The Crank Dolly

Here is a way to make an exact move with dollies of any size that smoothly accelerates, decelerates and always hits the mark. The principle of the crank dolly is the same as a piston in a cylinder of an engine. As you turn the crankshaft at the bottom of the stroke the piston moves very slowly, accelerates to maximum speed 90 degrees later and slows down at top dead center before moving the other direction. With an adjustable stroke and leverage you could move a train or a lens out of a mount to focus.

Small dollies made of linear bearings are quite expensive but are well worth it.

If you can find large electronic drawer slides that also work. Slide used ones in and out to check for bumps. Some times the rough spots are just dirt that can be cleaned. (Try C and H Sales, Pasadena, California) Adjustable stroke is important in your design. The ability to tilt the camera at the end of the dolly and to get the camera low and looking down is helpful for table top work.

Electronic Drawer Slide Crank Dolly
in Extended Position

Close-up in Back Position

Tilt Mechanism for Camera

Top View of CrankDolly

For moving a stage dolly you only need enough leverage. Most of a stage dolly crank can be made of Speed Rail #17 (tm) adjustable crosses if you modify them with a file to move 180 degrees. Kee Klamp #M50 and #F50 work well too. I replace the Kee Klamp factory pin with a 7/16 bolt after reaming the fitting out to 7/16". The original pins are too sloppy.

I don't recommend the Speed Rail (tm) Split Cross for anything else, but you can use it to attach your crank to 1-1/2" pipe dolly track. (Dexter Track) Use smooth headed 3/8"- 16 bolts or cap screws to avoid marking the dolly track with set screws. A knob would be handy for the small clearance under the dolly track pipe. For standard Elemack dolly track you shim 1-1/4 Speed Rail fittings to fit. Some 1-5/8" nylon or plastic will make a quick and dirty bearing for use with 1-1/4" Speed Rail. (See Speed Rail Bearings)

To coordinate a camera position with a camera assistant focusing, make and call numbers of positions of the lever arm. Once set, these will save time because every move will be the same. If anyone misses his mark he should say so, so that corrections will not be made if not necessary.

© Copyright 1999-2004 Ron Dexter. All Rights Reserved.