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Re: mark VII speeds up on its own above 1800 rpm
Posted: Fri Aug 04, 2017 7:38 pm
by lahola1
thanks for clarification.
i put my original belt back on,(od approx 30.25" w .530" thick.300").
tach now says 870rpm @700 setting; 2730rpm@2400 setting. belt ia approx 1/8" from top od of idler sheave is fully open; spring compressed max AND motor belt is slightly rubbing on frame for the worm gear that moves headstock back and forth. Sooo, I can only move the fan sheave closer to the motor.
Re: mark VII speeds up on its own above 1800 rpm
Posted: Fri Aug 04, 2017 7:43 pm
by lahola1
by the way; back to the original reason for this thread, I put in 'the fix' similar to yours, JPG ( drilled speed shaft and tapped for an 8-32 screw, made some washers and tightened speed control down. now the speed control works good.
thanks for your insight and engineering.
Re: mark VII speeds up on its own above 1800 rpm
Posted: Fri Aug 04, 2017 7:46 pm
by lahola1
i meant to say that the motor sheave is fully open and the spring is fully compressed.
Re: mark VII speeds up on its own above 1800 rpm
Posted: Fri Aug 04, 2017 8:17 pm
by JPG
Relocating the fan sheave closer to the motor will shift the speeds downward.
I feel it is 'risky' to not have hard control of the fan sheave/motor clearance. Risky in that it is subject to moving due to vibration.
IIWY, I would reposition the fan sheave 1/16" from the motor.
I believe that will also move the belt away from the headstock positioning parts.
Do realize with out an 'original' belt, the speed range will be 'different'.
The dial markings are an estimate at best.
Re: mark VII speeds up on its own above 1800 rpm
Posted: Fri Aug 04, 2017 9:11 pm
by lahola1
i moved the fan sheave closer to the motor (i guestimate it to be .090-.100" from motor cover.
tach now say 838rpm @ 700 setting; motor belt is now slightly away from worm gear frame and spring is not quite fully compressed on the moving motor sheave.
it's noisier now. i think the fan sheave is rubbing on the tin cover over the motor. i'll have to see if i can lock that down somehow.
btw, JPG, i think i read somewhere that you purchased an OEM motor belt.
did you install it? did you get rpm readings closer? what are the dimensions of the OEM belt?
Re: mark VII speeds up on its own above 1800 rpm
Posted: Fri Aug 04, 2017 11:41 pm
by JPG
I have not yet installed the oem belt.
I do not have much faith re measuring belt lengths - however the M VII belt (507490) is 3.5" longer than a Mark V belt (504193).
Since 26.5 is the 'nominal' agreed upon length of a Mark V belt, 30" seems to be what a Mark VII needs.
IIRC, I added the idler pulley stuff to effectively make the 30.5" belt act like a slightly shorter belt(also due to narrowness of belt).