What is the rectangular-shaped joint under the M1? What does it do, and what is its DOF?
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$\begingroup$ Hello parvin, this kind of sand clock is usually used for valves in mechanics diagram, but it makes little sense here. As these diagram shapes have old origin there is not so much of an international standard. Maybe if you precise possible country we could help better. $\endgroup$– N. StaubNov 19, 2018 at 7:52
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That diagram appears to be taken from the paper Design and development of a Mechanism of Robotic Arm for Lifting (Part 5), by M.H.Ismail et al - which is available to download as a pdf file from ResearchGate.
That paper also includes the following free-body diagram of the arm:
It is, perhaps, easier to see from this diagram that the 'rectangular-shaped' joint under M1 in your diagram:
is simply a revolute joint, constrained to rotate in a single plane, allowing the arm to be rotated on the base.
The depiction is different to the other revolute joints because it shows a side-view of the joint, rather than an end-view as we have in the other three revolute joints.
answered Nov 18, 2018 at 17:16