Abstract:
A fluid power load-clamping system includes at least one fluid valve for variably regulating the maximum fluid pressure causing closure of the clamp. Preferably the valve increases the maximum fluid pressure automatically in relation to the measured magnitude of the weight of the load to regulate the load-gripping force. A controller causes the valve to permit a relatively high maximum fluid pressure as the clamp closes toward the load to enable high initial clamp closure speed. Thereafter the valve automatically reduces the maximum pressure as the clamping surfaces close into a predetermined relationship with the load, and then increases the maximum pressure to regulate the gripping force. Other preferable features include continuous weight-responsive automatic regulation of the gripping pressure while the load is supported by the clamp, and compensation of the weight measurement for the longitudinally-extensible position of the lifting mechanism, to maximize the accuracy of the load-weight measurement. Gripping pressure regulation is operable in response to linear load-lifting or tilting load-lifting, without concurrent clamp closure actuation. Different predetermined relationships between the weight of the load and the maximum gripping pressure are selectable alternatively. A gravity-referenced tilt controller adjusts the load automatically to an attitude which is untilted with respect to gravity. Lowering of the lifting mechanism is automatically prevented in response to the setting down of the load.

Description:
[0001]    This is a continuation-in-part of patent application Ser. No. 09/168,358, filed Oct. 7, 1998, which is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to improvements in fluid power load-clamping systems for variably regulating maximum load gripping forces in a manner automatically adaptive to at least one characteristic of the load.  
           [0003]    Various types of such adaptive load-clamping systems have been proposed in the past. Such previous systems can be categorized as follows:  
           [0004]    (1) Systems which sense the existence of load slippage and respond automatically by gradually increasing the gripping force on the load by fixed force increments until the sensed slippage stops;  
           [0005]    (2) Systems which automatically vary the gripping force in proportion either to the sensed weight or to the resistance to gripping of the load, without regard to whether or not slippage is actually occurring; and  
           [0006]    (3) Systems which perform a combination of (1) and (2).  
           [0007]    Fluid power clamping systems of any of the above types regulate gripping force by gradually increasing gripping fluid pressure automatically from a relatively low threshold pressure. However such low threshold pressure limits the speed with which the load-engaging surfaces can be closed into initial contact with the load, thereby limiting the productivity of the load-clamping system. This problem occurs because high-speed closure requires higher closing pressures than the desired low threshold pressure, such higher pressures becoming trapped in the system by fluid input check valves during initial closure so that the desired lower threshold pressure is exceeded before automatic regulation of gripping pressure can begin. Although gripping pressure relief valve systems have in the past provided high and low relief settings selectable either manually, or automatically in response to clamp closure speed, to enable high-speed closure followed by low maximum gripping pressure, no such systems capable of automatically changing such settings in a manner compatible with automatic variable gripping pressure regulation have been known.  
           [0008]    Prior fluid power systems such as those disclosed in British Patent Publication No. 2312417 and German Patent Publication No. 3245715, which vary the gripping fluid pressure in proportion to the sensed weight of the load, obtain weight measurements by lifting the load. However such weight-sensing systems operate only in response to clamp closure actuation, and therefore do not continue to vary the gripping fluid pressure in proportion to load weight during subsequent manipulation of the load in the absence of continued clamp closure actuation. Furthermore, such prior systems do not weigh the load in response to lifting of the load by tilting which, in paper roll handling operations, is a commonly-used alternative way to lift the load. The system shown in the British publication is also susceptible to inaccurate weight measurements due to variations in lifting pressure which are inherent within the extensible lifting mechanism depending upon its degree of extension.  
           [0009]    Such prior weight-responsive systems also do not provide for different selectable predetermined relationships between the weight of the load and the gripping pressure, which are needed to account for variations in load fragility and stability.  
           [0010]    Although automatic load tilt adjustment systems have been provided in the past for leveling fragile loads to prevent edge damage when the load is being set down, such automatic adjustment systems have not been capable of sensing the tilt of the load with respect to gravity, leading to inaccurate automatic tilt adjustment depending on whether or not an industrial lift truck is level with respect to its supporting surface, or whether or not such surface is level.  
