TITLE : "SEALS FOR HYDRAULIC ASSEMBLIES"

TECHNICAL FIELD OF THE INVENTION

THIS INVENTION relates to improvements in hydraulic

mechanisms such as fasteners or nuts and in particular it relates to

improvements in the seals to such mechanisms and fasteners.

BACKGROUND ART

Hydraulic systems such as nuts and like type fasteners

are known. The nuts provide a means by which a stud or bolt can be

tensioned on being engaged by the nut, which nut is then hydraulically

actuated to apply a tensile force in the stud or bolt. The nuts often

operate under extremes of pressure and temperature.

Hydraulic nuts or like type fasteners are typically pre-

tensioned mechanically, after which a source of hydraulic pressure is

applied to a chamber within the structure to generate an hydraulic

force which applies an axial tensile force to a stud or nut engaged by

the fastener. A locking collar may be used to retain that tension after

release of the pressure from said chamber.

The magnitude of the added tensile force is dependent

upon the operative surface area of the hydraulic chamber in the nut

and that pressure which is introduced into the chamber and acts upon

it. Often the available operative surface area of the hydraulic chamber

is limited by the juxtaposition of adjoining features and the necessary

thickness of its internal structure to withstand stresses generated by

the introduced fluid pressure. In such cases a stacked array of

chambers may be utilised (see US 436826 - Bunyan).

The expansion chambers of the above style nuts need to

be sealed. In some assemblies pressurising fluid is retained within a

bladder (see US 4854798 - Snyder) . Most often seals are annular

rings (see US 4074923 - Lathara).

Seals for use with high pressure hydraulic devices are

typically made of elastomeric material such as nitrile rubber or

polyurethane. The means by which these seal against the passage of

fluid pressure can be divided into two distinct parts or mechanisms,

described herein as primary and secondary mechanisms. The primary

mechanism of sealing acts during the initial application of fluid

pressure, and simply blocks the passage of fluid, allowing internal

pressure to rise. As this pressure increases, the elastomeric seal is

deformed and is forced to a position where the seal bridges the gap

which is to be sealed, herein referred to as the extrusion gap, to

establish a secondary seal.

Prior art document US 5468106 Percival-Smith shows

seals, supposedly for operation at higher temperatures than is

achieved by conventional seals. Its seals are integrated with

components of the hydraulic assembly and these are therefore non

replaceable. Sealing is accomplished by deflecting a thin edge of a

component to bridge the extrusion gap. A very rapid surge of viscous

fluid is required to displace this edge.

It is typical of hydraulically activated piston and cylinder

arrangements that as operating pressures increase, the cylinder walls

are expanded radially, causing a proportional increase in the extrusion

gap between piston and cylinder. This is particularly a feature of the

above stacked configuration (Bunyan) . Stacking is effected because of

limitations to radial dimensions and the walls of such nuts are limited

as to thickness. The walls of these nuts are particularly subject to

increase in size of the extrusion gap as pressures are increased. There

is a need for a seal which is re-usably operative in these and other

systems, and at high temperature and pressures. The design of

Percival-smith does not achieve good low pressure performance and it

does not offer a useful re-usable seal.

A limiting feature in the operation of hydraulic nuts is the

effectiveness of their seals. Factors such as high pressures, high

temperatures, and service life under adverse conditions curtail their

application and effectiveness. If these factors become extreme, either

singly or in combination, then materials which are commonly used as

sealing agents will fail. A failure mode is the flow or movement of the

material of the seal into the extrusion gap under pressure and/or

temperature. At this point the seal can be lost.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an

hydraulic assembly, such as a fastener, with improved sealing

characteristics able to tolerate more extreme factors such as high

pressures, high temperatures, achieving a more extended service life

under such adverse conditions.

NATURE OF THE INVENTION

The invention achieves its object in the provision of a seal

for an hydraulic assembly wherein hydraulic fluid is to be contained in

a working chamber formed between at least two parts of the assembly

operating in at least two modes, a primary mode and a secondary

mode, the primary sealing mode being operative to contain hydraulic

fluid to a first pressure level, and the secondary sealing mode being

operative above said first pressure level with elastic deformation of a

metallic part of the seal to closely associate the seal material with any

gap between the at least two parts of the assembly.

