Improvements in multi-disc brakes

Abstract: A multi-disc brake suitable for an aircraft has a stack of rotor and stator discs in which the discs (5-9) of a set at one end of the stack each have a greater allowance for wear than the discs (1-4) of a set at the other end of the stack whereby the group of discs (1-4) are removed together from one end of the stack as a fully worn group at a brake servicing operation. An unworn group of discs is added to the other end of the stack. The provision of selective variation of wear allowance in the initial construction of the brake allows it to be lighter and more compact than conventional brakes of similar performance and expected disc wear life.

Description: This invention relates to multi-disc brakes, such as are used in aircraft brake systems. In recent years the rotor and stator discs for aircraft brakes increasingly have been made from carbon-carbon composite material, this material providing both the frictional surfaces of the discs and their structural integrity. The rotor discs are formed with slots at their outer peripheries to receive drive keys attached to the aircraft wheel, and the stator discs are correspondingly slotted at their inner peripheries to engage splines on a stationary torque tube. In order to provide allowance for wear of the braking surfaces, the initial thicknesses of the discs have to be greater than may be required by their structural function in transmitting torque from their respective keys or splines. Thus the axial length, and weight, of the disc stack is initially greater than the optimum. Further, in normal practice discs of a given type (i e rotors, single stators or double stators) are all worn concurrently from their original thickness to their fully worn thickness, which means that the initial size of a brake may be unnecessarily large. An example of a disc brake of this type is contained in GB-A-2161227. Another disc brake and procedure for operation is described in US-A-3480115. This provides that a group of rotor, or stator, discs is to comprise discs each of a different thickness such that the discs become fully worn at different occasions and are removed for replacement on an individual basis. One object of the present invention is to provide a multi-disc brake, such as one comprising carbon discs, which may be more economically operated, and a second object is to enable such a brake to be made lighter and more compact than conventional brakes of similar performance and the same expected disc wear life. The invention seeks also to provide an improved method of maintenance of a multi-disc brake. According to the invention there is provided a stack of interleaved rotor and stator discs disposed in an axially aligned relationship between a thrust device and a reaction member, said thrust device being operable to create axial brake applying pressure on the stack and wherein after replacement of a group of adjacent worn discs the wear surfaces of some adjacent discs of the stack are unworn, the wear surfaces of other adjacent discs are partly worn and the discs are arranged with only one disc of each group contacting a disc of the other group for generating friction therebetween. The present invention also provides for the maintenance of a multi-disc brake of the kind comprising a stack of interleaved rotor and stator discs provided between a thrust device and a reaction member in an arrangement in which friction is generated between rubbing faces of adjacent rotor and stator discs, a method comprising selecting and arranging the rotor and stator discs so that the wear allowance of a set of adjacent discs at one end of the stack is less than the wear allowance of a set of adjacent discs at the other end of the stack, removing a fully worn group of adjacent discs from said one end of the stack during a brake service operation following wear, axially relocating the remaining partially worn discs of the stack to a new position towards said one end of the stack, and adding an unworn group of adjacent discs to the other end of the stack. Preferably, the arrangement is such that upon replacement of worn discs the remaining discs in the stack are maintained in mutual relationship so that partly worn mating faces of adjacent discs remain in contact throughout their entire wear lives. The invention is based upon the concept that by increasing the wear allowance of the discs (e g progressively) from one end of the stack to the other it is possible to operate a schedule of maintenance in which certain discs may be moved progressively through the stack, sustaining wear at different positions, and becoming fully worn in a position at or adjacent the other end. In practice, the two end discs of an aircraft brake are single stators, i e they do not rotate about the axis with the wheel and are worn on one side face only, the other side face either being in contact with a hydraulic thrust or reaction member : for this reason, it is not normally practicable to replace a single stator disc, and the minimum group size is therefore two discs, a rotor disc and a stator disc. One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic axial cross-section showing part of an aircraft brake assembly in its initial unworn state comprising a piston assembly P, a reaction plate R, and a stack of interleaved stator and rotor discs 1-9; Figure 2 is a similar view to Figure 1 showing the brake in a worn condition prior to a service operation, and Figure 3 is a similar view to Figure 1 showing the brake re-assembled following the service operation with worn discs removed and new discs inserted. In the example illustrated in the drawings, carbon-carbon composite discs 1-9 as shown in Figure 1 are made with wear allowances as follows (where t is the normal wear allowance per wear face during a service period after which the disc position will be changed within the stack or the disc will be removed from the brake and where s, r, and d are the final thicknesses of the single stators, rotors, and double stators respectively which are greater than or equal to the thicknesses necessary to carry the torque loadings applied to the discs when the discs are in the fully worn state. Disc Original Disc Function Total Wear Allowance Required Thickness after First Wear Period 1 single stator 1 x t s 2 rotor 2 x t r 3 double stator 2 x t d 4 rotor 2 x t r 5 double stator 3 x t s + t 6 rotor 4 x t r + 2t 7 double stator 4 x t d + 2t 8 rotor 4 x t r + 2t 9 single stator 4 x t s + 3t The above example assumes that said wear allowance "t" is equal for rotors and stators. A brake which is initially assembled with disc wear allowances as indicated in the above table is intended to be run in service until all of the wear allowances of the group comprising discs 1-4 have been used (this will normally occur simultaneously for all of discs 1-4 as designed in accordance with the table). Discs 1-4 will then be removed as fully worn, and discs 5-9 moved along the axle to occupy the positions originally occupied by discs 1-5. New discs 10-13 are added to occupy the positions previously held by discs 6-9. The new discs 10-13 will have wear allowances similar to the original discs 6-9, thus re-establishing the initial condition of the brake. It will be noted that the special wear allowances for discs 1-5 apply only to the original brake fitted to the aircraft, subsequent disc replacements always, in this example, entail the insertion of a standard pack of four discs similar to the original discs 6-9. It should be noted that in order to maintain the torque-carrying capacity of discs 5, s + t should not be less than d. In the example described above the 'group' of discs for replacement comprises four discs : in other examples the number of discs for removal as a group may be different. In an alternative example involving a 7-disc brake group of three discs at each end will be provided with different wear allowances (considering the groups as a whole), and a group of four discs may be replaced at each service. The construction and maintenance schedule of the brake described above provides a light and compact brake having a good wear life between service intervals. A further important advantage of the brake in accordance with the invention is that where adjacent discs have been run initially with their wear surfaces in mutual engagement these discs may be retained in contact throughout their full service lives. The importance of this feature stems from the wear patterns which the mutually engaging surfaces develop during service : such patterns on each disc conform to those of the mating disc, and it is highly advantageous to retain them as a 'friction pair', giving better frictional characteristics than would be obtained by mating a worn surface with that of a new disc. (In the example described, the surfaces of discs 9 and 10 which make contact as shown in Figure 3 are both initially unworn surfaces.

