Supports – Hold-down – Looping or straddling
Reexamination Certificate
2000-09-13
2003-05-06
King, Anita (Department: 3632)
Supports
Hold-down
Looping or straddling
C248S503000
Reexamination Certificate
active
06557814
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of devices used to secure heavy objects to support structures, the objects having variable outer dimensions such as do cycling pressure vessels, more particularly to securing compressed natural gas (CNG) fuel tanks to vehicles.
BACKGROUND OF THE INVENTION
Natural gas is more widely distributed throughout the world than crude oil. Since the 1920's, natural gas has been used to fuel vehicles in such countries as Italy. In 1995, approximately 100,000 vehicles in the United States and half that in Canada were fueled by natural gas. Significant growth in natural gas-fueled vehicles, especially in fleets, is gradually occurring throughout the rest of the world.
Natural gas, as delivered through pipelines, cannot be used directly as a mobile vehicular fluid and must be first physically compressed, liquefied or adsorbed. Compressed natural gas (CNG) for use in CNG-fueled vehicles is compressed between 3,000 and 3600 psi (20.77-24.92 MPa) and stored in tanks—usually ferrous, aluminum or non-ferrous liners wrapped in a fiber-reinforced tensile composite.
National Fire Protection Association (NFPA) standards (See NFPA 52, Chapter 3—3.3-3.9) require that CNG tanks must be secured to vehicles in such as a way as to prevent damage from road hazards, slippage, loosening or rotation, using a method capable of withstanding a static force, in the six principle directions (up, down, left, right, forwards, backwards), of eight times the weight of a fully pressurized container. For moving vehicles this equates to imposed relative acceleration or deceleration on the tank of 8 times gravity (8 g). Further, under these conditions, the tanks cannot be displaced more than ½ inch (13 mm).
When a vehicle accelerates or decelerates, the inertia of an attached tank resists the change in velocity, restrained to follow the vehicle's change in velocity only by its mounting means. A rear-ending situation or head-on collision can impose significant forces on the mounting means. If they are not capable of restraining the tank, it could conceivably be sheared from the vehicle and become a projectile. The NFPA code anticipates that most such loads on a tank would not exceed 8 g and hence mounting means meeting the code are adequate.
Typically, for atmospheric or stationary tanks, the mounting means need only support the tank's weight or restrain minor movement. However, wide changes in pressure (say 0 to 3000 psi) cause expansion and contraction of CNG fuel tanks. Typically, metallic liner CNG tanks can expand 0.5% at maximum fill pressure; a 500 mm diameter tank expanding about 2.0±0.5 mm. Plastic liner tanks (such as those under Department of Transport (DOT) Federal Motor Vehicle Safety Standards FMVSS-304, NGV2-98 Std, Type 4 tank) can experience expansion of even 5 times that experienced with ferrous or aluminum liner tanks.
The restraining devices must be able to expand and contract with the tank through multiple tank expansion and contraction cycles. If they do not, then they lose the ability to continue to apply sufficient force to the tank after it contracts, resulting in non-compliance with the code and greater potential for prohibited and possible hazardous movement. Most recently, in Los Angeles, a transit vehicle actually lost a plastic-liner, composite CNG tank because the restraining device could not maintain a pre-load through the pressure cycles.
As CNG tanks are pressure vessels, they usually do not incorporate integrated mounting elements. Accordingly, and referring to prior retraining devices shown in
FIGS. 1 and 2
,
FIG. 1
illustrates a mere continuous strap formed around a tank. It is simple but is incapable to dealing with variable tank dimensions. If initially fitted to a depressurized tank with sufficient load to secure the tank, a failure eventually or immediately occurs in the strap or fastener at the mounted end when the tank grows larger under pressure.
As shown in
FIG. 2
, conventional multi-piece steel straps and spring attachments are also used to secure CNG fuel tanks to vehicles. The springs are adaptive to the expansion and contraction, however, multi-piece construction can result in excess noise (important in vehicular use), variable strap load around the tank, and fatiguing of the connections between the strap and the spring. Therefore, straps using spring attachments must be tested and replaced more frequently and are associated with increased installation time and labor cost. Characteristic of spring attachments, the compression of the spring to its designed deflection is not necessarily representative of the load throughout the strap. The spring responds only to the resistance to load at the ends of the strap, permitting the unwary to apply a pre-load to the spring representing only the frictional resistance of the strap leaving the tangent of the tank, leaving large areas of the tank having no pre-load at all (shown as a gap of exaggerated proportions in FIG.
2
).
U.S. Pat. No. 4,367,572 to Zielenski discloses a composite elastic strap for securing rectangular batteries in vehicles. This composite strap comprises an elongate, notched rubber member having a saw-tooth-shaped fabric reinforcement member embedded within it. The resilient rubber can be stretched to a maximum, governed by extension of the fabric reinforcement member. This non-metallic composite strap helps Zielenski meet the following objectives: avoiding application of a rigid holddown device to an otherwise fragile battery case (particularly at the right angle corners), avoiding placing conductive metal about an electrical device; and avoiding the associated corrosion common with batteries. The composite strap is fitted with plates and hooks at each end for attachment to the vehicle.
Zielenski's application relies heavily on an elastomeric composite for achieving its forgiveness, anti-corrosion and electrical insulating properties. Unfortunately, the composite strap is neither capable of imparting the loads necessary, nor providing the long term consistent loading required for use with CNG tanks. Such a strap cannot withstand acceleration and deceleration and maintain secure attachment of large mass tanks to the vehicle. Substitution of a stronger metal reinforcement member defeats the forgiving and anticorrosion advantages. In particular, as stated by Zielenski, a metal restraint imposes unacceptably significant stresses on the upper edges of a battery. Additionally, in the CNG tank environment, this prior art strap will harden at low ambient temperatures, resulting in an inability to react to dimensional changes. As a tank shrinks, an initial pre-load will reduce or a space can even form between the strap and tank. The use of separate hooks for attachment further weakens the strap for restraining large loads.
There is therefore a demonstrated need for a restraining device that can reliably secure a heavy tank to a supporting structure regardless whether the tank is in an expanded or contracted state and is capable of maintaining its ability to retrain through multiple expansion and contraction cycles.
SUMMARY OF THE INVENTION
In one preferred aspect, an improved restraining device is provided for securely attaching heavy pressure vessels or tanks of curved section to the supporting structure of a vehicle. Each restraining device is a strap comprising a thin elongate unitary metallic member which, in use, extends about the curved wall of the tank, the ends of which are fastened rigidly to the supporting structure. The otherwise planer body of the strap has one or more localized bends. Axial loads impose on the straps due to dimensional changes to the tank are converted in part to moments in the bends, increasing the dimensional ability of the strap to lengthen without yielding. Accordingly, pressurization and depressurization of a CNG tank cause the strap to elastically lengthen and contract repeatedly while meeting or exceeding a predetermined restraining capability, without the use moving elements.
Therefore, in
Baxter Gwendolyn
Dynetek Industries Ltd.
Goodwin Sean W.
King Anita
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