GB2078310A - Gas turbine rotor blade vibration damping system - Google Patents

Abstract

The damping means consists of a passage 22, 23 within the blade aerofoil within which fits a pin which is retained to the aerofoil at its root end 26-29. The pin chunk 24, 25 is free to permit relative sliding with respect to the passage and friction between the pin and the passage absorbs energy and hence damps the vibration, the pin arranged not to be on a radius of the rotor, centrifugal force enhancing the friction. <IMAGE>

Description

SPECIFICATION A rotor blade for a gas turbine engine This invention relates to a rotor blade for a gas turbine engine.

One of the problems which has consistently arisen in the design of rotor blades for gas turbine engines relates to the provision of damping means which reduce the amount of vibration of the blade. This vibration if unchecked can lead to reduction of the blade life and in some cases to rapid damage to the blades. It is clear that any such damping arrangement should be simple and lightweight and if possible it should be readily applicable to those portions of the blade where the amplitude of vibrational movement is likely to be highest.

The present invention provides a very simple construction which enables damping to be applied to a specific part of the aerofoil.

According to the present invention a rotor blade for a gas turbine engine comprises a spanwise passage through the aerofoil of the blade and a pin which fits closely in the passage, the pin being retained to the blade at its end closest to the root of the blade and being a sliding fit within the passage.

The passages and pins are likely to provide greatest damping effect if they are arranged usually closely adjacent to the leading and/or trailing edges of the aerofoil where they can deal with most types of vibration mode.

One particular convenient way of locating the pin in the passage entails the use of a pin having an enlarged head which engages with a seating at the inner end of the passage.

The invention will now be particularly described merely by way of example with reference to the accompanying drawings in which: Figure 1 is a partly broken away view of a gas turbine engine having rotor blades in accordance with the invention, Figure 2 is an enlarged perspective view of one of the rotor blades of the engine of Figure 1 and in accordance with the present invention, Figure 3 is a section on the line 3-3 of Figure 2 and, Figure 4 is a section through a further embodiment of rotor blade in accordance with the invention.

In Figure 1 there is shown a gas turbine engine which comprises a casing 10 which contains in flow series a compressor section 11, a combustion section 12 and a turbine 13 and which forms at its downstream extremity a propulsion nozzle 14. The operation of the engine overall is conventional. The casing is broken away to expose to view the downstream part of the combustion section 12 and rotor blades 15 of the turbine. Figure 2 shows one of the rotor blades enlarged.

Each rotor blade is seen to consist of a root 16, a root shank 17, a platform 18 and an aerofoil 19. The aerofoil 19 is of conventional form and has leading and trailing edges 20 and 21. It will be seen that in this instance the blades 15 are of the unshrouded type, that is the tip of the aerofoil 19 does not carry an integral shroud or shroud portion and is therefore not restrained by such structures.

Because of the absence of a shroud the blade 15 is prone to vibration which may be induced by engine resonances or other energy inputs. If the blade should vibrate the amplitude of vibration will be greatest at the very tip of the blade and in the thin leading and trailing edges regions 20 and 21.

Therefore in accordance with the present invention the leading and trailing edges 20 and 21 are provided with respective narrow passages 21 and 23. Within the passage 22 there extends a pin 24 and similarly a pin 25 extends within the passage 23. The pins 24 and 25 can best be seen from Figure 3 and it will be seen that both pins consist of a body portion and an enlarged head 26 and 27 respectively. The body portion extends through the respective passage with a reasonably close fit but is free to slide within the passage. The head portions 26 and 27 engage with respective seatings 28 and 29 which are formed at the ends of the passages 22 and 23.

Operation of these devices is quite simple in that when the blade is in operation centrifugal force will act on the blades and the pins. The passages 22 and 23 are arranged not to be precisely radial and therefore centrifugal loads on the pins will force them into contact with the side walls of their respective passages. Engagement between the enlarged heads and the respective seatings will retain the pins in position within the passages. Should the blade now vibrate relative sliding motion will occur between the pins and the passages within which they fit. The friction caused will absorb energy and will then tend to damp out the vibration of the blade.

Although as mentioned above in most circumstances it will be particularly useful to damp the leading and trailing edges of the aerofoil it is of course possible to apply this damping technique to other portions of the blade and Figure 4 shows this technique applied to the central section of the blade.

In this case the blade again has a root 30, a root shank 31, a platfrom 32 and an aerofoil 33 which in this case is hollow. An angled passage 34 extends from one face of the aerofoil adjacent to the platform to adjacent the outer face at the tip of the aerofoil. A pin 35 having an enlarged head 36 extends within the passage 34. Operation of the embodiment is exactly the same as described above although in this case the angle of the pin and passage to the radial is much greater and in this case damping of the central section of the aerofoil is provided.

It should be understood that in the embodiments described above the pins are solely retained by mechanical engagement between their heads and the seatings of the blade. It will of course be possible to provide alternative or additional attachments such as metallurgical bonds (for instance brazing or welding) or other forms of mechanical engagement.

Again although described in relation to shroudless blades the invention may be applied to shrouded blades and is particularly useful where these blades are of high aspect ratio and consequently prone to vibration of their centre span portions. It will also be noted that fretting has generally been regarded as undesirable and it is therefore a useful feature of the construction of the present invention that the pins may relatively easily be withdrawn to establish the degree of wear which has taken place.

Claims (6)

1. A rotor blade for a gas turbine engine comprising a spanwise passage through the blade aerofoil and a pin closely fitting in the passage, the pin being retained to the blade at its end closest to the blade root and being free to slide within the passage.

2. A rotor blade as claimed in claim 1 and in which the pin is retained to the blade by engagement between an enlarged head of a pin and a seating in the blade.

3. A rotor blade as claimed in claim 1 or in claim 2 and in which the passages and their pins extend adjacent the leading and/or trailing edges of the blade aerofoil.

4. A rotor blade as claimed in claim 1 or claim 2 and in which the passage extends diagonally across the central section of the blade aerofoil.

5. A rotor blade substantially as herein described with reference to Figures 1-3 of the accompanying drawings.

6. A rotor blade substantially as herein described with reference to Figure 4 of the accompanying drawings.