Lathe problem

Please help. I have a silly little problem that is causing me grief way beyond what it should be doing.

My Smart and Brown has a belt drive and high and low speed ranges which are controlled by shifting the belt from a small pulley to a big pulley at the bottom and big to small at the top.

So low speeds have the belt on the small pulley at the bottom and large at the top. Now I want to speed things up a bit so I have changed to the high speed range. The problem is that the belt won't stay on the pulley. It slips off every time I start the lathe.

The bottom pulleys are one piece with the smaller one furthest out on the shaft. This has a small lip on it so the belt can't slide off but the larger pulley has no such lip and when I start the lathe the sideways jerk of the belt pulls it off 9 times out of 10. I have tightened up the tension to the point where I can barely push the lever back up but it makes no difference.

A colleague seemed to think I could tilt the motor/gearbox assemly but if I can I can't work out how to. Most of it is out of reach of the hatch in the case and I can't get behind it as its up against a wall (and weighs 3/4 of a ton).

Its driving me nuts how do I sort this out.

Is it an old original belt?

A few pictures would probably help

No its a new belt. I can get some pics tomorrow.

Did the old belt slip in the same way?

Have you still got it, to compare with the new?

Perhaps these belts need to be "dressed" in some way

I'm only fumbling in the dark btw but toolroom lathes are high end gear.

Sometimes Apprentices weren't allowed anywhere near them

Edited By Ady1 on 19/07/2012 19:19:38

I don't recall the old belt doing it but it snapped shortly after I bought the lathe and I didn't keep it.

If it's flying straight off the pulley/s they must be very badly misaligned?

Hi there, Wolfie,

Is it a flat belt or a vee belt?

Pulleys for flat belt should be 'crowned', that is the centre of the belt path should be a bit bigger than the edges. The belt will always climb to the maximum diameter part of the pulley and stay there, a bit counter-intuitive. Flanges or cheeks on flat belt pulleys don't keep the belt on - the belt will promptly climb over them!

As Paul has written, vee belt pulleys need to be well aligned.

Best regards,

Swarf, Mostly!

Its a flat belt, will go up and videocam it later today (my workshop isn't at my house)

Found a picture on the net, this show the belt in the slow range position, the pulley for the high range is to the right of the belt behind the tensioning handle.

Edited By Wolfie on 20/07/2012 07:23:53

Edited By Wolfie on 20/07/2012 07:24:33

More pics here...

**LINK**

Wolfie, If your flat belt has a joiner in it you might take it out of the lathe and lay the joint on a flat surface. Check the two end parts of the belt are in line and not skew to one another, this can cause the belt to run off.

is this it?

Maybe I'm cock eyed but it looks like that belt is pulling very slighty back the way, so it rests against the lip of the larger rear pulley

http://www.lathes.co.uk/smartbrown/img11.jpg

You should be able to adjust your headstock pulley section independently of your spindle

I wouldn't make the belt too tight either, this is not good as far as the bearings are concerned

As a completely clueless tweaker it looks like the pulleys need to be slid along their respective shaft at the top OR bottom of the belt until there is a teeny amount of pressure placed upon the lip

So the bottom pulley is just a wee bitty further to the left than the top pulley

The only adjustment part may be at the headstock end, so the headstock pulley grubscrew is released from the spindle and the pulley is slid over ever so slightly to the right

2cents

gl

Edited By Ady1 on 20/07/2012 09:35:38

I've been for another play this aft.

Robert you are right, the threaded bar with a nut on to the right is the tensioner, the handle is just to change the belts. The pulleys are in line.

In the end I took the pulley off and found the larger pulley coated with some kind of slippery shiny stuff. Cleared that off and tensioned it up again and its a lot better. I'm gonna get some of that spray sticky stuff too and spray that on the back of the belt.

Back in business!

Hi Wolfie,

the belt should not need to be too tight, as Swarf M. says the pulleys are slightly crowned over the width, you can see that on the picture that Ady put the link to, the larger pulley shows the slight crown. It is that crowning (aka 'barreling' which keeps the belts on the pulleys, it doesn't need to be much in order to work. Too tight is likely to exacerbate the problem of the belt slipping off the pulleys. As Ady also says you will only shorten the life of the bearings on the spindle and motor.

When serving my apprenticeship I worked with a guy I can only call a 'master' turner. He had a lathe which originally ran from overhead shafting. The 'new' electric motor was mounted on a wall bracket and the original drive pulleys on a countershaft drove the lathe. The motor and countershaft were on a platform hinged at the centre, on the wall bracket and the motor was arranged so that it's weight tensioned the belt. My tutor changed speed while the lathe was running by hitting the belt sideways and it would either slip onto the smaller pulley or ride up onto the larger one. It would then move on the countershaft drive pulley until it was aligned. the belt was around 12 foot long when off the machine.

In the days of overhead shafting that was how the speed of your machine was changed. A machinist couldn't stop the whole shafting system just to change speed. The pulleys should be reasonable aligned to work, oil or grease on the pulley shouldn't make much difference unless it is really excessive, these belt systems would become covered in the stuff and leather belts were often dressed with oil to preserve them.

It is possible that your pulley shafts became misaligned when changing the belts, there is usually a method of setting the angle of the motor drive shaft usually a combination of adjusting screw threads or bolts. I'm not sure how this is managed on the S&B but perhaps another owner or Tony at Lathes.co.uk might be able to offer advice or a manual. It is also possible that a a manual is available through the inter-library loan system or stored on a user forum where files can be uploaded.

Good luck,

Terry

Edited By Terryd on 21/07/2012 02:38:54

It certainly looks out of alignment

Bob

It'll be an issue with belt tracking, often this is adjust by slight changes in the angle of one or both shafts (effectively adjusting how parallel tir axes are in relation to one another). If the machine has had a lot of use check that the crowning hasn't worn off too.

Wolfie,

As Terryd indicates ... flat belts should not be run tight

It is essential that the belt can "ride up" onto the crown of the pulley, which [maybe counter-intuitively] it will do if the pulleys are aligned.

Terry mentions the old line-shafting ... Over each machine there were two pulleys: one was "Fast" [i.e. fastened, or fixed] to the line-shaft, and the other was "Loose" upon it ... the belt was moved from one to t'other as a local on/off for that machine.

As you have probably deduced; his is the origin of the expression "Playing Fast and Loose"

MichaelG.

This is worth reading

Edited By Michael Gilligan on 21/07/2012 08:26:37

When I was at Tech in Dunedin in the early 1960s, our work shop was mainly run by line shafting, the old lathes were gear headed, but to stop the drivebelt had to be put over to the loose pully. The only lathe with its own motor was the Chip Master, I think the motor on that was about half of the motor that drove the rest of the workshop. When the school started the workshop was steam powered. Ian S C

After some use it started doing it again. Inspection showed more of that shiny stuff on the pulley. Its coming off the new belt I think. The pulleys are definitely in line

Anyway problem sorted, belt sprayed with spray glue!

Hello.

Just out of curiosity...
When I was a kid (12 or 13 years old) I saw looms in a weaving factory ran from overhead shafting. I could see the belts being cleaned with kind of a shoemakers knife edge (not the cutting surface) and after that a large (50mm diameter) stick made of some material was used against the contact surface of the belt. I remember the material the stick smelled strongly pine resin.

I know this was a regular procedure when the belt fall from the pulley.

Best regards
Dias Costa