Crank Design to Increase Leg Clearance
by
Bob Kuykendall

PDF File of Crank Improvement Design

The stock HP-18 uses (I think) an NA18B pinion with 18 teeth, so each turn of the crank moves the rack 18 teeth.

My crank system has a 2:1 reduction over the stock system, so each turn of the crank moves the rack only 9 teeth, essentially reducing the input torque by 50%. I also shortened the handle by 1/2" for even more leg clearance so that it is 3" in length instead of 3.5". That increases the input force by a little (about 17%). By my calculations, the input force at the handle should be about 58% of what would be required with the stock crank.

I was pretty happy with it once I got the clearances all right.

The gears are all stock Boston Gear pinions. Both of the small gears are NA18B, and the large gear is an NA36B that I turned a bunch of metal out of to lighten. I had the local TIG shop seam weld the two paired gears together.

One important thing is to make an aluminum or plastic cover for the gears to keep clothing and stuff from getting into them and binding the mechanism.

Previously Push-Button release system details were published in the Construction Tip section of the web site.