WO2013044394A1 - Fail-safe actuator assembly for an annular disk brake - Google Patents

Abstract

The actuator assembly (40) includes a substantially cylindrical housing (310), a first and second annular actuator devices (300, 302) respectively positioned inside inboard-facing and outboard-facing compartment (320, 322) of the housing (310), a plate (342) axially movable in the housing (310) between an inboard end position and an outboard end position, and an axially-movable annular member (48). One or more springs (340) are provided between the plate (342) and a cover (312) closing an inboard side of the housing (310). The plate (342) is set to its outboard end position when the first actuator device (300) is under a threshold internal fluid pressure, thereby automatically setting the brake (10) into a parking mode using the spring force. The brake (10) is in a normal operation mode when the first actuator device (300) is above the threshold internal pressure, the plate (342) then being at its inboard end position.

Description

FAIL-SAFE ACTUATOR ASSEMBLY FOR AN ANNULAR DISK BRAKE

CROSS-REFERENCE TO PRIOR APPLICATION

The present case claims priority over a patent application filed in the United States on 30 September 2011 under Serial No. 61/541,654 and entitled "FAIL-SAFE ACTUATOR ASSEMBLY FOR AN ANNULAR DISK BRAKE", the entire contents of which is hereby incorporated by reference.

TECHNICAL FIELD

The technical field relates generally to annular disk brakes, and more particularly to fail-safe actuator assemblies. BACKGROUND

An annular disk brake includes at least one rotor disk that is axially movable with reference to a fixed component. The rotor disk is in a torque-transmitting engagement with a rotating element, such as the wheel of a vehicle for instance. The rotor disk is axially movable between a set of fixed braking pads on one side, and a set of axially-movable braking pads on the opposite side of the rotor disk. The set of movable brake pads is axially pushed against the corresponding side of the rotor disk using an actuator assembly. A braking friction and heat are generated when the brake pads are in a clamping engagement with the opposite sides of the rotor disk. An example of an annular disk brake is described in PCT application published on 4 June 2009 under publication number WO 2009/067801, the content of which is hereby incorporated by reference in its entirety.

Some vehicles, particularly those using a pneumatic pressure as the source of energy for the brake, may require having a fail-safe system for automatically generating a full braking force in the event of a significant drop or a complete loss of pneumatic pressure from the vehicle. An example of such system is described in PCT application published on 28 April 2005 under publication number WO 2005/037618. Other systems also exist.

Although many of prior systems can be suitable in some designs, they are not completely satisfactory in others. Room for improvements always exists in this area.

SUMMARY

In one aspect, there is provided a fail-safe actuator assembly for an annular disk brake, the brake having an inboard side, an outboard side and a central axis, the actuator assembly including: a substantially cylindrical housing, the housing being coaxially disposed with reference to the central axis and having opposite opened inboard and outboard sides, the housing including inboard-facing and outboard-facing inner annular compartments that are coaxially disposed with reference to the central axis; a first annular actuator device positioned inside the inboard-facing compartment, the first actuator device having an inboard-facing side; a second annular actuator device positioned inside the outboard-facing compartment, the second actuator device having an outboard-facing side; a cover connected to the inboard side of the housing; an axially-movable plate coaxially disposed with reference to the central axis, the plate including an inboard portion and an outboard portion, the inboard portion having an outboard-facing side that is in engagement with the inboard-facing side of the first actuator device, the plate being movable between an inboard end position and an outboard end position, the brake being in a normal operation mode when the plate is at its inboard end position and the brake being in a parking mode when the plate is at its outboard end position; at least one spring provided between the plate and the cover , the plate being set to its outboard end position by the at least one spring when the first actuator device is under a threshold internal fluid pressure, and the plate being set to its inboard end position when the first actuator device is above the threshold internal fluid pressure; and an axially-movable annular member coaxially disposed with reference to the central axis, the axially-movable member having an inboard-facing side that is in engagement with the outboard-facing side of the second actuator device, the axially-movable member being engaged by the outboard portion of the plate and then pushed into an outboard position to set the brake into the parking mode when the plate is in its outboard end position.

