Sunday, December 25, 2011

Razor Wind: Shredding is a Pro for Guitarists, but not for Power Transmission

I've been running the Wheelpod system for about 3-4 weeks now and needless to say it has been an interesting experiment with pulley diameters and belt materials. I began with a 13T motor pulley, a 29T wheel pulley, and a 3/8" wide XL neoprene/fiberglass timing pulley. After a few runs, I noticed the drive producing extra grinding sounds and added a tensioner to absorb the thermally induced slack. Days later, the belt actually snapped.

All the teeth seemed to have rounded a bit (nominal trapezoidal shape) but the failure point was clear. The tension members all failed at one point, where the belt broke. At least I was in front of my apartment when it happened. Had I been at the Invention Studio, I might have been raging a bit.

So what happened? Easiest assumption is the wrong material. My second choice was a urethane belt, with kevlar tension member. Kevlar? HELLS YES! I also knew from experience that the urethane timing belts suffered less form expansion. I was hoping this combination alone would be able to solve my problems.

I did however have to change the motor pulley, because the Urethane belts came in intervals of 5 teeth (5 x .2" = 1" differences between belts) which meant I had to recalculate my belt distances. A new pulley with 11T was swapped for the 13T.

Can anyone guess what is going to happen next?

Well I blatantly ignored the minimum tooth requirement for most timing pulleys. Basically the number of teeth in contact with the motor pulley were not enough to resist the force required to move a human body at that ratio. Most (if not all the teeth) eventually sheered off like in the belt below.

And this belt too...

And this belt most recently. Luckily I had purchased enough belts to make it past my finals week (otherwise I would have surely been late to each one). The problem was clearly not belt or tension cord material. After consulting the problem from a few different angles I determined to cause to be the pulley diameter.

Increasing the pulley diameters (proportionally) would not change my overall ratio, but would allow a greater number of teeth in contact. This redistributes the sheer forces acting on each contact belt tooth.

While I was at it, I figured I would redesign the wheelpod chassis for 'real' belt lengths, and a dynamic belt tensioner.

However in the interest of saving time and material, I've elected to skip the experimental belt steps and revert to a method that has worked before: #25 chain. I will still be increasing the sprocket diameters to distribute the loads, but I will be using metal roller chain, which supports much higher working loads.

Starting with the largest motor side sprocket I could support and the center to center distance between the motor and wheel, I used this nifty chain length calculator to empirically determine a combination of chain and ratio that would suffice. The only limitations is that the sprocket tooth number and the # of chain links had to be integer values. In the end, I chose a 14T motor sprocket, 31T wheel sprocket, and a chain of 51 links. That gives an overall ratio higher than the current setup, but hell I know the motor can output enough power to make the scooter fly.

Machining to take place the moment I return to Tech. But until then, Happy Holidays!

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