Page 586 - Mechatronics with Experiments

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The worst case power loss in open center systems occurs if the flow has to go though
the relief valve. Relief valve pressure setting can be in the range of 3000–5000 psi. If we
assume the relief valve pressure is set at p relif = 3000 psi, and the main flow valve sized
such that Δp PT = 100 psi (to support full flow rate across the P–T orifice when the spool is
in neutral position), hydraulic power loss (wasted power) can be 30 times higher if the flow
has to go through the relief valve as opposed to P–T port of the main flow control valve.

P loss = Q ⋅ 100 flow through P–T (7.687)
s
P loss = Q ⋅ 3000 flow through relief valve (7.688)
s
Clearly, a hydraulic system with a fixed displacement pump, a relief valve, and a closed
center valve would also be functional. However, since in a closed center valve, the P–T
port does not exist (A PT = 0.0), the flow would have to go through the relief valve when
the main flow control valve is in neutral position. Such a system would be about 30 times
less efficient than the same system with an open center main flow control valve.
Let us consider the orifice geometries as a function of the spool position. There are
five orifice areas defined by the position of the spool: pump to tank (P–T), pump to cylinder
head end (P–C ), pump to cylinder rod end (P–C ), cylinder head-end to tank (C –T)
HE RE HE
and cylinder rod end to tank (C –T) (Figures 7.107 and 7.108). Independently controlled
RE
valves (IMVs) control each one of these orifice areas by controlling a separate spool for
each orifice. In a standard single spool valve, these orifice areas are directly a function
of the spool. Figure 7.108 shows a possible set of orifice area curves which are directly a
function of the geometry of the valve spool and valve body orifices. Let us consider the case
that the spool moves to the right side in the following increments of its total displacement:
0, 25, 50, 100%. As that happens, the orifice openings between ports, pump, tank, cylinder
head-end, and cylinder rod-end vary as follows,
A PT ≠ 0 (7.689)
A ≠ 0 (7.690)
PC HE
A C RE T ≠ 0 (7.691)

Orifice areas

A A P-C he
P-C re A P-T

A A C -T
C -T re
he

% 100 75 50 25 0 25 50 75 100 %
X - spool displacement (%)
s
FIGURE 7.108: Open-center valve and its orifice areas as a function of spool displacement:
P-T, P-C HE , P-C RE , C HE -T, C RE -T.