The present invention relates to the field of turbomachines used for the propulsion of an aircraft. The invention relates more particularly to an aircraft turbomachine rotor comprising a device for damping the rotor blades.

STATE OF THE ART

In known manner, an aircraft turbomachine extends longitudinally along an X axis and allows the aircraft to be moved from an air flow entering the turbomachine and circulating from upstream to downstream. Hereinafter, the terms “upstream” and “downstream” are defined with respect to the longitudinal axis X oriented from upstream to downstream. Likewise, the terms “inside” and “outside” are defined in the radial direction with respect to the longitudinal axis X. In a known manner, the turbomachine comprises a compressor, a combustion chamber and a turbine for rotating the compressor. . The compressor comprises one or more rotors, mounted in line along a longitudinal axis X to compress the air flow circulating from upstream to downstream in a primary stream of the turbomachine.

In practice, a rotor comprises a rotor disc, extending transversely with respect to the longitudinal axis X, which is integral with a power transmission shaft. The rotor disc comprises a plurality of housings, also called cells, formed at the outer periphery of the rotor disc, in which blades are respectively mounted by axial insertion along the longitudinal axis X. The blades extend in the same plane. transverse to the longitudinal axis X. Each blade extends radially with respect to the longitudinal axis X and successively comprises a mounting foot, radially interior, configured to be mounted in one of said housings of the rotor disc, a platform,

[0004] During operation of the turbomachine, each blade is subjected mainly to three types of stresses, namely centrifugal forces, aerodynamic disturbances and disturbances in the equilibrium of the rotor disc. Under the effect of centrifugal forces, each mounting foot is pressed against the outer part of the housing in which it is mounted and exerts a pressure dependent on the speed of rotation of the rotor. When changing

speed of the turbomachine, the centrifugal forces vary, resulting in pressure variations at the level of the housings, causing relative radial movements with respect to the longitudinal axis X between each mounting foot and the housing in which it is mounted. Aerodynamic disturbances, for their part, create variations in the constraints of the air flow on the blades, also inducing relative movements between each blade root and the housing in which it is mounted. Finally, when the mass of the rotor disc is not balanced, that is to say in the presence of unbalance, there is also a relative movement between each blade root and the housing in which it is mounted.

The repetition of relative movements leads to wear phenomena of the mounting feet of the blades and of the housings of the rotor disc, which reduces their service life. To limit these wear phenomena, it is known practice to introduce an additional part between a blade root and its housing.

It has been proposed by patent application FR2918702A1 to wrap the mounting foot of a blade with a foil comprising a viscoelastic material in order to absorb the relative movements. However, during its use, each foil is capable of moving axially and of coming into contact with another rotor disc positioned downstream of the rotor disc in which the foil is mounted. Such a movement is liable to increase the wear of the mounting feet of the blades and of the downstream rotor disc, which presents a drawback.

Alternatively, it has been proposed by the patent application FR2915510A1 to use a shock absorber comprising a sole, bearing on the outer periphery of the disc, a weight, helping to balance the mass of the rotor disc, and a spring connecting the sole and the weight so that the weight exerts a support on the platforms of two adjacent blades. The damper also makes it possible to limit the relative movements between the blades and the housings. However, as with the foil, each damper is capable of moving axially and of coming into contact with a rotor disc positioned downstream. As presented above, such a movement is likely to increase the wear of the mounting feet of the blades and of the downstream rotor disc, which presents a drawback.

In the field of turbine rotors, it is known from patent application WO2012 / 020195A1 to mount ring sectors in the groove of the rotor disc, which include ventilation orifices, radial locking wedges and axial locking teeth of the blade roots. Such wedges are however not suitable for damping the relative movement of the blade roots in their housing but for blocking them and are therefore mounted in the housings under each blade root.

Also in the field of turbine rotors, it is known from patent application US4781534A to press against one or both radial face (s) of the rotor disc a ring with tabs mounted in the housings under each blade root in order to reduce air leaks due to the misalignment of the disc and the blades, in particular in the shape of a fir tree. Such tongues have the same drawbacks as the blocking wedges described above.

[0010] There is therefore a need for a damping device making it possible to reduce the relative movements between the rotor blades and their housings without causing wear.

