GB2168505A - Fluid-pressure-operable servo systems - Google Patents

Abstract

A fluid-pressure-operable servo system, which comprises a fluid- pressure-operable motor 11 having an output member 10 and control valve means 25 therefor, includes a feedback member 24 which is connected between the motor and the control valve means and which, at least in part, is formed by a plurality of components 26, 30, 32, 37 certain of which together form a first load path and certain of which form a second load path. Upon mechanical failure of one of the components such relative displacement of at least two of the components takes place as to result in limited change in length of the feedback member. This modifies the effect of the control valve means thereby causing such adjustment of motor 11 as will afford an immediate and perceptible change in an operating condition of a device 5 controlled by the servo system. <IMAGE>

Description

SPECIFICATION Fluid-pressure-operable servo systems This invention relates to fluid-pressure-operable servo systems, suitable for effecting pitch-change of the blades of bladed rotors such as aircraft propellers.

Hitherto certain such servo systems have included a feedback member, connected between an output member of the system and the control valve means thereof, which so applies positional signals from the output member to an element of the control valve means, or to an element directly associated with the control valve means, as to neutralise the effect of a command signal to said valve means when the output member has reached a position demanded by that signal.

In certain aircraft propellers for example the blade pitch-change mechanism forming the servo system has hitherto included a feedback member which has taken the form of an elongate extension of the output member of the pitch-change motor of that mechanism. In this case the feedback member has usually been disposed with its axis coincident with the rotational axis of the propeller and has sometimes so extended rearwardly as at least in part to be housed within the hollow propeller shaft of an associated engine for co-operation with the control valve means.

In certain forms the feedback member has been designed to provide two generally parallel load paths so that if in operation of.the system a failure occurs, for example a fracture in the feedback member, whereby one load path is no longer available, the feedback member is still operable using the remaining load path which is automatically brought into use and thus the system continues to operate satisfactorily. However, since failure of the first load path is not readily evident to the operator of the system, the failure of that load path has remained undetected until dismantling of the components of the system has taken place.

The invention as claimed is intended to provide a remedy. It solves the problem of how to design a servo system in which the feedback member thereof is so provided with two load paths that, in the event that one of the load paths is no longer available, immediate indication of this situation is provided to the operator of the system.

According to this invention a fluid-pressureoperable servo system, comprising a fluidpressure-operable motor having an output member and control valve means therefor, includes a feedback member which is connected between said motor and said control valve means and which, at least in part, is formed by a plurality of components certain of which together form a first load path and certain of which together form a second load path, wherein upon mechanical failure of one of said components such relative displacement of at least two of said components takes place as to result in limited change in length of said feedback member which modifies the effect of said control valve means thereby causing such adjustment of said motor as will afford an immediate and perceptible change in an operating condition of a device controlled by said servo system.

The said components of said first load path may include a forked member and a primary load member pivotally-connected to the forks of said forked member and extending in a direction away from said output member and generally in alignment therewith, while said components of said second load path may include a secondary load member and a cylindrical member connected to said secondary load member in telescopic relation therewith, with a predetermined amount of lost motion therebetween.

Preferably said primary load member and said cylindrical member are connected to, or formed integrally with, a valve element associated with said control valve means.

Spring means may be operably engaged between said secondary load member and said cylindrical member.

Alternatively, spring means may be operably engaged between said forked member and said cylindrical member. In this case said spring means may engage said forked member through the intermediary of a slotted sleeve.

The secondary load member may be of cylindrical form and may be provided with a radially-inwardly-directed flange at its end remote from said cylindrical member, said flange being co-operable with upstanding means carried upon the exterior surface of said forked member at a position adjacent said flange.

The primary load member may be connected to said forked member by a transverse pin, and said pin may at its outer end portions pass through elongate slots in said cylindrical member and be located in seatings formed in said secondary load member, the co-operation of said pin and said elongate slots affording said predetermined amount of lost motion.

The fluid-pressure-operable motor may be of piston-and-cylinder type and said feedback member may then be an extension of said output member of said motor. In this case said output member may be hollow to enable a fluid under pressure passing through said feedback member to pass to the interior of said motor.

