View Full Version : AC Motor upgrade - pulley sizes

10-06-2013, 09:17 PM
As mentioned previously, I am converting a Warco WM16 milling machine spindle from a 600W DC motor to AC with VFD. After much contemplation, I opted for a 1.5kW 2-pole motor. This was because I will be mostly operating in the 1000RPM to 3000RPM range and feel more comfortable running a 2-pole within its rated frequency. Although a 4-pole motor would give me better torque at low speeds, I would have to run it at 100hz (i.e. double its rated frequency) in order to achieve the top end speed I seek, which would reduce its lifespan.

I'm thinking of using a 5mm x 15mm wide timing belt drive rather than the existing gear system. The machine has a hi/lo gearbox with ratios of 1:1 and 2:1, which I will probably keep. So the belt drive would only be joining the motor to the gearbox input shaft. The current DC motor runs at 4600RPM flat out and has a 10mm shaft with a 20T brass gear. The gearbox input shaft is 15mm and has a 37T plastic gear. My thinking is that I can use around a 1:1 pulley ratio to give me a max speed of 2800RPM in HI gear and 1400RPM in LO gear.

Soooo...the main question is...what is the smallest pulley I can get on the motor shaft which is 19mm dia and has a 6mm keyway?

Or maybe one of you experienced chaps can advise of a better solution?

11-06-2013, 05:56 AM
Hi To all
just fell on to this site while surfing. A quick intro I am Retired engineer with an ongoing interest in all thing mechanical.

Renovating old machinery. Modifying and improving machinery and making tools and fixtures.

This thread struck a familiar chord with me "HELP: AC Motor upgrade - pulley sizes"

Another solution that is possible with this mill driven with a VFD and 2850 rpm motor would be to remove the gear drive completely and convert to a " Poly Vee" belt drive with the driven pulley directly on to the spindle.

The drive ratio would have to be around 2 to 1 3 to 2 as a ballpark figure. Belt drive is much quieter than gears.

"Poly Vee" pulleys are fairly straight forward to fabricate and fit. You may wish to consider a two or three stepped pulleys.

Posted photo of my X2 mill with a Bigger DC motor and "Poly Vee" drive


15-06-2013, 01:48 AM
After MUCH deliberation, I've decided to go for a direct drive as suggested above. Main reasons are:

1. Maximum shaft centres is 46mm between motor and gearbox input. With shaft diameters of 19mm and 15mm, that doesn't leave much room for pulley adjustment.

2. Having removed the head, I've discovered that the gearbox is mostly plastic gears. I don't like plastic gears unless they're for damage limitation AND easy to replace.

But, of course, a direct drive raises new engineering problems...

My main issue is the length of the bearing journals/shaft as there's nowhere near enough meat to fit a pulley of any description. I want to keep the hand feed for drilling. I'd make a new piece, however I have no way to cut the internal splines. How do you guys suggest I extend this? Weld a lump on and machine it back? The gears will be redundant, but appear to be machined onto the same lump as the spline and bearing journals.


16-06-2013, 09:29 PM
Can I add a few more ideas for a belt drive conversion. Is it possible to machine the gear shaft to remove the top gear.

Looking at the photos there is enough metal to be able to turn off the gear and reduce the diameter to be able to press fit a sleeve over it and then turn the OD of the sleeve to fit the bearing and a pulley.

Have attached a sketch.


Lee Roberts
16-06-2013, 09:31 PM
Have attached a sketch. Eric

Eric, you should be able to upload your sketch to the post above now. Sorry your first post needed to be approved/moderated.


16-06-2013, 11:28 PM
I had a bright idea that I could turn the gears down and mount the spindle pulley inside the head, where the gears used to be. The motor would then be sat flush on top of the head which would look quite nice. Changing the belt would be a bit of a PITA but wouldn't be a regular occurrence. However, having drawn the design to scale, it turns out that the motor would foul the top of the spindle. Whatever I do, I'll have to raise the motor above the spindle.

Unfortunately, the O.D. is 45 (6209 bearing) and the I.D is 37mm, so only leaves a 4mm wall thickness. The only solution I can come up with is to use a piece of 50mm stock, turn about 10mm down to 37mm diameter, chamfer the edges, press it in and TIG weld around it. Then I can turn the O.D. down to 45mm and bore the centre out to 37mm. The pulley will have to be bored out to 45mm but that's not an issue as I can select the motor pulley to give a ratio of around 1.5:1. The 37mm bore is not critical as it's only clearance for the spring on the hand feed.

19-06-2013, 11:45 PM
I've now TIG welded a lump onto the original spindle. Bored it out to 35mm and 45.5mm O.D. Next job is to get it back in the lathe and turn it down to finished sizes. I think I'll go slightly under size on the new piece so that I don't have to press the bearing all the way down the shaft. The pulleys haven't arrived yet, but I can bore those out to 44.9mm or whatever O.D. I end up with.


21-06-2013, 10:49 PM
Hi birchy
This is all good for a belt drive conversion. No machining off gears with the associated problems.

Can i make a suggestion here.

"However, having drawn the design to scale, it turns out that the motor would foul the top of the spindle. Whatever I do, I'll have to raise the motor above the spindle".

Mount the motor on the side of the head with the spindle at the top same as a 'Seig X3" Mill Drill.

Not the prettiest solution just an alternative.

Please keep posting on this thread as this is useful way to make a vast improvement over noisy and fragile gear drives.