Rolling element gears
The rolling element gear is a type of non-positive gear . Here rolling rotationally symmetrical rolling elements against each other. In the simplest case, a rolling element drive consists of two shafts or rollers that roll on each other (friction gear). Friction gears can also be built with continuously variable transmission. Then one of the rollers can be moved or pivoted and this movement changes the rolling radii. Gearboxes for small outputs run dry. In gearboxes for higher outputs, an oil film prevents direct metallic contact between the bodies. In this oil film, the shear forces and the gradient of the shear forces between the driving and driven rolling elements increase with increasing torque .
The transmission ratio for the angular speeds ("speeds") behaves in reverse as the radii. As with other transmissions, the gear ratio for the transmitted torque is the reciprocal of the gear ratio for the speed.
One advantage of this gearbox is that the safety clutch is built in at the same time, since the friction pairing can slip in the event of an overload.
The main disadvantage is that in normal operation the slip is significantly greater than with other types of transmission and that the efficiency is lower as a result. Under certain circumstances - depending on the design - a higher level of wear is recorded. Another disadvantage is that the high contact forces required also result in high bearing loads; the contact forces required to transmit a torque also increase with decreasing roll radii, which means that there are limits to a compact and lightweight design.
Until the 1930s there were various automobile manufacturers who equipped their products with continuously variable friction gear drives. Before the First World War, they were particularly widespread among cyclecars . Frank Anderson Hayes patented a continuously variable friction gear with planetary rollers in 1929. The Hayes transmission was used on Austin vehicles, among others. Leyland started installing similar transmissions in trucks from the mid-1980s. The successors of this design are still offered today by the British manufacturer Torotrak for agricultural and construction machinery. In the 1990s Nissan realized such a transmission under the name "Extroid-CVT" and used it in vehicles.
Friction gear units are a special type of rolling element gear unit . One wheel rolls on another wheel or another body to be driven.
References and comments
- CVT = continuously variable transmission -> stepless transmission