Axle lift system and method of control

An axle lift system and a method of control. The axle lift system may include a liftable axle assembly and an axle securing unit. The axle securing unit may selectively secure the liftable axle assembly in a secured position in which the tires of the liftable axle assembly may not support the vehicle.

TECHNICAL FIELD

This patent application relates to an axle lift system and a method of control.

BACKGROUND

An axle lift assembly is disclosed in U.S. Pat. No. 8,695,998.

SUMMARY

In at least one embodiment, a method of controlling an axle lift system for a vehicle is provided. A first axle assembly may have a first tire and may be coupled to a chassis via a first suspension unit. A second axle assembly may have a second tire and may be coupled to the chassis via a second suspension unit. The first suspension unit and the second suspension unit may be refracted to lower the chassis. The second axle assembly may be held in a secured position with an axle securing unit. The first suspension unit may be extended to raise the chassis such that the second tire does not support the vehicle.

In at least one embodiment, a method of controlling an axle lift system for a vehicle is provided. The method may include retracting the first suspension unit to lower the chassis, releasing the axle securing unit from a second axle assembly, and extending first and second suspension units to raise the chassis such that first and second tires provided with the first and second axle assemblies, respectively, support the vehicle.

In at least one embodiment, an axle lift system for a vehicle is provided. The axle lift system may include a first axle assembly, a second axle assembly, and an axle securing unit. The first axle assembly may have a first tire and may be configured to be coupled to a chassis with a first suspension unit. The second axle assembly may have a second tire and may be configured to be coupled to the chassis with a second suspension unit. The axle securing unit may be disposed proximate the chassis and the second axle assembly. The axle securing unit may be actuated when the first suspension unit and the second suspension unit are retracted to lower the chassis, and the first tire and the second tire support the vehicle.

DETAILED DESCRIPTION

Referring toFIG. 1, an exemplary vehicle10is shown. The vehicle10may be a motor vehicle that may be used to transport cargo. For example, the vehicle10may be configured as a truck and may include a tractor12and/or a trailer14. The tractor12may receive a driver and may propel and steer the vehicle10. The trailer14may be coupled to the tractor12and may be configured to receive cargo. The vehicle10may also have an axle lift system16and a control system18.

The tractor12and the trailer14may each include a frame or chassis20. For clarity in the text below, a common reference number is used to designate the chassis20of the tractor12and the chassis20of the trailer14in the figures, although the chassis20of the tractor12may be separate from the chassis20of the trailer14.

The tractor12and the trailer14may include one or more axle assemblies. For example, the tractor12and/or the trailer14may include at least one non-liftable axle assembly30and a liftable axle assembly32. The non-liftable and liftable axle assemblies30,32may be disposed proximate or may be mounted to the chassis20.

The non-liftable and liftable axle assemblies30,32may each rotatably support one or more wheel assemblies34that may include a tire36that may be mounted on a wheel38. A non-liftable axle assembly30may be configured such that its associated wheel assemblies34may be disposed on a support surface40, like a road or the ground, during normal operation to support the vehicle10and facilitate movement of the vehicle10. A non-liftable axle assembly30may not be held in a lifted position or a secured position by the axle lift system16such that its associated wheel assemblies34are raised or lifted from the support surface40toward the chassis20and held in the secured position such that an associated wheel assembly34is lifted above and does not engage the support surface40or support the weight of the vehicle10. A liftable axle assembly32may be selectively held in a secured position by the axle lift system16such that its associated wheel assemblies34are raised or lifted from the support surface40toward the chassis20and do not engage the support surface40to support the weight of the vehicle10. As such, one or more non-liftable axle assemblies30or axle assemblies that are not held in the secured position may support the vehicle10when a liftable axle assembly32is held in the secured position. The wheel assemblies34of a liftable axle assembly32may be disposed on the support surface40when they are not held in a secured position by the axle lift system16to support the vehicle10and facilitate movement of the vehicle10.

