Abstract:
The rack is made essentially out of thermostructural composite material and comprises a baseplate ( 12 ), a partition ( 14 ) extending above the baseplate, and a plurality of support arms ( 20 ) fixed to the partition and extending substantially horizontally therefrom to their own free ends, so that parts to be treated (A) can be supported in cantilevered-out positions on said arms.

Description:
FIELD OF THE INVENTION 
     The invention relates to a rack or tooling for supporting parts in a heat treatment furnace. 
     A particular but non-exclusive field of application of the invention is that of tooling for supporting parts in a cementation furnace. 
     BACKGROUND OF THE INVENTION 
     In the above field, the tooling most commonly used is made of metal. It suffers from the following main drawbacks: 
     the tooling is itself subjected to cementation and rapidly becomes brittle, which can give rise to a large amount of disorder in a furnace; 
     it must be bulky in order to avoid deforming excessively under load, since such deformation can in turn cause the supported parts to become deformed, requiring them to be rectified subsequently and consequently losing thickness in the cemented layer; 
     tooling that is bulky makes gas exchange more difficult and decreases loading efficiency, i.e. reduces the working fraction of the volume which it occupies by the parts to be treated; 
     violent thermal shock can cause the metal to be deformed or to break; and 
     the inevitable variations in dimensions that are of thermal origin make it impossible for the operations of loading and unloading parts and of handling the tooling to be robotized because of the unacceptable lack of accuracy in positioning. 
     It is already known, in particular from document EP 0 518 746-A to use a thermostructural composite material instead of a metal when making the sole plates of heat treatment furnaces. A plurality of sole plates can be provided and spaced apart from one another by spacers likewise made out of thermostructural composite material. The composite material used is a carbon/carbon (C/C) composite material or a ceramic matrix composite (CMC) material. 
     Nevertheless, that known loading device is poorly adapted to achieving optimum loading, of the kind that can be desired when a relatively large number of identical parts are to be treated. In addition, that device does not lend itself to robotization of the operations of loading and unloading the parts. 
     OBJECT AND BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to remedy the above-mentioned drawbacks of prior art devices, and to this end the invention provides a rack made essentially out of thermostructural composite material and comprising: a baseplate; a partition extending upwards from the baseplate and comprising, for example, uprights with cross-members extending therebetween; and a plurality of support arms fixed to the partition and extending substantially horizontally therefrom to their ends which are free, the arms being disposed in substantially symmetrical manner on either side of the partition such that parts for treatment can be supported cantilevered out on said arms. 
     Because it is made of thermostructural composite material and because it has horizontal arms with free ends, the rack provides the positioning and accessibility accuracy required for robotizing the operations of loading and unloading the parts to be treated. Thermostructural composite materials such as C/C and CMC composites are characterized by their dimensional stability and by their bending strength, thus making it possible to load the parts in a cantilevered-out position. 
     In addition, such a rack can be made to be lightweight and open-structured, while providing a large amount of filling capacity. It is therefore easy to handle, provides great capacity for exchange with the parts to be treated, in particular during cementation or quenching operations, and presents high loading efficiency. 
     In addition, since the arms extend substantially symmetrically on both sides of the partition, loading can be balanced. 
     Furthermore, its structure is suitable for modular construction, making it easy from standard basic elements to adapt racks for parts of different dimensions and for different heat treatment installations. 
     According to a feature of the rack, pegs can be mounted on the support arms to mark locations for the parts to be treated. The parts can then be threaded or hooked onto the support arms if the parts have a through passage, or they can be suspended by resting on two adjacent arms. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the following description given by way of non-limiting indication and with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic perspective view of a first embodiment of a rack of the invention; 
     FIG. 2 is an exploded view showing some of the elements making up the FIG. 1 rack prior to being assembled together; and 
     FIG. 3 is a diagrammatic perspective view of a second embodiment of a rack of the invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the description below, reference is made to racks for metal parts for cementation. The invention is not limited to such an application and, more generally, covers carrying parts, whether made of metal or not, that are to be subjected to heat treatment. 
     The rack  10  shown in FIG. 1 is intended specifically for supporting annular parts A such as gears for gear boxes. Only a few parts A are shown in FIG.  1 . 
     The rack comprises (FIGS. 1 and 2) a support structure essentially formed by a baseplate  12 , a vertical partition  14  supported by the baseplate  12  in the middle thereof, lateral reinforcing gussets  16  and  18 , and horizontal support arms  20 . The central partition  14  comprises lateral uprights  140 ,  142  with horizontal cross-bars  144  extending between them. The support arms  20  are constituted by bars  22  whose central portions are supported by the cross-bars  144 . The bars  22  extend on either side of the partition  14  so that each forms two arms in alignment and of the same dimensions. At their ends remote from the partition  14 , the arms  20  are free. 
     In a variant, the horizontal support arms  20  could be screwed to the partition  14  on either side thereof. The arms are mounted substantially symmetrically about the mid-vertical plane of the partition. This means that the arms are of substantially the same dimensions and in the same number on both sides of the partition, but not necessarily aligned in pairs. 
     The above elements constituting the structure of the rack are made out of thermostructural composite material. 