In another aspect, there is provided a method of operating an actuator assembly for an annular disk brake having a central axis, the method including: generating a spring force inside the actuator assembly; receiving a first fluid pressure at a first annular actuator device provided inside the actuator assembly to generate a first actuator force opposing the spring force; maintaining the brake in a normal operation mode using the first actuator force while the first fluid pressure is above a threshold fluid pressure to overcome the spring force; receiving a second fluid pressure at a second annular actuator device provided inside the actuator assembly to generate a second actuator force, the second actuator force being substantially proportional to the second fluid pressure; generating a variable braking force inside the brake using the second actuator force when the brake is maintained in the normal operation mode; and automatically setting the brake from the normal operation mode to a parking mode using the spring force in the actuator assembly if the first fluid pressure falls under the threshold fluid pressure.

Further details on these aspects as well as other aspects of the proposed concept will be apparent from the following detailed description and the appended figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a partially-exploded isometric view illustrating an example of an annular disk brake incorporating the concept as suggested herein;

FIG. 2 is an isometric view of the brake shown in FIG. 1 once assembled, as viewed from the inboard side;

FIG. 3 is a view similar to FIG. 2 but showing the actuator assembly being detached from the rest of the brake;

FIG. 4 is an isometric cross-sectional view of the actuator assembly taken along lines 4-4 in FIG. 3;

FIG. 5 is an exploded isometric view of the actuator assembly of FIG. 1, as viewed from the inboard side; FIG. 6 is a fragmentary cross-sectional view of the actuator assembly of FIG. 1 and showing an example of a situation where the first fluid pressure is above the threshold fluid pressure but no braking force is being generated therein;

FIG. 7 is a view similar to FIG. 6 but showing the example in a situation where the first fluid pressure is still above the threshold fluid pressure and a braking force is now being generated therein using a second fluid pressure; and

FIG. 8 is a view similar to FIG. 6 but showing the example in a situation where the first fluid pressure is under the threshold fluid pressure and the actuator assembly then automatically set the brake into a parking mode.

DETAILED DESCRIPTION

FIG. 1 is a partially-exploded isometric view illustrating an example of an annular disk brake 10 incorporating the concept as suggested herein, as viewed from the outboard side of the brake 10. The illustrated brake 10 is designed to be used on a large vehicle, such as a truck or a bus. The brake 10 can also be designed for other kinds of vehicles. FIG. 1 shows a right side brake. The left side brake would be substantially a mirror image thereof.

It should be noted that the words "outboard" and "inboard" in the present context refer to the relative position with reference to the longitudinal axis at the center of the vehicle.

The brake 10 can be constructed as disclosed in PCT application published on 4 June 2009 under publication number WO 2009/067801. Variants are also possible as well. The brake 10 of the illustrated example includes a main support 12 to which the wheel (not shown) of the vehicle is attached. The main support 12 is bearing-mounted and can rotate around an internal central spindle (not shown) that is coaxially located with reference to the central axis 5 of the brake 10. Thus, the rotation axis of the wheel is coincident with the central axis 5 of the brake 10.

The main support 12 has a plurality of axisymmetric mounting bolts 16 outwardly projecting from a substantially radial portion of the main support 12. Ten mounting bolts 16 are shown in the illustrated example. Such configuration is common for large trucks. Variants are possible as well.

Most of the internal components of the brake 10 are located within a space generally defined by a casing 18. The casing 18 of the illustrated example includes an outboard casing part 20 and an inboard casing part 22. The outboard casing part 20 is connected to the inboard casing part 22 using a plurality of spaced-apart bolts 26 located on the periphery of the brake 10. The bolts 26 extend in a direction that is substantially parallel to the central axis 5. Variants are possible. The outboard casing part 20 and the inboard casing part 22 are fixed components, i.e. components that are not rotating with the main support 12 when the vehicle is in motion.

