Overpowered motor

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Tredders
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Joined: Wed Aug 08, 2018 4:15 am

Overpowered motor

Post by Tredders » Fri Aug 10, 2018 5:39 am

<t>Hi All<br/>
The motor on my 210 x 400 lathe is broken. <br/>
The motor spindle is 10 mm in diameter, but it sheared in two behind the bearing on the pulley side. <br/>
This happened because I replaced the V belt from the motor with a toothed belt drive and smaller pulley on the motor to get more torque.<br/>
Too much torque!!<br/>
Now I am planning to replace the broken motor with a treadmill motor rated at 2.5 HP. It has a 20 mm diameter spindle. I’ll use 20 mm wide multi-V belts between the motor and the lathe spindle. Also a separate cooling fan. <br/>
I’m thinking I’ll try the existing motor speed controller and if it breaks I’ll put a more robust speed controller on it. <br/>
My question is - what else I might break by doing this - change gear teeth?? Parting tool blades??<br/>
I’m not planning to drive it hard, just like decent torque. <br/>
Thanks for your thoughts.</t>

tornitore45
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Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: Overpowered motor

Post by tornitore45 » Fri Aug 10, 2018 6:54 am

Reducing the drive pulley diameter give more torque to the lathe spindle but does not increase the torque on the motor shaft. Actually if the spindle load is the same the motor needs to put out a lower torque.
Change the motor if you like but do not blame the broken shaft on the pulley reduction.
Mauro Gaetano
in Austin TX

John Hasler
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Location: Elmwood, Wisconsin

Re: Overpowered motor

Post by John Hasler » Fri Aug 10, 2018 9:15 am

Those changes may have increased the peak torque when starting with full voltage. It's a poor motor than can snap it's own shaft, though.

Since the other parts of the lathe are obviously stronger than that motor shaft you should be ok with your new motor. As a precaution always run the speed control down to zero before starting the motor.

That isn't really a 2.5 hp motor, of course. "Treadmill ratings" are about double real ones. Do you have the controller that was used in the treadmill?

Tredders
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Joined: Wed Aug 08, 2018 4:15 am

Re: Overpowered motor

Post by Tredders » Fri Aug 10, 2018 8:35 pm

I agree about snapped spindle being on a poor motor. Manufacturer said “Every product has a life” .... got to laugh!!

I don’t have the speed controller for the treadmill motor but I can get an electronic kit for a controller for up to 2.3 kw.

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liveaboard
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Re: Overpowered motor

Post by liveaboard » Sun Aug 12, 2018 5:01 am

10mm is a tiny shaft. You didn't state the power of that motor, but no matter. Scrap is scrap, why worry about it?

I sometimes wish for more power, and less belt slip.
As a novice machinist, I make mistakes quite often. My V belt drive has saved me from killing many things, some of them expensive. belt tension / slip point is a safety clutch.

Power ratings on electric motors seem to be very unreliable these days. KW, HP, torque, should be solid engineering numbers. But they aren't.
Sometimes I think someone just makes it up, then stamps it on the plate.
As load increases, so does current. At some point, the motor will lose speed, which is the point of overload.
But heat buildup also limits useful power; some motors overheat at 'full' load after some minutes, some are built to operate at rated load full time.

20mm shaft is good.

pete
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Joined: Tue Feb 10, 2009 6:04 am

Re: Overpowered motor

Post by pete » Sun Aug 12, 2018 7:06 am

What else would you break? That depends on you of course and how well you know that machine. If your only going to take cuts and feeds no larger than what the original motor was capable of then there's zero problems other than maybe a bit higher power bill. If the machine was properly engineered and designed the mechanical assembly's are able to do only so much and no more before a failure. The machines original motor size should have been chosen to suit the rest of the machines mechanical limits. Start getting greedy simply because you've got more torque available and stripped gearing, leadscrew nut failures or in the event of a major crash and no weak link in the system it could get real expensive. And crashes can and have happened to even the best sometimes. Yes normally the shear pin would break if it's got one, but I've seen times when for what ever reason it didn't. Stripping the teeth off a fairly cheap replaceable gear was the next cheapest part in the system. I see many complaining about "plastic" gearing in some lathes and many replace those gears with steel gearing thinking they have to be better. There were some very expensive and well made lathes that came from the factory with a couple of plastic or phenolic gears. There not in there to cut costs. While some of the cheaper off shore machines might have been built with under engineered gearing sizes or used the incorrect plastic type the expensive ones used them as a way to protect the machine from abuse or mistakes. The cheap ones copied the same idea but might not be as well engineered as they should be.

I'm planning on remotoring my single phase variable speed lathe with about 1 more hp and adding 3 ph and a good VFD simply to get more torque at the needed lower speed settings when turning larger diameters since mine wasn't built with a proper back gear system. If it had been I'd only remotor to that 3 ph set up with another motor and with the exact same horsepower. I've operated some very large and heavy duty mining equipment for a long time. It doesn't matter how tough and well built that equipment is. Over working any machine beyond it's designed capabilitys and you will start to drasticaly shorten it's life span. Just like Liveaboard said, having some type of safety clutch is a good idea and something I'd always want to have for the exact same reasons he has. If you have enough experience with machine tools it's not real hard to feel when a machines parts are starting to strain to get the fed and depth of cut done. That's when you better know when it's time to back things off a bit.

tornitore45
Posts: 1455
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Location: USA Texas, Austin

Re: Overpowered motor

Post by tornitore45 » Sun Aug 12, 2018 9:54 am

The machine size is given as 210 x 400 which is same size as an 8x16
As a point of reference I have a 9x20, the motor has a 1/2" shaft
There is no way a belt or a ball bearing slip clutch can transfer enough torque to break a 10mm shaft, unless there was a defect or it was threaded with a 6mm screw.
My machine has a 1HP 3PH motor and I never felt I used more than 1/2 the power available even when pushing to the limits of the machine rigidity.
A 2.5HP even discounted for the overstating typical of treadmill motors is plenty. Keep in mind that a DC motor can supply full rated torque at very low RPM
Mauro Gaetano
in Austin TX

John Hasler
Posts: 789
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: Overpowered motor

Post by John Hasler » Sun Aug 12, 2018 3:37 pm

He had replaced the v-belt with a toothed belt.

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