Patent Publication Number: US-2022219489-A1

Title: Wheel and brake assembly with mechanical stop

Description:
TECHNICAL FIELD 
     The present disclosure relates to brake systems of a vehicle. 
     BACKGROUND 
     Vehicles, such as aircrafts, may use a wheel brake system that includes a multi-disc brake system. For example, the multi-disc brake system may include a disc stack comprising plurality of rotor discs engaged with a wheel and a plurality of stator discs interleaved with the rotor discs. The rotor discs and wheel are configured to rotate around an axle, while the stator discs remain stationary. To decelerate rotational motion of a rotating wheel, the brake system may displace pistons against a pressure plate to compress the rotating rotor discs engaged with the wheel against the stationary stator discs, therefore producing torque that decelerates the rotational motion of the wheel. In some examples, the rotor discs may be engaged with the wheel via rotor drive keys positioned on an interior surface of the wheel. In some examples, stator discs may be engaged with a stationary torque tube surrounding the axle via splines positioned on the torque tube. In some such examples, the brake system may be configured to compress the rotor discs and the stator discs between the piston and a backing plate supported by the torque tube. 
     SUMMARY 
     The present disclosure describes example wheel assemblies configured to help limit a maximum displacement achievable between a piston housing and a brake disc stack when a brake system is utilized to reduce and/or substantially prevent a rotation of a wheel. The brake system is configured to compress a brake disc stack to reduce and/or limit rotational motion of the wheel about the wheel axis. For example, an actuator may be configured to compress the disc stack against a portion of a torque tube engaged with the disc stack. Under some circumstances, the compression force of the actuator may cause a linear deformation of the torque tube. The linear deformation may require additional piston travel to assure the compression force on the disc stack is maintained during certain operating conditions, such as a Rejected Take-Off (RTO) stop. The assembly disclosed here uses a mechanical stop configured to unload some portion of the compression force from the torque tube when the torque tube linearly deforms. The mechanical stop is configured to limit the maximum distance between the actuator housing and the disc stack at end of life, even during compression of the brake disc stack. 
     In some examples, an assembly comprises a wheel configured to rotate around a wheel axis; a brake system comprising: a disc stack comprising a rotor disc and a stator disc, wherein the rotor disc is rotationally coupled to the wheel, and wherein the wheel is configured to rotate relative to the stator disc; and an actuator defining an actuator housing and configured to compress the disc stack, wherein the brake system is configured to compress the disc stack by limiting the linear movement of the disc stack by exerting a reaction force on the disc stack when the actuator compresses the disc stack, and wherein a portion of the brake system is configured to linearly deform when the brake system exerts the reaction force; and a mechanical stop configured to reduce the reaction force exerted by the brake system when the linear deformation of the portion of the brake system causes the mechanical stop to encounter the brake system, such that the mechanical stop limits a linear displacement of the disc stack relative to the actuator housing. 
     In some examples, an assembly comprises: a wheel configured to rotate around a wheel axis; a brake system comprising: a disc stack; a torque tube configured to engage the disc stack; a backing plate attached to the torque tube, wherein the backing plate is configured to limit movement of the disc stack in a direction parallel to the wheel axis; and an actuator configured to compress the disc stack against the backing plate using a compression force, wherein the backing plate is configured to exert a reaction force on the disc stack when the actuator compresses the disc stack; and a mechanical stop configured to encounter the disc stack, wherein: the mechanical stop is configured to reduce the reaction force when the mechanical stop encounters the disc stack, the mechanical stop is configured to limit movement of the disc stack in the direction parallel to the wheel axis when the mechanical stop encounters the disc stack, such that the mechanical stop limits a linear displacement of the disc stack relative to the actuator; the actuator defines a first length between the actuator and the mechanical stop in the absence of the compression force, the actuator defines a second length between the actuator and the backing plate in the absence of the compression force, the first and second lengths being measured in a direction parallel to the wheel axis, and the first length is greater than the second length. 
     An example technique includes compressing a disc stack of a brake system using an actuator of the brake system, wherein the disc stack includes a rotor disc rotationally coupled to a wheel and a stator disc, wherein the wheel is configured to rotate around the stator disc, wherein the brake system exerts a reaction force on the disc stack when the actuator compresses the disc stack; and limiting the movement of the disc stack by reducing the reaction force exerted by the brake system by causing the brake system to encounter a mechanical stop using the compression of the disc stack by the actuator. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating an example wheel including a plurality of rotor drive keys on an interior surface of the wheel. 
         FIG. 2  is a schematic cross-sectional view illustrating an example wheel and brake system including the wheel of  FIG. 1 . 
         FIG. 3  is a plan view with selected cross-sections illustrating an example assembly including a mechanical stop. 
         FIG. 4  is an example perspective view of a disc stack. 
         FIG. 5  is a plan view with selected cross-sections illustrating the example assembly of  FIG. 3 . 
         FIG. 6  is a plan view with selected cross-sections illustrating displacements defined by an actuator. 
         FIG. 7  is a plan view with selected cross-sections illustrating an example assembly including a mechanical stop integral with a wheel. 
         FIG. 8  is a plan view with selected cross-sections illustrating an example assembly including a mechanical stop integral with a rotor drive key. 
         FIG. 9  is a plan view illustrating portions of the assembly relative to a wheel axis. 
         FIG. 10  is a flow diagram illustrating an example method of reducing a reaction force using a mechanical stop. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes articles, systems, and techniques relating to an assembly comprising a wheel and a brake system, and, in particular, an assembly including one or more structures configured to limit the axial displacement of the brake discs under certain conditions. The wheel is configured to rotate around a wheel axis. The brake system includes a disc stack which includes one or more rotor discs and one or more stator discs. For example, the disc stack may include a plurality of rotor discs interleaved with a plurality of stator discs. The rotor discs are rotationally coupled with the wheel, such that a rotation of the wheel around the wheel axis causes rotation of the rotor discs around the wheel axis. The stator discs are configured to remain substantially stationary relative to the wheel and the rotor discs. The brake system is configured to compress the disc stack to cause engagement of friction surfaces on the rotating rotor discs and the stationary stator discs, reducing a rotational speed of the rotor discs around the wheel axis. The rotor discs are configured to engage the wheel, such that the reduction in the rotational speed of the rotor discs causes a reduction in the speed of the wheel. 
     The brake system includes an actuator defining an actuator housing and configured to compress the disc stack to cause the slowing of the rotor discs and wheel. For example, the actuator may be configured to compress the disc stack against a backing plate supported by a torque tube, wherein the backing plate and the torque tube are configured to remain substantially rotationally stationary with respect to the stator discs. The braking system may be configured such that, when the actuator exerts a compression force to compress the disc stack against the backing plate (e.g., to slow the wheel), the backing plate transmits some portion of the compression force to the supporting torque tube. The portion of the compression force transmitted may cause a linear deformation of the torque tube (e.g., in a direction substantially parallel to the axis of the wheel), increasing a displacement between the actuator housing and the disc stack. 
     The assemblies disclosed herein are configured to help limit the displacement between the actuator housing and the brake disc stack when the actuator exerts the compression force. In examples, the assembly is configured to help limit the displacement when the compression force causes an excessive linear deformation of the torque tube. Hence, the assembly described herein may be configured to reduce and/or minimize additional piston travel that may be required to assure the compression force on the disc stack is maintained during certain operating conditions, such as a Rejected Take-Off (RTO) stop. The assembly may be configured to limit the maximum distance between the piston housing and the disc stack at end of life, even during compression of the brake disc stack. 
     The assemblies described herein include a mechanical stop configured to transfer load from the torque tube when the compression force of the actuator causes linear deformation of the torque tube. The mechanical stop may be configured to unload the torque tube in order to limit a displacement (e.g., a maximum distance) between the actuator housing and the brake disc stack. In some examples, the assembly is configured such that the actuator compresses the disc stack against the mechanical stop when the mechanical stop unloads the torque tube. For example, in some examples, the assembly is configured such that the brake system compresses the disc stack against a backing plate until the torque tube experiences a certain amount of linear deformation, at which point the mechanical stop encounters the disc stack to at least partially unload the backing plate and torque tube. In examples, the assembly is configured to reduce and/or substantially cease the linear deformation of the torque tube when the mechanical stop transfers load from the torque tube. In other examples, the mechanical stop is configured to limit or even prevent movement of the backing plate of the brake system in an axial direction (resulting from linear deformation of the torque tube), and the actuator continues to be configured to compress the disc stack against the backing plate even after the torque tube experiences a certain amount of linear deformation. 
