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New approach to gearing.....Belt Drive.
Just one story, but hopefully gives some perspective on gearboxes.
I have a Tamiya King Tiger. It's probably the heaviest Tamiya tank and it has added metal tracks making it all that much heavier. If I remember right, it weighs in at 12lbs.
Shortly after I got it, the main gearbox drive axle broke. It is 8mm in diameter where it enters the wheel, and 6mm in diameter inside the gearbox. But it broke at the c-clip groove, where the diameter is only 4mm! A definite weak spot. Tamiya was great - they replaced the gearbox for free (I had to send receipts, pictures, broken part).
A while later one of the gearboxes stripped a gear. I run "lightly" and not that frequently, and keep the gears clean and lubed with high quality synthetic grease. I bought a new gearbox. A few months ago, another gear stripped. Similar place...on a shaft nearer to the final output. I am NOT buying yet another gearbox.
Instead, I designed a timing belt/pulley drive.
MANY advantages to belt drives - see attachment. The big mechanical advantage is that timing belts engage the pulley at many, many teeth, vs gear trains where only a single tooth engages on each gear. To keep the loads small enough on the single teeth, the Tammy transmission has 8 separate geardown steps, at about an average ratio of 2.2:1 per step. That's what requires all those gears and shafts. A belt drive can do the same reduction easily in just 2 or 3 steps, with no risk of a breaking/stripping a single tooth and ruining the gear box.
I ran bench tests with a mock up drive using belts and pulleys I had, duplicating the reduction ratio of the one good Tammy gearbox I had left.
In the test, I ran 8.0 V to the belt drive set up and dead-lifted increasing weights. At three pounds vertical lift at high speed (a full 8V to the motor), the belt drive lifted with no hesitation and no significant RPM reduction.
Then I switched over and ran the Tamiya drive with the three pound load (exactly the same motor, same battery voltage and same reduction ratio as the belt drive). Yikes! The Tammy drive immediately stripped out. I didn't expect that, so now I have no good Tammy drives (3 stripped ones...anyone want to buy them for parts?).
Bottom line, the new miniature timing belt/pulley designs can handle tremendous loads, have very high efficiency, long life, require no lube or maintenance, are very quiet and easily built, modified and repaired.
More details as I get the parts and do the build. I ordered final parts and will be assembling the KT drive in a few weeks.
I'll be using GT2, 2mm pitch timing belts, 6mm wide with matching pulleys.
Parts for two drives adding up to about $80 from SDP/SI
http://www.sdp-si.com/products/Timin...leys/index.php
I'm looking for a lower cost source, but SDP offers a complete range of pulley and belt sizes needed for custom designs for tank drives.