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

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

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?

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

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

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

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

…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

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

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

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

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

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

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

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…

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

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

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

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

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

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

End of Week 2/Lesson 1