Abstract:
A lost motion piston assembly comprising a lost motion cavity formed within a piston and a lost motion piston disposed within the lost motion cavity. The lost motion piston forms a sealed gas cavity. The sealed gas cavity contracts when the actuator is extended under pressure into the cylinder space and expands under pressure when the actuator is retracted by relieving pressure in the cylinder space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This claims the benefit of U.S. Provisional Patent Application No. 61/477,676 filed Apr. 21, 2011, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present invention relates to hydraulic actuators and, more particularly, to hydraulic actuators with lost motion capability and, most particularly, to a lost motion piston assembly for use in cab tilt applications. 
     BACKGROUND OF THE INVENTION 
     Vehicles in which the cab tilts up to provide access to the vehicle&#39;s engine are generally referred to as “tilt-cab” vehicles. The cab, the enclosed space where the driver is seated, in a tilt-cab vehicle is typically positioned over the engine and front axle. As a result of this stacked design, tilt-cab vehicles are generally shorter than an equivalent conventional cab vehicle, where the cab is positioned behind the engine and front axle. 
     The cab in a tilt-cab design generally has two positions. In the driving position, the cab body is engaged and resiliently supported on the vehicle chassis and orientated such that the vehicle can be driven. In the maintenance position, the cab is tilted upward at a pivot point near the front of the cab to provide access to the engine and other mechanisms. 
     For tilting a resiliently mounted cab, it is known to use a hydraulic tilting device disposed between the chassis and the tilting cab to raise and lower the cab between the driving and maintenance positions. In order to ensure that the tilting device does not interfere with the spring movements of the cab relative to the chassis while the vehicle is being driven, tilting devices with a so-called lost motion capability are used. These tilting devices can be divided largely into mechanical types and hydraulic types. Mechanical types have, for example, a lost-motion arm, which is usually pivotably connected between the tilting cylinder and the cab, or a sort of pin-and-groove connection between the tilting cylinder and the cab. The lost motion capability is provided by the mechanical play in the pin-and-groove connection. 
     Hydraulic types have, for example, a dual acting hydraulic actuator to provide the lost motion effect. Dual acting hydraulic actuators contain two internal hydraulic cavities. Supplying hydraulic fluid to the push cavity will cause the actuator to extend in length, thereby exerting force to tilt the cab upward. Supplying hydraulic fluid to the pull cavity will cause the actuator to contract in length, thereby exerting force to lower the cab downward. To provide lost motion capability, it is known to place the push cavity in fluid communication with the pull cavity. As the resiliently mounted cab bounces on the chassis and pushing and pulling forces are exerted on the actuator, the motion of the hydraulic fluid between the push and pull cavities allows the actuator to extend and contract with relative ease, providing lost motion. 
     While a dual acting hydraulic actuator offers a means of providing lost motion in a tilt-cab design, a dual acting hydraulic actuator is not always necessary to fulfill the cab tilt functionality. A single acting actuator is sufficient to provide a means for cab tilt because gravity is sufficient to provide the downward force to lower the cab. In addition, dual acting actuators are generally more complex than single acting actuators. As a result, dual acting actuators are more costly and are less reliable than single acting actuators. Accordingly, it would be an advance in the state of the art to provide an alternative means of providing a hydraulic actuator with lost motion capabilities suitable for use in a tilt-cab design. 
     SUMMARY OF THE INVENTION 
     A hydraulic tilting device is presented. The hydraulic tilting device is used for tilting the cab of a vehicle, which is resiliently supported on the chassis of the vehicle, between a driving position and a tilted position. The hydraulic tilting device comprises an actuator. The actuator has a cylinder housing. The cylinder housing has a cylinder space. The actuator further has a piston rod. A first portion of the piston rod is disposed within the cylinder space and a second portion of the piston rod extends out from the cylinder space. A gas pressure cavity is provided in the cylinder space. The gas pressure cavity contracts when the actuator is extended under pressure into the cylinder space and expands when the actuator is retracted by relieving pressure in the cylinder space so as to provide for oscillating extensions and retractions of the piston rod relative to the cylinder housing in the driving position so as not to interfere with the operation of the suspension as the cab bounces up and down relative to the chassis when driving the vehicle. 
