Reassembling Mark V: problems with drive belt tension

fitzhugh · Post by **fitzhugh** » Wed May 14, 2014 10:36 pm

I'm finally getting my Mark V back together. Only problem I haven't been able to solve is the sheave and belt tension for the drive belt. I follow the various insrtuctions I've found, official and otherwise, and yet I still find the belt is loose when I manually rotate the shafts (sanding disk) and slowly run through the speeds and back. I start at high speed, as that is how the instructions say to put the belt on (and it won't go otherwise). I have noticed that on the motor sheave there is 1/4 inch between the belt and the outer rim instead of the 1/16 to 1/8 inch stated in the official instructions on setting the high speed adjustment.

Together, this seems to me that the lower sheave is not squeezing itself back together strongly enough. The spring is strong and the sheave moves very easily when there is no belt or spring on it - I had it apart and cleaned it, made sure there were no burrs, reassembled it and oiled it.

Any ideas?

I'm eager to use this machine. You all have gotten me really far, thank you!
Fitzhugh

JPG · Post by **JPG** » Wed May 14, 2014 10:54 pm

1) Is the motor belt slipping when it runs?

2) The high speed adjustment may be preventing the belt from rising on the motor pulley.

With the power off, back out the high speed adjusting screw. That will allow the control sheave to open further(you will have to adjust the speed control towards fast). Do this while manually rotating shafts. As long as the control sheave does not interfere with the speed control, all is well. There have been varying lengths of the HS adjustment screw so flush with the top of the nut will not always work. If when backing the screw out the control sheave begins clanging against the speed control, run the screw back in a bit. Look at the position of the belt on the motor pulley. It should be closer to the 'top'.

Now try adjusting the high speed if necessary.

fitzhugh · Post by **fitzhugh** » Thu May 15, 2014 12:55 am

I haven't run it since before taking it apart yet so this was all just rotating by hand. I went and tried again and this time changes speeds really slowly, having realized the motor will spin it many more revolutions per change in speed than when hand turning, and it seems better. I'll recheck the high speed bolt per your instructions.

Note I have to figure out why it is hard to crack in the slowing down direction. I don't recall it feeling that way before

JPG · Post by **JPG** » Thu May 15, 2014 8:54 am

fitzhugh wrote:I haven't run it since before taking it apart yet so this was all just rotating by hand. I went and tried again and this time changes speeds really slowly, having realized the motor will spin it many more revolutions per change in speed than when hand turning, and it seems better. I'll recheck the high speed bolt per your instructions.

Note I have to figure out why it is hard to crack in the slowing down direction. I don't recall it feeling that way before

That is normal. You are squeezing the belt. The idler pulley must rotate to allow the motor pulley to release the added tension. This is the reason for not turning while not running, and for slower adjusting towards slow.

fitzhugh · Post by **fitzhugh** » Fri May 16, 2014 1:10 am

Thanks. I have it an actual run through the speeds under power and all seemed fine.

I appreciate the ready help I receive here. It is what made me willing to try restoring two machines. A ton of great older threads plus new answers to whatever still stumps me.