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Oversize-rod cylinder as an intensifier
There are times when the operating pressure of a hydraulic system is too low to produce enough force on a cylinder. The pump’s rated pressure may be inadequate or the electric motor has too little horsepower for the higher pressure. Also, other actuators in the system may not be able stand higher pressure. One answer to this problem is a hydraulic cylinder piped as an intensifier.
When a single-rod cylinder extends, pressure in the rod end intensifies if there is any resistance to flow out of it. Resistance could be from a flow control, counterbalance valve, or simply a restriction. The amount of intensification depends on the area differential of the cap end to the rod end of the cylinder. A typical 4.0-in. bore cylinder with a 2.5-in. oversize rod is sold as a 2:1 ratio. All standard interchangeable cylinders use standard bore and rod sizes that are close to but not greater than a 2:1 ratio. The 4.0-in. bore, 2.5-in. rod combination actually has 1.64:1 area differential. With the rod-end port blocked, a 1.64:1-ratio cylinder produces 1640 psi at the rod end if the cap-end pressure is 1000 psi. This intensified fluid might cause problems in a typical circuit, but could supply a small volume of higher-pressure oil for a short, high-force work stroke from a cylinder.
The volume of oil entering and leaving the intensifier cylinder has the same ratio as the intensification. In the 1.64:1 example above, with a cylinder cap-end flow of 10 gpm, pressure intensified flow from the rod end is 6.1 gpm. The larger the cylinder rod, the higher the intensified pressure — and the lower the flow.
Figure 13-18 shows a schematic diagram of an oversize-rod cylinder used as an intensifier. Intensifier cylinder A has 5.0-in. bore with a 3.5-in. diameter rod. The area of the cap side is 19.64 in.2 and the rod annulus area is 10.01 in.2, giving a ratio of 1.96:1. Every 100 psi in the cap end produces 196 psi in the rod end. Also, 10 gpm entering the cap end pushes 5.1 gpm from the rod end. Stroke length of intensifier cylinder A must give enough volume to move work cylinder B through its high-pressure work stroke. If cylinder B has a 10.0-in. bore and a 0.5-in. stroke, the required volume is approximately 40 in. 3. Dividing the 40-in.3 work-stroke volume by a 10-in.3 intensifier volume indicates that a minimum stroke of 4 in. is needed from cylinder A. To allow for oil compressibility and leakage, specify an intensifier stroke of 6 to 8 in.
The cycle is automatic because sequence valves D and E control extension and retraction of the intensifier. Cycle time is slightly slower than the original low-force circuit.
Figure 13-19 shows solenoid A1 on directional valve C energized. Fluid flows directly to work cylinder B through the free-flow check on sequence valve E. Work cylinder B advances rapidly toward the work at low pressure.
At work contact, pressure builds to the setting of sequence valve D, Figure 13-20. Intensifier cylinder A extends and pressurizes oil in the cap end of work cylinder B to approximately twice system pressure. Before the intensifier bottoms out, it must give enough volume to complete cylinder B’s work stroke. For long holding cycles, calculate valve and cylinder leakage, then add extra intensifier stroke so pressure holds.
To retract work cylinder B, energize solenoid B1 to direct oil to its rod end, as in Figure 13-21. As cylinder B retracts, sequence valve E forces oil from its cap end to retract intensifier cylinder A. This saves pump fluid and retracts the intensifier within normal cycle time. When intensifier cylinder B retracts fully, external pilot-operated sequence valve E opens and the remainder of the oil in the work cylinder cap end goes to tank. The only added cycle time is while the intensifier boosts pressure in the work cylinder.