1. Fluid Power Engineering
Week 9/Lesson 1 –
Additional design scenarios
Additional hydraulic circuits

2. Additional design scenarios, circuits
In this lesson we shall
Investigate the design structure for additional applications and environments
As an application, design a hydraulic circuit to drive parallel cylinders

3. Additional design scenarios
So far all that has been considered from a design standpoint has been a single pump driving a single actuator
Many problems are more complicated than that
For instance, a back-hoe
Here we have one pump driving three separate loads
As systems become for complex, so do the choices
For instance with this system, will all joints be in operation simultaneously?
Will we have intermittent operation so that we could use accumulators to reduce the size of the pump?

4. Back-hoe hydraulic schematic
This is the hydraulic schematic for a back-hoe

5. An aircraft is even more complex
An aircraft is even more complex with an even greater need for reliability and redundancy
Also, with an aircraft, the big need for hydraulic power is at take-off and landing
The flaps are deployed or retracted
The control surfaces have to moved further because of the slow speed of the plane
The landing gear has to be retracted or deployed
And the whole system must fly, so it must be light
And if hydraulic pump power is lost, we want some reserve to continue to have control (accumulator)

6. An aircraft is even more complex
An aircraft is even more complex with an even greater need for reliability and redundancy
Three redundant systems
Ram Air Turbine for emergency power
These are accumulators
Each actuator is fed by two separate hydraulic pumps
There are multiple pumps, switching valves, accumulators, actuators

7. Design/Analysis principles
But the principles for designing such a complicated system we have studied
How to size motors to move a (given) load at a (given) speed
How to calculate flow losses in conduits, fittings, valves
How to size pumps and their prime movers to be able to overcome flow losses and deliver enough power to move a load
How to assess such a system upon completion of the design to calculate the pressure distribution in the system

8. Parallel systems
It’s worth mentioning that if we have the situation below, i.e. a parallel system
The pump will always move the easy load first and the hard one second
So if the cylinders are the same size and the pressures into them the same…
...then if F1 from Load 1 < F2 , Load 1 will travel its length to the end of the cylinder stroke before Load 2 starts to move
If the loads are exactly the same…well, the loads are never exactly the same
Even if the cylinders were pulling up identical weights, the construction details of the cylinders will make them close to identical but never exactly identical
The friction between the seals and the moving parts will never be exactly the same, even in supposedly “identical” cylinders

9. Coordinated motion
If the two loads need to move together in a coordinated motion
Note that the plumbing at left will not produce parallel motion if the cylinders are identical
The flow out of cylinder 1 will be less than the flow into it
So cylinder 2 will move at a slower rate than cylinder 1
Coordinated motion is possible if cylinder 2 is selected such that
A cyl2 = A cyl1 – A cyl1-rod
But from a practical standpoint, even that would be difficult to do, to find two commercial cylinders matched like that

10. Coordinated motion Below is shown a configuration that would work
Note, however, that the force exerted by cylinder 1 would be diminished
The rod side of cylinder 1 carries the pressure used to pull load 2 with cylinder 2
Also, from a practical standpoint, it might be hard to configure the cylinders with this geometry because of space

11. This cylinder carries no load
Coordinated motion
Below is shown another configuration that would work
This cylinder carries no load
But this requires an extra cylinder that carries no load
Practically, nowadays, this motion would be coordinated using two separate servo control valves and cylinders with position sensors, so that position feedback loops could be set-up and the motion coordinated by software

12. Coordinated motion – Just an example
This problem has been presented in some detail as an example of the infinite variety of situations that you are liable to encounter in designing hydraulic motion control systems
We do not have the time in this class to cover a large variety of these cases, only some
But you should be able to apply general principles learned to other situations

13. Notice that the rod end of the cylinder is connected to the input
Regenerative cylinder circuit
At right is shown a regenerative circuit
It is used to speed up the cylinder extension beyond what could be produced just with the pump acting directly
Notice that the rod end of the cylinder is connected to the input
Thus, when the left (extension) envelope of the 4/3 DCV is activated, both the cap end and the rod end of the cylinder are exposed to the pump discharge

