Abstract:
An anti-creep device for a trailer includes a support arm adapted to be mounted to a trailer frame proximate a trailer axle for rotational movement between a retracted position and an extended position. An actuator is associated with the support arm for rotating the arm at least to the extended position to thereby lock the frame at a selected height when the frame is raised with respect to the axle. A sensor switch is mounted to the frame in the vicinity of the support arm for sensing the position of the support arm and an indicator is operably connected to the sensor for indicating when the support arm is in the extended position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/US97/05976 filed Apr. 10, 1997, now abandoned which claims the benefit of U.S. Provisional Application Ser. No. 60/015,253, filed Apr. 10, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an anti-creep device for use in a trailer suspension, and more specifically in a low-ride trailer suspension. The invention serves to retard the forward and downward movement of the trailer during loading, loss of air spring pressure, or other “stationary” times to limit the creep of the trailer from a loading dock or parked position. 
     2. Description of the Prior Art 
     Many trailers have trailing arm suspensions with air springs to control the relative position of the frame with respect to an axle and also to cushion the relative movement of the axle toward the frame due to bumps in the road. In the loading or unloading of a low-ride trailer, the trailer is typically backed up against a dock by the tractor. The operator then pressurizes the air springs of the low-ride suspension so that the bed of the trailer is level with the loading dock. The operator then lowers the front dolly legs on the trailer until they touch the ground and then removes the tractor. In storage situations the tractor may or may not be connected to the trailer. Although the air springs are pressurized by the tractor&#39;s air compressor during transport and stationary times, the air compressor is normally off during loading or unloading operations. 
     As an empty trailer is loaded, the force from the weight of the goods being transferred to the trailer and the loading equipment, such as a forklift or a and car, lowers the rear portion of the frame rail with respect to the axle. Because the compressor is off during the loading operation, the air pressure in the air spring is not adjusted to compensate for the increased load. While the rear portion of the trailer frame moves downwardly, the front portion of the trailer frame is substantially fixed at the height of the dolly and the trailer frame effectively rotates about the contact point of the dolly with the ground. The downward movement of the rear portion of the trailer frame results in the pivotable movement of the pivotal connection between the trailer frame and the trailing arm. This pivotable movement results in the slight rotation of the trailing arm wheel to move the trailer forward. In other words, the trailer tends to move away from the loading dock. This movement is referred to as “creep.” Trailer creep can create hazards for loading. This problem is augmented in low-ride trailers, since the air spring is pressurized to a greater height to compensate for the difference between the dock height and trailer bed height. The background and possible solutions to this problem can be found in PCT/US94/10789, which is herein incorporated by reference. 
     U.S. Pat. No. 5,333,645, issued Aug. 2, 1994, discloses an apparatus for overcoming trailer creep by providing a dump valve to exhaust air from the air spring when the trailer door is opened. The trailer thus bottoms out on the suspension before loading begins and cannot creep away from the dock. Whereas this system has worked well to prevent creep, not all vehicles are equipped with this system, or cannot use such a system. Also, the door switch sometimes malfunctions and the dump valve does not always exhaust air from the air spring. 
     Therefore, there is a significant need to reduce or eliminate the creep associated with a trailer during loading. The anti-creep solution must also be simple, reliable and inexpensive if it is to be commercially viable. Further, the anti-creep solution must also not interfere with the normal function of the trailing arm suspension during normal operation thereof. 
     SUMMARY OF THE INVENTION 
     The invention relates to an anti-creep device for a trailer with a trailing arm suspension, and more especially for a low-ride trailer with a trailing arm suspension. It is simple, convenient and effective, and does not interfere with the normal operation of the trailing arm suspension during normal operation of the trailer. The anti-creep device is adapted for use in a trailing arm suspension comprising a trailing arm rotatably mounted at one of its ends to a trailer frame. At another end of the trailing arm, an air spring is disposed between the trailing arm and the trailer frame to resiliently resist the upward movement of the trailing arm with respect to the frame. An axle is mounted to the trailing arm and mounts a wheel. The anti-creep device comprises a pair of arms adapted to be rotatably mounted to the trailer frame at opposite ends thereof for movement between a retracted position and an extended position. In the extended position, the arms of the anti-creep device are adapted to extend between the frame and the suspension in order to prevent the movement of the axle with respect to the trailer frame and maintain the floor of the trailer level with the loading dock. In the retracted position, the arms are adapted to be disposed above the suspension so that the axle is free to move up and down through its normal range of motion under ordinary conditions. 