     A lost motion piston assembly is also presented. The lost motion piston assembly is used in an actuator for tilting the cab of a vehicle, which is resiliently support on the chassis of the vehicle, between a driving position and a tilted position. The lost motion piston assembly comprises a lost motion cavity formed within a piston and a lost motion piston disposed within the lost motion cavity. The lost motion piston forms a sealed gas cavity. The sealed gas cavity contracts when the actuator is extended under pressure into the cylinder space and expands under pressure when the actuator is retracted by relieving pressure in the cylinder space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the Drawings, of which: 
         FIG. 1  is a cross-sectional view of an embodiment of Applicant&#39;s actuator in a depressurized and retracted position while being axially compressed by an external force; 
         FIG. 2  is a cross-sectional view of the embodiment of  FIG. 1 , in an intermediate, partially extended position, and pressurized to lift, or tilt, a cab; 
         FIG. 3  is a cross-sectional view of the embodiment of  FIG. 1  in a depressurized and retracted position while being partially axially extended by an external force; 
         FIG. 4  is an illustration of an embodiment of Applicant&#39;s actuator in a depressurized and retracted position and with a locking mechanism to lock the cab in the maintenance position; and 
         FIG. 5  is an illustration of the embodiment of  FIG. 4  with the locking mechanism engaged, with the actuator in the maintenance position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention is described in preferred embodiments in the following description with reference to the Figures, in which like numbers represent the same or similar elements. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     The described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are recited to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
     Referring to  FIG. 1 , a cross-sectional view of one embodiment of Applicant&#39;s single acting actuator  100  is shown, wherein the actuator  100  is in a depressurized and retracted position while being compressed by an external force. A cylinder housing  102  is sealed at one end by a cap  104 , forming an interior cavity. In one embodiment, the cap  104  is integrated with the cylinder housing  102 . In another embodiment, the cap  104  is attached to the cylinder housing  102  by a threaded connector. In yet another embodiment, the cap  104  is welded to the cylinder housing  102 . In one embodiment, the combined length of the cylinder housing and the cap is about 20.4 inches and the outer diameter of the cylinder housing is about 3.5 inches. 
     The cap  104  includes a pressure port  160  and a pressure channel  110 . The pressure channel  110  connects the pressure port  160  to the interior of the cylinder housing  102 . In an embodiment, the pressure port  160  includes a standard threaded connector for connecting a source of hydraulic fluid under pressure. 
     In an embodiment, a mounting bracket  112  is attached to the cylinder housing  102 . The actuator  100  is attached to the chassis of a tilt-cab vehicle via the mounting bracket  112 . The mounting bracket  112  may includes pegs (not shown in this view) to facilitate attachment to the vehicle chassis. 
     A piston rod  114  is partially disposed within the interior cavity of the cylinder housing  102 . One end of the piston rod  114  extends from the rod end of the interior cavity and the other end of piston rod  114  is disposed in the bore end of the interior cavity. The exposed portion of the piston rod  114  may include mounting holes  150  and  152 . In an embodiment, mounting hole  150  may be used for mounting the rod to the cab of a tilt-cab vehicle and mounting hole  152  may be used for mounting a safety device, such as a locking bar, to the rod. 
     A collar  116 , or sealing gland, is attached to the cylinder housing  102 . The collar  116  may be is threadedly attached to cylinder housing  102 . To create a hydraulic seal, the interface between collar  116  and cylinder housing  102  may be sealed by a seal  120 . The collar  116  secures the piston rod  114  in a fixed concentric position in the mouth of cylinder housing  102 . The collar  116  contains a channel  118  in contact with the piston rod  114 . A seal  122  is disposed within the channel  118 . The O-ring  122  creates a hydraulic seal between the piston rod  114  and the collar  116 , thereby forming a pressure cavity  124 . In an embodiment, the piston rod  114  is about 24 inches long and about 2.2 inches in diameter. 
     In one embodiment, a spacer  108  is attached to and circumscribes the end of piston rod  114 . Spacer  108  holds the end of piston rod  114  in a fixed concentric position within the cylinder housing  102 . Channels  126  are formed through the spacer  108 , since the actuator  100  is a single acting actuator. The channels  126  connect the pressure cavity  124  on the sides of the piston rod  114  with pressure cavity  128  at the end of the piston rod  114  near the cap  104 . As such, the actuator  100  has a single internal hydraulic chamber, and can therefore be classified as a “single acting” actuator. It is noted that the invention could be used with a double acting actuator, if so desired. 
