What is in this article?:
- Shock absorbers
- Selecting a shock absorber
The purpose of an energy-absorbing device is to stop a moving load with minimum load rebound, with minimum shock to the load, and minimum shock to surrounding equipment
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.
Fixed or adjustable?
Two categories of shock absorbers are common: 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, Figure 2. Controlling the amount of fluid forced through the orifices control 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. Adjustment generally is made by observing energy absorption at different settings. Constant resistance to load should be evident throughout the stroke of the shock absorber.
Orifice design is critical to the operation a of shock absorber. A circular hole drilled in the inner cylinder wall will permit fluid to flow to the outer portion of the cylinder, but causes pressure drop or a change in fluid viscosity due to change in fluid temperature. A simple hole will produce laminar fluid flow, which is less efficient in dissipating energy and often cannot be controlled precisely. As a shock absorber cycles more and more frequently and if there is a high proportion of laminar flow through the orifice, operating temperature will increase and the resulting change in fluid viscosity require constant readjustment of the shock absorber. A knife-edge orifice is very short when compared to the thickness of the inner cylinder wall, Figure 3. These produce non-laminar flow which is not sensitive to changes in fluid viscosity, and is easily controlled. Not all shock absorbers are fluid-filled.
One design uses elastomeric pellets to absorb energy. On impact, the piston compresses the pellets. The curvature and length of each pellet determine the energy-absorbing characteristics of the shock absorber. The rate of return is slowed by the pellets. They store the energy and release it at a slower rate than they absorbed it. The release repositions the piston for the next stroke without any rebound.