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Motor drive - belt, gear or direct?

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Andy Carruthers20/06/2021 22:29:38
317 forum posts
23 photos

I am thinking of re-purposing a Mk1 Dore Westbury milling head to approximate a Tom Senior ELT style head, idle thoughts more than anything right now

Due to limited space, I am thinking of mounting the motor on top of the milling head dispensing with the pulleys

Can anyone appraise me of the pitfalls / right way to approach this please?

Nigel Graham 220/06/2021 23:11:22
1898 forum posts
26 photos

I can see your point but unless you use a variable-speed motor, you will have a machine that will run much too fast for a lot of work, and a bit slowly for some other tasks. Can you design and fit a gear-box between motor and spindle?

Andy Carruthers20/06/2021 23:23:47
317 forum posts
23 photos

Thanks for your reply Nigel

i am planning VFD which will take care of the speed issue, my question was more to do with “best practice”, pulleys / belts and gears give options for torque and speed whereas direct drive doesn’t - I’m probably answering my own question here and your gearbox question is pertinent and worthy of serious consideration

As must be obvious, I’m fairly new to this and figuring stuff out as I go along, in other words, more dumb questions to follow!

Kiwi Bloke21/06/2021 02:54:16
625 forum posts
1 photos

You are in danger of answering your own question re speeds and torque. One advantage of a belt drive is that the spindle housing and motor can be at opposite ends of a 'beam' that is supported near the assembly's centre of gravity, making the vertical slide and feed arrangement easier than if motor + housing are cantilevered from the vertical member.

John P21/06/2021 09:04:22
360 forum posts
238 photos

The photo's here shows the completely new head and spindle that
i made for my Dore Westbury mill when i cnc'd it ,there are three
speed ranges using 4 L type 3/8" belts and pulleys with the inverter
the speed range is from 100 rpm to 10,000 rpm .
Lot of work to do ,i doubt you would be able to fit something like
this on the standard Dore quill assembly.

dore west 1.jpg

dore west 6.jpg

Alan Jackson21/06/2021 10:07:30
240 forum posts
115 photos

This what I did

front belt cover removed.jpghead assembly - right side.jpg


Andy Carruthers21/06/2021 10:35:20
317 forum posts
23 photos


I have a long way to go to reach such engineering standards!

I have a copy of your excellent article Alan, which gives me a lot to think about. Unlike yourself, I'm not confident in using the Tom Senior horizontal milling capability so am toying with the idea of repurposing the horizontal bar to support the DW head, I haven't measured up properly yet so might not gain any height and quill use, I do know the DW arm is larger diameter than the TS horizontal bar

At the moment I'm at the ideas stage, the engineering may be well beyond my current capability, but I won't know until I try

old mart21/06/2021 19:24:23
3510 forum posts
216 photos

When I decided to change the spindle on the Tom Senior light vertical from MT2 to R8, I bought a 1hp three phase motor to use with a VFD. The motor specs are as follows.

6 pole motor 1 hp@ 50Hz 935rpm.

1 hp@ 75Hz 1402rpm.

0.43 hp@ 25Hz 467rpm

For my purposes the frequency of 25 to 75 Hz is sufficient. Note the drop off in power as the frequency drops. Because of this, the original set of 4 speed pulleys have been kept. The top spindle speed of 3000rpm is fine for the mill and the lowest is 174rpm. To get a decent torque throughout that range both the VFD and the 4 speed pulley system is essential.

Mike Poole21/06/2021 20:57:10
3162 forum posts
72 photos

While a VFD is a most useful and convenient method of speed control it does suffer from loss of power at low speeds. Fly cutting of steel will call for a low speed with enough power to drive the cutter through the material, larger milling cutters will also require lower speed and adequate power. Mechanical speed reduction has the advantage of always having maximum power available if we ignore losses in the drive train. In simple figures which ignore many other factors a VFD will have half the torque and thus half the power at half speed, a mechanical speed reduction will double the torque and thus maintain full power at half speed. Some people fit a more powerful motor than would be standard to compensate to some extent for the loss of power at low speed, this could put the machine more at risk of mechanical damage in the event of a crash. The Warco VMC mill has a high and low mechanical speed range on their VFD version and the Myford VFD drive has a couple of belt speeds and the back gear to make a very flexible range of speeds with useful power available.