           [0011]    Valves for automatically preventing excessive lowering of the lifting mechanism when a clamped load is set down, to prevent subsequent damage to fragile load surfaces by downward slippage of the clamp when it is opened to disengage the load, have been provided in the past as shown, for example, in U.S. Pat. No. 3,438,308. However, such previous systems lack the versatility needed for reliable protection of the load under variable circumstances, such as variations in the degree of extension of the lifting mechanism when the load is set down.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    In one preferred aspect of the invention, a controller automatically enables high initial clamp closure speed prior to automatic gripping pressure regulation by initially permitting relatively high fluid pressure to close the clamp, followed by an automatic reduction in the maximum fluid pressure as the clamping surfaces close into a predetermined relationship with the load, followed by an increase in the maximum fluid pressure pursuant to automatic maximum gripping pressure regulation.  
           [0013]    In another separate preferred aspect of the invention, the load-weight measurement is compensated to account for variations in extension of the lifting mechanism, also to maximize the accuracy of the load-weight measurement.  
           [0014]    In another separate preferred aspect of the invention, automatic weight-responsive gripping pressure regulation is operable without concurrent clamp closure actuation.  
           [0015]    In another separate preferred aspect of the invention, automatic weight-responsive gripping pressure regulation is operable in response to lifting of the load solely by tilting.  
           [0016]    In another separate preferred aspect of the invention, different predetermined relationships between the weight of the load and the maximum gripping pressure are selectable alternatively.  
           [0017]    In another separate preferred aspect of the invention, a gravity-referenced tilt controller automatically adjusts the load to an attitude which is untilted with respect to gravity.  
           [0018]    In another separate preferred aspect of the invention, an improved system is provided for automatically preventing further lowering of the lifting mechanism when the load is set down.  
           [0019]    In another separate preferred aspect of the invention, the speed of lowering of the lifting mechanism is limited automatically to aid the accuracy of the lowering prevention system.  
           [0020]    The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a front view of an exemplary embodiment of a fluid-powered load-handling clamp in accordance with the present invention.  
         [0022]    [0022]FIG. 2 is a top view of the load-handling clamp of FIG. 1.  
         [0023]    [0023]FIG. 3 is a schematic diagram of an exemplary electrohydraulic circuit for the clamp of FIG. 1.  
         [0024]    FIGS.  4 A- 4 F are an exemplary simplified logic flow diagram of an initialization sequence, a load clamping sequence, and a disengagement sequence utilized by the microprocessor-based controller in the circuit of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    An exemplary embodiment of a load-handling clamp in accordance with the present invention is indicated generally as  10  in FIGS. 1 and 2. The exemplary clamp  10  is a hydraulically-powered, pivoted-arm clamp having a base  15  adapted for mounting on a lift truck carriage which is selectively reciprocated linearly along an upright hydraulically-powered load-lifting mast indicated schematically as  11  in FIG. 3. The mast is selectively tiltable forwardly and rearwardly by a pair of tilt cylinders such as  13  in FIG. 3. The particular clamp  10  depicted in the drawings is for handling large paper rolls such as  12  in FIG. 2 used in the publishing and paper industries which, if deformed excessively as a result of overclamping to prevent slippage, will become too distorted for use on the high-speed printing presses or other machinery for which they are intended. On the other hand, under-clamping can cause the paper roll  12  to slip from the frictional grasp of the clamp  10 , particularly when the load-engaging surfaces  14  and  16  of the clamp  10  are oriented vertically by the clamp&#39;s rotator  18  which rotates the respective clamp arms  20  and  22  relative to the base frame  15  about an axis  24  (FIG. 2). Although the hydraulically-operated paper roll clamp  10  is described herein as the preferred embodiment, the present invention is also applicable to many other types of load clamps. For example, clamps in accordance with the present invention could alternatively have sliding rather than pivoted arms, and could handle rectilinear rather than round loads.  
