Step Pulley SFPM and Torque

H

HuffHK

Joined
Oct 28, 2013
Messages
18
Hi All,

I'm shopping for a motor and wondering if more experienced users have any advice. Short of a VFD setup I plan on a 1.5 hp with a pulley set up. I'm leaning towards 1725 rpm mainly because that's what I have found (Lesson TEFC farm duty for just over $200). I have 2-5" step pulley on my 4" drive wheel currently. Calculated SFPM are as follows:

Drive to Driven SFPM
6" to 2" 5419
6" to 3" 3613
6" to 4" 2710
6" to 5" 2168

5" to 2" 4516
5" to 3" 3009
5" to 4" 2258
5" to 5" 1806

4" to 2" 3613
4" to 3" 2409
4" to 4" 1806
4" to 5" 1445

3" to 2" 2710
3" to 3" 1806
3" to 4" 1355
3" to 5" 1084

2" to 2" 1806
2" to 3" 1204
2" to 4" 903
2" to 5" 723


My question is what size drive pulley should I get for the motor. If i can manage to find a 6" step pulley that would put my available SFPM around 5400 to 1080. Is a 5" pulley more ideal for stock removal on large blades (4500ish to 700ish SFPM)

Finally, and perhaps most importantly I'm a bit concerned about power/torque. With a 6" drive pulley and 3:1 ratio does that put my effective HP/torque around .5 (assuming a 1.5hp motor)? Is a drive pulley this large impractical. The other option would be a 3400 RMP motor with smaller drive pulleys.

Anyone have experience with a similar set up?

Thanks in advance.
 
Torque and HP are different things. The motor is still delivering 1.5HP into any size drive pulley. The amount of torque is a function of the pulley size, because the speed changes with pulley size.

To look at it in another way, power (HP) is a rating of how much work can be done, and torque is a rating of how fast it got done.

Thus, at 5000RPM, a 1.5HP motor delivers just about 1.5 pound feet of torque. At 2500RPM, it delivers twice the torque.

IIRC, HP = Torque X RPM / 5252 and Torque = HP X 5252 / RPM

Getting a faster motor won't get more torque, it just gets a faster RPM. Same HP and thus half the torque. I feel that a 1750RPM motor is better than a 3500RPM one if there is a choice......but for numerical comparison, they are the same.

In a race car, or a 4X4 truck, the torque is important.....in a belt grinder, it is all about HP and belt speed..........and very little about torque.

In summation, I think most folks greatly overthink this when applied to belt grinders. Pick a motor that is large enough for the type of grinding you plan on doing. For a 2X72 belt grinder, 1.5-2HP is plenty in most all cases. Pick a set of speed reductions that will allow you a belt speed capable of doing the work efficiently and safely. For most of us, that would be between 1000 and 5000SFM. Three or four steps is probably all you need, ............ so just pick the pulleys that place the belt speeds in that range and don't worry about the torque.

If more control is absolutely necessary, get a 3PH VS system.
 
Stacy said it very good
most folks greatly overthink this when applied to belt grinders
Click to expand...

I'm not sure of your experience level, but my level is low and I need the control of lower belt speeds for final blade work. Also, remember a fine grit belt requires a MUCH slower SFPM than does a coarse grit hogging belt. For my setup I've got a 4 step Vee pulley: 2-2.5-3.5-4 size (not the best, but was available) on the motor and the drive shaft. The drive wheel for the belt is a 2.7" diameter. I'm using a 2 hp 3450 RPM motor (from a junked air compressor). This gives me calculated belt speeds of 1219/1742/3414/4877. This works ok, but if I had a choice I think I'd go for a 1725 RPM motor with a 4.5" drive wheel for belt speeds of 1016/1452/2845/4064 SFPM.

Things are really humming at that 4877 SFPM speed and I think 4000 SFPM would be plenty for hogging, and perhaps the 1000 SFPM would be more precise? Not sure about that because the 1200 SFPM I've now got is pretty good. I do think I'd like the 4.5" drive wheel better than the 2.7" drive wheel I'm currently using..... When using a really slack belt for contouring I've had the 2.7" wheel to slip a tad, but I was pressing harder than should be at the time I'm sure.

Would a VFD work better? Not at all sure about that, I have the selection of 4 speeds and it's only a matter of a few seconds to change pulley positions. Motor is hinged and weight of motor puts required tension on Vee belt. You do have to wait for motor to stop, make change, then start motor again. With VFD no motor stopping is required - just turn knob and go.

Good luck and have fun:)

Ken H>
 
Thanks for the replies.

I'll shoot for 1000 to 5000 SFPM and not worry about what pulley configuration it takes to get there.
 
HuffHK said:
Thanks for the replies.

I'll shoot for 1000 to 5000 SFPM and not worry about what pulley configuration it takes to get there.
Click to expand...

But it is the pulley ratio that dictates sfpm, so you have to be concerned about the ratio. How would you shoot for a goal sfpm and not do the math on the pulley ratios and ultimate sfpm at the drive wheel?