GENERAL PRESENTATION OF THE INVENTION

The invention relates to a damping device for a turbomachine rotor, said turbomachine extending longitudinally along an axis X, said rotor comprising a rotor disc extending transversely with respect to the longitudinal axis X and a plurality blades, said rotor disc comprising a plurality of housings formed at the outer periphery of the rotor disc, each blade extending radially with respect to the longitudinal axis X and comprising a mounting foot, radially inner, configured to be mounted in one of said housings, a platform and a radially outer air deflection blade.

The invention is remarkable in that the damping device comprises:

A support ring configured to extend transversely with respect to the longitudinal axis X and configured to be positioned at the outer periphery of the rotor disc, and

a plurality of damping members, integral with the support ring and projecting upstream from the support ring, each damping member being configured to extend under at least one platform of a blade so as to exert a radial force outwardly so as to damp the radial displacement of said blade during operation of the turbomachine.

Thanks to the invention, there is advantageously no relative movement between the damping members given that they are connected by the support ring, which limits wear. In addition, their radial position is determined precisely. In addition, the damping members make it possible to guide the platforms of the blades during their assembly and to press them radially outwards during operation of the turbomachine. Such a damping device has a small footprint and a low mass. The sealing of the rotor is also improved.

Preferably, the damping device consists of a support ring and a plurality of damping members. Such a damping device has a simple structure, which makes it possible to reduce its mass, its size and its cost.

Preferably, each damping member is mounted in a prestressed manner under at least one platform of a blade so as to exert a radial force outwardly so as to damp the radial displacement of said blade during operation of the blade. the turbomachine. In other words, the damping members are mounted in contact with the blade platforms and under the effect of centrifugal forces, the damping members exert a large radial force on the blade platforms outwards to effectively dampen the blade. displacement of the blades. The plating of the blades during operation of the turbomachine is improved by the pre-stressing mounting of the damping members against the blade platforms.

Preferably, each damping member is in the form of a leaf spring configured to exert a radial force towards the outside. Such a shape makes it possible to achieve an optimum surface plating while limiting the mass and the bulk.

Preferably, the damping members are distributed uniformly around the circumference of the support ring. Thus, the radial force of the damping members is applied uniformly.

Preferably, at least one damping member comprises a proximal portion connected to the support ring, a distal portion mounted free and a central portion formed between the proximal portion and the distal portion.

According to a preferred aspect, the proximal portion has a thickness less than the thickness of the central portion. The proximal portion thus advantageously forms a flexible portion making it possible to articulate the damping member with respect to the support ring in order to achieve an optimum plating force.

According to another preferred aspect, the distal portion has a thickness greater than the thickness of the central portion. The distal end thus has a greater mass in order to exert a greater plating force under the effect of centrifugal forces. This is advantageous given that the distal end is located under a middle portion of the platform and thus makes it possible to act in the axis of its center of gravity. Damping is thus improved.

Preferably, at least one damping member comprises a flat outer face. Thus, the outer face makes it possible to produce a flat support guaranteeing a uniform plating limiting vibrations. More preferably, the interior face comprises a protuberance, in particular, at its distal end. Such a protuberance makes it possible to reinforce the plating by increasing the centrifugal effect, which is advantageous given that the distal end is located under a middle portion of the platforms of the blades. In addition, the protuberance forms a reserve of material which can be easily machined in order to correct an unbalance of the rotor.

The invention also relates to a rotor for a turbomachine extending longitudinally along an axis X, said rotor comprising a rotor disc extending transversely with respect to the longitudinal axis X and a plurality of blades, said disc of rotor comprising a plurality of housings formed at the outer periphery of the rotor disc, each vane extending radially with respect to the longitudinal axis X and comprising a mounting foot, radially inner, configured to be mounted in one of said housings, a platform and a radially outer air deflection blade, the rotor comprising a damping device, as presented above,the support ring of which is positioned at the outer periphery of the rotor disc and the damping members of which project upstream from the support ring, each damping member extending below at least a platform of a blade so as to exert a radial force outwardly so as to damp the radial displacement of said blade during operation of the turbomachine.

Preferably, each damping member extends under at least the platforms of two blades. Thus, a damping member makes it possible to apply a uniform plating between two adjacent blades.

Preferably, the rotor comprising a downstream rotor disc, positioned downstream of the rotor disc in which the blades are mounted, the support ring is positioned against the downstream rotor disc. Any downstream movement of the damping device is thus eliminated.

