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12 x 36 lathe vibration

I just adjusted the belt tension on my lathe. Mine has a bolt that goes through a clearance slot on the lower swing plate & threads into the motor plate. What keeps it into position are 2 nuts on either side of the swing plate. Kind of like a jack screw idea. The bolt is basically stationary but the combination of 2 nuts displaces the plate. I slightly increased belt tension but consciously left the upper nut loose. The motor bounced like it was on a bumpy road & you could even hear the RPM change. Bad news. Clamp the nut down so the plate is in fixed position, smooth as silk. Before you spend money on belts see if you can make the vibration disappear with a similar adjustment.
 
PeterT said:
I just adjusted the belt tension on my lathe. Mine has a bolt that goes through a clearance slot on the lower swing plate & threads into the motor plate. What keeps it into position are 2 nuts on either side of the swing plate. Kind of like a jack screw idea. The bolt is basically stationary but the combination of 2 nuts displaces the plate. I slightly increased belt tension but consciously left the upper nut loose. The motor bounced like it was on a bumpy road & you could even hear the RPM change. Bad news. Clamp the nut down so the plate is in fixed position, smooth as silk. Before you spend money on belts see if you can make the vibration disappear with a similar adjustment
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Thanks, I will take a look at that. The swinging motor mount on my lathe has one threaded stud attached to the mount, the end of this stud has a rubber tip on the end that buts up against the vertical surface of the lathe body. The rubber is fairly tight against the lathe but it does bounce with the motor, there are adjusting nuts I can try to adjust. I hesitated to apply more pressure on the motor mount, tightening the belt. I am not sure how much tension should be on the belt but I will give it a try.
 
What I'm trying to say is that it was less about belt tension & more about the motor mount plate being constrained. The difference is like night & day. Locked down its very smooth. Left untightened the motor does the harmonic bunny hop & the lathe wants to walk across the room. I suspect it will do this regardless of belt type, new or old. I specifically remember this same episode when I swapped belts & did not get the 2 nuts tight to the hinge plate which does the 'locking'. I misunderstood the principle. I thought the bolt screwed the plated in & out for tension. But really it could be a headless threaded stud. Its the nuts that sandwich the plate & keep it from pivoting. I'll get the picture off the camera but maybe your lathe has a slightly different arrangement anyways.

In terms of tension there are apparently some rules of thumb like how much deflection on what size of belt. I didn't bother looking them up. Mine was running pretty loose but if I had to guess its now ~1/2" just finger pushed mid span. Its a 2HP motor.
 
I tightened the motor mount set stud, it has significantly reduced the vibration! Thank you for the advice.
The belt is 1/2” wide but the distance between the pulley flanges at the top edge is close to 9/16” and the belt sits deep into the ‘V’, I think it rides on the bottom of the ‘V’ on the motor pulley and might be bottomed out on the lathe input pulley as well.
Previous responses stated the belt is not supposed to rest on the base of the pulley. Should I use a wider belt so that it bears on the sides of the pulley ‘V’ or replace with a link belt 21/32” wide?




 
canuck750 said:
Should I use a wider belt so that it bears on the sides of the pulley ‘V’ or replace with a link belt 21/32” wide?
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You can go with a wider belt, but since you have to change belts anyway, I agree with @YYCHM, and I'd go with a link belt.

Not sure you need the 21/32 though. That's damn near 3/4!
 
Looks like similar principle as mine, but you have even more room & access for hardware. It would benefit from flat washers on either side of the plate & maybe even lock washer on one side.

I seem to recall my lathe had a funky size, possibly metric based. And to confuse matters maybe the one you have now may not be the proper replacement. Wherever I got mine, maybe a NAPA or similar had a catalog & there was a bunch of similar sizes IMP & MET right in that range. I picked one with the same cross section & a bit shorter length than one I replaced to account for stretch. Before you order, see how much your pivot plate can accommodate. I'd say mine could take a belt 1" shorter than what I have. The section is important.

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