I might have missed your point of the above statement.
 
From the OP, I get the impression that you are looking to fit a single-groove pulley on the motor, using it to drive each step of the stepped pulley?

If so, it seems like hard work in terms of the engineering involved. In most cases, step pulleys are paired, giving a constant centre-centre distance and eliminating the need to move the motor axially to line it up with each step.

A pair of 4,3,2 step pulleys will give 3 speed ratios; 4:2 for twice motor RPM, 3:3 for motor RPM and 2:4 for half motor RPM.
 
Doc- I did a bunch of pulley to SFPM calculations in my OP. The statement in my second post was not referring to just using a random pulley configuration but that I should not worry about what size pulleys get used as long as the desired speeds are achieved. Obviously the correct ratios will be required depending on motor RPM and Drive Wheel circumference.

Tim- Based on my calculations it would be impossible to get a wide enough range of speeds with a single-groove pulley on the motor. Unless you had some crazy 8+ groove pulley on the driven shaft I doubt they make those. As far as center-center alignment, there are posts in this forum that describe how to make a sliding hinge so that the motor articulates for belt tension as well as adjusts side to side for belt alignment. You could probably use a longer drive belt resulting in a smaller angle coefficient for similar end results.

A set of two 3 step pulleys actually produces 7 different speed ratios.
 
Don't for the belt tension will change as a combination different than the standard 2-4; 3-3; 4-3 combinations is changed. i.e. - 2-3 will require a different length belt, unless the motor can slide a bit to accommodate. Not much, but a tad.

Ken
 
HuffHK said:
Doc- I did a bunch of pulley to SFPM calculations in my OP. The statement in my second post was not referring to just using a random pulley configuration but that I should not worry about what size pulleys get used as long as the desired speeds are achieved. Obviously the correct ratios will be required depending on motor RPM and Drive Wheel circumference.

Tim- Based on my calculations it would be impossible to get a wide enough range of speeds with a single-groove pulley on the motor. Unless you had some crazy 8+ groove pulley on the driven shaft I doubt they make those. As far as center-center alignment, there are posts in this forum that describe how to make a sliding hinge so that the motor articulates for belt tension as well as adjusts side to side for belt alignment. You could probably use a longer drive belt resulting in a smaller angle coefficient for similar end results.

A set of two 3 step pulleys actually produces 7 different speed ratios.
Click to expand...

With what you have in mind, what would be the sfpm for the 7 ratios?
 
With a 4" drive pulley and a 1725 motor, you could get:

2-4 = 903
2-3 = 1204
3-4 = 1355
3-3 = 1806
4-3 = 2409
3-2 = 2710
4-2 = 3613

Would that do it?
 
Those #s Ken H posted look right to me. I don't have one but I assume those are the speeds on a standard KMG.

Most people/posts I've seen suggest shooting for a top end of around 5000 SFPM. In order to achieve this with a 1725 rpm motor you need bigger pulleys on the motor side and/or a larger than the standard 4" drive wheel/pulley.
 
To be honest, I've found that grinding performance is more dependent on the belt and how new it is. A brand new 50 grit blaze on medium speed (I have 2-3-4 set) will cut curls when grinding, I haven't found much need to run high speed.

I have worked with variable speed machines for most of my career, but honestly for knife grinding I've been pretty happy with my 2x72 on medium or low. I still can't wait to go variable, but I don't feel that it is really hurting me in any way.

Edit: I guess what I'm saying is that I worried a lot about the step pulleys and speed ratio, and I don't think it is as important as I thought. It's nice to have options, but belt condition and technique are more important IMO.
 
From the KMG website (with 1725 RPM motor):
With the standard 4" drive wheel on the KMG, this set-up produces 3 ideal belt speeds of approx: 900, 1800, and 3600 ft/min
Click to expand...

Those do look like good speeds - I'd like to slow my grinder down on low to the 1,000 SFPM range, but it takes a 2.2" drive wheel for that with my 3450 RPM motor. I'm concerned that is too small... Anyone try using a 2-1/4" diameter drive pulley?

Ken H>
 
My KMG is the 3 speed pulley, 1725 RPM, 1 1/2 HP model, with the 4 inch drive wheel. Much like Ian mentioned above, I rarely find myself using the highest speed setting. About the only time I feel like I NEED to use it, is if I didn't get close enough to the profile with my bandsaw. A Cubitron ll belt in 36 grit, on high speed, will remove LARGE AMOUNTS of metal in a hurry. For the most part though, all of my grinding is done on the medium speed setting (60-220 grit). When I need to move beyond 220 grit or so, I slow it down. This set up has worked well for me so far.

Adam Buttry
 
I agree, 3600 SFPM is humming and will really cut metal. I used that speed for hogging and even cut just a tad at a calculated 4800 SFPM and boy that really does move some metal. I don't find a need for anything approaching 5000 SFPM speed. I can only imagine the folks who say they really need over 5,000 SFPM in a grinder. A belt breaking at that speed would really be scary!!

Ken H>
 
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