1

Lecture 17

DIRECTIONAL CONTROL VALVE [continued]

1.11.1 Applications of Pilot-Operated Valve to Control the Table of a Surface Grinder Figure 1.26shows the application of a pilot-operated DCV where the actuation of a double-acting cylinder

is used to reciprocate the table of a surface grinder. The table is fitted with adjustable stops as shown in

the figure. The pilot valve is a DCV that is actuated by a push button. During the operation when stop S1

hits push button B1, the pilot valve sends a pilot signal to the main valve to shift the configuration shown

in the right envelope of the main valve. This actuates the double-acting cylinder to extend. At the end of

the extension, stroke S2 hits push button B2, which causes the pilot signal directions to be reversed. Due to

this change in the pilot signal direction, the main valve moves to the configuration shown in the left

envelope of the main valve. This in turn actuates the double-acting cylinder to retract.

Thus, a pilot valve controls a main valve and the main valve used to control the double-acting cylinder.

Figure 1.26 Application of pilot-operated DCVs.

1.12Piston Overlap

The switching characteristics of a valve are decided by the piston overlap. A distinction is made between

the positive, negative and zero overlap. Overlap is defined as the longitudinal difference between the

Pilot valve

Main valve

S2 S1

Workpiece

Table

Electric motor

B2 B1

2

length of land and that of the port. The magnitude of overlap changes during unoperated and operated

conditions.

The piston overlap determines the oil leakage rate. Overlapping is significant for all types of valve. The

most favorable overlap is selected in accordance with the application.

1. Positive switching overlap: During the reversing procedure, all parts are briefly closed against

one another. Hence, switching imparts “pressure peaks” and make hard advance.

2. Negative switching overlap: During the reversing procedure, all ports are briefly interconnected.

Pressure collapses briefly (load drops down).

3. Zero overlap: Edges meet. Important for fast switching, short switching paths.

4. Pressure advanced opening: The pump is first of all connected to the power componentand then

the power component is discharged into the reservoir.

5. Outlet advanced opening:The outlet of the power component is first discharged to the reservoir

before the inlet is connected to the pump.

Figure 1.27Valve overlap: (a) Positive overlap; (b) negative overlap; (c) zero overlap.

1.12Miscellaneous Industrial Circuits This section examines some simple circuits that are commonly used in industry. This will help the reader

to develop the ability to read hydraulic schematics and to understand the operation of basic circuits.

Figure1.28shows a circuit in which a cylinder is used to raise and lower a large weight from above. The

cylinder is controlled by a four-way DCV with a tandem neutral. In Fig. 1.28(a), the DCV is in the

neutral.Therefore, the pump flow is unloaded to the tank at a low pressure. The cylinder should hold

position because the outlet ports from the DCV that connect to the cylinder are blocked. It does not hold

position, however, if the cylinder is in the orientation shown because the weight pulls the cylinder down,

causing pressure in the rod end line. The pressure causes a small amount of leakage within the DCV, and

the cylinder begins to creep downward. This can be remedied by placing a pilot-to-open check valve in

the rod end line, as shown. The pilot-to-open check valve does not allow flow out of the rod end of the

cylinder unless pressure is applied to the pilot line, thereby preventing cylinder creep. The check acts to

counterbalance the weight. When the DCV is shifted to extend the cylinder (lowering the weight), the

pump pressure from the blind end line holds open the check and allows flow to return to the tank from the

blind end. When the cylinder is retracted (the weight is raised), flow from the pump goes through the

check to the rod end. The check has no effect in this direction.

(a) (b) (c)

3

Figure 1.28 Raising and lowering large weights: (a Extend cylinder; (b) hold cylinder; (c) return cylinder.

Figure1.29 shows a circuit that utilizes a shuttle valve. This circuit allows either of the two three-way

buttons to operate a single-acting cylinder. The figure shows both three-ways in their normal positions.

The cylinder is vented to the tank and remains retracted under the force of the spring. In Fig. 1.30, three-

way number 1 is shifted and pump flow is sent to the cylinder through the path shown. In Fig. 1.31, the

cylinder is extended with valve number 2. This circuit could be used on a long machine with buttons on

either end for convenience. A shuttle valve is used in many other applications in which one of two flow

paths may supply a single branch of circuit.

-29 LPM 40 LPM

80 bar

4.5

bar

1.05 bar

80 bar

6 bar

(a)

1000 N

0 LPM 0 LPM

1000 N

40 LPM -53 LPM

1000 N

(b) (c)

80 bar

4

Figure 1.29Use of shuttle valves to control single-acting cylinders(return).