The non-liftable axle assemblies30and liftable axle assemblies32may be provided in various quantities and locations. InFIG. 1, one non-liftable axle assembly30and one liftable axle assembly32are provided with the tractor12and the trailer14, although it is contemplated that a greater number of non-liftable axle assemblies30and/or liftable axle assemblies32may be provided. InFIG. 1, a non-liftable axle assembly30is located to the right of each liftable axle assembly32from the perspective shown; however, it is contemplated that this positioning may be reversed in one or more embodiments. In addition, it is contemplated that the either the tractor12or the trailer14may not be provided with a liftable axle assembly32in one or more embodiments.

The non-liftable axle assemblies30and liftable axle assemblies32may be provided in different configurations. For example, a non-liftable axle assembly30or a liftable axle assembly32may or may not be configured to steer the vehicle10. In addition, a non-liftable axle assembly30or a liftable axle assembly32and may or may not be configured as a drive axle that may provide torque to at least one wheel assembly34that may propel the vehicle10.

Referring toFIG. 5, an exemplary pair of non-liftable and liftable axle assemblies30,32is shown in more detail. The non-liftable and liftable axle assemblies30,32may include a housing42that may rotatably support one or more wheel assemblies34. In the case of a drive axle, the housing42may receive a differential and an axle that may provide torque to one or more wheel assemblies34. The non-liftable and/or liftable axle assemblies30,32may also include multiple axles that may each support corresponding vehicle wheels. An example of such an axle assembly is the Meritor RideSentry™ MPA Series Trailer Air Suspension. Such an axle assembly may have two or more axles or axle assemblies that may be mounted on a common subframe, which may be referred to as a slider or slider assembly. The slider assembly may be mounted to the chassis20such that the slider assembly and move longitudinally with respect to the chassis20(e.g., back and forth in a direction extending from the front of the vehicle to the back of the vehicle). The axle assemblies will be described below primarily with reference to non-liftable and liftable axle assemblies that have a single axle; however, it is to be understood that multiple axle assemblies may be associated with a non-liftable axle assembly or a liftable axle assembly in one or more embodiments. As such, the axle lift system16may hold a liftable axle assembly that may include multiple axles that may each support different sets of wheel assemblies34in a secured position in which the wheel assemblies34are raised or lifted from the support surface40toward the chassis20and do not engage the support surface40to support the weight of the vehicle10.

The non-liftable and liftable axle assemblies30,32may each be coupled to or connected to the chassis20with an axle suspension system or suspension unit50. The suspension unit50and may dampen vibrations, provide a desired level of ride quality, and control ride height or the distance between the chassis20and the support surface40. In addition, the suspension unit50may be part of the axle lift system16or may cooperate with the axle lift system16to raise and lower the chassis20with respect to the support surface40so that a liftable axle assembly32may be secured or released. The suspension unit50may be configured as an air suspension unit or an air ride suspension unit that may employ air springs or air bellows that receive a pressurized gas as will be discussed in more detail below. As such, the suspension unit50may raise or lower the chassis20by inflating or deflating one or more air springs. The suspension unit50may be provided in various configurations. For example, the suspension unit50may include a suspension arm52, a hanger54, a shock absorber56, and an air spring58.

The suspension arm52may extend from the axle assembly30,32and may be fixedly positioned with respect to the axle assembly30,32. For example, at least one suspension arm52may be fixedly positioned on or with respect to the housing42. It is contemplated that the suspension unit50may be provided without a suspension arm in one or more embodiments.

The hanger54may be fixedly disposed or fixedly positioned with respect to the chassis20. For instance, a top surface of the hanger54may be mounted to the chassis20. The suspension arm52may be pivotally coupled to the hanger54. For example, the suspension arm52may be pivotally coupled to the hanger54with a pivot pin60that may extend through the hanger54and the suspension arm52. As such, the suspension arm52may pivot about the pivot pin60and may pivot with respect to the hanger54.

The shock absorber56may be provided to dampen shock impulses and dissipate kinetic energy. The shock absorber56may be mounted to the hanger54at a first end and to the suspension arm52at a second end.

