Patent Description:
A centre seeking actuator can be used to bias a first part of an assembly to move to a predetermined position relative to a second part of the assembly and oppose relative movement from the predetermined position. Once such example is an aircraft landing gear 'pitch trimming' actuator which is arranged to bias a bogie beam to adopt a predetermined orientation relative to a main strut and still allow the bogie beam to rotate in two directions.

A centre seeking actuator contains pressurised fluid which acts on internal surfaces of the actuator such that a pressure differential causes the actuator to adopt the intermediate condition. A centre seeking actuator will therefore oppose an applied force which acts to move the actuator from the intermediate condition.

The pressurised fluid can be provided by an external supply, such as a vehicle hydraulic supply. Alternatively, a centre seeking actuator can be self-contained, where the working fluid comprises a liquid and a compressible gas, the liquid being pressurised by the gas.

In both cases dynamic seals are provided to inhibit the flow of working fluid from the actuator.

<CIT> discloses a fluid pressure controlled strut or link of variable length. <CIT> discloses a known center seeking actuator.

The present inventor has identified that known centre seeking actuators can be improved to reduce the likelihood of working fluid leakage.

According to a first aspect of the invention, there is provided a centre seeking actuator according to claim <NUM>.

Thus, the centre seeking actuator according to the first aspect has second dynamic seals which act against the outside surface of the sidewall of the slave cylinder, rather than the inner surface of the sidewall of the outer cylinder as per prior art arrangements. This arrangement can provide an advantage over prior art centre seeking actuators in that an improved surface finish can be created on the outside surface of the sidewall of the slave cylinder in comparison to a surface finish that can be created on the inner surface of the sidewall of the outer cylinder. Thus, seal abrasion and therefore oil leakage may be reduced in comparison to prior art arrangements.

At least some of the outside surface of the sidewall of the slave cylinder that in use comes into contact with the second dynamic seals can be provided with a coating. Advantageously, a more uniform coating can be provided in a more repeatable manner on the outside surface of the sidewall of the slave cylinder in comparison to a coating that can be created on the inner sidewall of the outer cylinder.

The coating can comprise a high velocity oxygen fuel (HVOF) coating comprising metal, metal alloy, ceramic, plastic or composite materials. In one example, the HVOF coating can comprise chrome or tungsten carbide.

Alternatively, the second dynamic seals can be arranged to act on a bare outside surface of the sidewall of the slave cylinder. Advantageously, a more uniform machining profile can be provided in a more repeatable manner on the outside surface of the sidewall of the slave cylinder in comparison to that which can be created on the inner surface of the sidewall of the outer cylinder.

The outer cylinder can have a length and the one or more second dynamic seals can be located closer to the middle of the outer cylinder than to either end of it.

The outer cylinder comprises a first axial face and a second axial face connected by one or more first sidewalls to define a primary chamber, the first axial face having a first through-hole. Thus, the outer cylinder defines an inner volume within which a slave cylinder is slidably disposed so as to be movable relative to the outer cylinder.

The slave cylinder is slidably disposed within the inner volume, the slave cylinder comprising a third axial face and a fourth axial face connected by one or more second sidewalls to define a slave cylinder chamber, the third axial face having a second through-hole, the slave cylinder dividing the inner volume of the outer cylinder into a gas chamber and a working fluid primary chamber the slave cylinder being constrained to move only between first and second extremities within the inner volume of the outer cylinder so as to leave at least one free portion of the gas chamber, the outer cylinder including a venting port in fluid communication with the free portion of the gas chamber.

The piston of the piston-rod assembly is slidably disposed within the slave cylinder chamber and the rod of which extends through the second and first through-holes so as to project a free end from the outer cylinder.

The one or more first dynamic seals can be mounted at or adjacent to the first through hole and arranged to act on an external surface of the rod to inhibit working fluid within the chamber passing though the firsts through-hole. The one or more second dynamic seals are mounted on an inner surface of the the one or more first sidewalls of the outer cylinder and arranged to act on an external surface of the one or more second sidewalls of the slave cylinder to inhibit working fluid within the primary chamber passing to the free portion of the gas chamber.

The actuator can be arranged to be movable between an extended condition and a compressed condition and being biased to an intermediate condition between and distinct from the extended condition and the compressed condition. In embodiments of the invention the intermediate condition may be anywhere between but distinct from the extended and compressed conditions. In some embodiments the intermediate condition may be generally mid-way between the extended and compressed conditions. The actuator may be biased towards the intermediate condition from both contracted and extended conditions.

