Thread: Help with math

I know there's a thread discussing this, however i'm lousy at math.
I have a small .5 amp 1600 rpm motot with 1/4" shaft. I would like to use it for slow grinding if this is possible. If it is then what size pulleys would i use to bring a 3" gringing stone to approx. 90 to 160 rpm or what ever surface speed I would need.
Hope this explaines what I'm trying to do.
Thanks for any input.

There should be a direct linear relationship between the difference in rotational speed and the difference in pulley diameters. So for you to step down a 1600 rpm motor to a 160 rpm grinding wheel you will need to go from a 1" pulley on the motor to a 10" pulley on the grinding wheel shaft. This is probably better done with an intermediate "jack" shaft between the driving and driven members so you have 2 steps to reduce the size differential in pulleys.

good luck

Originally Posted by Bruce

There should be a direct linear relationship between the difference in rotational speed and the difference in pulley diameters. So for you to step down a 1600 rpm motor to a 160 rpm grinding wheel you will need to go from a 1" pulley on the motor to a 10" pulley on the grinding wheel shaft. This is probably better done with an intermediate "jack" shaft between the driving and driven members so you have 2 steps to reduce the size differential in pulleys.

good luck

Thanks, but what would the jack pully and new grind wheel pully be ?
As I said i'm a math idiot

How to Calculate Sheave Ratio | eHow.com

The issue here is physics, not math.

the linear grinding speed is not going to change with a simple two-pulleys system. the only thing you will accomplish is that the smaller wheel will make much more revolutions per minute than the larger wheel, and the relationship is linear, i.e. the wheel with twice smaller radius/diameter will spin twice as fast, the wheel with three times larger radius/diameter will spin three times slower.

however the speed that the belt is moving relative to the steel is going to be exactly the same on both wheels.

If you want to decrease the linear speed you'll have to use more complicated system where you have two pulleys stacked fixed on each other so that they always make the same number of revolutions per minute, and then the linear speed is directly proportional to the radius, i.e. twice smaller radius means twice smaller speed.

Originally Posted by gugi

the linear grinding speed is not going to change with a simple two-pulleys system. the only thing you will accomplish is that the smaller wheel will make much more revolutions per minute than the larger wheel, and the relationship is linear, i.e. the wheel with twice smaller radius/diameter will spin twice as fast, the wheel with three times larger radius/diameter will spin three times slower.

however the speed that the belt is moving relative to the steel is going to be exactly the same on both wheels.

If you want to decrease the linear speed you'll have to use more complicated system where you have two pulleys stacked fixed on each other so that they always make the same number of revolutions per minute, and then the linear speed is directly proportional to the radius, i.e. twice smaller radius means twice smaller speed.

Thanks but that really jumbled my head!

I presume you intend to use the motor to power a spindle arrangement on which the buffing wheels are used, with a pulley on both the motor and spindle? If so, what you are talking about is the speed of the outside of the buffing wheel going past the job, measured in surface feet per minute. The spindle speeds of the motor and spindle remain constant.

For instance, if the motor is fitted with the same size pulley as the spindle, then you have a direct 1:1 relationship. That is, the speed of the outer perimeter of the pulley on the motor is going at exactly the same speed as that on the spindle.

However, once you fit a buffing wheel to the spindle, the sfpm at the outer perimeter of the buffing wheel changes - it will be a constant rate at the driving (motor) and spindle but will vary with each different diameter wheel fitted to the spindle.

A rough formula for calculating sfpm is:

sfpm = 1/4 diameter of buffing wheel x spindle speed (rpm)

so for a 1600 rpm motor with a 4 inch diameter wheel:

sfpm = 1/4 of 4 x 1600 = 1x 1600 = 1600

If the buffing wheel is larger the speed increases and vice versa. To decrease it use a smaller wheel - in this instance a 2" diameter wheel would give a sfpm of 800. To decrease it use a smaller wheel or alter the relationship between the pulleys on the motor and the spindle - a sort of gearbox effect.

1600 sounds a pretty good speed to me. Just stick to using 4" diameter wheels.

The speed in rpm of the motor is not the critical speed (although it has of course a profound influence) - the speed of the outer edge of the buffing wheel going past the job is. It is this speed which will affect burning the job, cutting the job (cutting as in grinding, etc), and what abrasives you use.

Regards,
Neil

paco,

You'll have to have gugi and Neil Miller further explain this to you.

I must be in error, but I've made grinders with a pulley mounted on the motor connected by a Vee belt to a pulley mounted on a shaft with a grinding wheel mounted on it and the rotational speed of the grinding wheel varied from the rotational speed of the motor in direct proportion to the difference in size of the two pulleys. Say the grinding wheel shaft pulley is 4" and the motor shaft pulley is 2", the grinding wheel will rotate at 1/2 the speed the motor is turning.