Page 451 - Mechatronics with Experiments

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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 437
2000 10000
1000 5000
500
200 SAE 10W 1000
100 500
50 250
20 SAE 40 SAE 70 100
10 8 6 Hydraulic fluid MIL-0-5606 Bunker “C” & SAE 50 50
SAE 60
Kinetic viscosity 4 3 JP-5 Diesel fuel AN-0-9 grade 1010 SAE 10 SAE 20 40 Saybolt universal seconds
SAE 30
SAE 10
35
MIL-L-7808
Kerosene
No. 4 fuel oil
1.5 2 Aviation gasoline–average No. 2 fuel oil No. 3 fuel oil
JP-5– average
1
.9
.8
.7
.6
–60 –40 –20 0 20 40 60 80 100 150 200 250 300 350 400 500
Temperature °F
FIGURE 7.24: Viscosity of various hydraulic oils as a function of temperature. Notice the
nonlinear scale in the y-axis. In all cases, as temperature increases, viscosity decreases. At a
given temperature, different hydraulic oils have different viscosity. Higher viscosity increases
the friction losses (hence reduces the efficiency of the hydraulic power system), but reduces
leakage. Reproduced with permission from Parker Hannifin.

For agooddesign, V charge ≥ V disch .Duetothesymmetricratiointhecycletimesandcylinder
head-end and rod-end diameters, as well as the pump size selection, V charge = V disch in this
3
example. It may be better to select a slightly bigger pump, such as Q = 50 in ∕s, which
ave
would provide an extra capacity for the pump to be able to charge the accumulator. In this
case V > V , which is a desirable result in design.
charge disch

7.3 HYDRAULIC PUMPS

The functional block diagram and operating principle of a pump are shown in Figure
7.25a–d. The pump is the device used to convert mechanical power to hydraulic power.
A positive displacement pump concept is shown in Figure 7.25b–d. During the in-stoke
p < p , oil is sucked in from the “tank.” During the out-stroke p ≥ p , oil is pushed
3 1 3 2
out to the load. Notice that p , pump output pressure, is determined by the load pressure
3
p ≈ p = p + p . If there is no load resistance, then the pump cannot build up
3 2 load spring
pressure. In a hydraulic system including pump-valve-cylinder-tank, the pressure difference
between the pump outlet port and tank is determined by the pressure drop on the valve and
the load pressure created by the cylinder and load interaction. In this concept figure, the
check valves control the direction of the flow. The line relief valve (Figure 7.25d) limits
the maximum allowed line pressure as protection and returns the flow back to the tank if
the line pressure tries to exceed a set limit. Hence, the relief valve assures that the line
pressure stays less than or equal to maximum relief pressure set, p ≤ p max . Notice that
3
during the in-stroke, the volume is expanding. Similarly, during the out-stroke, the volume
is contracting. This phenemenon provides the suction and pumping action for the pump.