The circuit diagrammed in Figure 23-6 cycles a cylinder slowly in incremental steps as it extends, and then drops it very quickly to pick up another load. Feeding sheets of paperboard or plywood onto a conveyor is a common use for this circuit.

Figure 23-6

A small (3-gpm), fixed-displacement pump sends oil through a flow meter, normally open 2-way directional valve (A), and a filter to tank. In the condition shown in the diagram, the pump unloads to tank at almost no pressure, continually filtering all flow.

The pump can be small because it only has to extend the cylinder a short distance each feed cycle. Because pump flow is immediately available, it is always ready to raise the load.

Check valve (D) keeps cylinder fluid from going to tank while the pump unloads. This same check valve allows free flow to the cylinder when required.

Normally closed solenoid-operated relief valve (S) protects the system from excess pressure and gives a high-flow path to tank to lower the table for another load. It also decelerates the cylinder as it nears bottom and eliminates shock damage to the hydraulic components and the machine.

Normally closed 2-way directional valve (E) and needle valve (F) give a slow-down bypass to the normally closed solenoid-operated relief valve to make sure the cylinder bottoms out smoothly.

Cylinder extending — Energizing solenoid C1 on directional valve (A) sends pump flow to cylinder (C), making it extend as diagrammed in Figure 23-7. This action takes place as fast as directional valve (A) shifts because the pump is at full flow. Cylinder (A) extends at 1.50 ips at a maximum force of 10,000 lb, as long as solenoid C1 stays energized.

Figure 23-7

If the cylinder meets a resistance that calls for more than 1500 psi, pump flow goes to tank through normally closed solenoid-operated relief valve (B) and the cylinder stalls.

Solenoid C1 shifts as often as necessary to raise the load.

When the table gets all the way up, a limit switch signals the control circuit to lower for another load.

Cylinder retracting at high speed — To lower the table fast, the valves shift as shown in Figure 23-8.

Figure 23-8

Solenoid A1 on normally closed solenoid-operated relief valve (S) energizes to vent it. This allows the valve to open with about 20-psi backpressure. The 0.75-in. valve in this circuit passes about 30 gpm at 20- to 30-psi pressure drop, so the cylinder retracts rapidly from machine weight. On this particular machine, the weight gave it a speed of approximately 12 ips or about a 5-sec lowering time. Increasing load-induced pressure on the cylinder would make it even faster.

The solenoid on normally closed 2-way valve (E) also energizes during fast retract of the cylinder. It does not add much speed because its function is to let the cylinder descend slowly through the last 0.5 to 1.5 in. of travel. (Set the slow-down limit switch high enough to decelerate the cylinder before it hits bottom.) Fluid viscosity or weight changes give more or less flow to increase or decrease the slow-down distance.

Retracting cylinder slows down — The circuit in Figure 23-9 shows the valve positions as the retracting cylinder nears the end of stroke and makes the slow-down limit switch.

Figure 23-9

All solenoids are deenergized except C2 on normally closed 2-way (E). When solenoid A1 on normally closed solenoid-operated relief valve (S) deenergizes, the valve starts to close. When this happens, pressure from the load and high -flowing fluid builds to relief setting. The relief valve resists flow but does not close completely all at once (like a directional valve would). Backpressure at the cylinder causes it to decelerate smoothly and quickly with little or no shock.

With relief pressure set low, deceleration takes longer. With relief pressure set high, deceleration is quicker. In any event, cylinder slowing does not consistently stay the same.

Always position the slow-down limit far enough from end of stroke to decelerate the cylinder before it reaches bottom. This means it could stop before bottoming out when the only control is normally closed solenoid-operated relief valve (S).

Normally closed 2-way valve (E) provides another controllable flow path around the normally closed solenoid-operated relief valve. Set needle valve (F) to let the cylinder move quickly, but not fast enough to shock the machine when it reaches bottom. Use a cylinder cap-end cushion as a final shock absorber to shorten slow-down travel.

The slow-down part of the stroke is usually less than 2 in., so it poses no time problem.

After the cylinder bottoms out, solenoid (C2) on normally closed directional valve (E) deenergizes, ending the cycle.