An air spring58may be disposed proximate the suspension arm52. For example, the air spring58may be disposed proximate an end of the suspension arm52that may be disposed opposite the hanger54. The air spring58may support the chassis20. For instance, the air spring58may be disposed above an associated axle assembly30,32and under the chassis20. The air spring58may have a flexible bellows62that may at least partially define a chamber within the air spring58that may receive pressurized gas from a pressurized gas supply system70. Providing pressurized gas to one or more air springs58may extend a corresponding suspension unit50or move the suspension unit50toward an extended position and may raise or lift the chassis20away from the support surface40. Venting pressurized gas from one or more air springs58may retract a corresponding suspension unit50or move the suspension unit50toward a retracted position and may lower the chassis20toward the support surface40.

Referring toFIG. 1, the pressurized gas supply system70may provide a pressurized gas or pressurized gas mixture, such as air, to the air spring58. The term pressurized gas is used to generically reference a single gas or a gas mixture, such as air, that may be pressurized above atmospheric pressure by the pressurized gas supply system70. As is best shown inFIG. 1, the pressurized gas supply system70may include a pressurized gas source72, one or more conduits74, and one or more valves76.

The pressurized gas source72may include a tank or reservoir that contains a volume of pressurized gas and/or a pump or compressor that provides pressurized gas.

A conduit74may fluidly connect the pressurized gas source72to an air spring58. A conduit74may have any suitable configuration, such as a hose, tubing, pipe, or combinations thereof.

One or more valves76may be provided to control the flow of pressurized gas to and/or from an air spring58. For example, at least one valve76may enable or disable the flow of pressurized gas from the pressurized gas source72to at least one air spring58. The valve76may have any suitable configuration and may be actuated in any suitable manner, such as with a solenoid. InFIG. 1, a single valve76is associated with the air springs58of each axle assembly30,32; however, a different number of valves76or different valve configuration may be employed. For instance, a valve76may be associated with each individual air spring58or a single valve76may be associated with multiple axle assemblies30,32. In addition, it is contemplated that a common valve76may provide inflation and venting functionality or that separate valves76may be provided to control inflation and venting of an air spring58. An air spring58may be inflated by opening an associated valve76to provide pressurized gas from the pressurized gas source72to the air spring58. In addition, the valve76may enable or disable venting or the exhaust of pressurized gas from the air spring58. The air spring58may be deflated by positioning the valve76such that pressurized gas is vented or exhausted from the air spring58, such as by venting pressurized gas to the surrounding environment.

Referring toFIGS. 2 and 5, the axle lift system16may include an axle securing unit80. The axle securing unit80may be associated with or may be provided with a liftable axle assembly32, but may not be provided with a non-liftable axle assembly30. The axle securing unit80may be disposed proximate the chassis20. For example, the axle securing unit80may be disposed on a cross member82that may be fixedly positioned on or with respect to the chassis20. The axle securing unit80may include a hook84and an actuator86.

The hook84may be configured to secure an associated axle assembly in a secured position. The hook84may be configured move with respect to the chassis20and/or the cross member82. For example, the hook84may be movably disposed on the cross member82with a mounting bracket and may be configured to engage and receive the liftable axle assembly32. The hook84may be configured to move between a first position, which may also be called a released position, and a second position, which may also be called a secured position. The hook84may be disengaged from a liftable axle assembly32when in the released position as is shown inFIGS. 5 and 6. As such, movement of the liftable axle assembly32may be limited by the suspension unit50, but not the axle securing unit80when the hook84is in the released position. The hook84may engage or receive the liftable axle assembly32when in a secured position, such as is shown inFIGS. 7 and 8. As such, movement of the liftable axle assembly32may be limited by axle securing unit80when the hook84is in the secured position. For example, the axle securing unit80may inhibit or limit movement of the liftable axle assembly32away from the chassis20or toward the support surface40when in the secured position. Thus, the liftable axle assembly32may be free to move further away from the chassis20when the hook84is in the released position as compared to the secured position. The hook84may move in any suitable manner. For example, the hook84may be configured to move linearly or rotate or pivot about a pivot axis88.

