Motor, belt, and pulley questions

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Motor, belt, and pulley questions

Post by jacob » Tue Aug 20, 2019 9:01 am

I just acquired a 1725rmp 1/3HP split-phase A/C motor to upgrade my home-built lathe (I'll post pictures later) and save my power drill the abuse.

With a physics background, I have the 1000ft view of the theory (circuit diagrams, calculations, rotating magnetic fields, etc.) but none of the practice, so I know just enough to get myself in trouble. In the spirit of avoiding a Darwin award, I have a couple of tech questions.


First, what kind of belt do I need? Just from eyeballing it, it looks like the pulley is adjustable in width which is a bonus. I have not tried fiddling with it, but from the looks of it, there's a thread on the spindle leading into the outside of the pulley, so if I loosen the set screw, my guess is that I can accommodate different belt sizes. Correct?


Googling "a belt" is highly unproductive, but I eventually settled on the idea that I probably need a fractional horsepower V-belt of some sorts. More browsing leads to many versions of Gates fractional horsepower Truflex belts for under $10. Although the price is right, this is still too close to random for comfort and could probably stand some advice from an experienced person.

In connecting the belt to the drive side pulley (I figure I can use DIY and turn one myself) on the lathe, do I need any kind of tensioner. Is it rocket science how tense the belt should be or are we good to go as long as there's no slipping?

The motor came prewired with a jangy old cord. I am presuming the motor is wired correctly, but I can go in and take a look. Given the stern warnings about grounding, I took out my ground-o-meter and it's wailing red when I touch the casing with the motor running. OTOH, it's a piece of metal right next to a spinning B field. And it's not the first time the meter has detected a hot ground on something with large metal surfaces (a slowcooker), so maybe it's just overly sensitive?


PS: It's wired to a 2-prong plug.
PS: The outlet I hooked it up to is greenfield/clothwire as is the rest of the house. So basically, the metal conduit serves as the ground wire. There's no green wire leading to the receptacle in the junction box itself. An outlet tester shows it's wired correctly. I presume that neutral is therefore grounded in the fuse box?! (I hate old houses!)

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Re: Motor, belt, and pulley questions

Post by BMF1102 » Tue Aug 20, 2019 9:26 am

I don't have any specific recommendations. Your local library should have an Audel Millwright manual which should tell you everything you want to know and more relating to the belts and pulleys. Various home machinery simply has the motor mounted on a hinged base and the motor weight provides the tension not sure if this would be adequate in your application but it's a thought. As I said the Audel manual should tell you everything you want to know other than the electrical aspect... might even cover that as well have not looked through one in a while.

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Re: Motor, belt, and pulley questions

Post by C40 » Tue Aug 20, 2019 9:45 am

I used to work for Goodyear in R&D at a belt factory, managing testing of belts to check life durations, failure modes, etc.

I don't have links for you, but here are some points to get you started:

** 1/3 hp is pretty fairly low, so it should be pretty easy to get a belt that will carry that

** Two of the most common reasons for belt failure are:
1 - Pulley/sheave misalignment. - Make sure they are aligned on the same axis, both laterally and in the angle of the pulleys
2 - Belt flex. When a thick belt goes around a small pulley, there is internal motion/friction from the belt quickly bending tightly and unbending. Generally, a "cogged" V-belt can flex more because it has less material at what becomes the inner radius around the pulleys, and they cost more, but little V belts are all pretty cheap anyway.

** For tensioning, basically, it does just need to be tight enough to work and not slip (too much) and not force the sheaves into poor alignment. But also, there is detailed math and measurement. Ideally you measure with a strain gauge attached to one of the shafts holding a pulley, but the normal technique is to press down on the belt in the mid point with a certain force and check how much the belt deflects. There are calculators or at least formulas you can find to use for this.

** There usually isn't much risk of injury or damage when a belt does fail. Also, you can often see it coming before it happens. It will, in most cases, start making a mess, or stink from being too hot, or have some other sign of impending failure (cracks and/or chunks falling out of the inside edges, the tensile cords breaking, visible on the outside of the belt, if an enveloped belt: the envelope wearing off or coming loose, etc).

** I'm not totally sure how to go about determining the right size of belt. One important thing is to check the angle of the pulleys. Different belt types do have different angles. The main types are, ranging from oldest sizing types to newest:
- 2L, 3L, 4L, etc. (of which, if I recall correctly, you'd probably use a 3L or so)
- A, B, C, etc. (Of which you would probably use an A or B section belt)
- 3V, 5V, 8V (of which you'd probably use a 3V)

Cogged belts generally have an X added to the name. The belts also have a number added for the length of the belt.
SO, a 3VX500 is a 3V section belt, cogged (which also means bare rubber and no envelope, which is generally 'better'), and 500 whatever in length.

