View source for Bellcrank

{{Short description|Type of crank that changes motion through an angle}}
{{More citations needed|date=January 2021}}
[[File:Chambers 1908 Bell Crank.png|thumb|Illustration of a bellcrank from the 1908 Chambers's Twentieth Century Dictionary|240x240px]]
A '''bellcrank''' is a type of [[crank (mechanism)|crank]] that changes motion through an angle. The angle can range from 0 to 360 degrees, but 90-degree and 180-degree bellcranks are most common.
The name comes from its first use, changing the vertical pull on a rope to a horizontal pull on the striker of a [[bell]] to sound it.
==Design==
A typical 90-degree bellcrank consists of an L-shaped crank pivoted where the two arms of the L meet. Moving rods or cables are attached to the outer ends of the L. When one is pulled, the L rotates around the pivot point, pulling on the other rod.
A typical 180-degree bellcrank consists of a straight bar that pivots at or near its center. When one rod is pulled or pushed, the bar rotates around the pivot point, pulling or pushing on the other rod.
Changing the length of the bellcrank's arms changes the [[mechanical advantage]] of the system. Many applications do not change the direction of motion but instead amplify a force "in line", which a bellcrank can do in a limited space. There is a tradeoff between range of motion, linearity of motion, and size. The greater the angle traversed by the crank, the more the motion ratio changes, and the more non-linear the motion becomes.{{citation needed|date=May 2013}}
==Applications==
===Aircraft===
Bellcranks are often used in [[Aircraft flight control system|aircraft flight control systems]] to connect the pilot's controls to the [[Flight control surfaces|control surfaces]]. For example, on [[light aircraft]], the rudder often has a bellcrank (also called a control horn) whose pivot point is the rudder hinge. A cable connects one of the pilot's rudder pedal to one side of the bellcrank. When the pilot pushes the rudder pedal, the cable pulls the bellcrank, causing the rudder to rotate. The opposite rudder pedal is connected to the other end of the bellcrank to rotate the rudder in the opposite direction.
=== Architectural ===
Bellcrank mechanisms were installed at the top of entryway stairs in multi-unit [[Victorian architecture|Victorian]] and [[Edwardian architecture|Edwardian]] homes ({{circa|1890}} to 1930), particularly in the [[San Francisco Bay Area]], to allow residents to open and close the doors remotely so they would not need to walk down the stairs to welcome guests.<ref>{{cite web |last=McLean |first=Tessa |date=9 November 2020 |title=The story behind that weird lever at the top of the stairs in old SF homes |url=https://www.sfgate.com/local/article/bell-crank-levers-san-francisco-apartments-door-15708019.php |accessdate=9 November 2020 |website=SFGate}}</ref>
[[File:A.d.Tramontana Front Suspension.jpg|thumb|350x350px|Front suspension system of the [[Tramontana (sports car)|Tramontana]]; note the bellcranks coupling the suspension pushrods to the [[Coilover|coilovers]]]]
=== Automotive ===
Bellcranks are also seen in automotive applications, such as in the linkage connecting the throttle pedal to the [[carburetor]] or connecting the brake pedal to the [[master cylinder]].
In [[Car suspension|vehicle suspensions]], bellcranks are used in [[Pull-rod suspension|pullrod and pushrod suspensions]] in cars or in the [[Christie suspension]] in tanks. More vertical suspension designs such as [[MacPherson strut|MacPherson struts]] may not be feasible in some vehicle designs due to space, aerodynamic, or other design constraints; bellcranks translate the vertical motion of the wheel into horizontal motion, allowing the suspension to be mounted transversely or longitudinally within the vehicle.
=== Bicycles ===
Bellcranks are used in some [[Hub gear|internally geared hub]] assemblies to select gears.<ref>{{cite web |last1=Brown |first1=Sheldon |title=Sheldon Brown's Bicycle Glossary |url=https://sheldonbrown.com/gloss_ba-n.html#bellcrank |website=Sheldon Brown's Bicycle Technical Info |accessdate=17 February 2020}}</ref><ref>{{cite web |last1=Brown |first1=Sheldon |last2=Allen |first2=John |title=Shimano 3-speed Hubs |url=https://sheldonbrown.com/shimano333.html |website=Sheldon Brown's Bicycle Technical Info |accessdate=17 February 2020}}</ref> The motion from a [[Bowden cable]] is translated by a bellcrank to a push rod, which selects which portion of the [[Epicyclic gearing|epicyclic gears]] are driven by the bicycle's rear sprocket.
==References==
{{reflist}}
==External links==
* [https://web.archive.org/web/20121214014218/http://www.daerospace.com/MechanicalSystems/Bellcrank.php daerospace.com].
[[Category:Mechanical engineering]]
[[Category:Linkages (mechanical)]]