Ezy Mechanic

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Wednesday, July 29, 2015

Stiffness of a lever with eccentric loading - Part 2

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Let's continue from the previous post. We clearly see that the small post must be shortened because it the weakest point and creates torsional load to the lever arm. We're going to try redesigning with a bent lever arm so that the loading point stays in the middle of the center line of the arm.
bent lever's end to reduce torsion
Lever bending to eliminate torsion
The eccentric distance "r2" is shorter than previous design (r1). It's now the eccentric dimension with respect to the bent portion of the lever. Torsion still exists at end portion which has shorter length. This gives much less torsion compared with the earlier design. It also eliminate the torsion from the longer part of the lever since the loading point stays at the center line of the lever.
torsion elimination of the lever
Two lever designs at the same loading point
From the overlay, we can see that the pull rod connecting point is still at the same location. The lever arm still connect to the same hub. But we can reduce the length of the small post which is the pin for the pull rod connection. The longer portion of the lever is now under bending only. Most torsion has been eliminated. It exists at the small portion as explained.
FEA displacement of improved lever
Finite element analysis - displacement of the lever with bent end
Shortening the small post and keeping the loading point on the center line of the lever can reduce displacement from 2.4 mm to 1.6 mm (33% reduction). Small twisting is present at the end but there is no twisting over the long portion of the lever.
FEA von Mises stress of bent beam
von Mises stress of the bent lever
The von Mises stress reduces about 77%. This improvement can be used in most cases even when the lever shaft is short and the pull rod location is beyond the lever hub location and it's stiffer than the big lever with small post. The following picture is the example of bent levers on the machine.
example bent lever on machine
Example of a bent lever on the machine


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