Both mechanical and fluidborne shock and vibration can be very detrimental to equipment's performance and service life. Plan ahead to avoid or control them.
As load strokes shock absorber, piston blocks successive orifices into pressure chamber.
Virtually all machinery — whether it's industrial or mobile equipment — generates movement of some kind. The movement can be linear or rotary. At some point, these motions either change direction or come to a stop. Any moving object possesses kinetic energy as a result of its motion. When the object changes direction or is brought to rest, the dissipation of its kinetic energy can result in destructive shock forces within the structural and dynamic parts of the machinery.
Because kinetic energy is the product of mass and the square of velocity, the heavier an object is — or the faster it travels — the more energy it has and the greater the deceleration force that will develop as it stops. If kinetic energy can undergo controlled, smooth absorption and dissipation, equipment can:
- cost less to build,
- run faster,
- last longer,
- operate more efficiently, and
- require less maintenance.
In addition, noise pollution and energy costs will be reduced.
Older methods of mechanical-energy absorption include rubber bumpers, springs, hydraulic dashpots, and cylinder cushions. All of these are limited because they are non-linear and exhibit a high peak force at some point during their strokes. They cannot produce smooth deceleration.
Designed to control shock
Rubber bumpers and springs slow down moving loads by applying constantly rising reaction forces — up to the point of full compression. The peak stopping force occurs at the end of the stroke. These devices store energy, rather than dissipate it, which causes the load to bounce back after stopping.
Cylinder cushions and dashpots incorporate a metering orifice that abruptly slows the moving load at the start of their stroke — where the peak force and high shock load appear. The braking force then falls away rapidly.
Modern industrial shock absorbers improve on all these devices. They combine the proven strength of the hydraulic-cylinder configuration with a series of orifices that produce a constant resisting force throughout the shock absorber's stroke. This brings the moving load smoothly and gently to a stop. The load is decelerated with the lowest possible force in the shortest possible time, reducing peak force and shock damage.
Shock absorber construction
In its most general form, a shock absorber consists of a doublewalled cylinder with an inner pressure-chamber bore and a second chamber between the concentric inner and outer walls. It also has a piston, some mechanical means for returning the piston after the load is removed, and typically a mounting plate. The piston return usually is a spring, which can be mounted externally around the piston rod or on the inside of the cylinder body.
A series of orifices is drilled through the inner cylinder wall at exponential intervals. The reason for the exponential spacing is derived from the equation for kinetic energy:
where: KE is kinetic energy, W is the moving weight, and v is the impact-velocity.
All air must be bled from the fluid because air bubbles cut the efficiency of shock absorbers by causing spongy or erratic action. When a moving load contacts the piston rod, it drives the piston inward, forcing fluid through the orifices and into the outer pressure chamber. As the piston moves, it progressively blocks the orifices behind it. This reduces the effective metering area and maintains a uniform deceleration force while the load's energy is converted to heat in the fluid. This heat eventually dissipates to the ambient atmosphere.