Fluid Power: Hydraulic Cylinders
Bryan Shaub
Experiment conducted 02/14/2012
Report due 03/01/2012
Abstract
This experiment showed various simple hydraulic circuit setups and applications. Different hydraulic
circuits were set up to demonstrate fluid flow through hydraulic cylinders in both series and parallel
configurations as well as a directional control valve. A hydraulic lock out circuit was also constructed and
observed; observations and measurements were noted for each configuration.
Introduction
Fluid power circuits are extremely versatile and are also powerful when compared to mechanical
systems. This experiment involved constructing different hydraulic circuits on the Explorer II setup.
Inside the Explorer II is a hydraulic circuit (shown in Figure 1, inside the dotted box) which includes a
reservoir, electric motor, pump, pressure relief valve, and a system gauge. When energized, the electric
motor runs a pump which pumps oil out of the reservoir and to the pressure manifold, assuming the
pressure is not high enough that oil escapes through the pressure relief valve. The pressure can be
measured using the system gauge before it is sent out to the circuit. After the oil travels through the
hydraulic circuit setup, it returns through the return manifold and back to the reservoir.
Different hydraulic circuits were set up to demonstrate the following
- How the pressure relief valve can vary pressure to a cylinder and simulate load
- How hydraulic cylinders act in a series setup
- How hydraulic cylinders act in a parallel setup
- How a hydraulic lock out circuit works
For each of the four configurations, observations were noted and data was recorded.
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Discussion
Procedure 1 simulated a load being applied to a hydraulic cylinder. This simulation was done by
attaching a spring to the end of the cylinder, and then having the hydraulic circuit set up in a way to
make the cylinder contract; Figure 1 shows the circuit setup. The pressure relief valve was left
completely open to start, but was slowly closed to increase pressure in the system and to the cylinder,
causing it to contract. The relief valve was closed only enough each time to cause the cylinder to
contract by 1/4" and then each measurement was taken and recorded in Table 1. The process was
repeated until the spring compression was measured to be 1".
Figure 1: Shows the setup for Procedure 1. The dashed box on the left were components
contained within the Explorer II.
Table 1: Recorded Gage and System Pressures for each of the different spring deflections.
Spring
In-Line Gage
System
Force (lbf)
Compression (in)
Pressure (psi)
Pressure (psi)
0
180
165
146.30
1/4
280
250
221.66
1/2
360
395
350.22
3/4
500
475
421.15
1
600
580
514.25
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Force in Table 1 was calculated using Equation 1 and the given data that the cylinder had a bore of
1-1/16". The example shown is for a spring compression of 1/4".
𝐹
𝐹
𝐹
𝐹
= 𝑃𝐴
= 𝑃𝜋(𝑟)2
= (250 𝑝𝑠𝑖)𝜋(1.0625 𝑖𝑛/2)2
= 221.66 𝑙𝑏
Equation 1
Spring Compression vs. Force
600.00
Force (lb)
500.00
400.00
y = 374.16x + 143.64
R² = 0.9926
300.00
200.00
100.00
0.00
0
0.2
0.4
0.6
0.8
1
1.2
Spring Compression (in)
Figure 2: Shows Spring Deflection vs Force for the different measurements in Table 1.
Questions :
1- If fluid flow is increased the hydraulic cylinder would act faster, but would not have any increase in
power.
2- If pressure is increased, the force that the cylinder is able to exert also increases as a result of
Equation 1.
3- Figure 2 shows a linear relationship between spring deflection and force, which is to be expected
because the equation for force on a spring, F = kx would give F and x a linear relation.
4- The spring stiffness is shown in Figure 2 to be k = 374.1 lbf/in.
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For Procedure 2 a new circuit was constructed using a directional control valve and two hydraulic
cylinders connected in series, as seen in Figure 3.
Figure 3: Setup for Procedure 2, where hydraulic cylinders were connected in series.
When the pump was turned on the cylinders moved at different rates, because each cylinder had a
different internal diameter, one was 1-1/16" and the other was 1-1/2". The 1-1/16" cylinder had a
longer stroke. The motion of the extending cylinder could not be resisted because of the series
connection, the oil had only one way to travel making the cylinders more powerful.
Questions :
1- Series hydraulic cylinders can be used to perform multiple actions at once with cylinders of different
internal diameters.
2- The cylinder size related to the speed of motion. The larger cylinder moved at a slower pace
compared to the smaller one.
3a- In Figure 3, if the control valve is set to the left configuration, the cylinder on the left would be
extending, while the right cylinder would be retracting. Oil travels from the pump through the valve
and to the left cylinder causing it to extend. At the same time, oil in the top portion of the cylinder is
pushed to the right cylinder via the connection and it causes a retracting motion on the right
cylinder. Oil then flows out of the right cylinder through the control valve and back to the return. If
the control valve is set to the right configuration, the cylinder motions are opposite because the
flow of oil is reversed.
3b- The cylinders stop abruptly when the flow of oil is cut off in the middle control valve configuration.
3c- In either configuration the cylinders travelled at different speeds because of the internal diameter
difference. The flow rate was the same into and out of each cylinder, but the internal areas differed
because of the diameter difference.
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Procedure 3 involved changing the connection to parallel, seen in Figure 4.
Figure 4: Procedure 3 setup with both cylinders in parallel.
The motion of one cylinder could be resisted until the other cylinder was fully extended and then the
force was too great to be resisted. This was because in a parallel connection the oil had the option of
which cylinder to go to, so it took the path of least resistance first. Both cylinders moved at different
speeds, again because of the different internal diameters. When the cylinder with the smaller diameter
was fully extended, the flow was no longer being split between the two cylinders and was directed only
towards the larger cylinder - this caused the cylinder to move faster until it was fully extended.
Questions :
1- In parallel, when referring to hydraulic cylinders, means that all cylinders should be moving in the
same direction, and at the same speed, since each is receiving the same flow.
2- Since parallel cylinders move in the same direction and at similar speeds, there are different
applications that a parallel circuit might be used rather than a series. Parallel circuits may have more
possible options, but with more options comes a decrease in flow rate since the flow is being split
multiple ways.
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Procedure 4 had a cylinder controlled by a directional control valve, and then a solenoid to control the
flow to the control valve, this is known as a hydraulic lock out circuit. When the solenoid was energized,
fluid was permitted to flow through the valve to the cylinder, but when it was de-energized no flow
occurred.
Figure 5: Setup for Procedure 4 - hydraulic lock out circuit.
Questions :
1- The purpose of the lock out circuit is mostly for safety, for machine operators. Instead of only one
lever needing to be pulled, a separate button must be pressed to energize the system, and the lever
pulled simultaneously.
2- When the solenoid is energized the flow is directed to the cylinder, and when it is de-energized the
flow is sent back to the return.
Conclusions
For each setup there were small pressure losses through fittings and the tubing itself, but these were
not measured and were probably negligible. Also for each setup, whenever the motion of the cylinders
was completed, the system gauge pressure spiked. This was to be expected because the pump is still
pumping oil, yet there was nowhere for the oil to travel through the circuit, so it could only travel
through the pressure relief valve. Overall, the experiment demonstrated how different hydraulic circuits
operate, and also demonstrated the power that they can have.
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