Thus, according to an aspect of the invention, the centre seeking actuator arranged to be movable between an extended condition and a compressed condition and being biased to an intermediate condition between and distinct from the extended condition and the compressed condition.

According to a further aspect of the invention there is provided an assembly as defined in claim <NUM>.

The assembly may be an aircraft assembly, such as an aircraft landing gear assembly.

The first part may be a bogie beam. The second part may be a main strut, such as main fitting or a sliding tube of a shock absorbing strut.

<FIG> is a longitudinal cross section view of a known centre seeking actuator <NUM>. The actuator <NUM> is a "single acting" actuator i.e. when operated it moves to assume the intermediate condition, but is not arranged to be actuated to the compressed or extended conditions.

The actuator <NUM> includes an outer cylinder <NUM> defining an inner volume that is partitioned by a slave cylinder <NUM> into a gas chamber <NUM> and a primary chamber <NUM> for containing hydraulic or working fluid. The outer cylinder <NUM> has a first axial face 102a and a second axial face 102b connected by one or more first sidewalls 102c to define the inner volume. The first axial face 102a has a first through-hole. The outer cylinder <NUM> defines a first coupling region <NUM>.

The gas chamber <NUM> is vented to atmosphere via a venting port <NUM>, which can be formed in the second axial face 102b or the first sidewalls 102c.

The slave cylinder <NUM> comprises a third axial face 112a and a fourth axial face 112b connected by one or more second sidewalls 112c to define a slave cylinder chamber <NUM>. The third axial face 112a has a second through-hole. The slave cylinder <NUM> is slidably housed within the inner volume to move along an axis X between predefined end points or extremities defined by end stops E1, E2 which can for example be defined by abutment formations or lockup valves within the outer cylinder <NUM>. The venting port <NUM> is sufficiently open throughout movement of the slave cylinder <NUM> to provide pressure equalisation between the gas chamber <NUM> and atmosphere.

The outer cylinder <NUM> includes a hydraulic fluid port <NUM> on the opposite side of the slave cylinder <NUM> to the venting port <NUM>. The hydraulic fluid port <NUM> is arranged to be coupled to a hydraulic fluid circuit. The hydraulic fluid port <NUM> is sufficiently open throughout movement of the slave cylinder <NUM> to enable fluid to be supplied to the primary chamber <NUM>.

A second dynamic seal <NUM> is provided between the slave cylinder <NUM> and outer cylinder <NUM> to inhibit hydraulic fluid passing to the gas side <NUM>. The seal can be mounted on a gland nut arranged to be screwed into the aperture in the axial face of the outer cylinder <NUM>. A plurality of second dynamic seals can be provided in parallel with one another.

The slave cylinder <NUM> defines a slave cylinder chamber <NUM> within which a piston-rod assembly <NUM>, <NUM> is slidably housed to move along the axis X. The piston <NUM> of the piston-rod assembly is slidably disposed within the slave cylinder chamber <NUM> and free to move along it. The rod <NUM> of the piston-rod assembly extends though the second and first through-holes so as to project from the outer cylinder <NUM>. The free end of the rod <NUM> defines the second connector <NUM>.

A first dynamic seal <NUM> is provided between the rod <NUM> and outer cylinder <NUM> within the aperture formed through the axial face of the outer cylinder <NUM>, to inhibit hydraulic fluid from passing out of the outer cylinder <NUM> via the aperture. A plurality of first dynamic seals can be provided in parallel with one another.

The slave cylinder <NUM> includes one or more control apertures <NUM> arranged to enable fluid communication between the primary chamber <NUM> containing the hydraulic fluid and the slave cylinder chamber <NUM>.

In use, pressurised hydraulic fluid enters the hydraulic fluid port <NUM> and forces the slave cylinder <NUM> to an end stop adjacent to the venting port <NUM>, as well as passing through the control apertures <NUM> into the slave cylinder chamber <NUM> and in doing so forcing the rod <NUM> to extend outwardly with respect to the outer cylinder <NUM>. Thus, the actuator <NUM> is continually biased to an intermediate condition between and distinct from the fully extended condition and the fully compressed condition, and therefore can act as a shock absorber.

A dominant tensile force applied to the coupling regions <NUM>, <NUM> results in the slave cylinder <NUM> being drawn away from the end stop adjacent the venting port <NUM> against the spring force provided by the pressurised hydraulic fluid within the outer cylinder <NUM>. Thus, the actuator <NUM> can be forced to move to a fully extended condition. Upon the applied tensile force becoming inferior to the biasing force provided by the hydraulic fluid, the actuator <NUM> moves towards and assumes the intermediate condition.

