Abstract:
Disclosed herein is a vehicle including a frame, and a stabilizer apparatus including a pressure source, first and second fluid-actuated stabilizer assemblies connected respectively to the front and rear axles shiftably moveable on the frame, one of the first and second stabilizer assemblies normally being locked against extension and contraction in the absence of pressurization thereof by the pressure source, and the other of the first and second stabilizer assemblies normally being free to extend and contract in the absence of pressurization thereof by the pressure source, a hydraulic circuit connected between the pressure source and the first and second fluid activated stabilizer assemblies and including a flow controller operable between a first mode wherein the pressure source is disconnected from the first and second stabilizer assemblies, whereby the one of the stabilizer assemblies is locked against extension and retraction, thereby locking the axle connected thereto against shifting movement relative to the frame, and whereby the other of the stabilizer assemblies is free to extend and retract, thereby permitting shifting movement relative to the frame of the axle connected thereto, and a second mode wherein the pressure source is selectively connectible to the first and second stabilizer assemblies for selective pressurization thereof by the pressure source so as to selectively extend and retract the first and second stabilizer assemblies.

Description:
This is a division of U.S. patent application Ser. No. 986,145 filed Dec. 4, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to forklifts including rough terrain forklifts or other load moving vehicles, and, more particularly, to a stabilizing apparatus for the forklift frame. 
     2. Reference to Prior Art 
     Rough terrain forklifts are commonly used in a variety of industries for moving and lifting heavy or bulky loads. These vehicles are used at construction sites and may be used on relatively rough terrain or uneven surfaces, and generally include a boom or crane mechanism for raising and lowering a set of forks. 
     Rough terrain forklifts include front and rear axle assemblies each generally mounted on the forklift main frame so that the axle assemblies can shift with respect to the frame. These forklifts also often include at least one hydraulic leveling cylinder assembly between the frame and the front axle assembly for leveling the frame. The leveling cylinder assembly is manually operable to tilt the frame to the right or to the left so that the frame and load supporting boom can be leveled when the forklift is on uneven ground. 
     SUMMARY 
     This invention provides a stabilizer apparatus for a rough terrain forklift or other vehicle to restrict tipping of the vehicle main frame as the vehicle is driven over uneven surfaces. 
     An object of the invention is the provision of a stabilizer apparatus for use on a forklift having a main frame and an axle assembly supporting the main frame, the axle assembly being shiftable with respect to the main frame, the stabilizer apparatus having particular utility for mechanically restricting or preventing shifting movement of the axle assembly with respect to the main frame, thereby substantially reducing or preventing rocking or tilting of the frame when the forklift moves over an obstruction. 
     Another object of the invention is the provision of means for mounting the stabilizer apparatus on a forklift or other vehicle including a main frame and an axle assembly so that at least a portion of the load transmitted from the axle assembly to the stabilizer apparatus is transferred mechanically directly to the main frame. 
     The stabilizer apparatus includes a pair of fluidactuated stabilizing cylinder assemblies mounted on opposite sides of the forklift main frame. Each stabilizing cylinder assembly includes a cylinder and a reciprocable ram at least partially disposed in the cylinder. Each of the rams includes a rounded end, the rounded end of one of the rams engaging one end of a forklift axle assembly and the rounded end of the other of the rams engaging the other end of the forklift axle assembly. 
     Each of the stabilizing cylinder assemblies is mounted on the main frame by means for fixedly connecting the cylinder assembly to the main frame, and a shoe. The shoe includes a body portion fixed to the stabilizing cylinder assembly and a flange portion extending under part of the main frame and engaging the main frame for mechanically transferring at least part of the upwardly directed forces on the stabilizing cylinder assembly directly to the main frame. 
     A fluid pump and fluid reservoir provide pressurized fluid to the stabilizing cylinder assemblies. A pair of check valves, one between the pump and each of the stabilizing cylinder assemblies, permit fluid flow to each of the stabilizing cylinder assemblies but prevent flow in the reverse direction. A pair of restriction orifices and a pair of flow control valves, one restriction orifice and one flow control valve being located between the pump and each of the stabilizing cylinder assemblies, control fluid flow from the stabilizing cylinder assemblies. Each of the flow control valves is shiftable between a first position wherein fluid flow is blocked and a second position wherein fluid flow is permitted. In the second position, fluid flow from the stabilizing cylinder assemblies through the flow control valves is permitted, but is also restricted by the restriction orifices so that the rams engaging the axle assembly are allowed to retract only very slowly in response to upward pressure exerted on the rams from the axle assembly, and so that only very slow movement of the axle assembly relative to the main frame is permitted. When the flow control valves are in the first position, fluid flow from the stabilizing cylinder assemblies is prevented so that retraction of the rams in response to upward pressure on the rams from the axle assembly is prevented, causing the axle assembly to become rigid with respect to the main frame. 
     The invention also comprises a vehicle including a frame, an axle assembly mounted on the frame and including a front axle means and a rear axle means, each of the axle means being supported at its opposite ends by a wheel and being shiftable relative to the frame, a stabilizer apparatus including first and second fluid-actuated stabilizer assemblies connected, respectively, to the front and rear axle means, a pressure source, hydraulic circuit means connected between the pressure source and the first and second fluid-actuated stabilizer assemblies and including flow control means having a first mode wherein one of the stabilizer assemblies is reciprocated and the other of the stabilizer assemblies is selectively operable whereby one of the axle means is shifted and the other is selectively shiftable. The flow control means also has a second mode wherein both of the stabilizer assemblies are selectively operable. 
