A knuckle joint (as shown in figure- 188.8.131.52) is used to connect two rods under tensile load. This joint permits angular misalignment of the rods and may take compressive load if it is guided.
These joints are used for different types of connections e.g. tie rods, tension links in bridge structure. In this, one of the rods has an eye at the rod end and the other one is forked with eyes at both the legs. A pin (knuckle pin) is inserted through the rod-end eye and fork-end eyes and is secured by a collar and a split pin. Normally, empirical relations are available to find different dimensions of the joint and they are safe from design point of view. The proportions are given in the figure-184.108.40.206.
Mean diameter of the split pin = 0.25 d
However, failures analysis may be carried out for checking. The analyses are shown below assuming the same materials for the rods and pins and the yield stresses in tension, compression and shear are given by σt, σc and τ.
1. Failure of rod in tension:
2. Failure of knuckle pin in double shear:
3. Failure of knuckle pin in bending (if the pin is loose in the fork) Assuming a triangular pressure distribution on the pin, the loading on the pin is shown in figure- 220.127.116.11. Equating the maximum bending stress to tensile or compressive yield stress we have
4. Failure of rod eye in shear:
5. Failure of rod eye in crushing:
6. Failure of rod eye in tension:
7. Failure of forked end in shear:
8. Failure of forked end in tension:
9. Failure of forked end in crushing:
The design may be carried out using the empirical proportions and then the analytical relations may be used as checks.
For example using the 2nd equation we have We may now put value of d1 from empirical relation and then find F.S. =
which should be more than one.