Orifice design is critical to the operation of a 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 amount of laminar flow through the orifice, the operating temperature will increase. The resulting change in fluid viscosity will require constant readjustment of the shock absorber. A knife-edge orifice is very short when compared to the thickness of the inner cylinder wall. These produce non-laminar flow, which is not sensitive to changes in fluid viscosity, and is easily controlled.
Selecting a shock absorber
When choosing a shock absorber, the most important factor to consider is the type of load to be stopped. Basic loads encountered in shock absorber applications include: pure inertial, free-falling, rotating, and loads subject to an additional propelling force. Load weight and velocity are the next two most important factors in sizing a shock absorber. Potential shock to equipment, number of impacts per unit of time, and ambient conditions must also be considered to properly select a shock absorber.
Application conditions include extreme temperatures, load acceleration, maximum propelling force applied to the load, and time limitations imposed on the equipment. Time limitations would include minimum and maximum cycle times and the time required for the shock absorber to return to the extended position between strokes. Cycle rate is another important consideration. If the shock absorber must handle too many impacts within a given time, it will overheat, resulting in poor performance and premature failure. Rapid cycling may heat the fluid, reducing its ability to dissipate energy.
As a safety feature, most manufacturers recommend that shock absorbers be sized for 70% to 80% of capacity. Because the amount of impact the shock absorber can accommodate is inversely proportional to the length of its stroke; doubling stroke length will cut the impact of the load in half.
Shock absorbers must be bolted rigidly to a non-flexing mounting structure. Some type of external stop is required to provide a firm positioning point, and to prevent the shock absorber piston from bottoming out at the end of its deceleration stroke. Mounting can be achieved through a drilled hole and secured by a mounting stop collar, rear mounted into a drilled and tapped blind hole and secured by a jam nut, or via its own mounting flange.
Shock absorbers can be used in a myriad of places. Applications include straight-line functions, as well as rotary, free-falling, rolling, and sliding movements. It makes no difference if the action is driven mechanically, hydraulically, or pneumatically. One common situation for shock absorbers is on high-cycle automation machinery that use rotary motions. For this the shock absorbers should be positioned near the pivot point to provide more clearance for the work area. However, this placement subjects the shock absorbers to high effective weight conditions due to their low velocity. Most of the kinetic energy involved originates from the propelling force rather than from inertia. For such applications, specify shock absorbers designed to operate in a velocity range from 1/4 to 2 ft/sec.