DCV 1 DCV 2

400 bar

5

Figure 1.30Use of shuttle valves to control single-acting cylinders (forward).

DCV 1 DCV 2

2 bar

6

Figure 1.31 Use of shuttle valves to control single-acting cylinders (forward).

Figure1.32 shows a regenerative circuit that automatically switches off regeneration when full force is

necessary. This circuit could be used in a hydraulic press where the cylinder must extend quickly under

no load, then bottoms out and must apply full force to the work piece. Instead of using a four-way with a

regenerative neutral, this circuit uses a four-way DCV in conjunction with a three-way, pilot-operated

DCV. The three-way is shifted when sufficient pressure is applied to its pilot line, which is connected to

the blind end of the cylinder. In the figure, the four-way is shifted to the left position and flow is sent to

the blind end of the cylinder. Because the cylinder is not loaded, the pressure in the blind end is very low

and is not sufficient to shift the three-way. The flow from the rod end combines with the pump flow,

causing the cylinder to extend rapidly. When the cylinder bottoms out, pressure immediately builds up in

the blind end line to the relief valve setting because there is no other path for pump flow. The three-way

valve is then shifted into the left position and pressure is relieved from the rod side because it is

connected to the tank port. The pressure is then applied to the blind sidethat causes full force to be applied

to the work piece. When the four-way DCV is shifted into the right position, the cylinder retracts at a

normal speed. In this circuit, the reduction in force capability caused by regeneration is not an issue

because during the regeneration portion of the cycle, the cylinder is not loaded. The primary advantage of

using regeneration is that a smaller pump can be purchased that is less expensive to buy and operate.

DCV 1 DCV 2

7

Figure 1.32 Regenerative circuit (position 1).

Figure 1.33 Regenerative circuit (position 2).

8

Figure 1.34 Regenerative circuit (position 3).

1.13Direction Control Valve Mounting

DCVs can be mounted in two ways:

1. Inline: There are threaded connections in the valve itself. Fittings are screwed directly into the

valve. This method has several major disadvantages:

Each time the valve is disconnected; there is a probability of damaging the valve by stripping the

threads.

The threads wear each time the unit is disconnected, causing contamination and increased

probability of leakage.

2. Sub-plate: The bottom of the valve has unthreaded connections. The valve is then attached to a

sub-plate that has matching connections. The sub-plate has threaded connections to which the

fittings are attached. Sealing at the valve interface is by using O-rings, which fit into small

recesses around the DCV port. The advantages are as follows:

The sub-plate has less leakage, less contamination and a smaller probability of doing damage

during assembly and disassembly.

Valve replacement is simpler and easier.

Multiple valves can be connected on a manifold.

The valves and sub-plates are available with several standard patterns for the valve ports.

9

Figure 1.35 DCV mountings.

A manifold is a sub-plate that has connection for two or more valves. This method can be used to create

an integrated hydraulic circuit in which many connections are inside the manifold itself, eliminating the

need for fittings and plumbing between the valves. The advantages are a more compact design, less

leakage, less contamination, easy replacement of valves, etc. The only drawback is that it requires more

design and testing time thereby increasing the expense.

Figure 1.36 DCV mounting manifold.

Cartridge valves are also used in conjunction with manifolds. They are very compact and alternative to a

spool-type design. They screw directly into a cavity in a manifold and therefore do not require a separate

valve port and mounting holes. The advantages of manifold circuits are magnified when cartridge valves

are used. ******

1.8DCVSpecifications

10

The most critical specification when selecting a DCV is its maximum pressure and flow ratings. It is

common for a high pressure rating to be given for the pressure and outlet ports and a lower value to tank

port. Ratings of 3000–5000 psi are typical for the former, while the latter has ratings of 500–1000 psi.

The different ratings are because the spool seal is often exposed to the tank port, but no other ports.

Valves that can handle a high pressure to all of their ports are also available. Operating above the

maximum pressure rating leads to increased leakage and also permanent damage to the valve.

The flow rate is largely determined by the size of the valve itself. Larger valves can handle larger flow

rates but are heavy and expensive. Standard valves have ratings 10–250 GPM. Operating a DCV above its

maximum flow rating most likely results in a large pressure drop across the valve. This lost energy is

converted into heat and is not wasteful, but leads to increased component wear as the oil becomes thinner

and doesnot lubricate. Operating above the maximum flow rating leads to permanent damage to the valve

itself.