     The anti-creep device further includes an actuator adapted to be mounted between the trailer frame and the arms to control the movement of the arms between the retracted and extended positions. Preferably, the actuator is an air-operated actuator having a shaft adapted to mount to the arms so that the reciprocating movement of the shaft moves the arms between the retracted and extended positions. 
     In one embodiment, each arm comprises an elongated plate having one end adapted to be pivotally mounted to the trailer frame and having another end for contacting a load pad on the axle of the suspension when the elongated plate is in the extended position, preventing the upward movement of the axle with respect to the trailer frame. Preferably, the air actuator is adapted to be attached to a cross member, which forms part of the trailer frame. A drive cam is attached to the air actuator for rotating the arms of the anti-creep device. 
     In another embodiment, the anti-creep device comprises a pair of support arms that are adapted to be mounted at opposite sides of the trailer frame for rotational movement between a retracted position and an extended position. The support arms in the extended position are adapted to abut the axle at the opposite sides of the trailer frame to prevent movement of the axle toward the trailer frame and thereby prevent trailer frame creep. The support arms in the retracted position are disengaged from the axle so that the axle is free to move with respect to the frame. An actuator rod is adapted to extend between the opposite sides of the frame, and the pair of support arms are fixedly attached to opposite ends of the actuator rod for rotational movement therewith. An actuator is operably connected to the actuator rod for rotating the rod and thus the support arms between the retracted and extended positions. 
     According to another embodiment, an anti-creep device comprises a support arm adapted to be mounted to the trailer frame proximate to the axle for rotational movement between a retracted position and an extended position. The support arm includes a plurality of notches extending along a length thereof. A support member is adapted to be connected to the suspension and has at least one surface for receiving one of the support arm notches when the support arm is in the extended position to prevent movement of the axle toward the trailer frame and thereby prevent the trailer frame from creeping. The notches of the support arm in the retracted position are disengaged from the support member so that the axle is free to move with respect to the frame. An actuator is associated with the support arm for rotating the arm at least to the extended position to thereby lock the frame at a selected height when the frame is raised with respect to the axle. 
     Further according to the invention, a suspension for a vehicle includes a pair of trailing arms with a first end adapted to be rotatably mounted to opposite sides of a trailer frame, an air spring disposed on each trailing arm is adapted to be mounted to the trailer frame to resiliently resist the upward movement of the trailing arms with respect to the frame, and an axle connected to the trailing arms. The axle has opposite ends adapted for mounting wheels thereto. A support arm is adapted to be mounted to the trailer frame proximate to the axle for rotational movement between a retracted position and an extended position. The support arm in the extended position is adapted to abut the suspension to prevent movement of the axle with respect to the trailer frame and thereby prevent trailer frame creep. In the retracted position, the support arm is disengaged from the suspension so that the axle is free to move with respect to the frame. An actuator is operably associated with the support arm for rotating the support arm between the retracted and extended positions. The actuator is operably connected to a pressure switch that is at least fluidly connected to the air brake. The actuator is adapted to rotate the support arm in response to a change in air pressure at the pressure switch due to a change in air pressure at the air brake. 
     In a preferred embodiment, an air brake is mounted to the axle, and includes a spring brake actuator for preventing wheel rotation when air pressure is released from the spring brake actuator; and an air pressure source for supplying air under pressure to the spring brake actuator. In one embodiment, the pressure switch is a valve assembly for supplying pressurized air to and exhausting pressurized air from the actuator and air brake. In another embodiment, the actuator is an electrical actuator and the pressure switch controls operation of the actuator in response to the presence or absence of pressure on the air brake. 
