Rack and pinion

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Rack and pinion animation Rack and pinion animation.gif
Rack and pinion animation

A rack and pinion is a type of linear actuator that comprises a circular gear (the pinion ) engaging a linear gear (the rack), which operate to translate rotational motion into linear motion. Driving the pinion into rotation causes the rack to be driven linearly. Driving the rack linearly will cause the pinion to be driven into a rotation. A rack and pinion drive can use both straight and helical gears. Helical gears are preferred due to their quieter operation and higher load bearing capacity. The maximum force that can be transmitted in a rack and pinion mechanism is determined by the tooth pitch and the size of the pinion.


For example, in a rack railway, the rotation of a pinion mounted on a locomotive or a railroad car engages a rack placed between the rails and helps to move the train up a steep gradient.

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 (i.e. a toothed straight edge). [1]

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 Lock gate cogs, Montgomery Canal - geograph.org.uk - 1806427.jpg
Lock gate controls on a canal

Rack and pinion combinations are often used as part of a simple linear actuator, where the rotation of a shaft powered by hand or by a motor is converted to linear motion.

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 operate using the rack and pinion system.[ citation needed ]


Rack steering in an automobile Steer system.jpg
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 less 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

Rack railway axle Budapest, Varosmajor, pomnik zubacky.jpg
Rack railway axle

Rack railways are mountain railways that use a rack built into the center of the track and a pinion on their locomotives. This allows them to work on steep gradients, up to 45 degrees, as opposed to conventional railways which rely on friction alone for locomotion. Additionally, the rack and pinion addition provides these trains with controlled brakes, and reduces the effects of snow or ice on the rails.


Pneumatic rack and pinion actuators Pneumatic Rack and Pinion Actuators.JPG
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]

Arcuate rack

A rack gear that is curved is called an arcuate rack. [5] The term appears in many patent applications. [ which? ]


The Chinese Wu Pei Chih (1621) described Ottoman Turkish muskets that used a rack-and-pinion mechanism, which was not known to have been used in any European or Chinese firearms at the time. [6]

See also

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Starter (engine) device used to start an internal combustion engine

A starter is a device used to rotate (crank) an internal-combustion engine so as to initiate the engine's operation under its own power. Starters can be electric, pneumatic, or hydraulic. In the case of very large engines, the starter can even be another internal-combustion engine.


A camshaft is a rotating object— usually made of metal— that contains pointed cams, which converts rotational motion to reciprocal motion. Camshafts are used in internal combustion engines, mechanically controlled ignition systems and early electric motor speed controllers. Camshafts in automobiles are made from steel or cast iron, and are a key factor in determining the RPM range of an engine's power band.


A solenoid is a type of electromagnet, the purpose of which is to generate a controlled magnetic field through a coil wound into a tightly packed helix. The coil can be arranged to produce a uniform magnetic field in a volume of space when an electric current is passed through it. The term solenoid was coined in 1823 by André-Marie Ampère to designate a helical coil.

An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover".

Automatic transmission Type of motor vehicle transmission that automatically changes gear ratio as the vehicle moves

An automatic transmission is multi-speed transmission used in motor vehicles that does not require any driver input to change gears under normal driving conditions.

Hobbing Process used to cut teeth into gears

Hobbing is a machining process for gear cutting, cutting splines, and cutting sprockets on a hobbing machine, which is a special type of milling machine. The teeth or splines of the gear are progressively cut into the material by a series of cuts made by a cutting tool called a hob. Compared to other gear forming processes it is relatively inexpensive but still quite accurate, thus it is used for a broad range of parts and quantities.

Fluid power

Fluid power is the use of fluids under pressure to generate, control, and transmit power. Fluid power is subdivided into hydraulics using a liquid such as mineral oil or water, and pneumatics using a gas such as air or other gases. Compressed-air and water-pressure systems were once used to transmit power from a central source to industrial users over extended geographic areas; fluid power systems today are usually within a single building or mobile machine.

Transmission (mechanics)

A transmission is a machine in a power transmission system, which provides controlled application of power. Often the term 5-speed transmission refers simply to the gearbox, that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.

In motor vehicles, a power steering system helps drivers steer the vehicle by augmenting steering effort needed to turn the steering wheel, making it easier for the vehicle to turn or maneuver.

Linear actuator Actuator that creates motion in a straight line

A linear actuator is an actuator that creates motion in a straight line, in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer peripherals such as disk drives and printers, in valves and dampers, and in many other places where linear motion is required. Hydraulic or pneumatic cylinders inherently produce linear motion. Many other mechanisms are used to generate linear motion from a rotating motor.

Worm drive Gear arrangement

A worm drive is a gear arrangement in which a worm meshes with a worm gear. The two elements are also called the worm screw and worm wheel. The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.

Gresley conjugated valve gear

The Gresley conjugated valve gear is a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft. It enables a three-cylinder locomotive to operate with only the two sets of valve gear for the outside cylinders, and derives the valve motion for the inside cylinder from them by means of levers. The gear is sometimes known as the Gresley-Holcroft gear, acknowledging Holcroft's major contributions to its development.

Bevel gear

Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone.

A transmission control unit (TCU), also known as a transmission control module (TCM), or a gearbox control unit (GCU), is a type of automotive ECU that is used to control electronic automatic transmissions. Similar systems are used in various semi-automatic transmissions, purely for clutch automation. A TCU in a modern automatic transmission generally uses sensors from the vehicle, as well as data provided by the engine control unit (ECU), to calculate how and when to change gears in the vehicle for optimum performance, fuel economy and shift quality.

Starter solenoid

A starter solenoid is an electromagnet which is actuated to engage the starter motor of an internal combustion engine. It is normally attached directly to the starter motor which it controls.

Valve actuator

A valve actuator is the mechanism for opening and closing a valve. Manually operated valves require someone in attendance to adjust them using a direct or geared mechanism attached to the valve stem. Power-operated actuators, using gas pressure, hydraulic pressure or electricity, allow a valve to be adjusted remotely, or allow rapid operation of large valves. Power-operated valve actuators may be the final elements of an automatic control loop which automatically regulates some flow, level or other process. Actuators may be only to open and close the valve, or may allow intermediate positioning; some valve actuators include switches or other ways to remotely indicate the position of the valve.

Rotary actuator

A rotary actuator is an actuator that produces a rotary motion or torque.


  1. 1 2 Gear Nomenclature, Definition of Terms with Symbols. American Gear Manufacturers Association. 2005. p. 72. ISBN   1-55589-846-7. OCLC   65562739. ANSI/AGMA 1012-G05.
  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.
  5. Phillips, Jack (2003). General Spatial Involute Gearing. Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN   3662053020.
  6. Needham, Joseph (1986), Science & Civilisation in China, V:7: The Gunpowder Epic, Cambridge University Press, p. 446, ISBN   0-521-30358-3