When selecting a DCV for an application, we may also know what the pressure drop will be across the

valve at a particular flow rate. Manufacturers typically provide graphs that relate pressure drop to flow

rate through valve for each model. Separate curves are given for different port-to-port connections. These

curves represent data for a particular fluid and viscosity, most commonly standard hydraulic oil at 100

SSU. Manufacturers often give a correction factor for fluids at other viscosities. A fluid with a higher

viscosity has a higher pressure drop at a given flow rate because a thicker fluid is more difficult to move

through the valve.

1.9Material for DCVs

Following are the materials for DCVs:

1. Valve body: It is made of carbon steel, ductile cast iron and stainless steel. Aluminum alloys are

also preferred for low-pressure applications. High-strength aluminum alloys are used for aircraft

applications. Stainless steel is used for corrosive environment. Sometimes plastics are also used

for low-temperature applications.

2. Valve spool: It is made of hardened steel, ground and polished 15 Ni2Cr1Mo15 of hardness 60-

62 HRC, machined to 2–3 µm tolerance. Valve spool bore clearance is usually in the order of 5–

10 µm.

Example 1 A cylinder with a bore diameter of 7 cm and a rod diameter of 3.125 cm is to be used in a

system with a 45 LPM pump. Use the graph in Fig. 1.36to determine the pressure drops across the DCV

when the cylinder is retracting (P->B,A->T).

Solution The flow from P to B is the pump flow into the rod end, so this can be read from the graph

p =3.2 bar (approx.)

The flow from A->T is the return flow out of the blind end. This flow rate is greater than the pump flow

and must be determined by the following method:

(a) Calculate the piston area:

2 2 2

Pp  ( ) (7 ) 38.5 cm=

4 4A D

(b) Calculate the rod area:

2 2 2

R R( ) (3.125 ) 7.7 cm4 4

A D

(c) Calculate the return flow:

11

pump

return, R p

p R

pump

 

38.538.5 7.7

1.25 45

56.25 LPM

QQ A

A A

Q

The flow from A to T can now be read from the graph

p =6.2 (approx.)

Figure 1.36

Example 2 Derive an expression to estimate leakage through the spool and housing bore for concentric

leakage path. Refer Fig.1.37and the description of symbols.

Figure 1.37

L

d

r r

7.5 15 22.5 30 37.5 45 52.5 60 67.5 75

P - T

A – T, B - T

P – A, P – B

1.5

3.0

4.5

6.0

7.5

9.0

10.5 12.0

13.5

Flow in LPM

Pre

ssu

re d

rop

in b

ar

12

Solution

The typical pressure in the hydraulic system is in the order of 700–800 bar. The internal leakage is one of

the major problems and it results from wear of the components. Figure 1.37shows the internal leakage

through a radial clearance between two concentric cylindrical bodies, a spool and sleeve, for example.

Let

Constant (m/s)a

Radial clearance (m)c

Spool diameter (m)D

p Pressure force acting on the fluid element (N)F

r Shear force acting on the fluid element (N)F

3

L Leakage flow rate (m /s)Q

Length of the leakage path (m)L

Radial clearance from the midpoint of gap (m)c 5

L Resistance to leakage (Ns/m )R

Distance between the element side surface and solid boundary (m)y

Pressure drop across the radial clearance (Pa)p

Assuming the steady-state flow and forces at equilibrium, we can write the following:

The pressure force is p 2F r Ddp

The frictional force is r 2F Ddx

Also,

  0.5

 

r c y

du du

dy dr

From Newton’s law of viscosity,

 du du

dy dr

p

    or 

rF F

du r dP r dPdu dr

dr dx dx

The pressure gradient  

constantdP

dx

1 2

 , where 

dP PP P P

dx L

The velocity distribution in the radial clearance is found by integrating

 r dPdu dr

dx

2   

2

r dP r dPu dr a a

dx dx

Applying boundary conditions,

2

21    2 4

P cu r

L

The leakage flow rate

13

32

L

2

  

12

c

c

Dc PQ Ddr

L

LL L3

12 

 

LQP R Q

Dc

It is important to note that leakage is inversely proportional to the viscosity and directly proportional to

the cube of the radial clearance. If the radial clearance is doubled due to wear, the internal leakage

increases eight times. The power loss due to leakage is given by

L

3 2

22

L L

L

 

12

N Q P

Dc P

L

PR Q

R

The internal leakage reduces the effective flow rates and increases the power losses. The dissipated power

is converted to heat and leads to serious oil overheating problems. Therefore, it is important to keep the

oil viscosity within the predetermined limits over the whole operating temperature range. This is done by

using hydraulic oils of convenient viscosity index and implementation of oil coolers.