The brake 10 includes an actuator assembly 40 to generate the braking force inside the brake 10. This actuator assembly 40, in the present context, can be pneumatic or hydraulic. FIG. 1 shows a pneumatic actuator assembly. The actuator assembly 40 has a generally annular configuration and is designed to be connected to the inboard side of the inboard casing part 22. The inboard casing part 22 is thus positioned between the outboard casing part 20 and the actuator assembly 40. Bolts can be used to attach the actuator assembly 40 to the inboard casing part 22 but the actuator assembly 40 can also be connected differently to the casing 18. Other variants are possible as well.

As can be appreciated, mounting the actuator assembly 40 on the inboard side of the inboard casing part 22 can increase the compactness of the brake 10 compared to designs where an actuator assembly is provided inside the casing 18.

FIG. 1 partially shows the rotor disk 60 of the brake 10. The rotor disk 60 is coaxially positioned with reference to the central axis 5. The rotor disk 60 has opposite side surfaces against which brake pads will be engaged. In the illustrated example, the rotor disk 60 includes two parallel annular walls. These walls are connected together through a plurality of axisymmetric and spaced-apart ribs 61 forming air channels 63 between them. In use, the heated air will escape radially outwards through the air channels 63 while cooler air is admitted at a radially inner side of the rotor disk 60. The various parts of the rotor disk 60 are made integral with one another. Variants are possible as well. The rotor disk 60 can move in the axial direction with reference to the central axis 5 because the interior of the rotor disk 60 is designed to slide over the main support 12. However, the rotor disk 60 is also in a torque-transmitting engagement with the main support 12. In the illustrated example, the outboard wall surface of the rotor disk 60 is designed to engage a first set of axisymmetric brake pads 62 mounted inside the outboard casing part 20. The inboard wall surface of the rotor disk 60 is designed to be engaged by a second set of axisymmetric brake pads 64 mounted on a substantially axially-movable brake pad carrier 66. Rollers (not shown) are provided on the brake pad carrier 66 to engage corresponding slots (not shown) made in the inboard casing part 22 so as to guide the brake pad carrier 66 along the central axis 5. Variants are possible as well.

FIG. 2 is an isometric view of the brake 10 shown in FIG. 1 once assembled, as viewed from the inboard side.

FIG. 3 is a view similar to FIG. 2 but showing the actuator assembly 40 being detached from the rest of the brake 10. As can be seen, the actuator assembly 40 is provided as a removable unit incorporating the various components. Variants are possible.

In use, when the inboard brake pads 64 are urged against the inboard wall surface of the rotor disk 60, its outboard wall surface will be forced to move closer to the outboard brake pads 62 and will engage them if not already engaged. Increasing the force by which the inboard brake pads 64 are engaged against the inboard wall surface of the rotor disk 60 will increase the brake pad clamping force, thus the friction with the braking pads 62, 64 on both sides of the rotor disk 60. The kinetic energy resulting from the motion of the vehicle and/or being supplied by the vehicle's engine is then transformed into heat in the brake 10 until a full stop of the vehicle or until the braking force is discontinued. Heat in the brake 10 eventually dissipates in the atmosphere.

A part called an intermediary member 100 is located between the brake pad carrier 66 and the actuator assembly 40 of the illustrated brake 10. The intermediary member 100 includes a plurality of axisymmetric and axially-projecting helical ramp surfaces 102. The ramp surfaces 102 face the outboard side of the brake 10. These ramp surfaces 102 are engaged by corresponding rollers (not shown) provided on the inboard side of the brake pad carrier 66. Four pairs of ramp surface / roller are provided in the illustrated example. Variants are also possible. The torque generated at the intermediary member 100 of the illustrated example during braking causes the ramp surfaces 102 mounted thereon to push on the corresponding rollers, thus to move the brake pad carrier 66 in a substantially axial direction towards the rotor disk 60, i.e. in the outboard direction.

