9x20 clutch

savarin

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The clutch on my asian 9x20 slips badly on slow speed and not much better on the next speed.
I've dismantled it and all the balls have a definite wear pattern.
clutch3.jpg

The arrows point to a flat spot and a groove on one of the balls.
clutch1.jpg

They also just seem to sit on the holes rather than a small way inside.
clutch2.jpg

The holes in the pulley seem too large for the spring and ball and do show a fair bit of wear on the edge of the holes.
I have seen it said that the holes the balls sit on in the hub should be drill deeper but this would do nothing as the balls do not sit down inside the holes.
I assume they meant drill them to a larger dia as this would then allow the balls to sit deeper to sit deeper.
If I drill them to a wider dia, and use larger balls to fit better in the pulley would this improve things.
Has anybody enlarged and deepened the holes for the balls on this lathe?
 

markba633csi

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I see a few issues: those balls must not be sufficiently hardened to score like that. Secondly, I see wear around the holes so that piece too should have been made harder it seems
Thirdly, could you not install additional balls and springs- it looks like the factory had planned to do that but cheaped out and only used 5
 

RJSakowski

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I'm not familiar with this lathe but have recently had a similar experience with a tractor jumping out of gear when a load was a.pplied. Looking at the gear wear pattern on the motor(?) hub, it is clear to me what the problem is.

Whether the ball stays in a given hole or jumps to the next hole depends on the relative magnitude of two forces. The first is the spring force which is pushing the ball into the seat and the second is the component of the the force from from the applied load. The load force applied to the edge of the socket can be broken into two separate forces, one of which is opposing the spring force. If that force exceeds the spring force, the ball will jump. The magnitude of that force is a function of the angle of contact of the ball. As that angle become closer to perpendicular to the line of the spring force, the ball is more secure and vice versa.

In your case, the angle is becoming less perpendicular which means for a given load, the ball will be more likely to jump. The solution can go in several ways. Renew the surface of the hub make the angle closer to the perpendicular or increase the spring force, either through a stronger spring or with a shim above the spring to shorten the length, thus increasing the force.

In my case, the first attempt was to change the angle; that failed. The second was to provide a shim to increase the spring force (a stronger spring with the right dimensions was not practically available). This sort of worked. Finally, I drilled the blind pocket through and tapped the hole for an adjustable stop which seems to be working to date.

The root cause of the problem may very well be an oversized ho;le in the pulley in which case, replacing or reworking the hub maty just cause the problem to reoccur. A better solution would be to go a size up in the ball and remachine the pulley and hub for the new balls and springs. Also, it would appear that you are working the lathe fairly hard which is why the clutch slips in the first place. If there was no slippage, there would be no wear. I would suggest designing for a higher load if that is more in line with your work habits. Having set screws for adjustment of the spring force would be an easy way to set the right amount of force.
 

Illinoyance

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This appears to be an overload clutch to prevent damage to the machine. It apparently has been doing it job. I suggest replacing the balls and reassembling it. Do not deepen the pockets the balls sit in. That would raise the tripping torque of the clutch. It could prevent release of the clutch altogether and force some other component to fail. It is possible the springs have deformed a bit so it would be best to replace the springs,
 

C-Bag

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This appears to be an overload clutch to prevent damage to the machine. It apparently has been doing it job. I suggest replacing the balls and reassembling it. Do not deepen the pockets the balls sit in. That would raise the tripping torque of the clutch. It could prevent release of the clutch altogether and force some other component to fail. It is possible the springs have deformed a bit so it would be best to replace the springs,
I'm no expert and a quick search didn't seem to come up with anything. But I'm watching this closely because for whatever reason every time you run into a repair situation Savarin I swear my 9x20 follows suit.

I somewhat agree with Illinoyance that I'd not go drilling the holes for the balls as that would drastically change the amount torque needed to kick the clutch in. I think I'd try to replace the balls and maybe see about taking the groove out of the seats (after measuring how much chamfer is in each hole) and redo the chamfer to its original state. Like was noted it doesn't speak very highly of the balls that they were so worn. But it might be a lot more force than we know when that thing let go? As far as the springs I'd try to check the height and make sure they are all the same and if refacing and new balls doesn't work making some kind of spacer to drop in the hole to up their tension a bit like RJ mentioned.

Redoing the clutch is going to be an interesting challenge with the lathe down. You know somewhere I remember something about the clutch on the 9x20 and a mod but I as usual can't find it.
 

stioc

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Hmm, there's a clutch in your 9x20? I'm pretty sure mine doesn't.
 

savarin

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Its working a treat now.I used a reamer to smooth the insides of the pulley as there were lumps and wear.
Then I drilled the holes in the hub out to 4mm.
Then used a 5mm drill to just open up the first 2mm of the holes.
The original balls were 4.8mm dia but the new ones are 5.5mm dia and fit in the pulley holes with very little slop.
I kept the springs as they were all the same length.Everything was assembled with a standard grease and the shaft it all spins on and the bushes had very little wear so I havnt touched those.
I cut a short section of square tube, removed one side and drilled a hole to make a clamp to compress the springs to allow easy access to the circlip.
clutch4.jpg

Once assembled I tested it by making a brass scriber like clickspring.
 

C-Bag

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Good job, I knew you'd come up with something. The thing I think I'd have a hard time finding around here is separate balls. Just out of curiosity did you try pushing it hard enough to kick the clutch? Mine was a .080 depth of cut in mystery steel. It will do .070" all day, but not .080.
 

savarin

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I really pushed a blunt carbide bit into the outer edge of a 90mm dia chunk of mild steel till it stalled and triggered.
I didnt measure the depth of cut as it wasnt flat but it was at least 1.5mm
I forgot to mention the balls came from a cycle shop.
 

C-Bag

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How many times do you think you kicked the clutch before just out of curiosity?

Not until I redid my headstock bearings was I able to do greater than a .040-.050 DOC because of chatter. So I'd never had the clutch kick in before. So I'm wondering how many times can it kick in before its toast like yours?
 

stioc

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So where is this clutch located? is it on the carriage or in the headstock or the gearbox? I'll have to check my Grizzly G4000 if it has one but I don't think it does.
 

RJSakowski

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So where is this clutch located? is it on the carriage or in the headstock or the gearbox? I'll have to check my Grizzly G4000 if it has one but I don't think it does.
It does. Parts 210. 211. 212 in parts list.
 

stioc

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Thanks @RJSakowski - I see it now. I never knew there was a clutch in there. So this is to protect the motor or the leadscrew/gears etc if you take too deep of a cut?

1563806932196.png
 

savarin

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Yep but I've only ever had it slip on lowest speed and the next puller size up.
You will know when it happens as its very noisy.
Prior to it getting too weak it was kicking in many times on a large dia interupted cut.
 
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