Preferably, each damping member extends cantilevered upstream. There is thus no wear of the outer periphery of the rotor disc by the damping members.

The invention also relates to a turbomachine extending longitudinally along an axis X comprising a rotor as presented above.

The invention further relates to a method of using a turbomachine extending longitudinally along an X axis and comprising a rotor as presented above, the method comprising:

• a step of putting the turbomachine into operation, during which the rotor disc is rotated at a speed of rotation along the longitudinal axis X and each mounting root of a blade is moved radially towards the outer part of its housing by centrifugal effect and

• a radially outer plating step of each blade by at least one damping member so as to damp the radial displacement of the mounting foot of said blade in its housing.

The invention further relates to a method of manufacturing a damping device, as presented above, comprising:

• a step of cutting the flat contours of the damping device from a thin panel,

• a stamping step of the cut damping members so as to give them their relief and their thickness, and

• a step of folding the stamped damping members so as to orient them in the upstream axial direction.

A damping device can thus be manufactured in a practical, rapid and inexpensive manner.

PRESENTATION OF FIGURES

The invention will be better understood on reading the description which follows, given solely by way of example, and referring to the accompanying drawings given by way of non-limiting examples, in which identical references are given to similar objects and on which:

Figure 1 is a longitudinal sectional representation of a turbomachine according to one embodiment of the invention,

FIG. 2 is a three-dimensional schematic representation of a portion of a turbomachine rotor equipped with the damping device according to the invention and with two blades,

FIG. 3 is a three-dimensional schematic representation of a portion of a turbomachine rotor equipped with the damping device according to the invention with the blades in transparency,

Figure 4 is a schematic perspective view of the damping device according to the invention,

FIG. 5 is a schematic representation in cross section of a portion of a turbomachine rotor equipped with the damping device according to the invention,

Figure 6 is a schematic representation in cross section of the damping device according to the invention and

FIG. 7 is a schematic representation of a method of manufacturing a damping device according to the invention.

It should be noted that the figures show the invention in detail to implement the invention, said figures can of course be used to better define the invention if necessary.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Referring to Figure 1, there is shown a turbomachine T extending longitudinally along an axis X and allowing to move the aircraft from an air flow entering the turbomachine T and circulating upstream downstream. Hereinafter, the terms “upstream” and “downstream” are defined with respect to the axis X oriented from upstream to downstream. Likewise, the terms “inside” and “outside” are defined in the radial direction with respect to the longitudinal axis X. In a known manner, the turbomachine T comprises a compressor 3, a combustion chamber 4 and a turbine for driving in rotation of the compressor 3. The compressor 3 comprises one or more rotors 2, mounted in line along the longitudinal axis X to compress the air flow circulating from upstream to downstream in a primary stream of the turbomachine T.

As illustrated in Figure 2 showing an embodiment of the invention, a rotor 2 comprises a rotor disc 20 extending transversely with respect to the longitudinal axis X and integral with a power transmission shaft (not shown) as well as a plurality of blades 21. The rotor disc 20 comprises a plurality of housings 200, also called cells, formed at the outer periphery of the rotor disc 20, in which the blades 21 are respectively mounted by axial insertion along the longitudinal axis X. The blades 21 extend in the same plane transverse to the longitudinal axis X. In FIG. 2, the blades 21 are identical to each other but it goes without saying that the blades 21 could be different. Hereinafter, for the sake of clarity, a single vane 21 is described with reference to FIG. 2.

In known manner, a blade 21 extends radially along an axis Z defined relative to the longitudinal axis X and successively comprises a mounting foot 210, configured to be mounted axially in one of said housings 200 of the rotor disc 20, a platform 211, making it possible to maintain a predetermined blade 21 at a distance from the blades 21

adjacent and to reconstitute the internal surface of the primary stream and an air deflection blade 212 extending radially with respect to the longitudinal axis X. The blade 21 extends radially in the mounted position. The mounting foot 210 is thus qualified as radially inner while the blade 212 is qualified as radially outer relative to the longitudinal axis X.

In order to limit the relative movement of the blades 21 relative to the rotor disc 20, the rotor 2 comprises a damping device 1 positioned between the outer periphery of the rotor disc 20 and the platforms 211 of the blades 2 so as to flatten the vanes 21 radially outwardly in their housings 200.

