in driving straight the whole assembly will turn as one with no relative movement of the components, so it will not cause noise or other problems. Also the whole lot is properly lubricated, unless the owner has neglected to keep the oil level correct; in that situation any play will lead to further wear.
Robert Leigh

RE: Differential Planet Gears - Chris KC - 16-08-2019

For anyone who is curious, I pulled one of the planet shafts out of a scrap differential (too bent to be recoverable).

The shafts are secured only by a plain 0.2" dia mild steel pin, which is split and splayed on the inner side only. The pin takes a fair effort to drive out; rather like a rivet it spreads rather than punches through. I guess the pins were inserted before the outer diameter was turned, as the pin heads are almost invisible on most units.

I'd say there is a reasonable chance you could make new securing pins (forget re-use) and re-fix the shafts, but how to bring the shaft hole accurately back down to size is another matter. I wonder if they ever were tight - the shafts are ground but the holes are very roughly drilled. As a colleague pointed out, if all the pins were tight and square it would have been a devilish job assembling the two diff halves together, a little wiggle is necessary.

I'm inclined to agree with Ian, it all seems to go firm once the two halves are put together so probably nothing much to worry about. I might see if I can squeeze some Loctite bearing fit into the joint just to tighten them up though and eliminate fretting.

Update: Just in case anyone is quietly following with interest - I flushed the shaft to carrier joints out in situ with volatile solvent (brake cleaner) and allowed 24 hours for it to evaporate off; then flooded the joints with Loctite 641 (bearing fit, dismountable) and assembled to the other half of the housing so they would set in alignment. By the following morning any trace of looseness has disappeared. I'm not a huge fan of 'liquid engineering' but I feel pretty confident in the result.