The seal of the invention is particularly suited to

applications where there is a substantial extrusion gap increase with

applied pressure. The seal of the invention travels with the outer wall.

It does not separate from it. The shoulder of the seal maintains a

sliding contact with the radially expanding component of the

assembly.

In a preferred embodiment the seal may be one which

provides secondary sealing off sloped shoulders, a resolved force along

the slope creating the secondary mode of sealing. The slope and

material may be matched to achieve secondary sealing without the

down force causing the seal base to adhere under friction to the ramp

before the seal can react. The specific nature of a desirable seal is a

combination of factors involving angle of slope, choice of material,

target temperatures, and target pressures, such that no one design is

forced for any particular application.

In a further and different embodiment the seal may be

one wherein it is a pressed, cup shaped form, with flanges to form

primary seals, and reactive to pressure to form a secondary seal. The

cup form can be filled with materials with resistance to crushing such

as ceramics.

Ideally the seal of the invention is one wherein dis-similar

metals are chosen for the seal and the hydraulic assembly components

being sealed, to avoid fusing at extreme pressures.

It will be evident that an adaptation of the hydraulic

assembly, mechanism, or fastener, at the point where seals are fitted

to the expansion chamber, will be made to accommodate the selected

shape of the seal. A sloped shoulder will be effective in promotion of

the secondary mode of sealing. A slope sufficient to enhance

operation without inhibiting interaction between the respective

surfaces of shoulder and seal is desirable. It will be clear to those

skilled in the art that surface preparation will be important to

establishing what level of interaction might arise between two such

surfaces, in addition to choice of material, and geometry of the

interacting surfaces. Various combinations of the aforesaid factors

will provide increased radial thrust, to improve secondary sealing, in

any given assembly. A desired thrust is resolved from that action of

hydraulic forces directly on an angled base of the seal. Ramp angle for

the base is ideally calculated to both prevent frictional adhesion of the

seal to the ramp before it can react, or wedging of the seal resulting

impediment to free movement of the sliding components.

A number of seal constructions will be seen herein to be

possible. The seal may be a one piece all metal seal, having both

primary and secondary sealing functions. Alternately a one piece

metal seal might be a pressed, cup shaped seal with flanges to form

primary seals, reactive to pressure to form a secondary seal . Other

materials with resistance to crushing might be used such as ceramics.

The yield strength of the materials used is desirably in

excess of the sum of pressure and radial loads induced by operation,

for repeated operation. Otherwise the seal may be limited to a single

use. Below the yield strength the rings will possess elastic properties,

permitting re-use for a longer service life and durability.

Whilst the invention is described herein with particular

reference to hydraulic nuts, the seals of the invention have application

in any hydraulic assembly where there is a working chamber to be

operated, in use, under pressure of an hydraulic fluid, and it is desired

to achieve a higher level of performance, at higher temperatures and

pressures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to

various specific embodiments of the invention, as shown in the

accompanying drawings wherein:

FIGS. 1 to 3 are transverse sectional views through prior

art seals illustrating the manner of operation of such seals as are

common to hydraulic nuts;

FIG . 4 is a cross-section through a seal and elements of a

nut being sealed, showing details of a first embodiment of the present

invention;

FIGS . 5 and 6 are sectional, segmental views of further

sealing rings in accordance with the invention;

FIGS. 7 and 8 are still further sectional views showing

additional types of sealing rings in accordance with the invention;

FIG . 9 is a transverse sectional view through a composite

style sealing ring made in accordance with the invention;

FIG. 10 shows yet a further composite sealing ring in

accordance with the invention;

FIGS. 1 1 to 1 3 show cross-sectional details of further

seals which might be used in accordance with the invention;

FIG. 1 4 is an exploded view of an hydraulic nut assembly,

showing a system of seals to the expansion chamber of the nut, used

in accordance with the invention; and

FIG . 1 5 shows the assembled nut of FIG. 1 4.

PREFERRED EMBODIMENTS

In FIG . 1 is seen a prior art interference type seal

between two parts 1 1 and 1 2. The gap 1 3 is sealed against fluid

under pressure in gap 14. This is referred to herein as the Primary

Sealing Mechanism.

The Primary Sealing Mechanism allows seal media to

exert a light pressure against opposing surfaces to prevent passage of

pressurizing fluid at low pressures yet allowing easy sliding contact

between components. As the pressure increases, the force directed

against the surface of the seal acts to deform the shape of the seal.

