GB2050978A - Rotary assembly, particularly a helicopter tail rotor - Google Patents

Abstract

A rotary assembly such as a helicopter tail rotor has pitch links (14) for controlling the pitch of blades (4). The pitch links (14) rotate with the assembly and are parallel to but spaced from the rotary axis. Consequently there is a high centrifugal force on the pitch link, which can lead to rapid axial wear in a bearing (32) at the blade end of the link (14). In accordance with the invention, the other end of the link (14) is connected resiliently to the rest of the assembly and applies a preload force at the blade end of the link (14). This preload force wholly or partly cancels the load resulting from centrifugal force when the assembly rotates, so increasing the life of the bearing (32). The resilient connection may be provided by an elastomeric coupling (24) received in a misaligned hole in the pitch link (14). <IMAGE>

Description

SPECIFICATION A rotary assembly This invention relates to a rotary assembly including a link of which at least one end is spaced from the axis of rotation. A helicopter tail rotor is one such assembly, since such rotors usually have variable pitch blades, the pitch being controlled by pitch links which are spaced from the axis of rotation of the rotor. The present invention is not, however, limited to helicopter rotors, although it will be described with reference to them.

The function of the tail rotor of a helicopter is to prevent the helicopter body from rotating under the reaction torque exerted on the body by the main lifting rotor and to enable the pilot to point the helicopter in a desired direction. Conventionally, the tail rotor, like the main rotor, rotates at a fixed speed which, for the tail rotor, is typically about 2000 revolutions per minute. Control of the position of the body of the helicopter is achieved by varying the pitch of the blades of the tail rotor by means of pitch links, one for each blade. In the context of the present invention, the word "pitch" is used in the sense "angle of attack".

As the tail rotor rotates, its blades move not only through the airstream induced by the forward motion of the helicopter, but also through the downwash caused by the main rotor. During each revolution of the tail rotor, each blade is therefore subjected to cyclically varying forces which are transmitted to the pitch links. These forces cause cyclic loading of the points of attachment of the pitch link to the rest of the assembly. This cyclic loading is directed lengthwise of the link.

These points of attachment are also loaded by centrifugal force, because each pitch link, or at least one end of it, must be spaced from the rotary axis of the rotor in order to achieve the desired pitch change of the blades. This centrifugal force is a function of the mass of the pitch link, the speed of rotation of the rotor and the distance of the pitch link from the axis of rotation and is consequently substantially constant in flight. The blade end of the pitch link is commonly connected to the rest of the assembly by a bearing, and in many rotor assemblies, the centrifugal force has a component which is directed axially of this bearing. Since the bearings used usually have a smaller contact area in the axial direction than in the radial direction, the bearing stresses in the axial direction are more severe than those in the radial direction.

Furthermore, the bearing at the blade end of the pitch link experiences greater movements in operation than that at the other end of the link, and so the bearing atthe blade end is usually larger and more robust that that at the other end. This results in an increased mass at the blade end with consequent greater centrifugal loads and higher bearing stresses.

Dry self-lubricating self-aligning bearings, often incorporating po ly(tetrafluoroethylene) (PTFE) at the contact surfaces are increasingly being used atthe blade ends of pitch links. However, these bearings have an average life of about 200 to 300 hours, and consistent performance of 500 to 600 hours is considered exceptional. These performances compare unfavourably with the tail rotor and main rotor assemblies as a whole, where 1200 and 2500 hours is being specified. The pitch links are therefore considered to be unreliable components and are designated as "on-condition" maintenance items which are inspected frequently and likely to be replaced several times during the main overhaul intervals of the rotary assemblies.The tail rotor pitch links are almost always deemed unfit for further operational service because of excessive axial wear in the blade end bearings resulting from the combination of the stresses induced by the centrifugal loading and the blade motions.

Attempts to change the angle at which the loads impinge on the bearings so as to transfer a portion of the axial loading to a radial direction leads to problems of fatigue failure where angled rods are bent.

