GB2411710A

GB2411710A - Self balancing rotor

Info

Publication number: GB2411710A
Application number: GB0405006A
Authority: GB
Inventors: Andrew John Rose
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2004-03-05
Filing date: 2004-03-05
Publication date: 2005-09-07
Also published as: GB0405006D0; WO2005085672A1

Abstract

A self balancing rotor comprises a rotor shaft 12 coaxially arranged with a balancing means 14. The balancing means comprises at least one ring shaped member 16 provided with a balancing mass 18 and radially spaced from the rotor shaft by at least one bearing means 20 to be rotatable relative to the rotor. The balancing means is further provided with at least one braking means 30 such as a resilient member 32 with a radially inward bias and a foot 34 arranged to releasably engage a rotor surface feature 40 and configured such that when rotating below a predetermined rotational speed the braking means engages the surface feature and the balancing means rotates with the rotor. Above the speed centrifugal force disengages the foot and allows automatic balancing. Locking prevents migration of the masses at low speed that may increase out of balance.

Description

2411 710

SELF BALANCING ROTOR

The invention relates to a self balancing rotor.

More specifically, although not exclusively, the invention relates to a self balancing rotor for turbomachinery.

The self balancing rotor can be employed to balance a rotor in any suitable rotating machinery including, but not limited to, gas turbine engines.

Many mechanical devices comprise rotors which, despite efforts during manufacture will require balancing post manufacture because of imbalance introduced during assembly, fitting or mounting. Imbalance may also be introduced because of unpredictable loads applied in operation. By way of non limiting example a washing machine comprises a rotatable hollow drum that must be balanced to rotate at high speed without causing damage to the adjacent components, but will suffer imbalance because of unevenly distributed articles placed inside the drum. Similarly by way of non limiting example, a gas turbine engine rotor shaft may incur imbalance in the unlikely event of a rotor blade, or part thereof, becoming detached, such as a result of bird strike or other foreign object entering the engine. Because of the unpredictable nature of the imbalance, it is necessary to employ an "automatic" or "self balancing" mechanism. Such devices comprise moveable balance masses arranged around the circumference of a rotor and are free to distribute themselves around the rotor so as to achieve balance.

When a rotor rotates at low (or "subcritical") speeds it runs with the region of imbalance weight rotating at a larger radius than the normal inbalance radius of rotation of the rotor. This is often referred to as "heavy side out". Conversely, when the rotor rotates at high (or "supercritical") speeds it runs with the region opposite the imbalance weight rotating at a larger radius than the normal in-balance radius of rotations of the rotor.

This is often referred to as "light side out". - 2

When the rotor is at supercritical speeds the balancing masses "roll" around under the influence of centrifugal force so that they are at the maximum radius thus adding mass to the "light side", thereby bringing the rotor into balance. The balance masses are held in this position by the induced centrifugal force for as long as the rotor is at a supercritical speed.

The demerit of such devices are that while they produce a beneficial balancing effect at high (or "supercritical") rotor speeds, they tend to increase out of balance at low (or "sub critical") rotor speeds. This is obviously a problem as many machines run their rotors both at high speed and low speed. Thus, the prior art does not provide balancing for variable speed rotor applications.

According to the present invention a self balancing rotor comprises a rotor shaft coaxially arranged with a balancing means, said balancing means comprising at least one ring shaped member, the or each ring shaped member being provided with at least one balancing mass, said balancing means being radially spaced apart from said rotor shaft by at least one bearing means such that the balancing means is rotatable relative to said rotor, wherein said balancing means is further provided with at least one braking means configured such that when the balancing means rotates below a predetermined rotational speed the at least one braking means communicates with a braking surface feature provided on the rotor thereby causing the balancing means to rotate in unison with the rotor.

Preferably the at least one braking means comprises a resilient member configured such that its resilience acts in a radially inward direction in opposition to the centrifugal loading operationally exerted on said resilient member.

Preferably the at least one braking means comprises an axially extending resilient member configured such that its resilience acts in a radially inward direction in opposition to the centrifugal loading operationally exerted on said resilient member. - 3

Hereinbefore and hereafter a radial direction is taken to mean a direction perpendicular to the longitudinal axis of the rotor and an axial direction is taken to mean a direction parallel to the longitudinal axis of the rotor.

The invention provides a means for automatically balancing a rotor in operation such that it is balanced when operating at low and high rotational speeds. The self balancing rotor employs a ring shaped member provided with at least one balancing mass which is in communication with a rotor. A braking means is provided on the ring member which, at a predetermined speed, locks the ring member and the rotor together so that they rotate in unison.