     The assembly may allow a reduction in the amount of reserve actuator capacity (e.g., piston travel) typically provided for situations when the torque tube experiences larger linear deformations (e.g., in high heat environments following, for example, emergency braking events or other relatively high energy vehicle stopping conditions). The assembly may be configured to reduce a required stroke length of the actuator necessary to effectively engage the brake system (e.g., during a static hold of the vehicle by the brake system when the vehicle is parked) when the torque tube experiences the higher linear deformations. By reducing and/or substantially ceasing the linear deformation, the assembly may allow reducing the required stroke length of the actuator. This may result in some measure of space saving when the brake system is positioned within a wheel well of the wheel. Further, by reducing the required stroke, additional brake disc thickness may be added to unused spaces, increasing the amount of wearable brake disc thickness and potentially increasing the life of the brake assembly. 
     In some examples, the wheel supports the mechanical stop. In examples, the wheel supports the mechanical stop such that, when the actuator compresses the disc stack against the mechanical stop, the mechanical stop transmits a portion (e.g., substantially all) of the compression force to the wheel. Transmission of the compression force to the wheel reduces (or substantially eliminates) the amount of the compression force acting on the torque tube, reducing (or substantially eliminating) any further linear deformation of the torque tube and associated brake disc stack translation. In examples, the mechanical stop is configured to limit a maximum displacement which may occur between an actuator housing and the brake disc stack when the mechanical stop encounters the disc stack. 
     The assembly may be configured such that a linear deformation of the torque tube (e.g., by exertion of the compression force by the actuator) greater than or equal to a threshold linear deformation causes the mechanical stop to encounter the disc stack. The assembly may be configured such a first linear deformation of the torque tube maintains a displacement between the mechanical stop and the disc stack, and a second linear deformation of the torque tube greater than the first linear deformation causes the mechanical stop to contact the disc stack. The first linear deformation and the second linear deformation may each be a deformation in a direction substantially parallel to the axis of the wheel. In examples, the torque tube is configured such that the actuator causes the first linear deformation under a first heat load and causes the second linear deformation under a second heat load greater than the first heat load. The heat load may be defined by, for example, a temperature at one or more points of the torque tube, a temperature of another component in the wheel and brake system, a temperature within the wheel well of the wheel, or some other suitable parameter. In examples, the torque tube may have a configuration (e.g., may comprise a material and/or a geometry) causing the torque tube to experience a greater degree of linear deformation under a given heat load than other components of the brake system and/or wheel. For example, the torque tube may be relatively thin compared to a rotor drive key or other component of the brake system, which may result in relatively higher temperatures of the torque tube for a given thermal environment (e.g., the thermal environment following an emergency stop.) 
     Hence, the assembly may be configured such that the mechanical stop encounters the disc stack depending on a state of the torque tube. For example, the assembly may be configured such that, at relatively low heat loads, the compression force of the actuator is insufficient to generate a linear deformation of the torque tube necessary to cause the mechanical stop to encounter the disc stack (i.e., the linear deformation is less than the second linear deformation described above). Thus, the assembly may be configured to effectively operate under certain conditions wherein braking of the wheel is accomplished without transferring compression load from the torque tube to the mechanical stop. The assembly may be configured such that, at sufficiently high heat loads (e.g., following an emergency stop), the compression force of the actuator may generate a linear deformation of the torque tube sufficient to cause the mechanical stop to encounter the disc stack (i.e., the second linear deformation). 
     In examples, the actuator defines a first length between a portion of the actuator and the mechanical stop in the absence of the compression force. The actuator may define a second length between the portion of the actuator and a backing plate in the absence of the compression force. The first length may be greater than the second length. The first and second lengths may be measured in parallel directions, e.g., along a wheel axis of the wheel to which the brake assembly is attached. The portion of the actuator may be, for example, a contact area through which the actuator is configured to exert the compression force on the disc stack. 
     In some examples, the mechanical stop is rotationally coupled to the wheel. The mechanical stop may be configured to rotate with the wheel and configured to encounter a portion of the brake system which also rotates with the wheel (e.g., a rotor disc) in order to limit and/or avoid contact between rotating and relatively stationary portions of the system. For example, the mechanical stop may be configured to rotate substantially synchronously around the wheel axis when the wheel rotates around the wheel axis. In some examples, a pin engaged with the wheel defines the mechanical stop. In some examples, an interior surface of the wheel includes a ledge defining the mechanical stop. In yet other examples, a rotor drive key attached to the wheel and configured to engage a rotor disc defines the mechanical stop. The mechanical stop may be configured to encounter some portion of the disc stack rotationally coupled with the wheel. The mechanical stop may be configured to encounter a rotor disc or a rotor disc component (e.g., a drive insert) configured to rotate synchronously with the rotor disc. In examples, when the actuator is configured to compress the disc stack against the backing plate, the mechanical stop is configured to encounter a rotor disc adjacent to or nearest the backing plate. 
       FIG. 1  is a perspective view illustrating an example wheel  10 . In some examples, wheel  10  is a part of an aircraft vehicle. In other examples, wheel  10  may be a part of any other vehicle, such as, for example, any land vehicle or other vehicle. In the example shown in  FIG. 1 , wheel  10  includes a wheel rim  12  defining an exterior surface  14  and interior surface  16 . Wheel rim  12  includes tubewell  18  and wheel hub  19 . In some examples, interior surface  16  may include an inner diameter of tubewell  18  of wheel  10 . For example, in some cases, interior surface  16  may be referred to as an inner diameter surface of wheel  10 . Interior surface  16  and wheel hub  19  may define a wheel well  17  (e.g., a volume) between interior surface  16  and wheel hub  19 . In some examples, a tire (not shown) may be mounted on exterior surface  14  of rim  12 . Wheel  10  may include an inboard bead seat  20  and an outboard bead seat  21  configured to retain a tire on exterior surface  14  of rim  12 . In examples, wheel  10  may comprise an inboard section  22  (e.g., including inboard bead seat  20 ) and an outboard section  23  (e.g., including outboard bead seat  21 ). Wheel  10  is configured to rotate around the axis of rotation A. 
     Wheel  10  includes a plurality of rotor drive keys  24  on interior surface  16  of wheel  10 , such as rotor drive key  25  and rotor drive key  15 . In some examples, each rotor drive key of the plurality of rotor drive keys  24  extends in a substantially axial direction of wheel  10  (e.g., in a direction parallel to the axis of rotation A). The plurality of rotor drive keys  24  (“rotor drive keys  24 ”) and interior surface  16  are configured to be substantially stationary with respect to each other, such that when wheel  10  (and interior surface  16 ) rotates around axis of rotation A, each of the rotor drive keys (e.g., rotor drive keys  15 ,  25 ) translates over a closed path around axis A. Consequently, when wheel  10 , interior surface  16 , and rotor drive keys  24  are rotating around axis of rotation A, a force on one or more of rotor drive keys  24  opposing the direction of rotation acts to slow or cease the rotation. As will be discussed, rotor drive keys  24  may be configured to receive a torque from a braking system (not shown) configured to reduce and/or cease a rotation of wheel  10 . Rotor drive keys  24  may be integrally formed with interior surface  16 , or may be separate from and mechanically affixed to interior surface  16 . 
       FIG. 2  is a schematic cross-sectional view illustrating wheel  10  and an example brake system  40 . Wheel  10  includes wheel rim  12 , exterior surface  14 , interior surface  16 , wheel well  17 , wheel hub  19 , inboard beat seat  20 , outboard bead seat  21 , inboard section  22 , outboard section  23 , and rotor drive key  25 .  FIG. 2  illustrates wheel rim  12  as a split rim wheel with lug bolt  26  and lug nut  27  connecting inboard section  22  and outboard section  23 , however wheel rim  12  may utilize other configurations (e.g., a unified wheel rim) in other examples. 
     Wheel  10  is configured to rotate about axis A extending through axial assembly  28 . Axial assembly  28  is figured to support wheel  10  while allowing wheel  10  to rotate around axis A using bearing  29  and bearing  30 . For example, bearings  29 ,  30  may define a substantially circular track around axial assembly  28 . A torque tube  31  is coupled to axial assembly  28  (e.g., via bolts  32 ,  33 ), such that torque tube  31  remains substantially stationary when wheel  10  rotates around axial assembly  28  and axis A. Torque tube  31  may at least partially surround an exterior of axial assembly  28 . Axial assembly  28  may be mechanically coupled to a strut attached to a vehicle (e.g., a landing gear strut (not shown)). 
