Abstract:
A wear indicating friction disc assembly includes at least two friction discs and may include a plurality of clamping assemblies positioned around a circumference of the friction discs to clamp at least two of the friction discs together. At least one of the clamping assemblies may include a wear indicating washer with a wear indicating portion extending into a portion of one at least one of the friction discs. A height of the wear indicating portion is set to engage the opposing friction member at a predetermined wear level of the friction disc assembly and to emit a detectable signal upon sliding engagement with the opposing friction member.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a wear indicating friction disc, and more particularly, to a wear indicator for a multiple friction disc aircraft clutch.  
         [0002]     Wear indicators, including audible signaling elements, are known for indicating/signaling brake/clutch disc wear. See for instance, Beemer (U.S. Pat. No. 4,049,084), which discloses the use of a secondary frictional material between a primary frictional material and a backing plate for a clutch or brake system. Upon sufficient wear of the primary material, the reaction member comes into contact with the secondary material and produces an audible sound indicating that replacement of the friction member is due.  FIGS. 9, 10  and  11  show specific embodiments where the secondary friction material  94 / 52  is carried in apertures in the primary material.  
         [0003]     Mann (U.S. Pat. No. 4,604,604) discloses the use of a vapor emitting wear indicator for use with a brake lining or clutch pad.  FIG. 3  shows an embodiment where a capsule  18  including a vapor emitting material is positioned in a sleeve  24  of the lining  14  and will emit a distinct scent and/or colored smoke upon being the lining/pad wearing to the point that the capsule comes into contact with the reactive member.  
         [0004]     Campbell (U.S. Pat. No. 5,238,093), Heidenreich (U.S. Pat. No. 5,295,909 and U.S. Pat. No. 5,020,650), Grosspietsch (U.S. Pat. No. 5,908,097), DeVlieg (U.S. Pat. No. 6,659,233), Flotow (U.S. Pat. No. 5,181,593), Müller (U.S. Pat. No. 6,206,158), Gochenour (U.S. Pat. No. 6,296,099), Despres (U.S. Pat. No. 6,357,570), and Ziegler (USPA 2003/0062237) all disclose brake/clutch wear indicators that provide a visual indication of wear of the brake/clutch lining.  
         [0005]     Drexl (U.S. Pat. No. 6,040,768) discloses a system for monitoring clutch wear status that uses speed and position information. Hayashi (U.S. Pat. No. 6,276,197) discloses a system that uses an electromechanical sensor  19  for measuring clutch wear. Wang (U.S. Pat. No. 5,712,805) discloses a noise diagnostic system for a vehicle.  
         [0006]     Certain Mercedes automobiles use electrical brake wear indicators that insert into the side of the disc brake pads. When the brake pads wear to the point of uncovering the indicator, an electrical path is made through the indicator to signal that the pad needs replacement.  
         [0007]     Carbon/carbon composite friction discs are used in a variety of aircraft for brake disc material. As the discs are used, they wear away. Each disc has a designated amount of wear material that can be used. However, an aircraft clutch using the same material often experiences unequal wear between the plates (discs). Therefore, while one plate may still be within acceptable wear limits, another plate may be beyond acceptable wear limits and should be replaced to prevent failure of the clutch plate and clutch and/or to maintain the torque handling capability of the clutch within acceptable limits.  
         [0008]     In such an aircraft clutch, there is a need to determine whether one or more clutch plates of the clutch have reached their acceptable wear limits and to signal that this has occurred. Detection of the unequal wear assures proper condition based maintenance while the clutch is still functioning normally.  
       SUMMARY OF THE INVENTION  
       [0009]     In certain aircraft clutch discs, the disc is assembled from two stacked carbon discs to provide a desired thickness for the disc assembly. Such split discs utilize rivets and washers to clamp the two carbon discs together. In the present invention, one or more of the clamping washers is modified to have a wear indicating portion that extends into the disc beneath a disc surface that engages the friction surface of a mating disc assembly in a sliding manner. When the disc surface wears to the depth of the wear indicating portion, it creates a wear signal in the form of a unique noise or vibration that can be detected audibly and/or by an aircraft Preventative Health Maintenance (PHM) system, which then reports the need for maintenance. The depth of the wear indicating portion below the friction surface is determined to emit the wear signal while the disc assembly is still within acceptable wear limits and to provide sufficient warning so that preventative maintenance can be performed in due course before the disc assembly wears beyond acceptable limits.  
         [0010]     By using such a modified clamping washer in place of a conventional clamping washer, no additional components are necessary to install the wear indicator on conventional clutch discs. This provides for a simple, reliable, light-weight and cost effective wear indicator.  