Also according to this invention there is provided a bladed rotor, the pitch of the blades of which is adjustable by a fluid-pressure-operable servo system as set out in any of the eight next preceding paragraphs herein.The bladed rotor may be adapted for mounting upon the hollow output shaft of an associated engine and in this case part at least of said feedback member may be adapted sealingly to slide within the interior of that shaft in response to output movements of said motor.

Fluid under pressure may be conducted from the interior of said shaft into the interior of said feedback member under the control of said control valve means.

The advantages offered by the invention are mainly that a mechanical failure occurring in the primary load path of the servo system can more readily be detected than hitherto and the then necessary overhaul of the equipment can in consequence be initiated as soon as possible after the failure has taken place.

One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment, in which: Figure 1 is a cross-sectional side elevation of an aircraft propeller hub together with part of a blade and of associated spinner structure, and Figure 2 is an enlarged view of part of the construction shown in Fig. 1.

As shown the hub 1 of propeller 2 comprises a two-part main casing 3, 4 which carries a plurality of blades one of which is shown at 5 mounted in a ball bearing 6. Each blade carries a counter-weight 7 and has a crank pin 8 at its root which engages a crosshead 9 carried by the output member 10 of a pitch-change motor 11 of piston-and-cylinder type. The piston 12 of motor 11 is slidable in the larger diameter bore 13 of a stepped cylinder 14 bolted to the forward face 15 of casing 3, 4. Such sliding in the forward, blade-pitch-fining, direction is against the effort of coil springs 16, 17 and takes place under liquid pressure applied by way of the interior 18 of member 10 and through ports 19 to annular chamber 20 to the rear of piston 12. The range of the pitch-change movement is between reverse pitch defined by stop 21 and fully-feathered pitch defined by stop 22.Part of suitable spinner structure is shown at 23.

A feedback member 24 is connected between motor 11 and a propeller pitch-control valve means 25. Member 24 includes a hollow main portion 26 which is formed integrally with, and which is an aligned extension of, member 10. This portion passes rearwardly to the exterior of casing 3, 4 where it is adapted to extend into the interior 27 of a hollow rotary output shaft 28 of an associated propeller-driving engine, part of which is shown at 29. Member 24 also includes a plurality of components certain of which together form a first load path and certain of which together form a second load path.

The said components include a forked member 30 which is screw-threadedly secured at 31 to the rearward end of main portion 26 and a primary load member 32 which is balljointed at 33 at its forward end and there pivotally-connected as shown by means of a transverse pin 34 to the forks 35 of member 30. Member 32 extends in a direction away from output member 10 and is generally in alignment therewith. The said components also include a secondary load member in the form of a sleeve 36, which is parallel to member 32, and a cylindrical member 37 which is connected to sleeve 36 and in telescopic relation therewith, with a predetermined amount of lost motion therebetween.

Member 32 is ball-jointed at 38 at its other end and there pivotally-connected by means of a transverse bolt 39 and nut 40 to a valve element 41 which has a circular valving edge 42. Element 41 is in this embodiment formed integrally with cylindrical member 37 and is provided with ports 43. Member 30 is also provided with ports 44.

A pre-loaded coil spring 45 is operably engaged between sleeve 36 and member 37, and pin 34 extends at its outer portions through elongate slots 46, 47 in member 37, being located in circular seatings 48, 49 in sleeve 36 and retained there by circlips 50, 51. The co-operation of pin 34 and slots 46, 47 affords the said predetermined amount of lost motion between sleeve 36 and member 37.

The edge 42 of element 41 is co-operable, as feedback member 24 slides axially within the interior 27 of shaft 28, with a series of spill ports 52 provided in shaft 28 and forming part of valve means 25. This valve means also includes a non-rotative pitch control sleeve 53, suitably connected for control from the aircraft cockpit by means (not shown), which is mounted for limited axial sliding movement upon shaft 28, the forward inner circular edge 54 of sleeve 53 being co-operable with ports 52 which, when open to the exterior of shaft 28, are suitably placed in communication with drain.

