Crank arm
The crank arm is a gear from the four-bar chain (four swivel joints ) of the four-link coupling gear . In it, rotary movements are converted into oscillating (back and forth) rotations ("vibrations") and vice versa.
Applications in which the crank serves as the drive and a rocker as the output:
- Tool drive of shaping machines for machining material processing,
- in the windshield wiper of a motor vehicle (see schematic drawings).
Use cases with moving rocker arm:
- Foot-operated sewing machines ,
- Railroad trolleys ,
- Hand-powered bicycles and children's vehicles .
Function requirements
The crank can only be fully rotatable ( unfit if it is not too long, and the lengths of the parts of the crank mechanism in a specific ratio to) each other.
The German engineer Franz Grashof has formulated a doctrine: "The crank as the smallest link of a crank arm (four-bar chain) is fully rotatable if the crank together with the longest link of the crank arm is smaller than the other two links."
Basically, two cases have to be distinguished here, two inequalities must be fulfilled by the crank arm:
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From these two inequalities it also follows that a must in any case be less than c , namely by exactly the difference between b and d .
If a + b = c + d and / or a + d = c + b , the rocker arm comes to dead center in at least one of the two extreme crank positions . There is a risk that the rocker will turn in the opposite direction. If the crank arm had to start again in this position, problems would also arise.