     The piston rod  114  is configured to move axially in a reciprocating manner relative to the cylinder housing  102 . During such motion, the seal  122  slides along the length of piston rod  114  while maintaining the hydraulic seal between piston rod  114  and collar  116 . In addition, spacer  108  is configured to slide axially with piston rod  114  along the interior surface of the cylinder housing  102  while retaining the concentric position of the piston rod  114  within the cylinder housing  102 . 
     A lost motion bore is formed in the end of piston rod  114  that is enclosed within the cylinder housing  102 . In an embodiment, the lost motion bore is comprised of an outer chamber  130  and an inner chamber  132 . Outer chamber  130  has a diameter larger than inner chamber  132 . The end of chamber  130  may taper to join chamber  132 . Alternately, chamber  130  may abruptly transition to chamber  132  at a right angle. In an embodiment, the outer chamber is about 5 inches long and about 1.5 inches in diameter. In an embodiment, the inner chamber is about 1.3 inches long and about 0.8 inches in diameter. 
     A lost motion piston  134  is disposed within the outer chamber  130 . The lost motion piston  134  is configured to move axially in a reciprocating manner relative to the chamber  130 . A seal  136  circumscribes the lost motion piston  134 . The seal  136  creates a pressure seal that divides the lost motion bore into an isolated gas cavity  140  and a fluid cavity  142 . The isolated gas cavity  140  is formed by the inner chamber  132  and a portion of the outer chamber  130  to the left of the lost motion piston  134 . The fluid cavity  142  is formed by the remainder of the outer chamber  130  to the right of the lost motion piston  134 . In an embodiment, the lost motion piston  134  is about 1.3 inches wide. 
     In the illustrated embodiment, a channel  138  is formed in the lost motion piston  134 . Gas may be released through the channel  138  during insertion of the lost motion piston  134  to maintain atmospheric pressure in the isolated gas cavity  140 . Channel  138  may include a port capable of accepting a connector for the purpose of adjusting the amount of gas in the isolated gas cavity  140 . In one embodiment, when the connector is removed, the port may seal to create a hydraulic seal between the isolated gas cavity  140  and the fluid cavity  142 . In another embodiment, a plug is inserted into the port to create a hydraulic seal between the isolated gas cavity  140  and the fluid cavity  142 . A threaded hole  144  may be used to connect the lost motion piston  134  with an installation bolt or fixture used to position the piston to desired depth within the outer chamber  130 . In an embodiment, the lost motion piston  134  is positioned at a depth where the mouth of the fluid chamber  142  and the closest edge of the lost motion piston  134  is about 1.3 inches. In this position, the gas within the isolated gas cavity  140  is at atmospheric pressure. When the pressure chamber is initially filled with hydraulic fluid, the distance between the mouth of the fluid chamber  142  and the closest edge of the lost motion piston  134  is about 1.9 inches, which positions the center of the lost motion piston  134  about midway along the length of the outer chamber  130 . In this position, the gas within the isolated gas cavity  140  is at slightly greater than atmospheric pressure. 
     The cylinder housing  102  may also include a safety solenoid valve  148 . The safety solenoid value  148  opens when the pressure in the pressure cavity exceeds a predetermined limit, thereby venting the contents of the pressure cavity to the atmosphere. 
     A retaining ring  146  is disposed at the mouth of outer chamber  130 . The retaining ring  146  serves as a physical stop to retain the lost motion piston within the outer chamber  130 . The retaining ring  146  is a split ring and is open in the center to allow hydraulic fluid to freely flow between the fluid cavity  142  and the pressure cavity  128 . 
     When the tilt-cab vehicle is in the driving position, actuator  100  provides lost motion capabilities. Under normal operating conditions, the resiliently-mounted cab moves up and down relative to the vehicle chassis as the chassis is exposed to mechanical shocks. The cab tilt actuator is connected between the cab and the chassis. Without some play, or lost motion, in this connection mechanical shocks exposed to the chassis would be transmitted to the cab, resulting in a rougher ride for the driver. The lost motion feature allows the cab tilt actuator to extend and retract axially over a set distance so as not to interfere with the resilient connection of the suspension between the cab and chassis, so the cab can oscillate up and down relative to the chassis in normal driving conditions. 