     The invention also includes a method of stabilizing a vehicle having boom means pivotally mounted on the frame for movement through preselected vertical angles, an axle assembly mounted on the frame and including a front axle means and a rear axle means, each of the axle means being supported at its opposite ends by wheels and being shiftable relative to the frame, the method comprising the steps of permitting one of the axle means to shift relative to the frame and selectively shifting the other axle means relative to the frame or locking the axle means relative to the frame when the boom means is below a predetermined vertical angle, and selectively shifting both of the axle means relative to the frame when the boom means is above the predetermined vertical angle, the selective shifting of the front and rear axle means being at a substantially slower speed when the boom means is above said predetermined angle than when said boom is below said predetermined angle. 
     The invention also comprises a vehicle including a frame, a boom mounted on the vehicle, means for elevating the boom above a predetermined horizontal angle, an axle assembly mounted on the frame and including a front axle means and a rear axle means, each of the axle means being supported at its opposite ends by a wheel and being shiftable relative to the frame, a stabilizer apparatus including first and second fluid-actuated stabilizer assemblies connected respectively to the front and rear axle means, a pressure source, hydraulic circuit means connected between the pressure source and the first and second fluid activated stabilizer assemblies and including flow control means having a first mode wherein one of the stabilizer assemblies is reciprocable and the other of the stabilizer assemblies is selectively operable whereby one of the axle means is shiftable and the other is selectively shiftable, the flow control means having a second mode wherein the one stabilizer assembly is reciprocable at a slower rate than when the hydraulic circuit means is in its first mode, the flow control means including sensing means for sensing when the boom is elevated above the predetermined angle and for setting the flow control means in its second mode. 
     The invention further includes a method of stabilizing a vehicle having a frame, boom means pivotally mounted on the frame for movement through preselected vertical angles, an axle assembly mounted on the frame and including a front axle means and a rear axle means, each of the axle means being supported at its opposite ends by wheels and being shiftable relative to the frame, the method comprising the steps of permitting one of the axle means to shift relative to the frame at a first rate and selectively shifting the other axle means relative to the frame or locking the axle means relative to the frame when the boom means is below a predetermined vertical angle, and permitting the one axle means to shift relative to the frame at a second rate slower than the first rate and selectively shifting the other axle means relative to the frame when the boom means is above a predetermined vertical angle. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view of a rough terrain forklift embodying the invention; 
         FIG. 2  is a schematic view, partially in section, taken along line  2 — 2  in FIG.  1  and showing the stabilizer apparatus when the forklift moves over a bump; 
         FIG. 3  is a view similar to FIG.  2  and showing the stabilizer apparatus when the forklift is on an inclined surface; 
         FIG. 4  is a view taken along line  4 — 4  in  FIG. 2 ; 
         FIG. 5  is an enlarged partial view of the axle stabilizer apparatus shown in  FIG. 3 ; 
         FIG. 6  is a view taken along line  6 — 6  in  FIG. 1 ; 
         FIG. 7  is a front view of the front axle assembly shown in  FIG. 6 ; 
         FIG. 8  schematically illustrates an alternate embodiment of the invention; and 
         FIGS. 9 and 10  illustrate an alternate embodiment of the invention. 
       Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a load lifting vehicle or forklift  10  which embodies the invention and which is preferably capable of traveling over rough or uneven terrain such as may be encountered at a construction site. The forklift  10  includes a main frame  12  which has a right side  14  and a left side  16 , a rear pair of axle mounting members  18 , and a front pair of axle mounting members  20  (FIG.  6 ). Axle mounting members  18  and front member  20  each extend between the right side  14  and the left side  16  of the main frame  12 . In the illustrated arrangement, the main frame  12  also includes a leveling cylinder support member  21  located on the front of the main frame  12  and adjacent the right side  14  of the main frame  12 . 
     As shown in  FIG. 1 , forklift  10  also includes means for manipulating a load  22 . While various means for manipulating can be employed, in the illustrated arrangement, the means for manipulating includes a hydraulically-operated telescoping boom  24  which is connected at one end to the rear of the main frame  12  and which is pivotable relative to the main frame  12  about a generally horizontal axis  26  between a lowered position (shown in solid lines in  FIG. 1 ) and a raised position (shown in broken lines in FIG.  1 ). The means for manipulating also includes a carriage  28  which is attached to the other end of the boom  24  and which includes forks  30  for supporting the load  22 . The carriage  28  is pivotable relative to the boom  24  about a generally horizontal axis  32  and is also preferably pivotable relative to the boom  24  about a generally vertical axis  34 . 
     The means for manipulating a load  22  also includes hydraulic cylinder assemblies  38 ,  40  and  42 , which respectively rotate or pivot the boom  24 , telescope the boom  24 , and tilt the carriage  28  about the horizontal axis  32 . A pressurized hydraulic fluid source  44  is provided for supplying hydraulic fluid to the cylinder assemblies  38 ,  40  and  42 . As shown in  FIG. 2 , the fluid source  44  includes, a fluid reservoir  46 , a main valve  48  which directs fluid flow to the various hydraulic components, and a pump  50  between the reservoir  46  and the main valve  48 . 
     The forklift  10  also includes means for supporting the main frame  12  for movement along the ground. As shown in  FIG. 7 , the means for supporting the main frame  12  includes a front axle assembly  52  which includes a center section  53  having an upwardly extending center portion  54 , and right and left end sections  55  and  56  pivotally connected to the opposite ends of the center section  53  by vertically disposed pins  57  and  58 , respectively. The end sections  55  and  56  are respectively pivotable about the axes of the pins  57  and  58 . Means  63  ( FIG. 6 ) which are known in the art are provided for pivoting the end sections  55  and  56  to steer the forklift  10 . A first horizontal pin  59  extending through the upper part of the front axle assembly center portion  54  and through the front axle mounting members  20  connects the front axle assembly  52  to the main frame  12  so that the front axle assembly  52  is shiftable or pivotable about the axis of the first horizontal pin  59  relative to the main frame  12 . The front axle assembly  52  also includes right and left front wheels  60  and  61  for supporting the front axle assembly  52 . 