One embodiment of a damping device 1 according to the invention is shown with reference to Figures 2 to 6. As illustrated in Figure 3, the damping device 1 comprises a support ring 7 and a plurality of damping members 8 which are integral with the support ring 7. Preferably, the damping device 1 is made of metal, preferably of stainless steel so as to have high mechanical strength as well as flexibility to improve damping. However, it goes without saying that other materials could be used.

Preferably, the damping device 1 is in one piece. In other words, the damping members 8 are made from the material of the support ring 7. This makes it possible to achieve optimum damping by the spring effect while reducing the manufacturing cost.

According to the invention and with reference to Figures 2 to 6, the support ring 7 extends transversely with respect to the longitudinal axis X. Thus, the longitudinal axis X passes through the center of the ring support 7. The support ring 7 is configured to be positioned at the outer periphery of the rotor disc 20. The damping members 8 can thus be directly in contact with the platforms 211 of the vanes 21. In this example, the The support ring 7 is positioned downstream of the housings 200 so as to be able to be positioned on a rotor disc 20 'located downstream of said rotor disc 20 as illustrated in FIG. 2.

According to one aspect of the invention, the support ring 7 has a diameter slightly greater than the diameter of the rotor disc 20 so as to be positioned on the rotor disc 20 near the platforms 21 1 of the vanes 21 which are subsequently mounted. As illustrated in FIG. 5, the insertion of the blades 21 in the housings 200 of the rotor disc 20 advantageously ensures the radial and longitudinal locking of the support ring 7. The support ring 7 is in fact advantageously blocked. longitudinally downstream by the downstream rotor disc 20 ', radially outwards by the platforms 211 and radially inwards by the rotor disc 20. Thus, any relative movement between the damping device 1 and the pressure disc. downstream rotor 20 'is omitted, which eliminates the risk of wear. The support ring 7 is preferably flat and has a low thickness in order to be able to be positioned in a practical manner on the downstream rotor disc 20 ', without increasing the bulk. Preferably, the support ring 7 comprises a downstream flat surface 7 A intended to be positioned against an upstream surface, preferably substantially radial, of the downstream rotor disc 20 '.

According to the invention and with reference to Figures 2 to 6, the damping members 8 project upstream from the support ring 7. More precisely, each damping member 8 is configured to extend under the platforms 211 of two adjacent blades 21 so as to exert a radial force directed outwards so as to damp the relative radial movements of the mounting feet 210 of said adjacent blades 21 in their housing 200. Preferably , each damping member 8 is in the form of a leaf spring configured to exert a radial force outwards. As illustrated in Figures 2 and 3, each damping member 8 extends cantilevered upstream.

The damping members 8 are distributed uniformly around the periphery of the support ring 7 so as to allow the damping of all the blades 20. Preferably, there are as many members damping 8 as blades 21 to allow uniform damping.

Preferably, with reference to Figure 4, two adjacent damping members 8 are spaced tangentially with respect to the X axis by an angular distance Ay which approximately corresponds to the width of a mounting foot 210 of a dawn. In other words, each damping member 8 is configured to be located in a cavity formed between two mounting feet 210 of adjacent vanes 21.

Still with reference to Figure 4, each damping member 8 has a length Lx between 50mm and 60mm, a width Ly between 20mm and 25mm and a radial thickness between 0.5mm and 1mm. The length Lx is determined to allow optimal support of the platforms 211 of the blades 21, in particular, over more than 50% of their length. The length Ly is determined as a function of the tangential spacing between two adjacent mounting feet 21 1. In this example, the length Ly is substantially equal to the spacing between two mounting feet 210 of adjacent vanes 21 so as to constrain simultaneously the platforms 211 of the two adjacent blades 21. However, it goes without saying that a damping member 8 could extend only under a single platform 211 of the blade 21.

Preferably, the radial thickness is small compared to the other dimensions to reduce the mass of the damping device 1 while having a sufficient thickness to allow sufficient mechanical strength and a spring effect.

Thanks to the invention, during operation of the turbomachine T, the relative movements existing between the mounting feet 210 of the blades 21 and their housings 200 are greatly reduced. In addition, each damping member 8 is not capable of moving during operation of the turbomachine T given that it is integral with the support ring 7 which is itself blocked by the rotor discs 20. , 20 'and the platforms 211 of the blades 21. A damping member 8 is therefore not likely to increase the wear of the mounting feet 210 or that of the downstream rotor disc 20'.

