Motor HP upgrade, so what about belts?

[Chazz]

So, my stock .75 HP motor in my 9 x 20 Lathe bit the bullet, I'm upgrading to to a 2 HP motor and I can't seem to find a "Rule of Thumb" of belt size to HP ratio formula, (understanding the pulley depth\dia. will have to change too).

The idea is, the new motor will also be VFD controlled, and as such, yes I can control the 'Start-up-to-RPM' and 'Coast-to-stop' and I could actually use the .75 HP rated belt I have.

However, if I want to set my E-Stop to 1 second stop (with a resistor pack) and not the normal Stop button, how do you figure out how big a belt you need to handle the 'Shock Load' of a sudden stop?

Thanks for any help.
Chazz   

[Lew_Merrick_PE]

Chazz -- 1 HP, by definition, is 550 ft-lb/sec.  Thus, 2 HP is 1100 ft-lb/sec.  Torque (lb-ft) = HP * RPM/5252 (with minor rounding errors).  It is common to use the smallest pulley as the basis -- though it should not really matter as the RPM/Pulley-Radius is (almost) a constant (there are "losses" I am glossing over here).  Thus, if you have a .500 inch (.0417 ft) radius pulley turning at 1000 RPM (just to pull some numbers out of the air), then your equation becomes:

   Belt-Tension (lbs) * 0.417 (feet) = 2 (HP) * 1000 (RPM)/5252.

This works out to 9.14 lbs of tension in the belt.  (1) You will need to run this for the highest RPM range to get the worst case value.  (2) You will want a Murphy Phactor of at least 5 to account for start-up loads and this could easily go as high as 20 for braking.  The problem with figuring out the value used for braking is that you need the rotational mass moment of inertia to figure out the load.  This is not trivial and can be (somewhat) accounted for using a Murphy Phactor.

If you go to http://www.scribd.com/Lew%20Merrick/shelf you will find a document Rotational Dynamic Design Formulae that will give you more information on the subject than you likely would want to know...

[DavoDavo]

So, my stock .75 HP motor in my 9 x 20 Lathe bit the bullet, I'm upgrading to to a 2 HP motor and I can't seem to find a "Rule of Thumb" of belt size to HP ratio formula, (understanding the pulley depth\dia. will have to change too).

The idea is, the new motor will also be VFD controlled, and as such, yes I can control the 'Start-up-to-RPM' and 'Coast-to-stop' and I could actually use the .75 HP rated belt I have.

However, if I want to set my E-Stop to 1 second stop (with a resistor pack) and not the normal Stop button, how do you figure out how big a belt you need to handle the 'Shock Load' of a sudden stop?

Thanks for any help.
Chazz

Hi Chazz,
Good question.
FWIW, I have just replaced the single phase 550Watt, 1500RPM motor (0.75HP) on my 9 x 20 lathe with a 550Watt, 1500RPM (0.75HP) 3phase motor and VFD. I did consider putting a larger motor on but decided against it as the original motor comfortably handled anything I ever threw at it.
I can't imagine wanting to machine anything that would require over twice the grunt of the stock motor, I doubt that the lathe itself is big enough or sturdy enough to go nuts with it. So, do you really need 2HP? Given that you are considering (2?) new pulleys too, it seems like a lot of work for no good reason, even if you stick with the old pulleys you will need to bore out the motor pulley to accept the much bigger motor shaft.

However, if you did go for 2HP I'm sure that the stock 9 x 20 toothed belt (provided it's tensioned correctly) will handle it fine, how often would you be hitting the E-Stop anyway?
Those belts are plenty strong, both my belts handle an E-Stop with no problem, but buy a couple of spare belts if you are worried about breaking one, they aren't expensive.

One more thing, a 2HP motor is physically much bigger than a 0.75HP, installing that so that pulleys line up etc will be a pain.

Good luck. 

[nel2lar]

Chazz
The belt drive does not matter unless the belt starts to slip. The torque specs that are used for classing it to what it can power. You will never need all that torque. The biggest question is your lathe strong enough for that torque that you might put it to. In other words if you intend to take aggressive cuts or hogging you might put undue stress on the lathe. If that is the intent you will destroy your lathe. If you use the lathe the way you have in the past you will have nothing to worry about. If you can run 220V it will cost less to run and it will run cooler.
Good Luck
Nelson Collar

[Chazz]

Thanks for the input, the 240v, 3 Phase, 2HP motor was free.  I have a line on a 1.75, 2.5, 3.5" step cone pulley for 'A' size V belt for the driver but I'd really like to find a 7,6,5" step cone pulley for the spindle however, I can only seem to find 6,5,4" step pulley for the big end.

As to motor alignment, I have 2 journeymen millwrights and a journeyman machinest at my disposal.    I've already upgraded the spindle bearings, tumbler reverse and the lower feed-rate upgrade for the carrage feed.

Thanks again,
Chazz

[Noitoen]

Having a bigger motor with a VFD only means that you can run lower speeds without changing the pulley ratio. I would not hassle with a belt size change. Like it was said before, you will only require the amount of torque the lathe can handle.