Arack and pinion is a type oflinear actuator that comprises a circulargear (thepinion) engaging a linear gear (therack).[1] 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 inrack railways, where the pinion mounted on alocomotive or arailroad car engages a rack usually placed between the rails, and helps to move thetrain up a steepgradient. It is also used inarbor presses anddrill presses, where the pinion is connected to alever and displaces a vertical rack (theram). Inpipelines and other industrial piping systems, a rack displaced by alinear actuator turns a pinion to open or close avalve.Stairlifts,lock gates,electric gates, and the mechanicalsteering mechanism of cars are other notable applications.
The term "rack and pinion" may be used also when the rack is not straight butarcuate (bent), namely just a section of a large gear.[2]
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. Thisdouble rack and pinion mechanism can be used, for example, with a pair ofpneumatic actuators to operate avalve with minimum stress.[3]
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. Thesouth-pointing chariot from China and theAntikythera mechanism are evidence of these being well-known already a couple of centuriesBC.[citation needed]
In 1598, firearms designer Zhao Shizhen developed the Xuanyuanarquebus (軒轅銃), featuring a rack-and-pinionmatchlock mechanism derived from anOttoman Turkish matchlock design.[4] TheWu Pei Chih (1621) later describedOttoman Turkishmuskets that used a rack-and-pinion mechanism.[5][6]
The use of a variable rack (still using a normal pinion) was invented byArthur Ernest Bishop[7] in the 1970s, so as to improve vehicle response and steering "feel", especially at high speeds. He also created a low costpress forging process to manufacture the racks, eliminating the need to machine the gear teeth.
A rack and pinion has roughly the same purpose as aworm 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'spitch 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.[8]
The teeth of a rack and pinion pair may be either straight (parallel to the rotation axis, as in aspur gear) orhelical. On the pinion, the profile of the working tooth surfaces is usually an arc ofinvolute, 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 (ahob).[1]
