Dual piston actuator

Abstract

Actuator arrangement (126), comprising: a first piston (242) and a first cylinder (266), wherein the first piston (242) is received in the first cylinder (266), wherein the first piston (242) and the first cylinder (266) define a first chamber, wherein the first cylinder (266) defines an inlet in fluid communication with the first chamber; a second piston (246) and a second cylinder (270), wherein the second piston (246) is received in the second cylinder (270), wherein the second piston (246) and the second cylinder (270) define a second chamber, wherein the second piston (246) defines a fluid passage that fluidly couples the first and second chambers, and wherein the second cylinder (270) defines an outlet in fluid communication with the second chamber; a valve body (250) which is movable between a first valve position, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position, wherein the valve body (250) blocks the fluid passage to prevent fluid communication between the first and second chambers; and a drive mechanism (238) configured to axially displace the second piston (246), and configured to axially displace the first piston (242) when the pressure in the first chamber is less than a predetermined pressure, and not to displace the first piston (242) when the pressure in the first chamber is equal to or greater than the predetermined pressure; wherein the valve body (250) is prevented from moving into the second valve position and the first piston (242) provides a first fluid volume at a first pressure for the outlet when the drive mechanism (238) displaces the first piston (242) in a first axial direction; wherein the valve body (250) is allowed to move into the second valve position when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston (246) is displaced in the first axial direction; and wherein the second piston (246) provides a second fluid volume at a second pressure for the outlet, wherein the first fluid volume is larger than the second fluid volume and the first pressure is lower than the second pressure when the valve body (250) is in the second valve position and the second piston (246) is displaced in the first axial direction.

Description

AREA

[0001] The present disclosure relates to dual piston actuators for separable vehicle powertrains. BACKGROUND

[0002] This section provides background information related to the present disclosure that may not necessarily constitute prior art.

[0003] Hydraulically operated clutches generally have a piston that applies force to the clutch system to engage the clutch plates. In order to have low parasitic drag when the clutch is disengaged, there must be clearance between the plates rotating relative to each other. Since there are often many plates in a clutch, this means that the total axial space in a clutch can be substantial. Therefore, the piston must retract a considerable distance from the point of engagement in order for the clutch plates to achieve maximum separation for low drag torque. Typically, the piston must first compensate for this entire distance for the piston to move from the fully retracted position to engagement of the clutch plates before the clutch begins to transmit a usable amount of torque.In order for the piston to quickly compensate for this distance, a high flow of hydraulic fluid is required to the piston during initial actuation. Because the clutch discs are disengaged during this initial piston movement, the fluid can be provided at a lower pressure. After the clutch discs begin to engage, the piston has to travel a shorter distance before the clutch is fully engaged, so the high flow rate is no longer needed. Instead, a higher pressure is required to force the clutch discs into full engagement. These two requirements are diametrically opposed in the design of a hydraulic actuation system. The present invention relates to an actuator of such a hydraulic system capable of rapid adjustment and high pressures.

[0004] GB 2 123 503 A describes a master cylinder with variable displacement.

[0005] GB 348 691 A describes a hydraulic braking mechanism.

[0006] DE 42 37 852 A1 describes a hydraulic actuating cylinder for a friction clutch. SUMMARY

[0007] This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features.

[0008] The present teaching provides a power transmission component including a friction clutch, a hydraulic cylinder, and an actuator. The friction clutch may have a plurality of first clutch plates and a plurality of second clutch plates interleaved with the first clutch plates. The hydraulic cylinder may be coupled to the friction clutch. The hydraulic cylinder may have a cylinder chamber and a cylinder piston movable within the cylinder chamber between a first cylinder position and a second cylinder position. In the first cylinder position, the cylinder piston is retractable relative to the first and second clutch plates. In the second cylinder position, the cylinder piston is extended toward the first and second clutch plates to a greater extent than when the cylinder piston is in the first position.The actuator may include a first piston and cylinder, a second piston and cylinder, a valve body, and a drive mechanism. The first piston may be received within the first cylinder. The first piston and cylinder may define a first chamber. The second piston may be received within the second cylinder. The second piston and cylinder may define a second chamber in fluid communication with the cylinder chamber. The second piston may define a fluid passage fluidly coupling the first and second chambers. The valve body may be movable between a first valve position and a second valve position. When the valve body is in the first valve position, the fluid passage may be opened to allow fluid communication between the first and second chambers.When the valve body is in the second valve position, the valve body can block the fluid passage to prevent fluid communication between the first and second chambers. The drive mechanism can be configured to axially displace the second piston. The drive mechanism can be configured to axially displace the first piston when a pressure in the first chamber is less than a predetermined pressure and to not displace the first piston when the pressure in the first chamber is equal to or greater than a predetermined pressure. When the drive mechanism displaces the first piston in a first axial direction, the valve body can be prevented from moving to the second valve position, and the first piston can provide a first volume of fluid at a first pressure to the cylinder chamber.When the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction, the valve body can be allowed to move to the second valve position. When the valve body is in the second valve position and the second piston is displaced in the first axial direction, the second piston can provide a second fluid volume at a second pressure to the cylinder chamber. The first fluid volume can be greater than the second fluid volume, and the first pressure can be less than the second pressure.

[0009] The present teachings further provide a power transmission component including a friction clutch, a hydraulic cylinder, and an actuator. The friction clutch may have a plurality of first clutch plates and a plurality of second clutch plates interleaved with the first clutch plates. The hydraulic cylinder may be coupled to the friction clutch. The hydraulic cylinder may have a cylinder chamber and a cylinder piston movable within the cylinder chamber between a first cylinder position and a second cylinder position. In the first cylinder position, the cylinder piston may be retracted relative to the first and second clutch plates. In the second cylinder position, the cylinder piston may be extended toward the first and second clutch plates to a greater extent than when the cylinder piston is in the first position.The actuator may include a housing, a first piston, a second piston, and an elastic member. The housing may have a first cylinder and a second cylinder arranged about an axis. The first piston may be received within the first cylinder. The first piston and the first cylinder may define a first fluid chamber. The second piston may be received within the second cylinder. The second piston and the second cylinder may define a second fluid chamber in fluid communication with the cylinder chamber. The elastic member may have a first end coupled for axial displacement to the second piston and a second end coupled for axial displacement to the first piston.The elastic member may be configured to displace the first piston to provide a first volume of fluid under a first pressure to the cylinder chamber when a pressure in the first chamber is lower than a predetermined pressure and the second piston is displaced in a first axial direction. The elastic member may be configured to compress between the first and second pistons to allow relative axial movement between the first and second pistons when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction. The second piston may provide a second volume of fluid under a second pressure to the cylinder chamber when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction.The first fluid volume may be greater than the second fluid volume, and the first pressure may be less than the second pressure.

