A bellcrank is a type of crank that changes motion through an angle. The angle can be any angle from 0 to 360 degrees, but 90 degrees and 180 degrees 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 A bell is a directly struck idiophone percussion instrument. Most bells have the shape of a hollow cup that when struck vibrates in a single strong strike tone, with its sides forming an efficient resonator. The strike may be made by an inte ...

, used for calling staff in large houses or commercial establishments.

Angles

A typical 90 degree bellcrank consists of an "L" shaped crank pivoted where the two arms of the L meet. Moving rods (or cables or ropes) are attached to the ends of the L arms. When one is pulled, the L rotates around the pivot point, pulling on the other arm. A typical 180 degree bellcrank consists of a straight bar pivoted in the center. When one arm is pulled or pushed, the bar rotates around the pivot point, pulling or pushing on the other arm.

Mechanical advantage

Changing the length of the arms changes the

mechanical advantage Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...

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 non-linear the motion becomes (the more the motion ratio changes).

Applications

Aircraft

Bellcranks are often used in aircraft control systems to connect the pilot's controls to the control surfaces. For example: on light aircraft, the rudder often has a bellcrank whose pivot point is the rudder hinge. A cable connects the pilot's rudder pedal to one side of the bellcrank. When the pilot pushes on the rudder pedal, the rudder rotates on its hinge. The opposite rudder pedal is connected to the other end of the bellcrank to rotate the rudder in the opposite direction. Also referred to as a control horn.

Automotive

Bellcranks are also seen in automotive applications, as part of the linkage connecting the throttle pedal to the

carburetor A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meteri ...

, and connecting the brake pedal to the master brake cylinder. In vehicle suspensions, bellcranks are used in pushrod-style suspensions in automobiles or in the

Christie suspension The Christie suspension is a suspension system developed by American engineer J. Walter Christie for his tank designs. It allowed considerably longer movement than conventional leaf spring systems then in common use, which allowed his tanks t ...

in tanks. Vertically mounted suspensions 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 internally geared hub assemblies to select the gearing. The motion from a

Bowden cable A Bowden cable ( ) is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing. The housing is generally of composite construction, consisting of an inne ...

is translated by a bellcrank to a pushrod which selects which portion of the epicyclic gears are driven by the bicycle's rear sprocket.

Residential flats

Bellcrank mechanisms were installed at the top of entryway stairs in multi-unit Victorian and

Edwardian The Edwardian era or Edwardian period of British history spanned the reign of King Edward VII, 1901 to 1910 and is sometimes extended to the start of the First World War. The death of Queen Victoria in January 1901 marked the end of the Victori ...

homes (circa 1890 to 1930), particularly in the

San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area G ...

to allow residents to open and close the doors remotely so they would not need to walk down the stairs to welcome guests.

References

{{reflist

External links

daerospace.com
Mechanical engineering Linkages (mechanical)