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Just got an orbital back that did this, and sent another out to a customer who reported back that it's doing this. So I chased around some more, talked to a couple people about it, and found out what the deal is. I've actually done this now, in the shop, on two different valves, and so I'm pretty confident in it.

Shaking is caused by the gerotor section and the control section being out of time with each other. Inside the orbital, you've got a control section, with a centering spring between the two rotating parts.

My apologies on that some of these pics suck, my digital camera won't do this close up without problems. If you're looking at the parts, you can tell what they are; this isn't intended to be a "how to machine an orbital valve" post.

This pic is the control valve structure; the pen is pointing at the centering spring.

This pic is the control valve structure with the coupling pin partially slid out to one side. This pin is what couples to the dogbone (next pic), which drives the gerotor.



To re-time an orbital, take the dogbone out. Draw a line on the gear end of the dogbone, parallel to the slot in the other end that the pin rides in. This line will either be perfectly between drive teeth, or perfectly lined up with them--not partway between. Whether it's inline or in between, depends on the brand of valve that you've got.

Dogbone with magic-marker line on it:

Now put the first wear plate and dogbone back in the valve. Line up the line on the end of the dogbone, with valleys between the lobes, on the star of the gerotor, and put the gerotor star back in.

Now put the gerotor surround, spacer, and cover plate back on, and bolt it all back down.

Why'd I think this was important enough to post?

A certain major domestic manufacturer, well-known for their orbital valves, also makes hydraulic gear motors. They have a timing mark on their gerotor stars, to aid in assembly in timing them for motor use... and they use the same gerotors for orbital valves.

A motor is a "slave" unit; an orbital is a "master" unit. Timing them out of tune one tooth, changes "master" or "slave" orientation--a motor timed one tooth out, will simply be stalled. An orbital, however, has an extra control section on it, which doesn't work if it's not in-time with the rest of its parts.

Problem is, there's also a timing mark on the dogbones that this particular manufacturer makes... and the timing mark lineup is such that timing them together, is timed right for a motor, not for an orbital. So there's probably more than a few valves out there now, that are timed wrong, because somebody at the factory, followed the marks.

This particular manufacturer is the only one I know of, who actually internally marks their stuff.

Hopefully, this helps somebody chasing a problem, figure it out quickly and save some sanity--I know somebody else chased this recently, and ended up with a different solution, but I suspect they got lucky (there's a 50/50 chance of randomly putting the gerotor back on right--half the teeth lineups work, half are wrong) when reassembling it.

Rock Stacker
1,496 Posts
Safety note: if you loose timing of the valve and rotor, it can turn into a motor, it may be stalled and not move until you go to turn the wheel and then it will start spinning with your broken arms attached, slapping you in the face, reminding you why no body wants to rebuild them.
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