     While in the driving position and with no external force acting axially on the piston rod  114 , the lost motion piston  134  will be approximately centered along the length of the outer chamber  130 . As an external force is applied axially to push the piston rod  114  into the cylinder body  102 , pressure is exerted on the hydraulic fluid within pressure cavity  124  and pressure cavity  128 . This, in turn, exerts pressure on lost motion piston  134 , which forces the lost motion piston  134  deeper into the lost motion bore and compresses the gas within the isolated gas cavity  140 . 
     As the external force on the rod increases and the rod is pushed further into the cylinder housing  102 , the lost motion piston  134  will further compress the gas in the isolated gas cavity  140  and the pressure within the isolated gas cavity  140  will continue to increase. As a result, the resistance against further movement increases due to the increasing pressure in the isolated gas chamber  140 . 
     In similar fashion, as an external force is applied axially to force the piston rod  114  out of the cylinder body  102 , the pressure exerted on the hydraulic fluid is decreased. This, in turn, allows the gas in the isolated gas cavity  140  to expand, which forces the lost motion piston  134  toward the mouth of outer chamber  130 . 
     As the external force on the rod increases and the rod is pulled further out of the cylinder housing  102 , the lost motion piston  134  will exert less pressure on the gas in the isolated gas cavity  140  and the pressure within the isolated gas cavity  140  will continue to decrease. As a result, the resistance against further movement increases due to the decreasing pressure in the isolated gas chamber  140 . 
     As the piston rod  114  moves in and out of the cylinder housing  102 , hydraulic fluid is able to flow between pressure cavity  124  and pressure cavity  128  through channels  126  in spacer  108 . The displacement of hydraulic fluid through channels  126  causes the expansion of the gas in isolated gas cavity  140  (when piston rod  114  is pulled) and compression of the gas in isolated gas cavity  140  (when piston rod  114  is pushed), resulting in lost motion action. 
     Turning again to  FIG. 1 , actuator  100  is shown in lost motion mode under an external force  154  acting axially on the piston rod  114  and causing the actuator  100  to shorten in overall length. The force  154  on piston rod  114  displaces the fluid in pressure cavities  124  and  128 , which forces the lost motion piston  134  from its neutral position at the approximate center of the outer chamber  130 , away from the mouth of the outer chamber  130 . As a result, the gas within isolated gas cavity  140  is compressed. The compressed gas provides slightly increased resistance to force  154 . 
     Turning to  FIG. 2 , actuator  100  of  FIG. 1  is shown when pressurized and lifting a cab or, if fully extended, in the maintenance position. A source of hydraulic fluid  202  connected to pressure port  160  supplies hydraulic fluid under pressure to pressure cavity  128  through pressure channel  110 . The pressure is propagated into pressure cavity  124  through channels  126  in spacer  108 . As the pressure increases in the pressure cavities  124  and  128 , pressure on the lost motion piston  134  overcomes the pressure of the gas in the isolated gas cavity  140 . As a result, the lost motion piston  134  bottoms out against the back of outer cavity  130 , compressing the entire quantity of gas in the isolated gas cavity  140  into the volume of the inner chamber  132 . In a typical application, the maximum operating pressure within the pressure cavity  124  and  128  is approximately 3,500 psi. In the embodiment illustrated, the pressure within the isolated gas cavity when the lost motion piston  134  is bottomed out against the back of outer cavity  130  is more on the order of about 200 psi. 
     Once the lost motion piston  134  bottoms out, the lost motion action is overcome and any additional increase in pressure in the pressure cavity  124  and  128  will directly result in piston rod  114  extending out of the cylinder housing  102 . The extending piston rod  114  is used to tilt the cab up and to the maintenance position. In a typical application, the piston rod  114  can extend up to about 18 inches from the mouth of the cylinder housing  102 . 
     To lower the cab, the pressure within the pressure cavities  124  and  128  is slowly decreased. In an embodiment, the decrease in pressure is accomplished by exposing the hydraulic fluid to atmospheric pressure through a restricted orifice. The orifice forces the pressure within the pressure cavities  124  and  128  to decrease at a slow, controlled rate. The cab will lower under the influence of gravity at a rate determined by the release of hydraulic fluid from the pressure cavities  124  and  128 . Once the pressure in the pressure cavities  124  and  128  drops below the maximum pressure in the isolated gas cavity  140 , the gas will expand and the lost motion piston will move back toward the neutral position (midway along the length of the outer chamber  130 ). The actuator  100  will operate in lost motion mode when the lost motion piston is between, but not in contact with either of, the retainer ring  146  and the inner chamber  132 . 
