Lathe Drive Systems and Belts

Although the restoration of my recent lathe acquisition (a Flexispeed Mk 2) is not yet complete it's at the point where I starting to think about the drive system and the design of a suitable cabinet for mounting the whole assembly.

So I would appreciate a bit of advice.

The drive system will comprise motor >> countershaft >> lathe.

Question 1:
The advice on lathes.co.uk states:

"Just relying on the weight of a motor to do the job will invariable cause "bounce".... "

I fully understand the "bounce" risk. But it seems to me if the motor were used to maintain belt tension then in most cases that would be a pretty significant weight pulling on the belt/pulleys. So that got me thinking - how much tension should be provided and how to judge it. Is there any rule-of-thumb to follow here?

Question 2:
For the drive system mentioned above is tension between motor>>countershaft and countershaft>>lathe equally important and do both "halves" need a method of adjusting tension?

Question 3:
The physical layout of this drive system motor>>countershaft>>lathe obviously has a direct bearing on the total footprint of the setup (including the size of cabinet I need to make to house it). So this leads me to another question. Is there another rule-of-thumb on spacing between motor and countershaft and/or countershaft and lathe?

Question 4:
Lastly for the belts. The V-belts I currently have (smooth surface, 9mm and 13mm wide) seem perfectly serviceable but of course if I change the physical layout I'll likely need different size belts. Is there any general advice on best choice. For example, I see some use "toothed" belts on smooth pulleys - any advantage or personal choice?

The Myford ML7, and its successors rely on the weight of the motor to tension the primary belt (A section )

My present lathe uses B section belts, and the again the weight of the motor provides the tension for the primary drive. The secondary belt, inside the Headstock is tensioned by an over centre arrangement, where the length of the connection between the external lever and the countershaft carrier is adjustable in half turn increments.

The original single phase motor was 2 hp, the three phase motor is 1.5 hp.

In cas of mishaps / over loads, the gravity tensioned primary belt can slip, reducing the risk of damage.

An A section V belt in conventional V pulleys can transmit quite a lot of power A single belt can easily transmit 3/4 hp.

For our usual hobby machines, a single A section belt usually suffices.

My Mill Drill transmits 1.5 hp through single A section belts. The secondary is over 20 years old, so well within its capacity!

Tension should be such that halfway along one side, the belt can be moved no more than 12 mm. without using undue force.

For years, private motor cars used a single A section belt to drive coolant pump, alternator and cooling fan.

All of which could require more than 3/4 hp!

If more power needs to be transmitted (As on higher horsepower industrial machines, the belts are duplicated or triplicated. The two cylinder air compressor for the brakes and suspension on Bristol buses was driven by triple A section belts, with the compressor having a screw adjustment to set the tension. )

Howard

Is this to use or to admire? If for use you might want to look at DC or even brushless motors - more compact, more efficient, variable speed so no need for countershaft or belt changing. I have been considering this for mine, I tried out a recycled cordless drill motor which worked OK but I didn't like the aesthetics of it.

See also this recent discussion:

**LINK**

Howard. The standard ML7 motor was I/2 hp not 2 hp. The super 7 was 3/4hp.

Roy

I would try to avoid the issue of "motor bounce" if you can. My bandsaw suffers from it, and it drives me round the bend. So much so, I'm in the process of trying to fix it with some form of positive belt tension adjustment.

I first noticed this with the Atlas 12 x 24 which uses the swinging motor to accommodate changing the first stage belt double speed pulleys. Having a couple of offcuts of that tungsten alloy Densimet added to the motor weight did not interfere with changing speeds and should have reduced the problem.

Re bounce, remember 1HP is defined as the amount of power needed to lift 550lbs one foot into the air in one second. Motors are usually clamped because that amount of power can deliver a lot of force! How much bounce there is in practice depends on the geometry of the belt, but bounce is likely to cause trouble.

Rule of thumb from my dad: car fan belts should tightened such that the long stretch can be moved no more than 1/2" when pushed hard with a thumb. Seems about right.

Dave

Howard, The ML7 (& S7) do not rely on the weight of the motor to tension the belt. Well they shouldn't. There is a tie bar (also called base link) for the motor platform. Theis is used to set the belt tension.
Extract from ML7 manual:





The weight of the motor is suggested as a inital setting but the motor is held by the angled link seen in image above.
Letting the motor move is bad practice.

Robert.

That link might be added to the Atlas, I must have a look into the possibility.

Thanks for all the replies. There's some really useful guidance here.

It just leaves my Question 3
>>Is there another rule-of-thumb on spacing between motor and countershaft and/or countershaft and lathe?

Thinking about his a bit more, the only difference I can see is that the greater the distance between them (and therefore the longer the belt) there will be less belt wear - although I suspect this isn't a big issue. Is there some other consideration I am missing?

On the point:

>>Is this to use or to admire? If for use you might want to look at DC or even brushless motors

It is very much to use. But, and I accept this is purely where personal views and preferences come into play, my preference is to keep it original. I accept the benefits of alternative motors and electronically controlled speed but, well that's not the way it was designed and built in the first place. I suppose this attitude comes from my other interest of clock repair and restoration (which is the sole reason for having a lathe incidentally). To draw a parallel, no matter what the problems with an old clock I wouldn't dream of removing this original movement and replacing it with a quartz movement. As I said, not disagreeing with the benefits in any way, just explaining the reasons!

