Shock transmission and shock mitigation factors

During the preliminary design stages of Aerospace and Ground Equipment, a major consideration is determining the degree of shock transmitted to the payload.  This concerns:

  • The reduction of the highest anticipated input shock to the system to a level that the equipment can withstand
  • The analysis of the effects of periodic input shocks

The AEROL SAM SUSPENSION UNIT, THE BANJO MOUNT, CUSHION TREAD WHEELS, HIGH PROFILE TIRES, and SHOCK ABSORBING CASTERS are all designed to provide the optimum combination of features to reduce shock transmissibility and to minimize resonant response. Determination of the highest tolerable input shock involves consideration of the sprung weight (the vehicle chassis and payload), speed, the input pulse amplitude and shape, and the natural frequency of the system.

Vibration and shock dampening

The vibration and shock dampening characteristics of Aero wheel suspension assemblies and shock absorbing casters are extremely important factors in the mitigation of shocks imposed on AGE equipment. The vibration dampening characteristic or dampening ratio results from the sprung weight, the system friction, the inherent dampening characteristics of pneumatic and solid rubber tires, and the dampening characteristics of the metallic components of the suspension assembly.

Both ductile iron, and cast aluminum, used exclusively for the structural components of Aerol casters and suspension systems, possess excellent dampening properties. It should be remembered that while the dampening properties of a system increase the shock transmissibility, an undamped system would respond violently to an exciting frequency close to the natural frequency, and input shock amplitudes in excess of the deflection capacity of the spring system could not be tolerated.

In practice, dampening ratios of the order of 0.15 to 0.20 is desirable, and the dampening ratios of Aerol suspension units and shock absorbing casters fall within this range.

Sprung weight and dampening characteristics of a tire

As stated, the sprung weight and dampening characteristics of the tire contribute to the total dampening ratio of the suspension system. If the shock absorbing capability of the tire is considered isolated from the remainder of the system, then only that portion of the tire in contact with the road bet is unsprung weight. In addition, all parts of the suspension system including the wheel and most of the tire contribute to the total inertia force that opposes the input shock force of an obstacle striking the wheel.

It is readily seen that a shock-absorbing suspension system with solid rubber or pneumatic tire wheels represents a differential spring system. That’s because a portion of the input shock is attenuated by the tire, a portion of the remainder is absorbed in the spring system, and only the residual shock is transferred to the payload. In practice, the input shock loads of many obstacles of low to medium vertical profile and of short pulse duration are absorbed by the tire with negligible shock levels imposed on the spring system.