     The means for supporting the main frame  12  also includes a rear axle assembly  62  which has opposite right and left ends each respectively including a plate  64  and  66  secured to the upper surface thereof. The rear axle assembly  62  includes a center portion  68  extending upwardly between the rear axle mounting members  18 . A second horizontal pin  70  extends through the upper part of the rear axle assembly center portion  68  and through each of the rear axle mounting members  18  to connect the rear axle assembly  62  to the main frame  12 . The rear axle assembly  62  is shiftable or pivotable about the axis of the second horizontal pin  70  relative to the main frame  12 . Right and left rear wheels  72  and  74  each respectively support the right and left ends of the rear axle assembly  62 . 
     Each of the axle assemblies  52  and  62  is preferably coupled through a drive train (not shown) to a forklift engine (not shown) to drive the forklift wheels  60 ,  61 ,  72  and  74 . 
     The forklift  10  also includes means for leveling the main frame  12 . While various means for leveling can be employed, in the illustrated arrangement, the means for leveling includes a double-acting, hydraulic leveling cylinder assembly  76  which is pivotally interconnected between the support member  21  and one end of the center section  53  of the front axle assembly  52  (FIG.  7 ). The leveling cylinder assembly  76  includes a leveling cylinder  78  and a reciprocable rod  80  at least partially disposed within the leveling cylinder  78 . The leveling cylinder assembly  76  is supplied with hydraulic fluid from the fluid source  44 . Fluid flow to and from the leveling cylinder assembly  76  is preferably manually controllable by the operator, such as by a conventional joy stick control. Thus, the operator can selectively extend or retract the rod  80  to level the main frame  12  to compensate for hillsides and the like. 
     While in the illustrated arrangement the leveling cylinder assembly  76  is secured between the front axle assembly  52  and the support member  21 , in other arrangements, the leveling cylinder assembly  76  could be positioned in the rear of the forklift  10  between the rear axle assembly  62  and the main frame  12 . 
     As shown in  FIG. 2 , the forklift  10  also includes means for reducing tipping or tilting, or for stabilizing the main frame  12 . The means for stabilizing includes a stabilizer apparatus  82 . The stabilizer apparatus  82  includes right and left stabilizing assemblies  84  and  84 ′ respectively positioned on the right side  14  and the left side  16  of the main frame  12 . Although each of the stabilizing assemblies  84  and  84 ′ may have an arrangement different from that of the other, in the illustrated arrangement, the structure of the stabilizing assemblies  84  and  84 ′ is substantially identical, except for being mirror images of one another, and operation of each of the stabilizing assemblies  84  and  84 ′ is also substantially identical. Thus, only the right stabilizing assembly  84  will be described in detail. Common elements of the stabilizing assemblies  84  and  84 ′ will be given the same reference numerals, except numerals associated with the left stabilizing assembly are identified with a prime (′) notation. 
     The stabilizing assembly  84  includes a stabilizing cylinder assembly  86  which includes a cylinder  88 , a cylinder cap  90  which is on the top of the cylinder  88  and which includes fluid ports  92  and  94 , and a reciprocable ram  96  extending out from the bottom of the cylinder  88  and at least partially disposed within the cylinder  88 . The ram  96  includes a rounded end  98  engaging the plate  64  on the right end of the rear axle assembly  62 . The ram end  98  remains in constant, or nearly constant contact with the plate  64  during operation of the forklift  10  as will be further explained below. Likewise, the ram end  98 ′ also remains in constant, or nearly constant contact with the plate  66  during operation of the forklift  10 . 
     While in the illustrated arrangement the stabilizing cylinder assemblies  86  and  86 ′ are located toward the rear of the main frame  12  and engage the rear axle assembly  62 , in other arrangements, the stabilizing cylinder assemblies  86  and  86 ′ could be located elsewhere, such as toward the front of the main frame  12 , and could engage the front axle assembly  52 . 
     The stabilizing assembly  84  also includes means for mounting the stabilizing cylinder assembly  86  on the main frame  12 . While various means for mounting can be employed, in the illustrated arrangement, the means for mounting includes means for fixedly connecting the stabilizing cylinder assembly  86  to the main frame  12 . In the illustrated arrangement, the means for fixedly connecting includes fasteners such as bolts  100 . 
     The rear axle assembly  62  is capable of exerting upwardly directed loads on the stabilizing cylinder assembly  86  through the ram  96  so that the stabilizing cylinder assembly  86  acts as a load-bearing member for transferring a load from the rear axle assembly  62  to the main frame  12 . To accommodate the load which the stabilizing cylinder assembly  86  bears, the means for mounting the stabilizing cylinder assembly  86  on the main frame  12  also includes load carrying means for transferring at least part of the upwardly directed load on the stabilizing cylinder assembly  86  directly to the main frame  12 . In the illustrated arrangement the load carrying means includes a shoe  102  which is preferably integrally formed. 