         [0026]    Each of the clamp arms  20  and  22  is rotatable about its respective pivot pins  26 ,  28  selectively toward or away from the other clamp arm by the selective extension or retraction of respective pairs of hydraulic cylinders  30  and  32  associated with the respective arms  20  and  22 . The cylinders  30  which actuate the shorter clamp arm  20  are primarily used only to position the clamp arm  20  in advance for carrying rolls  12  of different diameters and different desired lateral positions. Therefore, closure of the clamp arms and their load-engaging surfaces to grip the load is normally accomplished solely by movement of the clamp arm  22  in response to extension of the cylinders  32 . In some clamps, the shorter clamp arm  20  could be fixed, and the cylinders  30  eliminated. In other clamps, particularly those with sliding arms, closure would normally be accomplished by moving both clamp arms simultaneously toward each other. Moreover, closure may be caused by retraction of cylinders instead of extension thereof.  
         [0027]    With reference to FIG. 3, hydraulic clamping cylinders  32  are controlled through hydraulic circuitry indicated generally as  34  to receive pressurized hydraulic fluid from the lift truck&#39;s reservoir  38  through a pump  40  and supply conduits  42  and  43 . Safety relief valve  44  opens to shunt fluid back to the reservoir  38  if excessive pressure develops in the system.  
         [0028]    A priority flow control valve  49  insures that a predetermined priority flow, for example one gallon per minute, of fluid is diverted to conduit  43  before excess flow is permitted to conduit  42 . The priority flow in conduit  43  is for automatic gripping pressure regulation, while the excess flow in conduit  42  supplies manually actuated load-clamping and hoisting selector valves  36  and  80  respectively, as well as a tilt control valve  82 .  
         [0029]    The clamp control valve  36  is controlled selectively by the operator to cause the cylinders  32  to open the clamp arms and to close the clamp arms into initial contact with the load  12 . To open the clamp arms, the spool of the valve  36  is moved downwardly in FIG. 3 so that pressurized fluid from line  42  is conducted through line  46  to the rod ends of cylinders  32 , thereby retracting the cylinders  32  and moving the clamp arm  22  away from the clamp arm  20 . Pilot-operated check valves  50  are opened by the pressure in line  46  communicated through pilot line  52 , enabling fluid to be exhausted from the piston ends of cylinders  32  through line  54  and valve  36  to the reservoir  38  as the cylinders  32  retract. Alternatively, to close the clamp arms, the spool of the valve  36  is moved upwardly in FIG. 3 so that pressurized fluid from line  42  is conducted through line  54  to the piston ends of cylinders  32 , thereby extending the cylinders  32  and moving the clamp arm  22  toward the clamp arm  20 . Fluid is exhausted from the rod ends of the cylinders  32  to the reservoir through line  46  via the valve  36 .  
         [0030]    During closure of the clamp arms by extension of the cylinders  32 , the maximum closing pressure in the line  54  is preferably regulated by a pilot controlled modulating pressure regulator valve assembly  75  of which the pilot control is by variably controlled relief valve assembly  74 . The variable relief valve assembly  74  preferably comprises a single relief valve whose relief setting is infinitely proportional to a variable signal received from the controller  70  through signal line  76 . Alternatively, the maximum closing pressure could be regulated by single or multiple relief valve and/or regulator valve assemblies with different settings automatically selectable by a signal from the controller  70 , or by an automatically-variable pressure-reducing valve assembly having one or more pressure-reducing valves in series with line  54  whose output pressure settings are variably regulated by the controller  70 .  
         [0031]    As the clamp arms are closed toward the load, the controller  70  operates in accordance with the steps of FIGS.  4 C- 4 E, and in accordance with the initialization values previously entered into the controller  70  by the operator pursuant to FIGS. 4A and 4B using keyboard switches such as  118 . Appropriate portions of these figures will be referenced in the following operational description of the clamp.  
