I've seen Hypoid gear sets back-driven but it really depends on the application. What are the parameters of this application?
Heckler
Sr. Mechanical Engineer
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WHITETOP: Are you talking about a hypoid gearset or a worm gearset. With a hypoid gear set there should be no problem , but hypoids are expensive to manufacture,are noisy and have a lot of friction. On the other hand they can carry a lot of load becasue it is divided among several teeth.
Worm gears can be back driven only if the tangent of the worm angle is above or below (I cannot rmemeber which) the coeffiecient of friction. In the late 1950's early 1960's Puegeot Automobiles had a worm drive differential.
Come to think of it, I've never seen a hypoid set at 25:1.
I have seen a Helicon set at 25:1, and they look similar to hypoids, except that the pitch surface of the pinion is a right circular cylinder and the pitch surface of the crownwheel is a planar ring. Like a hypoid, the axes are nonintersecting.
Hypoids, worm gears, etc. have a lot of friction as cessna1 says because the worm rubs against the wheel. This is why they are relatively inefficient and thus why they are difficult to back drive especially at higher ratios. The problem is that with any such gearset if there is any significant vibration present, back driving is very possible as it unloads the system. I have seen this happen in several instances.
I want to thank everyone for their responses and concerns. It sounds as though the application, the efficiency and maybe even some luck will determine if the unit can be back-driven. It appears that the only way to find out if the gear reducer in question can be back-driven by the load or intensionally by manual effort is to set it up on the research bench and test it.
This is an inherited project that may have gone too far and I am just trying to salvage the "to date" cost. I don't have a good feeling about the system; especially, since starting this thread my study has led to the fact that not all hypoid sets are even bidirectional. Well, if it can't be driven in both directions from the input shaft, what are the chances of being driven in either direction from the output side?
If there is any interest by you to know the outcome of the research give me a "yea" and I will be happy to post it. Due to my current work load that won't happen for at least two weeks.
Whitetop,
It may be easiest to install a "backrun brake" which is basically an overrunning clutch on your input shaft, either before or after it meshes with the driven wheel. These are often used even when it doesn't seem possible that it will be needed. As others have indicated, almost any gearset can be made to run, or at least creep, backwards.
