The four pistons are carried upon the gimbal ring, which connects, by means of pivots, the two chair couplings.
gimbal, gim′bal, n. a contrivance for suspending the mariner's compass, so as to keep it always horizontal.
1570s, "joints, connecting links;" alteration of gemel "twins" (late 14c.), from Old French jumel (Modern French jumeau) "a twin," from Latin gemellus, diminutive of geminus (see geminate). Related: Gimbals.