Download presentation
Presentation is loading. Please wait.
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
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.