### Regeneration Circuits

A regeneration circuit can double the extension speed of a single-rod cylinder without using a larger pump. This means that regeneration circuits save money because a smaller pump, motor, and tank can produce the desired cycle time. It also means that the circuit costs less to operate over the life of the machine.

A regeneration circuit can also replace a double rod-end cylinder in some circuits. With equal rod diameters, a double-rod cylinder’s area is the same on both ends. Equal areas mean identical force and speed both ways at a given pressure and flow. Reciprocating tables often use double rod-end cylinders for this reason. When the main function of a double rod-end cylinder is equal speed and power in both directions of travel, replace it with a regeneration circuit.

A double rod-end cylinder costs more than a cylinder with a single oversize rod; the extra rod needs space in which to move; and the second rod seal is another potential leakage source. To eliminate these objections, use the full-time regeneration circuit shown in Figures 17-6 and 17-8. Extension and retraction speed (as well as thrust) is the same, without the extra rod and its problems.

One disadvantage to using cylinders with a single oversize rod is that speed and thrust are not identical if the rod diameter ratio is not exactly 2:1. Most cataloged 2:1 rod diameters are only close to that ratio. A standard NFPA 3.25-in. bore cylinder comes with a 2.00-in. diameter rod as a 2:1 differential. If using this cylinder in a full-time regeneration circuit, speed is about 21% faster on the extension stroke, with about 21% less force than the retraction stroke.

This chapter explains regeneration principles and shows several common circuit designs, as well as some uncommon and unique circuits.

Regeneration principles
Applying fluid flow to both ports of a single-rod cylinder makes it extend -- or at least try to extend. Because areas on opposite sides of the piston are unequal, the cap end of the cylinder always has more force than the rod side.

Figure 17-1 shows forces and speeds that result when using a cylinder with a 10-in.2 piston area and a 2-in.2 rod area. This cylinder has a rod differential of approximately 1.25:1.

Applying 10-gpm flow to both ports of the cylinder makes it extend at a rate of 1155 ipm. The effective cylinder thrust as it moves forward at 1000 psi is 2000 lb. The same cylinder, with 10 gpm flowing into the cap end while the rod end is connected to tank, would extend at 231 ipm or 1/5 the speed of the regeneration circuit.

Figure 17-2 shows the forces and speeds of this same cylinder as it retracts. Thrust is 8000 lb, but speed is only 288 ipm.

Often when a cylinder with a small rod is piped for regeneration, it will not even try to extend at a reasonable working pressure. This is because the extension force is too low to overcome cylinder friction, machine force requirements, and pressure drop due to high flow from rod end to cap end. Even if the cylinder does extend, low force may make it useless. Notice also that extension speed is very fast and retraction speed is very slow.

Most regeneration circuits use cylinders with a 2:1 rod ratio. A 2:1 rod cylinder has a rod area equal to half the piston area. In actual use, extend and retract speed and power is identical with an area differential of exactly 2:1. All of the cylinders in the following examples have 2:1 ratios.

Regeneration with a 2:1 rod
Figure 17-3 shows a 2:1 rod cylinder in regeneration. Notice the difference in force and speed using the same flow and pressure as Figure 17-1. These figures assume an exact 2:1 rod. In actual practice only specially made cylinders have an exactly 2:1 area ratio.

With a 2:1 rod, force and speed on the extension and retraction stroke is identical. As Figure 17-4 shows, a regenerating cylinder acts like a ram-type cylinder. In effect, a cylinder in regeneration does not need a piston. Because both ports connect to the same power source, the effect of the piston area outside the rod diameter is cancelled. Pressure on both sides of the annulus area around the rod makes the piston area here useless. As the cylinder extends in regeneration, force is half while speed doubles.

During regeneration there is no flow to tank. All oil from the pump goes to the cylinder. All fluid from the rod end of the cylinder mixes with pump flow and goes to the cap end. The exchange of rod end fluid, added to pump oil going to the cap end, doubles the cylinder’s speed.

Figure 17-5 shows force and speed when a 2:1 oversize rod cylinder retracts. Pump flow of 10 gpm enters the rod port and the cylinder retracts using the 5-in.2 annulus area. Speed during retraction is the same as when the cylinder regenerates forward. Flow from the cylinder cap end port is twice pump flow as it retracts because cap end area is twice rod end area. A conventional regeneration circuit has double flow through the valving while the cylinder extends and retracts.

The rest of this section shows many regeneration circuits with explanations of how they work. Different circuits have various features that make them better for certain applications. Some circuits are just a different way of connecting the same parts.