Motor sizing/Torque requirements

[maintpro]

I know this is a little off topic but I could sure use some help in obtaining the proper motor size/torque specifications for a project that I am working on.

I am building a auger system to deliver powdered gypsum to a weigh station. My auger is 6" in diameter with a 3" pitch and is only 2' in length. The bulk density of the matherial to be conveyed is approx. 63lbs/cu.ft.
The auger system will be mounted directly to our mixer wich will have about 6000lbs of product in it feeding into the auger via an 8" diameter inlet.

I need the auger to turn at 125 rpm and would like to use a DC gearmotor to accomplish this.

How do I go about determining the following items:
1) The amount of torque that is required to turn the auger.
2) The horsepower and gear ratio of the gearmotor that is needed.

 

[randylud]

Boldy go where no man has gone before?

Try this file!

Or, maybe these guys!

Buying a screw conveyor for your needs is probably less expensive than building your own, but if you have the time, money, experience, guts, and other things required, then go for it!

At any rate, good luck!

 

[rsdoran]

I know the rule of thumb for a 6" auger at 500rpm is 1HP per 10ft for a dry material.

Your auger manufacturer should have data tables that will offer the specifics for this.

The actual engineering formula requires knowing:
Dc = Design capacity in cubic feet per hour
e = Drive efficiency
Fb = Hanger bearing factor
Fd = Conveyor diameter factor
Fm = Material Factor
Fo = Overload Factor
L = Total Length of Conveyor
N = Operating Speed in rpm
W = Weight of material conveyed in lbs/ft3

        L N Fd Fb                     
HPf  = -----------------               
           1,000,000 
  
            Dc L W Fm
HPm  = ----------------------- 
            1,000,000 

            (HPf + HPm) Fo
HPtotal  = -------------------
                  e

I did mention its best to contact the auger manufacturer and get the requirements from them.

 

[GregPLC]

Can you say.....Measure it with a micrometer, mark it with chalk, and cut it with an axe?

 

[DickDV]

maintpro, you've got lots of good advice in the above posts. If you will, let me propose another method, slightly back-door in style but probably just as good.

What you need when you get done with your figuring is conveyor speed and torque. Your post says max speed is 125 rpm so that's the first half.

Finding the torque is the hard part if you go straight at it. But consider this: the screw conveyor has a maximum torque limit. You can get that from its manufacturer. Regardless of how fast you go, how heavy the material is, how much grease is in the bearings, etc, etc, that's the torque you will have to stay below to avoid damage when the conveyor plugs.

At the small hp sizes you are dealing with, there is very little difference in system costs based on size so use that torque figure less 10% as your torque.

The formula for hp is torque times rpm divided by 5250. Crank the torque and speed into that formula and that's your hp.

It's at this point that the process differs for DC and AC. Since you said DC and overspeed is not easy to do in DC, figure motor nameplate speed to occur at the same point that the conveyor reaches max speed. Divide conveyor speed into motor speed and that's your gear ratio. Choose the next available lower ratio.

If this had been an AC motor, maximum conveyor speed should occur when the motor is running at 90Hz, typically 2700rpm for an 1800rpm motor. Dividing the conveyor speed into that 2700rpm will be your gearbox ratio.

Using the above technique, I would size the DC drive to provide only 10% shortterm overload capacity. This is because the actual running load would reasonably be less than the maximum torque rating of the conveyor and you can't use more torque than that anyway.

Hope this helps cut down the figuring a bit and still gives you a good working system.

 

[DickDV]

One other thing, maintpro. You need to make sure the motor you choose can remain cool enough at the MINIMUM speed you intend to run at. Generally, in the sizes you are talking about, a motor with its own shaft fan will be ok down to about 1/4 of nameplate speed on a constant torque load such as yours.

If you are going to run slower than that loaded and for more than a few minutes continuous, you will need to go to a motor rated for a wider speed range, say 10 to 1. Such a motor in the small sizes will likely be TENV.

Either way, be sure to check it.

 

[rsdoran]

He did say DC gearmotor so he could possibly find one that runs at 125rpm or slightly higher and gear down using belts and pulleys. The motor would run full speed all the time if maintained at 125rpm so cooling shouldnt be an issue.

In this case I like your approach Dick...a 2 foot 6 inch screw isnt likely to need to move 33,000 lbs 1 foot in 1 minute which is 1HP.

The auger manufacturer will have this data already worked out so thats the easy way to make sure.

I believe with this information you can probably figure out that what you were planning to use will or will not work...I am thinking it will.

 

[Peter Nachtwey]

Has anyone heard of unit analysis?

125 rev    min    3 in  1ft PI                 63 lb        ft-lb 
-------X ------ x-----X----X--x.5ft x.5ftx2ft x----- =12.885----- 
  min    60 sec   rev  12in  4                 ft^3          sec

That is much less than 550 ft-lb/sec which is one HP. However, no data was provide about efficiency. Maintpro can figure out the efficiency and scale up the HP requirements accordingly.
Now one can approach the RPM and gearing requirements.
Why not run an AC motor at 1800 or 1200 rpm and gear it down? Maintpro didn't say anything about needing a variable speed.

 

[DickDV]

Unit Analysis, Peter! That's the old "fractional-dimensional" method I learned in 11th grade chemistry!

It's very handy and I use it often. But, I thought I was all alone in its use!

How about that! There's two of us now!

 

[Steve Bailey]

Three of us and counting

I use it all the time. Maybe you just have to be old enough to have been in school when they taught the technique. Is it still being taught? Any recent grads care to comment?

 

[MikeW]

You don't need to be a rocket scientist to realize the value of the fractional- dimension method. Oh, wait, wasn't that the reason a Mars probe crashed a while back because they didn't know better than to convert feet to meters or vice versa?

I was lucky enough to have had to learn how to use a slide rule as I came of age in the early '70s so estimating results prior to "running the numbers" is second nature.