1. Week2/Lesson 1 – Elementary hydraulic circuits
Fluid Power Engineering
Week2/Lesson 1 –
Elementary hydraulic circuits

2. Elementary hydraulic circuits
In this lesson we shall
Look at an example of a hydraulic circuit
Further build our experience with hydraulic symbology
Discuss the operational features of several common valve types utilized in many hydraulic circuits
Explore the importance of filter placement to protect equipment in hydraulic circuits

3. Let’s start with an example
Q: We want a circuit with a pump driving a cylinder
We want hand-lever-actuated 4/2 valve with preference to retract cylinder
We want the pump protected from overpressure
Q: Can you draw this circuit?

4. Answer Note: p-relief for when cylinder hits limit each way
p would then build quickly, because closed volume

5. Answer – some observations
Note operation:
If not actuated, cylinder would retract, pump would relieve through PRV
If actuated, operator holds lever, cylinder extends to limit, pump relieves through PRV

6. Answer – some observations
Q: What is difference between operation of this circuit and one with the 4/3 valve below?
A: With this 4/3 can stop in between
Hand-lever would have two stopping position away from default
In fact, may want to have stop position be the default

7. Single-acting/spring-return cylinder
This is a simple cylinder with an internal spring for retraction:
With valve in this default position (P,A-T because of spring), cylinder retracts
Spring in cylinder
Valve is 3/2
Over-pressure protection
But in actuated position (P-A,T), cylinder extends
Actually, could use a normal cylinder with an external spring
Note that though have 3 reservoir symbols, they all imply the same tank

8. …with adjustable retraction speed
Modify this circuit so that the retraction speed of the cylinder is adjustable
We put a needle valve here to be able to adjust the flow rate back to the tank

9. Add stop-and-lock center envelope
Make the valve a 3/3 valve with a center envelope to stop and lock the cylinder. Make this the default envelope.
Cylinder locked here

10. Force delivered by spring-retract cylinder
As cylinder extends, what happens to the force it can deliver?
As cylinder extends, spring pushes against it with a greater force
Force out is p·Acyl - Fk
Fk is spring force
So the price we pay for a simpler operation (single-acting) cylinder is less force out

11. Open-center directional control valve
The DCV shown below has a half-open center
In the center envelope, we have P-T,A,B
With the center envelope active, the cylinder is locked…
…but the pump is unloaded
It does not develop pressure, so the pump load is absent

12. But, is this valve practical?
The entire flow of the pump must go through the open center of the valve
This might not be practical
Valve would have to be big

13. Completely open DCV
What would a complete open center envelope mean as far as system behavior goes?
In the center envelope, we have P-A-B-T
Since all ports are connected to tank, all have 0 pressure
Cylinder not locked, can be moved freely with this envelope active

14. Another example
Draw the symbol for the valve that matches this description:
Solenoid-actuated, spring-centered, 4/3 valve with built-in over-pressure protection through a half-open center
The valve could be characterized as P-A,B-T/P-T,A,B/P-B,A-T

15. Filter: where to put in circuit?
Put it on pump suction line
Here the filter protects the pump
But there is a p-drop across it, so it could cause pump cavitation
As filter gets dirty, Dp fltr gets greater
Too low p sctn can cause fluid to vaporize…

16. Pump cavitation
…the bubbles in suction collapse violently as p builds in pump
Damage caused to lobe pump by cavitation
This can erode pump parts exposed to these bubble collapsing

17. Filter: where to put in circuit?
Put it on pump discharge line
Here the filter does not protect the pump from anything picked up in reservoir
But it does protect delicate PCV and PRV
It has to be a high-p filter
It does not contribute to pump damage from cavitation

18. Filter: where to put in circuit?
Put it on return line to reservoir
In low-pressure part of circuit, so only need a low-pressure filter
It does not contribute to pump damage from cavitation
It does not protect anything from foreign material picked up in tank

19. Pump pumps against full pressure, even when system not moving
Unloading circuit
Problem with standard over-pressure protection by a PCV is that in this configuration, the pump is pumping against full pressure
Pump pumps against full pressure, even when system not moving

20. Oil through pump does not heat up because it’s not being compressed
Unloading circuit
Can add a check valve and lead the pilot line off after the check valve
As long as pressure maintained in this part of the line, PCV opens and relieves pressure at discharge of pump
Pump is unloaded, i.e. ported directly to tank, so no pressure builds up.
Oil through pump does not heat up because it’s not being compressed

21. Outside learning
To better understand this subject matter, view the following videos on the “Big Bad Tech Channel”
Don’t forget to turn the closed-captioning on to be able to understand better the details of the lectures
Watch: Hydraulic Cylinders
Watch: Check Valves
Watch: Pressure Relief Valves

22. End of Week 2/Lesson 1