GB1561361A

GB1561361A - Flexible laminated sheet

Info

Publication number: GB1561361A
Application number: GB710776A
Authority: GB
Original assignee: British Hovercraft Corp Ltd
Current assignee: British Hovercraft Corp Ltd
Priority date: 1976-02-23
Filing date: 1976-02-23
Publication date: 1980-02-20
Also published as: FR2341470A1; FR2341470B1; JPS52125589A

Description

(54) FLEXIBLE LAMINATED SHEET (71) We, BRITISH HOVERCRAFT COR PORATION LIMITED, of Yeovil, in the County of Somerset, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to flexible laminated sheet of the kind in which impermeable elastic material, for instance rubber, is bonded to a reinforcing web, and is particularly concerned with such flexible laminated sheet when used to form elements of a flexible skirt assembly for an air cushion vehicle.

It was during 1960, one year after the first experimental air cushion vehicle, designated SR.Nl, had proved that the basic air cushion concept was sound, that a flexible skirt was fitted around the periphery of this vehicle to form a part of the cushion sealing means. In the fifteen years that have elapsed since that time we have been involved in continuous research and development work directed towards improving flexible skirt design and the materials from which these flexible skirts are manufactured. At the inception of this work a survey of availabe materials, together with tests carried out in our laboratories, showed that a cloth-reinforced rubber material would be necessary to meet the property requirements anticipated.The tensile and tear strengths of rubber alone are comparatively low so that its reinforcement by a stronger material is necessary in order to meet the necessary strength requirements.

Although prior to this time fabric-reinforced rubbers were used for drive belts, conveyor belts and for many other arduous applications, none of these materials would stand up to the conditions imposed by air cushion vehicle operation, one factor being the inability of the bond between the fabric and its rubber coating to withstand these conditions. We have found it necessary, therefore, to investigate and to understand the problems associated with these materials when used in air cushion vehicle flexible skirt assemblies, to develop new technology and techniques for manufacturing these materials, and to develop new methods of testing these materials in order that they may be evaluated and compared one with another.

As the flexible skirt assembly makes an essential contribution to the performance and safety of the vehicle it is important that it is maintained in a reasonable condition during operation. It is desirable from the point of view of the operator that the maintenance is as simple and economical as possible. One well known flexible skirt assembly that has evolved from the major developments in skirt dsign which have taken place over the last fifteen years is our so-called "bag and finger" (or segment) combination skirt. This comprises a bag member formed of a flexible impermeable sheet suspended in a continuous loop between two spaced lines of attachment on the vehicle. A plurality of independent flexible skirt elements known as fingers are attached in a releasable manner to the underside of the bag member so as to depend downwardly therefrom and form the lower part of the skirt assembly.Thus, when a finger becomes worn or damaged it can be readily detached from the bag member and replaced by a new finger.

At present a finger or segment is manufactured as a flexible laminated sheet having an outline shape such that when the sheet is folded and attached to the underside of the bag member it will have the required configuation. This sheet is formed by cutting to the desired shape one or more sheets of a woven fabric, generally nylon, pretreating this fabric for rubber acceptance, laying the sheet or sheets of fabric between sheets of uncured natural or synthetic rubber, such as polychloroprene, that have been cut to the desired shape, and subjecting this assembly to heat and pressure so as to cure the rubber.

During this curing process the rubber flows into the interstices between the warp and weft threads of the woven fabric so as to form a composite sheet.

When the vehicle is operating over an un- dulating surface such as the sea, the fingers are continuously flexing. This repeated flexing which takes place at high frequency leads to a breakdown of the bond between the reinforcing fabric and the coating, and to subsequent failure of the reinforcing fabric.

One of the major factors giving rise to delamination is the dissimilar elongation properties of the rubber and the reinforcing fabric.

The elongation at failure of the woven nylon fabrics used as the reinforcing laminae in present-day flexible skirt materials is generally only a few per cent of the elongation at failure of the rubber, and rarely, if ever, exceeds ten per cent.

When the finger flexes under load the material is bent and this generates interlamina stress concentrations at the interfaces of the reinforcing fabric and the rubber. We have found that these oscillating stress concentrations lead to delamination of the material and subsequent load tearing and general attrition of the reinforcing fabric in fatigue. This behaviour also occurs on other parts of the skirt that are subjected to continuous flexing.

As a product of some fifteen years research and development we have found that as a result of increasing the stiffness of the flexible laminated sheet by the use of higher strength and higher stiffness reinforcing fabrics, the flexible laminated sheet shows a decrease in life and new types of failure are introduced.

On the other hand, we have found that by omitting the reinforcing fabric so that the sheet comprises only rubber no fatique cracks occur but, however, as might be expected, such material does not have adequate tensile strength and tear resistance.

It is an object of the present invention to provide improved independent flexible skirt element for use in the manufacture of flexible skirt assemblies for air cushion vehicles.

