Model Answer Question (1) [15 Marks] a ) The properties of a good hydraulic fluid are.

Favorable viscosity Minimum viscosity-temperature dependence Chemically stable Physically stable Good lubricity Compatible with different materials Have heat transfer capability (high specific heat and high thermal conductivity) High bulks modulus Low volatility Low foaming tendencies High flash point

b ) The advantages of power transmission by hydraulic systems are:

The force can be scaled by increasing actuator area. Power transmission to remote locations is possible. The heat generated by internal friction is a basic limitation of any machine. The hydraulic fluid also acts as a lubricant and increase the component life.

The disadvantages of using hydraulic systems in power transmission are:

Hydraulic power is not as easily available as that of electrical power. The hydraulic fluid has a maximum operating temperature The hydraulic systems are very sensitive to dirt and contamination. Hydraulic systems are not desirable for the low power applications of control system.

c ) Fixed displacement vane pump:

Vane pumps generate a pumping action by causing the vanes to move along a ring. The pump consists mainly of a rotor, vanes, ring and a port plate with inlet and outlet ports. The rotor in a vane pump is connected to the prime mover by means of a shaft. The vanes are placed in radial slots milled into the rotor, which in turn is placed off-center inside a cam ring. When the shaft rotates, the vanes move outward against the cam ring by the centrifugal force and track along the ring and therefore, providing a hydraulic seal.

d ) The velocity distribution in the radial direction is given by the following expression:

⎟⎟⎠

⎞⎜⎜⎝

⎛−= 2

2

4v rca

Where 4

Vv2

maxca==

For very small clearance DvdrdQ π=

drrcDaQc

c∫

−⎟⎟⎠

⎞⎜⎜⎝

⎛−=

2/

2/

22

4π

6

3caDQ π=

For the clearance area DcAc π= and the side area subjected to viscous friction DLAs π2=

The shear stress radrdv µµτ 2−==

The force required to overcome the friction forces cDLaAF s µπτ 2==

The required pressure difference= LAAFP

c

µ2==∆

Then

PL

DcQL ∆=µ

π12

3

Using the data given in the problem: ( )( ) ( )

smQL

376

3

363

1008.110151010015.012

1071012 −−

−−

×=×××××××

=π

Question (2) [15 Marks] a) The symbols of different methods used to operate directional control valve

b) The hydraulic pump flow rate and pressure are selected based on the following steps:

Select of the appropriate actuator based on the load Selection of system pressure Determine the required flow rate which is required to drive the actuator with the specified speed. Determine the pump speed and determine the pump size. Selection of the pump type based on the application. Determination of the total power to be delivered by the pump.

c) The parameters affecting volumetric efficiency of hydraulic pumps are:

Internal leakage Pump cavitations Fluid compressibility Partial filling of pump due to fluid inertia

d) Determine motor displacement and motor output torque for the hydraulic transmission system operating at pressure of 105 bar and has the following characteristics: [6 marks]

Pump Motor = 90 % = 92 %

= 85 % = 94 % =100 / =??

N = 1000 rpm N = 600 rpm The actual pump flow rate

0.85 100 1000 85000 / = 1.42 10 /s The actual power driven the pumps

∆ 1.42 10 105 100.9 166

The actual motor flow rate is

0.92 1.42 10 1.31 10 /s The motor geometric volume

. 2.18 10 /

131 Hydraulic power transmitted by the fluid

∆ 1.42 10 105 10 149100 Motor output

149100 0.92 137172 Motor angular velocity

260

2 60060 62.83

Motor output torque 137173

62.83 2183 Question (3) [15 Marks] The circuit with a sequence valve

The first cylinder exerts 10 kN to move and hold the work piece while the other cylinder is used to perform the pressing job with 180 kN. Both cylinders have bore diameter of 150 mm and a 100 rod diameter. Both cylinders have to move at velocity of 5 m/min. Pump pressure during work piece movement = 5.66

Pump pressure during pressing = 102

The capacity of the pump = 150 10 1.47 10 88 / The appropriate setting of the sequence valve at pressure above 5.66 bar, eg. 1.1 5.66 6.2 Question (4) [15 Marks] a) End caps have to withstand shock loads at the ends of piston travel. These shock loads can be reduced

with cushion valves built in the end caps. During the final small leg of the stroke, the oil must exhaust through an adjustable opening. The cushion design also incorporates a check valve in order to allow free flow to the piston during reversal in direction.