     The shoes  102  and  102 ′ are preferably identical and will be described with respect to shoe  102 ′. As shown most clearly in  FIG. 5 , the shoe  102 ′ includes a body portion  104 ′ which is fixed to the cylinder  88 ′ by means such as welding, and which is secured to the main frame  12  by at least one of the bolts  100 ′. The shoe  102 ′ also includes a flange portion  106 ′ projecting from the body portion  104 ′ and extending under and engaging a portion of the main frame  12 . An upwardly directed load transmitted through the ram  96 ′ is at least partially mechanically transferred directly to the main frame  12  through the flange portion  106 ′, thereby reducing the shear load on the bolts  100 ′. 
     As shown in  FIG. 2 , pressurized hydraulic fluid for operating the stabilizing cylinder assembly  86  is supplied by the pressurized hydraulic fluid source  44  through a main fluid line  108  leading from the mean valve  48 , and also through a second fluid line  110  leading from the main line  108 . Fluid returning to the fluid reservoir  46  from the stabilizing cylinder assembly  86  bypasses the pump  50  and the main valve  48  via fluid return line  112 . A first filter  114  located in the return line  112  preferably includes a bypass valve  116  to maintain a constant pressure in the lines  108  and  110  of approximately 25-50 psi. 
     A one way or check valve  118  is located between the second fluid line  110  and the fluid port  92 . Fluid flow to the cylinder  88  through the check valve  118  is permitted, while fluid flow from the cylinder  88  through the check valve  118  is prevented. In the event the rear axle assembly  62  moves in a direction away from engagement with the ram end  98  (FIG.  3 ), fluid flow through the check valve  118  to the cylinder  88  facilitates extension of the ram  96  to maintain contact between the ram end  98  and the plate  64 . 
     The stabilizing assembly  84  is provided with means for restricting fluid flow from the cylinder  88 . The means for restricting is in parallel with the check valve  118  and is located between the second fluid line  110  and the fluid port  94 . While various means for restricting can be employed, in the illustrated arrangement, the means for restricting includes a restriction orifice  120 . The restriction orifice  120  restricts fluid flow from the cylinder  88  to substantially slow retraction of the ram  96  within the cylinder  88 . In the event the rear axle assembly  62  attempts to move against the ram  96 , thereby exerting an upwardly directed force on the ram  96 , slowed retraction of the ram  96  will impede movement of the rear axle assembly  62 , so that the rear axle assembly  62  becomes generally rigid and shifts only very slowly with respect to the main frame  12 . The restriction orifice  120  is preferably of a size which does not interfere to any appreciable degree with the leveling of the frame  12  by the leveling cylinder assembly  76 . A second fluid filter  122 , is provided between the restriction orifice  120  and the port  94 . 
     While in some arrangements the means for restricting can include only the restriction orifice  120 , such as is preferred in arrangements wherein the carriage  28  is not pivotable relative to the boom  24  about a generally vertical axis such as the axis  34 , in the illustrated arrangement, the means for restricting also includes a flow control valve  124  which is positioned between the restriction orifice  120  and the second fluid line  110 . The use of a flow control valve  124  is preferred in arrangements which include a carriage  28  that is pivotable relative to the boom  24  about a generally vertical axis, such as the axis  34 , as will be further explained below. The flow control valve  124  is shiftable between a first position wherein fluid flow through the flow control valve  124  is blocked (FIG.  2 ), and a second position wherein fluid flow through the flow control valve  124  is permitted (FIG.  3 ). When the flow control valve  124  is in the first position, fluid flow is prevented both to and from the stabilizing cylinder assembly  86  through the flow control valve  124 , so that the ram  96  cannot retract into the cylinder  88  in response to movement of the rear axle assembly  62 . Thus, when the flow control valves  124  and  124 ′, are each in the first position the rear axle assembly  62  becomes rigid and generally immoveable relative to the main frame  12 . When the flow control valve  124  is in the second position, fluid flow from the stabilizing cylinder assembly  86  is permitted so that the ram  96  can retract into the cylinder  88  at the rate permitted by the restriction orifice  120 . 
     While in the illustrated arrangement the restriction orifice  120  and the flow control valve  124  are separate, in other arrangements, the restriction orifice  120  could be incorporated into the flow control valve  124  such that the orifice  120  is in use when the flow control valve  124  is in the second position. 
     Means are provided for shifting the flow control valve  124  between the first and second positions. While various means for shifting can be employed, in the illustrated arrangement, the means for shifting includes a solenoid  126 . Although the solenoid  126  can be actuated to shift the flow control valve  124  in response to a variety of conditions, and although actuation of the solenoid  126  can be automatic or manual, in the illustrated arrangement, the solenoid  126  is automatically actuated to shift the flow control valve  124  to the first position in response to movement of the boom  24  to the raised position. Preferably, the solenoid  126  is activated to shift the flow control valve  124  to the first position when the boom  24  forms an angle of about 10 degrees or more with the horizontal. When the boom  24  is in a lowered position and forms an angle of less than 10 degrees with the horizontal, the solenoid  126  is deactivated and the flow control valve  124  remains in, or is returned to the second position by a spring  128 . 
       FIG. 2  illustrates the operation of the stabilizer apparatus  82  when the rear axle assembly  62  encounters an obstruction such as a bump  130 . In  FIG. 2 , the flow control valves  124  and  124 ′ are shown in the first or blocked position, i.e., the boom  24  is in the raised position, and the right rear wheel  72  is shown on the bump  130  while the surface beneath the left rear wheel  74  is relatively flat. Blocking the exit of fluid from the stabilizing cylinder assembly  86  prevents retraction of the ram  96  when the bump  130  is encountered so that the rear axle assembly  62  is maintained in a fixed position relative to the main frame  12  and tipping, tilting or rocking of the main frame  12  is reduced or prevented. The rear wheels  72  and  74  rise and fall together when an obstruction is encountered and the forklift  10  is supported on the three wheels  60 ,  61  and  72  until the wheel  72  moves off the bump  130 . 
