Thursday, November 18, 2010

Broken Belt and Proper Tension

Back in April 2010 I was driving home from work and as I came to an intersection to make a right turn I noticed what looked like a self taping sheet metal screw in the middle of the lane. As any three wheeled car driver has learned it is important to remember there is a wheel in the center of your track. I remember thinking, "I hope I missed that. If I hit it just right it give me a flat tire." As I turned the corner and accelerated up over the bridge I heard a thunk, thunk, thunk sound that seemed to be in sync with the tire rotation. I thought that maybe the screw had stuck in the tire and had caused it to bulge out or something. I thought that maybe I could just make it the 2.5 miles and 450 feet increase in elevation to get home and see what was the issue. As I continued along I found that the sound was louder under load than when lightly loaded. Even then it continued to get louder the closer I got to home. I thought maybe the tire was losing air. When I got home I couldn't see any thing wrong so I jacked up the rear and turned the rear tire. Below is what I found.

A tear in the belt all the way to the middle. It looked like the center string was not severed but the rest were.
As you can see the belt lifted quite away off the pulley. I think that I was saved by the fact that the belt was on the drive pulley a short time so didn't have a chance to slide off and that the rear wheel pulley was large enough that the belt didn't slide off.

When I took the belt into the local store to pickup a replacement W-1280 belt the man who helped me said that it was very likely that the screw I saw got caught in the belt and tore it. Click the link to go right to the catalog page with the belt the Gizmo uses. I'm fortunate enough to have one of their stores right in town. The belts are not cheap but spending $95 for a belt rather than the retail of $130 I was happy. Maybe that is why there is retail pricing. It is so you feel great getting a deal when you pay less. In addition to the belt I asked about a tension measuring tool. The one they listed was a frequency measuring device and I didn't want to spend several hundred dollars for one so I asked about one which uses belt deflection. The guy who helped me called their supply place in Portland, OR and found out they had one for about $20 so I ordered it too. The next day I picked them up and was ready to replace the belt.

For those of you who have had the rear wheel off your Gizmo it takes quite a bit to get there. The tail piece is removed, next the rear wheel, then the e-brake, next the e-brake disk off the motor, then the belt shroud. Finally the bolts holding the motor on both the brush end and drive end have to be loosened, then the tensioning screw can be backed off allowing the belt to be removed. When installing the process is reversed. The problem is that if the belt isn't tight enough it will slip and most of this long process has to be repeated. When you think you have it things go great until it rains and you find out the belt slips again. Having the tensioner eliminates all of this. If the belt is too tight it will break sooner and wear out the bearings in the motor and wheel early so it is important to have the optimum tension.

I didn't relish doing the all the geometry calculations or hoping my measurements were right so I went looking for software that would tell me the proper tension for my setup. I found that GoodYear had MaximizerPro available on their website to do just what I wanted. I discovered that I could enter the two pulley sizes I had and have it force them to the proper spacing. I believe I had to enter what the power load was so I entered 15Hp which may be low given the potential on full acceleration and the fact that electric motors can put out a much higher horse power value than they are rated at but this value seems to work. The software then showed a Drive Layout with lots of information. A screen capture of the results is below.

Even though the picture above shows all relevant information it is nice to see drawn out exactly how the tension is figured and measured. I used a piece of 1/4" by 3/4" aluminum bar as my straight edge and pressed the tension meter in the center until it read 15Lbf at a deflection of 6.2mm. Yes you read that right. I think it is strange to mix systems of measurement but that is how it is done. I don't know why they don't just use Newtons and mm but this works.

I decided to go out and get a picture of the tension tester with my iPhone 4. I've found the HDR setting on the camera to be very useful for photos like this because it brings out the shadows much better and I don't have to do much editing later.

Here is the Industrial V-Belt Tension Tester I use on my Eagle belts. The end with the rubber cap is what you grab with your hand. The other end has a small short stub which pokes slightly into the belt surface so it doesn't slip. It works just fine for my application.