[0010] The present teachings further provide a power transmission component including a friction clutch, a hydraulic cylinder, and an actuator. The friction clutch may have a plurality of first clutch plates and a plurality of second clutch plates interleaved with the first clutch plates. The hydraulic cylinder may be coupled to the friction clutch. The hydraulic cylinder may have a cylinder chamber and a cylinder piston movable within the cylinder chamber between a first cylinder position and a second cylinder position. In the first cylinder position, the cylinder piston is retractable relative to the first and second clutch plates. In the second cylinder position, the cylinder piston is extended toward the first and second clutch plates to a greater extent than when the cylinder piston is in the first position.The actuator may include a housing, a first piston, a second piston, an elastic member, and a valve. The housing may have a first cylinder arranged about an axis and a second cylinder coaxial with the first cylinder. The first piston may be received within the first cylinder. The first piston and the first cylinder may define a first fluid chamber. The second piston may be received within the second cylinder. The second piston and the second cylinder may define a second fluid chamber fluidly connected to the cylinder chamber. The elastic member may have a first end coupled for axial translation to the second piston and a second end coupled for axial translation to the first piston. The valve may have a fluid passage and a valve body. The fluid passage may fluidly couple the first and second chambers.The valve body can be movable relative to the fluid passage between a first valve position and a second valve position. When the valve body is in the first valve position, the fluid passage can be opened to allow fluid communication between the first and second chambers. When the valve body is in the second valve position, the valve body can block the valve passage to prevent fluid communication between the first and second chambers. The elastic element can be configured to axially displace the first piston in a first axial direction when the second piston is displaced in the first axial direction and a pressure in the first chamber is less than a predetermined pressure.The elastic member may be configured to compress between the first and second pistons to permit relative axial movement of the first and second pistons between a first relative position and a second relative position when the second piston is displaced in the first axial direction and the pressure in the first chamber is equal to or greater than the predetermined pressure. The first piston may be configured to engage the valve body to prevent the valve body from moving to the second valve position when the first and second pistons are in the first relative position. The valve body may be permitted to move to the second valve position when the first and second pistons are in the second relative position.

