| Movement of the center sheave is affected by moving the entire assembly vertically. If the assembly moves up (toward the gearbox), belt tension on the gearbox side decreases, and tension on the motor side increases. The imbalance forces the center sheave to move axially along the center shaft away from the motor belt and toward the gearbox belt. In turn, this movement causes the gearbox belt to ride up higher on the pulley over a larger diameter while at the same time the motor belt rides down lower as the pulley groove on that side widens. This causes the saw to slow down. The opposite happens if the VSD unit moves down (toward the motor). | |
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This first picture shows something that has caused me some alarm
regarding the VSD. The center part has cracked.
Although
it's not visible in this picture, the
crack does propagate all the way to the center of the part.
I'm not sure whether this crack could become
catastrophic in a hurry when I place the saw back in service.
The
VSD is not made from metal, unfortunately, it's made from Bakelite,
which is what they called plastic before plastic was invented.
It is possible to repair Bakelite with certain epoxies. Later models
used aluminum sheaves. Some day I hope to be able to cast replacements
for these with aluminum. Fortunately, through a tremendous act of generosity from another member of Practical Machinist, I obtained a replacement pulley that's in better shape than this one. That pulley is of a later design and includes a number of excellent changes that make it significantly stronger than this one. I'm going to clean up both pulleys, starting with the original cracked one (more on the new pulley later). |
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Dismantling my original pulley (for inspection
and cleaning as a spare) is accomplished by removing a total of 8 set
screws. The sheave at each end has 2 set screw locations with
2
set screws in each location: one to hold the bakelite sheave to its
metal bushing, and one to hold the metal bushing to the inner shaft.
Once the set screws are removed, I carefully pressed one end
of
the shaft to free the inner sheave and verify that a pressing operation
could be accomplished without destroying the bakelite. Having
done so, I then mounted the pulley assembly in a vise using wood to
prevent damage and "gently" pounded the center shaft free of the inner
and outer pulleys. The inner pulley had an ancient seal on it (gasket, really), apparently to prevent oil (the pulley spindle is oil filled) from seeping onto the pulley shaft. |
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A quick look at the cutaway drawing from the parts manual indicates all
three sheaves should have seals (gaskets) on them, plus a piece of felt
whose location wasn't initially clear from the drawing.
However,
after disassembly and some very heavy cleaning, I located the piece of
felt. The arrow in the diagram below is actually pointing
right
at it: it's located in a large groove in the center of the center
sheave. Notice in the drawing there's a tiny hole that penetrates the pulley shaft. That little hole should point right into a gap in the felt ring and permits oil to travel (by centrifugal force) out from the pulley spindle to the felt, which absorbs quite a bit. In this way, the center sheave maintains proper lubrication so it slides freely during speed changes. This also explains the presence of the oil seals on the inner side of the two outer sheaves. This is a pretty crude but highly effective lubrication scheme. Unfortunately, it results in a buildup of oil and residue all over the inner portions of the sheaves, which eventually degrades and turns into goo. |
| In all applications of solid film lubricants, the performance of the film is limited by the care and preparation of the surface to which they are applied. If they can't adhere properly, they cannot perform, coming off in flakes and often jamming under flexible components. |
| It should be noted that the functions of a good solid film lubricant cannot overcome the need for better surface finishing. Contacting surfaces should be smooth and flat to insure long life and minimum friction forces. Generally, surfaces should be finished to no more than 24 micro-inches AA with wariness no greater than 0.00002 inch. |
| Here's the center shaft with the inner sheave removed, and all the parts cleaned up and ready to go. |  |
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Now, regarding the new pulley I got from my friend at Practical
Machinist, here's a comparison to show what I mean when I say the newer
pulley is of superior design. On the left is one of the new sheaves, and on the right is the old. Notice they put tabs on the metal center portion of the newer sheave to better grip the bakelite and provide metal threads for the set screws. The sheaves on the new pulley are in almost perfect condition, and they were much cleaner than the old ones that came with my saw. Very little effort was required to shine them up and ready them for use. I did find, however, that the center shaft on which the sheaves mount was in worse condition than my original - it had some pitting in it around where the center sheave slides. So I'll simply swap the improved sheaves over to my old shaft. |
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Here's an overall view of the complete VSD assembly (sans pulley): Of interest in this picture is what I call the gearshift lock mechanism. Printed above the shift lever for the transmission is a warning that reads, "shift only at low speed". Actually, you can only shift at low speed because of the lock mechanism. It consists simply of a rod, as shown, with a spring-loaded latch (of sorts) which engages a hole in the gear shift lever to prevent changing gears if the VSD is set too fast. In the picture, the latch is not visible but you can see one of the two holes in the gearshift lever if you look closely. The mechanism was not functional, but will be restored to normal functionality when I reassemble it. |
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Here's the main assembly removed from the saw, prior to cleaning. It's what might be called "elegantly simple". The speed adjustment wheel on the outside of the saw turns leadscrew "A" via universal joint "B". The leadscrew acts on slide "C", which in turn causes rotation of part "D" at the pivot. "E" is the attachment point for the gearshift lock mechanism. Note the oil port which permits lubrication of the variable speed pulley. |
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And here's what this assembly looks like cleaned, painted, and
partially reassembled: It's interesting to note the pivot pin had to be pressed out with a considerable amount of force prior to cleaning. During cleaning all the sludge was removed and the pin was polished up. On reassembly for this picture the pin slipped in with no pressing whatsoever. Before cleaning, a lot of force was required to rotate "D" about the pivot; now it moves freely. In the "dirty" picture above, "D" is staying so positioned on its own. In the clean picture, it flops down into the casting as shown. There's a tremendous amount of sludge on this saw, and it's remarkable how well things fit together once clean. The yellow number in this casting is the part number. There are several castings on this saw with the part number cast in. |
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