The intermediary member 100 is coaxially disposed with reference to the central axis 5. It can pivot in a radial plane within the inboard casing part 22. In the illustrated example, the pivot motion of the intermediary member 100 is guided in part by rollers 120 (FIG. 3) mounted on radially-extending axles 122. The axles 122 project inwardly from the inboard casing part 22. Variants are possible as well. This intermediary member 100 does not rotate with the main support 12 and is not movable in the axial direction.

In use, the actuator assembly 40 creates a force linearly moving an axially-movable annular member 48 located therein towards the outboard side of the brake 10. The force pushing on the axially-movable member 48 is substantially parallel to the central axis 5. In the illustrated example, the axially-movable member 48 is configured to pivot the intermediary member 100 around the central axis 5 through a cam interface. The axially-movable member 48 includes axisymmetric cam elements 50 provided to engage corresponding rollers 108 on the periphery of the intermediary member 100. Each cam element 50 includes an oblique ramp against which the corresponding roller 108 is engaged. The cam elements 50 extend substantially in an axial direction and extend through apertures 52 (FIG. 3) provided through the inboard side of the inboard casing part 22. Variants are also possible.

It should be noted that in FIG. 1, the apertures 54 that are on the outboard side of the inboard casing part 22 must not be confused with the apertures 52 on the inboard side. Also in the illustrated example, an additional element 56 is positioned close to a respective one of the cam elements 50. This additional element 56 is substantially a mirror image of the adjacent cam element 50. They are circumferentially spaced apart from one another so as to create a plurality of axially-extending slots 58. These slots 58 are configured to receive corresponding rollers 124 mounted on the axles 122, thus the same ones on which the rollers 120 are mounted. The slots 58 are slightly larger in width than the outer diameter of the rollers 124. This arrangement will guide the axially-movable member 48 and prevent it from pivoting. Variants are possible as well.

FIG. 4 is an isometric cross-sectional view of the actuator assembly 40 taken along lines 4-4 in FIG. 3. FIG. 5 is an exploded isometric view of this actuator assembly 40, as viewed from the inboard side, and shows its main components when separated from one another. As can be seen, two annular actuator devices 300, 302 are provided inside the actuator assembly 40. These actuator devices 300, 302 are coaxially disposed with reference to the central axis 5. The first actuator device 300 can generate a first actuator force and the second actuator device 302 can generate a second actuator force. In the illustrated example, the annular actuator devices 300, 302 include inflatable air bellows, for instance made of a flexible or semi-flexible material, namely a first annular air bellows 300 and a second annular air bellows 302. The first air bellows 300 is pneumatically connected to a corresponding pressure line that is in fluid communication with the source of pressurized air. This first air bellows 300 receives what is called the first internal fluid pressure, namely a reference basic pressure coming from the vehicle's pneumatic system, for instance from the air tank, and that is normally always above a minimum threshold or higher during normal operation of the vehicle. Having the first air pressure above the threshold air pressure is indicative that the pneumatic system of the vehicle functions and that the brake 10 is currently connected to the system, thus that the brake 10 is operational. The first air pressure is generally maintained at a relatively constant pressure when the brake is in a normal operation mode. Variants are also possible.

The second air bellows 302 is pneumatically connected to a corresponding pressure line in which air pressure varies in function of the required braking force. It receives a second internal fluid pressure, thus a second air pressure in the illustrated example. Depending on the implementation, this second air pressure is controlled by the driver, for instance through the brake pedal of the vehicle, and/or by another system, depending on the implementations. The second actuator force is substantially proportional to the second air pressure so as to generate the variable braking force. Variants are possible as well.

Air ports 304, 306, shown in FIGS. 2, 3 and 5, provide the connection points between the ends of the pressure lines and the corresponding air bellows 300, 302 of the illustrated example. Other configurations and arrangements are also possible. It should be noted that the internal air connections for the air bellows 300, 302 are not visible in the enclosed figures. The actuator assembly 40 includes a substantially ring-shaped housing 310 made of a rigid material. The housing 310 is coaxially disposed with reference to the central axis 5. It includes outboard and inboard opened sides. The outboard side of the housing 310 (at the right in FIG. 5) is connected to the inboard side of the inboard casing part 22 when the brake 10 is assembled. When the brake 10 is assembled, the inboard side of this housing 310 is closed by a cover 312, such as a cover 312 being removable using, for instance, bolts 314 as shown in FIGS. 3 and 4,. Variants are also possible.

