Gear with integral overcouple protection

A gear including a pinion, a shaft and a disc is disclosed. The disc includes a cleavable element arranged to disengage the pinion from the shaft in case of an overcouple between the pinion and the shaft. A guiding module of the pinion is arranged to maintain the pinion in rotation in the support in case of a disengagement of the pinion from the shaft. In a turbo-machine, a gear box including such a gear allows the driving of auxiliary machines in case of a breakdown of a shaft in the gear box.

The present invention relates to the gear field, particularly to the gears present in gearing boxes for driving auxiliary machines in a turbo-machine.

The gears are used in all branches of mechanics to transmit movements, from clocks to heavy industry reducers. A gear is composed of two toothed pinions, respectively mounted on rotatable shafts. A driving shaft rotatably drives a pinion, said pinion being integrally mounted with the shaft. The teeth of the driving pinion are engaged with the driven pinion teeth in order to transmit the rotary movement from the driving shaft to the driven shaft. When more than two toothed pinions are engaged, this is referred to as a gear train.

A gear train comprises a plurality of serial gears forming a kinematics chain. A motor shaft mounted upstream from the kinematics chain rotatably drives a pinion with which it is integrally arranged. The pinions of the gear train, placed downstream from the motor shaft in the kinematics chain, are rotatably driven step by step by the rotation of the motor shaft.

The pinions respectively situated upstream and downstream in the kinematics chain with respect to a given pinion, will be referred to as upstream and downstream pinions herein below, the upstream pinion transmitting the rotary movement to the given pinion, the downstream pinion being rotatably driven by the given pinion.

A gear train can advantageously rotatably drive a plurality of shafts from a motor shaft, each driven shaft driving respectively one machine. The invention particularly relates to a gear train present in turbojets in order to drive auxiliary machines.

In a serial mounting of the gears, if a driven shaft is stuck or immobilized, the pinion integral with the shaft cannot rotate. All the pinions of the kinematics chain are likely to be blocked. In order to let the gear train go on working in such hypothesis, it is known to provide fusible means.

There are in the prior art gears formed with two pinions mounted on shafts provided with rectangular wedges fitted with the shaft. The pinions respectively comprise rectangular notches at their internal ring corresponding to the shape of the wedges. In operation, the pinions are mounted on the shafts, the notches maintaining the wedges in order to make the pinion integral with the shaft.

If one of the shafts of the gear train is stuck, the pinion the shaft of which is immobilized, suffers from an overcouple generated by the upstream pinion. The overcouple has this effect that the wedge gives way, whereby the pinion is disengaged from its shaft. The upstream pinion rotatably drives the disengaged pinion around the shaft. The kinematics chain is maintained, but for a period that can be quite short, since the pinion tends to be out of true and to move transversally and radially, this being able to go sometimes up to the gear stop, the auxiliary machines mounted downstream being then no more supplied. The loss of couple has an incidence on the gears and particularly on the machines mounted on these gears.

One object of the invention is to allow a pinion, the shaft of which is immobilized, to transmit the rotary movement to a downstream pinion in order to maintain the transmission yield per gear.

For this purpose, the Applicant provides a gear comprising a pinion being integral with a rotatably mounted shaft in a support through a cleavable element arranged to disengage the pinion from the shaft in case of an overcouple between the pinion and the shaft, characterized by the fact that a pinion guiding module, being pending, is arranged to hold the pinion in rotation in the support in case of a disengagement of the pinion from the shaft.

Advantageously, if a shaft being integral with an auxiliary pinion from a gear train is stuck or immobilized, the pinion, being submitted to an important couple, is disengaged from the shaft by means of the cleavable element serving as a “fuse”. The guiding module, so far pending, is activated. This module allows for the disengaged pinion to be guided in its rotation inside the support. Despite the breakdown, the yield at the gear level is maintained.

Preferably, the cleavable element is removably mounted between the pinion and the shaft.

More preferably, the cleavable element is formed with one disc being hooped between the pinion and the shaft, the disc comprising a plurality of radial arms.

Still more preferably, the guiding module comprises guiding rollers the rotation axis of which is oriented perpendicularly or parallel to the rotation axis of the shaft.

The rollers allow advantageously for the axial and/or transversal movements of the disengaged pinion to be forced so as to guide its rotation in the support.

Still more preferably, the pinion comprises a surface portion cooperating with the rollers.

According to another aspect of the invention, a box of auxiliary gears comprises a plurality of gears, in which at least one of the gears is a gear according to the invention.

Preferably, the gear box drives auxiliary machines in a turbo-machine.

According to another aspect of the invention, a turbo-machine comprises such a gear box.

Referring toFIG. 3, in a preferred embodiment of the invention, a turbojet, non shown, comprises a gear box100. The gear box100serves to drive auxiliary machines mounted on shafts of a gear train supported in the box100. Such a box100is generally referred to as the letter symbol AGB for “Accessory Gear Box”.

Referring toFIG. 1, the gear train comprises herein three pinions10,20,30provided respectively with teeth11,21,31. The pinions10,20,30are arranged in series forming a kinematics chain. A motor shaft rotatably drives the pinion10. The rotary movement is transmitted to the pinion20by the teeth11engaged with the teeth21, the pinions10,20forming a first gear, the pinions20,30forming a second gear.

