Weird Hitec 7954sh issue

[ik632]

I have a 7954sh in my Wraith and it's been great all until yesterday. All of a sudden the servo basically let go. You could move the tires back and forth like there wasn't a servo in there. I figured I stripped the gears. So, I pulled them out and checked the gears. Everything looked good. Even the gear on the motor. So, I reassembled and it did the same and felt like the gears were slipping against each other. You could feel the teeth a little as it slid by, almost like a pinion that's too lose.

So, I pulled it apart again and checked everything and it looked good. The next time I put it back together I just tightened the screws so they were holding the cases together, but didn't crank down on it (like the prior time). This time it seems to be good.

So, the question is, could my case be wearing out or maybe the bushings are going bad? They looked ok upon inspection, but if it felt loose with the screws over-tightened, I'm wondering if the case was deflecting and causing that.

[Charlie-III]

Cases do wear out over time, that may be your issue.

I believe you can get replacement cases, I usually use www.ServoCity.com for servo parts.

Are you sure the servo arm isn't slipping on the output shaft?

[KMS]

The lower delron gear it is almost impossible to see if its stripped ,,I would say thats the problem ,,,,Robert

[ik632]

That was the first thing I checked. You could actually turn the splines with your hand and feel all the gears moving. The strange thing is, even with the motor out, it should be hard to turn the gears. Now that I think about it (and have it all together and cleaned up) I'm wondering if the intermediate shaft has worn or is broken somehow. That could cause the gears to pull apart. Think it's under warranty still?

[ik632]

Originally Posted by KMS

The lower delron gear it is almost impossible to see if its stripped ,,I would say thats the problem ,,,,Robert

The pinion on the motor? All of the gears in the top are steel in the 7954.

[JesterSpec]

I wouldn't trust your eyes, some of those gears are really hard to tell if they are stripped. Even the tiniest amount with do it in.

I'd start with another set of gears, and if that doesn't solve it, contact hitec. They are good people and will help you out.

[ik632]

Ok, took the gears back out and found that one of the gears wasn't stripped, but pulled apart. The smaller section that is pressed into the larger section totally separated. So, that seems to be the culprit. Time to order new gears.

Thanks for the help guys.

[FRENCHBLUE72]

If you have a welder you can weld that gear back together no problem... A few of here in oregon have done the welding mod and they have held for going on 2 years now...

[meatmonkey]

Fact: not all the gears in a 7954 are completely steel.

The weakest gear in all 7900 series Hitec servos is the "MK" gear. Hitec calls it Metal Karbonite, I've seen others refer to it as delrin/steel pressed together. This is the gear driven off of the motor's pinion gear.

A three pack is very cheap, about $12. The teeth are so small it is very difficult to identity stripped teeth, and it only takes two or three next to each other to make it all crunchy.

It is impossibly to weld this gear back together. I wish someone made a true steel gear to replace this problomatic MK gear. I've stripped four of them in 3 different 7954's. It's always landing small jumps at awkward angles that does them in, and I even have mechanical steering stops.

Also inspect how tightly the gears shafts fit into the case halves, they should be quite snug with no play.

[AddictedtoRC1]

Originally Posted by ik632

Ok, took the gears back out and found that one of the gears wasn't stripped, but pulled apart. The smaller section that is pressed into the larger section totally separated. So, that seems to be the culprit. Time to order new gears.

Thanks for the help guys.

I had the same thing happen to me on a 7954. You can fix it fairly easy on your own. Assemble the two gears and with a small punch, go along the edge of the smaller gear on the backside and lightly hammer it. It'll expand the smaller gear like a press fit.

[Olle P]

Originally Posted by Charlie-III

Cases do wear out over time...

How? In what way?
It's not a moving part so I really don't understand...

[Charlie-III]

Originally Posted by Olle P

How? In what way?
It's not a moving part so I really don't understand...

Some of the case parts have the "bushing" for gear shafts in them. Most of the time it's just plastic and the holes wear "egg shaped" that causing gear misalignment over time. This can cause teeth to "skip" which acts like broken gear teeth.