     Brake system  40  may be positioned within wheel  10  and configured to engage main torque tube  31  and rotor drive key  25 . Brake system  40  is configured to generate a torque to oppose a rotation of wheel  10  around axis A and transfer the torque to rotor drive key  25 , reducing and/or eliminating the rotation of wheel  10  around axis A. Brake system  40  includes a disc stack  42  which includes one or more rotor discs (e.g., rotor discs  43 ,  44 ,  45 ) and one or more stator discs (e.g., stator discs  47 ,  48 ). Rotor discs  43 ,  44 ,  45  and/or stator discs  47 ,  48  may have any suitable configuration. For example, rotor discs  43 ,  44 ,  45  and/or stator discs  47 ,  48  can each be substantially annular discs surrounding axial assembly  28 . Stator discs  47 ,  48  are coupled to torque tube  31  via spline  49  and remain rotationally stationary with torque tube  31  (and axial assembly  28 ) as wheel  10  rotates. Rotor discs  43 ,  44 ,  45  are rotationally coupled to rotor drive key  25  and interior surface  16  and rotate substantially synchronously with wheel  10  around axis A. 
     An actuator  39  is configured to compress disc stack  42  to bring friction surfaces of rotor discs  43 ,  44 ,  45  into contact with friction surfaces of stator discs  47 ,  48  generating shearing forces between the discs. A friction surface of a rotor disc and/or stator disc may also be brought into contact with a friction surface of pressure plate  51  or a friction surface of backing plate  50 . The shearing forces cause rotor discs  43 ,  44 ,  45  to exert a torque on rotor drive key  25  opposing a rotation of wheel  10 . In some examples, actuator  39  is configured to compress disc stack  42  using a pressure plate  51 . In these examples, actuator  39  may be configured to cause pressure plate  51  to translate toward disc stack  42  when actuator  39  compresses disc stack  42 . In examples, actuator  39  is configured to cause a piston  54  to translate relative to a body  52  of actuator  39  (“actuator body  52 ”) to exert to compress disc stack  42 . Actuator  39  may cause piston  54  to translate using any suitable method. In some examples, actuator  39  is configured to cause translation of piston  54  by supplying and/or venting a pressurized hydraulic fluid to or from a piston chamber. In addition or instead, in some examples, actuator  39  is configured to cause piston  54  to translate through a motion (e.g., a rotary motion) generated by an electric motor. 
     In the example shown in  FIG. 2 , actuator  39  is configured to compress disc stack  42  against a backing plate  50 . Backing plate  50  may be supported by torque tube  31 . For example, backing plate  50  may be configured to be substantially stationary with respect to torque tube  31 . Wheel  10  may rotate around backing plate  50  when wheel  10  rotates around torque tube  31 . Brake system  40  may be configured such that the compression force exerted on disc stack  42  by actuator  39  causes disc stack  42  to translate toward backing plate  50 . For example, the compression force may cause rotor discs  43 ,  44 ,  45  to translate over rotor drive key  25  toward backing plate  50  and cause stator discs  47 ,  48  to translate over spline  49  toward backing plate  50 . 
     Backing plate  50  is configured to resist the translation of disc stack  42  and exert a reaction force on disc stack  42  opposite the compression force exerted by actuator  39 , such that disc stack  42  is compressed by actuator  39  between pressure plate  51  and backing plate  50 . When torque tube  31  supports backing plate  50 , backing plate  50  further exerts a force on torque tube  31  in response to the compression force. In examples, actuator  39  is configured to exert the compression force on disc stack  42  toward backing plate  50  and substantially parallel to axis A. 
     Thus, brake system  40  may be utilized to reduce and/or eliminate the rotation of wheel  10  using a compression force by actuator  39  exerted on disc stack  42 . Backing plate  50  may be configured to react against the compression force, causing a compression of disc stack  42 . Torque tube  31  may be configured to support backing plate  50 , such that torque tube  31  experiences a force (e.g., substantially parallel to axis A) when actuator  39  exerts the compression force on disc stack  42 . 
     Wheel  10  may be used with any variety of private, commercial, or military aircraft or other type of vehicle. Wheel  10  may be mounted to a vehicle via, for example, axial assembly  28 . Axial assembly  28  may be mounted on a strut of a landing gear (not shown) or other suitable component of a vehicle to connect wheel  10  to the vehicle. Wheel  10  may rotate around axis A and axial assembly  28  to impart motion to the vehicle. Wheel  10  is shown and described to provide context to the brake system described herein, however the brake system described herein may be used with any suitable wheel assembly in other examples. 
       FIG. 3  illustrates an example assembly  70  including an example portion of wheel  10  and an example portion of brake system  40  within wheel well  17  defined by wheel  10 .  FIG. 3  depicts a cross-section of wheel  10  and selected portions of brake system  40 , with the cross-section taken parallel to axial direction A in  FIG. 1 . As shown in  FIG. 3 , in examples, rotor drive key  25  is supported by wheel  10  via one or more fasteners  35  (e.g., bolts) attaching rotor drive key  25  to wheel  10 , and a torque tube  31  is engaged with a portion of an axial assembly  28  by, e.g., bolt  32 . 
     Wheel  10  and rotor discs  43 ,  44 ,  45  are configured to rotate around torque tube  31  and axis A. Stator discs  47 ,  48 , actuator  39 , spline  49 , and axial assembly  28  are configured to remain substantially rotationally stationary with respect to torque tube  31 . When torque tube  31  is engaged with axial assembly  28  and disc stack  42  is in an uncompressed condition (e.g., actuator  39  is not compressing disc stack  42  in a direction towards backing plate  50 ), torque tube  31  is configured to maintain a displacement D between torque tube  31  and wheel  10 . The displacement D may serve to prevent contact between the rotationally stationary torque tube  31  and the rotating wheel  10  when wheel  10  rotates around axis A. As described above, such contact may cause premature maintenance or replacement of wheel  10  and/or brake system  40 . 
     Actuator  39  is configured to compress disc stack  42  to cause a reduction in the rotational speed of wheel  10 , and/or substantially prevent a rotational movement of wheel  10  (e.g., when wheel  10  is in a parked condition). Actuator  39  may be configured to exert a compression force on disc stack  42 , causing engagement of the friction surfaces on rotor discs  43 ,  44 ,  45  and stator discs  47 ,  48 . Actuator  39  may be configured to exert the compression force to cause engagement of a friction surface of a rotor disc and/or stator disc with a friction surface of pressure plate  51  or a friction surface of backing plate  50 . In examples, actuator  39  is configured to exert a compression force (e.g., the force F) substantially parallel to axis A on disc stack  42 . In examples, actuator  39  includes a body  52  (“actuator body  52 ”) and a piston  54 . Actuator  39  may be configured to cause piston  54  to translate relative to actuator body  52  to exert the compression force on disc stack  42 . In examples, actuator  39  is configured to cause a piston face  53  to exert the compression force on disc stack  42  (e.g., via pressure plate  51 ). Some portion of actuator  39  (e.g., actuator body  52 ) is configured to remain substantially stationary with respect to a portion of torque tube  31  and/or axial assembly  28  when actuator  39  exerts the compression force. In some examples, torque tube  31  and/or axial assembly  28  are configured to limit movement of actuator body  52  when actuator  39  exerts the compression force on disc stack  42 . In examples, actuator  39  is mechanically connected to torque tube  31  and/or axial assembly  28 . 
     For example,  FIG. 3  illustrates actuator  39  with piston  54  in a first position and in a second position relative to actuator body  52 .  FIG. 3  illustrates the first position of piston  54  in dashed line, and the second position of piston  54  in solid line. Actuator  39  may be configured to translate piston  54  from the first position to the second position to cause piston face  53  to exert the compression force on disc stack  42 . The first position and the second position may be displaced from each other by an amount of piston travel T. That is, actuator  39  may be configured to define a first displacement between piston face  53  and a point P on actuator body  39  in the first position, and define a second displacement between piston face  53  and the point P in the second position, wherein the first displacement and the second displacement define the piston travel T. Actuator  39  may be configured to cause piston  54  to translate over the piston travel T to exert the compression force on disc stack  42 . 
     Disc stack  42  is configured to cause a reduction in the rotational speed of wheel  10  and/or substantially prevent rotational movement of wheel  10  (e.g., in a parked condition) when actuator  39  compresses disc stack  42 . A compression force exerted by actuator  39  causes friction surfaces on the rotating rotor discs  43 ,  44 ,  45  to engage friction surfaces on the relatively stationary stator discs  47 ,  48 . Engagement with stator discs  47 ,  48  and/or friction surfaces of pressure plate  51  and/or backing plate  50  causes rotor discs  43 ,  44 ,  45  to exert a torque on wheel  10  (e.g., via rotor drive key  25 ), reducing the speed of wheel  10 . When wheel  10  is substantially stationary with respect to torque tube  31  (e.g., when the vehicle is in a parked condition) and actuator  39  is compressing disc stack  42 , rotor discs  43 ,  44 ,  45  may resist rotational motion of wheel  10 . 