The pinion20is integrally mounted on a rotatable shaft201. So, when the pinion20is rotatably driven by the upstream pinion10, it drives the shaft201by itself.

The shaft201is supported by bearings71,72, disposed respectively on each side of the pinion20, to guide the rotation of the shaft201in the box100. The bearings71,72are shown herein as roller bearings.

The pinion20comprises a cylindrical ring28having an external surface281, an inner surface283and two end transversal surfaces282. The external surface281is comprised of three strips, a central strip and two side strips. The teeth21are disposed on the central strip, both side strips being tooth-free.

A disc40is mounted between the shaft201and the ring28and makes the shaft201integral with the pinion20, the disc being advantageously mounted by hooping. The disc40comprises two concentric rings44,45, the external ring44being linked to the inner ring45by radial arms42.

The external surface of the ring44of the disc40abuts against the inner surface283of the ring28, the inner surface of the ring45being integral with the shaft201.

The arms42of the disc40are formed in such a way that they have a “cleavable” portion422arranged to give in for a couple value, between the shaft201and the pinion20, superior to a determined threshold value.

Referring toFIG. 1, a guiding module50is arranged for guiding the pinion20in its rotary movement inside the box100when it is no more integral with the shaft. The module50is arranged, being pending, parallel to the pinion20.

The guiding module50comprises a cylindrical part59forming a support for guiding rollers51-58, the rollers being divided into two sets of four guiding rollers51-54,55-58disposed respectively on each transversal face of the part59.

The four rollers51-54are disposed on each transversal face of the nearest pinion, the rotation axes of the rollers51-54being oriented parallel to the shaft axis201. The rollers51-54comprise a track on which the external surface281of the ring28is guided when the disc40is no more integral with the shaft201.

The four rollers55-58are disposed on the transversal face opposite to the rollers51-54, the rotation axes of the rollers55-58being oriented perpendicular to the axis of the shaft201. The rollers55-58comprise a track on which the side surface282of the ring28is guided when the disc40is no more integral with the shaft.

Referring toFIG. 2, the box comprises a second guiding module60arranged to guide the pinion20in its rotary movement when it is disengaged. The module60is disposed, being pending, parallel to the pinion20inside the box100.

The guiding module60comprises a cylindrical part69forming a support for guiding rollers61-68, the rollers being divided into two sets of four guiding rollers61-64,65-68disposed respectively on each transversal face of the part69.

The four rollers61-64are disposed on each transversal face of the nearest pinion, the rotation axes of the rollers61-64being oriented parallel to the shaft axis201. The rollers61-54comprise a track on which the external surface283of the ring28is guided when the disc40is no more integral with the shaft201.

The four rollers65-68are disposed on the transversal face opposite to the rollers61-64, the rotation axes of the rollers65-68being oriented perpendicular to the axis of the shaft201. The rollers65-58comprise a track on which the side surface282of the ring28is guided when the disc40is no more integral with the shaft201.

Upon a normal operation of the gear train, the upstream pinion10rotatably drives the pinion20, the couple received being lower than the threshold value. The rotary movement is transmitted to the downstream pinion30by the pinion20. The guiding modules50,60are pending, that is inactive, and stay at distance from the pinion20.

If the shaft201is stuck, the rotary movement is disturbed. An overcouple, exceeding the threshold value, is created between the pinion20and the shaft201and makes the arms42give in. The “fusible” portion422of each arm42gives in to disengage the pinion20from the shaft201, the pinion20being rotatably driven and the stuck shaft201remaining stationary.

After disengagement of the pinion20, this one is not free to move axially and transversally. If the pinion20is driven by the upstream pinion10transversally, the external surface281of the ring28contacts at least one of the guiding rollers with a parallel axis51-54,61-64. The transversal movements of the pinion20are forced and the rotation of the pinion20with respect to the shaft201is guided inside the box100.

If the pinion20is driven by the upstream pinion10longitudinally, the side surfaces of the ring28contact at least one of the guiding rollers with a perpendicular axis55-58,65-68. The axial movements of the pinion20are forced and the rotation of the pinion20with respect to the shaft201is guided.

Both guiding modules50,60allow the axial and transversal movements of the pinion20to be forced and only the rotation of the pinion20around the shaft201to be authorized. Thus, the disengaged pinion does not contact the box100and is guided inside it. Despite the breakdown of the shaft201, the yield of the gear is optimized and the kinematics chain is preserved.

Referring toFIG. 3, the guiding modules50,60comprise as many rollers with perpendicular axes as rollers with parallel axes. However, the number of rollers having radial axes can be increased compared to the number of rollers having parallel axes, or inversely, as a function of the desired guiding.

A single guiding module50can be sufficient to guide the pinion20, however when two guiding modules50,60are associated on each side of the pinion20, the guiding is optimized.

The disc40, placed between the pinion20and the shaft201, is easily removable after breaking. Its exchange is fast, simple and cheap. When a shaft201is stuck, the energy provided by the overcouple is absorbed by the disc40and it is not necessary to replace the expensive parts such as the shaft201and the pinion20.

At the turbo-machine level, the invention allows for the gear box to drive the auxiliary machines despite the breakdown of a shaft of the gear train.