Martians 494 cvt pics

[Jay TenBrink]

This CVT is a one of a kind that I designed on my dining room table with paper and pencil, so no CAD drawings are available at this time. I had never designed a transmission before this one. I used a book on tuning snowmobile clutches as my reference for my design. Unfortunately this book is out of print. If someone were interested in converting the paper drawings to CAD and making them available to the CD community, I might be able to fine a complete set and mail them to you. If there were more time, this could also make an interesting white paper.

Here’s a brief description of the function:

The drive side of the CVT responds to RPM and belt tension. One sheave is fixed to the shaft and the other is allowed to slide along the shaft but not slip. The faster the input shaft spins, the more the brass fly weights push the movable sheave toward the fixed sheave. The flyweight force is proportional to the square of the RPM and directly proportional to the mass of the weights. This device was designed to operate at about 1450 RPM. A snowmobile clutch operates at speeds up to 10,000 RPM. For this reason, the flyweights had to be rather large.

The driven side of the CVT responds to belt tension and torque on the output shaft. One sheave is fixed to the shaft and the other is free to slide and spin. The moving sheave is held against the fixed sheave by a spring that is inside of the aluminum cup with the cam surfaces. The movable sheave is kept from spinning on the shaft by three rollers that ride on the angled cam surfaces.

If the drive side is at full RPM and there is little or no load on the output shaft, the belt will be pulled down and the driven sheaves will be open. At this point, the CVT will be in high gear. If there is a load applied to the output shaft, the moving sheave will want to slip and will bear against the cam surfaces. The rollers will ride up the cam surfaces and push the sheaves back together. This will tighten the belt and pull the belt back down into the drive pulley and force the drive sheaves to open back up. This is known as “back shifting”

This CVT can function in either direction and does not have a neutral position. A snowmobile clutch only works in one direction and does not begin to bring the drive sheave together to contact the belt until the shaft is spinning about 2000 RPM.

The width of the V-belt and the angle of the side walls will determine the gear ratio change that is available. Because we wanted to use a small belt to keep parasitic losses to a minimum, we have a range of about 60% change from low to high gear.

We fitted this to one of our old robots and put the robot on our dynamometer to do some tuning of flyweight size and shape, cam profile, and driven clutch spring force. More development would would be required, but we are not pursuing it at this time.

Jay