     Turning to  FIG. 3 , actuator  100  of  FIG. 1  is shown. The actuator  100  is in a depressurized and mostly retracted position while being extended by an external force. As the piston rod  114  is extracted out of the cylinder housing  102 , the displacement of hydraulic fluid through channels  126  causes the pressure in the pressure cavities  124  and  128  to decrease. As a result, the gas in the isolated gas cavity  140  expands, which moves the lost motion piston  134  toward the mouth of outer cavity  130 . 
     Turning to  FIG. 4 , an embodiment of Applicant&#39;s actuator in a depressurized and retracted position (i.e., driving position) and with a locking mechanism to maintain the cab in the maintenance (i.e., lifted or tilted) position. A mounting bracket  112  is attached to the cylinder housing  102 . Pegs  404  extend from each side of the mounting bracket  112  (in and out of the page in the view of  FIG. 4 ). The pegs  404  may be used to mount the actuator  400  to the chassis of a vehicle so as to pivot as the actuator extends and retracts. 
     A piston rod  114  extends from the cylinder housing  102 . A mounting hole  150  is formed near the end of piston rod  114 . A locking bar  402  is mounted on piston rod  114  by pin  406 . The locking bar  402  allows the actuator to support weight in the extended position without being pressurized. 
     The pin  406  permits the locking bar to pivot about pin  406 . The locking bar  402  extends down the length of the cylinder body  102 . A notch  408  is formed at the end of locking bar  402 . The notch  408  allows the locking bar  402  to brace against the cylinder body  102  when the locking bar is engaged. 
     Referring to  FIG. 5 , the embodiment of  FIG. 4  is shown in the locked position. Once piston rod  114  is extended to near its maximum length, the locking bar  402  pivots about pin  406  with the end opposite the pin  406  contacting the portion of the actuator where the piston rod  114  and cylinder housing  102  join. Once the pressure provided to the actuator  400  is released the piston rod  114  retracts slightly and the locking bar  402  engages the lip of cylinder housing  102 . The notch  408  keeps the locking bar  402  in place. The locking bar  402  is configured to withstand the entire force of the cab acting to compress actuator  400  when the actuator is unpressurized. 
     To unlock and lower the cab, the actuator  300  is pressurized to extend piston rod  114  enough to clear the notch  408  and pivot the locking bar  402  clear of the lip of cylinder housing  102 . 
     Exemplary embodiments are described herein. While specific values chosen for the embodiment are recited, it is to be understood that, within the scope of the invention, the values of all of parameters may vary over wide ranges to suit different applications. 
     The embodiments described herein depict Applicant&#39;s lost motion piston assembly in a single acting hydraulic actuator. In other embodiments, Applicant&#39;s lost motion piston assembly may also be used in double acting actuators. Single acting actuators exert force in a single direction by pumping pressurized fluid into a cavity. Referring back to  FIG. 1 , this cavity is formed by  124  and  128  (which are connected by channels  126 ). As pressurized fluid is pumped into this cavity, the piston rod  114  is forced to extend out from the body  102 . While the use of pressurized fluid can force the piston rod  114  to extend, an external force is necessary to force the piston rod  114  to retract. The cavity is vented to a lower pressure allowing the external force to force the fluid from the cavity. 
     In contrast, a double acting actuator has two cavities (ex: Cavity  1  and Cavity  2 ) separated by a piston. Pumping fluid under pressure to Cavity  1  while venting Cavity  2  to a lower pressure forces the piston in one direction. Likewise, pumping fluid under pressure to Cavity  2  while venting Cavity  1  to a lower pressure forces the piston in the opposite direction. The single acting actuator with lost motion capability depicted in  FIGS. 1-3  may be modified into a double acting actuator with lost motion capability by disposing a piston ring around piston rod  114  to create two isolated chambers. Pressure port  160  (see  FIG. 1 ) would be used to pressurize/vent one chamber and another pressure port would be added to pressurize/vent the other chamber. A channel connecting the two chambers is also added. The channel would include a valve. The valve is closed (i.e., the two chambers are isolated) when the double acting actuator piston is being extended or retracted under the influence of pressurized fluid. The valve is open (i.e., the two chambers are connected to form a single chamber) when the double acting actuator is in lost motion mode. While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. Furthermore, disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s).