         [0032]    During initial clamp arm closure, the controller  70  sets the variable relief pressure of the valve assembly  74 , as indicated at step  200  of FIG. 4C, at a relatively high level previously selected by the operator at step  300  of the initialization sequence of FIG. 4B from among three alternative levels “1, 2, 3.” Such pressure level enables high-speed closure of the clamp arms toward the load prior to actually gripping the load. Thereafter, in response to contact of the load-engaging surfaces of-the clamp arms with the load, the clamp-closing pressure in line  54  as sensed by pressure sensor  78  increases above a minimum pilot pressure level previously selected by the operator at step  315 . At the same time the volumetric flow rate in line  54  decreases and causes a corresponding decrease in the positive differential, between the pressure reading by the pressure sensor  78  and the reading by the pressure sensor  66 , to a differential value below that previously selected by the operator at step  301  of the initialization sequence of FIG. 4B. In response to such changes, reflecting a predetermined resistance by the load to further closure of the arms, the controller  70  at steps  202  and  204  of FIG. 4C immediately reduces the relief setting of the relief valve assembly  74  to a relatively low threshold level previously selected by the operator from among three alternatives at step  302  of FIG. 4B. This decreases the pressure, between the pilot-operated check valves  50  and the cylinders  32 , to the reduced relief setting so that the high-speed initial closing pressure is not maintained between the check valves  50  and the cylinders  32 . Such reduced pressure is the threshold gripping pressure from which subsequent increases in gripping pressure will be automatically regulated as described below.  
         [0033]    Instead of reducing the closing pressure in response to load resistance as described, other predetermined relationships between the load and the load-engaging surfaces could trigger the pressure reduction, such as a predetermined proximity therebetween.  
         [0034]    After the desired threshold gripping pressure is established at step  204 , the operator moves the valve  36  to its centered, unactuated position and begins to lift the load, either by manually actuating the hoist-control valve  80  to move the load linearly upward, or by manually actuating the tilt control valve  82  to tilt the load rearwardly.  
         [0035]    In the case of the hoist valve  80 , its spool is moved upwardly to lift the load and downwardly to lower the load as seen in FIG. 3. When the valve  80  is actuated to lift the load, the valve  80  conducts pressurized fluid from line  42  through lines  84  and  88  to the base of one or more hoist cylinders, schematically indicated as  90 , of the mast  11 . A pressure sensor  92  senses a resultant increase in pressure in line  88  and signals the controller  70  that lifting has begun, as indicated at step  206  of FIG. 4C. In response, the controller actuates solenoid valve  94 , as indicated at step  208  of FIG. 4D, by moving its spool upwardly in FIG. 3 so that the priority flow in line  43  can flow through line  54  to the cylinders  32  to further close the clamp arms.  
         [0036]    The controller  70  senses the magnitude of the weight of the load through the signal from the pressure sensor  92 , and adjusts the relief setting of the valve assembly  74  upwardly in a predetermined relation to the sensed magnitude of the load weight in a manner to be explained more fully hereafter. Since solenoid valve  94  is actuated, this increases the maximum fluid gripping pressure in line  54  in a predetermined relation to the magnitude of the load weight. The cushioning effect of accumulator  87  minimizes dynamic effects on the load-weight measurement and thereby maximizes the accuracy of such measurement. If necessary, a restrictor (not shown) in the line  88  can be optionally included to limit lifting speed and thereby further minimize dynamic effects.  
         [0037]    After the foregoing maximum fluid gripping pressure has been achieved, the controller  70  deactivates the solenoid valve  94  as indicated at step  212  of FIG. 4E, moving the spool of the valve downward in FIG. 3 so that automatic gripping pressure regulation ceases. The valve  74  is set by the controller  70  to prevent any further gripping pressure increases which might otherwise result from the operator&#39;s manipulation of valve  36 , as indicated by step  214  in FIG. 4E. Thereafter, the system begins continuous monitoring of the fluid gripping pressure relative to the sensed load weight and, if necessary, readjusts the gripping pressure as explained more fully hereafter.  
         [0038]    Alternatively, the operator&#39;s manual actuation of the tilt control valve  82  to tilt the load rearwardly and thus lift it, by moving the spool upwardly in FIG. 3, also initiates the foregoing load-weighing and pressure-regulating operation in the same manner, since the pressure sensor  92  will sense a resultant increase in pressure in line  88  due to the lifting of the load and will initiate the above-described sequence beginning with step  206 .  
         [0039]    It will be recognized that sensors other than fluid pressure sensors  66 ,  78  and  92  could be used. For example, flow meters and/or electromechanical force sensors could be substituted as appropriate.  
