An overcentre valve — also called a counterbalance valve — does two jobs at once. It holds a load against gravity when the control valve is centred, and it controls the descent when the operator lowers it, by metering the return oil so the load cannot run away.

The difference between an overcentre valve and a plain lock valve is the second job. A lock valve is open or shut. An overcentre valve opens progressively in proportion to pilot pressure, which is what gives smooth, controllable lowering of a boom or a raised platform.

Internal or external pilot?

External pilot (OVCSE) takes its opening signal from the opposite working line — the normal arrangement, and the one you want for a boom or a lift cylinder. Internal pilot (OVCSEL) senses the pressure in its own line, which suits a motor circuit or anywhere the opposite line is not available.

Pilot ratio and setting

Set the overcentre pressure at least 30 per cent above the maximum load-induced pressure. Set it too close and the valve will chatter or lower jerkily; set it far too high and it will not open properly and the function will feel dead.

Manufactured by CBF Hydraulic, Montecchio Maggiore, Italy.