Mark VII Low Speed Too Fast

[miken]

Trying to get a predictable result with all the current varying belt designs is a z00.

I think I will set this issue aside until I have more ambition or boredom to resolve it. Was hoping for an easy adjustment fix.
Appreciate the replies, Mike

[lahola1]

I was going to try a metric belt but I got lazy and that 100 RPM wasn't that important to me anymore.
14MM = .551"; 13MM= .512".

[miken]

I was going to try a metric belt but I got lazy and that 100 RPM wasn't that important to me anymore.
14MM = .551"; 13MM= .512".

Yes, there are a lot of factors involved here that I admit I don't fully understand. In theory, with normal fixed pulleys, an increase in the larger pulley diameter would cause a decrease in the RPM of that pulley. So I was thinking a longer belt would actually be better, but there is that auto-adjusting spring on the end of the drive pulley that is taking up some of the additional length of belt, so it is not simply increasing the larger (eccentric) pulley alone.

Anyway, if belt width is the problem, I wonder if adding a spacer to the eccentric side of the idler would push the pulley (#15) slightly toward the speed cam effectively creating a wider belt at the top pulley only? Maybe I will give that a try just to see what happens? However, need to be careful because I think my spring (#20) is compressed about as far is possible, so that bottom pulley diameter can't really get much smaller. I think it may be a combination of a spacer on the top pulley, and a slightly longer belt that has some potential to lower the speed?

Mike -

[miken]

I was going to try a metric belt but I got lazy and that 100 RPM wasn't that important to me anymore.
14MM = .551"; 13MM= .512".

Just a follow-up, curiosity got the better of me so I threw a 1/8" +/- washer in between the eccentric and the spindle drive belt. It did lower the RPM's to 710, measured twice. It's hard to see in the below picture but the washer is there. I can no longer rotate the speed cam to exactly align 700 with the indicator arrow on the headstock, but that is an easy fix I think. My belt measured 15/32" wide. If I add 1/8" it is about 0.59". I don't think they make a belt that size, next size up is 21/32" (5L - 0.656), but a 15mm is 0.59". Just thought I'd share.

Thanks, Mike

[JPG]

Adding washers to alter the sheave positioning will create misalignment of the belt to the opposing pulley. The movable sheaves move in opposite directions. At the extreme positionS, the movable sheave aligns with the opposing pulley fixed sheave.

Adding the washer to the fixed end of the idler pulley is the same as moving the control sheave closer*(all else unchanged, but the belt will be slightly skewed.).* i.e. speed dial towards slow

Considering the small thickness of that washer, demonstrates how a small alteration creates a significant alteration of the end result. Belt width effect is doubled since it affects two different pulley diameters.

I went through the adding washers exercise also. Decided it was not the best thing to do.

Y'all need to realize we are using worn parts that change Lord knows what!

[jsburger]

And a 100 RPM difference effects what? Nothing in my opinion. That is why SS didn't put RPM's on the speed dial.

[JPG]

Ya it is ONLY about a 15% deviation.

[jsburger]

And again before we had cheap digital tachs nobody knew. Inventing problems that don't exist.

[JPG]

And again before we had cheap digital tachs nobody knew. Inventing problems that don't exist.

Perhaps, but do realize the MVII has actual rpm indications on the speed control dial.

I firmly believe with the correct belt and new condition parts the scale is reasonably accurate. 15% error is not reasonable.

And I am not one to think the numbers are all that relevant. They are a starting point to set the speed. The operation will tell if it needs to be altered.

[miken]

. . . I went through the adding washers exercise also. Decided it was not the best thing to do. . .

I agree JPG. I was only trying to determine what affect it would have, and try to figure out a belt width that may be better. I already stated the 100RPM is not very important. I wasn't trying to create any problems, just tinkering around.

Mike -