         [0040]    During the above described load-weighing and pressure-regulating operation, increased fluid gripping pressure causes some extension of the clamping cylinders  32 , requiring the exhaust of some fluid through line  46  from the rod ends of the cylinders  32 . Since the clamp control valve  36  would normally be centered during such operation, such fluid is exhausted to the reservoir  38  through a parallel line  48  and pilot operated check valve  58  which is opened by the pressure in line  54  transmitted through pilot line  60 .  
         [0041]    The accuracy of the load-weight measurement is enhanced by compensating for variations in extension of the mast  11  which vary the pressure reading of the sensor  92 . Such pressure variations can result from multiple causes, such as changes in effective pressure areas of the hoist cylinder or cylinders  90 , or the fact that telescopic sections of the mast  11  may or may not be supported by the hoist cylinder or cylinders  90 , depending upon whether the mast is in its lower “freelift” range of extension or in its higher “mainlift” range of extension. To account for these variables, as well as variables in the load-handling clamps that might be mounted interchangeably on the mast, the controller  70  is initialized according to FIGS. 4A and 4B to calibrate the load-weighing system with respect to such variables. Such initialization includes reading and storing the respective pressures sensed by the sensor  92  in both the freelift and mainlift ranges of extension of the mast while dynamically lifting the load-handling clamp, both without a load as shown in steps  304  and  306  of FIG. 4A to obtain P f  and P m  respectively, and with a load of known weight as shown in steps  308  and  310  to obtain P fw  and P mw  respectively. The controller  70  also reads respective pressures P fs  and P ms  sensed by sensor  92  with no load in the freelift and mainlift ranges, respectively, under static conditions, i.e. in the absence of dynamic lifting, and stores the pressures as indicated at steps  313  and  314  of FIG. 4B. Furthermore, the controller stores the known load weight W k  as indicated at step  312  in response to operator entry using keyboard switches such as  118 . Other operator entries using keyboard switches include one or more desired clamp-force-to-load-weight ratios CF/W ratio 1, 2, 3, as indicated at step  316 , and a “clamp factor” X at step  320  representing the total effective pressure area of the combined clamping cylinders  32  multiplied by the efficiency percentage of the clamp cylinders  32 . Such efficiency percentage corresponds to the ratio of the clamp force generated by the load-engaging surfaces  16  (after frictional and other mechanical losses) to the product of the effective pressure area of the combined clamping cylinders  32  and the applied fluid pressure.  
         [0042]    As indicated at step  324  at the beginning of the initialization process of FIG. 4A, all of the foregoing parameters-need be entered only for new installations or changes of load-handling clamps or masts. Otherwise, only the shorter list of entries designated as “Option 2” in FIG. 4 need be entered, or no entries if the operator does not wish to change any listed parameter.  
         [0043]    Returning to the load-clamping sequence of FIGS.  4 C- 4 E, the controller  70  controls the load-weight measurement and gripping pressure regulation processes by automatically accounting for the range of extension of the mast  11  (freelift or mainlift), different desired clamp-force-to-load-weight ratios, and the other variables mentioned in connection with FIGS. 4A and 4B. Immediately after clamp pressure is relieved at step  204  of FIG. 4C, the controller senses at step  218  whether a mechanical switch  219 , responsive to the degree of extension of the mast  11 , is closed. If the switch is closed, the controller  70  determines at step  218  that the mast is in its lower, or freelift, range of extension; otherwise the controller determines that the mast is in its higher, or mainlift, range of extension. Depending on such determination, the controller  70  sets the future load-weight calculation with parameters appropriate either for the freelift range of extension or the mainlift range of extension of the mast. After the actuation of solenoid  94  at step  208  in response to the operator&#39;s lifting of the load by actuation of the hoist valve  80  or the tilt valve  82  as previously described, the controller reads the lifting pressure P sensed by pressure sensor  92  as indicated at step  220 , and at step  222  calculates therefrom the load weight W using the appropriate freelift or mainlift calculation.  