     Had the flow control valves  124  and  124 ′ of  FIG. 2  been in the second position rather than in the first position as shown, the rear axle assembly  62  would still have been maintained in a generally rigid position relative to the main frame  12  as the right rear wheel  72  passed over the bump  130 . As fluid flow from the cylinder  88  is severely restricted by the restriction orifice  120 , retraction of the ram  96  within the cylinder  88  is severely impeded, thereby preventing or at least significantly slowing rotation of the rear axle assembly  62  when the wheel  72  travels over the bump  130 . Thus, when the flow control valves  124  and  124 ′ are in the second position and one of the rear wheels  72  and  74  encounters a transient obstruction such as the bump  130 , the rear axle assembly  62  is maintained in substantially the same position it was in prior to encountering the bump  130  so that the main frame  12  is not caused to tip. If the forklift  10  had come to rest with the right rear wheel  72  on the bump  130 , the rear axle assembly  62  would pivot very slowly relative to the main frame  12  until the left rear wheel  74  contacted the ground. 
       FIG. 3  shows the rear axle assembly  62  on an inclined surface when the flow control valves  124  and  124 ′ are in the second position, i.e. when the boom  24  is in the lowered position. Referring specifically to stabilizing cylinder assembly  86 ′ in  FIG. 3 , as the inclined surface urges the rear axle assembly  62  to rotate in a clockwise direction (as viewed in  FIG. 3 ) against the ram  96 ′, fluid is permitted to exit the cylinder  88 ′ through the flow control valve  124 ′ and the restriction orifice  120 ′, thereby allowing the ram  96 ′ to very slowly retract within the cylinder  88 ′. The very slow retraction of the ram  96 ′ allows the rear axle assembly  62  to adapt to contour changes in the terrain which are not merely transient, such as hillsides for example. 
     Referring specifically to stabilizing cylinder assembly  86  in  FIG. 3 , as the rear axle assembly  62  rotates in a clockwise direction (as viewed in FIG.  3 ), fluid flow through the check valve  118  permits the ram  96  to extend from the cylinder  88  so that the ram end  98  remains in constant or nearly constant contact with the plate  64 . The rounded end  98  of the ram  96  facilitates sliding contact with respect to the plate  64  so that only a generally upwardly directed load is exerted on the ram  96  by the rear axle assembly  62 . 
     When the forklift  10  is being driven from place to place, it is desirable that the boom  24  be in the lowered position so that the flow control valves  124  and  124 ′ are in the second position and the rear axle assembly  62  is permitted to slowly shift relative to the main frame  12  to follow the contour of non-transient terrain features. Allowing the rear axle assembly  62  to slowly shift to conform to hillsides and the like reduces the risk of forklift roll over. At the same time, when the flow control valves  124  and  124 ′ are in the second position, the rear axle assembly  62  remains generally rigid relative to the main frame  12  when one of the rear wheels  72  and  74  travels over transient terrain features such as holes or the bump  130 . This reduces or prevents tipping of the main frame  12 . When the flow control valves  124  and  124 ′ are in the second position the leveling cylinder assembly  76  can be operated to maintain the main frame  12  in a level position. 
     Before the boom  24  is elevated to the raised position to raise a load  22  or to pick up a load  22  which is already raised, the main frame  12  can be leveled by manually operating the leveling cylinder assembly  76 . Leveling of the main frame  12  when the boom  24  is elevated and the rear axle assembly  62  is rigid relative to the main frame  12  can cause undue stress on the leveling cylinder assembly  76 , can possibly raise one of rear wheels  72  and  74  off the ground, or can twist the main frame  12 . Once the main frame  12  is leveled the boom  24  can be raised. 
     The leveling cylinder assembly  76  can be operated to substantially fix the front axle assembly  52  relative to the main frame  12 . If the rear axle assembly  62  is shiftable relative to the main frame  12  and the front axle assembly  52  is in a fixed position relative to the main frame  12 , the main frame  12  will be generally supported against tipping to the left or right by the front axle assembly  52 . In addition, the weight of the main frame  12 , the boom  24 , and the load  22  will be supported by the front axle assembly  52  and also by the rear axle assembly  62  at the point of attachment of the rear axle assembly  62  to the main frame  12 . If the carriage  28  and the load  22  is supports are pivoted about the generally vertical axis  34 , the center of gravity of the load  22  will be shifted so that the load  22  may be supported predominantly by only one end of the front axle assembly  52  and by the rear axle assembly  62  at its point of attachment to the main frame  12 . Shifting of the rear axle assembly  62  under these circumstances could cause the main frame  12  to tip, or even roll over in severe cases where the load  22  is very heavy. When the rear axle assembly  62  is fixed relative to the main frame  12 , as is the case when the flow control valves  124  and  124 ′ are in the first position, the main frame  12  is further stabilized against tipping. Thus, the possibility that the main frame  12  will tip as a result of an eccentric load  22  which is created by the pivoting of the carriage  28  about the vertical axis  34  is reduced. 
     In the event the carriage  28  is not pivotable about the generally vertical axis  34  and the stabilizer apparatus  92  does not include flow control valves  124  and  124 ′, the main frame  12  can be leveled when the boom  24  is in either of the raised or lowered positions. The forklift  10  also avoids the risk of tipping which is caused by an eccentric load  22  on the end of the boom  24  and which could result if the carriage  28  were otherwise pivotable about the vertical axis  34 . 