The illustrated housing 310 includes a substantially cylindrical portion 310a inside which is located a double-sided annular inner flanged portion 310b. As shown for instance in FIG. 4, the inner flanged portion 310b has a substantially T-shaped cross section and creates two opposite annular compartments 320, 322 within the housing 310, namely an inboard-facing inner annular compartment 320 and an outboard-facing inner annular compartment 322. The inner flanged portion 310b of the illustrated example includes an inwardly-extending radial wall separating the two compartments 320, 322 and a tubular section coaxially disposed with reference to the central axis. Variants are also possible. The inboard-facing compartment 320 is configured to receive the first air bellows 300 with a tight fit and the outboard-facing compartment 322 is configured to receive the second air bellows 302 with a tight fit. The cylindrical portion 310a and the inner flanged portion 310b form a monolithic part in the illustrated example. Variants are also possible. One or more compression springs 340 are provided between the interior (outboard side) of the cover 312 and an axially-movable plate 342 located inside the housing 310. Four axisymmetric helical springs 340 are provided in the brake 10 of the illustrated example. Each spring 340 is coiled around a longitudinal axis that is substantially parallel to the central axis 5 once the springs 340 are mounted inside the housing 310. It should be noted that the exact number and/or the configuration of the springs 340 can vary, depending on the implementations. Other variants are also possible.

The plate 342 is coaxially disposed with reference to the central axis 5. The plate 342 of the illustrated example has generally circular shape but other shapes could be possible. The plate 342 includes an inboard portion 342a and an outboard portion 342b, both being made integral with one another. The inboard portion 342a has an outer diameter that is larger than the inner diameter of the inner flanged portion 310b of the housing 310 but that is also smaller than the interior diameter of the cylindrical portion 310a. The outboard portion 342b has an external diameter that is smaller than the internal diameter of the inner flanged portion 310b of the housing 310. This configuration allows the outboard portion 342b to move along the central axis 5 inside the central open space of the housing 310. The plate 342 can move between an inboard end position (FIGS. 6 and 7) and an outboard end position (FIG. 8). The inboard portion 342a of the plate 342 has an outboard-facing side that abuts against the inboard-facing side of the first air bellows 300. In the illustrated example, the outboard-facing side of the inboard portion 342a is generally annular and can fit inside the inboard-facing compartment 320, as shown for instance in FIG. 8. Variants are also possible. The cover 312 of the illustrated example also includes semi-cylindrical receptacles 312a covering the springs 340 and assist in holding one end thereof in place. Each receptacle 312a provides an internal seat for a corresponding one of the springs 340. Variants are also possible.

The inboard side of the axially-movable member 48 is in engagement with the second air bellows 302. As aforesaid, the axially-movable member 48 is annular. It includes a main outer annular portion 48a that is configured and disposed to fit loosely inside the outboard-facing compartment 322.

FIG. 6 is a fragmentary cross-sectional view of the actuator assembly 40 showing an example of a situation where the first air pressure is above the threshold air pressure but no braking force is being generated inside the brake 10, for instance the vehicle is not braking. As can be seen, the inboard-facing side of the axially-movable member 48 is almost completely inside the outboard-facing compartment 322 and the plate 342 is in its inboard end position since the second air bellows 302 is completed deflated. Forces, for instance spring forces, coming from other parts of the brake 10 push the axially-movable member 48 towards its inboard position. Also in FIG. 6, the plate 342 is pushed by the first air bellows 300 to its inboard end position against the combined spring forces of the springs 340, as depicted by the solid line arrow. The first air bellows 300, thus generate a linear force in the inboard direction capable of overcoming the combined spring return forces when inflated above a threshold fluid pressure. In the illustrated example, the plate 342 abuts against the interior of the cover 312 at its inboard end position. FIG. 7 is a view similar to FIG. 6 but showing an example of a situation where the first air pressure is still well above the threshold air pressure and a braking force is being generated in the brake 10 using the second air bellows 302. It shows that the position of the axially- movable member 48 was shifted towards the outboard side because the second air bellows 302 was inflated. The plate 342 is still in its inboard end position. FIG. 7 shows the second air bellows 302 being fully inflated, such as when the brake pedal is pushed to the maximum by the driver. Variants are also possible.