       FIG. 4  illustrates a perspective view of an example disc stack  42  illustrating stator discs  47 ,  48  interleaved with rotor discs  43 ,  44 ,  45 . Disc stack  42  may be positioned between pressure plate  51  and backing plate  50 . Axis A is included for reference to  FIGS. 1-3 . Disc stack  42  is illustrated in an uncompressed condition with opposing friction surfaces of adjacent stator and rotor discs disengaged. For example, as illustrated at  FIG. 4 , an air gap G exists between rotor disc  43  and adjacent stator disc  47  such that friction surface  56  of rotor disc  43  and friction surface  58  of stator disc  47  are substantially disengaged (e.g., not in contact with each other). The air gap G may have any suitable value (e.g., may be larger or smaller than illustrated, relative to disc stack  42 ). Each of stator discs  47 ,  48  and rotor discs  43 ,  44 ,  45  may have a first friction surface (e.g., friction surface  56  of rotor disc  43 ) and a second friction surface (e.g., friction surface  60  of rotor disc  43 ) on an opposite side of the respective disc from the first friction surface. In some examples, each of stator discs  47 ,  48  and rotor discs  43 ,  44 ,  45  may be substantially annular shaped discs, but may have other shapes in other examples. 
     Rotor discs  44 ,  45 ,  46  are configured to rotate substantially synchronously with wheel  10  ( FIGS. 1-3 ). In some examples, each of rotor discs  43 ,  44 ,  45  include a plurality of drive slots configured to engage a rotor drive key of wheel  10  to cause the rotation. For example, rotor disc  43  includes drive slot  62  on an outer perimeter  64  of rotor disc  43 . Drive slot  62  is configured to engage a rotor drive key (e.g., rotor drive key  25  ( FIGS. 1-3 ) to cause rotor disc  43  to rotate substantially synchronously with wheel  10 . Stator discs  47 ,  48  are configured to substantially remain rotationally stationary with respect to torque tube  31  ( FIGS. 2-3 ) as rotor discs  43 ,  44 ,  45  rotate. 
     Each of stator discs  47 ,  48  may include a plurality of spline slots configured to engage a spline of torque tube  31  to substantially maintain stator discs  47 ,  48  rotationally stationary relative to rotor discs  43 ,  44 ,  45 . That is, stator discs  47 ,  48  are configured to not rotate when rotor discs  43 ,  44 ,  45  rotate. For example, stator disc  47  includes spline slot  66  on an inner perimeter  68  of stator disc  47 . Spline slot  66  is configured to engage a spline (e.g., spline  49  ( FIGS. 2 and 3 ) to cause stator disc  47  to substantially remain rotationally stationary with respect to torque tube  31 . In similar manner, pressure plate  51  and/or backing plate  50  may include a plurality of spline slots (e.g., spline slot  71  on inner perimeter  72  of pressure plate  51 ) configured to cause pressure plate  51  and/or backing plate  50  to substantially remain rotationally stationary with respect to torque tube  31 . When actuator  39  ( FIGS. 2 and 3 ) exerts the compression force F (e.g., on pressure plate  51 ), disc stack  42  is compressed between pressure plate  51  and backing plate  50 , eliminating the gap G and causing the friction surfaces (e.g., friction surface  56  and friction surface  58 ) to engage. 
     Backing plate  50  is configured to exert a reaction force Fl on disc stack  42  in response to the compression force F against backing plate  50 . In some examples, backing plate  50  is configured to engage a portion of torque tube  31 , such that torque tube  31  substantially limits movement of backing plate  50  in a direction away from pressure plate  51  in a direction parallel to axis A. In some examples, backing plate  50  is configured to exert a force F 2  ( FIGS. 3 and 4 ) on torque tube  31  in response to the compression force F. Thus, torque tube  31  ( FIGS. 2 and 3 ) may be configured such that when actuator  39  exerts the compression force F on disc stack  42 , torque tube  31  experiences a force F 2  based on the compression force F. In examples, the compression force F and the force F 2  are substantially parallel to axis A. 
     Disc stack  42  may include components additional to those depicted in  FIGS. 2-4  and/or described above. For example, disc stack  42  can include one or more rotor drive inserts configured to insert at least partially within a drive slot of a rotor disc. For example, disc stack  42  may include rotor drive insert  76  ( FIG. 3 ) configured to insert at least partially within drive slot  62  ( FIG. 4 ) of rotor disc  43 . As another example, disc stack  42  may include one or more spline inserts configured to insert at least partially within a spline slot of a stator disc. For example, disc stack  42  may include spline insert  78  ( FIG. 3 ) configured to insert at least partially within spline slot  66  ( FIG. 4 ) of stator disc  47 . As used herein, disc stack  42  may include one or more rotor discs such as rotor disc  43 ,  44 ,  45 , one or more stator discs such as stator disc  47 ,  48 , and other components configured to rotate and/or translate as a substantially rigid body with at least one of the rotor discs (e.g., rotor discs  43 ,  44 ,  45 ) and/or the stator discs (e.g., stator discs  47 ,  48 ). 
     In examples, torque tube  31  ( FIG. 3 ) may comprise a material having a ductility that results in a linear deformation of torque tube  31  under some conditions (e.g., under a high heat load following an emergency stop of a vehicle using brake system  40 ). For example, when actuator  39  exerts the compression force F causing the force F 2  on torque tube  31 , the ductility of the material may result in the force F 2  causing the linear deformation. The linear deformation of torque tube  31  may be substantially parallel to axis A. In some examples, the linear deformation may cause torque tube  31  to extend towards wheel  10 , reducing the displacement D ( FIG. 3 ) between torque tube  31  and wheel  10 . The linear deformation of torque tube  31  may increase a displacement between actuator body  39  (e.g., point P) and disc stack  42 , increasing the piston travel T required by actuator  39  in order to assure that the compression force F is maintained. Assembly  70  is configured to limit the piston travel T which may be required by actuator  39  by limiting a maximum distance between actuator housing  52  (e.g., point P) and disc stack  42  under certain operating conditions, such as a Rejected Take-Off (RTO) stop. Hence, assembly  70  may act to reduce the reserve capacity requirements of actuator  39  which might be otherwise required when brake system  40  operates under certain conditions. 
     The reduction in the displacement D between torque tube  31  and wheel  10  may be more pronounced toward the end of the operating life for a given disc stack  42 . For example, as rotor discs  43 ,  44 ,  45  and stator discs  47 ,  48  wear and the individual disc thickness are reduced (e.g., thickness substantially parallel to axis A), the linear deformation of torque tube  31  may cause a greater reduction of the displacement D. For example, at the end of life, the greater reduction in the displacement D may occur as a result of higher temperatures caused by a reduction in heat sink material as disc stack  42  and/or other components of brake assembly  40  wear over life. Ensuring that at least some portion of the displacement D remains between torque tube  31  and wheel  10  may limit the dependence of the displacement D on the thickness of disc stack  42 , allowing for less frequent replacement of disc stack  42 . In addition, as the displacement D reduces, the available travel of piston  54  may become a factor as the individual disc thicknesses are reduced. By ensuring that some portion of the displacement D is maintained over the life of brake system  40 , the necessary translation of piston  54  in a direction parallel to axis A to cause sufficient compressing of disc stack  42  in end-of-life scenarios may be reduced, allowing for a measure of space saving within assembly  70 . 
     As shown in  FIG. 3 , assembly  70  includes a mechanical stop  74  configured to limit a translation of disc stack  42  when actuator  39  exerts the compression force F on disc stack  42  under certain operation conditions. In examples, mechanical stop  74  is configured to limit a translation of disc stack  42  in a direction substantially parallel to axis A. For example, mechanical stop  74  may be configured to limit the translation of disc stack  42  when torque tube  31  linearly deforms in a manner that leads to a reduction in the displacement D (e.g., when actuator  39  causes the linear deformation of torque tube  31 ). Mechanical stop  74  may be configured to limit the translation of disc stack  42  when torque tube  31  linearly deforms over a displacement less than the displacement D. In some examples, mechanical stop  74  is configured to exert a reaction force on disc stack  42  when mechanical stop  74  encounters disc stack  42 . For example, mechanical stop  74  may be configured such that, when torque tube  31  linearly deforms under the influence of force F 2 , mechanical stop  74  encounters and exerts a reaction force on disc stack  42  to reduce the force F 2  experienced by torque tube  31 . The reduction of the force F 2  on torque tube  31  may substantially cease the linear deformation and resulting extension of torque tube  31  toward wheel  10 , preserving at least some portion of the displacement D between torque tube  31  and wheel  10 , and limiting the linear translation of disc stack  42  when actuator  39  exerts the compression force F. 