       FIG. 8  shows an alternate embodiment of the invention wherein the stabilizing apparatus includes a first double-acting frame tilt cylinder  150  pivotally connected to one end of the front axle assembly  52  and a second double-acting frame stabilizer cylinder  152  is pivotally connected to the opposite end of the rear axle assembly  62 . Accordingly, the cylinders  150  and  152  are connected to the axle assemblies  52  and  62  at the diametrically opposite corners of the frame  12 . It will be appreciated that the base ends of each of the cylinders  150  and  152  are pivotally connected to the frame  12  in the manner illustrated in  FIGS. 6 and 7 , for example. 
     A hydraulic circuit  154  having first and second modes couples the cylinders  150  and  152  to a source of hydraulic pressure, such as a pump  156 . In its first mode, such as, for example, when the boom  28  is below a predetermined vertical angle, the frame tilt cylinder  150  locks the front axle assembly  52  unless manually operated and the frame stabilizer cylinder  152 , coupled to the opposite end of the rear axle assembly  62 , is free to float. In its second mode, when the boom  28  is elevated above the predetermined angle, the hydraulic circuit  154  is in its second mode wherein both cylinders  150  and  152  are locked unless manually operated to tilt frame  12 . However, in its latter mode, under normal operation, tilting movement of the frame  12  is at a substantially lower speed than when the hydraulic circuit  154  is in its first mode. 
     When the ignition of forklift  12  is turned “on”, and the boom  24  is below a predetermined angle, each of the solenoids of valves  191 ,  192  and  193  are energized to connect their through passages to its ports while the valve  173  is de-energized so that its ports are connected directly by through passages thereby connecting conduit  170  to conduit  185  and conduit  201  to conduit  202 . Unless manually operated, the valve  158  is centered by springs so that its ports are disconnected. This is the condition of the hydraulic control  154  as shown in FIG.  8 . As a result, cylinder  150  is locked while cylinder  152  is floating so that axle assembly  62  can tilt. 
     Assume that while the hydraulic circuit is in its first mode, so that cylinder  152  is floating, the rear axle assembly  62  is tilted counter-clockwise as a result of wheel  74  hitting a bump or wheel  72  a depression, for example. This moves the piston in cylinder  152  upwardly, forcing hydraulic fluid to flow from the base end through conduit  206 , valve  193 , conduit  207 , and to the conduit  201 . A first portion of the fluid will pass upwardly through conduit  202 , valve  192 , conduit  203  and to the rod side of cylinder  152 . However, because the rod side of cylinder  152  cannot accept all of the hydraulic fluid from the base side, a second portion of the hydraulic fluid will pass through valve  191 , pressure relief valve  215 , conduit  212  and to the sump  178 . 
     On the other hand, should the axle  62  be pivoted clockwise, fluid forced from the rod side of the cylinder  152  will flow through conduit  203 , valve  192 , conduit  202 , valve  173 , conduit  207 , valve  193 , and conduit  206  to the base side of cylinder  152 . However, because the base side of cylinder  152  can accept more hydraulic fluid than that discharging from the rod side, make-up fluid will flow from pump  156  through conduit  195 , pressure reducing valves  197  and  198  and valve  191  to conduit  207  and thence to the base side of cylinder  152 . The pressure reducing valves  197  and  198  will sense a drop in pressure because base side of cylinder  152  can receive more hydraulic fluid than that flowing from the rod side. As a result, conduit  201  will be connected to the pump through valve  191  and pressure reducing valves  197  and  198  so that the requisite make-up oil to the base side of cylinder  152  will be provided. 
     If it is desired to pivot the front axle assembly  52  relative to the frame  12 , the frame tilt valve  158  is manually operated. Movement of the valve  158  to the left will connect the pump  156  to the base side of cylinder  150  through conduit  161 , valve  158 , conduits  181  and  170 , valve  173 , conduit  185 , check valve  187  and orifice  186 . The rod side of cylinder  150  is connected to the sump  178  through conduits  164  and  176 , orifice  167 , check valve  166  (piloted open by the pressure in conduit  185 ). This will tilt the axle assembly  52  counter-clockwise as viewed in FIG.  8 . 
     Movement of the valve  158  to the right connects port  163  to port  182 , port  169  to port  175  and port  160  to port  180 . As a result, the pump  156  is connected to the rod side of cylinder  150  and the base side of cylinder  150  is connected to the sump  178 . This tilts the axle assembly  52  clockwise as viewed in FIG.  8 . 
     In this manner, when the hydraulic circuit is in its first mode, that is, when the boom  28  is below the predetermined vertical angle, the front axle assembly  52  may either be locked or tilted relative to the frame  12  while the rear axle assemble  62  is free to float. This allows the frame to tilt as the vehicle  12  moves over uneven terrain and with the boom  28  down. The speed at which the front axle  52  tilts is controlled by the orifices  167  and  186 . 
     When the boom  28  is elevated above the predetermined angle, such as 40° for example, an interlock (not shown) de-energizes the solenoids of valves  191 ,  192  and  193  so that each of the valves is moved by their respective springs to a position wherein check valves are disposed between their ports. In addition, valve  173  is energized so that it is moved to the right as viewed in FIG.  8 . This connects conduit  170  to conduit  202  and conduit  185  to conduit  201 . However, when the valve  158  is in its neutral position, cylinder  150  is disconnected from the pump  156 , while the check valve in valves  192  and  193  prevent the flow of hydraulic fluid between the rod and base sides of cylinder  152 . As a result, both cylinders are locked to the frame. In this mode, the center of gravity is less likely to move outside the base formed by the wheels  60 ,  61 ,  72  and  74  than if the axle assembly  62  is free to tilt. 