Changing the orientation of the bearings by rotating the ends of the rods through 90" leads to other unsafe engineering solutions at the points of attachment to the blade hub which are unacceptable in the helicopter industry as the pitch links are classified as vital parts, the failure of which, in flight, is almost certain to result in loss of life and of the aircraft.

According to the present invention there is provided a rotary assembly including a link of which one end, which is spaced from the axis of rotation, is connected to the rest of the assembly by a bearing, the other end of the link being connected resiliently to the rest of the assembly in such a way as to exert a preload force on the rest of the assembly at the said one end when the assembly is at rest, this preload force having a component directed radially inwardly of the axis of rotation, whereby it acts in opposition to the centrifugal force acting at the said one end of the link when the assembly rotates.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure lisa perspective view of a helicopter tail rotor assembly; and Figure 2 is a partly sectioned view of a pitch link for the assembly of Figure 1.

The assembly shown in Figure 1 comprises a rotor shaft 2 carrying two blades 4, of which only one is shown in the Figure. The blades 4 are mounted on blade connecting sleeves 6 which are mounted rotatably on an inner member 8 which is connected to the shaft 2. The sleeves 6 carry control levers 10 which are connected to a sliding sleeve 12 by pitch links 14. The sliding sleeve 12 is rotationally fixed to the shaft 2 but can be slid along it under the control of the pilot of the helicopter by means of a linkage including a bell crank 16. Lubricant is retained in the region of contact between the sliding sleeve 12 and the shaft 2 by rubber boots 18 which also provide protection against the ingress of dirt.

Each link 14 is mounted on a stub shaft 19 extending from the sleeve 12, and is retained on the stub shaft 19 by means of a retaining ring 20 and a nut 22.

There is an elastomeric coupling 24 (see Figure 2) between the stub shaft and the pitch link 14 to accommodate the angular deflections which occur at this location.

At the other end, the pitch link 14 is mounted on a pivot shaft constituted by a bolt 26 passing through two lugs 28 and secured by a nut 30. The pitch link 14 is mounted on the bolt 26 by means of a selflubricating spherical self-aligning bearing 32 (see Figure 2) to accommodate the larger angular deflec tionswhich occur at this end of the pitch link when the pitch of the blade changes, as well as other deflections which result from other movements of the blade in flight.

One of the pitch links 14 is shown to a larger scale in Figure 2. As can be seen in that Figure, the bearing 32 comprises an inner member 34 received in an outer member 36, this outer member 36 being located in a hole 38 in the end of the pitch link 14.

The inner member 34 fits closely around the bolt 26 and can rotate relatively to the outer member 36 about an axis 40, as well as being angularly displaceable relatively to the outer member 36 about axes perpendicular to the axis 40 so as to be selfaligning.

The elastomeric coupling 24 comprises two coaxial cylindrical metal sleeves 42 and 44 between which there is an elastomeric element in the form of a rubber bush 46. The sleeve 44 is a close fit about the stub shaft 19 and the sleeve 42 is received in a hole 48 in the pitch link 14.

The axis of the stub shaft 19 is substantially parallel to that of the bolt 26 and is represented in Figure 2 by a line 50. However, the axis 52 of the inner sleeve 44, when the bush 46 is unstressed, is inclined to the axis 50 by an angle cz . This misalignment is obtained in the embodiment shown in Figure 2 by forming the hole 48 in an inclined position, as shown. However, it would be possible, as an alternative, to form the hole 48 coaxial with the axis 50 but to construct the coupling 24 so that the inner sleeve 44 was misaligned.

When the pitch link 14 is in position in the rotor assembly, the elastomeric bush 46 is stressed because the sleeve 44 is forced into a position coaxial with the axis 50. The bush 46 therefore exerts a torque on the pitch link 14 which results in the bearing 32 being pressed to the right as viewed in Figure 2. In the assembled rotor as shown in Figure 1,the end of the pitch link 14 mounted between the lugs 28 is pressed towards the rotary axis of the shaft 2. This applies an axial preload to the bearing 32.

