What alternative size motor pulley?

The top speed of my Myford M type is meant to be 840 RPM. I tested it today and found it is 600 RPM.

The motor plate states 1425 RPM.

I do not wish to change the large countershaft pulley which is 8.25" dia.

So I'm thinking I can increase the motor pulley which is currently 1.75" dia.

Having looked on line at cast iron pulleys (I don't want Ali) I currently have two pulley options for the motor. One is 2.25" dia, the other 3.25" dia.

As my arithmetic abilities are less than great, could someone please advise which is likely to be my better option to increase the top speed by around 240RPM?

Chris.

Of the ones you suggest 2.25"

3.25" would be doubling the top speed. If my thinking is correct.

As Hollowpoint says, 2.25 is the better option on the face of it. The arithmetic says 2.45 ins PCD is needed, so 3.25 is a step too far.

The arithmetic is simple enough - new pulley dia = 1.75 x 840/600 = 2.45

But I'm profoundly suspicious that all is not that simple for the speed to be that far off the mark. Lathes.co.uk says 874 rpm is the top speed, and 600 rpm is the next speed down. Seems a bit basic, but are we missing a trick here?

What speed is the input motor actually doing? If you can check the spindle speed can you check what the motor is doing?

Is this a flat belt final drive?

Some thing is wrong ! 8.25/1.75 = 4.71 1425/ 4.71 = 302 RPM

The ideal would be 4.75. 1425/840 = 1.696

8.25/1.696= 4.86" Should give you 820 Hope that helps noel

I'm assuming there is another second stage step down (actually step up), and the OP is simply considering an alteration to the first stage drive from the motor to the countershaft.

I'm just wondering if one of these stages is actually slipping, hence my wondering if a flat belt is in the mix.

So a list of the pulley train diameters and a measurement of the input speed would allow us to model the behaviour and see if it makes sense.

Edited to add "Step up\"

Edited By Simon Williams 3 on 14/02/2021 14:23:35

Ok great thank you all for trying to help me.

First off Simon, yes my mistake my top desired speed is 874 RPM not 840 RPM.

Yes its all flat belts.

I just went out and checked and the motor is indeed running at 1425 RPM as near as I can see using my vintage Smiths RPM counter.

Whats missing? Well my large countershaft pulley although period is not the original and is one step only.

So looks like I'm achieving one step down from top speed as stated on Lathes.co.uk. That's stating 12 speeds utilising back gear as well, I have 6 speeds which is fine for now.

Oh the countershaft speed I just checked is 285 RPM.

Hope that clarifies!

PS the 3 step pulleys on both the lathe and counter shaft are 2.25", 3.25" & 4.5" dia.

Chris -

No point in worrying about 840 or 874, I rather doubt if this exercise is that accurate!

On the basis of 1425 rpm in, step down is presently 1.75/8.25, so countershaft speed would be 302 rpm. There are several approximations in this calculation, so achieving 285 rpm as you have measured is within the realms of believability. Motor should be running a bit faster than the rated full load speed anyway.

To change the top speed, as discussed before the answer we've come up is go for the 2.25 inch option. By my arithmetic you should should end up with 600 x 2.25/1.75 = 771 rpm (ish).

But we still haven't understood why your present top speed is so far awry. If you could advise the outside dia's of the two pulleys making up the second stage drive we can follow the logic.

Edit:

Sorry, hadn't seen you had added that info when I pressed the button to post.

Edited By Simon Williams 3 on 14/02/2021 15:02:13

Based on your declared pulley diameters the final spindle speed works out to be 605 RPM. Which begs the question of why 874 rpm was ever mooted, but it is what it is.

Still everything points to the choice of 2.25 ins dia as the motor driving pulley. Go for it!

Thanks so much Simon!

I thought in a broad sense I had cracked why the higher speed was stated, in that the CS pulley was originally a 2 step, of what diameter I do not know, only that mine is a single and being of the right period/look and cast iron I am keen to keep it. If the original two step pulley's larger diameter was smaller than mine presumably the lathe would run faster?

I also presume the motor would have had to have had a 2 step pulley??

Lastly if the 2.25" option is going to only get me part way to my top desired speed, an I correct in my understanding a 2.5" pulley = 857 RPM?

(I'm thinking I might wait a bit to see if I can find one nearer the right size to achieve my desired speed increase.)

Agreed.

However there is a complication which confuses things, particularly with significant differences between driver and driver (correction driven) diameters.

The actual running ratio of flat pulleys is properly calculated using the effective neutral radius of the belt wrapped round the pulley. That means that the smaller pulley in the drive (the driven pulley in the second stage, the driver pulley in the first stage) is effectively BIGGER than it's outside diameter. The same is true of the bigger pulley, but this has a smaller effect.

In this example you got two corrections which must nearly cancel each other. But don't be too surprised if the calculated speed isn't spot on with the reality.

I'm sure I'm not alone in looking forward to seeing how the results turn out in due course. Do let us know.

Your comments about needing double pulleys for the first stage are right, at least to maintain approximately the same centre distance of the pulleys. But with flat belts this will make the whole arrangement bigger and bulkier.

Edit - typo in italics

Edited By Simon Williams 3 on 14/02/2021 15:43:12

Thank you so much Simon for explaining this so I can understand.(not always easy with me!)

Well the good news already is I have located a 2.5" dia pulley so that should be near enough!

Thanks again!

Chris.

The original pully sizes are motor pully 3 3/8" and 2 1/8"

Intermediate pully 8" and 9"

This is the pully drawing for the motor I made.

David

Ah thank you David, that's good to know for reference!

Chris.

Coming in late.

1425/600 = 2.375. 1425 /870 = 1.637931 1.75 x 1.637931 = 2.5375

So by changing just the motor pulley to a 2.5 " you would get 857, or close to your desired 870 rpm

With a 2.5 " motor pulley, all other speed would b be increased by a factor of 1.638.

Howard

Thank you Howard!

Chris.

Feedback! (-: Success!

Once the supplier re sent the correct size pulley that I'd ordered ie 2.5" I bored it out this morning and fitted the new belt. Checked the top spindle speed and its 860 RPM, quite noticeably faster, so job done.

Many thanks to all that helped me with this!

Chris.