Smooth sailing on rough seas

Advanced shock absorbers expected to reduce injuries on fast attack boats

Crews delivering U.S. Special Forces for “tactical insertions” into global hotspots often suffer some pretty inhospitable conditions themselves. In fact, piloting high-speed combatant craft over rough seas can result in serious neck, back, shoulder and knee injuries.

Active Shock Inc., Manchester, N.H., is in the process of solving this problem with its advanced shock-absorbing system, which is being evaluated for potential use in the crew seats of U.S. Special Operations Command (SOCOM) fast attack boats. The company is currently conducting sea trials with an 11-meter, rigid inflatable boat (RIB).

The Active Shock system uses a proprietary algorithm to adjust shock absorber valving 1,000 times per second based on data from sensors monitoring the position, velocity and acceleration of the suspension components, as well as lateral and longitudinal movements. The algorithm, which has the intelligence to anticipate motion and calculate the optimum damping frequency in real time, provides just enough resistance to prevent bottoming regardless of load or ride height. It continuously adjusts the amount of damping within a range that extends from very soft for comfort on smooth seas to very high to control bottoming when landing in large wave troughs.

The damping action of conventional passive shock absorbers, governed by their internal construction, varies only with the velocity of vertical movements.

The five forward crew seats on the Active Shock test boat are each mounted on a large vertical strut capable of withstanding the weight of a 300-lb. occupant with body armor and gear. In addition, the oversize struts can handle up to 20 Gs of vertical force from rough seas, and up to 12 Gs of lateral force from hitting waves at the wrong angle.

Active Shock uses 75 mm DP4™ bushings from GGB, Thorofare, N.J., on strut housing to take side loads and a 16mm DP4™ bushing on the shock absorber shaft to keep it centered in the hydraulic seal. GGB application engineers assisted the company with tolerancing, so it could use stock drawn-over-mandrel (DOM) steel tubing for its strut housings, thereby eliminating the need for costly internal finishing operations prior to installing the bearings.

Struts exert high bending loads on the shock absorber body as it slides in the strut housing. The DP4™ bearings are swiped with grease to provide boundary lubrication and low breakaway force or stiction. Retaining seals on each end of the shock continuously relubricate the shaft.

“A shock absorber doesn’t work until it’s moving,” explains LaPlante. “If it doesn’t break free under light loads, ride quality is severely affected, and the shock isolation is negated. The challenge was to ensure the stiction from the large bushings and the grease seals would not affect ride quality.”

Providing good wear and friction performance over a wide range of load, speed and temperature conditions, DP4™ bearings are especially well suited for applications such as struts and shock absorbers. Their structure consists of a rigid steel backing, to which is bonded a porous bronze sinter interlayer impregnated and overlaid with a filled PTFE bearing layer. Exceptional resistance to both flow and cavitation erosion makes them ideal for start/stop applications. And because they are lead-free, they are also more suitable for use in applications subject to corrosion.

“Essentially this bearing material, the industry standard for struts and shocks, is the only thing that meets all of our requirements,” explained LaPlante, who has been working with GGB for three years. “Using it in a large-diameter strut is pushing the limits of our experience. We’re pleased to find that it is working so well with very low striction and wear. Its inherent lubricity helps reduce maintenance requirements, and the strut never needs to be regreased.”

In addition to the SOCOM fast attack boat project, LaPlante uses GGB bearings, both DP4™ and self-lubricating DU®, in its line of semi-active shock absorbers for heavy ground vehicle seating, automotive aftermarket high-performance retrofit suspension kits and commercial and private boat seats. They are also being evaluated for use in the road wheel suspension components for the SOCOM HMMWV.

LaPlante indicated he plans to continue working using GGB bearings as Active Shock expands into shock absorbers for construction and agricultural equipment, as well as additional military vehicles.