Shopsmith Forums

First a "fact check":

1) The sheaves are 5 1/2" in diameter (rim to rim).

2) It has been stated that the motor belt should ride 1/8" inside of that rim. Therefore, the diameter of this pulley is a maximum of 5.25 (radius of 2.625").

3) The motor belt is 26 3/4" long.

4) The center to center separation of the sheaves is 7 1/4". Therefore, 12 1/4" of the motor belt rides with the pulleys at all times (at all speeds).

5) If operating at Low Speed, the belt length riding in the upper sheave is 9.0321" (5.75*3.1416/2). This leaves 3.6106" of belt to ride in the lower pulley which means that the lowed sheave is effectively a pulley with a diameter of 1.1493".

Therefore the ratio between the two sheaves is 2.5016. WITH A MOTOR SPEED OF 3450rpm, THE MINIMUM SPEED OF THE UPPER SHAFT (idler) IS 3450/2.5016 or 1379rpm.....then the drive shaft is 1379*.6 or 827rpm. (1379rpm and 827rpm)

The charts show these two speeds to be 1120rpm and 700rpm. Likewise, the maximum speed for the drive shaft is 8255rpm and not 8400.

Which is incorrect - my calculations or the charts.

I am not an expert. I have a question though. What would the calculated ratio be if using the inside of the Drive Belt at the narrow point? It seems the sheave angle which remains the same it would make no speed difference, but would it affect rotational speed of output?

Everett

Dusty: The belt length riding on each pulley will be (12 1/4)/2 ONLY when the pulley diameters are equal(3450/3450) when the belt contacts 1/2 of each pulley(180²). When the speed is altered the length of the belt on each pulley will vary as the belt to pulley angle changes.
I do not wish to get into the effect of that angle change(s).

I agree that the available belt length available to contact the two pulleys IS 12 1/4".

The thing common to both pulleys at ANY speed is the belt fpm, not the belt wrap.

It is late and I am away from home so this is as far as I will take this presently. I am not on a 'pleasure' trip(work to do ).

everettdavis wrote:I am not an expert. I have a question though. What would the calculated ratio be if using the inside of the Drive Belt at the narrow point? It seems the sheave angle which remains the same it would make no speed difference, but would it affect rotational speed of output?

Everett

I've done as you suggested and it does alter the numbers but the results still do not come close to matching the speed charts that Shopsmith has created.

I thank you though for the input. Every little bit of constructive input helps.

I'm not going to give up on this but I don't really ever expect to understand how Shopsmith came up with their numbers.