Thanks

Andy

If it helps: the motor for my Flexispeed is screwed firmly to the wall behind the machine. The belt to the counter-shaft is a a tight stretchy silicon belt. The belt from the counter-shaft to the lathe itself is another, slacker, stretchy silicon belt. I have a couple of roller bearings on a crudely hinged arm arranged so that the bearings push down on this belt to tension it. I can adjust the tension by slackening off the pivot bolt and moving the bearing arm. It is really crude, but effective.

A larger distance between motor and countershaft will not influnce belt wear much. Wear is most dependent on load and pulley size. Bigger pulleys are better.
A longer belt will cause more variation in tension due to stretch and thermal effects. Generally short belts are better.
Do consider a 3 phase motor and VFD. the motor is no different to contempary ones and you can put the VFD in a box. Get a motor with low speed.minmum is 4 pole but I'd use a 6 pole. This won't need so much reduction = bigger motor pulley.

Robert.

A belt drive should be fixed and not over-tight. The standard ML7 motor mounting uses a slotted link then clamped to hold the motor steady, much as the alternator belt on a car.

Some machines with vertical shafts use a spring to tension the belt correctly, at which point the motor on its hinged plate or guide-bars, is then clamped. This happens on the Myford VMC milling-machine and the Meddings bench-drill; the latter have a warning notice not to tighten the belt further.

Nor should belts be bar tight! That will cause early wear on the belts, pulleys and bearings... and your wallet It should be possible to deflect the centre of the longest leg of the belt by light finger pressure, to about half an inch.

Note that this applies whether you keep the motor original or replace it with a 3-phase one fed from a VFD inverter. If you do that, consider it not replacing the belt drives but complementing them, particularly as motors should not be run at too low a speed. Use the pulleys and back-gear to keep the motor running as it should, even at very low spindle speeds.

(My Harrison L5 has a single-ratio belt drive to the geared headstock, and in bottom gear the spindle ambles round at maybe 70rpm with the motor at a healthy 900+ rpm, according to the VFD speed-control markings.)

Roy,

To clarify matters.

I did NOT say that a ML7 had a 2 hp motor.

I had a ML7. The single phase motor was 3/4 hp. Three phase motors were 1/2 hp.

Don't get confused with my lathe which is oriental, bigger, and specified a 2 hp single phase as standard. I specified a 1.5 hp for the VFD conversion.

Belt changes, and Back Gear, are used to provide variations iu speed, in additiuon to the VFD.

Mini lathes tend to use 400 - 500 watts, brushed or brushless, motors.

Other lathes will use motors of different powers, depening upon what is required of the machine, and what can be accomodated within the overall envelope.

Howard.

Some years ago I fitted a VFD to a Schaublin 70 lathe belonging to Anthony Randall a well known British clockmaker.

He said it transformed the performance of the lathe and particularly when he needed to re-pivot arbors where the lowest speed setting of the Schaublin's standard drive arrangement was too fast and a drill would tend to burnish the arbor rather than bite in.

Early lathes had simple drive systems because of cost as variable speed motor systems where expensive unlike modern BLDC motors or 3 phase with VFD. One lathe I had for a short time had a Ward-Leonard system where a 3 phase AC motor drove a DC generator driving the lathe's DC motor. Quite a sight to see but oh so heavy. I sold the lathe on after refurbishment and would have loved to have retained the original drive system but it needed a 3 phase supply and not many electricians would have been able to repair it if it had gone wrong. So I used the 3 phase motor to drive the lathe directly and fitted a VFD so it could be powered from domestic 240v supply.

I would advise anyone with a lathe/mill/drill to find some with a machine fitted with variable speed drive and try it. They won't be disappointed.

CS

If there is a substantial difference between pulley diameters a short centre distance will give not much wrap on the smaller pulley. Increasing the centre distance too much will result in the belt flapping, it's all a compromise, with wide limits of acceptability. The 'toothed' vee belts are more flexible, so good for small pulleys.

Howard. Sorry but that's the way I read it

Roy

The Boxford AUD BUD & CUD use a lever and rod system. Lever is on the end of the lathe cabinet just below the headstock to put tension on the motor plate. The rod is adjustable so as to give correct tension to the drive belt feeding the countershaft. There is no adjustment on the countershaft to spindle belt.

regards

One of our club members a gaugeoneist as they are known there, has a Simat which i believe to be pretty much the same as the Flexispeed. he did have belt problems unti he bought some orange coloured round material from a chap on ebay who sells them for Unimat lathes. He got the seller to make up the belts and join them at the length he estimated and has been turning out (sorry) engines on it for some time with no problems. His lathe has a very small motor on it I believe only 1/6 hp. I'll see him on Sunday and will try and find out the motor size for definite. The countershaft is very rudimentary but works and I know he does not have to loosen the belt to change speeds.

Dave

Update to above post. the motor is indeed 1/6 hp and is from a bacon slicer! the belts are round silicon he thinks 8mm dia and are orange. previously he used green ribbed belt that worked ok but was prone to part at the joints, possibly he admits through faulty heat jointing. The small motor is he says is probably slightly underpowered but pefectly adequate for what he does, his model of simat has back gear which he uses for cast iron wheels. He has just bought a Flexispeed Venus milling machine, which is a new one on me, after looking at the lathes website it looks ideal for the smaller gauges, crying out for a little vertical head attachment!