Rephasing of Hydraulic Cylinders

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When cylinders are connected in series, it is necessary to provide a feature for rephasing these cylinders when they are fully retracted. Otherwise, leakage will cause the downstream cylinder to not fully extend.

An example of the need for rephasing is an agricultural implement (planter, cultivator, disk harrow, grain drill, etc.) that must be folded to an 8-ft width for road travel and unfolded to 24-ft width (or wider) for field operation. After several cycles, the downstream cylinders might not extend completely so that the sections make proper ground contact.

One technique used for rephasing is shown in Fig. 7.8. The cylinder is designed with a passageway for oil to flow from the cap end to the rod end when the piston reaches full extension. This passageway has a small diameter, as it is not expected to pass a large flow. Basically, it passes the flow required to make up leakage from the cap end of the cylinder immediately downstream so this cylinder will then extend completely.

rephasing-cylinders

In some cases, three cylinders are connected in series. All three need the rephasing feature, not just the first and second cylinders. The third cylinder must have rephasing, because it may reach full extension before the two upstream cylinders both reach their full extension. It will then stop and block flow so that the other cylinders cannot fully extend.

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