replacing lower Mark 5 belt with common automotive belt

[rpd]

I'm trying to better understand the mechanical actions of the variable sheaves.

When I change to high speed, I would have expected the same relationship between the top of the motor belt and the outer edge of the lower sheave. That is not the case. There is about 3/16" difference (the belt being below the edge of the sheave).

Is there a logical reason for these anomalies?

The the movement is restrained by the high speed stop in the quadrant assembly to prevent the spindle from going faster than 5200 rpm.
The normal range is 1/8" to 1/16" from the outer rim of the motor sheaves.
Here is a link to the document. How To Do A High Speed Adjustment

[charlese]

Here's an old thread that Nick left on the forum:
(It talks about the Shopsmith belts compared to a Gates belt - With photos.)

http://www.shopsmith.com/ss_forum/viewt ... 700#p36700


Sorry - I mis-remembered the manufacturer of the Shopsmith belts at the time. They were Goodyear - not Goodrich. Must be an age thing.

[JPG]

Yes all 4 sheaves have the same physical dimensions(od, face angle etc.)

The positioning of the fan and floating sheave is pretty much fixed.

The control and idler sheave can however be somewhat positioned by the idler shaft and eccentric.

Actually the thing which determines everything else is the idler sheave. The control sheave moves relative to it.

The result of moving the control sheave causes the floating sheave to move to equalize belt tension.

So you have two sheaves that do not move and two that move in opposite directions. It is the position of those fixed sheaves that determine the resulting positions of the other two. Belt length also affects the resulting positions.

Now for Dusty's belt/pulley differences at lo/hi speed, the idler pulley being too far 'in' would cause that(the control sheave is too close to the idler sheave causing the belt to ride high in the pulley groove).

The high speed adjustment could also affect the belt positioning since that determines the control sheave positioning at the hi speed limit. i.e. allow the control sheave to open up further and the belt will ride higher on the motor pulley.

A pulley consists of two sheaves. The movable sheaves move to open/close the pulley. The idler pulley consists of the fixed idler sheave and the movable control sheave. The motor pulley consists of the fixed fan sheave and the movable floating sheave.

Only things 'independent' are the position of the fan sheave and essentially the idler sheave(it can be moved by moving the idler shaft in/out of the eccentric bushing).

[dusty]

I have created a monster. At least for myself it is. The sheaves are nothing more than pulleys. They are variable size pulleys but pulleys just the same. But for some reason, they do not seem to behave as simple pulleys.

I have been playing with an online calculator with the objective of trying to determine what size drive belt should work properly in the Mark V. I have been doing this based on some fixed dimensions. At low speed the drive belt should be high in the sheave and in some schools of thought it will be flush with the outer circumference of the upper sheave. Mine is that way and the upper sheave rotates at 1196rpm. The measured rpm of the motor sheave at this time is 3583rpm. I was surprised to learn that this is not a fixed rpm. The motor rpm actually varies a few rpm as I move through the speed range. The motor shaft and the idler shaft are 7 1/2" apart. The belt is approximately 26.5".

So I plug the following into this calculator to see what I get.

7.5" separation between pulley shafts.
5.5" diameter of large pulley.
3583 rpm of motor shaft.
1.5" diameter for the motor pulley. (this is a variable that I am going to change)

The calculator says that the upper pulley is rotating at 977 rpm which I know is wrong. It is rotating at 1196rpm. So I have to change the diameter of the motor pulley. Note that the calculated belt length is also wrong. The belt on my machine measures a tiny bit more that 26.5".

Changing the diameter of the motor pulley to 1 13/16" yields a rotation speed of 1180.8 rpm and a belt length of 26 15/16".

Not exact but close. Now I need to figure out why the Mark V does not behave as the calculate things it should.

http://www.blocklayer.com/pulley-belteng.aspx

[JPG]

Your 'calculator' assumes fixed pulleys which means the angle of the belt entering and exiting both pulleys remains the same. Not so with a variable pulley and doubly so with both pulleys varying. The amount of the belt length actually in contact with the pulleys changes.

I certainly do not wish to dig into the geometry to determine(quantitatively) why that makes a difference.

FWIW, the belt also moves 'along the shaft' as the pulleys are opened/closed. The fixed sheaves are on opposite sides to allow for that.

There are good reasons why the Mark 5/V speed control dial is calibrated with letters(somebody neglected to inform the Mark VII injuneers of them). As Nick has said, there is a lot of 'stuff' going on(or words to that effect).

I am surprised your motor runs within 17 rpm of synchronous speed(3600).


I suggest you let the monster go back to hibernating!


I literally spent months gaining what understanding I have with my Mark VII trying to get the belt size 'correct' as well as obtain a suitable belt.

Reeves drives ain't as simple as they appear. Easy to make yes, calibration is a different matter.

[dusty]

Your 'calculator' assumes fixed pulleys which means the angle of the belt entering and exiting both pulleys remains the same. Not so with a variable pulley and doubly so with both pulleys varying. The amount of the belt length actually in contact with the pulleys changes.

I certainly do not wish to dig into the geometry to determine(quantitatively) why that makes a difference.

FWIW, the belt also moves 'along the shaft' as the pulleys are opened/closed. The fixed sheaves are on opposite sides to allow for that.

There are good reasons why the Mark 5/V speed control dial is calibrated with letters(somebody neglected to inform the Mark VII injuneers of them). As Nick has said, there is a lot of 'stuff' going on(or words to that effect).

I am surprised your motor runs within 17 rpm of synchronous speed(3600).


I suggest you let the monster go back to hibernating!


I literally spent months gaining what understanding I have with my Mark VII trying to get the belt size 'correct' as well as obtain a suitable belt.

Reeves drives ain't as simple as they appear. Easy to make yes, calibration is a different matter.

At a given speed, the pulleys are fixed and the belts enter and exit the pulley exactly as though the pulleys were fixed.

Not a chance.

[rpd]

The effective size of the pulley is not the inner or outer diameter but a point in between called the pitch. see link below
V-Belt Transmission Notes and Design

[JPG]

Your 'calculator' assumes fixed pulleys which means the angle of the belt entering and exiting both pulleys remains the same. Not so with a variable pulley and doubly so with both pulleys varying. The amount of the belt length actually in contact with the pulleys changes.

I certainly do not wish to dig into the geometry to determine(quantitatively) why that makes a difference.

FWIW, the belt also moves 'along the shaft' as the pulleys are opened/closed. The fixed sheaves are on opposite sides to allow for that.

There are good reasons why the Mark 5/V speed control dial is calibrated with letters(somebody neglected to inform the Mark VII injuneers of them). As Nick has said, there is a lot of 'stuff' going on(or words to that effect).

I am surprised your motor runs within 17 rpm of synchronous speed(3600).


I suggest you let the monster go back to hibernating!


I literally spent months gaining what understanding I have with my Mark VII trying to get the belt size 'correct' as well as obtain a suitable belt.

Reeves drives ain't as simple as they appear. Easy to make yes, calibration is a different matter.

At a given speed, the pulleys are fixed and the belts enter and exit the pulley exactly as though the pulleys were fixed.

Not a chance.

Foul!!!!

Tain't fair to take a static example and apply it to a dynamic one. My point was that that the calculator(which includes belt length) assumes a static state(pulley dimensions and spacing). Only one belt length will 'compute' for a given pulley pair and spacing. Change the pulley dimensions and the belt length will change.

Unless I missed it, that calculator does not provide for belt length as an input. There are 4 interdependent variables here: pulley 1 od, pulley 2 od, pulley spacing, and belt length. The calculator takes the first three and calculates the fourth.

It also calculates the ratio based upon only the first two.

Independently it also calculates other things using a single pulley rpm as well as pulley separation.

What you need is a calculator that takes pulley 1 od, belt length, pulley spacing and calculates pulley 2 od. The belt angles I mentioned will affect the answer.

[dusty]

Yeah, yeah. I understand all of that. I was putting in variables and even altering the small pulley dimension looking for the combination that would utilize a 26.5" belt.

I know the pulley dimensions on my machine (when the drive belt aligns with the edge of the idler pulley) and I know the length of the two belts that I have been working with.

The hang up is that the empirical data and the calculators do not agree. I trust the calculators and unlike some folks i accept that my "empirical data" must be in error. The fun is in searching for the answer as to why.

[JPG]

Yeah, yeah. I understand all of that. I was putting in variables and even altering the small pulley dimension looking for the combination that would utilize a 26.5" belt.

I know the pulley dimensions on my machine (when the drive belt aligns with the edge of the idler pulley) and I know the length of the two belts that I have been working with.

The hang up is that the empirical data and the calculators do not agree. I trust the calculators and unlike some folks i accept that my "empirical data" must be in error. The fun is in searching for the answer as to why.

What you have no way of 'knowing' is what the creator of the calculator used nor any assumptions made. Without that determining why a difference is impossible.

Suffice to say the 'calculator' does not take into account pulley side slope nor belt thickness.

I do not trust the calculator to accurate predict a Reeves drive.