     For example,  FIG. 5  illustrates assembly  70  with mechanical stop  74  encountering disc stack  42 . Actuator  39  has translated piston  54  from a first position (indicated in dashed line) to a second position (indicated in solid line) to cause piston face  53  to encounter disc stack  42  (e.g., pressure plate  51 ) to exert the compression force F on disc stack  42 . Actuator  39  has caused piston  54  to translate over the piston travel Tm to exert the compression force F. The compression force F causes a linear translation of disc stack  42  toward backing plate  50 . 
     Mechanical stop  74  may be configured to encounter one or more rotor discs (e.g., rotor discs  43 ,  44 ,  45 ) and/or one or more stator discs (e.g., stator discs  47 ,  48 ) when the compression force F causes torque tube  31  to linearly deform under certain conditions. With mechanical stop  74  encountering disc stack  42  (e.g., drive insert  76  and/or rotor disc  45 ), mechanical stop  74  exerts a reaction force F 3  opposing compression force F. The exertion of the reaction force F 3  reduces and/or substantially eliminates the necessary reaction force imparted by backing plate  50 , reducing and/or substantially eliminating the force F 4  on torque tube  31 . Reducing and/or substantially eliminating the force F 4  on torque tube  31  may reduce and/or substantially eliminate further linear deformation of torque tube  31 . Reducing and/or substantially eliminating the further linear deformation of torque tube  31  limits further linear translation of disc stack  42  in a direction away from actuator body  52  (e.g., point P), limiting the amount of piston travel Tm required to maintain a sufficient compression force F. Hence, with mechanical stop  74  having encountered disc stack  42 , brake system  40  limits a maximum distance between actuator housing  52  (e.g., point P) and disc stack  42  under certain operating conditions. This may reduce the reserve capacity requirements of actuator  39  which might be otherwise required in the absence of mechanical stop  74 . 
     In examples, mechanical stop  74  is configured to reduce and/or substantially eliminate the force F 4  on torque tube  31  (e.g., by exerting the reaction force F 3 ) when actuator  39  causes torque tube  31  to experience a threshold linear deformation. Mechanical stop  74  may be configured to encounter disc stack  42  to exert the reaction force F 3  when torque tube  31  experiences a linear deformation sufficient to reduce the displacement D ( FIG. 3 ) to the displacement D 2  ( FIG. 5 ). Mechanical stop  74  may be configured to at least partially unload torque tube  31  (e.g., by exerting the reaction force F 3 ) to substantially cease the linear deformation, in order to limit the maximum distance between actuator housing  52  (e.g., point P) and disc stack  42 . In examples, assembly  70  is configured such that actuator  39  compresses disc stack  42  at least partially against mechanical stop  74  when mechanical stop  74  encounters disc stack  42 . When mechanical stop  74  encounters disc stack  42 , assembly  70  may be configured such that actuator  39  compresses disc stack  42  against both backing plate  50  and mechanical stop  74 , such that mechanical stop  74  substantially acts to reduce the amount of compression force F transmitted to torque tube  31 . 
     Mechanical stop  74  may be configured to possess a greater axial stiffness than torque tube  31  in a direction substantially parallel to the direction of the compression force F. For example, mechanical stop  74  may comprises a material and/or have a geometry causing the axial stiffness of mechanical stop  74  to exceed that of torque tube  31 . The greater axial stiffness may act to ensure mechanical stop  74  sufficiently unloads torque tube  31  without allowing additional linear deformation of mechanical stop  74  and/or torque tube  31 . 
     Mechanical stop  74  may be configured to encounter any portion of disc stack  42 . In some examples, mechanical stop  74  is configured to encounter a drive insert (e.g., drive insert  76 ) of disc stack  42  when actuator  39  compresses disc stack  42 . As another example, mechanical stop may be configured to encounter a spline insert (e.g., spline insert  78 ) of disc stack  42  when actuator  39  compresses disc stack  42 . In other examples, mechanical stop  74  is configured to encounter a disc in disc stack  42  having a greater displacement from actuator  39  than another disc in disc stack  42 , in order that compression force F causes a greater number of friction surfaces within disc stack  42  to engage (e.g., to substantially preserve braking power). 
     Mechanical stop  74  may be configured such that, when mechanical stop  74  encounters disc stack  42 , a plurality of rotors discs and/or stator discs is compressed between actuator  39  and mechanical stop  74 . In examples, mechanical stop  74  is configured to encounter a rotor disc (e.g., rotor disc  45 ) of disc stack  42  adjacent to or nearest backing plate  50 . For example, mechanical stop  74  may be configured to encounter rotor disc  45  when torque tube  31  experiences the threshold linear deformation, such that compression of disc stack  42  against mechanical stop  74  continues to cause engagement of friction surfaces between rotor disc  43  and stator disc  47 , between stator disc  47  and rotor disc  44 , between rotor disc  44  and stator disc  48 , and between stator disc  48  and rotor disc  45 . 
       FIG. 6  illustrates assembly  70  with disc stack  42  in an uncompressed condition (e.g., without a compression force exerted by actuator  39 ). Assembly  70  is configured such that, absent a compression force from actuator  39 , assembly  70  substantially maintains a clearance (measured in a direction parallel to axis A) between disc stack  42  and mechanical stop  74 . Assembly  70  may be configured such that, when actuator  39  compresses disc stack  42 , disc stack  42  encounters backing plate  50  prior to mechanical stop  74 . In examples, in the absence of a compression force from actuator  39 , actuator  39  defines a first length L 1  between actuator  39  and mechanical stop  74  and a second length L 2  between actuator  39  and backing plate  50 , with the first length L 1  greater than the second length L 2 . The first length L 1  and the second length L 2  may be defined by a specific point on actuator  39 , such as point P on actuator body  52 . The first length L 1  and the second length L 2  may be substantially parallel. In examples, the first length L 1  and the second length L 2  may be substantially parallel to axis A. 
     Disc stack  42  is configured to translate substantially parallel to axis A when compressed by actuator  39 . Assembly  70  may be configured such that the first length L 1  and the second length L 2  are substantially parallel to the direction of translation of disc stack  42 . Hence, assembly  70  may be configured such that compression of disc stack  42  causes disc stack  42  (e.g., rotor disc  45 ) to encounter backing plate  50  prior to encountering mechanical stop  74 . Further, assembly  70  may be configured such that a threshold linear deformation of torque tube  31  substantially causes disc stack  42  to encounter mechanical stop  74  (e.g., a linear deformation over a distance AL between the first distance L 1  and the second distance L 2 ). Thus, assembly  70  may be configured such that, when actuator  39  causes torque tube  31  to experience a linear deformation below the threshold linear deformation (or substantially no linear deformation), actuator  39  causes compression of disc stack  42  against backing plate  50  while maintaining a displacement between disc stack  42  and mechanical stop  74 . When actuator  39  causes torque tube  31  to experience a linear deformation substantially equal to or above the threshold linear deformation, actuator  39  causes compression of disc stack  42  against backing plate  50  and mechanical stop  74 . 
     Although  FIGS. 3, 5 and 6  discuss a linear deformation of torque tube  31  in a direction substantially parallel to axis A reducing a displacement D ( FIG. 3 ) substantially parallel to the axis A, the disclosure herein is not so limited. Mechanical stop  74  may be configured to reduce and/or elimination a linear deformation of torque tube  31  occurring in any direction, and the displacement D may be a displacement in any direction and between any portions of assembly  70 . In examples, the displacement D is a displacement between a first portion (e.g., a first component) of assembly  70  and a second portion (e.g., a second component) of assembly  70 , wherein the first portion is configured to rotate when wheel  10  rotates and the second portion is configured to remain substantially stationary relative to the rotation of the first portion. 
     Mechanical stop  74  may be supported in any manner sufficient to allow mechanical  74  to encounter disc stack  42  and exert reaction force F 3  on disc stack  42 . Mechanical stop  74  may be configured such that a movement of mechanical stop  74  is substantially limited in response to the exertion of reaction force F 3 . The movement of mechanical stop  74  may be substantially limited by any portion of wheel  10 , brake system  40 , axial assembly  28  ( FIG. 1 ), or some other component or system sufficient to limit the movement of mechanical stop  74  when mechanical stop  74  exerts the reaction force F 3 . Mechanical stop  74  may be configured such that, when mechanical stop  74  exerts the reaction force F 3  on disc stack  42 , some portion of wheel  10 , brake system  40 , axial assembly  28  ( FIG. 1 ), or another component or system exerts a substantially equal and opposite force on mechanical stop  74  to limit the movement of mechanical stop  74 . The portion of wheel  10 , brake system  40 , axial assembly  28  ( FIG. 1 ), or other component or system supporting mechanical stop  74  may be configured to possess a greater axial stiffness than torque tube  31  in a direction substantially parallel to the direction of the compression force F, in order to allow mechanical stop  74  to transfer load from torque tube  31  in a manner limiting any additional linear deformation of torque tube  31 . 
