An aircraft brake unit usually comprises alternating stators coupled to a torsion tube and rotors coupled to the corresponding wheel, with a front lateral stator at one end adjacent to a braking ring fitted with pistons capable of travelling over a predetermined wear stroke, and with a rear lateral stator at the other end.
An objective of the manufacturers of such carbon disk brake units is to optimize the degree of wear on each individual disk while keeping axial size as small as possible, with a minimum wear stroke for the pistons of the braking ring. Over recent years, manufacturers of brake units have set about defining disk arrangements and ways of substituting disks that make it possible, after operation for a certain length of time, to reuse at least some of the disks so as to increase the operating lifetime of the disks used.
Documents U.S. Pat. No. 4,613,017 and U.S. Pat. No. 4,742,895 thus show one way of replacing some of the brake disks. Those documents teach that previously brake units had been used having a brake ring fitted with pistons capable of a long predetermined wear stroke, and that once said wear stroke had been reached, all of the rotors and stators making up the brake unit were replaced. Those patents teach that under such circumstances it is more advantageous to use rotors that are thicker than the stators, together with two end stators that are thinner than the central stators in the set of disks. Once the predetermined wear stroke has been reached, constituting 50% to 65% of the prior-art wear stroke, then the very thick rotors can have both of their worn faces re-machined and they can be conserved in a new set of replacement disks which then include new, thicker stators, so as to return the set of disks to substantially the same initial total length, and thus be in a position to use up a new wear stroke after reinitializing the piston assemblies. That teaching thus makes it possible to use the rotors twice over, after appropriate resurfacing, and to replace only the stators of the brake unit. Thus, by giving the disks of the brake unit several "lives", each individual disk is used to a considerable extent, while nevertheless retaining a wear stroke that is small.
That technique has the advantage of good equilibrium for the heat sink, but it suffers from the drawback of re-machining all of the contact faces when replacing the stators after a first wear stroke of the pistons, in order to avoid contact between a new face and a worn face on an adjacent disk, since that would give rise to expensive loss of carbon.
An analogous technique is described in document U.S. Pat. No. 5,551,534 in an arrangement that includes thick disks (at both ends and in the middle of the stack) and thin disks. At the end of a half-life, the thick disks change position within the stack to be reused as thin disks, and the thin disks are either discarded or else are associated with one another to build up new thick disks. This suffers from the same drawbacks as those mentioned for the preceding documents.
Another replacement technique is taught in document U.S. Pat. No. 5,323,880. In that technique, replacement is performed step by step by progressively shifting groups of worn disks rearwards, removing the rearmost disks that are the most worn, and putting into place a new group of thick disks at the front. These thick new disks thus move progressively rearwards from the front every time a substitution is performed after the pistons have used up a wear stroke, thereby enabling them to "live" a plurality of "lives" until they are very worn and at the rear, from which they are removed. An analogous technique is described in document CA-A-2 004 091.
That substitution technique with translation presents the advantage of a long period of use for the disks, while conserving paired faces for disks constituting a portion of a replacement group, thereby avoiding the need to machine off carbon and consequently avoiding the above-mentioned drawbacks of carbon losses. Nevertheless, such an arrangement is not always satisfactory from the point of view of thermal equilibrium for the heat sink in so far as the disks at the front are very thick while the disks at the rear are very thin. This effect is particularly marked in that the very thick disks are located in a zone that is less sensitive to heating because it is well ventilated, unlike the very thin disks disposed at the rear of the series of disks.
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