Above the predetermined rotational speed the braking member is configured to unlock from the rotor, permitting the rotor and the ring member so rotate relative to one another. In this configuration the balancing masses are free to distribute themselves around the circumference of the ring, where they will reach a state of balance, thereby reducing imbalance in the system. As the rotor slows the braking means re-engages with the rotor and such that the rotor ring member, and the balancing masses, rotate together in unison. The balance achieved during supercritical running is thus held all the way down through the speed range.

Hence, unlike the prior art, the present invention does not suffer from the disadvantage of increased vibration levels at low rotational speeds.

The invention will now be described by way of example only with reference to the accompanying diagrammatic non scale drawings, in which: Figure 1 is a cross sectional view of a self balancing rotor according to the present invention having two ring members provided radially inwardly of a hollow rotor shaft; Figure 2 Is a perspective view of part of one ring member as shown in Figure 1; and - 4 Figure 3 shows a cross sectional view of a self balancing rotor according to the present invention having two ring members provided radially outwardly of a rotor shaft.

Presented in Figure 1 is a cross sectional view of a self balancing rotor 10 according to the present invention which comprises hollow rotor 12 and a self balancing means 14 coaxial with the rotor 12. In the embodiment presented in Figure 1 the self balancing means 14 is provided radially inwardly of the hollow rotor 12. The balancing means 14 comprises two ring shaped members 16 provided with at least one balancing mass 18 radially inward of a bearing means 20.

The ring shaped members 16 are identical in ail major respects. They may be arranged as shown in Figure 1, that is to say identical but as a reflection of one another, spaced axially along the rotor 12. Alternatively identical ring shaped members 21 may be spaced axially along the rotor 12. The balance masses 18 should be kept close to each such that they are as close to being in the same axial plane as possible.

The bearing means 20 are in communication with the rotor 12 and ring shaped member 16 and permit relative rotational movement between the ring shaped member 16 and the rotor 12. A bearing location means 22, on the shaft 12, provided as a track 24 in this example, prevents relative axial movement of the balancing means 14 and the rotor 12.

A braking means 30 is provided on the ring member 14. In this example the braking means 30 takes the form of a resilient member 32 extending axially away from the ring member 16. The distal end of the resilient member 32 is provided with a foot 34, which axially overlaps with a braking surface feature which takes the form of a "L" shaped flange 40 provided on the radially inward face of the hollow rotor 12.

Figure 2 shows a perspective view of a portion of the balancing means 14, shown without the rotor 12 for clarity. The bearing means 20 comprises a plurality of roller - 5 bearings 20a,b,c,d etc mounted in a track 42 formed in ring member 16. For clarity not all of the roller bearings are shown in this diagram.

At rest and at low rotational speeds, the foot 34 is held in contact with the flange 40 by the resilience of the resilient member 32 which acts radially inwards. This acts as a braking means, locking the rotor 12 and the balancing means 14 together such that they rotate in unison.

At a first predetermined speed the centrifugal force acting on the foot 34 and resilient member 32 overcomes the resilience of the resilient member 32 to such a degree that the foot 34 moves radially outwards, and hence disengages with the flange 40. The rotor 12 and the balancing means 14 are thus able to rotate relative to one another, and the balancing masses 18 distribute themselves around the rotor so as to achieve balance.

When the ring member 16 slows to a second predetermined speed, the process is reversed and as the resilience of the resilient member 32 overcomes the centrifugal force induced by the rotation of the ring member 16, the foot 34 makes contact with the flange 40, braking the ring member 16 and causing the balancing means 14 to rotate in unison with the rotor 12.

The first predetermined speed may be equal to or different to the second predetermined speed.

A cross sectional view of another embodiment of the same invention which operates on the same principle as the first embodiment is presented in Figure 3. in this embodiment a self balancing rotor 50 comprises a balancing means 52 and coaxially arranged around the outside of a rotor shaft 54.

The balancing means 52 comprises two ring shaped members 56 provided with at least one balancing mass 58 radially outwardly of a bearing means 60. As with the first embodiment the axial location of the balancing means is achieved by the location of the - 6 bearing means 60 in a track 62 provided in the rotor shaft 54, and a track 64 provided in the ring member 56.

A braking means 70 is provided on the ring member 56 which takes the form of a resilient member 72 extending axially away from the ring member 56. The distal end of the resilient member 56 is provided with a foot 74, which axially overlaps with a braking surface feature 76 provided on the rotor.

At rest and at low rotational speeds the foot 74 is held in contact with the braking surface feature 76 by the resilience of the resilient member 72 which acts radially inwards. This acts as a braking means, locking the rotor 54 and the balancing means 52 together such that they rotate in unison.