[0011] The disclosure of the present invention further includes the following embodiments: Embodiment 1: Power transmission component comprising: a friction clutch having a plurality of first clutch plates and a plurality of second clutch plates interleaved with the first clutch plates; a hydraulic cylinder coupled to the friction clutch, the hydraulic cylinder having a cylinder chamber and a cylinder piston movable in the cylinder chamber between a first cylinder position in which the cylinder piston is retracted relative to the first and second clutch discs and a second cylinder position in which the cylinder piston is extended toward the first and second clutch discs to a greater extent than when the cylinder piston is in the first position; and an actuator, including: a first piston and cylinder, the first piston received in the first cylinder, the first piston and cylinder defining a first chamber; a second piston and cylinder, the second piston being received within the second cylinder, the second piston and cylinder defining a second chamber in fluid communication with the cylinder chamber, the second piston defining a fluid passage fluidly coupling the first and second chambers; a valve body movable between a first valve position, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position, wherein the valve body blocks the fluid passage to prevent fluid communication between the first and second chambers; and a drive mechanism configured to axially displace the second piston and configured to axially displace the first piston when a pressure in the first chamber is less than a predetermined pressure and not to displace the first piston when the pressure in the first chamber is equal to or greater than the predetermined pressure; wherein the valve body is prevented from moving into the second valve position and the first piston provides a first volume of fluid under a first pressure to the cylinder chamber when the drive mechanism displaces the first piston in a first axial direction; wherein the valve body is allowed to move to the second valve position when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction; wherein the second piston provides a second fluid volume under a second pressure to the cylinder chamber, the first fluid volume being greater than the second fluid volume and the first pressure being less than the second pressure when the valve body is in the second valve position and the second piston is displaced in the first axial direction. Embodiment 2: The power transmission component of embodiment 1, wherein the drive mechanism includes a lead screw rotatable about an axis and having a plurality of threads, the second piston having a plurality of mating threads, the threads and the mating threads being configured to cooperate to axially translate the second piston when the lead screw is rotated about the axis. Embodiment 3: The power transmission component of embodiment 2, wherein the drive mechanism comprises an elastic member, the elastic member having a first end coupled for axial displacement to the second piston and a second end coupled for axial displacement to the first piston, and wherein the elastic member is configured to displace the first piston when the pressure in the first chamber is less than the predetermined pressure and the second piston is axially displaced, and is configured to compress between the first and second pistons to allow relative axial movement between the first and second pistons when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction. Embodiment 4: The power transmission component of embodiment 1, wherein the first piston is configured to engage the valve body to prevent the valve body from moving to the second valve position when the first piston is in a first position relative to the second piston, and wherein the valve body is permitted to move to the second valve position when the first piston is in a second position relative to the second piston. Embodiment 5: The power transmission component of embodiment 4, wherein the valve body comprises a sealing element and a control element, wherein the sealing element is configured to engage with a first side of the second piston proximate the second chamber to seal the fluid passage, wherein the control element extends from the sealing element through the fluid passage to a second side of the second piston proximate the first chamber, wherein the control element is configured to engage the first piston proximate the second side of the second piston. Embodiment 6: The power transmission component of embodiment 1, wherein the first piston has an annular shape defining a central bore and the second piston extends through the central bore. Embodiment 7: The power transmission component of embodiment 1, wherein the first chamber is fluidly coupled to a fluid reservoir. Embodiment 8: Power transmission component comprising: a friction clutch having a plurality of first clutch discs and a plurality of second clutch plates interleaved with the first clutch plates; a hydraulic cylinder coupled to the friction clutch, the hydraulic cylinder having a cylinder chamber and a cylinder piston movable in the cylinder chamber between a first position in which the cylinder piston is retracted relative to the first and second clutch plates, and a second position in which the cylinder piston is extended toward the first and second clutch discs to a greater extent than when the cylinder piston is in the first position; and an actuator comprising: a housing having a first cylinder and a second cylinder arranged about an axis; a first piston received in the first cylinder, the first piston and the first cylinder defining a first fluid chamber; a second piston received in the second cylinder, the second piston and the second cylinder defining a second fluid chamber in fluid communication with the cylinder chamber; and an elastic member having a first end coupled for axial displacement to the second piston and a second end coupled for axial displacement to the first piston; wherein the elastic member is configured to displace the first piston to provide a first volume of fluid under a first pressure to the cylinder chamber when a pressure in the first chamber is less than a predetermined pressure and the second piston is displaced in the second cylinder in a first axial direction, and wherein the elastic member is configured to compress between the first and second pistons to allow relative axial movement between the first and second pistons when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction; wherein the second piston provides a second fluid volume under a second pressure to the cylinder chamber when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston is displaced in the first axial direction, wherein the first fluid volume is greater than the second fluid volume, and wherein the first pressure is less than the second pressure. Embodiment 9: The power transmission component of embodiment 8, wherein the actuator further comprises a lead screw rotatable about the axis and having a plurality of threads, the second piston having a plurality of mating threads, the threads and the mating threads being configured to cooperate to axially translate the second piston when the lead screw is rotated about the axis. Embodiment 10: The power transmission component of embodiment 8, wherein the actuator comprises a valve, the valve having a fluid passage and a valve body, the fluid passage fluidly coupling the first and second chambers, the valve body being movable between a first valve position relative to the fluid passage, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position relative to the fluid passage, the valve body sealing the valve passage to prevent fluid communication between the first and second chambers. Embodiment 11: The power transmission component of embodiment 10, wherein the fluid passage is formed in the second piston. Embodiment 12: The force transmitting component of embodiment 10, wherein the first piston is configured to engage the valve body to prevent the valve body from moving to the second valve position when the first piston is in a first position relative to the second piston, and wherein the valve body is permitted to move to the second valve position when the first piston is in a second position relative to the second piston. Embodiment 13: The power transmission component of embodiment 12, wherein the valve body comprises a sealing element and a control element, wherein the sealing element is configured to engage with a first side of the second piston proximate the second chamber to seal the fluid passage, wherein the control element extends from the sealing element through the fluid passage to a second side of the second piston proximate the first chamber, wherein the control element is configured to engage the first piston proximate the second side of the second piston. Embodiment 14: The power transmission component of embodiment 8, wherein the first piston has an annular shape defining a central bore, and the second piston is received by the central bore. Embodiment 15: Power transmission component comprising: a friction clutch having a plurality of first clutch plates and a plurality of second clutch plates interleaved with the first clutch plates; a hydraulic cylinder coupled to the friction clutch, the hydraulic cylinder having a cylinder chamber and a cylinder piston movable in the cylinder chamber between a first cylinder position in which the cylinder piston is retracted relative to the first and second clutch plates, and a second cylinder position in which the cylinder piston is extended toward the first and second clutch plates to a greater extent than when the cylinder piston is in the first cylinder position; and an actuator comprising: a housing having a first cylinder arranged about an axis and a second cylinder coaxial with the first cylinder; a first piston received in the first cylinder, the first piston and the first cylinder defining a first fluid chamber; a second piston received in the second cylinder, the second piston and the second cylinder defining a second fluid chamber in fluid communication with the cylinder chamber; an elastic member having a first end coupled for axial displacement to the second piston and a second end coupled for axial displacement to the first piston; and a valve having a fluid passage and a valve body, wherein the fluid passage fluidly couples the first and second chambers, wherein the valve body is movable relative to the fluid passage between a first valve position, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position, wherein the valve body blocks the valve passage to prevent fluid communication between the first and second chambers: wherein the elastic member is configured to displace the first piston in a first axial direction when the second piston is displaced in the first axial direction and a pressure in the first chamber is less than a predetermined pressure, and the elastic member is configured to compress between the first and second pistons to allow relative axial movement of the first and second pistons between a first relative position and a second relative position when the second piston is displaced in the first axial direction and the pressure in the first chamber is equal to or greater than the predetermined pressure; wherein the first piston is configured to engage the valve body to prevent the valve body from moving to the second valve position when the first and second pistons are in the first relative position, and wherein the valve body is permitted to move to the second valve position when the first and second pistons are in the second relative position. Embodiment 16: The force transmitting component of embodiment 15, wherein a pressure in the second chamber maintains the valve body in the second valve position when the first and second pistons are in the second relative position. Embodiment 17: The power transmission component of embodiment 15, wherein the first piston is configured to provide a first volume of fluid under a first pressure through the fluid passage and to the cylinder chamber when the first piston is displaced in the first axial direction, and the second piston is configured to provide a second volume of fluid under a second pressure to the cylinder chamber when the valve body is in the second valve position and the second piston is displaced in the first axial direction. Embodiment 18: The power transmission component of embodiment 15, wherein the actuator further comprises a lead screw rotatable about the axis and having a plurality of threads, the second piston having a plurality of mating threads, the threads and the mating threads being configured to cooperate to axially translate the second piston when the lead screw is rotated about the axis. Embodiment 19: The power transmission component of embodiment 15, wherein the fluid passage is formed in the second piston. Embodiment 20: The power transmission component of embodiment 15, wherein the first piston has an annular shape defining a central bore and the second piston extends through the central bore.

[0012] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for illustrative purposes only and are not intended to limit the scope of the present disclosure. DRAWINGS

[0013] The drawings described herein are intended only for illustrative purposes of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure. Fig. 1 is an exemplary vehicle having a powertrain component constructed in accordance with the present teachings; Fig. 2 is a sectional view of the power transmission component of Fig. 1 in a first state; Fig. 3 is a sectional view of the power transmission component of Fig. 1 in a second state; Fig. 4 is a sectional view of the power transmission component of Fig. 1 in a third state; and Fig. 5 is a detailed view of part of the power transmission component of Fig. 1. Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

[0014] Corresponding reference numerals indicate corresponding parts in the several views of the drawings. DETAILED DESCRIPTION

[0015] Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0016] With reference to Fig. 1 of the drawings, an exemplary vehicle having clutches actuable by a power-transmitting component constructed in accordance with the teachings of the present disclosure is generally designated by the reference numeral 10. The vehicle 10 may have a powertrain 14 and a drive system or driveline 18. The powertrain 14 may be conventionally constructed and may include a power source 22 and a transmission 26. The power source 22 may be configured to provide motive power and may include, for example, an internal combustion engine and / or an electric motor. The transmission 26 may receive motive power from the power source 22 and may output power to the driveline 18. The transmission 26 may have a variety of automatically or manually selected gear ratios.The powertrain 18 is an all-wheel drive configuration in the particular example provided, but those skilled in the art will understand that the teachings of the present disclosure are applicable to other powertrain configurations, including, for example, four-wheel drive configurations, rear-wheel drive configurations, and front-wheel drive configurations.