     In some examples, some portion of wheel  10  is configured to substantially limit movement of mechanical stop  74  when mechanical stop  74  encounters disc stack  42 . For example, wheel  10  may support mechanical stop  74  such that, when actuator  39  compresses disc stack  42  against mechanical stop  74 , mechanical stop  74  transmits at least a portion of compression force F to wheel  10 . In examples, mechanical stop  74  is configured to transmit some portion or substantially all of reaction force F 3  to wheel  10 . Wheel  10  may be configured to exert a substantially equal and opposite force on mechanical stop  74  to limit the movement of mechanical stop  74 . Hence, mechanical stop  74  may be configured to at least partially unload a portion of the compression force F from torque tube  31  by transferring a portion of the compression force F from torque tube  31  to wheel  10  when mechanical stop  74  encounters disc stack  42 . 
     In some examples, mechanical stop  74  is rotationally coupled to wheel  10 , such that a rotation of wheel  10  causes a rotation of mechanical stop  74 . For example, mechanical stop  74  may be configured to rotate synchronously with wheel  10 . In some examples, mechanical stop  74  is rotationally coupled with wheel  10  and configured to encounter a portion of disc stack  42  (e.g., rotor disc  45  and/or drive insert  76 ) rotationally coupled to wheel  10 , in order to substantially avoid contact between mechanical stop  74  and portions of disc stack  42  configured to be rotationally mismatched with mechanical stop  74 . This may serve to minimize or even prevent contact between components configured to rotate with wheel  10  (e.g., mechanical stop  74 ) and components configured to remain substantially stationary relative to torque tube  31 . Such contact may result in a need for unscheduled maintenance or early replacement of one or more components of wheel  10  or brake system  40 . 
     Mechanical stop  74  can have any suitable configuration. In some examples, mechanical stop  74  is an elongated member engaged with wheel  10  and defining a shank  77 . Shank  77  is configured to encounter disc stack  42  and exert the reaction force F 3  on disc stack  42 . Mechanical stop  74  may be configured and positioned such that shank  77  extends in a direction from interior surface  16  of wheel  10  toward disc stack  42 . Shank  77  may be configured to encounter disc stack  42  when mechanical stop  74  encounters disc stack  42 . In some examples, mechanical stop  74  includes a pin  73  defining shank  77  and having a head  75  attached to shank  77 . Wheel  10  may be configured to engage some portion of pin  73  to limit movement of at least shank  77  relative to wheel  10 . For example, wheel  10  may be configured to substantially trap a portion of mechanical stop  74  (e.g., head  75 ) between inboard section  22  and outboard section  23  of wheel  10 . Wheel  10  may be configured to limit the movement of pin  73  in any suitable manner, including through internal or external threads defined by wheel  10 , a fastener and/or locking device configured to engage pin  73  and wheel  10 , an interference and/or engineering fit between pin  73  and a recess defined by wheel  10 , or a welded, soldered, or other connection between pin  73  and wheel  10 . 
       FIG. 7  illustrates another example mechanical stop  80 , which is configured as a step or ledge engaged with wheel  10 . Mechanical stop  80  is an example of mechanical stop  74 . Mechanical stop  80  extends from the interior surface  16  of wheel  10  (e.g., extends radially inward from interior surface  16 ). Mechanical stop  80  may be configured to act as a substantially rigid body relative to wheel  10 . In some examples, mechanical stop  80  defines a bearing surface  82  configured to encounter disc stack  42  when torque tube  31  experiences a threshold linear deformation (e.g., a linear deformation causing the displacement D 2  ( FIG. 5 )). Bearing surface  82  may be configured to exert the reaction force F 3  ( FIG. 5 ) on disc stack  42  when bearing surface  82  encounters disc stack  42 . In examples, bearing surface  82  is configured to substantially face some portion of disc stack  42 . Bearing surface  82  may be configured to substantially abut some portion of disc stack  42  when mechanical stop  80  encounters disc stack  42 . 
     Bearing surface  82  may be a substantially planar surface in some examples and a curvilinear surface in other examples. In some examples, bearing surface  82  defines a surface complementary to a surface of disc stack  42 . For example, bearing surface  82  may be configured to complement a surface of a brake disc (e.g., rotor disc  45 ) when bearing surface  82  encounters the surface of the brake disc. As an example, in some examples, support bearing surface  82  comprises a first planar surface and the surface of the brake disc defines a second planar surface, where the first and second planar surfaces are substantially parallel (e.g., parallel or nearly parallel to the extent permitted by manufacturing tolerances) when disc stack  42  is positioned within wheel well  17 . As another example, one of the first bearing surface or the second bearing surface can be a convex surface, with the other being a concave surface configured to receive and at least partially mate with the convex surface when mechanical stop  80  encounters disc stack  42 . As another example, one of the first bearing surface or the second bearing surface can define a protrusion, and the other of the first bearing surface or the second bearing surface can define a recess configured to receive and at least partially mate with the protrusion when mechanical stop  80  encounters disc stack  42 . 
     Bearing surface  82  defines a height measured in a direction perpendicular to axis A, where the height is sufficient to cause bearing surface  82  to encounter disc stack  42  when actuator  39  causes a threshold linear deformation of torque tube  31 . Bearing surface  82  may define a width measured in a direction perpendicular to the height and axis A. In some examples, the width of bearing surface  82  defines an arc around axis A. Bearing surface  82  may define a substantially constant height over the width of bearing surface  82 . In some examples, bearing surface defines a varying height over the width of bearing surface  82 . 
       FIG. 8  illustrates another example mechanical stop  84 , which is shown as being engaged with a component  86  of wheel  10  (“wheel component  86 ”). Mechanical stop  84  is an example of mechanical stop  74  and/or mechanical stop  80 . In the example shown in  FIG. 8 , mechanical stop  84  is engaged with wheel component  86 . Wheel component  86  may be a portion of rotor drive key  25 . Wheel component  86  may be configured to be substantially stationary with respect to interior surface  16  of wheel  10 , such that when wheel  10  (and interior surface  16 ) rotates around axis A, wheel component  86  rotates around axis A. Thus, rotation wheel of component  86  around axis A causes a rotation of mechanical stop  84  around axis A. Wheel component  86  may be, for example, the portion of rotor drive key  25 , or some other component of assembly  70  configured to rotate around axis A when wheel  10  rotates around axis A. 
     Wheel component  86  is configured to limit movement of mechanical stop  84  when mechanical stop  84  exerts a reaction force (e.g., reaction force F 3  ( FIG. 5 )) on disc stack  42 . In some examples, mechanical stop  84  is configured such that, when actuator  39  compresses disc stack  42  against mechanical stop  84 , mechanical stop  84  transmits a force to wheel component  86  caused by the compression force of actuator  39  acting on mechanical stop  84 . Wheel component  86  may be configured to exert a substantially equal and opposite force on mechanical stop  84  to limit the movement of mechanical stop  84  in a direction parallel to axis A. 
     Wheel  10  is configured to limit movement of wheel component  86  relative to wheel  10  when wheel component  86  exerts a force on mechanical stop  84  (e.g., when actuator  39  causes mechanical stop  84  to transmit force to wheel component  86 ). For example, wheel  10  may be configured to limit motion of wheel component  86  in a direction substantially parallel to the direction of a compression force (e.g., compression force F ( FIGS. 3, 5 ) exerted by actuator  39  on disc stack  42 . Wheel  10  may be configured to exert a substantially equal and opposite force on wheel component  86  when mechanical stop  84  transmits force to wheel component  86 . Hence, mechanical stop  84  may be configured to substantially transfer some portion of the compression force F exerted by actuator  39  from disc stack  42  to wheel  10 . Mechanical stop  84  may be configured to reduce and/or substantially eliminate the load on torque tube  31  caused by actuator  39 , such that linear deformation of torque tube  31  reduces and/or substantially ceases and a maximum distance between actuator housing  52  (e.g., point P) and disc stack  42  is limited. 
     In examples, wheel component  86  is attached to or defined by rotor drive key  25 . For example, wheel component  86  may be portion of rotor drive key  25 , e.g., mechanical stop  84  may be a unitary (e.g., substantially inseparable) portion of rotor drive key  25  or separate from and attached to rotor drive key  25 . As discussed above, wheel  10  may be configured to limit movement of rotor drive key  25  relative to wheel  10  using, e.g., one or more fasteners  35  (e.g., bolts) attaching rotor drive key  25  to wheel  10 . In examples, instead of or in addition to fasteners  35 , wheel  10  may be configured to receive a protrusion  88  of rotor drive key  25  within a recess  90  defined by wheel  10 . 
