another pulley question

[naughtyboy]

still trying to work out right speed for project im working on.
i know if i use 1" pulley on motr and 4" on what im driving that will give me 1/4 motor speed at driven bit.
but how do i work out pulley size on motor?
for example if motor runs at 100rpm and as 1" shaft and i put 4" pulley onto shaft will speed at outside edge of pulley be 25% of shaft speed?

sorry if its bit long winded.
cheers

 

[rtbcomp]

Depends what you mean by speed.

Speed in RPM will be the same at the shaft as the edge of the pulley, but the linear speed will increase as the pulley size increases.

For your purposes ignore linear speed and use RPM in your calculations.

 

[mike os]

Pulley size does not matter too much, it is the ratio of the sizes that matters

 

[rtbcomp]

Larger pullies grip the belt better because they have more belt in contact with the pulley, the running tension in the belt is less and belts are less likely to slip.

 

[rikrobson]

It may be better to ask what size pulleys to use for the application.
what is the desired speed of the driven shaft and what speed is the motor spindle. This will give the ratio needed. Then from the power/torque requirements we could work out the best sizes to use.
You need to measure the shaft speed in RPM and forget about the linear speed. It will make things easier.
What is the motor power, size etc?
Also remember that the torque will also be affected by the ratio of the pulleys.

 

[FranV8]

Just be a bit careful with smaller pulleys - belts have a minimum bending radius, below which you'll break the back of the belt.

 

[aametalmaster]

Just type in some numbers and change them to suit...Bob
http://www.forestryforum.com/members/donp/pulleycalc.htm

 

[Hitch]

You need to work out the circumference of your pulleys..... You either need to work in M/S or RPM, and need to know the periphial speed of what size pulleys you put on there.

For instance,

A 102mm(4") pulley, at 100rpm, will travel 314 mm per rev, so in a minute, it will have traveled 18840mm per min.

The shaft at 25mm(1") will travel 78mm per rev, in a min it will have traveled 4713mm.

Divide 18840 by 4713 = 3.997

So if the shaft had a pulley at 25mm, and you changed it for a 102mm, you would effectively reduced the rpm by 4 times. Assuming you keep the driven pulley the same as whatever it was when you worked out the speed of the drive pulley.

Chances are, you wont get a 25mm pulley for a 25mm shaft,

Think thats worked out right (bit of rounding off happened there to ease the sums)

 

[rtbcomp]

That's too complicated Hitch, I don't see why peripheral speed and cirumference are relevant. Motor speeds are quoted in rpm and the driven machine will have a required RPM so (Motor RPM x Motor Pulley dia) = (Load RPM x Load Pulley dia).

The only reason I can think of where you need to know the linear speed would be for something like a conveyor belt where you need to know how many widgets per hour it can deliver.

 

[Hitch]

Just re read...
Yes, your right, way over complicated that didnt i.

 

[scott0031]

Um well its really very simple.......... this is the way it was taught to me at college all those years ago. still the standard way as far as i know without fancy words overly complicating it.


Driver divided by driven x input rpm gives you output rpm you can use diam or circumference makes no odds from memory.

So driver being the one on the motor shaft, the driven being the output shaft. Anything in between is an idler and does nothing other than change direction.

It really is that simple.


Kind regards,


Scott

 

[graffian]

A 4" pulley, at 100rpm, will travel 314 mm per rev, so in a minute, it will have traveled 18840mm per min.

The shaft at 1" will travel 78mm per rev, in a min it will have traveled 4713mm.

Divide 18840 by 4713 = 3.997


(bit of rounding off happened there to ease the sums)

4/1:=3.997
Nicely rounded :-)