     With the boom  28  elevated, it may be necessary to tilt the frame  12 , such as, for example, if it is necessary to shift the load slightly in the horizontal direction for alignment purposes. This is accomplished by operating the frame tilt valve  158 . Specifically, movement of the valve  158  toward the right connects the rod side of cylinder  150  to the pump  156  through orifice  167 , valve  166 , conduit  164 , valve  158 , conduits  181  and  161 . The base side of cylinder  150  is connected to the base side of cylinder  152  through a path defined by orifice  186 , check valve  187 , conduit  185 , valve  173 , conduits  201 , conduit  207 , check valve  210 , orifice  211  and conduit  206 . The rod side of cylinder  150  is connected to the sump  178  through a path defined by conduit  203 , orifice  206 , check valve  205  (which is piloted opened by the pressure in conduit  207 ), conduit  202 , valve  173 , conduit  170 , valve  158  and conduit  176 . This will rotate both of the axle assemblies  52  and  62  clockwise as viewed in FIG.  8 . 
     If it is desired to rotate the axle assemblies  52  and  62  in the counter-clockwise direction, the frame tilt valve  158  is moved to the left as viewed in FIG.  8 . This connects the pump  56  to the rod side of cylinder  152  through a path defined by conduit  161 , valve  158 , conduit  181 , conduit  170 , valve  173 , conduit  202 , check valve  205 , orifice  206 , and conduit  203 . The base side of cylinder  152  is connected to the base side of cylinder  150  through a path defined by conduit  206 , orifice  211 , check valve  210  (which is piloted open by the pressure in conduit  202 ), conduit  207 , conduit  201 , valve  173 , conduit  185 , check valve  187  and orifice  186 . The rod side of cylinder  152  is connected to the sump through orifice  167 , check valve  166  (which is piloted open by the pressure in conduit  185 ), conduit  164 , valve  158  and conduit  176 . 
     It can be seen that in the second mode of operation, when the boom  28  is above the critical angle, flow through each of the valves  191 ,  192  and  193  is checked so that the hydraulic fluid must pass through orifices  206  and  211 . These orifices are sized to restrict fluid flow so that in the second mode, tilting movement of the axle assemblies  52  and  62  is about one-third the speed that axle assembly  62  tilts in the first mode. This is possible because one of the cylinders  150  or  152  is slaved to the other depending upon the position of valve  158 . As a result, frame tilting motion is relatively slow, thereby minimizing the possibility of tipping. 
     While the embodiment shown in  FIG. 8  includes two double-acting cylinders,  150  and  152 , those skilled in the art will appreciate that a pair of single-acting cylinders coupled to each axle may also be employed. 
       FIGS. 9 and 10  show an alternate embodiment of the invention which includes a hydraulic control system  254  for controlling frame tilting speed and rear axle tilting depending upon the angular position of the boom  24  and whether the vehicle&#39;s braking system is engaged. Moreover, frame tilting is overridden if the vehicle tilts a predetermined angle from the vertical. The hydraulic system  254  controls the flow of hydraulic fluid from a pump  256  to the double-acting frame tilt cylinder  150  connected to one end of the front axle assembly  52  and a second double-acting frame stabilizer cylinder  152  connected to the opposite end of the rear axle assembly  62 . 
     As seen in  FIG. 9 , the hydraulic circuit  254  includes a manually operable, two-way, three-position tilt valve  258  having a first port  260  connected by a conduit  261  to the pump  256 ; a second port  263  connected by conduit  264  to the rod side of cylinder  150  through check valve  266 ; a third port  269  connected by conduit  270  to a first port of a solenoid-operated control valve  273 ; and a fourth port  275  connected by conduit  276  to the sump  278 . The base side of cylinder  150  is connected through check valve  287  and conduit  288  to a second port of control valve  273 . 
     The hydraulic control circuit  254  also includes one-way, two-position solenoid-operated valves  291 ,  292 ,  293 ,  294  and  295  for controlling the flow of hydraulic fluid to the frame stabilizer  152 . Valves  292 ,  293 ,  294  and  295  are normally closed, that is, when energized, each provides a through passage between their inlet and outlet ports and when de-energized are returned by their respective springs to an alternate position wherein there is a check valve between said ports. Valve  291  is normally open so that, when de-energized, it provides a through passage and when energized a check valve is positioned between its ports. Valve  273  provides parallel flow when de-energized and cross flow when energized. 
     A first conduit  295  connects one port of valve  291  to the pump  256  through pressure-relief valve  298  and second and third conduits  301  and  302 , respectively, connect the other port of valve  291  to a second port of valve  273  and to a first port of valve  292 , the other port of which is connected by conduit  303  to the rod side of cylinder  152 . A check valve  305  and an orifice  306  are connected between conduits  302  and  303 . The base side of cylinder  152  is connected by conduit  306  to a first port of valve  293  while its other port is connected by conduit  307  to a fourth port of valve  273 . A check valve  310  and orifice  311  connect conduit  306  to conduit  307  and bypass valve  293 . Valve  294  and orifice  320  are connected between conduits  302  and  303 , and valve  295  and orifice  321  are connected between conduits  302  and  303 . 
       FIG. 10  is an electrical schematic for the embodiment of FIG.  9 . Here, it can be seen that the coils of solenoid valves  273  and  291  are connected in parallel, as are the coils of valves  292  and  293  and the coils of valves  294  and  295 . For purposes of identification, the coils of these valves will be identified by the same reference numeral as that used for the valve, but will be distinguished by a prime (′). The parallel combination of coils  273 ′ and  291 ′ is connected to a first contact  324  of solenoid-operated stabilizer lock switch SW 1  and the parallel combination of coils  294 ′ and  295 ′ is connected to the second contact  325  of stabilizer lock relay SW 1 . One end of the parallel combination of relay coils  292 ′ and  293 ′ is connected to a first contact  327  of a boom switch relay SW 2 . The other ends of each of the coils  273 ′ and  291 ′- 295 ′ are connected to the negative bus  329 . 
