Pulley size help required.

We have a problem with a 2speed pulley system on the Atlas 12 X 24 lathe. The first set of pulleys from the motor is paired for two speed operation. The high range pair is satisfactory, but the smaller pulley on the motor is smaller than it should be, and is outside the adjustment range for the belt tension. I think the motor pulley pair is not the correct size and it also looks out of place being machined from solid rather than the rest of the pulleys which are castings. The drawings for the machine are no help as only a single speed is shown at that point. There is room inside the casing for a double pulley system from the motor.

The undersize pulley could be machined off and be replaced with larger to keep the spindle centre distance the same in both speed settings if I could calculate the size. I could even make one if I knew the diameter,any help would be appreciated.

Edited By old mart on 16/06/2021 18:04:56

Assuming someone had it working at some point in time, could it be that either the belt has stretched/worn or is the wrong belt fitted?

Do you have any speed plate or table of speeds? you should be able to calculate the correct diameter.

If one pair works, then all you need is to make the other pair use the same ‘Lap distance’

Hard sums required if you’re doing it mathematically, but a doddle [to get close enough] if you can do a few iterations using CAD … just draw tangents.

MichaelG.

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Edit: or you could try this: https://www.omnicalculator.com/physics/belt-length

Edited By Michael Gilligan on 16/06/2021 18:29:04

FYI ... calcs to work out your driver pulley to give you the req'd final speed...

Scanned from my Newnes Mech.Eng. pocket book.

George.

Those pulleys are outboard so just buy another belt and hang it on a nail. Swap them over if you're using the slow speed.

Pete you are a genious, that is the easiest solution by far.

Old Mart,

I've got an Atlas with 2 speed primary drive. picture attached. Daft question but "do you have the motor pulley as my configuration? small motor pulley to large driven pulley, If not, try reversing the motor pulley on its shaft to look more like mine That works fine for me.

Hope this helps

Bob D

1. rt,

Nah … Pete is disqualified : He didn’t answer the question

MichaelG.

Question.

In my photo is the belt set to high or low speed ? With the second cone set to top speed it does about 600rpm with a 1400 rpm motor.

Bob D

He did better, by suggesting a simple practical solution. Which does require disqualification....

The pullys are in the same configuration as the photos, yours is in low speed I will take a picture tomorrow. Buying a belt to fit the mismatched sizes will save me a lot of time and effort. I'm thinking that the larger of the motor pulleys is bigger than it should be. It is a bit of a mystery, as Mike bought the belt and it matched the size in the manual, and the higher speed is the one that works, the small motor pulley is too small. The belt guard door is having to be lowered 20mm and moved rearward 25mm to make room for the drive system.

It will be interesting to find out just how fast top speed will turn out to be.

Edited By old mart on 16/06/2021 22:05:25

Even buying a second belt will require you to calculate the length so you don't have to adjust the tensioner when changing belts.

Calculations can be tedious when you want two belts for two "random" sized pulley pairs to operate at the same distance. Done it a couple or three times over the years with mixed results. Back in teh early 1980's I discovered that you can't rely on Picador and Machine Mart pulley diameter specifications to have the same relationship to pitch diameter. Thought I was so clever when my spreadsheet said mix'n match imperial Picador with metric Machine Mart A section pulleys worked just so.

Some comments when I discovered the specification discrepancy spoilt my maths leaving one belt slack.

Ended up shoving a tensioner pulley on a swinging arm in the system. Operated via a spring and over-centre lever it not only sorted the tension problem but also made an adequately effective clutch. Obviously not safe to leave running free whilst playing set up but just the job for easing into drive or quickly stopping. I made mine from scratch but a quick search of the car parts catalogues for an auxiliary drive belt tensioner should provide most of the parts in a pre assembled unit relatively affordably.

I've since wondered whether the common turnbuckle tension setting system could be re-worked to include a spring so a factory tension release lever could be safely used in that manner.

