shield, pole
杆 centers on a long, rigid object used for support or defense: a pole or a shield. The two meanings connect through the idea of a sturdy shaft or barrier.
杆 combines 木 (tree/wood) with 干, which likely contributes the sound and the idea of a shaft or shield. The exact historical development is uncertain, but the component structure suggests a wooden pole or defensive implement.
A wooden 木 shaft 干 makes a sturdy pole, or a shield held by a wooden frame.
For カン, imagine a can placed on top of a wooden pole: can -> カン, and the pole holds it up like a shield.
lever