Abrading – Abrading process – Utilizing fluent abradant
Reexamination Certificate
2000-11-17
2002-01-08
Morgan, Eileen P. (Department: 3723)
Abrading
Abrading process
Utilizing fluent abradant
C451S165000, C451S910000, C451S106000, C451S113000
Reexamination Certificate
active
06336844
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of so-called “ultrasonic” peening employing a mist of microbeads inside a chamber and, more particularly, relates to a method for peening parts on a wheel, such as the aerofoils of turbo machine blades on a rotor. The invention also relates to a peening machine essential in implementing the present method.
2. Summary of the Prior Art
The term “wheel” is to be understood as meaning an object with an overall shape that is axis symmetric about a geometric axis, it being possible for this object to be rotated about this axis.
To improve the fatigue strength of mechanical parts, it is known practice for their surface to be peened by blasting with microbeads. This technique is widely used in aeronautics. Peening involves impacting the surface of the part at a low angle of incidence with respect to the perpendicular to this surface. Thus, with sufficient kinetic energy, the microbeads cause permanent surface compression over a shallow depth. This surfaces compression inhibits the initiation and propagation of cracks on the surface of the part, which improves its fatigue strength. Typically, this angle of incidence has to be smaller than 30° for the impacts to be able to transmit sufficient energy from the bead to the impacted surface. The exposure of the part to peening passes through an optimum level which gives this part the best strength. Insufficient peening does not give the anticipated strength, but it is still possible to achieve the optimum by performing additional peening. By contrast, excessive peening causes surface damage to the part with a drop in its strength. This damage cannot be recovered and the part has to be scrapped.
Peening is customarily performed using nozzles supplied both with compressed gas and with microbeads. The compressed gas propels the microbeads towards the parts. This method of peening has two disadvantages per se:
the peening parameters are unstable and the peening machine has to be checked and adjusted frequently in order to achieve peening that is close to the optimum level,
the peening method has to be performed inside a booth which is large enough to allow the parts and the peening nozzles to be handled.
It is known practice in the aerospace industry for the flanks of the blades of aircraft turbine engine rotors to be peened. When the blades are manufactured separately, they each comprise a thin aerofoil and a root for holding the blade. To peen the thin aerofoil, the blade is held by the root and peening is performed with two nozzles pointing towards one another on each side of the acrofoil. One of the nozzles peens one Hank of the acrofoil and the other nozzle peens the other flank of the acrofoil. The two nozzles sweep across the flanks of the aerofoils and are moved with the best possible synchronisation so that peening progresses symmetrically.
When this symmetry is not achieved, stress peaks appear under the most peened flank. These stress peaks reduce the ability of the blade to withstand the loadings and lead to blade deformation. Peening symmetry is tricky to achieve and to maintain because of the spread and drift in the peening parameters inherent to this nozzle peening method. It will be understood that close to optimum blade peening is a lengthy and expensive operation because it has to be performed blade by blade with great precision.
Attempts have also been made at peening the blades of bladed wheels directly in the manner described above. These wheel and blade assemblies are made as a single piece, the acrofoils of the blades projecting from the wheel. Peening has to be achieved on the flanks of the aerofoils and the surfaces of the wheel known as the “tinter-blade space”, ie the surfaces located between two side-by-side aerofoils and possibly around these aerofoils. The blades can be mounted on the wheel or alternatively the aerofoils may be integral with the wheel.
Peening on bladed wheels is performed blade by blade as follows:
the two flanks of each blade are peened in synchronism using two deflected-jet nozzles entering the inter-blade spaces, that is to say the spaces located between two side-by-side aerofoils, said nozzles each comprising a reflector for deflecting the stream of microbeads through 90° and directing it normally onto the flanks of the blades.
then using a direct-jet nozzle to peen the inter-blade surface of the wheel.
One disadvantage with the above is the inevitable overlap of the peening of the flanks of the aerofoils and the peening of the inter-blade surface in the transition zone between said flanks and said inter-blade surface. It will therefore be understood that this transition zone is peened twice.
One major disadvantage of the above is that it is impossible to use the method when the inter-blade space is too narrow for the peening nozzles to gain access, which is often the case with present-day bladed wheels.
International patent application WO 95/17994, particularly designating the United States of America, discloses an ultrasonic peening machine employing a titanium bowl. The bottom of this titanium bowl is vibrated by a sonotrode associated with a magnetostriction vibrator. The parts that are to be peened are suspended from a lid placed over the opening of the bowl. The entire bowl is vibrated and, along with the lid. constitutes a peening chamber inside which a mist of microbeads is thus sustained. This peening chamber does not allow the peening of thin parts, such as the aerofoils of bladed wheels, as the mist of microbeads is heterogeneous, not least because the distribution of vibrational energy is highly complex and has antinodes and nodes. In addition, a very large titanium bowl is needed to form a peening chamber capable of peening a complete bladed wheel. Such a bowl is very expensive and requires a powerful vibration generator.
In French patent 2 689 431, a method for peening the teeth of a pinion is disclosed. The pinion is rotated past a sonotrode, the teeth of the pinion passing the sonotrode in turn. The sonotrode is surrounded by a curtain of spring-loaded rods, which come into contact with the teeth and with the pinion to form a sealed chamber around the sonotrode. The deformable edges of the chamber follow the profile of the teeth and of the pinion. The method of FR 2 689 431 cannot be applied to a bladed wheel because:
the peening of the flanks of the aerofoils would be highly asymmetric,
the rods would not automatically be able to follow the flanks of the aerofoils which are too closely spaced and close to the radial position.
In addition, optimum peening is difficult to achieve because peening would have to be halted precisely in order to treat the entire periphery of the wheel without subjecting that part of the wheel which was exposed to the peening first to any additional peening.
A first problem to be solved is that of peening the flanks of the aerofoils of a bladed wheel when said flanks are too close together for peening nozzles to gain access.
A second problem is that of peening the flanks of the aerotbils and the inter-blade surfaces of the wheel without peening the transition zones between said flanks and said inter-blade surfaces twice.
A third problem is that of speeding up the peening of the aerofoils and of the inter-blade surfaces of a bladed wheel.
A fourth problem is that of improving the symmetry of the peening on the opposite flanks of the aerofoils.
SUMMARY OF The INVENTION
The invention proposes a method for ultrasonic peening of parts on a wheel comprising an annular surface centered on the geometric axis of rotation of the wheel, the method including the steps of:
a) providing the parts on said annular surface of the wheel aligned on a geometric circle centered on the geometric axis of rotation so that the parts of the wheel define a geometric surface of revolution,
b) providing at least three chambers with respective openings having lateral edges,
c) providing said wheel simultaneously above the openings of said at least three chambers so that at least some of the sa
Duquenne Catherine Dominique Béatrice
Giffard Véronique Christiane Raymonde
Gueldry Gerard Michel Roland
Mons Claude Marcel
Morgan Eileen P.
Snecma Moteurs
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