Title: The Case for a Universal Adjustable Shock Mount
Authors: John K. Stenard, President, Sten-‐Tek Corporation, Anthony Paulic, Senior Mechanical Engineer, Advanced Acoustic Concepts Inc.
Problem Statement: The present technology used for resiliently shock-‐mounting shipboard electronic equipment has drawbacks in terms of performance and Total Ownership Cost (TOC). These shortcomings are examined, and the features of a new, improved technology to be developed are conceptualized.
Present technology performance -‐ how well the mounting protects against shock -‐ is impacted due to the six degrees-‐of-‐freedom (6DOF) allowed by the suspended rack/cabinet. The 6DOF means six global response modes, any of which may be amplified by the shock pulse. Of the six global response modes, the three rotational modes are especially detrimental to shock performance, as a tipping cabinet is more likely to hard-‐bottom for a given excursion range. Hard-‐bottoming typically results in damage to the equipment and shock test failure. The vulnerability to induced rotational modes is largely masked by the present testing methods, since the angle of the strongest shock vector in the barge test is known and thus the set of resilient elements comprising the mount may be selected so as to substantially counter-‐balance torques about the center-‐ of-‐gravity (CG) of the suspended load. It is at first surprising that present mount technology impacts the ship’s TOC, considering the low cost of individual mount elements – often simply a few coils of wire rope clamped between mounting bars? The initial cost is low, but TOC is impacted since the entire procedure of design/test/SHIPALT/provision whenever the mounted equipment configuration changes throughout the life of the ship. Any changes to the equipment or equipment arrangement requires a design review, and if the changes are deemed significant, the whole mount must be re-‐designed, re-‐shock-‐tested, replaced on all affected ships, and re-‐provisioned logistically. The cost, not including the actual shipboard modifications, is on the order of $250K per case. This increases the complexity of and cost of ship integration, as any changes to shock-‐ mounted gear must be redesigned and re-‐tested, etc. Weight is also impacted, as in sometimes installing a whole new foundation and rack is more expedient than disturbing (and having to re-‐design and re-‐test) existing racks. Since new electronic gear is continually being developed, the same rack of equipment may have to be modified multiple times during the ship’s service life. These factors explain why MIL-‐S-‐901 prefers equipment to be shock hardened, explicitly allowing shock mounting only when shock hardening is infeasible, (Para 6.4.(o)). Shock hardness is a Top Level Requirement for all warships, yet shock deficiencies are nearly invisible during normal operation. As such, other high-‐visibility issues have taken priority in funding, and there is now a backlog of several thousand line items in the Shock Deficiency Tracking System (SDTS). Shock deficiencies become immediately obvious if the ship is struck by a naval weapon, a very poor time to discover that a large portion of the ship’s electronic suite has been rendered inoperable.
This paper examines these factors, and conceptualizes an ideal resilient shock mounting method which would overcome each of the above shortfalls.
The new method should ideally prevent or substantially restrain relative rotational excursions, while allowing relative translation of the CG. Relative translation of the CG is required for shock mitigation, but relative rotation is superfluous, and exorbitantly consumes available excursion range, presenting the most significant vulnerability to hard-‐bottoming.
The suspension stiffness of the new method should be easily adjustable to accommodate the weight of suspended equipment throughout the ship’s service life. This will obviate replacing the entire mount when equipment changes.
The stiffness settings should be pre-‐engineered and pre-‐shock-‐tested to known, safe, acceleration limits for the weight of the equipment. This would obviate re-‐testing every mounted configuration to verify the design. Logistics impact would simply be to documentation, a note of the new stiffness setting for that rack. No SHIPALT or re-‐provisioning would be necessary, as no mounting equipment changed. Together, these features of a new mounting technology would obviate the costs of re-‐designing, re-‐ testing, performing SHIPALT’s, or re-‐provisioning electronic mounts throughout the ship’s service life. Anticipated benefits are reduction of TOC, improved shock protection, weight savings from consolidation of heavy foundations, and simplified ship integration.