Claims: A method of carrying out the maintenance of a multi-disc brake of the kind comprising a stack of interleaved rotor and stator discs provided between a thrust device and a reaction member in an arrangement in which friction is generated between rubbing faces of adjacent rotor and stator discs, characterised in that said method comprises selecting and arranging the rotor and stator discs (1-9) so that the wear allowance of a set of adjacent discs (1-4) at one end of the stack is less than the wear allowance of a set of adjacent discs (5-9) at the other end of the stack, removing a fully worn group of adjacent discs (1-4) from said one end of the stack during a brake service operation following wear, axially relocating the remaining partially worn discs (5-9) of the stack to a new position towards said one end of the stack, and adding an unworn group of adjacent discs (10-13) to the other end of the stack. A method according to claim 1 characterised in that when removing worn discs (1-4) the remaining discs (5-9) in the stack are maintained in a mutual relationship so that mating faces of adjacent partly-worn discs remain in their original mating relationship throughout their entire wear lives. A method according to claim 1 or claim 2 characterised in that the thicknesses of the respective discs (1-9) are selected such as to provide appropriate wear allowances in accordance with the positions to be occupied by the discs during their wear lives. A method according to claim 3 characterised in that the stack comprises nine discs and the wear allowances of the respective discs are such as to enable the brake discs to be replaced in groups of four discs. A method according to claim 3 characterised in that the stack comprises seven discs and the wear allowances of the respective discs are such as to enable the brake discs to be replaced in groups of four discs. A method according to any one of the preceding claims characterised in that an unworn wear surface of an end disc of the unworn group of discs (10-13) added to the stack is arranged to confront an unworn wear surface of an end disc of said partly worn group of discs (5-9). A method according to any one of the preceding claims characterised in that it comprises utilising discs (1-13) of a kind in which the friction surfaces of the discs and their structural integrity are provided by carbon-carbon composite material. A multi-disc brake comprising a stack of interleaved rotor and stator discs (5-13) disposed in an axially aligned relationship between a thrust device (P) and a reaction member (R), said thrust device being operable to create axial brake applying pressure on the stack, characterised in that after replacement of a group of adjacent worn discs (1-4) the wear surfaces of some adjacent discs (10-13) of the stack are unworn, wear surfaces of other adjacent discs (5-9) are partly worn and that the discs (5-13) are arranged with only one disc of each group contacting a disc of the other group for generating friction therebetween. A multi-disc brake according to claim 8 characterised in that the stack of discs is comprised by a first group (1-4) of rotor and stator discs and a second group (5-9) of rotor and stator discs with the two groups in an axially-aligned relationship, and the wear surfaces of discs within the first group being thicker than the wear surfaces of discs within the second group. A multi-disc brake according to claim 8 or claim 9 characterised in that a first group (5-9) of discs close to the thrust device have first thicknesses and a second group (1-4) of discs remote from the thrust device have second thicknesses which are less than the first thicknesses. A multi-disc brake according to claim 10 characterised in that the first and second thicknesses have a difference at least equal to a thickness of wear by which the second group of discs has to be worn until a next change of discs. A multi-disc brake according to any one of claims 8-11 characterised in that the discs are made from carbon-carbon composite material, said material providing the frictional surfaces of the discs and their structural integrity. A multi-disc brake according to any one of claims 8-12 characterised in that in each group of discs at least two discs of the group have different wear thicknesses. A multi-disc brake according to any one of claims 8-13 characterised in that in each group of discs at least two discs of the group have substantially the same wear thickness. A multi-disc brake according to any one of claims 8-13 characterised in that it comprises four rotor discs interleaved with five stator discs, four discs being of different thicknesses and said thicknesses being different from the thicknesses of each of the other discs of the brake. A multi-disc brake according to any one of claims 8-13 characterised in that it comprises three rotor discs interleaved with four stator discs wherein four discs are of substantially different thicknesses and said thicknesses being different from the thicknesses of each of the other discs of the brake.

Inventors: WARING ANTHONY JOHN, WELLS TREVOR CHARLES, WARING, ANTHONY JOHN, WELLS, TREVOR CHARLES
Applicants: DUNLOP LTD, DUNLOP LIMITED
Application year: 1989
Main classification: F16D55
IPC codes: F16D65, F16D55, F16D66