Example 3 Write an expression to estimate leakage through spool and housing bore for eccentric leakage

path. Refer Fig.1.38and the description of symbols. Compare with concentric leakage path and comment.

Figure 1.38

Solution In the case of eccentric mounting, the radial clearance is not constant and the flow rate is given by

14

33

L

  3  1

12 2

Dc PQ

L c

This is highly significant finding because if the inner cylinder just touches the outer cylinder, the flow rate

is increased by 2.5 times the valve with the concentric cylinders assuming the same pressure drop.

Example 4 The land in a spool valve separates two fluid passages. The land has a 25mm length and

18.7325 0.005mm diameter and operates in a 18.75 0.01 mm bore. We assume that the spool is

concentric. The pressure difference across the land is 20.68 MPa. Calculate the leakage flow rate past this

land for minimum, nominal and maximum leakage conditions assuming a fluid with minimum, nominal

and maximum viscosities of 5.84, 32.1 and 800 mm2/s.

Solution

The minimum height of the passage is achieved for the maximum diameter of the spool and the minimum

diameter of the bore

min

(18.75 0.01) (18.7325 0.005)0.00125 mm

2c

Thenominal height of the passage is achieved when both the spool and bore have their nominal

dimensions

nom

(18.75) (18.7325)0.00875 mm

2c

Themaximum height of the passage is achieved for the minimum diameter of the spool and the maximum

diameter of the bore

max

(18.75 0.01) (18.7325 0.005)0.01625 mm

2c

Conversion of viscosity

cSt (mm2/s) cP

(SG = 0.9)

Ns/m2

Pascal seconds

5.84 5.256 0.005256

32.1 28.89 0.02889

880 792 0.792

Case 1: Minimum height of the passage

3

min

3 3 6

8  3

 

12

(0.01875) (0.00125 10 ) 20.68 10

12 0.005256 0.0025

1.5088 10 m /s

Dc PQ

L

Case 2: Nominal height of the passage

3

nom

3 3 6

7  3

 

12

(0.01875) (0.00875  10 ) 20.68 10

12 0.02889 0.0025

9.4146 10 m /s

Dc PQ

L

15

Case 3: Maximum height of the passage

3

max

3 3 6

7  3

 

12

(0.01875) (0.01625  10 ) 20.68 10

12 0.792 0.0025

2.199 10 m /s

Dc PQ

L

Review Questions

1. Explain briefly the function of DCVs.

2. Draw a schematic of 4/3 DCV that is direct operated electrically and briefly explain its function.

3. Draw a schematic of 3/2 DCV that is manually operated and briefly explain its function.

4. State the different ways of control of DCVs.

5. How are DCVs classified?

6. Explain the construction and operation of electric solenoids and compare the DC and AC solenoids.

7. Cite the classification of check valves and explain the function of pilot-operated check valve, giving the

necessary drawing.

8. What is the difference between an open-center and closed-center type of DCV?

9. What is a shuttle valve? Name one application.

Objective-Type Questions

Fill in the Blanks

1. A valve is a device that receives an external signal to release, ------ the fluid that flows through it.

2. DCVs determine-----through which a fluid transverses a given circuit.

3. A check valve allows flow in --------, but blocks the flow in the opposite direction.

4. In 4/3 DCV,for-------- the pump line is blocked so that the flow must pass over the pressure relief valve

the pressure is at the system maximum.

5. In 4/3 DCV, for float neutral,the ------- is blocked and the outlet is connected to the tank.

6. In 4/3 DCV, for open neutral, the pressure port and the outlets are both connected to the ----.

State True or False

1. Pressure control valves protectthe system against overpressure, which may occur due to a sudden surge.

2. A pilot-operated check valve always permits flow in one direction only.

3. A shuttle valve allows two alternate flow sources to be connected in a one-branch circuit.

4. In 4/3 DCV, tandem neutral, the pump flow is allowed to flow to the system.

5. The purpose of the regenerative neutral is that instead of sending the return flow back to the tank, it

sends it into the inlet side of the cylinder, thereby decreasing its speed.

Answers

Fill in the Blanks

16

1.Stop or redirect

2.Path

3.Only one direction

4.Closed neutral

5.Pressure port

6. Tank

State True or False

1.True

2.False

3.True

4.False

5.False