[0017] The driveline 18 may include a front axle assembly 30, a transfer case (PTU) 34, a propshaft 38, and a rear axle assembly 42. An output of the transmission 26 may be coupled to an input of the front axle assembly 30 to drive an input member 46 of the front axle assembly 30. The PTU 34 may have a PTU input member 50 that may receive rotational power from the input member 46 of the front axle assembly 30 and a PTU output member 54 that may transmit rotational power to the propshaft 38. The propshaft 38 may couple the PTU output member 54 to the rear axle assembly 42 such that rotational power output by the PTU 34 is received by the rear axle assembly 42. The front axle assembly 30 and the rear axle assembly 42 could be operated on a continuous basis to drive vehicle front and rear wheels 58 and 62, respectively.However, it should be understood that the driveline 18 could include one or more clutches to interrupt the transmission of rotational power through a portion of the driveline 18. In the particular example provided, the driveline 18 includes a first clutch 66, which may be configured to interrupt the transmission of rotational power into or through the PTU 34, and a power-transmitting component 70, which may be configured to control rotation of components within the rear axle assembly 42.

[0018] The front axle assembly 30, the PTU 34, and the first clutch 66 may be assembled into a housing assembly 74. The front axle assembly 30 may include the input member 46, a two-speed transmission 78, a front differential assembly 82, and a pair of front axle shafts 86. The input member 46 may be a hollow shaft that may be configured to engage the output member of the transmission 26. The input member 46 may be configured to engage the two-speed transmission 78. The two-speed transmission 78 may be configured to engage the first clutch 66 and the front differential assembly 82.

[0019] The front differential assembly 82 may be coupled to the front axle shafts 86 and allow for a speed differential between the front axle shafts 86. In the example provided, the front differential assembly 82 is an open differential. However, it should be understood that other means for speed differential could alternatively be used, such as one or more clutches, a limited-slip differential, or a limited-slip differential.

[0020] The PTU 34 may include the PTU input member 50, a pinion gear 90, and the PTU output member 54. The PTU input member 50 may include a beveled ring gear mounted within the housing assembly. The pinion gear 90 may mesh with the beveled ring gear of the PTU input member 50 and may be oriented along an axis generally perpendicular to the rotational axis of the input member 46. If desired, the pinion gear 90 may be a hypoid pinion. The PTU output member 54 may be coupled to the pinion gear 90 for associated rotation.

[0021] The first or mode clutch 66 may be any type of clutch, including a friction clutch or a constant velocity device. In the particular example provided, the mode clutch 66 is a dog clutch having a clutch input member 94 and a clutch output member 98. The clutch input member 94 may be coupled to the two-speed transmission 78 for associated rotation therewith. The clutch output member 98 may be non-rotatably coupled to the bevel gear of the PTU input member 50. The mode clutch 66 may be operable to selectively transmit rotational power between the clutch input member 94 and the clutch output member 98.

[0022] The rear axle assembly 42 may include a drive pinion 102, a bevel gear 106, a second differential assembly 110, a pair of second shafts 114, and the power-transmitting component 70. The drive pinion 102 may be coupled to one end of the propeller shaft 38 for associated rotation. The second bevel gear 106 may mesh with the drive pinion 102. The second differential assembly 110 may be configured to receive rotational power transmitted through the second bevel gear 106 and to transmit rotational power to the second shafts 114. The second differential assembly 110 may have a means for permitting speed differential between the second shafts 114. In the example provided, the means for speed differential comprises an open differential.

[0023] The power-transmitting component 70 may include a second clutch 118, a cylinder 122, and a hydraulic actuator 126. The power-transmitting component 70 may further include a fluid reservoir 130 and a vent 134. The reservoir 130 may be configured to contain hydraulic fluid, and the vent 134 may vent to atmosphere or may be configured to selectively vent to atmosphere based on a predetermined pressure of the reservoir 130. The reservoir 130 and the hydraulic actuator 126 may be fluidly coupled by a hydraulic line 138. The hydraulic actuator 126 and the cylinder 122 may be fluidly coupled by a hydraulic line 142. The second clutch or axle disconnect clutch 118 of the power transmission component 70 may be configured to selectively interrupt power transmission through the second differential assembly 110.The axle disconnect clutch 118 may be any type of clutch and may be mounted coaxially with the second differential assembly 110. In the particular example provided, the axle disconnect clutch 118 includes a clutch input member 146 coupled to the bevel ring gear 106 for associated rotation therewith, a plurality of first friction plates 150 non-rotatably coupled to the clutch input member 146, a clutch output member 154 coupled to the second differential assembly 110 for providing rotational power thereto, and a plurality of second friction plates 158 non-rotatably coupled to the clutch output member 154.The first and second friction plates 150 and 158 may be nested, and the cylinder 122 may be employed to compress the first and second friction plates 150 and 158 so that they frictionally engage each other, allowing rotational power to be transmitted from the bevel ring gear 106 through the axle disconnect clutch 118 and to the second differential assembly 110. When the cylinder 122 is disengaged, so that rotational power is not transmitted through the axle disconnect clutch 118, the rear wheels 62 drive the second shafts 114, but the axle disconnect clutch 118 prevents the transmission of rotational power into the bevel ring gear 106. In this manner, operation of the vehicle 10 in a front-wheel drive mode will not allow the rear wheels 62 to "reverse drive" the bevel ring gear 106.

[0024] With reference to the Fig. 2 to Fig. 5 of the drawings, the power transmission component 70 is illustrated in more detail. The power transmission component 70 may further include a valve 210 disposed in series with the line 138 between the reservoir 130 and the hydraulic actuator 126. The cylinder 122 may include a cylinder piston 214, a thrust washer 218, and a cylinder housing 222 defining a cylinder chamber 226.The cylinder 122 is selectively operable to move the thrust plate 218 between a first cylinder position in which the first and second friction plates 150, 158 are disengaged so that rotational power is not transmitted between the clutch input member 146 and the clutch output member 154, and a second cylinder position in which the first and second friction plates 150, 158 are frictionally engaged to transmit rotational power from the clutch input member 146 to the clutch output member 154. The present teachings also permit the minimization of frictional drag by allowing a greater separation of the friction plates 150, 158 than typical cylinders.

