SHOCK ABSORBERS
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builders are always on the lookout for ways to run equipment faster and increase throughput and productivity. However, components moving at high speeds often must decelerate and stop without damaging the equipment or payload. Otherwise, the consequences are excessive loads, vibration and noise that can compromise safety and machine reliability. Engineers can sometimes dampen motion with products like inexpensive elastomeric bumpers, simple air cushions or gas-spring linear dampers. But these typically have a limited ability to absorb energy and decelerate objects. Shock absorbers, in contrast, provide controlled deceleration by converting kinetic energy to thermal energy. In action, motion applied to a hydraulic shock absorber’s piston forces pressurized fluid through specially designed internal orifices. That restricts flow and generates heat which, in turn, transfers to the metal body and dissipates to the environment. After impact, a spring typically returns the piston rod to the starting location. Shocks are used in a wide range of applications,
from automotive manufacturing and lumber processing to robots, cranes and packaging equipment. When choosing a shock absorber, one must specify the stroke length, compressed length, extended length, cylinder (body) diameter and rod diameter. The stroke length is the distance between the compressed and extended length. The cylinder diameter is an important factor in determining whether the cylinder will fit into the desired location, and how the shock absorber will be affixed to the adjacent structure. HOW DO YOU SIZE A SHOCK ABSORBER?
Sizing a shock absorber is relatively straightforward. Several reputable manufacturers offer online calculators, but here are a few guidelines to quickly come up with suitable products for a given task. Manufacturers’ web sites and data sheets typically list products by parameters like stroke, usable velocity range, maximum amount of energy that can be absorbed per cycle, maximum force capacity, and the maximum propelling force it can handle, as well as dimensions and other relevant details. Before sizing a shock absorber, however, users first need to determine the relevant operating www.fluidpowerworld.com
conditions, including the weight and velocity of the moving mass and how frequently the shock is loaded. For simplicity, let’s look at a linearmotion application and use Imperial units for the calculations. Determine kinetic energy in the system from:
Ek = W/(722)(V2)
where Ek = kinetic energy, lb-in.; W = weight of moving mass, lb; and V = velocity of moving mass, in./sec. This equation represents the amount of kinetic energy that the shock absorber will convert to thermal energy on each impact. Next calculate the work energy in the application, defined as the amount of energy an external device generates to move the load: Ew = Fd(S) where Ew = work or drive energy, lb-in.; Fd = drive force, lb; and S = stroke of the shock absorber, in. Note that Fd should not exceed the unit’s maximum rated propelling force. If it does, select a larger size and recalculate the work energy.
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