     Rotor drive key  25  may be configured to exert a force on wheel  10  (e.g., via fasteners  35 , protrusion  88 , or another attachment mechanism) when mechanical stop  84  transmits a force to rotor drive key  25  (e.g., caused by the compression force of actuator  39  acting on mechanical stop  84 ). Wheel  10  may be configured to exert a substantially equal and opposite force on rotor drive key  25  when mechanical stop  84  transmits the force to rotor drive key  25 . Hence, rotor drive key  25  may be configured to substantially transfer some portion of the compression force F exerted by actuator  39  from disc stack  42  to wheel  10 , when mechanical stop  84  encounters disc stack  42 . 
     In some examples, rotor drive key  25  is configured to possess a greater axial stiffness than torque tube  31  in a direction substantially parallel to the direction of the compression force F. For example, rotor drive key  25  may comprise a material and/or have a geometry causing the axial stiffness of rotor drive key  25  to exceed that of torque tube  31 . The greater axial stiffness may enable rotor drive key  25  to sufficiently unload torque tube  31  without a linear deformation of rotor drive key  25  and/or an additional linear deformation of torque tube  31 . 
     In the example shown in  FIG. 8 , mechanical stop  84  defines a bearing surface  92  configured to encounter disc stack  42  when torque tube  31  experiences a threshold linear deformation (e.g., a linear deformation causing the displacement D 2  ( FIG. 5 )). Bearing surface  92  may be an example of bearing surface  82 , and may be configured relative to disc stack  42  in the same manner as that discussed for bearing surface  82  and disc stack  42 . 
     Example wheel assemblies described herein can include any suitable number of mechanical stops.  FIG. 9  schematically illustrates a portion of assembly  70  having a plurality of mechanical stops including mechanical stop  102 , mechanical stop  104 , and mechanical stop  106 . Assembly  70  is shown from a direction looking down axis A. That is,  FIG. 9  illustrates wheel  10  with axis A perpendicular to the page. Mechanical stops  102 ,  104 ,  106  are each examples of mechanical stop  74 , mechanical stop  80 , and/or mechanical stop  84 .  FIG. 9  further illustrates interior surface  16 , wheel hub  19 , inboard bead seat  20 , and plurality of rotor drive keys  24  engaged with interior surface  16 . Brake system  40  is omitted from  FIG. 9  for clarity. Mechanical stops  102 ,  104 ,  106  are positioned around axis A within wheel well  17  of wheel  10 . 
     Each of mechanical stops  102 ,  104 ,  106  may be configured to exert a reaction force (e.g., reaction force F 3  ( FIG. 5 )) on disc stack  42  when mechanical stops  102 ,  104 ,  106  encounter disc stack  42 . Mechanical stops  102 ,  104 ,  106  may be configured around axis A to distribute the exertion of the reaction force on disc stack  42  (e.g., to reduce stress and/or eliminate concentrations on portions of disc stack  42 ). For example, mechanical stops  102 ,  104 ,  106  may be configured to exert the reaction force at one or more locations on rotor disc  43 ,  44 ,  45  and/or stator disc  47 ,  48 . Mechanical stops  102 ,  104 ,  106  may be configured to exert the reaction substantially force around a perimeter of rotor disc  43 ,  44 ,  45  and/or stator disc  47 ,  48 . Assembly  70  may include any number of mechanical stops arranged in any manner sufficient to cause one or more of the mechanical stops to encounter disc stack  42  ( FIGS. 3, 5, 6 ) when actuator  39  compresses disc stack  42  (e.g., when actuator  39  causes torque tube  31  to experience the threshold linear deformation). 
     Each of mechanical stops  102 ,  104 ,  106  may be radially displaced from axis A. For example,  FIG. 9  illustrates mechanical stop  102  radially displaced from axis A by the radius R. Each of mechanical stops  102 ,  104 ,  106  may be radially displaced from axis A by an individual radius from axis A. The individual radii may define a different displacement for each mechanical stop and/or substantially similar displacements for one or more mechanical stops. In examples, mechanical stops  102 ,  104 ,  106  are positioned with wheel well  17  to define a substantially circumferential pattern around axis A. Mechanical stops  102 ,  104 ,  106  may be evenly or unevenly spaced around axis A. Mechanical stops  102 ,  104 ,  106  may be spaced such that a spacing distance (e.g., an arc length) between adjacent mechanical stops is substantially equal around axis A. Mechanical stops  102 ,  204 ,  106  may be spaced such that the spacing distance (e.g., the arc length) between adjacent mechanical stops varies around axis A. The spacing distance and/or arc length may be defined in a place substantially perpendicular to axis A. 
     In examples, each mechanical stop  102 ,  104 ,  106  is configured to reside substantially between adjacent rotor drive keys in the plurality of rotor drive keys  24 . For example,  FIG. 9  illustrates mechanical stop  102  positioned substantially between rotor drive key  25  and the adjacent rotor drive key  15 . Mechanical stop  102  may be configured to reside closer to rotor drive key  15  than rotor drive key  25 , further away rotor drive key  15  than rotor drive key  25 , or substantially equidistant from rotor drive key  25  and rotor drive key  15 . In some examples, mechanical stop  102  may extend substantially from rotor drive key  25  to rotor drive key  15 , such that mechanical stop  102  is configured to exert the reaction force on disc stack  42  over a larger area (e.g., to reduce stress on disc stack  42 ). 
     Assembly  70  may include any suitable number of mechanical stops between adjacent rotor drive keys (e.g., rotor drive key  25  and rotor drive key  15 ) and the mechanical stops may be arranged in any pattern relative to the adjacent rotor drive keys. For example, a first mechanical stop and a second mechanical stop can be between rotor drive key  25  and rotor drive key  15 , with the first mechanical stop closer to rotor drive key  25  than rotor drive key  15  and the second mechanical stop closer to rotor drive key  15  than rotor drive key  25 . 
     Mechanical stops described herein, including mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106 , as well as wheel  10  and brake system  40 , and the components thereof, may be made from any suitable material. For example, the material may be any material of suitable strength for the intended use of mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106 , wheel  10 , brake system  40 , and the components thereof. In some examples, the material includes a metal or a metal alloy. For example, the material may include a nickel alloy or steel alloy. As one example, the material may include stainless steel. 
     Mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106 , wheel  10 , brake system  40 , and the components thereof can be formed using any suitable technique. Mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106 , wheel  10 , brake system  40 , and the components thereof may be forged, casted, made from bar stock, additive manufactured (e.g., three-dimensionally ( 3 D) printed), extruded, drawn, or be produced using other suitable methods. In some examples, mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106 , wheel  10 , brake system  40 , and the components thereof may be machined to define the configurations described herein. In other examples, Mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106 , wheel  10 , brake system  40 , and the components thereof may be formed without having to be substantially machined. 
     In some examples, wheel  10  may be finish machined from a near-net-shaped aluminum forging and contain an axial assembly and/or wheel rim for assembly of brake system  40  and/or mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  onto wheel  10 . In other examples, wheel  10  may be manufactured in a different manner. In yet other examples, wheel  10  may be obtained rather than manufactured. Wheel  10  may be made of any suitable material. In some examples, wheel  10  includes a metal or a metal alloy. For example, wheel  10  may include aluminum, a nickel alloy, a steel alloy (e.g., stainless steel), titanium, a carbon-composite material, or magnesium. 
     Brake discs described herein, including rotor discs  43 ,  44 ,  45  and stator discs  47 ,  48 , may be manufactured from any suitable material. In some examples, the brake discs described herein may be manufactured from a metal or a metal alloy, such as a steel alloy. In some examples, the brake discs may be manufactured using a ceramic material, such as a ceramic composite. In some examples, the brake discs may be manufactured from a carbon-carbon composite material. In some examples, the brake discs may be manufactured using a carbon-carbon composite material having a high thermal stability, a high wear resistance, and/or stable friction properties. The brake discs may include a carbon material with a plurality of carbon fibers and densifying material. The carbon fibers may be arranged in a woven or non-woven as either a single layer or multilayer structure. 
       FIG. 10  is a flow diagram illustrating an example technique for compressing a disc stack using an actuator. While the technique is described with reference to specific example wheel  10 , brake system  40 , and mechanical stops  74 ,  80 ,  84 ,  102 ,  104 ,  106  described herein, the technique may be used with other components in other examples. 