FIG. 8 is a view similar to FIG. 6 but showing an example where the first air pressure is well under the threshold air pressure, for instance being completely lost, and the actuator assembly 40 then automatically set the brake 10 into a parking mode, for instance in a fully braking position. Accordingly, the parking mode is the default state of the brake 10 and the brake 10 always returns to the parking mode as soon as the first air pressure is lost, i.e. the first air pressure received at the first air bellows 300 falls under the threshold air pressure. The actuator assembly 40 can automatically put the brake 10 into the parking mode if the first air pressure is lost for unplanned reasons, even while the vehicle is in motion. Triggering the parking mode is thus an inherent part of the fail-safe capability of the actuator assembly 40.

As can be seen in FIG. 8, when the first air pressure in the first air bellows 300 is lost, the plate 342 is urged to move towards the outboard side under the combined spring forces from the springs 340. The plate 342 is configured and disposed so that when it moves to its outboard end position, as in FIG. 8, its outboard portion 342b transfers the spring forces to the inboard-facing side of the axially-movable member 48, thereby pushing it towards the outboard side, and generating a full braking force in the brake 10 setting it in the parking mode. The axially-movable member 48 includes an inner flanged portion 48b against which the outboard portion 342b can abut.

Although the second air bellows 302 is shown in FIG. 8 as it would be inflated, it receives no air pressure from the corresponding pressure line in this example. It was set into the illustrated position simply because the axially-movable member 48 was moved to the outboard side and the second air bellow 302 is connected thereto. Inflating the second air bellows 302 is possible but this would have no influence on the axially-movable member 48. Variants are possible as well. To set the brake 10 back in a normal operating mode, one simply needs to inflate the first air bellows 300 once again by pressurizing the corresponding air pressure line connected to the air port 304 above the threshold air pressure.

If desired, an override arrangement 350 can be provided to manually move the plate 342 against the forces created by the springs 340 in the event that the first air pressure cannot be put back on but the full braking force inside the brake 10 must be removed or lowered, for instance in the event that the vehicle must be towed. In the illustrated example, the override arrangement 350 includes one or more pulling bolts capable of progressively pulling the plate 342 towards its inboard end position, thus towards the cover 312. The bolts are schematically illustrated in FIG. 3 at 352. The bolts 352 can be manually rotated by a maintenance technician from outside the cover 312 during a maintenance operation or by a towing operator. The threaded shank of each bolt 352 is loosely inserted in a corresponding hole 354 (FIGS. 3 and 8) provided through the receptacles 312a of the cover 312. The ends of the bolts 350 pass inside the springs 340 and engage a corresponding threaded hole 356 provided through the outboard portion 342b of the plate 342. Using other kinds of hand-operated components is also possible. Other variants are possible as well. As can be appreciated, the suggested concept provides a simple and practical way of designing an annular disk brake while still keeping the brake compact, safe and efficient.

The present detailed description and the appended figures are meant to be exemplary only, and a skilled person will recognize that many changes can be made while still remaining within the proposed concept. For instance, the exact shape, number and/or configuration of some of the parts can be different compared to what was shown and/or described herein.

The brake can be configured for using hydraulic fluid pressure instead of air pressure to operate the first and/or second annular actuator units. Still, the air bellows can be constructed or configured differently than what is shown and described herein.

