Actuator force

For the maximum load force (stall force), the system pressure and [[actuator]] size may be determined. Factors to consider when choosing system pressure are the mounting style , duty cycle, utilisation, performance, reli­ability and cost.
For an ideal actuator,

where suffices H and L refer to the high and low pressure sides. ii) The force available at the load is reduced by friction. Frictional forces are difficult to predict, but may well be of the order of 20% of the load under operat­ing conditions and higher for starting conditions.
iii) An allowance should be made for the efficiency of any mechanical linkages or gears connected to the output.
iv) In valve controlled systems (e.g., meter-in), the inlet pressure will reduce with increasing actuator velocity. Thus, for systems in which the force is varying, the velocity may vary as a consequence.

In applications where the load is unguided, transverse loads may be a acting. A stop tube, or spacer, is sometimes fitted to reduce the stroke in such instances but in any case if such loads are to be expected on the actuator the application should be discussed with the actuator manufacturer.

Cushioning

To retard inertial loads and increase the fatigue life of actuators, some form of internal cushioning is often used. An example of actuator cushioning can be seen in Figure 1.

When the actuator outlet flow is directed through the restrictor, the pressure drop generated will create a backpressure on the actuator, thus causing it to be retarded. The restrictor must be sized such that the maximum pressure, which occurs when the plunger first blocks the normal outlet port, does not exceed the safe value for the actuator.

For simple inertial loads, with no other forces acting, the actuator velocity decays exponentially, as does the actuator outlet pressure. This can be shown by simple analysis assuming an incompressible fluid and neglecting friction. Thus from Newton's Law we have:

The velocity and pressure variations with the distance, X, which the actuator has moved after cushioning has commenced are shown in Figure 2. At the start of the cushioning the pressure rises to a maximum value, PCm, when the flow is a maximum. For a given mass and initial velocity, the maximum cushion pressure is determined by the size of the adjustable restrictor. This also determines the distance that is required for the actuator velocity to reduce to an acceptable value.
It is normal that Pc max should not exceed 350bar which is a normal fatigue pressure rating for 106 actuator cycles. The change in the pressure, and velocity, will be slightly modified by the effect of the fluid compressibility but in most systems this effect will be small and the cushion performance can be calculated using the equations.

Some cushioning systems employ a long tapered plunger that maintains a higher mean pressure throughout the cushioning stroke and, consequently reduces the cushioning distance. In others the plunger has stepped diameters to give al­most the same effect. The performance of these cushion methods is usually given in manufacturer's literature in terms of the energy that is to be absorbed and the pressure level on the supply side of the actuator.

The shortest cushion length would be obtained from one that creates a constant pressure at the maximum permissible value. The performance of some cushion systems can be found in the papers by Chappie1 and Lie, Chappie and Tilley2. A comparison with a tapered cushion shows that the cushion length can be re­duced by around 30%.