Belt Grinder Belt RPM

S.Grosvenor

S.Grosvenor

Fulltime KnifeMaker
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Jan 24, 2010
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I was curious if anyone else has checked there 2x72" belt grinder RPM
not motor but actual belt RPM. I have a non contact photo tach that reads a small piece of reflective tape that can be applied to anything so you can measure RPM, I checked mine and its right @ 1200rpm (actual belt rpm) with VFD turned all the way up.

Wondering if that's a good speed?
 
Stroke feet or linear feet per minute, need to know circumterance of drive wheel times by R P M . So if you have a 4 inch drive wheel times pi or 3.14---- is about 12 inches times times 1000 r p m = 12000 inches divide by 12 for feet and it is moving about a thousand feet a minute.

May need to change pulley's to get right speed if your machine is not varible speed driven.
 
Seems pretty much ideal to me.

It's a 6' belt, turning at 1200 RPM, so works out at 7200 sfpm, unless I'm missing something blindingly obvious.

I assume the OP just stuck one piece of tape on a 72" belt and zapped it with the tacho.

Works out at around 36.5 metres/second. My contact wheels are rated for 36M/sec, and I think they're pretty standard in that respect, so it's about as fast as most of us can reasonably go without having to worry about things breaking.
 
I'm intrigued! The math sounds right Tim; 7200 sfm is smoking fast.

What motor and drive wheel are you using or is it a direct drive setup?

7200 is not to fast for quality ceramic belts; they are made to be used at high sfm along with a lot of pressure.

At that speed you will find out if your bearings are of good quality or not.

Fred
 
Fred.Rowe said:
I'm intrigued! The math sounds right Tim; 7200 sfm is smoking fast.

What motor and drive wheel are you using or is it a direct drive setup?

7200 is not to fast for quality ceramic belts; they are made to be used at high sfm along with a lot of pressure.

At that speed you will find out if your bearings are of good quality or not.

Fred
Click to expand...

KMG ish setup with a 1hp motor with 3" pulley to a 3" pulley that drives the 4" belt drive.

so 7200, no wonder I'm bouncing sparks of the floor
 
I'm guessing the SFM is not 7200, unless we are talking a 3450 RPM motor, even then, I'm not sure. What is the motor RPM?

I have to look up my formula for motor RPM to pulley to drive pulley to SFM. I have a 1750rpm motor running a 4 step pulley to a 4 step pulley on a 6" drive wheel. My speeds are roughly 1000, 2000, 3700, 6700 SFM. I plan on getting a 4" drive so I can run around 700, 1600, 2500, 5000 SFM.
 
No I think 7200sfpm is correct. Lets see 72" belt equals 72 x 1200rpm =86,400 inches per minute.
86,400/12= answer in surface feet per minute.
86,400/12=7200SFPM

Yes I agree its pretty fast, I personally go 4,680sfpm and feel its pretty good for Ceramic and slow it down for anything else.
 
Let's start here - SFM is a constant that we can all use. It determines how fast the belt of any length passes across our workspace. Direct drive and pulley drive may use different formulas. Belt circumference is irrelevant. Motor RPM, motor pulley dia., drive pulley dia., drive wheel dia. are the things that matter. Direct or on a belt, there is a difference.
 
As described, it's a 1:1 ratio (vee?) belt drive onto the grinding belt drive pulley, which is 4" diameter.

4" x Pi = 12.566" per motor revolution

7200 SFM x 12"/ft = 86400 IPM

84600 IPM / 12.566 "/Rev = 6732 RPM

Sounds like it's a 2-pole motor running at 120 Hz max on the VFD.
 
OK, I was trying to go off of memory as to pulley sizes but I should have known better to not try and remember something,

Here are the actual numbers,
Motor is 1 1/2 HP 3450rpm
Motor drive pulley is 5"
Shaft drive pulley is 3"
Belt drive pulley is 4"

All this equals 1197 Actual Belt RPM, couldn't get it to hit 1200 this morning and that was measuring with 1, 1/4" square piece of reflective tape on a 72" belt.


WOW you guys are impressive with the numbers, the only reason i used my photo tach was to get the actual rpm of the belt without trying to cipher this pulley + this drive wheel should equal X , and was not aware of the SFM standard number or the formula to figure it out. My caveman like instincts were to just measure the belts actual rpm.
If 6500 sfm is good for max I will just run it @ 80% or so. Thank you guys for the explanation, I'm gonna go hammer on something hot now!
 