[0025] In the example provided, the cylinder piston 214 is an annular piston, and the cylinder chamber 226 is an annular chamber, although it is understood that other configurations may be used. The cylinder piston 214 may be movable within the cylinder chamber 226 and coupled to the thrust washer 218 such that the cylinder piston 214 is retracted relative to the first and second friction discs 150, 158 when the thrust washer 218 is in the first cylinder position, and the cylinder piston 214 is extended relative to the first and second friction discs 150, 158 when the thrust washer 218 is in the second cylinder position. The cylinder housing 222 may have a cylinder port 230 for receiving hydraulic fluid into the cylinder chamber 226 on a first side 234 of the cylinder piston 214.Introducing fluid into the cylinder chamber 226 at the first side 234 of the cylinder piston 214 may cause the cylinder piston 214 to move from the retracted position to the extended position to cause the thrust washer 218 to move from the first cylinder position to the second cylinder position. Removing fluid from the cylinder chamber 226 at the first side 234 of the cylinder piston 214 may cause the cylinder piston 214 to move from the extended position to the retracted position to cause the thrust washer 218 to move from the second cylinder position to the first cylinder position. The cylinder piston 214 may also be biased toward the retracted position by a biasing member (not shown), such as a spring.In this manner, the introduction and removal of fluid within the cylinder can move the cylinder piston 214 within the chamber to selectively engage and disengage the clutch 118.

[0026] The hydraulic actuator 126 may include an actuator housing 238, a first piston 242, a second piston 246, a valve body 250, and a drive mechanism 238. The actuator housing 238 may have a first end 258 and a second end 262 and may define a first chamber 266, a second chamber 270, and a first port 274. The housing may also define a second port 278, an intermediate chamber 282, and a drive cavity 286. The first port 274 may be proximate the second end 262 and fluidly couple the second chamber 270 to the conduit 142 to fluidly couple the hydraulic actuator 126 to the cylinder 122. The second port 278 may be between the first and second ends 258, 262 and may fluidly couple the first chamber 266 to the conduit 138 to fluidly couple the hydraulic actuator 126 to the reservoir 130.The valve 210 may also be arranged in series with the conduit 138 between the reservoir 130 and the second port 278. In the example provided, the valve 210 is a check valve configured to allow fluid to flow between the reservoir 130 and the first chamber 266 when the first chamber 266 is not pressurized and to prevent fluid from flowing between the reservoir 130 and the first chamber 266 when the first chamber 266 is pressurized.

[0027] The first and second chambers 266, 270 may have a generally cylindrical shape and may be arranged coaxially about an axis 290. The cross-sectional area of ​​the first chamber 266 may be larger than the cross-sectional area of ​​the second chamber 270. The second chamber 270 may be near the second end 262, and the first chamber 266 may be axially between the second chamber 270 and the first end 258. The intermediate chamber 282 may have a generally cylindrical shape, may be coaxial with the first and second chambers 266, 270, and may be axially between the first and second chambers 266, 270. The cross-sectional area of ​​the intermediate chamber 282 may be less than the cross-sectional area of ​​the first chamber 266 and greater than the cross-sectional area of ​​the second chamber 270. The drive cavity 286 may be coaxial with the first chamber 266 and axially between the first chamber 266 and the first end 258 of the actuator housing 238.

[0028] The first piston 242 may be received in the first chamber 266, and the second piston 246 may be received in the second chamber 270. The cross-sectional area of ​​the first piston 242 may be larger than the cross-sectional area of ​​the second piston 246 such that the first piston 242 has a greater displacement than the second piston 246. The second piston 246 may have a generally cylindrical shape and may have a driven end 294 and a compression end 298 opposite the driven end 294. The second piston 246 may also define a radial passage 302 and an axial bore 306 in fluid communication with the radial passage 302. The driven end 294 may be proximate the first end 258 of the actuator housing 238 and may be coupled to the drive mechanism 238, as explained below. The driven end 294 of the second piston 246 may extend within the first chamber 266.The compression end 298 may be proximate the second end 262 of the actuator housing 238 and may extend into the second chamber 270. The compression end 298 may have a closure bore 310 with a sealing seat surface 314 that is proximate the second chamber 270 and in fluid communication with the axial bore 306 and the second chamber 270. A radially outer surface 318 of the second piston 246 may seal with a radially inner surface 322 of the second chamber 270. In the example provided, the seal is formed by an O-ring 326 received in an annular groove 330 in the second chamber 270, although other configurations or seals may be used. The radial feedthrough 302 extends through the radially outer surface 318 of the second piston 246 between the driven end 294 and the compression end 298.The axial bore 306 extends axially through the compression end 298 and intersects the radial passage 302 near the driven end 294 and intersects the closure bore 310 near the compression end 298 to allow fluid to pass between the first and second chambers 266, 270 through the second piston 246.

[0029] The first piston 242 may have a main body 334 and an extension member 338. The main body 334 may be generally annularly shaped with a radially outer surface 342 and defining a central bore 346 with a radially inner surface 350. The main body 334 may have a driven side 354 proximate the first end 258 of the actuator housing 238 and a compression side 358 opposite the driven side 354 and in communication with the first chamber 266. The radially outer surface 342 of the main body 334 may seal with a radially inner surface 362 of the first chamber 266. In the example provided, the seal is formed by an O-ring 366 received in an annular groove 370 in the radially outer surface 342, although other configurations or seals may be used.The second piston 246 may be received by the central bore 346 of the first piston 242, and the radial outer surface 318 of the second piston 246 may seal with the radial inner surface 350 of the central bore 346. In the example provided, the seal is formed by an O-ring 374 received in an annular groove 378 in the radial inner surface 350, although other configurations or seals may be used. The extension member 338 may extend from the compression side 358 of the main body 334 toward the second end 262 of the actuator housing 238 to engage the valve body 250, as explained below. In the example provided, the extension member 338 is a cylindrically shaped body that extends axially from the main body 334 and is slidable along the radially outer surface 318 of the second piston 246, although other configurations may be used.