     The movable contact  330  of switch SW 1  is connected to the second stationary contact  332  of relay SW 2  and the movable contact  333  of switch SW 2  is connected to the positive supply bus  334 . 
     The coil  336  of relay SW 1  is connected between the negative bus  329  and one terminal  338  of a service brake switch SW 3 . The other terminal  340  of switch SW 3  is connected to the positive bus  334 . Connected in parallel with SW 3  is a neutral switch SW 4  and the wiper  342  and stationary contact  343  of double-pole double-throw park brake switch SW 5 . A parking brake valve coil  345  is connected between the negative bus  329  and a second contact  346  of switch SW 5 . The vehicle  10  may be stopped by operation of the service brake or the parking brake in a manner well-known in the art. In either case, operation of the service brake or the parking brake will close the respective switches SW 3  or SW 4 . 
     The wiper  347  of the other pole of switch SW 5  is operative to connect and disconnect a starter solenoid to the ignition switch SW 6 . Operation of the ignition switch couples the positive bus  334  to the positive terminal of the battery B. Finally, the boom relay switch coil  348  is coupled to the negative bus  329  and to the positive bus  334  through a proximity switch  350 . 
     The proximity switch  350  is normally open and is closed when the boom is elevated above a predetermined angle. When the boom  29  is down, the boom relay switch SW 2  and the stabilizer lock relay SW 1  are both de-energized so that the movable contacts  330  and  333  are in the position shown in FIG.  10 . As a result, solenoid valve coils  292 ′ and  293 ′ are energized and coils  273 ′,  291 ′,  294 ′ and  295 ′ are de-energized, whereby the valves are in the position shown in  FIG. 9  with valves  292 ,  293 , and  291  open to flow, valve  273  set for parallel flow, and valves  294  and  295  are closed. As a result, the opposite ends of the rear cylinder  152  are connected through the path defined by conduit  306 , valve  293 , conduit  307 , valve  273 , conduits  301  and  302 , valve  292 , and conduit  303 . The front cylinder  150  may be manually frame-tilted by operation of the valve  258  in the manner discussed with respect to the embodiment of FIG.  8 . 
     If the boom is elevated above a predetermined angle while the vehicle is in motion, proximity switch  350  is closed to energize the coil  348  of switch SW 1 . This will actuate the solenoid to move wiper  333  from contact  327  to contact  332 . As a result, solenoid coils  292 ′ and  293 ′ are de-energized and solenoid coils  294 ′ and  295 ′ are energized. This opens valves  294  and  295  to flow and sets valves  292  and  293  to block flow. The opposite ends of cylinder  152  are thus connected through a path which includes orifices  320  and  321  so that while the rear axle remains free to float, its movement is dampened. The tilting of the front axle may be achieved by operation of valve  258  as described above. 
     The vehicle  10  may be stopped either by operating the service brake, which closes the service brake switch SW 3 ; actuating the parking brake switch, which steps switch SW 5 ; or placing the vehicle&#39;s transmission in neutral, which closes switch SW 4 . In any event, the stabilizer lock relay is stepped from contact  325  to contact  324 . This de-energizes coils  294 ′ and  295 ′ to close valves  294  and  295 . If the boom  24  is down so that movable contact  333  of switch SW 2  is on contact  327 , coils  273 ′ and  291 ′ remain de-energized so that valve  273  is in parallel flow mode and valve  291  is open. The rear axle  62  remains free to react to the terrain and the front axle  52  may be tilted by operation of valve  258 . 
     If the vehicle is stopped when the boom is up, that is, when proximity switch  350  is operated to energize coil  348  of switch SW 2  so that movable contact  333  is on stationary contact  332 , and relay coil  336  is energized so that movable contact  330  of switch SW 1  is on contact  324  as a result of the closure of switches SW 3 , SW 4  or SW 5 . In this condition, valves  291 - 295  will all be closed, that is, with a check valve between their outlet ports, and valve  273  will be set for cross flow. With the valves set in this position, the rear cylinder  152  is locked in position, since flow between its opposite ends is blocked by check valve  311 , valves  292 - 295 , and the cross-flow position of valve  273 . If the front axle is tilted by operation of valve  258 , the rear cylinder  152  is slaved to the front cylinder  150  through a path defined by conduit  288 , valve  273 , valve  311 , orifice  310 , conduit  303 , orifice  306 , valve  305 , conduits  302  and  301 , and valve  273 . As a result, both frame tilting of the front and rear axles  52  and  62  are dampened. 
     An inclination switch SW 7  is connected between terminal  330  of SW 1  and ground  329  and “E” of inclination relay “IRI”. The switch SW 7  is of a well-known type which closes when the vehicle tilts at a predetermined angle, such as 3°-4°, for example. AS a result, if the vehicle tilts when the boom is up so that proximity switch  350  is closed and the vehicle is moving, switch SW 7  will close to energize coil IRI. This moves contact “C” to contact “A” energizing SWI moving contact  330  to contact  324 , closing valves  291 ′- 295 ′ and placing valve  273  in its cross flow position so that the rear axle is locked. In this manner, if the angle of inclination approaches a critical value, the rear axle is locked to provide a more stable platform and thereby minimized the tendency for the vehicle to tip. 
     Various features of the invention are set forth in the following claims.