Last edited:
Where I see the problem is the motor pulley. It is too large. Switch it to a step pulley and you can go from slow to fast. A 2/3/4/5 would give a full range from slow to screaming fast.

As an alternative, switch your shaft and motor pulleys, and you will drop the speed 33%. That will give you a belt speed of 800RPM which equals 4800 SFM.
If the two pulleys are the same shaft bore, this will cost nothing. If not, just change the motor pulley to a 3" or 4", or the step pulley suggested above.
 
On the amended numbers:

Life's too short to worry about 1197 vs 1200 RPM, so we'll call the belt speed 1200 RPM as before, giving 7200 SFM

7200 SFM x 12"/ft = 86400 IPM

84600 IPM / 12.566 "/Rev = 6732 RPM at the drive pulley shaft, so at the driven pulley on the first drive.

Motor pulley is 5" and shaft pulley is 3", so motor runs at 3/5ths of the shaft speed.

3/5 * 6732 RPM = 4039 motor RPM.

So, assuming it's a 2-pole motor, it looks like the VFD is set to give "about" 70 Hz at maximum speed.

As the VFD gives speed control already, there's not very much to be gained by fiddling with drive ratios, unless lack of torque is a problem.

If things slow down noticably, particularly when grinding on a flat platten, reducing the first stage drive ratio will increase the available torque in line with the speed reduction.

For instance, reducing the motor pulley from 5" to 4" will reduce belt speed by 20% and increase the torque available at the (grinding belt) drive pulley by 20%.

It bears repeating that it's only worth doing if the extra torque is actually needed.
 
timgunn1962 said:
On the amended numbers:

Life's too short to worry about 1197 vs 1200 RPM, so we'll call the belt speed 1200 RPM as before, giving 7200 SFM

7200 SFM x 12"/ft = 86400 IPM

84600 IPM / 12.566 "/Rev = 6732 RPM at the drive pulley shaft, so at the driven pulley on the first drive.

Motor pulley is 5" and shaft pulley is 3", so motor runs at 3/5ths of the shaft speed.

3/5 * 6732 RPM = 4039 motor RPM.

So, assuming it's a 2-pole motor, it looks like the VFD is set to give "about" 70 Hz at maximum speed.

As the VFD gives speed control already, there's not very much to be gained by fiddling with drive ratios, unless lack of torque is a problem.

If things slow down noticably, particularly when grinding on a flat platten, reducing the first stage drive ratio will increase the available torque in line with the speed reduction.

For instance, reducing the motor pulley from 5" to 4" will reduce belt speed by 20% and increase the torque available at the (grinding belt) drive pulley by 20%.

It bears repeating that it's only worth doing if the extra torque is actually needed.
Click to expand...

Well, the VFD is on a 15amp 110 circut and when I want to Hog and press a little harder, it pops the breaker immediatly so maybe going to a 4" drive pulley may help with this, thanks for the explanation.
 
Dang, I totally missed the VFD sentence. (Note to self - slap forhead)

Yes just reverse the pulleys or change the motor pulley to 4". Problem solved.
 
greets

I know this is old thread but I need help understanging it...

I'm looking at this calculations and I'm bit confused with conversion... so if anyone is willing to simplyfy this to me I would be greatefull

direct drive...
motor RPM = 2860
pulley / wheel = 3,15" or 80mm
belt lenght is 72" or 6' or 182cm

so, 3,15" x 3,14Pi = 9,89" per revolution
9,89" x 2860 rpm = 28285" per minute
28285" /12 = 2357' per minute

so, is that IT - is that surface feet per minute? or I need to calculate belt lenght and width in it?
what am I missing here?

thank you
 
The length of the belt doesn't matter. 6' or 10'...it will be traveling at 2357ft/min.

Your math looks good.
 
ft/min is actualy SFPM or SFM? surface feet per minute?

so, for universal fixed speed grinder, is it any good for starters?

until I upgrade with VFD, but then I'll need to step up wheel diameter?

thank you again
 
Yes SFM. That speed will be a medium speed for a grinder. Most fixed speed grinders run at 4000-4500 SFM. Some even faster. Can you change the drive wheel to 5"?
 
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