[0030] The valve body 250 may include a radial member 382, ​​an axial rod 386, and a closure 390. The axial rod 386 may be disposed within the axial bore 306 of the second piston 246 and may extend into the radial passage 302 near the driven end 294 and may extend into the closure bore 310 near the compression end 298. The axial rod 386 may have a diameter less than the diameter of the axial bore 306 to allow fluid to flow around the axial rod 386 and through the axial bore 306. The closure 390 may be coupled to the axial rod 386 near the compression end 298 of the second piston 246. The closure 390 may be received within the closure bore 310 and may be larger than the axial bore 306.The closure 390 may be movable within the closure bore 310 and configured to engage the sealing seat surface 314 such that the closure 390 may block the flow of fluid through the axial bore 306 when engaged with the sealing seat surface 314. The radial member 382 may be disposed within the radial passage 302, may be coupled to the axial rod 386, and may extend radially from the axial rod 386 through the radial passage 302. The radial member 382 may extend radially outward from the radial outer surface 318 of the second piston 246 and may engage the extension member 338 of the first piston 242. The radial member 382 may be configured to allow fluid to flow around the radial member 382 and through the radial passage 302.In the example provided, the radial member 382 is a rod that extends in the radial direction from the axial rod 386, although other configurations may be used.

[0031] The drive mechanism 238 may be configured to axially translate the second piston 246 and may be disposed at least partially within the drive cavity 286. The drive mechanism 238 may include a lead screw 394, a threaded body 398, a resilient member 402, a motor 406, and a gearbox 410. The lead screw 394 may be disposed about the axis 290 and may have a plurality of external threads 414 configured to mesh with a plurality of internal threads 418 formed in the threaded body 398. The threaded body 398 may be non-rotatable within the actuator housing 238 and may be coupled for linear movement to the second piston 246. In the present example, the second piston 246 is integrally formed with the threaded body 398, although other configurations may be used.The threaded body 398 may have a boss 422 extending radially outward from the threaded body 398 and proximate the first end 258 of the actuator housing 238. The resilient member 402 may be a coil spring disposed about the axis 290 and axially between the boss 422 and the driven side 354 of the first piston 242. A first end 426 of the resilient member 402 may be coupled to the boss 422 for associated axial movement, and a second end 430 of the resilient member 402 may be coupled to the driven side 354 of the first piston 242 for associated axial movement. The motor 406 may be received within the drive cavity 286 and may be configured to drive rotation of the lead screw 394 about the axis 290. The motor 406 may be any type of motor, such as a DC motor, and may have an output shaft (not shown).The transmission 410 may have an input member (not shown), a plurality of reduction gears (not shown), and an output member 434. The output shaft may be coupled to the input member of the transmission 410 to transmit rotational power through the plurality of gears to the output member 434. The output member 434 may be rotationally coupled to the lead screw 394 to rotationally drive the lead screw 394.

[0032] The operation of the power transmission component 70 will now be explained in detail. Fig. 2 illustrates the power transmission component 70 in an unactuated state. Fig. 3 shows the power transmission component 70 in an intermediate state. Fig. 4 illustrates the power-transmitting component 70 in an actuated state. In the unactuated state, the first and second pistons 242, 246 are retracted relative to the first end 262 of the actuator housing 238, and the cylinder piston 214 is retracted relative to the first and second friction discs 150, 158, causing the second clutch 118 to disengage, as previously discussed. In the unactuated state, the first and second chambers 266, 270 may be depressurized, allowing fluid flow between the reservoir 130 and the first chamber 266. The extension member 338 of the first piston 242 also engages the radial member 382 to prevent axial movement of the radial member 382 within the radial passage 302 further toward the driven end 294 of the second piston 246.In this way, the extension element 338 prevents the closure 390 from sealing the axial bore 306, and thus fluid can flow between the first and second chambers 266, 270.

[0033] A current may be applied to the motor 406 to rotationally drive the lead screw 394 in a first rotational direction. Rotation of the lead screw 394 in the first rotational direction may cause the threaded body 398 and the second piston 246 to translate axially toward the second end 262 of the actuator housing 238. As the threaded body 398 moves axially toward the second end 262, the elastic member 402 may also be translated axially. The axial translation of the elastic member 402 causes the second end 430 of the elastic member 402 to press against the driven side 354 of the first piston 242 and translate the first piston 242 within the first chamber 266 axially toward the second end 262 of the actuator housing 238.Displacement of the first piston 242 in the first chamber 266 can cause the pressure of the fluid in the first chamber to increase, which can close the valve 210 to prevent fluid from flowing through the second port 278 to the reservoir 130. The first piston 242 can displace fluid in the first chamber 266 and cause fluid to flow generally from the first chamber 266 through the radial passage 302, the axial bore 306 and the second chamber 270, through the first port 274 and into the cylinder chamber 226. As the first and second pistons 242, 246 move toward the second end 262 and to the positions shown in FIG. Fig. 3, relative to the intermediate state, the extension member 338 may continue to engage the radial member 382 to prevent the closure 390 from blocking the axial bore 306. Because the first piston 242 has a greater displacement than the second piston 246, a relatively large volume of fluid under a relatively low pressure may flow from the first and second chambers 266, 270 into the cylinder chamber 226 to quickly move the cylinder piston 214 a large distance. This may cause the distance between the first and second friction discs 150, 158 to quickly equalize, as previously discussed.

[0034] After the first and second friction discs 150, 158 begin to engage, the fluid pressure in the first and second chambers 266, 270 increases. When the lead screw 394 begins to rotate in the first rotational direction, the threaded body 398 and the second piston 246 may continue to move axially toward the second end 262. At this point, the pressure in the first chamber 266 may begin to overcome the spring force of the resilient member 402 and compress the resilient member 402 between the first piston 242 and the boss 422. By compressing the elastic element 402, the first piston 242 no longer moves axially at the same speed as the second piston 246. The pressure in the first chamber 266 may reach a pressure such that the first piston 242 stops axially moving toward the second end 262 while the elastic element 402 is compressed.As the second piston 246 continues to move further toward the second end 262 relative to the first piston 242, the extension member 338 becomes farther away from the compression end 298 of the second piston 246 and can no longer engage the radial member 382 to prevent the closure 390 from sealing the axial bore 306. At this point, the pressure in the second chamber 270 may force the closure 390 to seal against the sealing seat surface 314 and block fluid from flowing from the second chamber 270 back toward the first chamber 266. In this manner, the valve body 250 and the second piston 246 may act as a check valve. The lead screw 394 may be rotated further to move the second piston 246 further toward the second end 262.With the sealed axial bore 306, advancement of the relatively smaller displacement second piston 246 can further compress the fluid in the second chamber 270 to deliver fluid to the cylinder chamber 226 at a low volume and high pressure. This low volume and high pressure fluid can move the cylinder piston 214 to a fully extended position within the cylinder chamber 226, as shown in FIG. Fig. 4 with respect to the actuated state to cause the first and second friction discs 150, 158 to fully engage. The valve body 250 can thus allow power to the motor 406 to be shut off while maintaining pressure within the cylinder chamber 226. In this way, the second clutch 118 can remain engaged without continuously applying force to the motor 406.

