Slowing a motor on a potter’s wheel

[Peter Cook 6Participant @petercook6]

Could you make a frame to support a countershaft above the motor? Move the motor pulley to the countershaft, and drive the countershaft using a suitably sized pair of timing pulleys and cogged belt.

It would need a frame fabricating to carry the countershaft in a couple of bearings, but there looks to be space.

[noel shelleyParticipant @noelshelley55608]

It’s belt drive ! All this talk VFDs and new motors when all that’s needed is to change the pulleys and a belt. Since the smaller pulley is already small I would change the larger one if there is room, if not then it will have to be a smaller small one. Picador is the name to look for, HAYLEY group may be able to help. Noel.

[HopperParticipant @hopper]

+1 on what Noel said. VFD on such a crude machine would be waxing a dirt floor.

Looks like quite a large pulley on the motor now. I would look for something half that size, or turn one up. Going to a smaller cross section Z series belt might allow even smaller.

Or could you adapt a DC variable speed motor off a treadmill and run it on a selected set voltage for lower rpm?

And as the original motor is just 1/3HP, it would be worth looking into cheap sewing machine motor with variable speed, even if you have to disable the rapid stop/start features.

I reckon I would be looking at that motor pulley first though.

[Michael GilliganParticipant @michaelgilligan61133]

I will stay out of the discussion

… but if my Wife was an exhibiting potter, I would want to do the best reasonable job of upgrading that machine for her.

MichaelG.

[Howard LewisParticipant @howardlewis46836]

Changing to a smaller section belt should allow a smaller pulley tom be fitted to the motor.

Failing that, how abnout arranging a two stage belt drive using a countershaft?

There appears to be plenty of room to fit a countershaft above the motor, with the secondary belt running down to the existing driven pulley.

Howard

[noel shelley]

On noel shelley Said:

It’s belt drive ! All this talk VFDs and new motors when all that’s needed is to change the pulleys and a belt. Since the smaller pulley is already small I would change the larger one if there is room, if not then it will have to be a smaller small one. Picador is the name to look for, HAYLEY group may be able to help. Noel.

I’m liking this idea.

Will search for pulleys following your suggestion.

Someone asked about how much speed reduction; she’d like 50% or more if possible.

(it should be noted that this is her secondary wheel; she has a fancy expensive modern one, but wants two so she can easily switch between terracota-type clays and white porcelain or white stoneware clays.)

[FulmenParticipant @fulmen]

50% (as in half) sounds ambitious. Can you raise the drive shaft to give room for a larger pulley? A smaller belt might also allow for a smaller motor pulley.

[mrcharlyParticipant @mrcharly]

Not possible to raise the drive shaft, it has to be parallel with the radius of the platen.

[Adrian R2Participant @adrianr2]

An intermediate shaft could be made to sit at the same approximate location as the motor is now. The motor could then be raised up on blocks above the intermediate shaft with a new belt arrangement to drive it, select pulley sizes to suit the preferred new speed range? You could even get a pair of stepped pulleys as per many old lathes to give std/low/lower choice.

[noel shelleyParticipant @noelshelley55608]

Looking at the picture the reduction ratio is 5:1 ? If you want another 50% then one option would be a 10:1 wormdrive gearbox, this type of box is found in alsorts of machines, Kiddies rides that rock back and forth, a redundant one may be found at an amusement arcade at the sea side. This gives the initial reduction, it is a simple matter then to fine tune the speed by pulley ratio if need be. The input to the box could be direct or belt drive. Whilst modern ones are often italian, Croft are a British maker, beautiful ! Good Luck Noel.

[SillyOldDufferModerator @sillyoldduffer]

I guess MrCharly is hoping for a cheap easy to fit answer to this problem.  The picture of the mechanism is discouraging, not because it;s complicated, but because there’s no room for simple modifications.

For example the motor is already fitted with a tiny pulley, whilst the wheel drive shaft pulley must already be as big as will fit : forget simply changing the pulleys!   Same problem at the wheel end – the leather drive wheel and clutch have to align with the drive disc.

