Hydraulic Press Control Circuit

Objectives:

• Discuss the operation of this hydraulic press control circuit.

• Connect the circuit in the laboratory.

• Operate the circuit using toggle switches to simulate limit and pressure switches.

LABORATORY EXERCISE

Name ___ Date _____

Materials Required:

Three-phase power supply

Control transformer

Three-phase motor starter with at least two normally open auxiliary contacts

5 double-acting push buttons (N.O./N.C. on each button)

Pilot light

3 toggle switches that can be used to simulate two limit switches and one pressure switch 1 three-phase motor or equivalent motor load

2 solenoid coils or lamps to simulate solenoid coils

3 control relays with three sets of contacts (11-pin) and 11-pin sockets

3 control relays with two sets of contacts (8-pin) and 8-pin sockets

The next circuit to be discussed is a control for a large hydraulic press ( Ill. 1). In this circuit, a hydraulic pump must be started before the press can operate. Pressure switch PS closes when there is sufficient hydraulic pressure to operate the press. If switch PS should open, it will stop the operation of the circuit. A green pilot light is used to tell the operator that there is enough pressure to operate the press.

Two run push buttons are located far enough apart so that both of the operator's hands must be used to cause the press to cycle. This is to prevent the operator from getting his hands in the press when it's operating. Limit switches UPLS and DNLS are used to determine when the press is at the bottom of its downstroke and when it's at the top of its upstroke.

In the event one or both of the run push buttons are released during the cycle, a reset button can be used to reset the press to its top position. The up solenoid causes the press to travel upward when it's energized, and the down solenoid causes the press to travel downward when it's energized.

Ill. 1 Hydraulic press.

Ill. 2 The circuit with pump operating.

Ill. 3 Circuit's started.

Ill. 4 The up limit switch opens.

To understand the operation of this circuit, assume that the press is in the up position. Notice that limit switch UPLS is shown normally open held closed. This limit switch is connected normally open, but when the press is in the up position it's being held closed. Now assume that the hydraulic pump is started and that the pressure switch closes. When pressure switch PS closes, the green pilot light turns on and UPLSCR (Up Limit Switch Control Relay) energizes, changing all UPLSCR contacts ( Ill. 2). When both run push buttons are held down, a circuit's completed to CR1 relay, causing all CR1 contacts to change position ( Ill. 3). The CR1 contact connected in series with the coil of DNCR closes and energizes the relay, causing all DNCR contacts to change position.

The DNCR contact connected in series with the down solenoid coil closes and energizes the down solenoid.

As the press begins to move downward, limit switch UPLS opens and de-energizes coil UPLSCR, returning all UPLSCR contacts to their normal position ( Ill. 4). When the press reaches the bottom of its stroke, it closes down limit switch DNLS. This energizes the coil of the down limit switch control relay, DNLSCR, causing all DNLSCR contacts to change position ( Ill. 5). The normally open DNLSCR contact connected in series with the coil of CR2 closes and energizes that relay, causing all CR2 contacts to change position.

The normally closed DNLSCR contact connected in series with DNCR coil opens and de energizes that relay. All DNCR contacts return to their normal positions. The normally open contact connected in series with the down solenoid coil opens and de-energizes the solenoid.

The normally closed DNCR contact connected in series with UPCR coil recloses and provides a current path to that relay.

The UPCR contact connected in series with coil DNCR opens and prevents coil DNCR from re-energizing when coil DNLSCR de-energizes. The normally open UPCR contact connected in series with the up solenoid closes and provides a current path to the up solenoid. When the press starts upward, limit switch DNLS reopens and de-energizes coil DNLSCR. A circuit's maintained to UPCR coil by the now closed UPCR contact connected in series with the CR1 contact ( Ill. 6).

The press will continue to travel upward until it reaches its upper limit and closes limit switch UPLS, energizing coil UPLSCR ( Ill. 7). This causes both UPLSCR contacts to change position. The UPLSCR contact connected in series with coil UPCR opens and de energizes the up solenoid. Notice that control relays CR1 and CR2 are still energized.

Before the press can be re-cycled, one or both of the run buttons must be released to break the circuit to the control relays. This will permit the circuit to reset to the state shown in Ill. 2. If for some reason the press should be stopped during a cycle, the reset button can be used to return the press to the starting position.

Ill. 5 DNLSCR and CR2 relays energize.

Ill. 6 Limit switch DNLS reopens.

Ill. 7 The press completes the cycle.

Connecting the Circuit

In this exercise toggle switches will be used to simulate the action of the pressure switch and the two limit switches. Lights may also be substituted for the up and down solenoid coils.

1. Refer to the circuit shown in Ill. 1. Count the number of contacts controlled by each of the control relays to determine which should be 11-pin and which should be 8-pin. Relays that need three contacts will have to be 11-pin, and relays that need two contacts may be 8-pin.

2. After determining whether a relay is to be 11-pin or 8-pin, identify the relay with some type of marker that can be removed later. Identifying the relays as CR1, CR2, and so on can make connection much simpler.

3. Place the pin numbers on the schematic in Ill. 1 to correspond with the contacts and coils of the control relays. Circle the numbers to distinguish them from wire numbers.

4. Place wire numbers beside each component on the schematic.

5. Connect the circuit. (Note: When connecting the two run push buttons, connect them close enough together to permit both to be held closed with one hand.) Testing the Circuit To test the circuit for proper operation:

1. Set the toggle switches used to simulate the pressure and down limit switch in the open (off ) position. Set the toggle used to simulate the up limit switch in the closed (on) position.

2. Press the "pump start" button and the motor or simulated motor load should start operating.

3. Close the pressure switch. The pilot light and UPLSCR relay should energize.

4. Press and hold down both of the run push buttons. Relays CR1 and DNCR should energize. The down solenoid should also turn on.

5. The press is now traveling in the down direction. Open the up limit switch. This should cause UPLSCR to de-energize. The down solenoid should remain turned on.

6. Close the down limit switch to simulate the press reaching the bottom of its stroke. DNLSCR, CR2, and UPCR should energize. The press is now starting to travel upward.

7. Open the down limit switch. DNLSCR should de-energize, but the UPCR should remain energized.

8. Close the up limit switch to simulate the press reaching the top of its stroke. The up solenoid should turn off. Control relays CR1 and CR2 should both remain on as long as the two run buttons are held closed.

9. To restart the cycle, release the run buttons and reclose them.

QUIZ:

1. Assume that the hydraulic pump is running and the pilot light is turned on indicating that there is sufficient pressure to operate the press. Now assume that the up limit switch isn't closed. What will be action of the circuit if both run buttons are pressed?

2. Assume that the press is in the middle of its downstroke when the operator releases the two run push buttons. Explain the action of the circuit.

3. Referring to the condition of the circuit as stated in question 2, what would happen if the two run push buttons are pressed and held closed? Explain your answer.

4. Referring to the condition of the circuit as stated in question 2, what would happen if the reset button is pressed and held closed? Explain your answer.

5. Assume that the press traveled to the bottom of its stroke and then started back up.

When it reached the middle of its stroke, the power was interrupted. After the power has been restored, if the two run buttons are pressed, will the press continue to travel upward to complete its stroke, or will it start moving downward?