equilibrium-ring

from The Century Dictionary.

• noun A balancing-ring; a metal ring placed on the back of a slide-valve, or in the casing of a steam-chest, to keep the steam-pressure from acting over an area on the back of the valve equal to the area of the exhaust-port. This so relieves the valve of pressure that it is practically balanced. The ring is usually placed in a recess and kept tight by placing springs behind it.