The interior 27 of shaft 28 is in communication in suitable manner with a source (not shown) of liquid under pressure, for example a pump driven by the engine.

At its forward end, sleeve 36 is provided with a radially-inwardly-directed flange 55 which is co-operable with a ring 56 carried by upstanding from forked member 30. Shims 57 are provided for accurate axial setting of ring 56.

On assembly of the components of the feedback member 24, spring 45 is pre-loaded to enable bolt 39 and its nut 40 to be fitted and that preload is maintained, thereby placing member 32 under slight tension.

During operation of propeller 2, the counterweights 7 bias blades 5 in the coarse pitch direction in opposition to the inherent finepitch-seeking centrifugal twisting moments in the blades, while springs 16, 17 bias piston 12 rearwardly in the pitch-coarsening direc tion. Thus in order to move the blades in the pitch-fining direction the liquid present in chamber 20 requires to be sufficiently raised in pressure. Accordingly the servo system, comprising motor 11, valve means 25 and feedback member 24, is operable by adjustment of sleeve 53 to the left in the drawings, thereby appropriately reducing the extent of liquid spilling through ports 52, to raise the pressure in the interior 27 of shaft 28.Pressure liquid passes from interior 27, through ports 43, the interior of members 37, 36 and ports 44 into the hollow interior of main portion 26 and the interior 18 of member 10, and thence through ports 19 into chamber 20.

When motor 11 and thus blades 5 reach the position demanded by the setting of sleeve 53, the feedback member 24, in following up this movement, reaches a position in which its valving edge 42 uncovers certain of ports 52, this causing a drop in pressure in the interior 27 of shaft 28 so that blade pitchchange movement ceases and the blades are held in their new fine pitch condition. If, subsequently, it is required to move the blades to a desired position in the pitch-coarsening direction, sleeve 53 is moved to the right in the drawings causing an increase in liquid spilling through ports 52 and thus a lowering of pressure in the interior 27 of shaft 28 so that the pitch-change system operates in the converse sense.

During such pitch-fining and pitch-coarsening operation feedback force is transmitted from motor 11, by way of main portion 26, and through the first load path, that is through forked member 30, the forks 35 of that member, pin 34, ball joint 33, member 32, ball joint 38 and bolt 39 to element 41. With the first load path so in use, the second load path is ineffective, its sleeve 36 being positioned as shown in Fig. 2. Any tendency of that sleeve then to chatter is resisted by preloaded spring 45.

If, during operation of the propeller, member 32 fractures and thus the first load path is severed, the pre-loading in spring 45 causes member 37 to move to the right with respect to sleeve 36 as far as the lost motion provided between the slots 46, 47 and pin 34 permits. Hence the second load path comes automatically into use, this through forked member 30, forks 35, pin 34, sleeve 36, spring 45 and member 37. However since the member 37 has been caused to move to the right with respect to sleeve 36, its valving edge 42 has instantly adopted a new setting in relation to ports 52, that is the overall length of the feedback member has been slightly increased.This new setting of edge 42 results in a small increase in pressure arising in the interior 27 of shaft 28 which causes sufficient pitch-fining of blades 5 for the propeller now to exhibit an abrupt increase in rotational speed, which in this embodiment is of the order of 300 r.p.m. Such an increase is readily noticeable in the cockpit and can be taken as indicative that a failure in the first load path has occurred, and gives warning that replacement of the feedback member should be made as soon as possible.

If, instead of member 32 fracturing, the forks 35 of member 30 fracture forwardly of pin 34, a new second load path comes into use automatically.

In this case the clearance between flange 55 and ring 56 is taken up by movement of sleeve 36 and member 37 to the right in the drawings. This new second load path is in consequence through ring 56, flange 55, sleeve 36, pin 34, ball joint 33, member 32, ball joint 38 and bolt 39. Thus the overall length of feedback member 24 is increased slightly as before to cause an increase in the rotational speed of the propeller which is indicative of the failure in the first load path.

