Oversize rod cylinders
Cylinders with oversize rods can end up with dangerously high intensification pressures under certain conditions. The cutaway in Figure 15-17 shows a cylinder mounted vertically with its rod down. The rod area is approximately half the piston area so the annulus area around the rod is also approximately half the area of the piston. The left circuit shows the cylinder extending with a meter-out flow control circuit. For the 5-in. bore cylinder with a 3 1/2-in. rod and the pump compensator set at 3000 psi, pressure in the rod end would be approximately 5880 psi as the cylinder approaches the work. If this is a hydraulic press, middle circuit, with 5000 lb of platen, tooling a load-induced pressure of 499 psi would bring the rod end pressure to 6379 psi. This much pressure could damage the cylinder seals, over-pressure the flow control, and exceed the rating of pipe connections.
The circuit on the right eliminates all of the above problems but still allows speed control of the cylinder. A counterbalance valve on the cylinder rod end set at 100 to 150 psi above load-induced pressure would keep the platen from falling while at rest or while it is approaching the work. A meter-in flow control sets cylinder speed and as fluid enters it, pressure in the cap end only rises to approximately half the counterbalance valve setting. The cylinder starts extending when pressure in the rod end reaches approximately 574 psi, which is well below the rating of all components.
Each of the circuits in Figure 15-17 would control cylinder speed but the counterbalance circuit is the best choice for the reasons given plus it has a lot less energy loss.
Another reason for using an oversize rod is for regeneration circuits like the ones in Figure 15-18. Any single rod cylinder will at least attempt to extend with equal pressure at both ports and this is called regeneration. Whether it actually extends depends on the load it must overcome, maximum system pressure available, and what the rod diameter is. This is because the maximum force during regeneration is pressure times the area of the rod. The piston is in balance during regeneration and serves no function during the process.
The standard rod cylinder in Figure 15-18 would extend at the rate of 12.25 in./sec at a maximum force of 3142 lb. Even if this is ample force to extend the present load, the amount of flow in regeneration is excessive. As the cylinder is regenerating forward with flow of 10 gpm from the pump, there would be 52 gpm coming from the head end for a total of 62 gpm to enter the cap port. This high flow would cause excessive back pressure and keep cylinder speed slow because the circuit relief valve would be bypassing at system pressure.
Another reason using a standard rod cylinder is not good practice is that its retract speed would only be 2.33 in./sec, so overall cycle time would not increase as much as first thought.
The above scenario is the prime reason for using 2:1 rod area ratios for regeneration. The lower cylinder in Figure 15-18 is the same bore but has a 31/2 in. oversize rod. This rod is not exactly 2:1 area ratio because it uses NFPA standard sizes for interchangeability. All NFPA cylinders have the largest standard rod that is up to but not over 2:1 area ratio.
The figures for the 2:1 ratio rod now show a net force on extend of 9621 lb. at a speed of 4 in./sec. During regeneration, flow from the head end is 10.4 gpm with a total flow to the cylinder of 20.4 gpm. This is a good measure of force and a reasonable flow rate that usually overcomes work resistance at easy-to-handle flow rates.
Retract speed would be 3.8 in./sec, making extend and retract speeds almost equal. When the rod diameter is exactly 2:1, extend and retract speed and force are identical -- the same as a double rod-end cylinder. However, getting exactly 2:1 area ratios requires an odd size bore or rod that may require special seals.
The circuits in Figure 15-19 show some standard regeneration setups used for particular needs. The full-time regeneration is a replacement for a double rod-end cylinder circuit. With a 2:1 area ratio rod, it will have identical speed and force in both directions. Even with standard rod diameter cylinders, force and speed are within 10% to 12% of the same, which is often satisfactory.
The full force at pressure buildup example uses a sequence valve to indicate work resistance and direct head-end oil to tank. A check valve in the regeneration flow line allows regeneration flow to the cap end and prevents pump flow to tank during the full force portion of the cycle. This circuit extends fast until work contacts, no matter the size of the part.
The full force at limit switch circuit uses a normally closed two-way directional control valve to send head-end flow to tank when a limit switch is made. This cuts cylinder speed in half so part contact is less abrupt. This circuit protects tooling, allows more time for visual inspection of alignment, and can give an operator more time to respond to unsafe conditions.
The circuits in Figure 15-19 may need a counterbalance valve to retard running-away conditions when the cylinders are vertically mounted. When this is necessary, the counterbalance valve must be externally drained to eliminate backpressure in the pressure adjustment chamber.
Adding a bleed-off flow control to the line between the head-end port and the check valve and after a counterbalance valve allows cylinder speed reduction when required. For complete coverage of regeneration circuits see the author’s upcoming e-book Fluid Power Circuits Explained.