     In each of the above embodiments, a support arm position sensor or switch can be provided for alerting an operator that the support arm is in the extended position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the drawings wherein: 
     FIG. 1 is a side view of a trailing arm suspension incorporating the anti-creep device according to the invention; 
     FIG. 2 is a partial sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a side view looking toward the trailing arm suspension of FIG.  1  and showing the actuator cylinder of the anti-creep device according to the invention; 
     FIG. 4 is a schematic side view of the trailing arm suspension shown in FIG. 1, with the anti-creep device in retracted and extended positions; 
     FIG. 5 is a schematic view of an air control circuit according to the invention. 
     FIG. 6 is a side view of a trailing arm suspension incorporating the anti-creep device according to a second embodiment of the invention; 
     FIG. 7 is a view similar to FIG. 6 showing the different positions of the axle with respect to the flipper plate; 
     FIG. 8 is a partial sectional view taken along line  8 — 8  of FIG. 6; and 
     FIG. 9 is a block diagram of a system for operating the anti-creep device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a trailing arm suspension  10  incorporating the anti-creep device  12  according to the invention. The trailing arm suspension  10  is mounted to a longitudinal frame rail  14  of a trailer frame and supports an axle  16  to which wheels  18  are mounted on opposite ends of the axle  16 . In a typical trailer application, two trailing arms are used to mount the axle  16  to the frame rail  14 . The trailing arms are mounted on opposite sides of the frame rail and support opposing ends of the axle  16 . Only one of the trailing arms will be described in detail. 
     The trailing arm suspension  10  comprises a hanger bracket  20  fixedly mounted to the frame rail  14  and to which is rotatably mounted a trailing arm  22  by means of a bushed connection  24  at the forward end of the trailing arm  22 . The rearward end of the trailing arm  22  mounts an air spring  30  at a lower end thereof, which is connected to the frame rail  14  at its upper end. The air spring  30  resiliently resists upward movement of the trailing arm  22  with respect to the frame and comprises an air bag  32  mounted to the frame rail  14  and a piston  34  mounted to a platform  36  on the trailing edge of the trailing arm  22 . As the trailing arm  22  rotates, the piston  34  is urged into the air bag  32  to resiliently retard movement of the trailing arm  22  toward the frame rail  14 . 
     The axle  16  is mounted to the trailing arm  22  by an axle mounting bracket  40  having opposing plates  41 , which are connected to the trailing arm  22  through two bushed connections  42  and  44 . A shock absorber  46  can be mounted between the frame rail  14  and the axle bracket  40  to dampen the movement of the trailing arm  22  with respect to the frame. 
     Referring to FIGS. 1-3, two anti-creep devices  12  are associated with the axle  16  on opposite sides of the frame  14 . Only one of the anti-creep devices will be described in detail, it being understood that the other anti-creep device is constructed in the same manner. The anti-creep device  12  includes a flipper plate or support arm  50  pivotally attached to frame rail  14  for engaging axle  16 . As seen most clearly in FIG. 3, a support plate  52  is welded or otherwise attached at a support plate first end  54  and support plate second end  56  to frame rail  14 . The support plate  52  has a central bore  58  therein. Frame rail  14  has a corresponding bore  60  in alignment with central bore  58 . Central bore  58  is larger than frame rail bore  60  for receiving a bushing housing  62  therein. Bushing housing  62  is welded or otherwise attached between support plate  52  at the periphery of the central bore  58 , and frame rail  14 . A bushing  64  is positioned in bushing housing  62 . The bushing  64  is preferably constructed of bronze, but other materials may be used. A main rod  66  is positioned in bushing  64  for rotation relative thereto. Flipper plate  50  is attached to main rod  66  for rotation therewith. Although FIG. 3 shows only one anti-creep device  12 , it is to be understood that a similar anti-creep device  12  is mounted on an opposite side of the frame rail  14 . An extension rod  68  is attached between main rods  66  located on opposite sides of frame rail  14 . The main rod  66  has a larger diameter than extension rod  68  since main rod  66  will ultimately support the weight of the trailer and its contents when flipper plate  50  is rotated to engage axle  16 . 
