Hydraulic Cylinder Cushioning

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When cylinders reach the end of their stroke, the pressure rises quickly, creating a shock wave in the hydraulic circuit. Cushioning is done to reduce this stock. The concept, shown in Fig. 7.6, is quite simple. First, we consider the case in which the cylinder is retracting. The spear closes off the large opening where the fluid is exiting the cap end of the cylinder. Fluid must now flow out the small opening past a needle valve. This valve adjusts the orifice and sets the back pressure that develops in the cap end. The resultant force slows the piston so that it “coasts” to a stop. The resultant pressure shock in the main circuit is significantly reduced.

cushioning-hydraulic-cylinder

The same technique is used to cushion the cylinder when it is extending. In this case, a sleeve is mounted on the rod to close the main opening so that flow goes through the orifice.

To understand cushioning, it is appropriate to review a basic principle of fluid power. Always ask the question, what is happening at the relief valve? We will consider the case where a fixed displacement pump is supplying the flow to extend the cylinder.

When the spear closes the large opening, the fluid must flow past the needle valve. The resultant pressure drop increases the back pressure on the rod end. The pump must build a higher pressure on the cap end. This cap end pressure must build to the point where the relief valve cracks open before the cylinder will slow. Remember, a fixed displacement pump puts out a given volume of oil for each revolution. Neglecting pump leakage, this oil either goes to the cylinder or through the relief valve. The adjustments made to the needle valve on the cylinder interact with the characteristics of the relief valve (and to a lesser degree with other components in the circuit) to produce a given deceleration rate.

A number of techniques have been developed to cushion cylinders. Large cylinders are cushioned with some type of stepped procedure that decelerates the piston in increments. Manufacturer’s literature can be referenced for the back-pressure curves generated by these techniques.

The ANSI symbol for a cylinder cushioned on both ends is given in Fig. 7.7. The arrow through the cushion block indicates that the cushioning is adjustable.

ANSI-symbol-cylinder

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