[0035] The motor 406 can be reversed, or the transmission 410 can be configured to have a reverse gear (not shown) to drive the output member 434 in a second rotational direction opposite to the first rotational direction. Driving the output member 434 in the reverse rotational direction can rotate the lead screw 394 in the reverse rotational direction to translate the threaded body 398 axially toward the first end 258 to reverse the previously described operation and disengage the second clutch 118.

[0036] Those skilled in the art will understand that while the power transfer component 70 is described with reference to a rear axle assembly 42, the power transfer component 70 may be adapted to selectively transfer power between other vehicle driveline components, such as a front axle assembly, or between a transmission and a drive system.

[0037] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but are interchangeable and may be used in a selected embodiment, even if not expressly shown or described. They may also be modified in a variety of ways. Such variations are not to be regarded as a departure from the disclosure, and all such changes are intended to be included within the scope of the disclosure.

[0038] Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those skilled in the art. Numerous specific details are set forth, such as examples of specific components, devices, and methods, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms, and none should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0039] The technical language used herein is for the sole purpose of describing particular embodiments and is not intended to be limiting. As used herein, the singular forms "a" and "an" and "the" may also be intended to include the plural forms unless the context clearly indicates otherwise. The terms "comprises," "comprising," "having," and "having" are inclusive and therefore indicate the presence of the specified functions, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other functions, integers, steps, operations, elements, components, and / or groups thereof.The method steps, processes, and operating procedures described herein should not be construed as requiring their performance in the order discussed or illustrated, unless specifically identified as such. It is also understood that additional or alternative steps may be employed.

[0040] When an element or layer is described as being "on," "engaging with," "connected to," or "coupled to" another element or layer, it may be directly on, engaging with, connected to, or coupled to the other element or layer, or intervening elements or layers may be present. Conversely, when an element is described as being "directly on," "directly engaging with," "directly connected to," or "directly coupled to" another element or layer, no intervening elements or layers may be present. Other words used to describe the relationship between elements should be interpreted in a like manner (e.g., "between" versus "directly between," "adjacent" versus "directly next to," etc.).As used herein, the term “and / or” includes any and all combinations of one or more of the related listed items.

[0041] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers, and / or sections, these elements, components, regions, layers, and / or sections should not be limited by these terms. These terms may only be used to distinguish one element, component, region, layer, or section from another region, layer, or section. Terms such as "first," "second," and other numerical terms, when used herein, do not imply a sequence or order unless the context clearly indicates otherwise. Thus, a first element, component, region, layer, or section discussed below could be called a second element, component, region, layer, or section without departing from the present teachings of the exemplary embodiments.

[0042] Spatially relative terms such as "inside," "outside," "below," "under," "lower," "above," "upper," and the like may be used herein for convenience of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation illustrated in the figures. For example, when the device in the figures is inverted, elements described as being "below" or "below" other elements or features are then oriented "above" the other elements or features. Thus, the example term "below" can encompass both an above and below orientation.The device may be oriented differently (rotated 90 degrees or with other orientations), and the spatially relative descriptors used herein may be interpreted accordingly.

