What size pulley should I use with DC treadmill motor?

[jdsantiagojr]

Hello! I would like to make a poly v-belt pulley for DC treadmill motor, but unsure how big the pulley would need to be . I have searched YouTube but a lot of the videos on the subject are in a different language.

Hoping someone would be able to point me in the correct direction. Setting this up on a 1924 11” south bend. Mounting motor about one or two feet behind headstock.


Thank you!

 

[Hawkeye]

You need to start with the RPM of the new motor. Next comes the RPM that you need to put into the lathe. If the original lathe motor was rated at 3450 RPM and the treadmill motor is 6000 RPM, that gives you a ratio of 1.74:1. If the pulley on the new motor is, say, 1.5", the new pulley on the lathe would be 2.6" in diameter.

An original RPM of 1725 and a new RPM of 7000 would give a ratio of about 4:1, so the new lathe pulley would be 4 times the diameter of the treadmill motor pulley. Easy enough to work out what you need.

 

[jdsantiagojr]

You need to start with the RPM of the new motor. Next comes the RPM that you need to put into the lathe. If the original lathe motor was rated at 3450 RPM and the treadmill motor is 6000 RPM, that gives you a ratio of 1.74:1. If the pulley on the new motor is, say, 1.5", the new pulley on the lathe would be 2.6" in diameter.

An original RPM of 1725 and a new RPM of 7000 would give a ratio of about 4:1, so the new lathe pulley would be 4 times the diameter of the treadmill motor pulley. Easy enough to work out what you need.

Would this be the same if I wanted to go directly from the motor pulley to the cone pulley on the lathe ? The original setup looked like the attached .

I removed everything that was mounted behind the machine. The original pulley on the AC motor was 3" and the others on the back measured about 8".

 

[Bi11Hudson]

The likely spec for the OEM motor on the lathe is probably 1800(~1740), a 4 pole motor. What will be needed is the rated "base speed" of the DC motor. It most likely has a 'permanant magnet' field. . . Then you must figure in the installation of a speed controller, either on install or some(possibly remote) time into the future. With a 'PerMag' field, you cannot easily overspeed the motor. I don't know your particular machining circumstances, as a rule I would try for a ~2400 or so RPM at base speed.

The reduction arrangement of multiple belts/pulleys may well serve several functions. Direct drive from the motor to the spindle is not recommended. Motor torque will fall off dramatically at speeds below 40-50% base. You will need to keep the motor speed up to some nominal speed and use the belts for reduction. Older machines were basically designed to be run from a line shaft, the speed of that source would have been much lower than an electric motor. Even then, there was a small reduction between the drive and the spindle.

The desired pulley sizes will be a ratio rather than absolute sizes. Figure the size of the OEM pulley. Then, knowing the base speed of the DC motor, find a pulley, either larger or smaller, to give a similar speed on the first stage of the countershaft. Making the allowance for above base speed as desired. That should get you started, it may be necessary to adjust the drive pulley up or down from there.

.

 

[jdsantiagojr]

The likely spec for the OEM motor on the lathe is probably 1800(~1740), a 4 pole motor. What will be needed is the rated "base speed" of the DC motor. It most likely has a 'permanant magnet' field. . . Then you must figure in the installation of a speed controller, either on install or some(possibly remote) time into the future. With a 'PerMag' field, you cannot easily overspeed the motor. I don't know your particular machining circumstances, as a rule I would try for a ~2400 or so RPM at base speed.

The reduction arrangement of multiple belts/pulleys may well serve several functions. Direct drive from the motor to the spindle is not recommended. Motor torque will fall off dramatically at speeds below 40-50% base. You will need to keep the motor speed up to some nominal speed and use the belts for reduction. Older machines were basically designed to be run from a line shaft, the speed of that source would have been much lower than an electric motor. Even then, there was a small reduction between the drive and the spindle.

