against the end covers as the piston approaches the ends at high velocity.
Often, the impact occurs at the both end points of cylinder and generates the
destructive shock within the structural operating members of the machine or
equipment. Some form of cushioning is normally required to reduce the rate
of travel of a cylinder before the piston strikes the end cover. Reducing the piston velocity at the end of its travel lowers the stresses on the cylinder while reducing vibration in the structure of which it is a part. To slow an
action and prevent shock at the end of a piston stroke, some actuating cylinders are constructed with a cushioning device at either or both ends of a cylinder. This cushion is a flow control valve that does not operate until the cylinder piston reaches a certain point in the cylinder. Then, the cushionrestricts airflow to slow the cylinder movement. This allows it to move to the end of its travel at a slower speed. This adjustment is normally on the end of the cylinder head. Cushioning of cylinders at one or both ends of piston stroke is used to reduce the shock and vibration. All the double-acting cylinders except for small sizes are provided with end position cushioning
arrangement.
Working of Cushions
The working of cushions is explained with the help of a double-acting cylinder with cushions at both ends (Refer to Figure 6.19)The piston moving in the cylinder has cushion noses installed on both the sides. These cushion noses are designed such that they mate with the cavity made in cap end, which is called as cushion chamber. Cushion assembly consists of a needle valve mechanism. Needle valve adjusts the cushioning effect, i.e., by rotating needle of the needle valve; the intensity of
shock can be decreased or increased. When the piston is moving backwards, i.e., it is proceeding in the direction of end cap shown in figure, the cushion nose enters inside the cushion chamber. The locked up fluid just before the cap end is allowed to pass through a carefully designed flow resistance path. This produces the back up pressure hence opposing movement of piston and piston rod. This force decelerates the motion and result in a smooth operation.Now when the process is over and cushion nose fully enters into the
cushion chamber then the forward stroke will take time to start because the area of nose is great, whereas the entrance from where fluid is coming in is intentionally kept small. In order to overcome this problem sometimes a
check valve gallery is provided below the cushion chamber to initiate the process without consuming much time.
Ideal Cushioning means that there is no end of stroke bounce, i.e., the direction of travel of the piston is the same throughout the entire cushioning sequence, and that its velocity is exactly zero when it reaches the end of its
travel. The sound of end cover contact is negligible and the total cycle time
is minimized. Cushioning is an important aspect of cylinder life. Cushioning not only ensures smooth operation but also extends the life of cylinders by preventing shocks. In Figure 6.20, a pneumatic cylinder is shown which has both conventional pneumatic and elastic cushions