     A load pad  70  preferably constructed of steel is attached to axle  16  by welding. An indentation  51  extends across a portion of a bottom edge of the flipper plate  50  for contacting load pad  70 . 
     Referring now to FIGS. 2 and 3, a rear cross member  82  extends to opposite sides of frame rail  14 . A long stroke actuator cylinder  80  is attached to rear cross member  82  through a rear cylinder bracket  84  and forward cylinder bracket  86 . The brackets  84 ,  86  firmly engage cylinder  80  and are rigidly attached to cross member  82 . Actuator cylinder  80  includes a shaft  88  which extends from a first shaft end  90  inside cylinder  80  to a second shaft end  92  outside cylinder  80 . First shaft end  92  is attached to a fluid tight plunger  94 . A compression spring  96  located between plunger  94  and cylinder rear wall  98  biases shaft  88  outwardly of cylinder  80 . The second cylinder shaft end  92  is slidably located in slot  102  of drive cam  100 . Drive cam  100  is in turn fixedly secured to extension rod  68  for rotatable movement therewith. Slot  102  in drive cam  100  permits shaft  90  to linearly reciprocate through the arcuate movement of cam  100 . An actuator air port  104  is preferably in fluid communication with a pressurized air source, such as the air source for the brake actuators. Thus, when the pressure to the brake actuators is removed, the air is exhausted from the cylinder  80  and the shaft  88  is forced toward the extended position (to the right as viewed in FIG. 3) by the compression spring  96 . 
     As shown in FIGS. 1 and 2, a normally closed sensor switch  110  is directly mounted on frame rail  14  or may be mounted through an extension plate  112 . The sensor switch  110  is mounted for lateral adjustment along the frame rail  14  so as to be proximal to flipper plate  50 . Sensor switch  110  includes a pressure protection valve port  116  and a shuttle valve port  118 . A plunger  114  moves between open and closed positions to direct pressurized air to either port  116  or port  118 . Plunger  114  is pushed inward when flipper plate is in the extended position, thus opening sensor switch  110 . In the open position, sensor switch  110  serves to signal to the operator that flipper plate  50  is in the extended position so as to avoid possible suspension damage when pulling or backing the trailer. Sensor switch  110  may in addition or alternatively disable the tractor or trailer from operating when flipper plate  50  is extended. 
     With reference now to FIG. 9, a block diagram of a system  150  for operating the anti-creep device is illustrated. The system  150  includes an operator interface  152  for releasing and setting an air-operated parking or emergency brake  156  that forms part of an air brake actuator (not shown), often referred to as a spring brake actuator. The operator interface is connected to a valve assembly  155  having an air supply valve  154  and an air release valve  158 . The air supply and release valves are each in turn connected to the parking brake  156  and the anti-creep actuator  80 . The parking brake  156  includes a chamber having a rod (not shown) that is normally biased outward of the chamber to apply braking pressure to the wheels of the vehicle in a well known manner. When the air supply valve is open and the air exhaust valve is closed, air under pressure is supplied to the chamber from an air pressure supply source  160 , which is connected to the valve  154 , to push the rod inwardly of the chamber when the parking brake is released. The air supply  160  typically comprises a compressor for generating compressed air and a tank for storing the compressed air. When an operator sets the parking brake, the air supply valve  154  is closed and the exhaust valve is simultaneously opened to exhaust air from the chamber through the valve  158 . The parking brake and its method of operation is well known in the art and, therefore, will not be described in further detail. Since the actuator  80  and parking brake  156  are both connected to the air supply valve  154  and air exhaust valve  158 , air pressure is simultaneously supplied to the actuator and parking brake when the valve  154  is opened and the valve  158  is closed and is simultaneously released from the actuator  80  and parking brake  156  when the valve  154  is closed and the valve  158  is opened. The valve assembly  155  thus functions as a pressure switch for controlling operation of the actuator in response to setting and releasing the parking brake. 