     The technique includes compressing a disc stack  42  using an actuator  39  ( 1002 ). Brake system  40  may be configured such that compression of disc stack  42  by actuator  39  causes engagement of the friction surfaces of rotor discs  43 ,  44 ,  45  and stator discs  47 ,  48 , reducing and/or substantially preventing the rotation of wheel  10 . Rotor discs  43 ,  44 ,  45  and stator discs  47 ,  48  may be configured to translate in a direction substantially parallel to an axis A of wheel  10  when actuator  39  compresses disc stack  42 . Actuator  39  may be configured to exert a compression force F against disc stack  42  to compress disc stack  42 . In examples, actuator  39  includes an actuator body  52  and a piston  54  configured to translate relative to actuator body  52  to cause the compression of disc stack  42 . Actuator  39  may be configured to cause a translation of piston  54  relative to actuator body  52  using a supply of a pressurized hydraulic fluid, one or more electric motors, or some other appropriate methodology. 
     Actuator  39  may be configured to compress disc stack  42  against backing plate  50  using the compression force F. Backing plate  50  may be configured to transmit at least some portion of the compression force F to torque tube  31 . Torque tube  31  may be configured to exert a reaction force on backing plate  50  in response to the compression force F. Torque tube  31  may be configured such that, under certain conditions (e.g., a high heat load), torque tube  31  linearly deforms as torque tube  31  exerts the reaction force on backing plate  50 . 
     Linear displacement of disc stack  42  is limited using a mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  ( 1004 ). For example, disc stack  42  may encounter mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  when actuator  39  compresses disc stack  42 , due at least in part to the stretching of torque tube  31  in a direction parallel to axis A. Mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  may be configured to exert a reaction force F 3  ( FIG. 5 ) against disc stack  42  when mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  encounters disc stack  42 . Mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  may be configured to reduce the reaction force exerted by torque tube  31  on backing plate  50  when mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  exerts the reaction force F 3 . Thus, mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  is configured to limit the linear displacement of disc stack  42  when mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  encounters disc stack  42 . Mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  may be configured to limit and/or substantially cease the linear deformation of torque tube  31  when mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  encounters disc stack  42 . 
     Assembly  70  may be configured such that, when actuator  39  compresses disc stack  42 , disc stack  42  encounters backing plate  50  prior to mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106 . In some examples, assembly  70  defines a first length L 1  between actuator  39  and mechanical stop  74  and a second length L 2  between actuator  39  and backing plate  50 , with the first length L 1  greater than the second length L 2 . In examples, the first length L 1  and the second length L 2  may be substantially parallel to axis A. Assembly  70  may be configured such that a threshold linear deformation of torque tube  31  caused by the compression F from actuator  39  substantially causes disc stack  42  to encounter mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  . 
     Some portion of the reaction force F 3  exerted by mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  is transferred to wheel  10 . Wheel  10  may be configured to substantially limit movement of mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  when mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  exerts the reaction force F 3 . Wheel  10  may be configured to exert a force on mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  in response to the reaction force F 3  exerted by mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  on disc stack  42 . In some examples, mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  is an elongate pin supported and/or defined by wheel  10 . In some examples, mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  is a step or ledge supported and/or defined by wheel  10 . In some examples, mechanical stop  74 ,  80 ,  84 ,  102 ,  104 ,  106  is supported by a wheel component  86  of wheel  10 . Wheel component  86  may be a rotor drive key  25 ,  15  in a plurality of rotor drive keys  24  coupled to an interior surface  16  of wheel  10 . 
     The present disclosure includes the following examples. 
     Example 1: An assembly comprising: a wheel configured to rotate around a wheel axis; a brake system comprising: a disc stack comprising a rotor disc and a stator disc, wherein the rotor disc is rotationally coupled to the wheel, and wherein the wheel is configured to rotate relative to the stator disc; and an actuator defining an actuator housing and configured to compress the disc stack, wherein the brake system is configured to compress the disc stack by limiting the linear movement of the disc stack by exerting a reaction force on the disc stack when the actuator compresses the disc stack, and wherein a portion of the brake system is configured to linearly deform when the brake system exerts the reaction force; and a mechanical stop configured to reduce the reaction force exerted by the brake system when the linear deformation of the portion of the brake system causes the mechanical stop to encounter the brake system, such that the mechanical stop limits a linear displacement of the disc stack relative to the actuator housing. 
     Example 2: The assembly of example 1, wherein the mechanical stop is configured to encounter the disc stack when the mechanical stop encounters the brake system. 
     Example 3: The assembly of example 1 or 2, wherein the mechanical stop is configured to transmit at least a portion of the reaction force to the wheel. 
     Example 4: The assembly of any of examples 1-3, wherein the mechanical stop is rotationally coupled to the wheel. 
     Example 5: The assembly of any of examples 1-4, wherein the mechanical stop is configured to maintain a displacement between the brake system and the wheel. 
     Example 6: The assembly of example 5, wherein the brake system includes a torque tube, wherein the wheel is configured to rotate relative to the torque tube, and wherein the mechanical stop is configured to maintain the displacement between the torque tube and the wheel. 
     Example 7: The assembly of any of examples 1-6, further comprising a wheel component attached to the wheel, wherein the wheel component includes the mechanical stop. 
     Example 8: The assembly of example 7, wherein the wheel component is a rotor drive key. 
     Example 9: The assembly of any of examples 1-8, wherein the mechanical stop includes a bearing surface defined by an interior surface of the wheel, wherein the bearing surface is configured to encounter the brake disc stack. 
     Example 10: The assembly of any of examples 1-9, further comprising a backing plate configured to encounter the disc stack when the actuator compresses the disc stack, wherein the wheel is configured to rotate relative to the backing plate, and wherein the backing plate is configured to move in an axial direction of the wheel when the portion of the brake system linearly deforms. 
     Example 11: The assembly of any of examples 1-10, wherein the brake system includes a torque tube configured to exert a portion of the reaction force, and wherein the mechanical stop is configured to reduce the portion of the reaction force exerted by the torque tube when the mechanical stop encounters the brake system and the actuator compresses the disc stack, such that the mechanical stop reduces the reaction force exerted by the brake system and limits the linear displacement of the disc stack relative to the actuator housing. 
     Example 12: The assembly of any of examples 1-11, wherein: the actuator is configured to compress the disc stack using a compression force, the actuator defines a first length between the actuator and the mechanical stop in the absence of the compression force, the actuator defines a second length between the actuator and the backing plate in the absence of the compression force, the first and second lengths being measured in a parallel direction, and the first length is greater than the second length. 
     Example 13: The assembly of any of examples 1-12, wherein the mechanical stop is configured to encounter a portion of the disc stack rotationally coupled to the wheel. 
     Example 14: The assembly of any of examples 1-13, wherein the mechanical stop is configured to encounter a portion of the disc stack, and wherein the wheel is configured to rotate relative to the portion of the disc stack. 
     Example 15: The assembly of any of examples 1-14, wherein the mechanical stop is configured to encounter a drive insert within a drive slot of the rotor disc. 
     Example 16: An assembly comprising: a wheel configured to rotate around a wheel axis; a brake system comprising: a disc stack; a torque tube configured to engage the disc stack; a backing plate attached to the torque tube, wherein the backing plate is configured to limit movement of the disc stack in a direction parallel to the wheel axis; and an actuator configured to compress the disc stack against the backing plate using a compression force, wherein the backing plate is configured to exert a reaction force on the disc stack when the actuator compresses the disc stack; and a mechanical stop configured to encounter the disc stack, wherein: the mechanical stop is configured to reduce the reaction force when the mechanical stop encounters the disc stack, the mechanical stop is configured to limit movement of the disc stack in the direction parallel to the wheel axis when the mechanical stop encounters the disc stack, such that the mechanical stop limits a linear displacement of the disc stack relative to the actuator; the actuator defines a first length between the actuator and the mechanical stop in the absence of the compression force, the actuator defines a second length between the actuator and the backing plate in the absence of the compression force, the first and second lengths being measured in a direction parallel to the wheel axis, and the first length is greater than the second length. 
     Example 17: The assembly of example 16, wherein the mechanical stop is rotationally coupled to the wheel, and wherein the mechanical stop is configured to encounter a portion of the disc stack rotationally coupled to the wheel. 
     Example 18: The assembly of example 16 or 17, wherein the mechanical stop is configured to transmit at least a portion of the reaction force to the wheel when the mechanical stop encounters the disc stack. 
     Example 19: A method comprising: compressing a disc stack of a brake system using an actuator of the brake system, wherein the disc stack includes a rotor disc rotationally coupled to a wheel and a stator disc, wherein the wheel is configured to rotate around the stator disc, wherein the brake system exerts a reaction force on the disc stack when the actuator compresses the disc stack; and limiting the movement of the disc stack by reducing the reaction force exerted by the brake system by causing the brake system to encounter a mechanical stop using the compression of the disc stack by the actuator. 
     Example 20: The method of example 19, wherein the mechanical stop rotates substantially synchronously with the rotor disc, the method further comprising: causing the mechanical stop to encounter the brake system by contacting the rotor disc and the mechanical stop. 
     Various examples have been described. These and other examples are within the scope of the following claims.