crableg,Lshin,wshin,Lthigh,wthigh,thick,ymod,pois,sel
Design of a crab leg suspension

Lshin         length of shin leg of beam in m
wshin         width of shin leg of the beam in m
Lthigh       length of thigh leg of the beam in m
wthigh       width of the thigh leg of the beam in m
thick          thickness of the beam in m
ymod         Young's modulus of the material of the beam in GPa
sel             number denoting the selected result.
                  Use 1 for spring constant in X axis and 2 for spring constant in Y axis and 3 for spring constant in Z axis

crab leg suspension

Notes

Crab-leg suspension has four L shaped beams as shown above. By varying the beam dimensions, it is possible to vary the stiffness in all three axes. Force, moment and torque balance are applied to each leg of the beam to create the model.

In this analysis all four beams are considered in calculating the effective spring constant of the suspension. If you wish to find the spring constant for just one beam, divide the result by 4 since the four beams are in parallel.

The suspension allows lateral movement in the X axis and vertical movement in Z axis. It is often used in comb drives, MEMS switches and MEMS varactor. The stiffness of this suspension in all the three axes directions can be found out using this design interface. The model loses accuracy in Y direction for small thigh lengths since axial stress in shin is not included in calculation of ky.

The plot shows the stiffness variation in all three axes with increase in thigh length. The stiffness in all the three axes decreases with increasing thigh length. The vertical stiffness kz decreases rapidly compared to the lateral stiffness kx as torsion in the thigh beam has a greater influence on kz than bending has on kx. By changing the beam dimensions the most complaint axis can be changed.

 

Assumptions

-The beam has uniform rectangular cross section.
-The beam has small deflection and has small angle or rotation
-Axial stress in shin is not included.
-Residual stress is not considered.
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