Rack and pinion

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For rack and pinion railways, see Rack railway.
Rack and pinion animation

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move relative to the pinion, thereby translating the rotational motion of the pinion into linear motion.

For example, in a rack railway, the rotation of a pinion mounted on a locomotive or a railcar engages a rack between the rails and forces a train up a steep slope.

For every pair of conjugate involute profile, there is a basic rack. This basic rack is the profile of the conjugate gear of infinite pitch radius.[1] (I.e. a toothed straight edge.)

A generating rack is a rack outline used to indicate tooth details and dimensions for the design of a generating tool, such as a hob or a gear shaper cutter.[1]


Lock gate controls on a canal


The rack carries the full load of the actuator directly and so the driving pinion is usually small, so that the gear ratio reduces the torque required. This force, thus torque, may still be substantial and so it is common for there to be a reduction gear immediately before this by either a gear or worm gear reduction. Rack gears have a higher ratio, thus require a greater driving torque, than screw actuators.


Most Stairlifts today are operating using the Rack & Pinion system.


Rack steering in an automobile

A rack and pinion is commonly found in the steering mechanism of cars or other wheeled, steered vehicles. Rack and pinion provides a less efficient mechanical advantage than other mechanisms such as recirculating ball, but less backlash and greater feedback, or steering "feel". The mechanism may be power-assisted, usually by hydraulic or electrical means.

The use of a variable rack (still using a normal pinion) was invented by Arthur Ernest Bishop,[2] in the 1970s, so as to improve vehicle response and steering "feel," especially at high speeds. He also created a low cost press forging process to manufacture the racks, eliminating the need to machine the gear teeth.

Rack railways[edit]

Rack railway axle
For more details on this topic, see Rack railway.

Rack railways are mountain railways that use a rack built into the centre of the track and a pinion on their locomotives. This allows them to work on steep gradients, up to 1 in 2 (50%), far in excess of those a conventional railway relying on friction alone can achieve.

Although the extra grip of the rack system is obviously important for climbing, it has perhaps a more important use in also allowing controlled braking on these steep lines and for being much less affected by snow or ice on the rails.


Pneumatic rack and pinion actuators

A rack and pinion with two racks and one pinion is used in actuators. An example is pneumatic rack and pinion actuators that can be used to control valves in pipeline transport. The actuators in the picture on the right are used to control the valves of large water pipeline. In the top actuator, a gray control signal line can be seen connecting to a solenoid valve (the small black box attached to the back of the top actuator), which is used as the pilot for the actuator. The solenoid valve controls the air pressure coming from the input air line (the small green tube). The output air from the solenoid valve is fed to the chamber in the middle of the actuator, increasing the pressure. The pressure in the actuator's chamber pushes the pistons away. While the pistons are moving apart from each other, the attached racks are also moved along the pistons in the opposite directions of the two racks. The two racks are meshed to a pinion at the direct opposite teeth of the pinion. When the two racks move, the pinion is turned, causing the attached main valve of the water pipe to turn.[3][4]

See also[edit]


  2. ^ "Rack and pinion variable ratio steering gear". Google Patent Search. Retrieved 2007-03-22. 
  3. ^ "The Role of Rack and Pinion Actuators in On-Off & Modulating Controls". Valve Magazine (Valve Manufacturers Association) (Spring 2010). 
  4. ^ "Automax SuperNova Series Pneumatic Rack & Pinion Actuators" (PDF). Flowserve Corporation. Retrieved 7 July 2014.