The actuator86may be configured to move or actuate the hook84between the released position and the secured position. The actuator86may be of any suitable type, such as a pneumatic, hydraulic, electrical, or electromechanical actuator. In at least one embodiment, the actuator86may include an actuator shaft that may be operatively connected to the hook84. In the embodiment shown inFIG. 2, the actuator shaft extends through a hole in the cross member82.

The axle securing unit80may not include an actuator that lifts an axle assembly toward the chassis20. As such, the axle lift system16described herein may be smaller, lighter, less expensive, easier to package, and easier to install than an axle lift assembly that has an actuator that lifts a heavy axle assembly toward the chassis rather than lowering the chassis toward the support surface40prior to securing or releasing an axle assembly.

Referring toFIG. 1, the control system18may monitor and control operation of components and systems of the vehicle10. The control system18may include at least one microprocessor-based controller or control module that may monitor and/or control various components or systems of the vehicle10, such as the axle lift system16and/or the pressurized gas supply system70. For example, the control system18may be configured to control the operation of the valves76to control the flow of pressurized gas to the air spring58and to control venting of pressurized gas from the air spring58. The connection or communication between the control system18and the valves76is represented with connection nodes A, B, C and D.

The control system18may also communicate with various sensors or input devices. For instance, the control system18may be configured to receive a signal or data from a speed sensor90, a gear selector sensor92, a load sensor94, and an operator communication device96.

The speed sensor90may be configured to detect or provide data indicative of the speed of the vehicle10. For example, the speed sensor90may detect the rotational speed of a drivetrain component or a wheel assembly34. The speed sensor90may be of any suitable type and may provide data indicative of whether the vehicle10is stationary or moving.

The gear selector sensor92may be configured to detect or provide data indicative of the selection of a transmission gear ratio or whether a transmission drive gear has not been selected or engaged (e.g., the transmission is in a neutral or park position). As such, the gear selector sensor92may provide data that may be indicative of whether the vehicle10is stationary (e.g., a neutral or park position is selected). A gear selector sensor92may be associated with a gearshift lever or similar operator input device for selection of a transmission gear ratio.

One or more load sensors94may be provided to detect or provide data indicative of axle load and/or vehicle load. The load sensor94may be of any suitable type. For example, the load sensor94may include one or more physical sensors that may be disposed on the vehicle10that may detect or provide data indicative of the pressure of pressurized gas that is disposed in or supplied to an air spring58. As such, axle load or vehicle load may be based on data indicative of pressure in an air spring58. A load sensor94may be associated with or provided with one or more non-liftable axle assemblies30or with one or more non-liftable axle assemblies30and one or more liftable axle assemblies32. Alternatively, the load sensor94may be a virtual sensor that may receive vehicle load data that may be wirelessly transmitted to the vehicle10, such as from a scale that may be equipped with suitable communication equipment. Communication between the control system18and each load sensor94is represented by connection nodes L1through L4inFIG. 1.

The operator communication device96may be provided to receive an input from an operator or an operator command. The operator communication device96may be of any suitable type or types, such as a switch, button, sensor, display, touchscreen, keypad, voice command or speech recognition system, or the like. The operator communication device96may be used to input data that may not be predetermined or provided by a sensor or other input device, such as may be the case when a vehicle10is not equipped with one or more of the sensors discussed herein. In addition, the operator communication device96may be used to allow manual entry of vehicle load data and/or a command to lift and secure or lower and release one or more liftable axle assemblies32. It is also contemplated that the operator communication device96may provide a warning message when the operator attempts to secure or release a liftable axle assembly32under conditions that fall outside of target axle load levels or predetermined operating parameters.

Referring toFIG. 3, a flowchart of an exemplary method of controlling an axle lift system is shown. The method may be used to inflate or deflate one or more air springs to extend or retract one or more suspension units to facilitate securing and lifting of one or more liftable axle assemblies and/or releasing one or more liftable axle assemblies so that its associated wheel assemblies may engage the support surface and support the vehicle. As will be appreciated by one of ordinary skill in the art, the flowchart represents control logic which may be implemented or affected in hardware, software, or a combination of hardware and software. For example, the various functions may be affected by a programmed microprocessor. The control logic may be implemented using any of a number of known programming and processing techniques or strategies and is not limited to the order or sequence illustrated. For instance, interrupt or event-driven processing may be employed in real-time control applications rather than a purely sequential strategy as illustrated. Likewise, parallel processing, multitasking, or multi-threaded systems and methods may be used.

