frictional back gear

例句：

1. 1. The car's engine is designed with a frictional back gear to improve efficiency.
   汽车的发动机设计有摩擦式后齿轮来提高效率。
2. 2.In the old days, people used frictional back gears to transmit power from one machine to another.
   在旧时代，人们使用摩擦式后齿轮将一个机器的动力传递到另一个机器上。
3. 3.The engineer had to replace the frictional back gear with a new one because it was worn out.
   工程师不得不用一个新的摩擦式后齿轮替换掉磨损的旧齿轮。