Can I reduce or remove backlash between horn and shaft?

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Can I reduce or remove backlash between horn and shaft?

Postby servouser » Sun Nov 20, 2011 2:18 am

Hi forum members, is there a way to reduce or remove the backlash between the servo horn and the servo shaft? Using super glue is not a good option as I may need to separate the parts later on.
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Re: Can I reduce or remove backlash between horn and shaft?

Postby Sid Sideslip » Sun Nov 20, 2011 2:38 am

Any backlash should be in the gear train, not the fit of the horn to the output shaft, which should be tight on the splines, even before you tighten the screw.

If it is loose, then I guess releasable Locktite might work (there are heaps of different types for different applications)

Other than that...buy better servos?
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Re: Can I reduce or remove backlash between horn and shaft?

Postby Aussie Joe » Sun Nov 20, 2011 5:37 am

Maybe you accidently mixed servo horns from different manufacturers. Standard Hitec splines are 24 tooth and standard Futaba splines 25 tooth. The right servo horns are allways a good fit to the spline.
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Re: Can I reduce or remove backlash between horn and shaft?

Postby servouser » Sun Nov 20, 2011 9:10 am

Perhaps I did not use the term "backlash" correctly. I got several high-end, expensive motors from a well-known japanese company. The servo horn is made of aluminum and it is compatible with two series of motors from the same company. I tried the same horn on different motors from these two series. In one series, let's called A (the one the horn is meant to be used with), I can insert the horn to the motors of this series easily without effort. In other words, if I flipped the motor over, the horn would have fallen. So, even before I can actually rotate the gears inside manually, I can turn the horn back and forth by about +-3 to 4 degrees because of the gap between the horn and the shaft. This happens to all the motors in this series. When I inserted the same horn to the motors in Series B. I can insert the horn and rotate about +-3-4 easily of some motors. However, for the rest, I need to use just a little bit of force to insert the horn to the shaft. As the connection is tight, I cannot rotate the horn by a few degrees without actually turning the gears inside. If I flipped the motor, the horn would not have fallen. This is what I want. Maybe there are some tolerance due to manufacturing? Is this common? I want to know if there is a 'simple fix' to this issue. Not sure if they will let me return the motors because they do not 'fit tightly' with the horn.
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Re: Can I reduce or remove backlash between horn and shaft?

Postby Keith » Sun Nov 20, 2011 11:20 am

You do not mention the screw that holds the horn on...? If the horns are not a tight press fit then they are the wrong ones... The screw stops them coming off... I suppose the screw would hold a loose horn if tightened enough but I have never tried it....
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Re: Can I reduce or remove backlash between horn and shaft?

Postby servouser » Sun Nov 20, 2011 11:50 am

I bought everything directly from the manufacturer. They are the right ones. Even with the screw, I can turn the horn back and forth by about +-3 to 4 degrees easily without force. Is this common? How many degrees of such turning is considered to be acceptable? Of course, I prefer one that is perfectly, tightly fit.
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Re: Can I reduce or remove backlash between horn and shaft?

Postby Aussie Joe » Sun Nov 20, 2011 12:29 pm

servouser wrote:When I inserted the same horn to the motors in Series B. I can insert the horn and rotate about +-3-4 easily of some motors. However, for the rest, I need to use just a little bit of force to insert the horn to the shaft.

You have two different servos, it fits one but not the outher. The differnce may only be one spline, that is enough to strip the spline/horn and cause a crash.
servouser wrote:I want to know if there is a 'simple fix' to this issue.

Change servo or change the horn. The ones you have are incompatible with each outher. Thay must be a firm fit / press fit.
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Re: Can I reduce or remove backlash between horn and shaft?

Postby kaptain_zero » Mon Nov 21, 2011 5:34 am

servouser wrote:I bought everything directly from the manufacturer. They are the right ones. Even with the screw, I can turn the horn back and forth by about +-3 to 4 degrees easily without force. Is this common? How many degrees of such turning is considered to be acceptable? Of course, I prefer one that is perfectly, tightly fit.


No this is not common and there should be no significant play in the spline joint. If you are sure the horn is meant to fit the servo in question but it is loose, it is a manufacturing problem. Either the shaft is undersized or the horn is oversize. As you say the horn fits correctly on the other servo it is meant to fit, it sounds like the shaft is undersized on the servo in question. You have purchased directly from the manufacturer so the best course of action is to contact the manufacturer with your problem and have them rectify the issue by either replacing the faulty servo or sending the correct horn if indeed it turns out that there was an error in the documentation and that the servo in question DOES require a different horn. That is the correct way to solve the problem.

In a pinch, the problem could be fixed, but only as a last recourse and it's not easy. You would have to make up the space by using a metal filled epoxy and use a release agent on one surface of the splines. Doing this would in theory allow it to release so you could take it apart but it is by no means a sure thing as the joint is small and hard to make sure it is fully coated with a release agent. Any spot not coated properly would be permanently bonded which is not a good thing.

So I would contact the manufacturer and if needed, return servo and/or horn for replacement. If the manufacturer does not stand behind it's product, you should take your business elsewhere.


Regards

Christian
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Re: Can I reduce or remove backlash between horn and shaft?

Postby ergocentric » Wed Nov 23, 2011 1:49 am

Sounds like they gave you the wrong or a faulty servo horn.

try putting a layer of plastic bag over the shaft before pushing on the servo arm
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