A rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert between rotational motion and linear motion: rotating the pinion causes the rack to be driven in a line. Conversely, moving the rack linearly will cause the pinion to rotate. The rack and pinion mechanism is used in rack railways, where the pinion mounted on a

locomotive A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...

or a

railroad car A railroad car, railcar (American English, American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and International Union of Railways, UIC), also called a tra ...

engages a rack usually placed between the rails, and helps to move the

train A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...

up a steep

gradient In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...

. It is also used in arbor presses and drill presses, where the pinion is connected to a

lever A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...

and displaces a vertical rack (the ram). In pipelines and other industrial piping systems, a rack displaced by a linear actuator turns a pinion to open or close a

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

. Stairlifts, lock gates, electric gates, and the mechanical

steering Steering is the control of the direction of motion or the components that enable its control. Steering is achieved through various arrangements, among them ailerons for airplanes, rudders for boats, cylic tilting of rotors for helicopters, ...

mechanism of cars are other notable applications. The term "rack and pinion" may be used also when the rack is not straight but arcuate (bent), namely just a section of a large gear. A single pinion can simultaneously drive two racks, parallel but opposite; which will always be displaced by the same distance, only in opposite directions. Conversely, by applying opposite forces to the two racks one can obtain pure torque on the pinion, without any force component. This double rack and pinion mechanism can be used, for example, with a pair of pneumatic actuators to operate a

with minimum stress.

Applications gallery

File:Lock gate cogs, Montgomery Canal - geograph.org.uk - 1806427.jpg, Lock gate lifter on a

canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...

File:Sliding electric gate-001.JPG, Electric gate File:Steer system.jpg, Automobile steering mechanism File:Budapešť, Városmajor, pomník zubačky.jpg, Rack railway wheelset File:Muzeum MHD, dvoukolí vozu petřínské lanovky 1891.jpg, Funicular wheelset and brake File:Pneumatic Rack and Pinion Actuators.JPG, Double-rack pneumatic valve actuator

History

The time and place of the invention of the rack-and-pinion mechanism are unknown, but it presumably was not long after the invention of gears. The south-pointing chariot from China and the

Antikythera mechanism The Antikythera mechanism ( , ) is an Ancient Greece, Ancient Greek hand-powered orrery (model of the Solar System). It is the oldest known example of an Analog computer, analogue computer. It could be used to predict astronomy, astronomical ...

are evidence of these being well-known already a couple of centuries BC. In 1598, firearms designer Zhao Shizhen developed the Xuanyuan arquebus (軒轅銃), featuring a rack-and-pinion

matchlock A matchlock or firelock is a historical type of firearm wherein the gunpowder is ignited by a burning piece of flammable cord or twine that is in contact with the gunpowder through a mechanism that the musketeer activates by pulling a lever or Tri ...

mechanism derived from an Ottoman Turkish matchlock design.Zhao Shizhen (1598): ''Shen Qi Pu'' (神器譜) The '' Wu Pei Chih'' (1621) later described Ottoman Turkish muskets that used a rack-and-pinion mechanism. The use of a variable rack (still using a normal pinion) was invented by Arthur Ernest Bishop 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.

Comparison with Worm gear

A rack and pinion has roughly the same purpose as a worm gear with a rack replacing the gear, in that both convert torque to linear force. However the rack and pinion generally provides higher linear speed — since a full turn of the pinion displaces the rack by an amount equal to the pinion's pitch circle whereas a full rotation of the worm screw only displaces the rack by one tooth width. By the same token, a rack and pinion mechanism yields a smaller linear force than a worm gear, for the same input torque. Also, a rack and pinion pair can be used in the opposite way, to turn linear force into torque; whereas a worm drive can be used in only one way.ZHY Gear(1023):
Worm Gears vs. Rack and pinion
. Online article a
the company's websiteArchived on 2024-07-28

Geometry

The teeth of a rack and pinion pair may be either straight (parallel to the rotation axis, as in a spur gear) or helical. On the pinion, the profile of the working tooth surfaces is usually an arc of

involute In mathematics, an involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An involute of a curve is the Locus (mathematics), locus of a point on a piece of taut string as the string is eith ...

, as in most gears. On the rack, on the other hand, the matching working surfaces are flat. One may interpret them as involute tooth faces for a gear with infinite radius. In both parts the teeth are typically formed with a gear cutter (a hob).

References

{{DEFAULTSORT:Rack and Pinion Automotive steering technologies Gears Actuators Turkish inventions

Applications gallery

History

Comparison with Worm gear

Geometry

See also

References