14. Regenerative cylinder circuit
If both ends of the cylinder are exposed to the pump pressure, why would it move?
Well, the rod-end area is less than the cap-end area, and so the cylinder will extend
But it will not have the force that it would have because of the pressure in the rod-end of the cylinder
In fact, the non-equal part of the area on either side of the piston is the area of the rod: it’s exposed on the cap-end side but not on the rod-end side
Thus the net force on the piston is
𝐹 𝑝𝑖𝑠𝑡𝑜𝑛 = 𝑝 𝑝𝑚𝑝 ∙ 𝐴 𝑟𝑜𝑑

15. Regenerative cylinder circuit
So why do this?
The flow into the cap-end side of the cylinder is augmented by the flow out of the rod-end side of the cylinder
𝑄 𝐴 = 𝑄 𝑝𝑚𝑝 + 𝑄 𝐵
Thus the cylinder will extend faster than it would with Q pmp alone
This is often used when a tool or end effector needs a fast approach speed to begin working on an object

16. Regen modification circuit
The circuit to the right is a regen circuit too
But the regeneration is done inside the middle envelope of the DCV
In its default position (shown), the cylinder retracts
When the lever actuates the valve to its central envelope, the cylinder advances rapidly due to regeneration
Then when the lever advances further to activate the left envelope, regeneration is turned off and the piston rod-end is exposed to tank pressure to bring full force onto the load
Such a DCV would have two detents to make the activation of the envelopes clear to the user

17. A counterbalance valve is placed in the circuit
Counterbalance valve circuit
A counterbalance valve circuit is used with a vertical load
A counterbalance valve is placed in the circuit
The danger with such a load is that if the cylinder is shifted to the retract envelope without the counterbalance valve, the load will just fall, propelled not only by the pump but also by its own weight
The pilot pressure on the counterbalance valve is set higher than the pressure from the weight load
For extension, the pump supplies flow through the check valve
Also, when the valve is shifted to the center envelope, the pressure relief part of the counterbalance valve is closed, so the load stays up, even though the center envelope connects everything to the tank

18. Counterbalance valve circuit
A counterbalance valve circuit is used with a vertical load
This is the drain for the valve’s pilot flow
To retract the cylinder, i.e. lower the weight, the flow into the rod side of the cylinder increases the pressure beyond the setting on the relief valve
Thus, pressure is needed on top of the piston to lower the load, and the danger of a falling weight is avoided
With the pressure on the rod side of the cylinder, this PRV is opened

19. Cylinder sequencing circuit
Often the situation arises where we want to clamp a part securely and then do some work on it (bend it or drill a hole in it)
The entire operation can be automated into a step 1 – step 2 procedure with a sequencing circuit
Step 1: The clamping cylinder actuates until it runs out of travel
Step 2: The working cylinder actuates and does its work
Step 3: The working cylinder retracts
Step 4: The clamping cylinder retracts, releasing the part
We’ll call this a clamp-and-work circuit

20. Cylinder sequencing circuit
The circuit works as follows
The left envelope is the extension envelope
When activated, flow enters the cap side of the clamp cylinder because the PRV prevents flow to the work cylinder
But once the part is clamped, the pressure on the cap side builds and opens flow to the cap side of the work cylinder

21. Cylinder sequencing circuit
Retraction:
The right envelope is the retraction envelope
When activated, flow enters the rod side of the work cylinder because the PRV prevents flow to the clamp cylinder
But once the work cylinder is retracted, the pressure on the rod side builds and opens flow to the rod side of the clamp cylinder

22. Outside learning
To better understand this subject matter, view the following videos
Don’t forget to turn the closed-captioning on to be able to understand better the details of the lectures
Watch:
Series and Parallel Hydraulic Circuits
Regenerative Extension
Counterbalance Valves
Sequence Valves

23. End of Week 9/Lesson 1