This causes a transition of the actual point of contact at which sealing

occurs from the low pressure point of contact to that area immediately

adjacent the extrusion gap. At such time the seal material can be said

to act simply as a barrier or plug to prevent loss of pressurizing fluid

through the extrusion gap. This effect is herein described as the

Secondary Sealing Mechanism.

FIG . 2 shows the shape ultimately adopted by the seal

under increased pressure. The material of the seal 10 is flattened

against shoulder 1 6 and is squeezed over or into gap 1 3 at 1 5 to

effect the Secondary Sealing Mechanism. At such times should

pressures and/or temperatures rise, the usual materials employed in

the seal 1 0 will eventually extrude into and through the extrusion gap

and the seal will become ineffective or fail totally.

In FIG . 3 is seen a prior art composite seal 1 8 with a seal

expander 1 7 and sealing lips 1 9 and 20 to effect an interference

contact. In operation these act as the primary seal and the base of the

seal 1 8 forms an extrusion barrier or secondary seal in use, akin to

that of FIGS. 1 and 2 above. This form of seal will fail at higher

pressures and temperatures depending on the choice of materials by

extrusion at the gap as described above.

In FIG . 4 is seen a section through a seal 24 in

accordance with the invention applied between components 21 and

22 which are acted on by hydraulic pressure in use, with a shoulder

23 supporting seal 24 which is angled as shown towards extrusion

gap 27. The primary seal is via the upwardly tapered, outwardly

acting extensions, or lips 28 and 29. The seal may be provided with a

variety of geometries at this point to achieve a primary seal, with

shoulders, flanges, protrusions, lips, etc., acting outwardly against the

adjoining surface it is to seal against. During the application of fluid

pressure over the seal, seal 24 is subject to an applied force thereby

which produces a translated force 26 over the slope of shoulder 23,

with a horizontally resolved vector or component thereof actioning,

forcing or driving the seal, expanding or extending it over the gap 27,

against the wall of component 21 . This provides an improved

secondary seal in accordance with the invention.

As seen in FIGS. 4 to 1 2, this principle may be applied to

seals having a variety of different shapes, constructions, and/or

materials. Both primary and secondary seals are produced. The seals

utilise a vector component of the applied force to press the seal

radially outwards against the wall of the cylinder. FIG. 1 3 shows an

arrangement in which an annular seal, cup shaped in section, existing

either singly or in multiples, may variously make primary and

secondary seals in similar fashion to that described previously. The

sectional shape of such seal components may be other than what is

illustrated, for example, a V-form.

Ideally the secondary seal involves metal to metal

contact. These seals are ideally formed in materials or combinations

of materials so as to be reusable, or replaceable.

There are a number of ways in which the primary sealing

effect can be generated as seen in FIGS. 4 to 1 2 (described in greater

detail below) .

When the seal acts on an angled shoulder, the angle of

the base of the seal, and corresponding angle of the shoulder, is

critical in providing radial thrust against the respective wall against

which the seal is to act. The optimum angle is determined by factors

including operating pressure, width and composition of seal and nut

which determine the co-efficient of friction between the sliding

surfaces. Ideal thrust force develops pressure of the seal against the

cylinder wall, which together with that acting on the angled shoulder,

resists the passage of pressurised fluid between these elements. The

optimum configuration of these elements is that which effectively

seals without exerting an excessive amount of force of the seal

against the cylinder wall, which would cause sticking friction, and

thence galling of the surfaces.

In FIGS. 5 and 6 are seen sections of sealing rings 30 and

31 which can have an O-ring type seal added to respective grooves 32

and 33 (one acting radially outwardly, the other radially inwardly) over

an angled lower surface 34 and 35. The O-rings and corresponding O-

rings in the nut components form the primary seal. As pressure is

increased, the resolved force pushes the base outwardly (ring 30) or

inwardly (ring 31 ) to form a secondary seal.

In FIGS. 7 and 8 are seen sections of sealing rings 36 and

37 which have a thin upper section 38, 39 respectively, which can

deflect to give primary sealing at low pressures. The angled bases

establish the secondary seals.

