FW: Wood wheels

["Steve Tyng" <stevet-at-stcharlesmd.com>]

Dee wrote:

>My motors are 3910rpm w/o load how would I go about reducing the rpm's
using timing belts or chains.  My knowledge of spockets and such is limited. What 
would be a good rpm and what size spockets would I need.

Dee, I wish I could pull the information out of my head that your looking for but I 
don't have the training for it (I'm not an engineer and don't have the math 
memorized) but more importantly, I don't know the maximum size you can use for your 
final drive sprocket.  All I can do is tell you how I arrived at my current 
configuration which was a process of trial and adjustment (not error ;-).  Forgive 
me if parts of the following seem simplistic.

I knew I would need to reduce rpm's and the way this is done is by turning a large 
wheel with a smaller wheel.  When I say wheel it can be a sprocket, pulley, or even 
a gear, it doesn't matter - their all round.  So the first step was to figure the 
maximum size chain sprocket I could fit on my drive wheels (I decided to use a chain 
drive).  This turned out to be a 33 tooth sprocket (chain sprocket sizes are given 
by the number of teeth it has).  I then determined the smallest sprocket that would 
fit on my motor shafts which was a 9 tooth sprocket.  This would give me an initial 
3.66 to 1 reduction (33 divided by 9), for every full turn of the large sprocket the 
smaller one will turn 3.66 times.  I mounted the large sprocket to a drive wheel and 
the smaller one to a motor (see 
http://www.rctankcombat.com/tanks/T005/48DriveWheelChain-large.jpg for an example).  
I put everything together and ran it on the bench.  The track went round but seemed 
to be going awfully fast and it didn't seem to have much power (applying pressure to 
the track would slow it down easily).  I decided another stage of reduction was 
needed.

Another stage of reduction means another large wheel driven by a smaller wheel.  For 
this stage I decided to use a pulley/belt drive setup (see 
http://www.rctankcombat.com/tanks/T005/49DriveInside-large.jpg).  Again, I had to 
determine the maximum size large wheel (or pulley in this case) I could fit and this 
was a 4" diameter pulley (pulleys are measured by their diameter) (FYI: pulleys can 
also be called sheaves and many catalogs list them as such).  I ordered the 4" 
pulleys along with several different sizes of smaller pulleys (1.5", 2", and 2.5") 
so I could experiment with different reduction ratios.  I built a mounting for the 
pulleys and motor and I now had a two stage reduction system.  The sequence is: the 
small motor pulley drives the large pulley via v-belt, the large pulley drives the 
small chain sprocket via a common drive shaft, the small sprocket dives the large 
chain sprocket via a roller chain.  Through experimentation I determined that the 2" 
pulleys on my current motors (see 
http://www.rctankcombat.com/tanks/T005/85NewMotors-large.jpg) are giving me a good 
combination of speed and power.  The total reduction ratio with my current setup is 
9 to 1, for 9 turns of the motor the drive wheel turns once.

For your 3910rpm motors I would think sprockets or pulleys in the size ranges I used 
should work fine.  Just leave room in your design so you can fine tune the final 
ratio with different size motor pulleys or sprockets.

As far as design, there are many ways to build a reduction system.  All you need is 
a solid way of mounting your pulleys/sprockets, shafting, and motors.  My design 
works well but is a little complex in that there is some rather creative millwork 
involved in building the bearing and motor mounts.  Tri-Pact has a simplified design 
using off the shelf components that also works very well.  Check out the following 
pics of Paul Pittelli's KV1 for an example.

http://www.rctankcombat.com/tanks/T018/12RearAxle-large.jpg (older pic using KCM's)

http://www.rctankcombat.com/tanks/T018/24new_motors-large.jpg


Steve 'Merry-Go-Round' Tyng