Although in the embodiment above described with reference to the drawings the spring 45 is operably engaged between member 37 and sleeve 36, in other embodiments spring 45 may with advantage directly be operably engaged between member 37 and forked member 30, or alternatively between member 37 and a slotted sleeve which itself bears against forked member 30 with pin 34 passing through the slots of that sleeve. In both cases spring load is allowed additionally to operate for a failure of primary forks 35.

Although in the embodiment above described with reference to the drawings the servo system of this invention has been applied to an aircraft propeller, in alternative embodiments of the invention the servo system may with advantage be applied to other devices which require a feedback member having first and second load paths.

Claims (17)

1. A fluid-pressure-operable servo system, which comprises a fluid-pressure-operable motor having an output member and control valve means therefor, including a feedback member which is connected between said motor and said control valve means and which, at least in part, is formed by a plurality of components certain of which together form a first load path and certain of which together form a second load path, wherein upon mechanical failure of one of said components such relative displacement of at least two of said components takes place as to resuit in limited change in length of said feedback member which modifies the effect of said control valve means thereby causing such adjustment of said motor as will afford an immediate and perceptible change in an operating condition of a device controlled by said servo system.

2. A system as claimed in claim 1, wherein said components of said first load path include a forked member and a primary load member pivotally-connected to the forks of said forked member and extending in a direction away from said output member and generally in alignment therewith, while said components of said second load path include a secondary load member and a cylindrical member connected to said secondary load member in telescopic relation therewith, with a predetermined amount of lost motion therebetween.

3. A system as claimed in claim 2, wherein said primary load member and said cylindrical member are connected to, or formed integrally with, a valve element associated with said control valve means.

4. A system as claimed in either claim 2 or claim 3, wherein spring means is operably engaged between said secondary load member and said cylindrical member.

5. A system as claimed in either claim 2 or claim 3, wherein spring means is operably engaged in said forked member and said cylindrical member.

6. A system as claimed in claim 5, wherein said spring means engages said forked member through the intermediary of a slotted sleeve.

7. A system as claimed in any one of claim 2 to 6, wherein said secondary load member is of cylindrical form and is provided with a radially-inwardly-directed flange at its end remote from said cylindrical member, said flange being co-operable with upstanding means carried upon the exterior surface of said forked member at a position adjacent said flange.

8. A system as claimed in any one of claims 2 to 7, wherein said primary load member is connected to said forked member by a transverse pin, and said pin at its outer end portions passes through elongate slots in said cylindrical member and is located in seatings formed in said secondary load member, the co-operation of said pin and said elongate slots affording said predetermined amount of lost motion.

9. A system as claimed in any one of the preceding claims, wherein said fluid-pressureoperable motor is of piston-and-cylinder type and said feedback member is an extension of said output member of said motor.

10. A system as claimed in claim 9, wherein said output member is hollow to enable a fluid under pressure passing through said feedback member to pass to the interior of said motor.

11. A bladed rotor, the pitch of the blades of which is adjustable by a fluid-pressure-operable servo system as claimed in any one of the preceding claims.

12. A bladed rotor as claimed in claim 11, said rotor being adapted for mounting upon the hollow output shaft of an associated engine.

13. A bladed rotor as claimed in claim 12, wherein part at least of said feedback member is adapted sealingly to slide within the interior of said shaft in response to output movements of said motor.

14. A bladed rotor as claimed in claim 13, wherein fluid under pressure is conducted from the interior of said shaft into the interior of said feedback member under the control of said control valve means.

15. A fluid-pressure-operable servo system substantially as hereinbefore described with reference to the accompanying drawings.

16. A bladed rotor, the pitch of the blades of which is adjustable by a fluid-pressure-operable servo system substantially as hereinbefore described with reference to the accompanying drawings.

17. An engine and bladed rotor in combination suitable for propelling an aircraft or the like, the pitch of the blades of said bladed rotor being controlled by a fluid-pressure-operable servo system, substantially as hereinbefore described with reference to the accompanying drawings.