In FIG. 9 is a composite seal formed in two parts, in two

materials, preferably polyurethane 42 with a steel backing 43. The

steel backing 43 is a shell with an angled base (oriented outwardly in

this case but it could be reversed) . The steel backing may be pressed

to shape or machined from preferred materials for the application. The

polyurethane insert 42 may have tapered extensions or lips 45 and 46.

It forms the primary seal and deflects under pressure to force the shell

43, as described above, to bridge the extrusion gap and establish the

secondary sealing. It might be used for temperatures up to 1 20°C

and/or high working pressures

In FIG. 1 0 is a further composite seal seen in cross-

section. A pressed metal seal part 48 with lips 49,50 forms a primary

seal. A machined support ring 47 adjoined thereto, is forced under

pressure over extrusion gap 51 to form a secondary seal there over.

The machined support ring 47 might be advantageously in bronze.

This seal will be effective at temperatures to 400°C and at high

pressures.

The seal of FIG. 1 1 uses a cup-shaped metal pressing 65

over a machined support ring 66. In FIG. 1 2, the cup-shaped metal

pressing has flanges 68 and 69 for primary sealing and a sloped base

67 for the aforementioned secondary seal.

The seal of FIG. 1 3 uses one or more of a simpler metal

pressing operating to seal a gap, as before between components 70

and 71 of an hydraulic assembly. The seals act over a

complementary, generally concave shoulder 72. Two cup shaped

seals 73, 74 are loaded over the shoulder (shown in their un-

pressurised state) . On application of pressure the first cup 73

achieves a primary seal. As the pressure rises the seals are

compressed into the concavity 72 to adopt the shape indicated in bold

and numbered 74. A spring effect in the seal achieves primary sealing

at the edge of the seal at contact with component 70. A horizontal

vector of the downward force expands the sea! to form a secondary

seal against the wall of component 70 and the lip of the seal groove

on component 71 .

The existing range of prior art seals for high temperature

applications is somewhat limited. Seals to working temperatures of

300°C are of prohibitive individual cost. A 5-stack hydraulic fastener

of the Bunyan kind (see US 4854798) would have 1 0 of these. Many

applications are far beyond such operating range. With the present

proposal, simple yet effective seals have been developed from readily

available materials such as sheet metal, bronze and steel alloys. The

seal of the prior art document US 54681 06 Percival-Smith is

inadequate being integrated with components of the hydraulic

assembly non replaceable. They do not have any primary sealing

mechanism and they cannot be activated at low pressure. The

number of operating cycles of such seals is limited, and component

replacement cost is high.

A seal in accordance with one embodiment of the

invention is shown (see below) , fitted to a single chamber type

hydraulic nut. When used in the configuration below (see FIG. 1 4),

the invention allows use of simple seals, functioning at great

pressures. Two rings are used and they are typically of bronze or

nickel alloy. When pressure is introduced into the sealed cavity during

charging, the thin lip portion of the seals in contact with the cylinder

walls form a primary seal. However, as pressures increase, the metal

seal is driven by a simple vector of the hydraulic force and geometry

into the gap to bridge and seal the extrusion gap. In this seemingly

simple arrangement, there are number of substantial benefits, not least

being reasonable cost. When used at high temperatures, the bronze

will anneal thereby solving problems of strain hardening which may

otherwise occur with use. Other rings can be made from hardened

steel, gunmetal, aluminium etc, depending upon the application. The

essential principle lies in using a vector of the hydraulic force derived

from the sloped shoulder to stretch the metal seal to form a metal to

metal secondary seal during operation. The seals may be physically

retained in place where necessary by retention means such as is used

in some prior art seals.

FIGS. 14 (expanded) and 1 5 (assembled) show a section

through an hydraulic nut with piston 51 fitted into cylinder 52 forming

chamber 53. When chamber 53 is charged with fluid under pressure

(by any of the usual charging means) the piston is forced upwardly to

expand the chamber. The locking ring 59 can be screwed upwardly

on thread 58 against the flange 60 of the piston to hold the extension.

In use, a bolt or stud (not shown) is projected through a central bore

61 to engage the nut thread 62. On extension of the nut, the bolt is

tensioned with cylinder 52 abutted against that part being clamped

between the nut and the support for the stud. This assembly is

shown with 'metal only' seals 63, 64 which might be simply

manufactured nickel alloy annular rings. Testing has shown that these

nuts can typically withstand operating temperatures in excess of

650°C and extreme pressures.