At a first predetermined rotational speed the centrifugal force acting on the foot 74 and resilient member 56 overcomes the resilience of the resilient member 32 to such a degree that the foot 74 moves radially outwards and hence disengages with the braking surface 76. The rotor 54 and the balancing means 52 are thus able to rotate relative to one another and the balancing masses 58 are able to distribute themselves around the rotor so as to achieve balance.

When the ring member 56 slows to a second predetermined speed the process is reversed and as the resilience of the resilient member 72 overcomes the centrifugal force induced by the rotation of the ring member 56, the foot 74 makes contact with the braking surface 76 thereby braking the ring member 72 and causing the balancing means 52 to rotate in unison with the rotor 54.

The advantage of the present invention over the prior art is that balance achieved during high rotational speeds (otherwise referred to as "supercritical" speeds) is maintained at lower speeds. Hence the present invention does not suffer the - 7 disadvantage of the prior art which suffers increased vibration levels at low rotational speeds (otherwise referred to as "sub critical" speeds). Hence the present invention achieves improved balance performance across a wide range of operating speeds, both in the sub critical and super critical speed ranges.

It will be appreciated that the braking means could be achieved by an alternative means to that presented in the above examples. By way of non limiting example, braking means utilising the centrifugal force of the rotating ring members could be achieved by a series of tangential spring loaded fingers or a series of springs and masses configured to translate radially. The centrifugal force of the shafts rotation would be used to disengage the braking means by using it to overcome the spring stiffness and preload. Hence the centrifugal force would counter the resilience and preload of the spring and cause the mass to move radially outwards and inwards at predetermined speeds.

It will also be appreciated that braking means could be achieved by some suitably configured mechanical or electro mechanical device, designed to engage and disengage a brake at predetermined speeds. This would be of particular use if in a given application it is desirable that the engagement speed (the first predetermined engine speed) should be different to the disengagement speed (the second predetermined engine speed).

It will be appreciated that a balancing means provided on a rotor may comprise more than one pair of ring shaped members.

It will be appreciated that the present invention may be used in any rotating machinery application where it is important to suppress vibration by out of balance loads.

The configurations shown in these figures are diagrammatic. Although aspects of the invention have been disclosed with reference to the embodiments shown in the accompanying drawings, it is to be understood that the invention is not limited to the - 8 precise embodiments and that various changes and modifications may be affected without further inventive skill and effort. - 9 -

Claims

1 A self balancing rotor comprises a rotor shaft coaxially arranged with a balancing means, said balancing means comprising at least one ring shaped member, the or each ring shaped member being provided with at least one balancing mass, said balancing means being radially spaced apart from said rotor shaft by at least one bearing means such that the balancing means is rotatable relative to said rotor, wherein said balancing means is further provided with at least one braking means configured such that when the balancing means rotates below a predetermined rotational speed the at least one braking means communicates with a braking surface feature provided on the rotor thereby causing the balancing means to rotate in unison with the rotor.

2 A rotor as claimed in claim 1 wherein the at least one braking means comprises a resilient member configured such that its resilience acts in a radially inward direction in opposition to the centrifugal loading operationally exerted on said resilient member.

3 A rotor as claimed in claim 1 or claim 2 wherein the at least one braking means comprises an axially extending resilient member configured such that its resilience acts in a radially inward direction in opposition to the centrifugal loading operationally exerted on said resilient member.

4 A rotor as claimed in claim 1, claim 2 or claim 3 wherein the at least one braking means is configured such that above a first predetermined rotational rotor speed a clearance is maintained between the at least one braking means and the braking surface feature.

A rotor as claimed in any one of the preceding claims wherein the at least one braking means is configured such that below a second predetermined rotational rotor speed the at least one braking means communicates with the braking - 10 surface feature provided on the rotor thereby causing the balancing means to rotate in unison with the rotor.

6 A rotor as claimed in any one of the preceding claims wherein the balancing means is provided radially inwardly of a hollow rotor shaft.

7 A rotor as claimed in any one of the preceding claims wherein the braking surface feature is a flange with an extension that axially overlaps the at least one braking means.

8 A rotor as claimed in any one of the preceding claims wherein the at least one balancing mass is provided radially inward of the bearing means.

9 A rotor as claimed in any one of claims 1 to 5 wherein the balancing means is provided radially outward of the rotor shaft.

A rotor as claimed in any one of claims 1 to 5 or 9 wherein the braking surface feature is a region on the radially outer surface of the rotor.

A rotor as claimed in any one of claims 1 to 5, 9 or 10 wherein the at least one balancing mass is provided radially outward of the bearing means.

12 A rotor as claimed in any one of the preceding claims wherein said balancing means comprises two ring shaped members.

13 A rotor as claimed in any one of the preceding claims wherein the rotor forms part of a gas turbine engine.

14 A rotor as hereinbefore described with reference to the accompanying drawings.