Calculating Pulley Size for Band Saw Motor

[Doug Mungham]

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Hi all

I am trying to find the diameter for a pulley on a drive motor for a band saw the motor turns 2800 rpm the next pulley is 11 1/2" away that is 5" dia on the same shaft is the band saw wheel which is 10" dia and must turn 900 rpm .

any help would be great.

Doug.

 

[CWatters]

If I have understood your description correctly the pulley needed should have a diameter equal to...

= 5 * 2800/900

Edit: Sorry this is wrong. See below..

 

[CWatters]

Length of belt calculator...
http://www.calculatoredge.com/mech/vbelt length.htm

 

[Doug Mungham]

thank you for information but looking for motor drive pulley diameter not belt length

 

[Doug Mungham]

I already have it running but it appears to fast.
the drive pulley is 3" dia that I turned in wood, when running smoke comes from the wood being cut so I am assuming that it is running to fast

 

[CWatters]

Sorry I had that equation in #2 the wrong way up. Should be...

5 * 900/2800 = 1.6"

If you have a 3 inch already the output rpm would be 2800 * 3/5 = 1680 which is too fast if you want 900

 

[CWatters]

1.6" is probably too small to work well so you may have to make the 5" pulley bigger.

 

[Doug Mungham]

would a 1.6" pulley be the right speed to get 900 rpm at 10" wheel is that what you are saying

 

[Doug Mungham]

I might have to try and find a 1725 rpm motor ?

 

[NTL2009]

Can you fit a 9" pulley to the shaft of the 10" wheel? If you put your 3" pulley on the motor, you get 933 RPM at the drive wheel (2800* 3/9).

 

[Dr.D]

Your question raises issues about where you are. If you have a motor with a nominal 2800 rpm speed, that suggest that you have a two pole motor in a 50 hz system. But later you speak of looking for a 1725 rpm motor which suggests a four pole machine in a 60 Hz system. Be sure that you get a motor designed for the system frequency where you intend to apply this motor.

 

[Doug Mungham]

Hi Dr.D
the application is a 1953 Delta craftman 28-110 band saw.
the band saw wheels are 10" dia on a shaft with a 5" drive wheel, 11 1/2" away is the motor with a shaft speed of 2800 rpm at this time but if I can not get the drive pulley on the motor shaft reach the right rpm then I will look at changing the motor for a 1725 rpm and return a new drive pulley for it.

Thank you for your interest

 

[Asymptotic]

Found a copy of the manual online.
http://www.vintagemachinery.org/pubs/1141/4395.pdf
http://www.vintagemachinery.org/pubs/1141/4395.pdf
10" saw wheels. The lower wheel is shafted to a 5" V-belt pulley, and (at least, originally) driven by a 1/3 HP, 1725 RPM cap start 115V/60Hz single phase motor fitted with a 2-3/4" pulley via a 39-7/8" outer circumference V-belt. The manual states a 2400 FPM blade speed when the lower wheel (hence, 5" drive pulley) is operating at 900 RPM.

A 2800 RPM motor would provide a blade surface speed of approximately 3900 FPM if it were using the originally specified 2-3/4" pulley, and overheating in the cut would be a serious issue.

One way to determine surface speed is to measure blade length (circumference), make an easily visible mark on it, and time how long it takes for it to make a complete revolution. At the specified 900 RPM (15 RPS) and 2400 FPM (40 FPS) the blade should travel 2.67 feet (40/15) per saw wheel revolution, and (for a 71-3/4" long saw blade) the mark should return home every 2.24 seconds.

The 3" pulley you mention, is this the outer diameter of the pulley on the motor shaft? If yes, this may actually be the original stock part. V belt pulley diameter usually refers to the pitch diameter (where the belt comes into contact with it), and a 3" OD would be just about right for a 2-3/4" pulley.

Changing the motor from 2800 RPM to 1725 RPM should solve your problem.

 

[CWatters]

would a 1.6" pulley be the right speed to get 900 rpm at 10" wheel is that what you are saying

Yes.

2800 * 1.6/5 = 900 rpm approx.

However a 1.6" diameter motor pulley is probably impractical. For one thing it would mean a very tight bend radius for the belt to go around.

The lower wheel is shafted to a 5" V-belt pulley, and (at least, originally) driven by a 1/3 HP, 1725 RPM cap start 115V/60Hz single phase motor fitted with a 2-3/4" pulley

The original numbers from the manual give you...
1725 * 2.75/5 = 950 rpm

As Asymptotic said, going back to a 1725rpm motor is probably the best bet. If you want to stick with the 2800 rpm motor one option might be to use the 3" motor pulley and replace the 5" pulley with a 9" pulley...

2800 * 3/9 = 933 rpm

 

[Asymptotic]

As CWatters points out, extreme bend radius limits the diameter of the "small" pulley. This is one of the reasons manufacturers don't offer them under a certain size (others include reduced power transfer, an excessive tensioning requirement to prevent slip, and premature belt failure). For instance, T.B. Woods smallest 5/8" bore cast iron pulley (part # BK2558) is 2-5/8" OD with a pitch diameter of 1.9" (for 4L, A, and AX belt profiles), and 2.3" pitch diameter (for 5L, B, and BX belt cross-sections).

I like CWatter's idea of using a larger diameter sheave on the driven side, but it occurs to me neither of us have addressed Dr.D's observation about motor speed and line frequency. 2880 RPM sounds like a 2 pole, 50 Hz motor, and it would be turning a lot faster than nameplate (on the order of 3500 RPM) if fed from a 60 Hz source. This pushes pulley diameter to 11", and the question then becomes, "is there enough room for it?"

What is the AC line frequency of your power supply?

 

[Doug Mungham]

Hi ALL
thank you for all the information.
I am afraid that you are asking questions of some one who has NO idea that is why I posted in the first place.
I have NO equipment to do any test on what I am trying to achieve, I am trying to put together a 1950's American band saw, I have 98% together but the drive pulley I have turned one with NO way to check diameter, the motor I got on ebay used wood hollow mortiser drive motor, it is all a bit Heath Robinson so please treat me gently

 

[Asymptotic]

Surface speed and saw blade characteristics (material, and tooth count) are the key numbers. Different materials cut best at different speeds - generally, the harder the material, the slower the cutting speed should be, although there are exceptions. 2400 FPM is a good mid-range speed (1000 to 5000 FPM range) for cutting wood, while the range is more like 60 to 300 FPM for hard metals, and up to 1600 FPM for aluminum.

What is your power line frequency?
60 Hz is standard in Canada, the USA, and upper half of South America. With several exceptions, it is 50 Hz everywhere else.

This saw was originally built with a four pole, 1725 RPM/60 Hz motor. Using original equipment pulleys, and supplied with 60 Hz AC power, saw speed is 2400 FPM.
A 1725 RPM motor would run more slowly, at about 1438 RPM - (1725 RPM* (50 Hz/60 Hz)) - if supplied with 50 Hz power.
What you have now appears to be a two pole, 2800 RPM/50 Hz motor, and spins at about 3360 RPM if supplied with 60 Hz power.