A countershaft would do it, but not much room for one.  I think it would have to be bespoke, a fairly significant amount of work even if one has a junk-box full of bearing blocks and other goodies.

A fixed gearbox might be made to fit, but the input and output shafts have to connect the motor and wheel in alignment, unlikely to ‘just happen’.

Possible to speed control some types of single-phase motor, but not straightforward.  As many single-phase motors can’t be speed controlled, MrCharly might waste a lot of time and money discovering that his motor is unsuitable.

I think the easiest option is to replace the motor with one that has speed control.   Either a industrial sewing machine motor assembly or a 3-phase + VFD.    The sewing machine assembly isn’t plug and play because their motors spin at 4000 or 5000 rpm.    In comparison, a 3-phase motor spins at much the same speed as the existing single-phase, and a basic VFD is easy to fit.   Main complication I think is the small pulley – the existing one might not fit a new motor.   Not a problem, because pulleys to fit new motors are widely available.

If money is no object, buy the good lady a new potters wheel.   For a mere £3500 you get a built in seat with a 550W motor, 0-240rpm, and reverse.    More important, ‘She Who Must be Obeyed’ cannot object to MrCharly spending the same on his hobbies!!!

Dave

 

 

 

 

 

 

[peak4Participant @peak4]

Whilst I earlier mentioned single phase VFDs, now I’ve seen the photos, I’d also agree with others that Poly V belts might be the easiest solution.
Pulleys down to 20mm and possibly smaller are ready available from eBay, and larger ones are available, both as plain and taper lock for the driven end.
When I needed some for a wire wheel/polishing gizmo, I used a pulley from a car alternator and I think one from a water pump, though in this case a larger difference in sizes would be needed.
This link suggests that driving pulleys down to 9mm are useable.
https://www.acorn-ind.co.uk/insight/poly-v-belts/#:~:text=Designed%20to%20have%20a%20larger,with%20a%20traditional%20V-belt.

Bill

[Ian PParticipant @ianp]

Dave, although we do not know what skills, equipment and budget MrCharly has, its hard to advise on the best approach but to me I would not describe the motor pulley as tiny, yes it is small but I think it could be half the diameter, possibly even with the same belt.

Highly unlikely though that there would be an off-the-shelf pulley so one would have to be made. Converting to PolyVee would involve the purchase of two pulleys (with the right bore sizes) and a belt whereas just modifying or making the motor one smaller would be a cost effective and involve the leastest engineering, although the motor to base slots might need extending.

Running a Vee belt over a tiny pulley is generally not good practice but this is not a precision machine and shortened belt life is not a major drama. A better compromise would be to change to an ‘A’ section belt running on the correct profile motor pulley and leave the large pulley as it is, the larger wrap will be more tolerant of the belt to sheave contact errors.

Ian P

 

[John Doe 2Participant @johndoe2]

Any progress on this ?

I am wondering how domestic fans change speed ?

They have single phase induction motors – the type with the two copper links around the core plates, and usually four switches to give three fan speeds plus Off.

How do these electrically change the fan speed ? – is it just crude series resistors in the mains feed to the motor, and if so, would that work for this application ?

Or would torque be reduced too much ?

 

[mrcharlyParticipant @mrcharly]

I’ve been waiting for pulley to turn up.
Arrived on Sat, but no time free to fit it (too many grandchildren visiting).

Might manage to fit it tonight and will see how that goes (fitting time could be 5min, or several hours, depending on how long it takes me to get old pulley off; it has been operating for many decades in a damp, dirty environment).

Should reduce the speed by a third.

[bernard towersParticipant @bernardtowers37738]

Course you could always go for the 800rpm version of that motor

[bernard towers]

On bernard towers Said:

Course you could always go for the 800rpm version of that motor

You are the first person to suggest that.

 

Sources for said motor?

[John Doe 2]

On John Doe 2 Said:

…

I am wondering how domestic fans change speed ?

They have single phase induction motors – the type with the two copper links around the core plates, and usually four switches to give three fan speeds plus Off.