Claims

[1] Actuator assembly (126) comprising: a first piston (242) and a first cylinder (266), the first piston (242) being received within the first cylinder (266), the first piston (242) and the first cylinder (266) defining a first chamber, the first cylinder (266) defining an inlet in fluid communication with the first chamber; a second piston (246) and a second cylinder (270), the second piston (246) being received within the second cylinder (270), the second piston (246) and the second cylinder (270) defining a second chamber, the second piston (246) defining a fluid passage fluidly coupling the first and second chambers, the second cylinder (270) defining an outlet in fluid communication with the second chamber; a valve body (250) movable between a first valve position, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position, wherein the valve body (250) blocks the fluid passage to prevent fluid communication between the first and second chambers; and a drive mechanism (238) configured to axially displace the second piston (246) and configured to axially displace the first piston (242) when a pressure in the first chamber is less than a predetermined pressure and not to displace the first piston (242) when the pressure in the first chamber is equal to or greater than the predetermined pressure; wherein the valve body (250) is prevented from moving into the second valve position and the first piston (242) provides a first volume of fluid under a first pressure to the outlet when the drive mechanism (238) displaces the first piston (242) in a first axial direction; wherein the valve body (250) is allowed to move into the second valve position when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston (246) is displaced in the first axial direction; and wherein the second piston (246) provides a second fluid volume under a second pressure to the outlet, the first fluid volume being greater than the second fluid volume and the first pressure being less than the second pressure when the valve body (250) is in the second valve position and the second piston (246) is displaced in the first axial direction. [2] The actuator assembly (126) of claim 1, wherein the actuator assembly (126) includes a lead screw (394) rotatable about an axis (290) and having a plurality of threads (414), the second piston (246) having a plurality of mating threads (418), the threads (414) and the mating threads (418) being configured to cooperate to axially translate the second piston (246) when the lead screw (394) is rotated about the axis (290). [3] The actuator assembly (126) of claim 2, wherein the drive mechanism (238) includes a resilient member (402), the resilient member (402) having a first end (426) coupled for axial displacement to the second piston (246) and a second end (430) coupled for axial displacement to the first piston (242), the resilient member (402) being configured to displace the first piston (242) when the pressure in the first chamber is less than the predetermined pressure and the second piston (246) is axially displaced, and being configured to compress between the first and second pistons (242, 246) to allow relative axial movement between the first and second pistons (242, 246) when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston (246) is displaced in the first axial direction. [4] The actuator assembly (126) of claim 1, wherein the first piston (242) is configured to engage the valve body (250) to prevent the valve body (250) from moving to the second valve position when the first piston (242) is in a first position relative to the second piston (246), and wherein the valve body (250) is permitted to move to the second valve position when the first piston (242) is in a second position relative to the second piston (246). [5] The actuator assembly (126) of claim 4, wherein the valve body (250) includes a sealing element (390) and a control element (386), the sealing element (390) being configured to engage a first side of the second piston (246) proximate the second chamber to seal the fluid passage, the control element (386) extending from the sealing element (390) through the fluid passage to a second side of the second piston (246) proximate the first chamber, the control element (386) being configured to engage the first piston (242) proximate the second side of the second piston (246). [6] The actuator assembly (126) of claim 1, wherein the first piston (242) has an annular shape defining a central bore (346) and the second piston (246) extends through the central bore (346). [7] The actuator assembly (126) of claim 1, wherein the first chamber is fluidly coupled to a fluid reservoir (130). [8] Actuator assembly (126) comprising: a housing (238) having a first cylinder (266) and a second cylinder (270) arranged about an axis (290), the housing (238) defining an inlet and an outlet; a first piston (242) received in the first cylinder (266), the first piston (242) and the first cylinder (266) defining a first fluid chamber in fluid communication with the inlet; a second piston (246) received in the second cylinder (270), the second piston (246) and the second cylinder (270) defining a second fluid chamber in fluid communication with the outlet; and an elastic member (402) having a first end (426) coupled for axial displacement to the second piston (246) and a second end (430) coupled for axial displacement to the first piston (242); wherein the elastic element (402) is configured to displace the first piston (242) to provide a first volume of fluid under a first pressure to the outlet when a pressure in the first chamber is less than a predetermined pressure and the second piston (246) is displaced in a first axial direction in the second cylinder (270), and wherein the elastic element (402) is configured to compress between the first and second pistons (242, 246) to allow relative axial movement between the first and second pistons (242, 246) when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston (246) is displaced in the first axial direction; wherein the second piston (246) provides a second fluid volume under a second pressure to the outlet when the pressure in the first chamber is equal to or greater than the predetermined pressure and the second piston (246) is displaced in the first axial direction, wherein the first fluid volume is greater than the second fluid volume, and wherein the first pressure is less than the second pressure. [9] The actuator assembly (126) of claim 8, wherein the actuator assembly (126) further comprises a lead screw (394) rotatable about the axis (290) and having a plurality of threads (414), the second piston (246) having a plurality of mating threads (418), the threads (414) and the mating threads (418) being configured to cooperate to axially translate the second piston (246) when the lead screw (394) is rotated about the axis (290). [10] The actuator assembly (126) of claim 8, wherein the actuator assembly (126) comprises a valve, the valve having a fluid passage and a valve body (250), the fluid passage fluidly coupling the first and second chambers, the valve body (250) being movable between a first valve position relative to the fluid passage, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position relative to the fluid passage, the valve body (250) sealing the valve passage to prevent fluid communication between the first and second chambers. [11] The actuator assembly (126) of claim 10, wherein the fluid passage is formed in the second piston (246). [12] The actuator assembly (126) of claim 10, wherein the first piston (242) is configured to engage the valve body (250) to prevent the valve body (250) from moving to the second valve position when the first piston (242) is in a first position relative to the second piston (246), and wherein the valve body (250) is permitted to move to the second valve position when the first piston (242) is in a second position relative to the second piston (246). [13] The actuator assembly (126) of claim 12, wherein the valve body (250) includes a sealing element (390) and a control element (386), the sealing element (390) being configured to engage a first side of the second piston (246) proximate the second chamber to seal the fluid passage, the control element (386) extending from the sealing element through the fluid passage to a second side of the second piston (246) proximate the first chamber, the control element (386) being configured to engage the first piston (242) proximate the second side of the second piston (246). [14] The actuator assembly (126) of claim 8, wherein the first piston (242) has an annular shape defining a central bore (346), and the second piston (246) is received by the central bore (346). [15] Actuator assembly (126) comprising: a housing (238) having a first cylinder (266) disposed about an axis (290) and a second cylinder (270) coaxial with the first cylinder (266), the housing defining an inlet and an outlet; a first piston (242) received in the first cylinder (266), the first piston (242) and the first cylinder (266) defining a first fluid chamber in fluid communication with the inlet; a second piston (246) received in the second cylinder (270), the second piston (246) and the second cylinder (270) defining a second fluid chamber in fluid communication with the outlet; a spring (402) having a first end (426) coupled for axial displacement to the second piston (246) and a second end (430) coupled for axial displacement to the first piston (242); and a valve having a fluid passage and a valve body (250), wherein the fluid passage fluidly couples the first and second chambers, wherein the valve body (250) is movable relative to the fluid passage between a first valve position, wherein the fluid passage is open to allow fluid communication between the first and second chambers, and a second valve position, wherein the valve body (250) blocks the valve passage to prevent fluid communication between the first and second chambers: wherein the spring (402) is configured to displace the first piston (242) in a first axial direction when the second piston (246) is displaced in the first axial direction and a pressure in the first chamber is less than a predetermined pressure, and the spring (402) is configured to compress between the first and second pistons (242, 246) to allow relative axial movement of the first and second pistons (242, 246) between a first relative position and a second relative position when the second piston (246) is displaced in the first axial direction and the pressure in the first chamber is equal to or greater than the predetermined pressure; wherein the first piston (242) is configured to engage the valve body (250) to prevent the valve body (250) from moving to the second valve position when the first and second pistons (242, 246) are in the first relative position, and wherein the valve body (250) is permitted to move to the second valve position when the first and second pistons (242, 246) are in the second relative position. [16] The actuator assembly (126) of claim 15, wherein a pressure in the second chamber maintains the valve body (250) in the second valve position when the first and second pistons (242, 246) are in the second relative position. [17] The actuator assembly (126) of claim 15, wherein the first piston (242) is configured to provide a first volume of fluid under a first pressure through the fluid passage and to the outlet when the first piston (242) is displaced in the first axial direction, and the second piston (246) is configured to provide a second volume of fluid under a second pressure to the outlet when the valve body (250) is in the second valve position and the second piston (246) is displaced in the first axial direction. [18] The actuator assembly (126) of claim 15, wherein the actuator assembly (126) further comprises a lead screw (394) rotatable about the axis (290) and having a plurality of threads (414), the second piston (246) having a plurality of mating threads (418), the threads (414) and the mating threads (418) being configured to cooperate to axially translate the second piston (246) when the lead screw (394) is rotated about the axis (290). [19] The actuator assembly (126) of claim 15, wherein the fluid passage is formed in the second piston (246). [20] The actuator assembly (126) of claim 15, wherein the first piston (242) has an annular shape defining a central bore (346) and the second piston (246) extends through the central bore (346).

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