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.

Why vented?

In a standard overcentre valve the spring chamber sees the return line pressure, so any back pressure on the return — a cooler, a filter, a long return line, or another function running at the same time — adds directly to the setting. The valve effectively tightens up and the load stops lowering smoothly.

A vented valve drains the spring chamber separately, so the setting stays where you put it no matter what the return line is doing. If your circuit has a return filter or cooler, or several functions sharing a return, this is the version you want.

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.

Do not forget the drain line. A vented valve needs its drain port piped back to tank with as little back pressure as possible. Leaving it plugged defeats the whole point.

Manufactured by CBF Hydraulic, Montecchio Maggiore, Italy.