According to the invention, the thickness of the damping member 8 is not constant over its length so as to allow optimum damping. As illustrated in Figures 5 and 6, each damping member 8 comprises a proximal portion 80, having a proximal thickness E80, connected to the support ring 7, a distal portion 82, having a distal thickness E82, freely mounted and a central portion 81, having a central thickness E81, formed between the proximal portion 80 and the distal portion 82. According to one aspect of the invention, the proximal thickness E80 is smaller than the central thickness E81, preferably at least. minus 50%, so as to make it possible to offer flexibility to the central portion 81 which can thus pivot because of its flexibility in the manner of a leaf spring. In other words, the proximal portion 80 forms a hinge which makes it possible to improve the damping. The thickness E80 of the proximal portion 80 remains sufficiently large to ensure mechanical strength.

In this example, the distal thickness E82 is greater than the central thickness E81, preferably by at least 100%. Advantageously, the distal portion 82 has an extra thickness to increase its mass, which increases the clamping force at the level of the distal portion 82 under the effect of centrifugal forces and of the leverage effect due to the door mounting. -only. Thanks to this feature, the platform 21 1 of each blade 21 is optimally pressed even if the length of a damping member 8 is shorter than the length of a platform 210.

In this example, still with reference to FIG. 6, the damping member 8 comprises a flat upper face 8A and a lower face 8B comprising a protuberance 83, or bead, corresponding to the extra thickness. Thus, the extra thickness only affects the lower face 8B and in no way the upper face 8A intended to be in contact with the platforms 211 of the vanes 21. Advantageously, the protuberance 83 can advantageously be machined in order to balance the mass of the blade. rotor disc 20 equipped with the damping device 1, that is to say, to correct unbalances linked to dynamic damping.

Such a damping device 1 can be obtained at a reduced cost. With reference to FIG. 7, an exemplary implementation of a manufacturing method successively comprises a cutting step E1, a stamping step E2 and a folding step E3. During the cutting step E1, the plane contours of the damping device 1 are cut, by means of a cutting member, from a thin metal panel so as to form the support ring 7 and the members d 'damping 8, the damping members 8 extending radially relative to the axis of the support ring 7. Following the cutting step E1, the damping members 8 are stamped so as to give them relief and calibrated thicknesses. Then, following the stamping step E2, each stamped damping member 8 is folded so as to be oriented in the upstream axial direction during the folding step E3. In this example, the damping members 8 are bent by approximately 90 °.

The damping device 1 is mounted radially outside the rotor disc 20, the blades 21 of which are to be damped. With reference to FIG. 5, the downstream face 7A of the support ring 7 is pressed against an upstream face of the rotor disc 20 'positioned downstream of the rotor disc 20, the blades 21 of which must be damped. The damping members 8 are positioned circumferentially between the housings 200 so as not to interfere with the assembly of the vanes 21.

Following the positioning of the damping device 1, the damping members 8 project upstream and make it possible to participate in the guiding of the platforms 211 during the insertion of the blades 21 in the housings 200 of the rotor disk 20. When mounting the vanes 21, the distal portion 82 of the damping members 8 is forced radially inwards by taking advantage of the proximal portion 80 which is flexible. Thus, due to this pre-stress, each damping member 8 acts as a leaf spring to press the platforms 211 of the vanes 21 outwardly.

Following the positioning of the vanes 21, each damping member 8 extends tangentially between two adjacent mounting feet 210 and radially between the outer periphery of the rotor disc 20 and the platforms 21 1 of the adjacent vanes 21.

Following the positioning of the blades 21, the support ring 7 is blocked on the one hand longitudinally downstream by the downstream rotor disc 20 'and, on the other hand, radially between the platforms 21 1 and the outer surface of rotor disc 20.

Preferably, a damping device 1 according to the invention can be used in an existing turbomachine instead of existing dampers. According to a preferred aspect, the mounting foot 210 of the blades 21 can be machined, for example by forming a recess, in order to allow the blade 21 to rest on the support ring 7.