         [0044]    For the freelift range of extension of the mast  11 , the calculation is as follows:  
       W   =         (     P   -     P   f       )                     (     W   k     )         (       P   fw     -     P   f       )                             
 
         [0045]    For the mainlift range of extension of the mast  11 , the calculation is as follows:  
       W   =         (     P   -     P   m       )                     (     W   k     )         (       P   mw     -     P   m       )                             
 
         [0046]    In the foregoing calculations, P f  and P m  are the values which were previously entered during steps  304  and  306 , respectively, of the initialization sequence of FIG. 4A, while P fw  and P mw  are the values previously entered during steps  308  and  310 . W k  is the weight of the known load used during initialization and previously entered at step  312  of the initialization sequence.  
         [0047]    After calculation of the load weight W at step  222  of FIG. 4D, the controller determines which predetermined clamp-force-to-load-weight ratio CF/W was previously selected by the operator at step  316  of FIG. 4B, and determines at step  224  of FIG. 4E the desired maximum clamp force CF by the equation:  
           CF=W ( CF/W ).  
         [0048]    Having determined the desired maximum clamp force CF at step  224 , the controller  70  determines the clamp factor X previously entered by the operator at step  320  and calculates the maximum fluid gripping pressure CP at step  226  by the equation:  
         
       CP=CF/X.  
     
         [0049]    At step  228  the controller then adjusts the maximum pressure relief setting of valve  74  to the desired maximum fluid gripping pressure CP. This process repeats continuously until the controller determines that the actual fluid gripping pressure sensed by sensor  66  equals or exceeds the desired fluid gripping pressure CP, as indicated at step  230 . The controller  74  then deactivates the solenoid  94  at step  212  and sets the valve  74  at step  214  to prevent manually activated pressure increases as described previously.  
         [0050]    Instead of manual keyboard selections of different clamp-force-to-load-weight ratios at step  316  of FIG. 4B, or different initial threshold gripping pressures at step  302 , different relationships between maximum gripping pressure and load weight to account for differences in fragility or stability of the load can be selected automatically in response to an electronic code reader  120  which senses characteristics of a load by reading a coded label on the load. Such variable relationships can also be selected automatically by a proximity sensor  122  which senses the distance between the load-engaging surfaces of the clamp arms to determine the size of the load being gripped. Accordingly, different types of predetermined relationships between fluid gripping pressure and load characteristics are contemplated by the present invention, as well as different types of mechanisms for selecting such different relationships.  
         [0051]    After initial automatic regulation of the gripping pressure during initial clamp closure, the system continually senses whether the clamped load is being supported by the clamp by comparing the hoist pressure sensed by sensor  92  with the appropriate unloaded static hoist pressure P fs  or P ms  previously stored at steps  313  and  314 , depending on whether the switch  219  is closed, as indicated at step  240 . As long as the hoist pressure at sensor  92  is greater than the appropriate stored unloaded static hoist pressure, indicating at step  242  of FIG. 4F that the operator has not set the load down, the system repeatedly recalculates the desired fluid gripping pressure CP as before, and compares it to the actual fluid gripping pressure at sensor  66 . In the comparison at step  244 , the system determines whether the actual fluid gripping pressure is at least a predetermined percentage (such as 95%) of what it should be. If not, the system automatically readjusts the relief setting of the valve assembly  74  upwardly to the new desired maximum fluid gripping pressure CP and readjusts the fluid gripping pressure and resultant gripping force, beginning at step  218  of FIG. 4C, by recalculating the gripping pressure CP and reactivating the solenoid valve  94 . On the other hand, if the actual fluid gripping pressure is still within the predetermined percentage at step  244 , the controller merely continues to recalculate and compare the actual fluid gripping pressure, without also readjusting it. This automatic repetitive monitoring and correction of the fluid gripping pressure and resultant gripping force corrects for such variables as leakage in the clamp cylinders  32  which could decrease the gripping force, or the possibility that the load was not fully supported by the clamp during the initial automatic regulation of the gripping pressure. The priority flow from the priority flow control valve  49 , and the parallel exhaust line  48 , insure the reliability of the continuous gripping-force correction feature, even though the clamp control valve  36  is in its centered, unactuated condition.  