How do these electrically change the fan speed ? – is it just crude series resistors in the mains feed to the motor, and if so, would that work for this application ?

Or would torque be reduced too much ?

 

Domestic fans reduce speed by limiting the power available to the motor with a resistor or, more likely, a capacitor.   When the fan is spinning, it does work (scientific definition!), by moving air.  Reducing motor power drops the fan’s RPM because the air acts a brake, and the motor is pushed back towards a stall.  Power, torque and speed all vary horribly with load, but the arrangement is ‘good enough’ in a fan because the braking force is more-or-less constant, allowing a stable power/rpm balance to emerge.

The same power-reducing speed control method isn’t ‘good enough’ for a potters wheel because the braking force caused by throwing clay varies wildly depending on what the potter is doing.   And a drive system that doesn’t maintain power, torque and speed will make the potters job much harder.

Worth mentioning that there are several different types of single-phase motor, and many of them can’t be speed controlled.  For example, the motor supplied by Myford contains a centrifugal switch needed to disconnect the run winding once the motor is up to speed.  Applying any form of speed control to this type of motor will confuse the hell out of the centrifugal switch and in due course is likely to overheat the motor and burn the switch contracts out.

Dave

 

[MacolmParticipant @macolm]

Fans are a special case because the “cube law” drag falls off faster than the motor torque as the current is reduced, so this method of speed control only works for a fan.

[not done it yetParticipant @notdoneityet]

So far, I’ve only seen that the OP’s wife wants it to run it slower. There was real reference to whether the top speed was never again expected to be used.

Changing the motor drive pulley to a smaller one would reduce the top speed.  Indeed, changing to a multi-V pulley system would be a simple change to achieve a slower top speed.

It is only a fractional HP motor.  No real problem, apart from cost, to change to a 3 phase motor with VFD.  The variable speed would still be available, so what is the problem?  I don’t see any.  The VFD needs to be in a dry enclosure.  The programmable speed control can be operated by a low voltage controller – basically a rheostat/potentiometer in some form (likely a 1k potentiometer).

My lathe and mills, with mechanical variable speed/gear selection drives, are fitted with variable speed control through 3 phase motors.  I rarely need to change the mechanical options from the median variable belt position or intermediate gear.

[John Doe 2Participant @johndoe2]

Thanks SoD and Malcolm for the explanations – I suspected something like that.

As for those suggesting VFDs and new 3ph motors etc. – think of the cost and all the trouble it would involve ! Fitting a new motor, making brackets to hold it, enclosures and wiring for a VFD with all the associated switch-gear and controls. He doesn’t want a Saturn V rocket; just needs the wheel to go a bit slower !

 

[John Doe 2]

On John Doe 2 Said:

[…] As for those suggesting VFDs and new 3ph motors etc. – think of […]

 

I did think about it, when the question was first asked

… I thought then, as I think now [diametrically opposed to your opinion]

MichaelG.

[Howard LewisParticipant @howardlewis46836]

DAFT suggestion from a mechanical Engineer.  Since it is an AC motor, how about inserting a diode into the Live lead at the motor end?

This will only feed one half of the cycle into the motor, probably reducing power, as well as speed

Howard

[ChrisLHParticipant @chrislh]

If I were faced with the problem I would look very carefully at the polyvee solution (electrical ignorance ruling out any moves in that direction) but with one simplification. I would use a large pulley with a plain cylindrical surface which, if I were lucky, could be fashioned from the existing large pulley. Not original but noted as working very well on several of the washing machines and tumble driers I have owned.

[mrcharlyParticipant @mrcharly]

I fitted the new pulley last night. Only a bit of percussive maintenance required to get old pulley off.

The pulley sizes have changed from 67mm to 43mm, so about a drop of speed to 2/3rds the original.

The potter hasn’t checked it yet.

As some people have suspected, steady speed under varying load is absolutely key.

If this speed reduction isn’t sufficient, I suspect I’ll go down the route of adding a third pulley. Although that would involve messing with brackets etc, it would be easier than changing motor and controllers; it is difficult to create a dry clean enclosure inside the wheel.

 

 

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