Andy Carruthers21/06/2021 21:21:56
317 forum posts
23 photos

Every day I set out to learn something new and today has been a bonus day - thank you for your input, very helpful

I'll think about how to incorporate belts or gearbox

Andrew Johnston21/06/2021 22:08:38
6404 forum posts
682 photos
Posted by Mike Poole on 21/06/2021 20:57:10:

.............a VFD will have half the torque and thus half the power at half speed..............

Given that power is torque times angular velocity (aka speed) then half torque at half speed is a quarter of the power. In reality the VFD will have the same torque at half speed, giving half power.


not done it yet22/06/2021 07:54:44
6518 forum posts
20 photos

In addition to Andrew’s correct explanation, one also need to possibly take into account the reduced cooling of the motor when it may be operating less efficiently than at design specification. Halving the cooling fan speed may well reduce the cooling air flow by rather more than half these fans are not positive displacement items), leading to possible overheating of the motor windings - especially so if the motor is run under full available power.

I believe some VFDs may actually enhance the torque at lower speeds, so the cooling aspect might well become even more acute under heavy load, or extended running time, at those reduced motor speeds.

Mike Poole22/06/2021 09:24:26
3162 forum posts
72 photos
Posted by Andrew Johnston on 21/06/2021 22:08:38:
Posted by Mike Poole on 21/06/2021 20:57:10:

.............a VFD will have half the torque and thus half the power at half speed..............

Given that power is torque times angular velocity (aka speed) then half torque at half speed is a quarter of the power. In reality the VFD will have the same torque at half speed, giving half power.


Foot in mouth again, thank you for the correction Andrew. With sensorless flux vector control the impact of lower frequencies has been reduced but will never match mechanical speed reduction. In real life the VFD will cope with many setups with no problems but if the option for a mechanical speed reduction is unavailable then there is nowhere to go if the task stalls the motor.


Howard Lewis22/06/2021 14:19:56
5748 forum posts
13 photos

One advantage of belt drive, is that in the event of a jam up, the belts can slip reducing damage.

Belt drive is also quieter than gear drive.

For all the reasons listed above, I would avoid direct drive from a VFD set up. By all means have it, but driving into belts or gears to give a range of speeds, and torques , capable of being modified by the VFD.

My lathe has VFD; of the six speeds available from belt and back gear. Direct drive would much too restrictive, and unsatisfactory.


Andy Carruthers22/06/2021 14:55:14
317 forum posts
23 photos

From my research so far, and informed by comments here, belt drive is both possible and preferred, thank you all for contributing and putting me on the right track

I'm not in a hurry to start the build, lots to do first not the least of which is to get my design written up, drawn out in Tinkercad and gather some materials

After I have built my new (temporary) workshop!

old mart22/06/2021 15:06:37
3510 forum posts
216 photos

I'm not sure if this has come up, but remember that there must be space above the spindle to access the drawbar. Should you decide to use a VFD and belt drive, you could get away with only 2 belt speeds with about a 4:1 ratio. If you then bought a 6 pole three phase motor, running the spindle from1870rpm to 5600rpm in high, and 467rpm to 1868rpm in low gear, if the VFD was set from 25Hz to 75Hz.The easiest way to run that would be with a belt for each speed as mentioned by Pete Rimmer in another post. You can work out the ratios needed simply by the diameters of vee belt pulleys. For instance, a 2:1 ratio would be achieved by running a 3" and a 6" together. For the belt lengths, see my thread "pulley size help required".

One advantage of vee belts verses toothed belts is they can be tensioned to slip before too much damage is done, as already mentioned.

Edited By old mart on 22/06/2021 15:15:04

Andrew Johnston22/06/2021 22:06:41
6404 forum posts
682 photos
Posted by not done it yet on 22/06/2021 07:54:44:
.....some VFDs may actually enhance the torque at lower speeds....................

Correct, at least in the short term. As a first approximation torque is proportional to current. At slow speeds (below the base speed of the motor) the currents can be boosted beyond normal as there is more voltage available than needed to reach the nominal maximum currents. However, heat dissipation is I2R, so the increase in current causes a disproportionate increase in heating. Ok in the short term, but not good in the longer term. On my CNC mill the VFD drops the speed slightly and increases current if it detects a short term overload.

As mentioned by Mike vector control can also help. At slow speeds it can compensate for the reduction in currents caused by losses due to rotor resistance.

Vector control is essentially a 3 to 2 dimensional transform which eliminates the frequency component of the measured currents. So the calculations are reduced to a DC problem with two currents to be controlled D (in-phase) and Q (quadrature phase).


NB: dunno why the maths uses D and Q whereas the rest of the signal processing world uses the more logical I (in-phase) and Q (quadrature).

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