Page 31 - Fluid Power Engineering

8 Cha pte r O n e

During the time period, Δt, the piston travels vertically a distance y.
The volume of oil that entered the cylinder during this period is V = A y.
p
Then, the oil flow rate that entered the cylinder is

Q = V = Ay = Av (1.4)
p
t Δ t Δ p
Assuming an ideal cylinder, then the hydraulic power inlet to the
cylinder is

N = Fv = pA Q A =/ Qp
p p (1.5)
where A = Piston area, m 2
p
p = Pressure of inlet oil, Pa
3
Q = Flow rate, m /s
V = Piston swept volume, m 3
The mechanical power delivered to the load equals the hydraulic
power delivered to the cylinder. This equality is due to the assump-
tion of zero internal leakage and zero friction forces in the cylinder.
The assumption of zero internal leakage is practical, for normal con-
ditions. However, for aged seals, there may be non-negligible internal
leakage. A part of the inlet flow leaks and the speed v becomes less
than (Q/A ). Also, a part of the pressure force overcomes the friction
p
forces. Thus, the mechanical power output from the hydraulic cylin-
der is actually less than the input hydraulic power (Fv < Qp).

1.3 Basic Hydraulic Power Systems
Figure 1.11 shows the circuit of a simple hydraulic system, drawn in
both functional-sectional schemes and standard hydraulic symbols.
The function of this system is summarized in the following:
1. The prime mover supplies the system with the required mechan-
ical power. The pump converts the input mechanical power to
hydraulic power.
2. The energy-carrying liquid is transmitted through the hydrau-
lic transmission lines: pipes and hoses. The hydraulic power is
controlled by means of valves of different types. This circuit
includes three different types of valves: a pressure control
valve, a directional control valve, and a flow control (throttle-
check) valve.
3. The controlled hydraulic power is communicated to the hydrau-
lic cylinder, which converts it to the required mechanical power.
Generally, the hydraulic power systems provide both rotary and
linear motions.