The brake as illustrated can also be modified for use on many different kinds of vehicles, including vehicles that are not intended for road traveling, such as airplanes. Furthermore, using the brake in a machine that is not a vehicle is possible as well. Such machine can have, for instance, a pulley or another rotating element to which the brake is connected. The uses of the word "vehicle" or its equivalents in the present text only refer to the illustrated example and do not necessarily exclude using the brake in other environments. The brake can also be designed so that the braking force is transmitted directly from the axially-movable member to the brake pad carrier, thus not through pivoting components. Still, the braking force can be transmitted indirectly between them but without using cam interfaces as described. Still, many other variants of the proposed concept will be apparent to a skilled person, in light of a review of the present disclosure.

REFERENCE NUMERALS

5 brake central axis

10 annular disk brake

12 main support

16 mounting bolts

20 outboard casing part

22 inboard casing part

26 bolts

40 actuator assembly

48 axially-movable member

48a main outer annular plate

48b inner flange portion

50 cam elements

52 apertures

54 apertures additional elements

slots

rotor disk

brake pads (first set)

brake pads (second set)

brake pad carrier

intermediary member

ramp surfaces

rollers

rollers

axles

rollers

first annular actuator (first air bellows) second annular actuator (second air bellows) air port

air port

housing

a cylindrical portion

b double-sided angular inner flanged portion cover

a receptacles

b cylindrical portions 314 bolts

320 inboard-facing inner annular compartment

322 outboard-facing inner annular compartment

340 spring

342 plate

342a inboard portion

342b outboard portion

350 override arrangement

352 bolts

354 hole

356 threaded hole

Claims

CLAIMS:

1. A fail-safe actuator assembly (40) for an annular disk brake (10), the brake (10) having an inboard side, an outboard side and a central axis (5), the actuator assembly (40) including:

a substantially cylindrical housing (310), the housing (310) being coaxially disposed with reference to the central axis (5) and having opposite opened inboard and outboard sides, the housing (310) including inboard-facing and outboard-facing inner annular compartments (320, 322) that are coaxially disposed with reference to the central axis (5);

a first annular actuator device (300) positioned inside the inboard-facing compartment (320), the first actuator device (300) having an inboard-facing side;

a second annular actuator device (302) positioned inside the outboard-facing compartment (322), the second actuator device (302) having an outboard-facing side;

a cover (312) connected to the inboard side of the housing (310);

an axially-movable plate (342) coaxially disposed with reference to the central axis (5), the plate (342) including an inboard portion (342a) and an outboard portion (342b), the inboard portion (342a) having an outboard-facing side that is in engagement with the inboard-facing side of the first actuator device (300), the plate (342) being movable between an inboard end position and an outboard end position, the brake (10) being in a normal operation mode when the plate (342) is at its inboard end position and the brake (10) being in a parking mode when the plate (342) is at its outboard end position;

at least one spring (340) provided between the plate (342) and the cover (312), the plate (342) being set to its outboard end position by the at least one spring (340) when the first actuator device (300) is under a threshold internal fluid pressure, and the plate (342) being set to its inboard end position when the first actuator device (300) is above the threshold internal fluid pressure; and

an axially-movable annular member (48) coaxially disposed with reference to the central axis (5), the axially-movable member (48) having an inboard-facing side that is in engagement with the outboard-facing side of the second actuator device (302), the axially-movable member (48) being engaged by the outboard portion (342b) of the plate (342) and then pushed into an outboard position to set the brake (10) into the parking mode when the plate (342) is in its outboard end position.

The actuator assembly (40) as defined in claim 1, characterized in that the at least one spring (340) includes a plurality of axisymmetric helical compression springs (340) positioned inside the housing (310).

The actuator assembly (40) as defined in claim 2, characterized in that each spring (340) generally extends in a direction that is substantially parallel to the central axis (5).

The actuator assembly (40) as defined in any one of claims 1 to 3, characterized in that the first and second actuator devices (300, 302) each include a pneumatic air bellows.

5. The actuator assembly (40) as defined in any one of claims 1 to 4, characterized in that the outboard-facing side of the inboard portion (342a) of the plate (342) is generally annular and fits inside the inboard-facing compartment (320).