     Suitable composite materials are carbon/carbon (C/C) composites and ceramic matrix composite (CMC) materials. C/C composites are obtained by making a fiber preform out of carbon fibers and densifying the preform by forming a carbon matrix in the pores thereof. The carbon matrix can be obtained by a liquid method, i.e. by impregnating the preform with a liquid composition (such as a resin) that is a carbon precursor, and by applying heat treatment to transform the precursor into carbon,-or by a gas method, i.e. chemical vapor infiltration. CMCs are obtained by making a fiber preform out of refractory fibers, e.g. carbon fibers or ceramic fibers, and densifying the preform to form a ceramic matrix within its pores. In well-known manner, the ceramic matrix, e.g. of silicon carbide (SiC) can be obtained by a liquid method or by chemical vapor infiltration. 
     An advantage of thermostructural composite materials lies in their excellent mechanical properties, in particular their bending strength. 
     Consequently, it is possible to support the annular parts A by threading them onto the arms  20  from the free ends thereof, with each part A resting in a cantileveredout position, and without causing the arms to bend. is Advantageously, the load as a whole is kept in balance by distributing the parts equally on both sides of the partition  14 . 
     Another advantage of thermostructural composite materials lies in their great dimensional stability, even when exposed to large variations of temperature. This makes it possible for the support arms  20  to conserve practically invariable position references and thus to have the precision required for robotizing loading and unloading operations. The way in which the parts A are supported on the arms  20  has the further benefit of making such robotization easy. 
     Making the rack with arms  20  that extend on either side of the partition  14  in substantially symmetrical manner thereabout also makes it possible to perform loading and unloading simultaneously and symmetrically on both sides of the partition. This leads to a significant saving of time when performing such operations. 
     It will be observed that the parts A can be placed on the arms  20  side by side or in predetermined locations, with such locations being marked, for example, by notches formed in the arms. 
     As can be seen more particularly in FIG. 2, the uprights  140 ,  142  have end portions  140   a ,  142   a  which engage in corresponding housings  12   a ,  12   b  formed in the baseplate  12 , while the cross-bars  144  have end portions  144   a ,  144   b  which engage in housings such as  142   c  formed in the uprights  140 ,  142 . Such housings  142   c  can be provided at regular intervals along the uprights  140 ,  142  so as to enable the cross-bars  144  to be mounted at a determined pitch as a function of the size of the parts A in the vertical direction. The gussets  16 ,  18  have tenons  16   a ,  18   a  along their bottom edges which are engaged in corresponding housings  12   c ,  12   d  formed in the baseplate  12 . The uprights  140 ,  142  engage the gussets  16 ,  18  via setbacks  140   d ,  142   d  formed in their outside edges. 
     Each bar  22  has a notch  22   a  in its central portion for co-operating with the notch  144   c  formed in a crossbar  144  so as to engage the bar on the cross-bar. Each cross-bar has notches  144   c  distributed along its length so as to enable the bars  22  to be mounted on a given cross-bar at a pitch which is determined by the size of the parts A in a horizontal direction. 
     The modular nature of the rack can be extended by making each upright  140 ,  142  not as a single piece, but as a plurality of pieces that are assembled end to end. 
     In a variant, the uprights  140 ,  142  and the crossbars  144  of the partition  14  can be made as a single piece, e.g. by machining a plate of thermostructural composite material. 
     FIG. 1 shows that the rack possesses very great filling capacity while nevertheless presenting a structure that is lightweight and open, and holes can be formed in the structural elements such as the baseplate  12  and the gussets  16 ,  18 . It is thus easy to handle a complete rack. Furthermore, when the heat treatment includes allowing a gas to diffuse in contact with the parts, gas exchange with the parts is facilitated. 
     The rack of FIG. 3 differs from that of FIG. 1 in that it is designed more particularly for supporting parts that are solid and elongate, such as shafts B which are disposed vertically (in FIG. 3, the parts B are shown on one side only of the rack). In addition, the locations for the parts B are marked by pegs  26  on which the parts rest. 
     The rack is built in identical manner to that shown in FIG. 1, with the baseplate  12  supporting the central partition  14  on which the bars  22  that form the horizontal arms  20  with free ends are mounted. The number of cross-bars  144  in the central partition, between the uprights  140 ,  142 , and the spacing between the cross-bars are determined as a function of the vertical size of the parts B. The spacing between the arms  20  is determined as a function of the horizontal size of the parts B. 
     It will be observed that each part B rests via a shoulder on two pegs  26  carried by adjacent arms  20  at the same locations along said arms, each part being inserted for loading purposes in the gap between two arms. The pegs  26  are distributed along each arm at a spacing that is a function of the horizontal size of the parts B in the direction parallel to the arms  20 . 
     The pegs  26  can be made out of a thermostructural composite material, e.g. the same material as the other elements of the rack, or they can be made of a refractory metal material. The pegs  26  can be in the form of clips that are merely placed with a small amount of force on the arms  20 , with no adhesive being required. 
     Although FIGS. 1 and 3 show racks each supporting parts that are all identical, it is naturally possible to put parts of different shapes on a single rack.