Control logic may be independent of the particular programming language, operating system, processor, or circuitry used to develop and/or implement the control logic illustrated. Likewise, depending upon the particular programming language and processing strategy, various functions may be performed in the sequence illustrated, at substantially the same time, or in a different sequence while accomplishing the method of control. The illustrated functions may be modified, or in some cases omitted, without departing from the spirit or scope intended.

Referring toFIG. 3, a flowchart is shown that illustrates steps associated with lifting a liftable axle assembly32and holding the liftable axle assembly32in a secured position with the axle securing unit80.FIGS. 5-8are provided to help illustrate associated method steps. For simplicity,FIGS. 5-8illustrate the method in the context of a vehicle10that has one non-liftable axle assembly30and one liftable axle assembly32. For illustration purposes, the method will be described beginning with the vehicle10in an initial configuration shown inFIG. 5in which liftable axle assembly32is released and not secured by the axle lift system16with the tires36of the non-liftable axle assembly30and the liftable axle assembly32disposed on the support surface40such that the non-liftable axle assembly30and the liftable axle assembly32support the chassis20.

At block100, the method may determine whether any liftable axle assemblies32are available to lift. A liftable axle assembly32may be available to lift when it is not held in a secured position with an associated axle securing unit80. As such, a tire36of a liftable axle assembly32may be disposed on the support surface40and the hook84of the axle securing unit80may be disengaged from the liftable axle assembly32. A determination as to whether a liftable axle assembly32is available to lift may be based on data associated with the axle securing unit80, such as whether the hook84is in the secured position or the actuator86has been actuated to move the hook84to the secured position. Alternatively or in addition, a determination as to whether a liftable axle assembly32is available to lift may be based on data from an associated load sensor94. For instance, a liftable axle assembly32may not be available to lift when a corresponding axle load is not detected (i.e., no axle load may be detected when a liftable axle assembly32is secured with an associated axle securing unit80and the liftable axle assembly32does not support the vehicle10). If a liftable axle assembly is not available to lift (e.g., all liftable axle assemblies32are secured by an associated axle securing unit80), then the method or iteration of the method may end at block102. If a liftable axle assembly is available to lift (e.g., at least one liftable axle assembly32is not secured by an associated axle securing unit80), then the method may continue at block104.

At block104, the method may determine whether the vehicle10is stationary. A determination as to whether the vehicle10is stationary may be based on data from the speed sensor90and/or the gear selector sensor92. If the vehicle is not stationary, then the method or iteration of the method may end at block102. If the vehicle is stationary, then the method may continue at block106. It is also contemplated that block104may be omitted in one or more embodiments to facilitate raising or lowering of an axle assembly if the vehicle is not stationary. As such, block100may proceed to block106instead of block104.

At block106, one or more axle loads may be compared to an axle load threshold. An axle load may be based on data from a load sensor94that may support the vehicle10. As such, an axle load may be determined for a non-liftable axle assembly30or a non-liftable axle assembly30and one or more liftable axle assemblies32that are not secured by the axle securing unit80and are supporting the vehicle10. The axle load threshold may be a predetermined value that may be based on vehicle development testing. For example, the axle load threshold may be set at a predetermined value, such as 80% of a maximum load rating of the axle assembly. If the axle load or axle loads are not less than the axle load threshold (e.g., at least one of a non-liftable axle load and a liftable axle load is greater than or equal to a corresponding axle load threshold), then the method or method iteration may end at block102. If an axle load is less than the axle load threshold (e.g., at least one of a non-liftable axle load and a liftable axle load is less than a corresponding axle load threshold), then the method may continue at block108.