6. The actuator assembly (40) as defined in any one of claims 1 to 5, characterized in that the compartments (320, 322) are juxtaposed to one another and are separated by an inwardly-extending radial wall.

7. The actuator assembly (40) as defined in any one of claims 1 to 6, characterized in that the housing (310) includes an outer cylindrical portion (310a) and a double-sided annular inner flanged portion (310b) extending inside the cylindrical portion, the compartments (320, 322) being formed between the cylindrical portion (310a) and a corresponding side of the inner flanged portion (310b).

8. The actuator assembly (40) as defined in any one of claims 1 to 7, characterized in that the plate (342), when at its outboard end position, abuts against an inner flanged portion (48b) of the axially-movable member (48).

9. The actuator assembly (40) as defined in any one of claims 1 to 8, characterized in that the axially-movable member (48) includes a main outer annular portion (48a) fitting inside the outboard-facing compartment (322).

10. The actuator assembly (40) as defined in any one of claims 1 to 9, characterized in that the cover (312) is removably connected to the housing (310).

The actuator assembly (40) as defined in any one of claims 1 to 10, characterized in that the actuator assembly (40) includes a manually-operated override arrangement (350).

The actuator assembly (40) as defined in claim 11, characterized in that the override arrangement (350) includes at least one bolt (352) having a threaded shank to be inserted in an axial direction through a hole (354) in the cover (312) and engaging a corresponding threaded hole (356) provided on the outboard portion (342b) of the plate (342).

A method of operating an actuator assembly (40) for an annular disk brake (10) having a central axis (5), the method including:

generating a spring force inside the actuator assembly (40);

receiving a first fluid pressure at a first annular actuator device (300) provided inside the actuator assembly (40) to generate a first actuator force opposing the spring force;

maintaining the brake (10) in a normal operation mode using the first actuator force while the first fluid pressure is above a threshold fluid pressure to overcome the spring force;

receiving a second fluid pressure at a second annular actuator device (302) provided inside the actuator assembly (40) to generate a second actuator force, the second actuator force being substantially proportional to the second fluid pressure;

generating a variable braking force inside the brake (10) using the second actuator force when the brake (10) is maintained in the normal operation mode; and automatically setting the brake (10) from the normal operation mode to a parking mode using the spring force in the actuator assembly (40) if the first fluid pressure falls under the threshold fluid pressure.

14. The method as defined in claim 13, characterized in that generating the variable braking force inside the brake (10) includes increasing the second fluid pressure to move an axially-movable member (48) towards an outboard side of the brake (10).

15. The method as defined in claim 13 or 14, characterized in that the first actuator force and the second actuator force are in opposite directions.

16. The method as defined in claim 15, characterized in that the opposite directions are substantially parallel to a central axis (5).

17. The method as defined in any one of claims 13 to 16, characterized in that maintaining the brake (10) in the normal operation mode includes maintaining the first fluid pressure at a substantially constant fluid pressure that is above the threshold fluid pressure.

18. The method as defined in any one of claims 13 to 17, characterized in that the method further includes:

while the first fluid pressure is under the threshold fluid pressure, overriding at least the parking mode of the brake (10) using at least one hand-operated component to be inserted at the actuator assembly (40).

19. The method as defined in any one of claims 13 to 18, characterized in that the first fluid pressure is received as a first pneumatic pressure and the second fluid pressure is received as a second pneumatic pressure.

20. The method as defined in any one of claims 13 to 19, characterized in that automatically setting the brake (10) to the parking mode includes constantly generating a full braking force inside the brake (10) using only the spring force from within the actuator assembly (40).

21. The method as defined in any one of claims 13 to 20, characterized in that generating a spring force inside the actuator assembly (40) includes combining individual spring forces from a plurality of springs (340) located inside the actuator assembly (40).

22. The method as defined in any one of claims 13 to 21, characterized in that the method further includes:

setting the brake (10) from the parking mode back to the normal operation mode if the first fluid pressure rises again above the threshold fluid pressure.