Thread: Good vibrations .........maybe not

My old Sheraton 9 lathe has always had a vibration , at just about any spindle speed above 400 rpm , it seems to get worse as the rpm's go higher

The motor is single phase and I think the motor is the main problem , it is bolted directly to the swing bracket arm that tensions the V belt to the headstock .

Maybe rubber motor mounts would be an idea ? Or a 3 phase motor and a VFD ?

Any suggestions to fix this ?

Mike

I would find the cause before contemplating remedies.
E.g. Take the belt off and see if the motor vibrates first. Then disconnect the clutch or first gear train to see if the driven pulley has an imbalance (very likely). Follow the drive train one element at a time - from either end - to find the cause.
At the other end, the chuck may be the problem. Remove it and see if the spindle vibrates without it. Then disconnect the spindle somehow (e.g. disengage backgear AND direct drive) see if the vibrations stop.... etc.
Once you identify the element that is causing the vibration, see if you can mount it between centres and find the heavy spot and either drill into that part or stick on balance weights opposite.
Good luck. This is often tedious, but just replacing one component on a hunch is rarely efficient.

Mike, if it is the motor,
- then the vibrations must be there even when the drive belt is removed? (assuming it is a belt drive lathe)
- then the vibrations would not get worse at higher spindle speeds, since the motor always runs at the same rpm?

The torque vibrations inherent to all single phase induction motors, remain the same regardless of spindle rpm. They exhibit themselves as a pattern on surface finish. They can often be made visible by a glass water put on the headstock (faint concentric circles on the water surface).

From your description, me thinks you may have an off balance chuck. Have you removed the chuck? Are you using a spliced Vee belt? Try a better quality belt or even better a link belt.

Chris

As others have said sounds like an out of balance problem if it gets worse with speed.

Ok thanks for the replies

Today, I did some tests with various parts disconnected . I ran the lathe with:

1. Motor only running and V belt removed to the two speed 10 " counter shaft pulley .... the motor and bed behaved smoothly with minimal vibrations

2. I then ran the lathe with the V belt connected from motor to the 10" Countershaft pulley .. the vibrations returned .

I have concluded that the 10" twin diameter pulley is out of balance , or the counter shaft pulley ? The counter shaft bushes and 7/8" shaft show scoring , but wear is minimal , about .001"

I have made up a static balancing jig ..will use my milling parallels as a track , I noticed the students at TAFE checking out grinding wheels from surface grinders etc, using this method

BTW does anyone know of a source for 7/8" shafts ? I guess mild steel would be OK ? The hardened stuff is hard to find in imperial I have found out .

Mike

Originally Posted by morrisman

...........I have concluded that the 10" twin diameter pulley is out of balance , or the counter shaft pulley ? The counter shaft bushes and 7/8" shaft
show scoring , but wear is minimal , about .001"

I have made up a static balancing jig ................
Mike

Mike,

just in case you are using one of these old automotive style V belts. I mean the spliced A-section ones as used for water pumps and small alternators. Especially if it is already somewhat old and the rubber has become stiff. These can cause bad vibrations on a lathe, that may easily be confused with an out of balance pulley. It comes from the thump-thump-thump as the stiffened splice in the belt passes each pulley. They are made from rubber that perishes on its own, regardless if you use it a lot or not. Very much unlike the modern serpentine polybelts used in cars today, which are made from Polyurethane and do not perish and are solvent resistant and are made as an endless one piece (eg not spliced together). Chris

I managed to remove the pulley from the countershaft , it had a tapered pin locking it but i pressed off the pulley in the super crap auto press and the pin broke

I used the German level to level the static balancing stand , the mill vice spacers sat on top .

The pulley exhibits a definate out of balance . I drilled 3 holes but that isnt enough . A weight added opposite has done the trick

i had a similar problem on an A model hercus it turned out to be the pulley on the motor wasnt on tight.I too blamed the counter shaft pulley.then the belts even changed the vertical bracket
How come its always the last thing you try that fixes it???
JOHN

I can't recollect EVER having an electric motor out of balance.

They are universally as smooth as.

Pulley or wonky belt is generally the cause.

Good to see you got it sorted.

Rob

I had the same problem (again with a 9inch Sheraton) I finished up drilling about 20 9mm holes in the heavy side of the pulley. See my post/thread (I can never work out which is which) lathe balance problem 12/04/2011
Ken

Originally Posted by morrisman

I managed to remove the pulley from the countershaft , it had a tapered pin locking it but i pressed off the pulley in the super crap auto press and the pin broke

I used the German level to level the static balancing stand , the mill vice spacers sat on top .

The pulley exhibits a definate out of balance . I drilled 3 holes but that isnt enough . A weight added opposite has done the trick

Originally Posted by Kentower

I had the same problem (again with a 9inch Sheraton) I finished up drilling about 20 9mm holes in the heavy side of the pulley. See my post/thread (I can never work out which is which) lathe balance problem 12/04/2011
Ken

Hi Ken

Yes that is the same pulley that I have on mine .



http://www.woodworkforums.com/f65/la...almost-133876/

You would think that Sheraton would have been aware of this problem and feedback from customers would have alerted them to this

Mike

I have the lathe back together after balancing the counter pulley .

There is a definate improvement of about 70% . I would like to remove the last 30% which I feel is in the bull gear on the main spindle . The lathe is now useable up to the 2nd highest rpm around 1000 .

I tried sticking neo magnets onto the bull gear perimeter and watching for a sweet spot , but it does not seem to have a high iron content ? The magnets dont stick very well and come off above 800rpm

I am watching a bottle of liquid on the bed as my test reference ! The liquid shakes more with more vibrations !

Mike

I have been stuffing around with the old Sheraton , it can be frustrating but I think I have it running smoothly at last.

I did a search and found a useful thread on the PM forum regarding balancing older lathes

The poster said he did this balancing as his profession .

Anyway he went on about " natural frequencies " . He suggested that by altering the V belt tension on the main spindle , this will change the natural frequency of the lathe .

I tried it and damn me if he is correct The change is remarkable - the vibrations I had around 800 rpm have dissapeared .

Mike

It is the splice in the belt that causes this effect. If you replace it with a link belt, the tension will not matter anymore.

Originally Posted by cba_melbourne

It is the splice in the belt that causes this effect. If you replace it with a link belt, the tension will not matter anymore.

You are correct Chris.

The same " expert " suggested that a link belt is the better way to go . I will try and find that thread again and link it here .... Mike