At block108, one or more liftable axle assemblies32may be selected to lift. A liftable axle assembly32may be manually or automatically selected. A liftable axle assembly32may be manually selected by an operator or based on a user input that may be provided via the operator communication device96. For example, the operator communication device96may permit an operator to select one or more liftable axle assemblies32to lift and secure. A liftable axle assembly32may be automatically selected by the control system18. For example, the control system18may automatically select a single available liftable axle assembly32when the axle load on the non-liftable axle assembly30does not exceed the axle load threshold. If more than one liftable axle assembly32is available to lift, then the method may determine whether to secure one or more liftable axle assemblies32based on load data from one or more load sensors94. For example, if there are two liftable axle assemblies32and one non-liftable axle assembly30and none of the liftable axle assemblies32are secured with a corresponding axle securing unit80, then the two liftable axle assemblies32and one non-liftable axle assembly30support the weight of the vehicle10. As such, the load sensors94associated with each axle assembly may detect an axle load. The total axle load or the maximum axle load may be compared to additional axle load thresholds or may be used to reference data in a lookup table to determine whether one or more liftable axle assemblies32may be lifted. As an example, two liftable axle assemblies32may be selected to lift when the maximum axle load detected is less than 50% of an associated maximum axle load rating. One liftable axle assembly32may be selected to lift when the maximum axle load detected is between 50% and 80% of an associated maximum axle load rating. These values and ranges are examples and it is contemplated that additional or different values or ranges may be provided depending on the number of axle assemblies and the configuration of the vehicle.

At block110, the chassis20may be lowered. The chassis20may be lowered by deflating the air spring or air springs58associated with one or more suspension units50which in turn may retract those suspension units50toward the chassis20. An air spring58may be deflated by venting pressurized gas to the surrounding environment as previously described. In at least one embodiment, all air springs58of the tractor12and the trailer14except for those provided with the front steering axle assembly of the tractor12may be deflated when the vehicle10includes a tractor12and a trailer14. Similarly, all air springs58of the tractor12except for those provided with front steering axle assembly may be deflated when the vehicle10includes a tractor12without a trailer14. Lowering of the chassis20is best shown by comparingFIG. 5toFIG. 6. InFIG. 6, the air springs58of the non-liftable axle assembly30and the liftable axle assembly32are deflated, thereby allowing the associated suspension units50to retract or move toward a retracted position in which the suspension arm52may pivot about the pivot pin60and move closer to the chassis20.

At block112, the selected liftable axle assembly or assemblies32may be secured with the axle lift system16. Securing of a liftable axle assembly32is best shown inFIG. 7. InFIG. 7, the actuator86of the axle lift system16may move the hook84into engagement with the liftable axle assembly32to hold the liftable axle assembly32in the secured position.

At block114, the chassis20may be raised. The chassis20may be raised by inflating the deflated air springs58of the suspension units50of the axle assemblies that are not secured by an associated axle securing unit80. As such, the air springs58of the non-liftable axle assembly or assemblies30that were deflated and the air springs58of the liftable axle assemblies32that were deflated but are not currently secured by an associated axle securing unit80may receive pressurized gas and may be inflated. Inflation of the air springs58may extend the suspension units50and may move the chassis20away from the support surface40and may lift the secured liftable axle assemblies32away from the support surface40such that the wheel assemblies34and tires36of the secured liftable axle assemblies32may disengage the support surface40and not support the vehicle10. An illustration of raising of the chassis20is best shown inFIG. 8. InFIG. 8, the air spring58of the non-liftable axle assembly30is inflated while the air spring58of the liftable axle assembly32that is held in the secured position by the axle lift system16is not inflated. Inflation of the air spring58of the non-liftable axle assembly30causes the chassis20and the liftable axle assembly32to move away from the support surface40, thereby allowing the tire or tires36of the secured liftable axle assembly32to lift away from and disengage the support surface40. Lifting of one or more liftable axle assemblies32may help improve vehicle fuel economy by reducing or eliminating frictional drag forces between one or more tires36and the support surface40that may otherwise be present if the tires36remained in engagement with the support surface40.