         [0052]    The foregoing repetitive monitoring and, if necessary, correction operation continues until the system senses, at step  242  of FIG. 4F, that the operator has set the load down. Thereafter, once the operator has opened the clamp, as sensed at step  232  by a pressure rise at sensor  98 , the load clamping sequence returns to its origin at step  200  of FIG. 4C where the relief pressure of valve  74  is reset at the relatively high level needed for high speed closure, as described previously.  
         [0053]    To minimize the possibility of setting a fragile load down onto a supporting surface in a tilted attitude such that the edge of the load would be damaged, a gravity-referenced tilt sensor  124  is optionally mounted on the base frame  15  of the clamp  10  to determine whether or not the load is tilted forwardly or rearwardly with respect to gravity and to cause the controller  70  to automatically adjust the load to a level attitude by corrective solenoid actuation of the tilt control valve  82 . Mounting the gravity-referenced tilt sensor  124  on the clamp structure, rather than on the mast  11 , allows the sensor to determine whether or not the load is tilted with respect to gravity irrespective of any tilting of the mast  11  due to mast deflection or other factors. The gravity-referenced sensor is also independent of whether or not the lift truck is level with respect to its supporting surface, or whether or not such surface is level. However, despite its foregoing advantages, the gravity-referenced sensor  124  is also susceptible to instability and long settling times if subjected to dynamic disturbances during lift truck travel, such as acceleration or braking, or vertical dynamic disturbances caused by ramps or uneven surfaces. For this reason, the controller  70  actuates the tilt control valve  82  correctively only in response to a decrease in load-weight detected by pressure sensor  92  (i.e. a negative pressure slope) in response to lowering of the load by the mast  11  to set the load down. During such lowering of the load, dynamic disturbances are minimized due to stoppage of the lift truck.  
         [0054]    Another problem which can lead to load damage while setting the load down onto a supporting surface is the possibility that the operator may continue to lower the mast  11  after the load has been set down but before the operator has opened the clamp arms. In such case, the chains of the mast which normally support the clamp will become slack because the clamp is then supported by the clamped load rather than the mast. Thereafter, when the operator finally opens the clamp arms to disengage the load, the load engaging surfaces of the clamp arms slide down the surfaces of the load, causing external damage to fragile loads such as paper rolls. To minimize the possibility of such damage a solenoid valve  47  downstream of a priority flow control valve  45  is preferably provided so as to be automatically controlled by the controller  70 , in response to the setting down of a clamped load, to prevent further lowering of the mast until after the clamp arms have been opened to disengage the load. In the normal lowering mode, fluid flows through the priority path of the priority flow control valve  45 , and flows through conduit  84  and hoist control valve  80 , in its lowering position, through line  56  to the reservoir  38 . The priority flow control valve  45  is of a design where the priority flow requirements must be satisfied before the valve will permit any flow to bypass through its excess flow port and the excess flow conduit  51 . With reference to FIG. 4E, when the controller  70  detects through sensor  92  at step  240  that the hoist pressure has declined to a level equal to or less than the unloaded static pressure P fs  or P ms , this indicates that a clamped load has been set down on a supporting surface. Accordingly, pursuant to step  242  of FIG. 4F the controller  70  activates the solenoid valve  47  at step  236  thereby blocking the priority flow path. Without the priority flow condition being fulfilled, the priority flow control valve  45  blocks excess flow from returning to the reservoir alternatively through conduit  51  and thereby prevents the mast from lowering further. When the clamp is subsequently opened, as automatically determined at step  232  by sensing a pressure rise at sensor  98 , the controller deactivates the solenoid valve  47  at step  238 , and the mast and clamp can thereafter be further lowered by the operator without damaging the load. During lowering of the mast  11 , an optional restrictor  55  can be employed to limit lowering speed to maintain the accuracy of the pressure sensed by sensor  92  even if the operator opens the lowering control valve rapidly and fully. The foregoing lowering prevention system is also applicable to other types of loads and load-engaging structures, such as forks, to prevent free-fall of the load-engaging structure when disengaged from the load.  
         [0055]    The foregoing lowering prevention system can alternatively be implemented without the priority flow control valve  45  and excess flow conduit  51  by employing a solenoid valve  47  capable of a larger volumetric flow rate.  
         [0056]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.