In motion control, it is often desirable — or necessary — to stop a moving load smoothly. A rubber bumper, a compression spring, or a dashpot all can accomplish this by absorbing energy. The bumper 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 at a controlled flow. 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 the machine’s moving elements or to the load itself. In order to dissipate the energy uniformly, the use of a shock absorber is required.
The graphic shows plots of force versus stroke for the same load moving at the same velocity striking a rubber bumper, 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 decelerate loads linearly
Shock absorbers convert the kinetic energy of a load into heat that 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 return is usually a spring, which can be mounted externally around the piston rod or internally on the inside of the cylinder body. A series of orifices are drilled in the inner cylinder wall at exponential intervals. The reason for the exponential spacing is derived from the equation for kinetic energy: KE=1/2 mv2.
The cylinder is filled with fluid; 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 behind 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. There is no rebound because the shock absorber stores no energy. To return to its extended position, several events must happen. First, the load must be removed from the piston. The spring then pushes the piston outward, opening a check valve, which permits fluid to flow from behind the piston to the space the piston was in when retracted.
Smaller shock absorbers, with bores under 3 in., have a ball check valve to control fluid flow. Larger models use a piston-ring check valve.
Fixed vs. adjustable shocks
There are two basic types of shock absorbers: those with a fixed orifice, and those with an adjustable orifice. The fixed type, sometimes referred to as non-adjustable, has orifices drilled along the inner cylinder wall at distances determined by the manufacturer. While generally less expensive, they are designed for a specific application’s load range and cannot be changed to meet the requirements of other applications. They are more economical in high volume applications where the exact operating parameters will not change significantly over time.
Adjustable-orifice shock absorbers can accommodate a range of loads - as much as 30 times the range of a non-adjustable type. They are adjusted by moving a graduated dial on the outside of the shock absorber. This moves a ring around the orifices to control the size of the openings.
Controlling the amount of fluid forced through the orifices controls the deceleration rate. The dial rotates through 90° or 180°, and is calibrated on a scale from 1 to 10. Usually, the higher the number, the greater the resistance to impact. Generally, adjustments are made by observing energy absorption at different settings. Constant resistance to the load should be evident throughout the stroke of the shock absorber.