Motion control often requires stopping a moving load smoothly. A rubber snubber, a compression spring, and a dashpot all can accomplish this by absorbing energy. The snubber and spring store energy and release it after they are compressed, resulting in a rebound. A dashpot is a fluid-filled cylinder with an opening through which fluid may escape in a controlled manner. Any force acting against the piston in the cylinder encounters high resistance from the fluid at the beginning of the stroke, then much less as the piston retracts. However, none of these three items dissipate the energy uniformly. The impact of a moving load against a resisting force produces peak forces which are transmitted to plant equipment, or to the load itself. In order to dissipate the energy uniformly, the use of a shock absorber is required.

Figure 1 shows plots of force versus stroke for the same load moving at the same velocity striking a rubber snubber, a spring, a dashpot, and a shock absorber. The kinetic energy to be absorbed is the same in each case, but it is dissipated at differing rates. A linear rate of deceleration is the most efficient combination of force, space, and time that can be used to stop a moving object. The ideal rate is an almost square curve, where a constant force resists the load, until it is slowed to a stop.

### Shock absorbers provide linear deceleration

Shock absorbers convert the kinetic energy of a load into heat which is dissipated into the atmosphere. They stop a moving load with no rebound and without transmitting potentially damaging shocks to equipment. In its most general form, a shock absorber consists of
a double-walled cylinder with space between the concentric inner and outer walls,
a piston,
some means of mechanical return for the piston, and
a mounting plate. The piston usually is returned to its initial position by a spring, which can be mounted externally around the piston rod or internally on the inside of the cylinder body.

A series of orifices is drilled in the inner cylinder wall at exponential intervals. The reason for the exponential spacing is derived from the equation for kinetic energy: KE= ½ mv2. The cylinder is filled with fluid, and all air is bled from the fluid because air bubbles cut the efficiency of the shock absorbers by causing spongy or erratic action.

When a moving load contacts the piston rod, it moves the piston inward, forcing fluid through the orifices in the inner cylinder wall. The fluid is forced through the oil return passages, into the space be hind the piston head. As the piston retracts, it closes the orifices behind it, reducing the effective metering area, and maintaining a uniform deceleration force as the load loses its energy.

Fluid pressure is constant in a shock absorber, providing constant resistance to the load. The load slows to a stop as its kinetic energy approaches zero. No rebound occurs because the shock absorber stores no energy. Several events must happen for the piston to return to its extended position. First, the load must be removed. The spring then pushes the piston outward, opening a check valve, which lets fluid flow from behind the piston to the space the piston was in its retracted position. Smaller shock absorbers, (bores less than 3 in.) have a ball check valve to control fluid flow. Larger models use a piston-ring check valve.