     In operation, as the trailer is being pulled by a tractor, the air supply valve  154  is normally open and the actuator  80  is pressurized by air from the air supply  160  which enters the air port  104  and biases the plunger  94  inward against the compression spring  96 , thus keeping flipper plate  50  in the retracted position. With the flipper plate  50  in the retracted position, the trailer is backed into a loading dock area so that the end of the trailer abuts the end of the loading dock. The axle  16  of the low-ride suspension  10  can be in the axle full up position illustrated in full lines in FIG. 4, and the flipper plates  50  are in the retracted position (also illustrated by the phantom lines in FIG. 4) during trailer pulling or backing. The operator then pressurizes the air springs of the low-ride suspension such that the rear axle is in the full down position where the frame is fully raised (illustrated by the phantom lines in FIG.  4 ), and the bed of the trailer is level with the loading dock. The air is then exhausted from the parking brake by simultaneously closing the air supply valve  154  and opening the air exhaust valve  158  to set the parking brake, whereupon the pressure in the cylinder  80  is also exhausted and the shaft  88  is extended due to the force of spring  96 , thereby pivoting drive cam  100 , which in turn pivots the flipper plates  50  to the extended position (illustrated in phantom lines in FIG.  4 ). In the extended position, edge  51 A of flipper plate channel  51  abuts an edge of load pad  70 , thereby stopping further rotation of the flipper plate  50 . 
     As the uncoupled trailer is loaded, the weight placed on the frame rail  14  by loading devices, such as hand carts and forklifts, and the goods that are carried on the loading devices lower the frame rail  14 . As the frame rail drops, flipper plate channels  51  of flipper plates  50  move towards the load pads  70 . Once flipper plates  50  contact the load pads  70 , the frame rail  14  is prevented from moving downwardly with respect to the trailing arm  22  and thus prevents creeping of the trailer away from the dock. 
     When trailer loading or unloading is completed, the air supply is turned on, the air supply valve  154  is opened, and the air exhaust valve  158  is simultaneously closed, to pressurize actuator cylinder  80  to thereby rotate the flipper plates to the retracted position illustrated in full lines in FIG.  4  and release the parking brake. The air spring is then depressurized to lower the frame rail  14  to its travel height. 
     As shown in FIG. 4, an optional extension spring  106  can be connected between flipper plate  50  and frame rail  14 . Extension spring  106  can replace or add to compression spring  96  located in actuator cylinder  80 , depending on the amount of force required to rotate the flipper plates. Although two forms of springs are contemplated, it is preferred that the compression spring  96  be used, since it is protected from outside contamination within actuator cylinder  80 . 
     Referring now to FIG. 5, air control circuit  120  incorporates sensor switch  110 . Sensor switch  110  is connected to shuttle valve  122 , which is in turn connected to an on/off valve  124  to raise or lower the trailer. On/off valve  124  is also connected to a pressure protection port  126  connected to a pressure protection valve (not shown). A valve box  128  includes a pilot valve  130  connected to shuttle valve  122 , a normally open valve  132  connected to a quick exhaust valve  133 , a normally closed valve  134  connected to a pressure protection valve (not shown), and a cylinder valve  136  connected to air springs  30 . Quick exhaust valve  133  is connected to height control valve  138 . Height control valve  138  has a pressure protection port  140  connected to a pressure protection valve (not shown). 
