Spur gear diff rotation.

[Steve CrowParticipant @stevecrow46066]

A bit of help needed.

Here is a picture of a spur gear differential I nicked off the 'net.

My question is, if shaft A is held stationary and cage B is rotated clockwise, which direction would shaft C move?

Logic and intuition tell me clockwise but I may be missing something!

Cheers

Steve

[Steve CrowParticipant @stevecrow46066]

[ChrisLHParticipant @chrislh]

Logic and intuition win through! Shaft C will rotate in the same direction as Cage B (i.e. clockwise in this case) but at twice Cage B's speed.

[Martin ConnellyParticipant @martinconnelly55370]

The second shaft will go clockwise. It is easier to consider the case where the cage is stationary first and what is labelled as fixed goes anti-clockwise at rpm X. Then you will see that shaft C goes clockwise. In order to then see the case where the first shaft is fixed everything else has to go clockwise at whatever rpm X you imagined the fixed shaft to be going in the first case plus what it was doing in the first case. So the cage is rpm X plus zero clockwise and the output is clockwise amount plus rpm X.

Martin C

[Steve CrowParticipant @stevecrow46066]

Thank you Chris but I don't understand why the rpm is doubled.

[Pete RimmerParticipant @peterimmer30576]

Posted by Steve Crow on 31/10/2021 15:33:40:

Thank you Chris but I don't understand why the rpm is doubled.

In a differential whatever speed one side is slowed by is added to the opposite side.

[Steve CrowParticipant @stevecrow46066]

Thanks for you help. I am ok with a bevel diff, (here's one I made earlier – **LINK**), I'm just finding it hard to get my head around the spur gear type. It's the fact that there are two gears counter rotating transmitting the power rather than a single bevel gear.

Using the picture above. If cage B is fixed and shaft A is rotated once clockwise, I presume shaft C will rotate once anti-clockwise, the same as a bevel diff?

If so, would putting an idler gear between the two small pinions reverse that?

Cheers

Steve

[Peter Cook 6Participant @petercook6]

Posted by Steve Crow on 07/11/2021 10:46:18:

It's the fact that there are two gears counter rotating transmitting the power rather than a single bevel gear.

If I understand it, the two pairs of "contra rotating" pinions are not actually in mesh. The two meshing with the shaft A gears drive the cage round. The two meshing with the shaft B gear are driven by the cage and drive shaft B.

Presumably you could add more pairs at right angles if you wanted to transmit more torque.

[Steve CrowParticipant @stevecrow46066]

If I understand it, the two pairs of "contra rotating" pinions are not actually in mesh.

Hi Peter, this animation shows the pinions in mesh. **LINK**.

Steve

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