Thus the invention provides an independent flexible skirt element for an air cushion vehicle flexible skirt assembly, said element being formed from flexible material comprising at least one elastomeric lamina bonded to at least one lamina of a reinforcing web having in at least one direction an elongation at failure at least 25 % of the elongation at failure of the elastomeric lamina.

We have found that worthwhile improvements in the life of such flexible skirt elements are obtained when the web lamina has an elongation at failure that is as small as 25 % of the elongation at failure of the elastomeric material. However, by using web laminae having elongations at failure that are at least 50 % of the elongation at failure of the elastomeric material, further improvements in skirt element life are obtainable. Web laminae having such elongation characteristics may conveniently be formed from knitted fabrics.

Such a fabric may be knitted from a textured nylon yarn, so as to impart additional elasticity to the web material.

Preferably the skirt element is manufactured so that the elongation properties of the reinforcing web and the elastomeric laminae are most closely matched in the direction of the maximum operational load carried by the element.

Thus, when the element is of the type known as a finger, so that the developed profile of the element is a generally triagnular shape, the reinforcing web lamina is preferably constructed so as to have elongation properties that are most closely matched to the elongation properties of the elastomeric lamina in a direction that extends normal to the base of the generally triangular shape of the developed profile of the element.

Conveniently the reinforcing web lamina is cut from a rib knitted fabric so that the rib extends in a direction that is substantially parallel to the base of the geneally triagndlar shape of the developed profile of the element.

If desired, the skirt element may be manufactured by a moulding process so that it is moulded to an operational configuration.

The invention will now be further described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a transverse cross-section through an air cushion vehcile bag and finger combination flexible skirt assembly, and Figure 2 shows the developed profile of a finger.

Our known form of bag and finger combination skirt, shown in Figure 1 attached to rigid structure 11 of an air cushion vehicle, comprises a bag member 12 forming the upper part of the skirt and a plurality of fingers 13 attached to the underside of the bag member so as to depend downwardly therefrom and form the lower part of the skirt assembly. The bag member 12 is manufactured from two sheets of flexible reinforced material 14 and 15, respectively. The sheet 14 forms that part of the bag that faces the cushion area beneath the vehicle and it is attached along one edge 16 to the rigid structure 11. The sheet 15 forms that part of the bag which faces ambient atmosphere and is atached along one edge 17 to the rigid structure 11. Those edges of the sheets 14 and 15 opposite to the edges 16 and 17, respectively, are overlapped and bonded or bolted together to form a joint 18. The edge of the sheet 15 is allowed to extend beyond the joint 18 so that it hangs downwardly to form an apron 19 facilitating the outerface attachment of the fingers 13.

Each finger 13 is formed from flexible laminated material and has a developed profile of generally triangular form as shown in Figure 2. The edges 20 and 21 of the finger are bolted to flaps 22 (Figure 1) attached to and extending downwardly from the bag member. The apex of the generally triangular shaped finger is formed as a profiled edge 23 so as to facilitate bolting to the apron 19 of the bag member. When the finger is attached to the bag member the edge 24 depends downwardly in a loop so that the central portion 25 of this edge forms the lowermost tip of the finger.

Five test fingers having the profile shown in Figure 2 were manufactured with reinforcing web laminae cut from knitted fabric.

This fabric comprises a warp knitted 18-2 oz square yard nylon fabric made on a 10 gauge machine (10 needles/inch) using 5 ends of 2/70 denier crimped nylon (textured) yarn. Prior to cutting to profile the fabric was heat set at 60 inches width to fix it against shrinkage, extension or other deformation during the heat bonding/curing process, and it was then pretreated for natual rubber receptiveness which also acts to bond the knitted yarn sufficiently to prevent it from falling apart by fraying or running when cut.

Each profiled web lamina was reinforced at the attachment edges 20, 21 and 23, respectively, with a 30 oz./square yard nylon woven fabric doubler. The profiled web lamina was then positioned between two similarly profiled lamina of 0-06 inch thick natural rubber and cured under heat and pressure in a press so as to effect bonding of the rubber to the web.

The five test fingers were fitted to operational craft and throughout their service lives these fingers were almost completely free of delamination.

This improvement can be accounted for by the fact that we have more nearly matched the stiffness of the reinforcing web with the stiffness of the rubber coating material than has been the case in flexible laminated sheet previously used in the manufacture of components and elements for flexible skirt assemblies. Thus under flexing of the material the web and the rubber coating are able to extend by similar amounts, thereby avoiding interlamina stress concentrations at the interface of the web and coating material. This results in a substantial reduction in delamination and breakdown of the bond between the web and the coating material is very nearly eliminated.

Another suitable knitted fabric for use as the reinforcing web lamina has the following basic construction: YARN:- Multi-folded, crimped, textured nylon.

FABRIC CONSTRUCTION:-- Flat bed knitted 1 x 1 rib construction.

FINISHING -Heat set and remove contaminants which affect adhesion of rubber to nylon.

PRE-TREATMENT:-- Pretreat fabric for rubber receptiveness. Other suitable knitted fabrics have the following basic constructions YARN:-- Synthetic multi-filament yarn of the polyamide/polyester types.