However, when the rotor is rotating, the pitch link 14 is subjected to a centrifugal force which tends to fling it away from the axis of the shaft 2. At the end of the pitch link mounted between the lugs 28, this centrifugal force acts in a direction opposite to the preload applied by the elastomeric bush 46, and the two may cancel each other out so that, in operation, there is substantially no net axial loading on the bearing 32. In any case, the axial load resulting from centrifugal forces will have a reduced effect on the bearing 32 as a result of the opposing pre-load applied by the elastomeric bush 46.

Since PTFE self-lubricating bearings are generally considered to have a wear rate proportional to the square of the stress level at the bearing surface, a four-fold increase in life should be obtained by reducing the axial bearing load by 50%. Theoretically, therefore, the average life of a tail rotor pitch link should be increased from 250 to 1000 hours.

The angle by which the hole 48 is misaligned can be determined in the following manner. A pitch link 14 is constructed with the hole 48 aligned with the axis 50. This link is then supported on a pivot received in the sleeve 44 and the bearing 32 is subjected to a force equal to that which the resilience of the bush 46 is to apply. For example, this force may equal the whole axial loading which will result, in operation, from centrifugal force or it may equal only part of that load. This loading can be calculated from a knowledge of the mass of the pitch link and the specification of the tail rotor assembly as a whole.

The force applied to the bearing 32 will cause the pitch link to deflect by deformation of the bush 46, and this angular deflection a0 can be measured; it is the angle by which the hole 48 must be misaligned.

Typically, the centrifugal loading on the bearing 32 will be about 50-1001b.f. To oppose this fully, the angle a0 may be about 5 to 10 depending on the stiffness of the elastomeric coupling.

Claims (17)

1. A rotary asseinbly including a link of which one end, which is spaced from the axis of rotation, is connected to the rest of the assembly by a bearing, the other end of the link being connected resiliently to the rest of the assembly in such a way as to exert a preload force on the rest of the assembly at the said one end when the assembly is at rest, this preload force having a component directed radially inwardly of the axis of rotation, whereby it acts in opposition to the centrifugal force acting at the said one end of the link when the assembly rotates.

2. A rotary assembly as claimed in claim 1, in which the said other end of the link is connected to the rest of the assembly by a coupling comprising an elastomeric element.

3. A rotary assembly as claimed in claim 2, in which the coupling comprises two coaxial cylindrical steel sleeves between which is disposed the elastomeric element.

4. A rotary assembly as claimed in claim 3, in which the elastomeric element comprises a rubber bush.

5. A rotary assembly as claimed in claim 3 or4, in which the elastomeric element is deformed whereby the axis of the inner sleeve is inclined to the position occupied by that axis when the elastomeric element is unstressed.

6. A rotary assembly as claimed in claim 5, in which the axis of the inner sleeve is substantially parallel to the axis of the bearing.

7. A rotary assembly as claimed in claim 5 or 6, in which the angle by which the axis of the inner sleeve is inclined to the position occupied by that axis when the elastomeric element is unstressed is approxi mately5to 100.

8. A rotary assembly as claimed in any one of the preceding claims, in which the magnitude of the preload force is approximately 50 to 100 Ibf.

9. A rotary assembly as claimed in any one of the preceding claims, in which the bearing is a selfaligning bearing.

10. A rotary assembly as claimed in any one of the preceding claims, in which the bearing is a selflubricating bearing.

11. A rotary assembly as claimed in claim 10, in which the bearing has a contact surface of poly(tetrafluoroethylene).

12. A rotary assembly as claimed in any one of the preceding claims, which is a helicopter rotor.

13. A rotary assembly as claimed in claim 12, in which the rotor is a tail rotor.

14. A rotary assembly as claimed in claim 12 or 13, in which the link is a pitch link.

15. A rotary assembly as claimed in any one of claims 12 to 14, in which the link is connected between a sliding sleeve on a shaft of the rotor and a blade of the rotor, the bearing being at that end of the linkwhich is attached to the blade.

16. A rotary assembly substantially as described herein with reference to, and as shown in, the accompanying drawings.

17. A pitch link for a helicopter tail rotor, substantially as described herein with reference to, and as shown in, Figure 2 of the accompanying drawings.