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Counterbalance valves

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how an overcentre valve works

1. LOAD HOLDING

OVC1.jpg
fig. 1

The load induces a pressure (Pc) in the cylinder. The overcentre valve is closed. In this way the load keeps its position (Fig, 1).
As the valve is set to a certain pressure (Pt), once the load induced pressure nears this setting, the valve starts to open, lowering the load (the valve, in this way, acts like a pressure relief valve). To avoid this “load lowering”, it is important to know the maximum load induced pressure in order to choose the correct setting.
Setting pressure must be at least 1.3 times the maximum load induced pressure:

Pt >1.3 Pt

Pressure setting is regulated by a spring; each spring has its own adjustment range.
CBF standard springs, called 01 and 02, have these adjustment ranges respectively:
Spring 01: 20-200 bar (standard setting 100 bar)
Spring 02: 50-350 bar (standard setting 280 bar)

2. LOAD LIFTING

OVC2.jpg
fig. 2

The pump gives the pressure to lift the load, the check valve opens, and the oil flow fills the pressured cylinder chamber; return oil is free (fig. 2).
Pv pressure (i.e. pump pressure) comes from the load, while pressure drops through the valve (pressure drop depends on flow rate).

Pv = Pc + DPv

3. LOAD LOWERING

OVC3.jpg
fig. 3

Load lowering is controlled by the pilot pressure (fig. 3).
Rp = pilot ratio
Pc= load induced pressure
Ppil= pilot pressure
Pt= setting pressure
j= area ratio= cylinder bore side area / cylinder annular area

With the following formulas, it is possible to calculate the pilot pressure:

1. Load pressure on bore cylinder chamber (fig. 3):

Ppil=(Pt-Pc)/(Rp+1/j)

2. Load pressure on annular cylinder chamber:

Ppil=(Pt-Pc)/(Rp+j)

3. Hydraulic motors, or actuators with j= 1:

Ppil=(Pt-Pc)/(Rp+1)

- single effect overcentre valves

- double effect overcentre valves

- overcentre valves, cartridge type

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