     In normal operation, i.e. when the trailer is being pulled or backed up, the on/off valve  124  is normally in the off position. This permits pressurized air from the height control valve  138  to enter and exit the air springs  30  through the normally open port  132 . When the valve  124  is turned on by the operator, high pressure air is channeled through the shuttle valve  122  to the pilot valve  130 . High pressure from the pilot valve closes valve  132 , opens valve  134  and permits pressurized air to pass through the cylinder valve  136  to pressurize air springs  30 . Air springs  30  in turn raise the bed of the trailer to the dock height, whereupon the on/off valve  124  is turned off. Flipper plate  50  is then rotated to its extended position. Flipper plate  50  in the extended position biases against sensor plunger  114  which opens the normally closed sensor switch. In this condition, high pressure from sensor switch  110  is directed through shuttle valve  122  through pilot valve  130 . This maintains high pressure to air springs  30 , assuring that flipper plate  50  can be retracted when the trailer is fully loaded. After loading or unloading the trailer, flipper plate  50  is retracted. This causes sensor switch  110  to close again, which creates a low pressure at pilot valve  130 . Port  132  opens, permitting pressurized air from air springs  30  to escape through quick exhaust valve  133  until the pressurized air in air springs  30  is equal to the pressurized air present in the height control valve, thus lowering the trailer to its ride height. 
     In the event that flipper plate  50  is not retracted, the sensor switch  110  sends an audible and/or visible signal to the operator by indicator  135  that the flipper plate must be rotated to the retracted position. 
     Although only one actuator  80  is shown, it is to be understood that two or more actuators may be used with a single or plurality of drive cams  100 . Moreover, although only one sensor switch  110  is shown, it is contemplated that two sensor switches, each associated with its respective flipper plate, may be provided. 
     Referring now to FIGS. 6-8, there is shown an alternate embodiment of an anti-creep device  212  according to the invention. Like parts in the previous embodiment are represented by like numerals. Two anti-creep devices  212  are associated with the axle  16  on opposite sides of the frame  14 . Only one of the anti-creep devices will be described in detail, it being understood that the other anti-creep device is constructed in the same manner. The anti-creep device  212  includes a flipper plate  250  pivotally attached to a mounting assembly  214 . 
     As seen most clearly in FIG. 8, the mounting assembly  214  includes a first plate  216  shaped to fit around the side  15  and bottom  17  of frame rail  14 . The first plate  216  is preferably welded to the frame rail but may be secured through other well-known means. A second flat plate  218  is welded or otherwise secured to the first plate at a top portion thereof. A bore  220  at a lower portion of first plate  216  is in alignment with a bore  222  at a lower portion of second plate  218 . A gusset  224  is welded to first plate  216  and includes a bore  226  in alignment with apertures  220  and  222 . A bushing  228  is positioned in bore  226 . The bushing is preferably constructed of bronze, but other materials can be used. A main rod  230  is positioned in bushing  228  for rotation relative thereto. Flipper plate  250  is non-rotatably attached to main rod  230  at a position between the first and second plates for rotation with the main rod. A flange  232  is attached to opposite ends of the main rod at a location outwardly of the second plates to prevent the main rod from sliding laterally. An extension rod  234  is attached between main rods  230  located on opposite sides of the frame rail  14 . As in the previous embodiment, the main rod  230  has a larger diameter than extension rod  234  since main rod  230  will ultimately support the weight of the trailer and its contents when flipper plate  250  is rotated to secure the axle  16  against movement. The main rod  230  is rotated in the same manner as main rod  66  in the previous embodiment. A centerline  234  of the swing plate  250  is ideally in alignment with the side  15  of frame rail  14  for minimizing any torque that may be present on the first and second plates. 
     As illustrated in FIGS. 6 and 7, the flipper plate  250  includes a plurality of notches  252  for engaging a pawl  236  welded to the outside plate  41 . One end of an extension spring  238  is attached to frame  14  and another end is attached to flipper plate  250 . The extension spring serves to bias flipper plate  250  against pawl  236 . As in the previous embodiment, extension spring  238  can replace or add to compression spring  96  in actuator cylinder  80 , depending on the amount of force required to rotate the flipper plates. 