A dominant compressive force applied to the coupling regions <NUM>, <NUM> results in the piston <NUM> of the piston-rod assembly moving towards the piston <NUM> of the slave cylinder <NUM> against the spring force provided by the pressurised hydraulic fluid within the outer cylinder <NUM>. Upon the applied compressive force becoming inferior to the biasing force provided by the hydraulic fluid, the actuator <NUM> moves towards and assumes the intermediate condition.

The present inventor has identified that the second dynamic seals <NUM> can be prone to leakage due to the fact that they are arranged to act against a counter-face which is defined by a bore, namely the inner surface of the outer cylinder sidewall 102c. It is technically challenging to machine and/or apply a uniform coating to a bore surface in order to provide a fine surface finish counter-face, due to the long and narrow bore profile. Consequently, the second dynamic seals <NUM> run against a bare metal surface. The inventor has identified that this can lead to premature wear, thereby resulting in premature oil leakage.

<FIG> shows an actuator <NUM> according to an embodiment of the present invention. The actuator <NUM> is similar to the known actuator <NUM> and therefore, for brevity, the following description will focus on the differences between the actuator <NUM> and the known actuator <NUM>. Like parts have been give the same reference numerals. For clarity purposes, the end stops E1 and E2 are not shown.

The one or more second dynamic seals <NUM> in an actuator according to embodiments of the invention are mounted on an inner surface of the one or more first sidewalls of the outer cylinder <NUM> so as to sealing engage the exterior surface 14a of the one or more second sidewalls of the slave cylinder <NUM>. The seals <NUM> can be mounted in any suitable manner, such as within a reinforced mounting formation <NUM>. The <NUM> outer cylinder has a length L, which can for example be at least <NUM> and the one or more second dynamic seals <NUM> are located closer to the middle of the outer cylinder <NUM> than to either end of it.

In use, as the actuator <NUM> is extended and compressed, a contact region CR of the slave cylinder exterior surface 14a will move in sliding engagement relative to the stationary second dynamic seals <NUM>.

The contact region CR of the exterior surface 14a of the slave cylinder <NUM> is provided with a coating C that improves the dynamic sealing relationship with second dynamic seal <NUM>. The exterior surface 14a of the slave cylinder <NUM> is analogous to the rod <NUM> of the piston-rod assembly in terms of ease of applying a coating to it.

In some embodiments just some of the contact region CR can be provided with a coating C and in other embodiments the entire slave cylinder exterior surface 14a can be provided with a coating C.

Any suitable coating C can be provided that reduces wear of the second dynamic seals <NUM> and/or exterior surface 14a of the slave cylinder <NUM> relative to the arrangement illustrated in <FIG>. For example, the coating C can comprise a high velocity oxygen fuel (HVOF) or an electrolytic process coating comprising metal, metal alloy, ceramic, plastic or composite materials. In one example, the coating can comprise chrome or tungsten carbide.

The coating C can have a thickness of between <NUM> and <NUM> and in some cases between <NUM> and <NUM>.

<FIG> shows an actuator <NUM> according to a further embodiment of the present invention. The actuator <NUM> is similar to the actuator <NUM> of <FIG> and therefore, for brevity, the following description will focus on the differences between the actuator <NUM> and the actuator <NUM>. For clarity purposes, the end stops contact region CR and coating C are not shown.

The actuator <NUM> of this embodiment is self-contained in that it is not energised by an external supply. Therefore, the outer cylinder <NUM> does not include a fluid supply port. Instead, the piston rod <NUM> includes a blind hole separated by a second slave cylinder <NUM> into a separating piston <NUM> and the slave cylinder chamber <NUM>. Gas, such as nitrogen, within the separating piston <NUM> forces the second slave cylinder <NUM> against fluid within the slave cylinder chamber <NUM> to bias the actuator <NUM> to its central position in a conventional manner. The gas can be compressed and expanded to permit the actuator to be compressed and expanded.

<FIG> shows an actuator <NUM> according to a further embodiment of the present invention. The actuator <NUM> is similar to the actuator <NUM> of <FIG> and therefore, for brevity, the following description will focus on the differences between the actuator <NUM> and the actuator <NUM>.

The actuator of this embodiment includes an external cylinder connected to the outer cylinder <NUM> containing a separating piston <NUM> and second gas chamber <NUM> which functions analogously to the separating piston <NUM> and second gas chamber <NUM> of <FIG>.

The actuator of this embodiment includes a second end stop E2' which lies beyond the venting port <NUM> in terms of slave cylinder <NUM> travel. The inner diameter ID of the outer cylinder <NUM> is greater than the outer diameter OD of the slave cylinder <NUM> in at least the region of the venting port <NUM> so as to provide a relief channel RC between them which permits pressure equalisation within the venting chamber <NUM> throughout compression of the actuator <NUM>.

