Shopsmith Forums

JPG wrote: Wed Sep 29, 2021 7:57 pm 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

That's the funny thing about it. I couldn't move the speed cam any further, it was max'ed out and about to come off the yoke, and the RPM was 820. I am not pretending to understand this, but adding the washer dropped it 110 RPM, and now the speed cam is not as max'ed out as before.

Mike -

miken wrote: Wed Sep 29, 2021 9:58 pm

JPG wrote: Wed Sep 29, 2021 7:57 pm 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

That's the funny thing about it. I couldn't move the speed cam any further, it was max'ed out and about to come off the yoke, and the RPM was 820. I am not pretending to understand this, but adding the washer dropped it 110 RPM, and now the speed cam is not as max'ed out as before.

Mike -

Bear in mind the control sheave is aptly named. It controls the idler pulley and the amount of room for the motor belt to ride in the pulley.

By repositioning the fixed sheave of the idler pulley, that decreased the amount of room on the pulley for a given position of the speed control. So the speed is decreased from that before the washer was added(for a given position of the speed control cam). The floating sheave closed the motor pulley a bit more.

Do I understand correctly that adding the washer lowered the speed with and more additional adjustment towards slow is available on the speed control dial? I am curious how thick the washer was.

JPG wrote: Thu Sep 30, 2021 4:51 pm Do I understand correctly that adding the washer lowered the speed with and more additional adjustment towards slow is available on the speed control dial? I am curious how thick the washer was.

JPG, I have been carefully reading your previous journey through this same exact process here: viewtopic.php?f=10&t=13906
Sounds like I was starting down the same path and considering the same potential solutions. I never thought to just push the eccentric in a bit as opposed to using a washer, and I see you tried both. I could have saved some time disassembling. Anyway, I am not likely to find a new-old-stock Shopsmith 507490 belt as you did. The 29.5" length is counter-intuitive to me at this point, but I am going to try to find a 14mm (0.55") X 750mm (29.52") which should be a close match if the advertised dimensions are true. I'm considering adding the idler/rack protection as well.

Yes, I am going to go remove the washer now and measure it. I ran about 6 RPM tests last night and they were all right around 710. I am not using a digital laser RPM meter, I am using an old Starrett 104, so RPM is based on how fast you can react to a stopwatch. When I remove the washer I am going to take another RPM reading and if it is back to 820 RPM than I will know I am not going crazy.

Yes, in one of my previous pictures you can see the speed cam is completely max'ed out without the washer. I will take another pic of the position before I remove the washer but I am sure it displays more exposed cam at the end. Note though, that it is not really possible to rotate that cam any further without likely damaging the plastic cam. There is one additional available detent left on the dial, but it cant be turned.

Also note, that the first set of pictures in this post are from my first Mark VII, and the others are from a backup headstock. I see that could be confusing.

Thanks, Mike -

JPG wrote: Thu Sep 30, 2021 4:51 pm Do I understand correctly that adding the washer lowered the speed with and more additional adjustment towards slow is available on the speed control dial? I am curious how thick the washer was.

Ok, the picture below is the speed cam with the washer installed. There is a bit more cam end visible than without washer (bottom picture), which makes sense. I removed the washer and I am back up to about 825 RPM. The washer was 0.156" thick (my mistake previously estimating 1/8" thick).

So to summarize; the 0.156" thick washer will drop the RPM about 110 RPM without any other changes. I can't explain it. Maybe because I am effectively only adding belt width to the top pulley that somehow makes a difference?

I don't think I am really adding 0.156" to the width of the belt because when I add the washer I can't really get the speed cam rotated to the exact same spot (detent), so maybe the difference is the washer thickness minus the distance between detents? Complete guess.

Interesting puzzle.

Hopefully, I can just find a 14mm X 750mm belt.

Mike -

miken wrote: Thu Sep 30, 2021 6:27 pm

JPG wrote: Thu Sep 30, 2021 4:51 pm Do I understand correctly that adding the washer lowered the speed with and more additional adjustment towards slow is available on the speed control dial? I am curious how thick the washer was.

Ok, the picture below is the speed cam with the washer installed. There is a bit more cam end visible than without washer (bottom picture), which makes sense. I removed the washer and I am back up to about 825 RPM. The washer was 0.156" thick (my mistake previously estimating 1/8" thick).

So to summarize; the 0.156" thick washer will drop the RPM about 110 RPM without any other changes. I can't explain it. Maybe because I am effectively only adding belt width to the top pulley that somehow makes a difference?

I don't think I am really adding 0.156" to the width of the belt because when I add the washer I can't really get the speed cam rotated to the exact same spot (detent), so maybe the difference is the washer thickness minus the distance between detents? Complete guess.

Interesting puzzle.

Hopefully, I can just find a 14mm X 750mm belt.

Mike -

What is puzzling me is the speed control appears to have been moved also????

Yes, my mistake, I forgot I moved that bit between that bottom pic taken yesterday and the top pic from today, but the results from today still stand. I haven't rotated the speed control since I took the top pic with the washer installed, and the cam looks pretty much the same now as the bottom pic from yesterday with the washer removed. Will add a new pic without the washer shortly.

Metric belts seem to come in 10, 13, 17, 22 mm widths.

JPG wrote: Thu Sep 30, 2021 8:34 pm Metric belts seem to come in 10, 13, 17, 22 mm widths.

Yes, I have searched a little and haven't found anything yet either. I have a clausing lathe belt that measures 0.51" but its 36" long, and still not wide enough. Maybe clausing has some other options? I appreciate your willingness to go through this again, but I am thinking that my options are limited on this one. Either find a belt that works (trial and error), use a washer on the backup headstock and let the belts run slightly misaligned, or just forget about it. I can say that after seeing what 710 RPM looks like I do like it a lot better than 820. I may still make the idler to protect the plastic gear rack. Maybe I will go back to considering a version of the speed reducer Scott Markwood builds here: https://www.youtube.com/watch?v=5ZNIDkewy0U

Below is a new pic of the max'ed out speed cam with the control dial thing in the same location.

Appreciate the help JPG.
Mike -

That 710 rpm? Looks very reasonable. How about dial calibration? How about other end(fast)?