     In operation, as the trailer is being pulled by a tractor, the long stroke actuator cylinder  80  is pressurized by the air supply  160  that is in fluid contact with the chamber of the parking brake  156  which enters the air port  104  and biases the plunger  94  inward against the compression spring  96 , thus keeping flipper plate  250  in the retracted position, as in the previous embodiment. With the flipper plate  250  in the retracted position, the trailer is backed into a loading dock area so that the end of the trailer abuts the end of the loading dock. The axle  16  of the low-ride suspension  10  is in the axle full up position where the frame is lowered, as shown in phantom lines in FIG. 7, and the flipper plates  250  are in the retracted position during trailer pulling or backing. The operator can then either pressurize the air springs of the low-ride suspension to move the axle toward the full down position such that the frame is raised, as shown in solid line in FIG. 7, or exhaust the air pressure from actuator cylinder  80  to rotate the flipper plates toward pawl  236 . In a preferred operation of the present embodiment, the air is first exhausted from actuator cylinder  80  (FIG. 3) by exhausting air from the air brake system  150 , thereby setting the parking brake. As in the previous embodiment, the valve assembly  155  functions as a pressure switch for controlling operation of the actuator in response to setting and releasing the parking brake. When air is exhausted from the brake system and actuator, the shaft  88  is extended due to the force of spring  96  and/or tension spring  238 , thereby pivoting drive cam  100  which in turn rotates the flipper plates  250  in a clockwise direction as shown in FIGS. 6 and 7 to engage the pawl  236 . In the rotated position, edge  253  of flipper plate  250  abuts a corner  237  of pawl  236 , thereby stopping further rotation of the flipper plate  250 . The air springs  30  are then pressurized to raise the frame  14  with respect to the axle, until the bed of the trailer is even with the loading dock. As the air springs are pressurized, the flipper plate edge  253  rides along the corner  237  of pawl  236  until a first notch  252 A receives the corner. At this point, if the trailer bed is at the proper height, the pressure in the air springs is released such that the pawl firmly seats in the notch to support the trailer weight. If the first notch  252 A does not define the required trailer bed height, the air springs continue to be pressurized to raise the frame  14  even further with respect to the axle  16 . As the frame  14  is raised, flipper plate  250  is forced to swing in a counter-clockwise direction as viewed in FIGS. 6 and 7 due to the notch ramped surface  254  until the second notch  252 A is reached. This operation is continued until the proper trailer bed height is reached. Thus, the pawl and flipper plate notches function as a ratchet mechanism for positioning the trailer bed at the desired height. In a preferred arrangement, five notches are provided at every inch for locking the axle with respect to the frame. Once the pawls are seated in a corresponding notch, the frame rail  14  is rigidly supported by the flipper plates  250  on the axle  16  and thus creeping of the trailer away from the dock is prevented. 
     When trailer loading or unloading is completed, the air supply is turned on again to pressurize the air springs to thereby disengage the pawl and flipper plate notch. The air brake system and actuator cylinder  80  are then simultaneously pressurized to thereby rotate the flipper plates to their retracted position and release the parking brake. The air spring is subsequently depressurized to lower the frame rail  14  to its travel height. 
     The anti-creep device according to the invention provides a simple and reliable solution to the trailer creep problem. The anti-creep device also can be installed as original equipment or as a retrofit. 
     While the invention has been described with reference to its preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. For example, the actuator cylinder  80  can be replaced by an air brake actuator to accomplish similar results to either embodiment. Instead of an air-pressured actuator cylinder, it is contemplated that an electrically operated actuator can be used in place thereof, such as a spring-loaded solenoid. In this instance, an electrical pressure switch (not shown) can be in fluid communication with the chamber of the spring brake actuator to supply electrical power to the actuator in response to a reduction in air pressure in the spring brake chamber, such as when the parking brake is set, and to cut off electrical power from the actuator when pressure is restored to the spring brake chamber. When power is supplied to the actuator, a solenoid shaft extends in the same manner as the air pressured actuator to rotate one or both flipper plates toward the extended position. Many other modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from the scope of the invention.