         [0011]     The material selected for the wear indicating portion/washer is preferably one that will not damage the carbon/carbon friction discs when they come into contact with the wear indicating portion/washer but will produce the noise/vibration as they wear away that will be detected by the PHM system. The present invention can be used with other types of friction discs and friction discs made of alternative materials.  
         [0012]     The invention is preferably applied to each clutch disc in a multi-disc aircraft clutch to signal when any one of the discs is approaching its wear limit. The number and circumferential spacing of the wear indicators can be varied from disc to disc to provide unique vibratory wear signals for each particular disc. This can then be used to specifically pinpoint the worn disc. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0013]      FIG. 1  is an axial side view of one embodiment of a clutch disc assembly of the present invention;  
         [0014]      FIG. 2  is a partial sectional view of the clutch disc assembly of  FIG. 1 , taken along section line  2 - 2  of  FIG. 1 ;  
         [0015]      FIG. 3  (Prior Art) is a side elevational view of a known clutch disc assembly clamping washer;  
         [0016]      FIG. 4  is a side elevational view of one embodiment of the wear indicating washer of the present invention;  
         [0017]      FIG. 5  is a side elevational view of a further embodiment of the wear indicating washer of the present invention; and  
         [0018]      FIG. 6  is a schematic view of a multiple friction disc clutch assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1  shows an axial side view of one embodiment of a clutch disc assembly  10  of the present invention. The clutch disc assembly  10  is constructed of two carbon fiber discs  14  and  16  (see  FIG. 2 ) that are clamped together by a plurality of clamp assemblies  18 . In a preferred embodiment, each disc  14  and  16  has an axial relief  12  at its outer circumferential edge to accommodate the clamping assemblies  18  without interfering with the clamping assemblies of adjacent disc assemblies  10  during operation. Although the number of clamp assemblies  18  shown in  FIG. 1  is  14 , this number can be changed to provide the desired clamping strength to the disc assembly  10 , as can the positioning of the clamp assemblies  18  around the circumference of the clutch disc assembly  10 . Other configurations of disc assemblies  10  can also use the present invention, as can disc assemblies constructed of other materials.  
         [0020]     Each clamp assembly  18  preferably includes two clamping washers  20  connected together by a rivet  22  which passes through bores  24  in the clamping washers  20  to create a clamping force on the two friction discs  14  and  16 .  FIG. 3  shows a side elevational view of a known clamping washer  20  for clamping together a clutch disc assembly  10 . Other fasteners, such as threaded fasteners, can be used in place of the rivet  22 , although the rivet  22  is preferred because of its simplicity, strength, low cost, and reliability. Preferably, the bore  24  of the clamping washer  20  has a countersunk portion  26  to accommodate the head of the rivet  22 , which reduces the height of the rivet  22 . Preferably, the rivet  22  will be made flush with an outer surface of the clamping washer  20  as shown to reduce possible interference between rivets  22  of adjacent disc assemblies  10 . Although the use of two opposing clamping washers  20  is presently preferred to help distribute the clamping forces, in an alternative embodiment, the head of a headed fastener can be substituted for one of the clamping washers  20 .  
         [0021]     Certain of the clamping washers  20  are provided with sideways extending wear indicating portions  30  to become wear indicating washers  28 . Here, the wear indicating portions extend radially inward but they can be configured for a particular application as desired. The wear indicating portions  30  are configured with the wear indicating washers  28 , so that the wear indicating portions  30  will be uncovered when the disc assembly  10  is near a wear limit, thereupon emitting a wear signal caused by the running contact between the adjacent disc assembly and the uncovered wear indicating portions  30 . This wear signal will be in the form of a vibration detectable by the PHM system. The wear signal can be sensed by electronic sensors such as accelerometers, noise sensors, and the like.  
         [0022]     Furthermore, the frequency range of the indicating portion  30  can be tuned to generate sound waves in the audible range for human ears. It is also contemplated that the wear signal can be generated by a proximity sensor or probe that can sense the distance of the wear indicating portion  30  from the wear surface of the friction disk  14 . This wear signal will cause a variation in the vibration that normally results from frictional sliding contact between adjacent disc assemblies when the clutch is engaged and allowed to slip. Of course, when the clutch is locked up, there will be no vibration signals resulting from disc to disc sliding unless the torque capacity of the clutch is surpassed. It is preferred that the wear indicating portions  30  be configured to be uncovered and begin emitting wear signals before the absolute wear limit of the assembly disc is reached. In this manner, a comfortable cushion of time can be provided to allow non-disruptive scheduling of the appropriate maintenance after the wear signals are first emitted.  