The desired pulley sizes will be a ratio rather than absolute sizes. Figure the size of the OEM pulley. Then, knowing the base speed of the DC motor, find a pulley, either larger or smaller, to give a similar speed on the first stage of the countershaft. Making the allowance for above base speed as desired. That should get you started, it may be necessary to adjust the drive pulley up or down from there.

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Thank you. I am just getting started in hobby machining .. off and on for two years. This is my first old lathe and was not sure how to proceed with conversion to dc motor. Some of the video I have watched do direct to cone pulley,but I was unsure with this approach. Really appreciate the information. Going to think about it and start sketching something out.

should I just look at also reusing original setup with only motor and motor pulley replaced ? Only problem I have is space with the original setup

 

[Hawkeye]

The first mistake we all make with machining is in not getting the cutting speed (RPM) right. Different materials can stand different cutting speeds - stated in Surface Feet per Minute. Different cutter materials (HSS or carbide) can stand different speeds. Exceeding those speeds by much of a margin will ruin the cutting edge.
As already mentioned, treadmill motors run at a much higher speed than your original motor. If the manufacturer needed a jackshaft with extra pulleys, your new installation definitely will. Your easiest installation will be to turn the jackshaft at the same speed as before. This will involve using the right size pulley feeding into the jackshaft. Simple math as outlined above to design that diameter.

 

[Grendel]

I may be wrong, but I think the south bend literature for that era quotes the countershaft speed (I seem to recall thats what I worked to, so I then use that speed, in the end my slowest non backgeared speed was about 600rpm at the spindle,

 

[RJSakowski]

I converted my G0602 lathe to run on a dc treadmill motor last year. Rather than use the OEM poly v pulley, I adapted it to with the lathe's OEM motor. The old motor was 1725 rpm 1 hp motor @ 120 volts rms while the new motor was rated at 4090 rpm, 2.5hp at 130 volts. Calculating torque, the OEM motor developed 3.02 lb-ft. and the new motor developed 3.21 lb-ft. My controller runs directly off the line voltage, rectifying via a full wave bridge and has an output of 176 volts with no load.

I retained my OEM pulley configuration and designed my own controller. The controller uses pulse width modulation with feedback to set operating speed. Operating on the medium-low belt position, I can generate useful torque from 20 rpm to almost 950 rpm at the spindle. While the motor would normally lose torque at lower speeds, the feedback circuitry senses a decrease in speed under increased load and adjusts the control pulse width to maintain the set speed. This effectively maintains torque at low speed.

Using the OEM pulley configuration, the range of spindle speeds are from 10 rpm to 7500 rpm (an insane speed , btw. I wouldn't try using that speed under any circumstance. The OEM spindle speed in the highest belt configuration is 2400 and the bearings are rated at 5600 rpm.) So far, all my lathe work has been on the medium low belt configuration and between 20 and 700 rpm at the spindle)

This will all be published in gory detail on a dedicated future thread. As of now, I am working on a Prony brake to measure spindle torque. The plan is to measure torque vs. motor speed at various controller settings. This will be used to optimize the controller feedback. and determine actual performance vs. the theoretical.

 

[jdsantiagojr]

Really appreciate everyones response. I do have another probably dumb question, but I was curious if someone could explain to me the setup in the attached picture highlighted by the purple rectangle. I noticed the pulleys are two different sizes (4", 8") driven by same size pulleys (3", 3") below. What is the purpose of this ?

 

[Bi11Hudson]

I can't see the entire gear train in the photo, but the general area appears to be for a "back gearing" mechanism. If so, there will be a shaft with a gear at either end and a latch mechanism to allow the cone pulley to rotate individually from the shaft. Please note that back gearing and the latch must never be engaged at the same time.

My machine is ~25-30 years newer and of different manufacture, so I am just guessing about most of what I'm saying. The only thing I am sure of is that back gearing and the spindle latch should no be used at the same time. As with most things, there will be exceptions. But for a novice, NEVER is a good rule until you have some idea about what you're doing.

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