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
Selecting a shock absorber
When choosing a shock absorber, the most important factor to consider is the type of load to be stopped. Basic types of 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. Additionally, potential shock to equipment, number of impacts per unit of time, and ambient conditions must 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 to 50% to 60% 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. Most manufacturers recommend the use of an external stop to prevent damage both to their product and to the user's equipment. 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 to save time and space. In this instance, 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 ¼ to 2 ft/sec.