Vibration when using higher speed ranges on Myford S7

I needed to turn some 1/8" diameter bar and set the motor-to-clutch belt to the higher of the two ranges. Even with the clutch disengaged the vibration was horrendous. I engaged the clutch to start turning and feared that the lathe was about to walk out of the shop.

On the slow range, it runs sweetly and the motor and clutch pulleys look as though they are running true. But on the high range, the motor and its pulley are wobbling about all over the place. Both belts are fairly new and I can't detect any lumps or irregularities in them.

Before I buy another belt, is there anything else I should check?

Thanks

Remove motor belt and check (clock) the motor pulley is running dead true, both axially and radially. Even though the belts are "new" there may be a "lump" somewhere along the length, the tighter the belt, the more it will be felt.

My S7 can be run flat out OK and as it is VFD controlled, at any other speed.

Remove chuck and run a bare spindle to illiminate out of balance there.

Check motor and or pulley for security on their respective mounts.

Remove everything from the motor drive train, run motor and observe shaft for run out.......

I changed to the linked belts, makes doing any work on the head stock very easy, no more hard spots in the belt and the vibration at the headstock is virtually nil. Should have done it years ago.

Not a lover of linked belts, so many pieces to have problems with.

I vaguely remember mention that myford back off the belt tension over the speed range. Perhaps you should check the coutershaft bearings for play. To check the large pulley fully for run out I suspect it needs to be removed and rotated on V blocks to see what the problem is. It will run true in place if the shaft is at an angle but the belt wont. A common simple way of checking that is by using some sort of straight edge

I was sucked in by a simple dodge on an ML7 but aimed at masking head stock bearing looseness - the belts were arranged to be much tighter than the they should be. Not that it masks it that well but it does "improve" things a bit.

John

-

Thanks for the replies. I managed to get a dial gauge into the large groove in the motor pulley and got about .018" TIR out of true. Took the pulley off and put some 5/8" silver steel in the chuck and fixed the pulley onto it using the pulley grub screw.

Using a mandrel handle, I re-cut both grooves - the larger at 38 deg included angle and the smaller at 34 deg.

What a difference! Not perfect on the higher speed, but much better and usable.

The only difficulty I had in re-cutting the grooves was trying to see the angle scale of the top slide. Need another scribed line at 90 deg to the standard one, I think.