Given the huge range of belt lengths available the discrepancy in centre distances for the same belt on nearly the same pulley pairs is unlikely to be more than about 1/4".

Clive

Since there were several pulleys knocking around in the junk boxes, I thought I would try out a likely candidate. First, I measured the pulleys. The motor pulleys are 2" and 5 1/4" and the layshaft ones are 7 1/2" and 9". It is the 2" one that is too small. One of the single pulleys was 2 3/4", and when I tried it, it was too small. There was also a triple made by Picador and the centre pulley was 3", and was exactly right. I was surprised that the small 1/4" diameter increase would make such a difference.

I shall be machining both pulleys and fixing them together, it will cost nothing.

Edited By old mart on 17/06/2021 18:02:51

Hi, well the 3" one would give you about 3/4" of extra belt travel for every RPM, that it does over the 2-3/4" one, multiply that by the motor RPM, which soon adds up to a lot more revs on the driven pulley.

Regards Nick.

Old mart,

Your photos show your Atlas to be a later model than mine and I'm glad you can see a solution.
A note of caution regarding your speed range with the 3" pulley on the lower speed pair. You will then have raised your lowest spindle speed by 50% (3" instead of 2" > This may suit your purposes Ok but it depends on what your motor speed is and the type of work you will be doing as to what part of the overall speed range you will use.
I used an online belt length calculator ( https://www.omnicalculator.com/physics/belt-length ) and from it, assuming my scaling estimate of 8" for your centre distance in the photo is right and presuming a motor speed of 1400rpm your 2 choices of layshaft speed would be 466 or 985 rpm.
If you opted to keep the 2" pulley and fitted an inch shorter belt ((35" instead of 36" as indicated for 8" centres) and then made a 4.4" dia. pulley as the larger drive pulley you could expect a slower and wider ranging layshaft speed of 311 or 823 rpm.
As I noted before the choice depends very much on what your expected work range is.

Regards Bob D

Edited By Robert Dodds on 18/06/2021 12:24:06

Edited By Robert Dodds on 18/06/2021 12:49:20

Edited By Robert Dodds on 18/06/2021 12:50:33

I get the impression that the whole set of speeds for this lathe will be from 10 to 20% faster than they should be. The guard door is having to be lowered and moved rearwards to clear that 5 1/4" motor pulley. Fortunately there is plenty of clearance around the upper 7.5"-9" pulley to do this. The motor is in working condition to test the range of speeds, the last owner had simply fitted a plug to the cable from the motor. Not exactly my idea of a proper way of running a machine tool.

I could keep the 2" pulley and reduce the 5 1/4" as the motor pulley is one piece and machined from plate, not a casting. It would be easy to mount it on a faceplate if some tooling holes were drilled in it.

The list of options available to me has now reached three. I am starting to lean in the reduction of the larger motor pulley size as suggested by Robert, as being my easiest solution.

More measuring will be done tomorrow.

Edited By old mart on 18/06/2021 20:33:14

Edited By old mart on 18/06/2021 20:35:45

Edited By old mart on 18/06/2021 20:38:16

Edited By old mart on 18/06/2021 20:55:59

I had a look at the belt system on the Atlas. Keeping the 5 1/4" motor pulley will give a maximum speed of about 1980 rpm. This fits in quite well with the speed of 2080 rpm on the speed label. The difference might be between 50 and 60 Hz.

So I decided to replace the 2" motor pulley with the 3" one, they both share the same 8.5" shaft spacing. The pulleys are fitted together with Loctite 638, and I am part way through adding four 5mm screws for luck.

We will still have to move the door to clear the 5 1/4" motor pulley.

The 5 1/4" and 3" pulleys are now joined, I used Loctite 638, but have added four screws for extra security. The screws I had are going to be replaced with better ones which will be secured with Loctite 270. I will be leaving that plain diameter but will remove the centre part to just above the screw heads.