Some of the really cheap belts are indeed cheap and will fail much quicker than quality belts. Performance varies by both companies and sizes. Generally, I recall Gates, Dayco, and Goodyear as being the best performers in most cases.

The simplest and quickest way to get it going could be just taking your pulley to a store, finding some different belt sections (A, B, 3V) putting them in the pulley and seeing how they fit the pulley angle. Then you just need to figure out how long of a belt to get. And - maybe, a driven pulley setup(?). You can, of course, use the pulleys for modifying the drive ratio of the system and increasing/decreasing torque vs. speed. Just don't go too small on the driven pulley and create too much belt flex. How small is too small? IDK/it depends. (if you can find a good belt system design tool/software, it tells you that kind of thing. I used to use Goodyear's, but I don't know if/how widely is is publicly available)

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Re: Motor, belt, and pulley questions

Post by Campitor » Tue Aug 20, 2019 10:00 am

I only have experience working with car engine pulleys and none of them ever had an adjustable width pulley. I don't see how that would work properly because any exposed thread would hamper the belt's ability to grip on the shaft properly and possibly eat away at the belt unless the adjustability was intended to NOT have any pulley threads exposed. I would prefer to use a smooth pulley with a fixed diameter.

In regards to belt tension, you should read this write up that includes charts regarding pulley sizes, rpm ranges, and proper belt tensioning technique which maximizes belt life:

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Re: Motor, belt, and pulley questions

Post by C40 » Tue Aug 20, 2019 10:12 am

The belt grips on the pulley walls, not the shaft. In an application like this, I believe the belt should not touch the shaft.

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Re: Motor, belt, and pulley questions

Post by Ego » Tue Aug 20, 2019 10:13 am

Check the trash bin behind an auto repair shop and/or an auto parts store. You may find a variety to test. It looks like the pulley will accommodate many. What kind of pulley do you have on the lathe side?

Old VW's have shims in the pulley. Removing shims closes the gap and increases tension in the belt.

Another alternative. I've heard stories of people doing this when their generator belt broke on old VWs.
Last edited by Ego on Tue Aug 20, 2019 10:34 am, edited 1 time in total.

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Re: Motor, belt, and pulley questions

Post by ffj » Tue Aug 20, 2019 10:34 am

This is for a wood lathe, correct?

Go buy a basic V-groove belt at your nearest automotive store that can handle those rpm's which should be easy since you have such a small motor. On my lathe, the weight of the motor is the tension, so no need for tensioners. My motor sits on a hinged platform that I lift up to change speeds (putting the belt on another set of pulleys) and the alignment is not an issue, even though I am sure it's not perfect. At a 1/3 horsepower, it's not a big deal.

The motor is grounded through its housing, so no need to re-wire it. If you plug it in and it works, I'd leave it alone.




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Re: Motor, belt, and pulley questions

Post by 7Wannabe5 » Tue Aug 20, 2019 11:37 am

Zero lathe skills, but my librarian skills suggest searching the American Association of Woodturners (AAW)forum for answer.

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Re: Motor, belt, and pulley questions

Post by sky » Tue Aug 20, 2019 12:00 pm

I would add a ground wire to the case (or a new three strand wire) and put a three prong outlet on it. If you can find the L2 post, use that for ground connection, otherwise use some bolt on the case.

Pulleys are cheap, you can find them at Ace or Tractor Supply. You may want several sizes to experiment with the most desirable turning speed.

Since your drive pulley is adjustable, your final pulley determines the belt width. Set up your motor and lathe and wrap a rope around the pulleys. Measure the rope in inches, that is approximately your belt number. Buy a v-belt of the width and length, test it, and if it doesn't work, return it and get another.

Remember you can also put a new drive pulley on if the old one is not working right.

My guess is you want the inner side of the belt about halfway up the face of the pulley.

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Re: Motor, belt, and pulley questions

Post by Sclass » Wed Aug 21, 2019 12:08 pm

I just went through this for my custom truck AC.

I picked my belt from this table. ... 000-000000

Probably the ones C40 worked on. The critical parameters were the belt cross section. I picked the 0.5” V belt. Then the length which is the outer circumference of the belt. I wrapped a piece of masking tape around the outer circumference of the pulley system and taped it tight. Marked it with a pen then unrolled it on the driveway and measured it with a tape measure. The local auto parts store stocked the Gates 2440 for some oddball car. Didn’t matter, it is stocked by its Gates #.


Took a look at the width of the pulley and it was a little under 1/2” at the top of the groove.

4L profile seemed to fit the bill.

Your pulley looks adjustable BTW.

Used this table and selected a Gates 2440. Worked great. I never had to do this before because usually you start with the cars make and model year and then get a Gates part number. Unfortunately I built this AC from scratch and ad hoc mounted it to the engine so I had no idea what to use. All you need is a tape measure, pen and masking tape.

I built a tensioning mechanism from a ACE Hardware turnbuckle to symmetrically tension the belt.


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