A novel solution to incorrect form roller diameter

I was recently trying to set the form (inking) roller height on an SP15 and found that I was unable to properly set the gear-driven one (nearer the impression drum). There was no way the back end (closer to the gear) could drop down far enough. I measured the rollers and found that, instead of being 2.5″ diameter as they should be, they were 2.382″. The back end of the form roller could not drop down because the drive gear was bottoming out in its rack.

Other than being the wrong diameter (inexplicably so) the form rollers were in good condition. Having the rollers recovered would have cost $270, although I could have got away with half that by just recovering the gear-driven one.

I found a less expensive solution, though: replace the gear with one that is the right size for the roller. This worked out in this case because the roller height mechanism has enough range to hold the roller axes 1/16″ lower than normal, and also because the actual roller diameter corresponded to another gear size.

The standard gear is a 40-tooth, 16 DP, 14.5° PA gear, with a pitch diameter of 2.5″ (40/16) to match the standard form roller. Applying the 16 pitch to 2.382″ diameter comes to 38.112 so a 38-tooth gear with the same pitch and pressure angle would do the trick. A gear this size is not generally available with a hub (like the original gear) but is is available as a “change gear” (usually used in machine tools to adjust the speed ratio between two shafts). This is a flat (hubless) gear with a 3/4″ hole and 2 keyways. I also bought a 1/2″ hub (for welded-on pulleys and sprockets), and used my lathe to fit the hub to the gear with a press fit. The combined parts look just like the original gear (except for the tooth count, of course) and the press now inks properly.

The slight difference between the ideal tooth count (38.112) and the actual tooth count (38) will cause a slight amount of slurring, but the amount is very small, about 0.05″ total slippage over the full length of the form.

Total cost: about $43, plus my time on the lathe. Of course, if I had to pay someone to do this, getting the roller recovered would likely have been cheaper.

Hub and Change gear as purchased, and original 4-tooth gear
Left to right: The 1/2″ hub, the 38-tooth change gear, and the original 40-tooth gear.
New gear assembled - 2 views
Two sides of the new gear assembled. Rather than pressing the parts together I heated the gear up so it would slip over the hub but a side effect is the brown staining from some factory coating that was on the gear. The keyway in the hub is not used by the SP15 but nor is it detrimental.
Kevin Martin
the Papertrail Handmade Paper & Book Arts
New Dundee, Ontario, Canada
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Eric Holub
11 years ago

Paul, there is a 219 OS here in SF that has the NS gears and gear rack.

Paul Moxon, Moderator
11 years ago

Nice DIY, Kevin. This should also work for the SP20/25 and Universals which have 3″ gears.

It might work for the 15-21 and the 219 New style, which have two 3″ gears. Both would need to be swapped, but then the rubber may not contact the steel riders.

Other models like the Nos. 4 and 3 and the 219 OS have gear profiles that are no longer standard.

Eric Holub
11 years ago

Good approach to the geometry of the problem. My concern is that an undersize roller is usually aged and shrunken, and consequently harder than what is wanted to give good inking (though it could also be reground or just made off-spec).
Even so, with the two sizes of gear to use, you could regrind 2-1/2″ rollers more times for a much longer lifespan.

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