Regenerative circuit (for fast extension of a double-acting cylinder)

REGENERATIVE CIRCUIT (FOR FAST EXTENSION OF A DOUBLE-ACTING CYLINDER)

1. Functional and Operational Requirements

• A regenerative circuit is used to speed up the extending speed of the double-acting cylinder.

• Requirements: A double-acting hydraulic cylinder is to be actuated such that it has a fast extend speed, intermediate 'hold' facility, and manual actuation.

2. Circuit

Fig. 8.4 illustrates a regenerative circuit that can be used to speed up the extending speed of the double-acting cylinder. This circuit uses a manually-operated, three position, four way directional control valve (closed center position), and a double-acting cylinder. It should be noted in this circuit that the pipelines to the cylinder are connected in parallel and one of the ports of the DCV is blocked.

3. Operation

Extension: When the 4/3 DC valve is shifted to the left mode, the oil flows from the pump to the blind end of the cylinder. This pump flow extends the cylinder.

Retraction: When the 4/3 DC valve is shifted to the right mode, the oil from the pump bypasses the DC valve and enters into the rod end of the cylinder. Oil in the blank end drains back to the tank through the DC valve as the cylinder retracts.

4. Why is Extension Stroke Faster than Retraction Stroke in the Regenerative Circuit ?

The speed of extension in the regenerative circuit is greater than that for a regular double- acting cylinder. But the speed of retraction is similar to the regular double-acting cylinder. This is because oil flow from the rod end (QR) regenerates with the pump flow (QP) to provide a total flow rate (QT), which is greater than the pump flow rate to the blank end of the cylinder.

5. Ratio of Extending and Retracting Speeds

Let

QP = Pump flow rate,

QR = Flow from the rod end,

QT = Total flow rate,

VPext = Extending speed of piston,

vpret  = Retracting speed of piston,

Ap = Cross-sectional area of piston,

Ar = Cross-sectional area of rod, and

P = Pressure setting of the relief valve.

Dividing equation (8.1) by (8.2), and simplifying, we get

From equation (8.3), it may be noted that when the piston area equals two times the rod area, both the extending and retracting speeds are equal.

Example 8.1 

A double acting cylinder is hooked up in regenerative circuit as shown in Fig.8.5. The relief valve setting is 105 bars. The piston area is 130 cm2 and the rod area is 65 cm2. If the pump flow is 0.0016 m3/s, find the cylinder speed and load carrying capacity for (1) Extending stroke, (2) Retracting stroke. 

Given Data: 

P = 105 bars; Ap = 130 cm2; Ar = 65 cm2; QP = 0.0016 m3/s.

Solution: 

(1) Cylinder speed and load carrying capacity for extending stroke:

(2) Cylinder speed and load carrying capacity for retracting stroke: