Wheel Pair

one of the main units of the running gear of a car or undercarriage of a locomotive. A wheel pair consists of an axle with two wheels mounted on it; it directs the motion of the car or locomotive along the rails. The wheel pairs accept and transmit all loads of the rolling stock to the rails and vice versa. Wheel pairs are joined in groups of two, three, or more in the trucks of a railroad car.

The wheels of the wheel pair usually are immovably attached to the axle. In some cases, the wheels may rotate on the axle or be displaced along it, moving together or apart depending on the track gauge. The wheel pairs of electric and diesel locomotives and of motor railcars are load bearing and accept the torque from the traction electromotors through a one-speed or two-speed gear drive or through a hydraulic reduction transmission. In this case a large transmission is mounted on the wheel pair axle.