Referring toFIG. 4, a flowchart is shown that illustrates steps associated with releasing a liftable axle assembly32from the secured position. Releasing a liftable axle assembly32from a secured position generally reverses many of the method steps shown inFIG. 3. The steps associated with this flowchart will be described beginning with the vehicle10in the configuration shown inFIG. 8.

At block200, the method may determine whether any liftable axle assemblies32are lifted or held in a secured position by an associated axle securing unit80. If no liftable axle assemblies are lifted (e.g., no liftable axle assembly32is secured by an associated axle securing unit80), then the method or method iteration may end at block202. If at least one liftable axle assembly is lifted (e.g., at least one liftable axle assembly32is secured by an associated axle securing unit80), then the method may continue at block204.

At block204, the method may determine whether the vehicle10is stationary as previously discussed with respect to block104. If the vehicle is not stationary, then the method or iteration of the method may end at block202. If the vehicle is stationary, then the method may continue at block206. It is also contemplated that block204may be omitted in one or more embodiments to facilitate raising or lowering of an axle assembly if the vehicle is not stationary. As such, block200may proceed to block206instead of block204.

At block206, one or more axle loads may be compared to an axle load threshold. An axle load may be based on data from one or more load sensors94as previously described. The axle load threshold may be a predetermined value as previously described. If the axle load or axle loads are less than or equal to the axle load threshold, then the method or method iteration may end at block202. If an axle load exceeds the axle load threshold, then the method may continue at block208.

At block208, one or more liftable axle assemblies32may be selected to release. A liftable axle assembly32may be manually or automatically selected. A liftable axle assembly32may be manually selected by an operator as previously discussed. A liftable axle assembly32may be automatically selected by the control system18. For example, the control system18may automatically select a single available liftable axle assembly32for release when the axle load on the non-liftable axle assembly30is greater than the axle load threshold. If more than one liftable axle assembly32is available to release, then the method may determine whether to release one or more liftable axle assemblies32based on load data from one or more load sensors94. For example, if there are two liftable axle assemblies32and one non-liftable axle assembly30and both of the liftable axle assemblies32are secured with a corresponding axle lift system16, then the one non-liftable axle assembly30supports the weight of the vehicle10. As such, the load sensor94associated with the non-liftable axle assembly30may detect an axle load. The axle load may be compared to one or more axle load thresholds or may be used to reference data in a lookup table to determine whether one or more liftable axle assemblies32may be released. As an example, two liftable axle assemblies32may be selected for release when the axle load detected is greater than 80% of an associated maximum axle load rating. One liftable axle assembly32may be selected for release when the axle load detected is between 50% and 80% of an associated maximum axle load rating. These values and ranges are examples and it is contemplated that additional or different values or ranges may be provided depending on the number of axle assemblies and the configuration of the vehicle.

At block210, the chassis20may be lowered by deflating the air springs58associated with one or more suspension units50as previously discussed with respect to block110. As such, the chassis20may move from the position shown inFIG. 8to the position shown inFIG. 7.

At block212, the selected liftable axle assembly or assemblies32may be released by the axle lift system16. The axle lift system16may release one or more liftable axle assemblies32by moving the hook84out of engagement with a selected liftable axle assembly32. Releasing a liftable axle assembly32is best shown inFIG. 6.

At block214, the chassis20may be raised. The chassis20may be raised by inflating the air springs58of the suspension units50of the axle assemblies that are deflated and are not secured by an associated axle securing unit80. Raising of the chassis20is best shown inFIG. 5.

Inflation of the air springs58may move the chassis20away from the support surface40and may lift any remaining secured liftable axle assemblies32away from the support surface40such that the wheel assemblies34and tires36of the secured liftable axle assembly or assemblies32may disengage the support surface40and not support the vehicle10. Released liftable axle assemblies32may remain on the support surface40and may support the vehicle10. The release of a liftable axle assembly32may provide additional axle assemblies to support vehicle loads and may help avoid exceeding maximum axle load ratings and/or may help better distribute loads between multiple axle assemblies.