Although the actuators described with reference to <FIG> have coatings C applied to the slave cylinder exterior surface 14a of the one or more second sidewalls, in other embodiments the second dynamic seals <NUM> can be arranged to act on a bare exterior surface of the one or more second sidewalls of the slave cylinder. Advantageously, a more uniform machining profile can be provided in a more repeatable manner on the exterior surface of the one or more second sidewalls of the slave cylinder in comparison to that which can be created on the inner surface of the one or more first sidewalls of the outer cylinder.

<FIG> shows a landing gear assembly <NUM> according to an embodiment of the present invention. The landing gear <NUM> includes a main strut <NUM>, having an upper portion (not shown) arranged to be coupled to the underside of an aircraft (not shown) and a lower portion 62b telescopically mounted with respect to the upper portion. A bogie beam <NUM> is pivotally coupled to the lower portion of the main strut 62b, the bogie beam <NUM> having axles <NUM> mounted on it for carrying one or more wheel assemblies (not shown). A landing gear assembly according to embodiments of the present invention may have any suitable number of axles and wheels per axle.

A linkage <NUM> is pivotally coupled to the bogie beam <NUM> at a first coupling region 72a and pivotally coupled to the lower strut portion 62b at a second coupling region 72b. In the illustrated embodiment the linkage is defined by an actuator <NUM> according to an embodiment of the present invention, such as those described with reference to <FIG>. As will be appreciated, pivotal movement of the bogie beam <NUM> relative to the strut <NUM> results in a change in the condition i.e. the effective length of the actuator <NUM>. The term "effective length" may refer to the distance between the pivot axis of first and second coupling regions 72a, 72b. Equally, a change in the effective length of the actuator <NUM> results in pivotal movement of the bogie beam <NUM> relative to the strut <NUM> and the actuator <NUM> can thus be used to "trim" the position of the bogie beam <NUM> for stowing. In alternative embodiments the coupling regions 72a, 72b could be reversed and may in other embodiments be coupled between any part of the bogie on the one hand and any part of the strut <NUM> on the other hand. In some embodiments the linkage <NUM> may include a multi bar linkage that is movable by an actuator so as to change the angular position of the bogie relative to the strut.

While the actuator <NUM> is described with reference to a landing gear assembly pitch trimming actuator, an assembly according to embodiments of the present invention may be any assembly including a central seeking actuator according to an embodiment of the invention arranged to bias a first part of the assembly to assume a predetermined position relative to the second part, the assembly being arranged, in use, to force the actuator to assume the first condition when in a first state and having a second state in which the actuator moves the first part of the assembly to assume the predetermined position relative to the second part and where maintenance can be difficult and/or expensive; for example, a flaps or slats in an aircraft wing, an oil rig boom, a vehicle suspension system.

Claim 1:
A centre seeking actuator (<NUM>) comprising:
an outer cylinder (<NUM>) comprising a first axial face (102a) and a second axial face (102b) connected by one or more first sidewalls (102c) to define an inner volume, the first axial face (102a) having a first through-hole;
a slave cylinder (<NUM>) slidably disposed within the inner volume of the outer cylinder, the slave cylinder comprising a third axial face (112a) and a fourth axial face (112b) connected by one or more second sidewalls (112c) to define a slave cylinder chamber (<NUM>), the third axial face (112a) having a second through-hole,
the slave cylinder dividing the inner volume of the outer cylinder into a gas chamber (<NUM>) and a working fluid primary chamber (<NUM>), the slave cylinder being constrained to move only between first and second extremities within the inner volume of the outer cylinder so as to leave at least a free portion of the gas chamber, the outer cylinder including a venting port (<NUM>) in fluid communication with the free portion of the gas chamber (<NUM>); a piston-rod assembly (<NUM>, <NUM>), the piston (<NUM>) of which is slidably disposed within the slave cylinder chamber and the rod (<NUM>) of which extends through the second and the first through-holes so as to project a free end from the outer cylinder; and
one or more first dynamic seals (<NUM>) arranged to act on an external surface of the rod to inhibit working fluid leaking from the outer cylinder;
one or more second dynamic seals (<NUM>) mounted on an inner surface of the one or more first sidewalls of the outer cylinder and arranged to act on an exterior surface of the one or more second sidewalls of the slave cylinder to inhibit working fluid leaking from the outer cylinder,
wherein the one or more second dynamic seals are arranged to inhibit working fluid within the primary chamber passing to the free portion of the gas chamber.