FABRIC CONSTRUCTION:-- Knitted- warp, weft or circular knitted construction.

FINISHING:-- Heat set and remove contaminants which affect adhesion of rubber to nylon.

PRE-TREATMENT:-- Pretreat fabric for rubber receptiveness. Polychloroprene is another suitable material for the elastomeric lamina.

In an air cushion vehicle flexible skirt component the elongation properties of the reinforcing web should most closely match the elongation properties of the elastomeric lamina in the direction of the maximum stress in the material. In the case of a finger the maximum stress occurs in the direction extending between the base and the apex of the substantially triangular developed profile of the finger. Therefore, if a ribbed knitted fabric is used for the reinforcing web the web lamina is cut so that the rib runs parallel to the base of the substantially triangular developed profile, since the extension of the web lamina is greater across the rib than with the rib.

The five test fingers hereinbefore described with reference to the accompanying drawings were manufactured by curing in a platten press so that they were produced as substantially flat sheets. Each sheet was then folded and attached to the bag member to produce the operational configuration of the finger. In an alternative method of manufacture a finger is produced by laying up a suitably profiled reinforcing web lamina between two similarly profiled elastomeric lamina on a mould of male form. This assembly is held in position by a vacuum bag and heated to effect curing of the elastomeric material. The finger so produced is moulded to the operational configuration.In the manufacture of a finger or other flexible skirt component by moulding the knitted construction of the reinforcing web lamina provides an added advantage in that it permits the web to be stretched to more readily conform to the curvature of the mould.

The foregoing description of flexible laminated material in accordance with the invention has been principally with reference to the manufacture of those independent flexible skirt elements known as fingers, however, it will be appreciated that the invention is not limited thereto. For instance, other components of an air cushion vehicle flexible skirt assembly may be manufactured from flexible laminated material in accordance with the invention, such as sections of the bag member, and those independent flexible skirt elements of generally tubular form that are known in the art as cones.

WHAT WE CLAIM IS: 1. An independent flexible skirt element for an air cushion vehicle flexible skirt assembly, said element being formed from flexible material comprising at least one elastomeric lamina bonded to at least one lamina of a reinforcing web having in at least one direction an elongation at failure at least 25 % of the elongation at failure of the elastomeric lamina.

2. An independent flexible skirt element as claimed in Claim 1, wherein the or each lamina of the reinforcing web has an elongation at failure that is at least 50% of the elongation at failure of the elstomeric lamina.

3. An independent flexible skirt element as claimed in Claim 1 or Claim 2, wherein the reinforcing web is a knitted fabric.

4. An independent flexible skirt element as claimed in Claim 1, 2 or 3, wherein the developed profile of the element is a generally triangular shape.

5. An independent flexible skirt element as claimed in Claim 4, wherein the elongation properties of the or each lamina of the reinforcing web are most closely matched to the elongation properties of the or each elastomeric lamina in a direction extending substantially normal to the base of the generally triangular shape of the developed profile of the element.

6. An independent flexible skirt element as claimed in Claim 5, wherein the or each lamina of the reinforcing web comprises a rib knitted fabric with the rib extending substantially parallel to the base of the generally triangular shape of the developed profile of the element.

7. An independent flexible skirt element as claimed in Claim 3 or Claim 6, wherein the fabric is knitted from multi-folded, crimped textured nylon yarn.

8. An independent flexible skirt element as claimed in Claim 3 or Claim 6, wherein the fabric is knitted from synthetic multi-filament yarns of the polyamide/polyester types.

9. An independent flexible skirt element as claimed in any preceding claim, wherein the or each elastomeric lamina comprises a layer of polychloroprene.

10. An independent flexible skirt element as claimed in any one of Claims 1 to 8, wherein the or each elastomeric lamina comprises a layer of natural rubber.

11. For an air cushion vehicle flexible skirt assembly, a finger or segment comprising an independent flexible skirt element as claimed in any one of Claims 1 to 10.

12. A finger or segment as claimed in Claim 10 or Claim 11, wherein the finger or segment is manufactured as a substantially flat profiled sheet that is subsequently folded to form an operational configuration.

13. A finger or segment as claimed in Claim 10 or Claim 11, wherein the finger or segment is manufactured by moulding to an operational configuration.

14. A finger or segment substantially as hereinbefore described and with reference to the accompanying drawings.

15. For an air cushion vehicle, a flexible skirt assembly comprised at least in part by a plurality of fingers or segments according to any one of Claims 10 to 14.

16. An air cushion vehicle including a flexible skirt assembly comprised at least in part by a plurality of fingers or segments according to any one of Claims 10 to 14.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. bag member, and those independent flexible skirt elements of generally tubular form that are known in the art as cones. WHAT WE CLAIM IS:

1. An independent flexible skirt element for an air cushion vehicle flexible skirt assembly, said element being formed from flexible material comprising at least one elastomeric lamina bonded to at least one lamina of a reinforcing web having in at least one direction an elongation at failure at least 25 % of the elongation at failure of the elastomeric lamina.