How to achieve ideal cushioning
Most people determine cylinder bore based primarily on the thrust required, with no regard for kinetic energy. Usually, this produces a cylinder cushioning capacity that well exceeds that necessary for the application. This will suffice if ideal pneumatic cushioning is not required. But if it is required, pneumatic cushioning will be unachievable with certain combinations of bore, load, and operating pressure.
If the cushioning energy required is within 10% of the cylinder's initial setting, the adjusting screw can be opened to allow the piston to strike the end cover. The shock will be moderate due to the low load, the impact time will be short, and the braking effect of the cushioning will be low because of the short time available for backpressure to develop.
However, if the required cushioning energy exceeds 10% of the initial setting, but is less than 80%, a different scenario applies. If required cushioning is 10% to 80% below the initial setting, the piston can be cushioned partially with air, with impact cushioning accounting for the remainder of the energy absorption. Some impact shock will occur, and the adjusting screw will be almost completely open.
If required cushioning is 10% to 80% higher than the initial setting, the adjusting screw will have to be closed almost completely. The kinetic energy will be damped, and the piston will travel toward the end cover at a certain velocity. However, the shock will be less than in the case described above. The condition is referred to as rebound cushioning. With rebound cushioning, the piston changes direction two or three times as deceleration progresses, which makes the total cycle time somewhat longer.
A better approach
To this point, scenarios have dealt with cases in which the cylinder was already selected, so the pneumatic cushioning had to be adjusted. Achieving ideal pneumatic cushioning is much less complicated if certain factors are taken into account when specifying the cylinders.
First, cylinders should be specified according to cushioning charts contained in manufacturers' catalogs. A typical example is shown in Figure 3.
For this discussion, we will assume a horizontal installation, with a minimum stroke of 200 mm, and an operating pressure of 6.3 bar. A good rule of thumb for choosing the correct cylinder is that the ratio of the load mass (in kg) to the piston area (in cm2) should not exceed four. This limit is represented by the vertical line for each bore size shown in Figure 3. Ideal cushioning can be achieved, without changing the operating pressure, if the intersection of mass and velocity-is on or just below the inclined line in the chart.
Effect of mounting orientation
The horizontal cushioning charts also can be used when the piston operates vertically downward. If the cylinder is installed with piston traveling upward, the cushioning capacity will be less, due to the reduction in cushioning pressure. The force of gravity, which acts downward, reduces the cushioning capacity. Figure 4 serves as a guide, as does this rule of thumb:
m/A < 2
where m = mass, kg and
A = piston area, cm2
Influence of cylinder stroke
Figure 5 shows that full cushioning capacity is achieved only when our specified cylinder has a stroke of at least 200 mm. Note that the cushioning energy decreases sharply when stroke is less than the recommended minimum.
Again, the operating pressure, mass, and velocity govern the characteristics of cushioning. Once ideal cushioning has been achieved, the application parameters must remain unchanged — otherwise, any variation would require cushioning energy to be readjusted.
Phil O'Neill is Industrial Products Manager at Bosch Rexroth Corp., Pneumatics Div., Lexington, Ky. Bosch Rexroth Pneumatics is now Aventics; click here for more information.
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