During operation of the turbomachine T, the rotor disc 20 of the compressor 3 of the turbomachine T is rotated along the longitudinal axis X. By centrifugal effect, each mounting foot 210 of the blade 21 is moved radially towards the outer part of its housing 200. Similarly, each damping member 8 is driven in rotation by the rotor disc 20. By centrifugal effect, each damping member 8, which extends under two platforms 211, exerts a radial force towards the outside, which makes it possible to achieve an outward plating which is optimal.

When the speed of the turbomachine T drops, the speed of rotation of the rotor 2 decreases, resulting in a decrease in the centrifugal effect developed. A mounting foot 210 of a blade 21 is then capable of moving in its housing 200. Thanks to the damping device 1 according to the invention, the platforms of the blades 21 are kept pressed radially outwards even when the forces centrifuges are reduced. Thanks to the spring effect of the damping members 8, any radial variation is dynamically compensated.

The extra thickness of the distal portion 82 of each damping member 8 makes it possible to achieve effective plating under the effect of centrifugal forces, in particular, at mid-length of the platforms 21 1.

Thanks to the invention, the phenomena of wear at the level of the blades 21 and of the rotor disc 20 are greatly reduced. Furthermore, advantageously, a damping member 8 is not capable of increasing the wear of the rotor disc 20 'located downstream of the rotor disc 2 because it is integral with the support ring 7 to it. even stably positioned against the downstream rotor disc 20 '. The service life of the downstream rotor disc 20 ', the cost of which is high, is thus preserved.

CLAIMS

1. A compressor rotor (2) for an aircraft turbomachine (T) extending longitudinally along an X axis, said rotor (2) comprising a rotor disc (20) extending transversely with respect to the longitudinal axis X and a plurality of vanes (21), said rotor disk (20) comprising a plurality of housings (200) formed at the outer periphery of the rotor disk (20), each vane (21) extending radially with respect to the longitudinal axis X and comprising a mounting foot (210), radially inner, configured to be mounted in one of said housings (200), a platform (211) and a radially outer air deflection blade (212), the rotor (2) comprising a damping device (1) which comprises:

• a support ring (7) which extends transversely to the longitudinal axis X and which is positioned at the outer periphery of the rotor disc (20) and

• a plurality of damping members (8), integral with the support ring (7) and projecting upstream from the support ring (7), each damping member (8) ) extending under at least one platform (21 1) of a blade (21) so as to exert a radial force outwardly so as to damp the radial displacement of said blade (21) during operation of the turbomachine (T).

2. A rotor according to claim 1 wherein the damping device (1) consists of a support ring (7) and a plurality of damping members (8).

3. Rotor according to one of claims 1 and 2, wherein each damping member (8) is mounted in a prestressed manner under at least one platform (21 1) of a blade (21) so as to exert a force. radial outwardly so as to damp the radial displacement of said blade (21) during operation of the turbomachine (T).

4. Rotor according to one of claims 1 to 3, wherein the damping members (8) are distributed uniformly around the circumference of the support ring (7).

5. Rotor according to one of claims 1 to 4, wherein at least one damping member (8) comprises a proximal portion (80) connected to the support ring (7), a distal portion (82) mounted free and a central portion (81) formed between the proximal portion (80) and the distal portion (82).

6. A rotor according to claim 5, wherein the proximal portion (80) has a thickness less than the thickness of the central portion (81).

7. Rotor according to one of claims 5 and 6, wherein the distal portion (82) has a thickness greater than the thickness of the central portion (81).

8. Rotor according to one of claims 1 to 7, wherein at least one damping member (8) comprises a flat outer face (8A) intended to come into contact with a platform (211), and an inner face. (8B), opposite the outer face (8A), comprising a protuberance, in particular, at its distal end.

9. Rotor (2) according to one of claims 1 to 8 wherein each damping member (8) extends under at least the platforms (211) of two blades (21).

10. Rotor (2) according to one of claims 1 to 9, comprising a downstream rotor disc (20 ') positioned downstream of the rotor disc (20) in which the blades (21), the ring are mounted. support (7) is positioned against the downstream rotor disc (20 ').

11. Rotor (2) according to one of claims 1 to 10 wherein each damping member (8) extends cantilevered upstream.

12. Rotor (2) according to one of claims 1 to 11 wherein each damping member (8) is in the form of a leaf spring configured to exert a radial force outwardly.