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Fluid Power Lab

2003

 

Lab 1

Experiment 1: Automatic reciprocating circuit without limit valves

1)      What is the role of the quick exhaust valve (OR) in this circuit?

The purpose of the OR valve:

®    When the lever valve is released, flow is directed into right side of main switching valve, which retract the cylinder

®    Or, when the cylinder extends fully yet the lever is still engaged, flow from the rod end of the cylinder runs out and ceases to pressurize the right 3/2 valve, which results in a state change as the left side is always subject to 10psi pressure. This state change allows flow to enter the right pilot port of the right 3/82 valve as before, which retracts the cylinder.

Therefore the Or valve serves to retract the cylinder when either the lever valve is released of the cylinder has fully extended.

2)      Although R1 is set to 10 psi, why does the pressure at pilot lines P1 and P7 change when line (10) is pressurized?

Pilots P1 and P7 are always subjected to a nominal 10psi as line 10 is always pressurized. P1 and P7 both act to actuate the left pilot ports of the right and left valve (not main switching valve). The right pilot ports of these two valves are subject to intermittent pressure of 0 or 60psi depending on the cylinder position. As one of these right ports drop to 0 pressure in one valve, the pressure increases in the right pilot of the other valve to 60psi, which serves to switch both spools simultaneously (net volume change in line equal 0). This change results in the volume of line 10 increasing in one area and dropping in another area, which induces momentarily localized pressure spikes and drops. This quickly returns to an equilibrium pressure of 10psi.

Experiment 2: Automatic reciprocating circuit with limit valves

1)      Briefly explain how the circuit works.

The cylinder always starts completely retracted assuming it was stopped using one of the stop levers, not by switching the system power off (if this happened the cylinder will continue where it left off as the main switching valve has not moved and will stop at the retracted position after the cylinder completes the rest of it cycle).

The circuit serves to continuously retracted and extend between the limit switches so long as the start has been activated. As soon as one the stops are activated, the flow into LV1 ceases, which means that as the cylinder retracts and contacts LV1, no flow through LV1 can switch the main switch valve to actuate the cylinder forward. The cylinder will always complete it motion whether the stop has been engaged while it was extending or retracting; the stop just ensures that the cylinder won’t extend again. Theoretically, the cylinder can cycle once completely before it stops (finishes in complete retraction) As soon as start is pressed again, the cylinder, sitting on LV1, allows flow to the extending pilot of the main switching valve to move the cylinder forward.

2)      During system operation, if the start valve is activated again what will happen? 

Nothing will happen to the circuit as the valve supplying LV1 has already been set by the start. The start only performs a task after the stop has been activated, which spools the valve to the left.

3)      During extension, if the stop valve is activated, does the cylinder immediately retract? Why?

As previously discussed, the cylinder always completely its cycle after the stop has been hit. The stop only serves to prevent the cylinder from cycling again.

Experiment 3: Time delay circuit

1)      If flow control valve FLC1 is opened more than one turn, valve P8 gets triggered before cylinder C2 is fully extended. Why?

The accumulator works on the principle that once the C1 has fully extended to make contact with LV2, flow is permitted to pass the limit switch and towards both the accumulator and reverse pilot of the main switching valve. Depending on the flow rate into the accumulator set by the flow control, the time to build up pressure on the pilot can vary. However, if the accumulator cylinder is sticky, this serves increase the backpressure, which may trigger the solenoid too early.

2)      If the exhaust port of cylinder C2 is blocked (left side of C2 in Figure 4), is the delay time increased or decreased? Why? 

Blocking the exhaust port decreases the delay time by virtually removing the accumulator as a functioning part. The backpressure will rise very rapidly as it cannot flow into the cylinder and will actuate the pilot with virtually no time delay. The blocked accumulator is insignificantly better than no cylinder.

Lab 2

1)      Draw the step-position diagram

The sequence of operations is:

C1+ C1- C2+ C3+ C2- C3-

 

2)      Draw the circuit

The full circuit schematic is attached to this report

3)      Briefly explain the circuit and the way the logic of the project is implemented

The cascade solenoids are arranged in the typical fashion in order to activate/deactivate pressure lines one, two, and three. There are four pressure pilots in total to start and end each group of operations. There are three groupings, and in each group the corresponding line is the only one pressurized (for example: in group one, only line one is pressurized). As the systems starts, line one is pressurized, and cylinder one extends. Once cylinder one is fully extended, group one ends and group two begins, pressurizing line two. Cylinder one is retracted, and once fully retracted, cylinder two extends. Once cylinder two is fully extended cylinder three extends. Once cylinder three is fully extended, group two ends and group three begins, depressurizing line two and pressurizing line three. Cylinder two retracts, and once fully retracted, cylinder three begins to retract. Once cylinder three retracts group three is ended and group one started again (depending on the position of the start toggle). The logic is implemented so that as each solenoid activates a cylinder to retract or extend, pressure is also supplied to the limit switch so that when the cylinder is fully retracted or extended the next operation can begin. By sequencing this logic, the whole system can be implemented.

4)      State your observations and comments. Suggest ways for enhancing laboratories of ME 561 in the future. Any constructive criticism and suggestions are welcome. 

Some of our observations and comments are as follows: For the double piston we used the two limit switches on one end, and not one on either end. This is because the limit switch on one end is in the middle of the stroke only. Some of the solenoids on the circuit were 5 port / 2 way, and since our design used 4 port / 2 way, we had to take this into consideration when building our circuit. The cascade solenoids had to be initialized before we ran the circuit; otherwise the system would not run properly. Once these solenoids were initialized, the circuit would set them to the desired location by the end of the system sequence. This way the circuit could be run repeatedly. Once the circuit was built, and the solenoids were initialized, we had to run through the sequence one in order to synchronize the pistons. After running the system once, the pistons were at their correct initial positions, and the system could be run with the desired effects.

Overall, our group found the ME 561 labs to be extremely helpful. In each case we were able to work through the problems, and were challenged to explain the operation of the circuit by the teaching assistant. The labs definitely helped our understanding of the course material. Some suggestions for enhancing the laboratories in the future would be to make available more kinds of solenoids, valves, and hydraulic/pneumatic components. Even if these components were not used in the lab, being able to see them and physically hold them would be beneficial to understanding how some of these components interact.