Chain Driven Gears

    Building the Aztec Calendar was my first experience making wooden gears and they turned out rather well.  I became interested in making things with moving pieces.  I have several projects planned that involve gears in different configurations and various moving parts.  So I began experimenting with different gears and mechanisms for future use in these projects.  One of the things that I wanted to experiment with was a homemade "chain drive" system -- a way to transfer power from one axle to another parallel axle.  Below are some of my trials and errors and some lessons learned.


    I decided to use short sections of dowel that were connected by string.  In my first attempt I cut sections of dowel that were 2" in length.  I drilled a tiny hole through the dowel at 1/4" from each end.  I drilled these holes at my drill press using a special jig made to hold cylindrical objects.  The jig has sides that are sloped at a 45-degree angle to hold these round objects steady while drilling.


    These gears were slightly different than the ones I had made before.  The first time I cut gears I used a band saw to cut out the teeth from the edge of the circle.  This time I chose to make gears by adding the teeth rather than cutting them out.  I started with two circles that were 3" in diameter.  Each circle was 1 & 1/2" thick (two pieces of 3/4" plywood glued together).  Then I held each gear in a clamp on my drill press as I drilled holes into the edge.  Each hole would then be fitted with a 3/8" dowel that extended from the edge of the circle by about 1".  When finished, these gears looked like the old Tinker Toys I used to play with as a kid -- a circle with a peg protruding from the edges along a radius every 45-degrees.  These 8 radii would be the "teeth" that would drive the chain.


    To prevent having to tie off each dowel, I decided to use small beads as spacers in between each dowel.  This would ensure the correct spacing and also might add some visual interest while the chain is in motion.  When I purchased the small beads at the craft store, I also purchased some clear plastic cord that is used to string the beads.  My first version of this chain drive utilized this plastic cord.  Unfortunately, this plastic cord tends to stretch just a little when enough force is applied.  When the first chain was placed around the two gears and turned, I noticed that the plastic started to stretch in certain places.  This chain worked for a while, but eventually the stretching cord caused the dowels to be spaced improperly and the chain no longer meshed smoothly with the gears.


    In my second attempt I decided to use 8 lb test fishing line in stead of the plastic cord.  That would be stronger and would not stretch.  Again I chose to use beads as spacers, although this time I used different colors.  I also shortened each dowel segment -- from 2" down to 1 & 1/2".  I wanted them to fit a little more tightly around the gear teeth.


    These changes seemed to help somewhat.  First of all, the fishing line was much stronger and, of course, did not stretch at all.  Second, the dowels were now the same width as the gears which looks more appropriate.  However, the chain still has a tendency to come off the gears as I'm turning it.  It might help if the gear teeth were a little longer.  Also, the beads, while excellent as spacers and ensuring the correct distance between the dowels, sometimes tend to bind a little while going around the gear.  Perhaps these gears simply have too small a diameter for them.  Larger gears would cause the chain to turn at an angle that was not so sharp and that might be easier on the chain.  My next attempt will involve slightly larger gears and dowels that have no beads as spacers -- just fishing line tied off around each dowel segment.


    The rack that I used to run this chain drive on is just a rectangular frame of 1 & 1/2" x 1 & 1/2" wood screwed together at the corners.  I cut dadoes in the underside so that I can stand the frame on two pieces of plywood and change the height as necessary.  The four pieces the hold the 1/2" axles are just scrap plywood with 1/2" holes drilled through them.  These are just clamped in place.  That way I can slide the axles closer together or farther apart to install the chain and then to vary the tension on the chain once it's in place.  The whole assembly breaks down quickly and easily for storage.

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