         [0023]     In one embodiment shown in  FIG. 4  and the upper portion of  FIG. 2 , the wear indicating portion  30  is the same height as the clamping washer  20 . In an embodiment shown in the lower half of  FIG. 2 , the wear indicating portion  30  is of a shorter height than the clamping washer  20 . This embodiment is of lighter weight than the above embodiment. In the embodiment shown in  FIG. 5 , the wear indicating portion  30  is taller than the clamping washer  20 . In this embodiment, the wear indicating portion  30  will be uncovered upon disc wear and begin signaling disc wear before the adjacent disc assembly  10  makes contact with the clamping washer  20 . In an alternative embodiment, each disc assembly can be provided with two or more sets of wear indicating washers  28 , with each set being uncovered at a different wear level of the disc assembly. In this manner, the progressive resulting wear signals can provide indication of progressive wear of the disc assembly.  
         [0024]     For instance, one set of wear indicating washers can be set at a level to be uncovered and begin emitting wear signals at 10% of the remaining allowable wear of the disc assembly  10 , while a second set of wear indicating washers can be set at a level to be uncovered and begin emitting wear signals at 5% or even 0% of the remaining allowable wear of the disc assembly  10 . The same progressive wear signaling can also be accomplished through the use of layered wear indicating washers  28 , where each layer is constructed of a different material to provide a different wear signal when uncovered. See  FIG. 4 , wherein possible upper and lower layers of the wear indicating portion  30  are indicated by the phantom line.  
         [0025]     The wear indicating portion  30  can have the same width as the clamping washer  20  or a different width. As shown in  FIG. 1 , the wear indicating portions  30  can be narrower than the clamping washers  20 . Although the wear indicating portions  30  and clamping washers  20  are shown as having generally flat, rectangular configurations, other configurations can also be used. Preferably, the wear indicating portion  30  and washer  20  are integral to one another on wear indicating washers  28 . In one embodiment, the wear indicating washers  28  are constructed of Monel®, although other materials can also be used.  
         [0026]     Such wear indicating portions  30  are accommodated in channels  32  formed in the discs  14  and  16 . The channels  32  are shown as being open only radially at the relief  12  of the outer circumference of the discs, but in an alternative embodiment, the channels  32  can be open axially outward as well. The channels  32  can be machined into the discs using known machining methods, can be cast into the discs during the manufacture of the discs or formed with other manufacturing methods as is known to those skilled in the art. The channels  32  are preferably configured to have the same shape as the wear indicating portions  30  so as to securely support the wear indicating portions  30  when they are uncovered and begin to be contacted by the adjacent moving disc assembly. In one embodiment, adhesive can be used during assembly of the disc assemblies to secure the wear indicating portions  30  in the channels  32 , although this may not be necessary if the washers  20  are securely clamped and the wear indicating portions  30  are securely supported in the channels  32  by the surrounding disc material. In an alternative embodiment, the wear indicating washers can be cast into the discs during disc manufacture.  
         [0027]     In the embodiment shown in  FIG. 1 , two wear indicating washers  28  are used on the shown side of the disc assembly  10 , positioned at an angle of about 133° to one another. As discussed above, this number and positioning of wear indicating washers  28  can be varied as desired. In a further aspect of the invention, each clutch disc assembly  10  of a multi-disc clutch can be provided with a different number and/or positioning of the wear indicating washers  28  so that each disc assembly  10  will produce a unique wear signal that can be differentiated by the PHM system to help identify the problem disc assembly  10  in a multi-disc clutch.  
         [0028]     That is, if one disc assembly  10  has two wear indicating washers positioned approximately 50° apart, it will produce a different wear signal (vibration or tone) than a disc assembly  10  that has two wear indicating washers positioned about 180° apart. Under one maintenance approach, the wear of one disc assembly  10  to the wear limits can be used as an indicator to replace all of the disc assemblies in the clutch, though this may not be required under an alternative maintenance approach. In one embodiment, the positioning of the wear indicating washers  28  on one side of a disc assembly will be generally circumferentially opposite the positioning of wear indicating washers on the other side of the disc assembly  10  to assist in maintaining the rotational balance of the disc assembly  10 .  
         [0029]     In an alternative embodiment, the wear indicating washers  28  need not clamp two friction discs together, but can clamp one friction disc to another clutch assembly component, such as an inner or outer disk hub, a disk carrier or other component. Alternatively, the wear indicating washers can be clamped only to a single disk.  
         [0030]     Although the present invention has been discussed above in reference to use in a multi-disc clutch of an aircraft, it can be used with other friction discs in other applications. Various aspects of the embodiments described above can also be combined in different manners to create new embodiments.