Page 477 - Mechatronics with Experiments

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JWST499-c07
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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 463
6. main control purpose of valve: pressure or flow rate,
7. mounting method of the valve into the circuit: stand alone body, multi function inte-
grated valve block, subplate mounting, manifold block, stacked (sandwich) block. The
mounting standards specify the mechanical dimensions of the ports, their locations
as well as screw holes on the mounting plate. Mounting standards are specified by
NFPA, ISO, and DIN. NFPA standards are referred to with letter D, and DIN standards
are referred to by NG, ISO standards are referred to by CETOP. Equivalent standard
specifications are DO3/NG6/CETOP 03, D05/NG10/CETOP 05, D07/NG16/CETOP
07, D08/NG25/CETOP 08, D010/NG32/CETOP 10.

7.5.1 Pressure Control Valves

Pressure control valves involve two or three port connections. The spool which controls the
orifice area is actuated based on a sensed pressure. Relief valves, pressure reducing valves,
counterbalance valves, sequence valves, brake valves are examples of pressure control
valves (Figures 7.45–7.50). Relief valves limit the maximum pressure output of the valve
by venting the flow to tank. In a mechanical relief valve, a spring sets the maximum pressure.
In an EH controlled relief valve, the current level of the solenoid sets the maximum pressure,
hence an EH controlled relief valve can be used to control a variable line pressure. There
are two major types of pressure relief valves: direct and indirect acting (Figure 7.46). In a
direct acting relief valve, the relief pressure is set by the spring. The line pressure directly
acts on the area of the poppet spool. When the line pressure exceeds the set pressure, the
poppet moves up and opens the line to tank.

p line ⋅ A poppet ≤ k spring ⋅ x spring ; valve closed (7.167)
p ⋅ A ≈ k ⋅ x ; valve open (7.168)
line poppet spring spring
p ≈ p = (k ⋅ x )∕A ; (7.169)
line spring spring spring poppet
where x spring = x preload +Δx spring

The indirect acting relief valve uses an intermediate orifice and spring between the
poppet valve section and the line pressure. The function of the orifice in the indirect pressure
relief valve is as follows: when the set pressure of the pilot stage is exceeded (spring 2 in
Figure 7.46), the poppet spool opens and flow crosses the orifice. The role of the orifice is
effectively to create a pressure drop between A and B locations, and hence allow the flow to

M

Clamp Drill

M

(a) (b) (c)

FIGURE 7.45: Pressure relief valve applications in a hydraulic circuit: (a) unloading valve,
(b) sequence valve, (c) counter balance valve.