Abstract:
A load sensing safety device comprises a pulley shaft carrying a pulley. The pulley, in turn, carries a belt that effectuates movement of the patient lift boom. A biasing element applies a spring force against the pulley shaft to move the pulley shaft in a first direction. A switch that opens and closes responsive to movement of the pulley shaft, whereby the switch opens where the pulley shaft is moved to a first position by movement of the spring in the first direction.

Description:
BACKGROUND OF INVENTION 
   This invention relates in general to lifts for lifting and transporting patients. More particularly, the invention relates to a load sensing safety device for patient lifts and more particularly for vertical lifts. 
   Lifts for lifting and transporting patients are well known. Such lifts typically include a base that is usually supported for movement relative to a supporting surface, such as a floor. Extending upwardly from the base is a mast and extending forward from the mast is a boom. From a forward end of the boom is suspended a rigging of some sort, such as a sling hanger for supporting a sling, which in turn is provided for supporting the patient. The boom is displaceable to vertically move the rigging to raise and lower a patient supported by the rigging. With the patient supported by the rigging, the lift may merely support the patient in a fixed position, or the lift may be moved relative to the supporting surface to transport the patient. 
   There are two common ways in which the boom is typically displaced. In one manner, the boom is supported for pivotal movement relative to the mast and an actuator is angularly disposed between the mast and the boom. Extending the actuator causes a free end of the boom to be raised and contracting the actuator causes a free end of the boom to be lowered. In another manner, the boom is supported for vertical movement relative to the mast. An actuator causes the entire boom to be raised and lowered. The actuator often includes a flexible belt for providing a lifting force for raising and lowering the boom. 
   The boom and patient support rigging are often very heavy. If the patient should come into contact with an underlying obstruction and the actuator continues to lower the boom, the weight of these components can be applied against the patient, resulting in patient injury. When the lift employs a flexible belt to transmit lifting force, further potential for injury exists if a patient encounters an underlying obstruction because the belt may continue to be extended and accumulate as slack. If the obstruction is suddenly removed, the patient may move abruptly downward until the slack of belt is taken up. 
   SUMMARY OF INVENTION 
   The present invention is directed towards a lift for lifting and transporting patients, and a load sensing safety device for the lift. The load sensing safety device comprises a pulley shaft carrying a pulley, which, in turn, carries a belt that effectuates movement of the patient lift boom. A biasing element applies a spring force against the pulley shaft to move the pulley shaft in a first direction. A switch opens and closes responsive to movement of the pulley shaft, whereby the switch opens when the pulley shaft is moved to a first position by movement of the spring in the first direction. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of a lift for lifting and transporting patients. 
       FIG. 2  is an enlarged bottom perspective view of the base with the base housing shown in hidden line to make visible a leg articulation actuator therein for pivotally displacing the legs. 
       FIG. 3  is a side elevational view of the lift with the mast and the base housing shown in hidden line to make visible a lift actuator therein for raising and lowering the boom. 
       FIG. 4  is an enlarged top perspective view of a motor and pulley arrangement forming a part of the actuator shown in  FIG. 3 . 
       FIG. 5  is an enlarged rear perspective view of a carriage and pulley arrangement forming a part of the actuator shown in  FIG. 3 . 
       FIG. 6  is an exploded perspective view of the pulley arrangement illustrated in  FIG. 5 . 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings, there is illustrated in  FIG. 1  a lift  10  for lifting and transporting patients. The lift  10  generally comprises a base  12  supported for movement relative to a supporting surface, a mast  14  extending vertically upward from the base  12 , and a boom  16  supported for movement relative to the mast  14 . Although not shown, a forward extending free end of the boom  16  is subject to support rigging, such as a cradle or sling hanger, which are well known in the art of the invention. 
   The base  12  generally includes a housing  20  and legs  22  extending forwardly from the housing  20 . The rear end of the base  12  and the forward ends of the legs  22  are supported relative to a supporting surface by rear and front wheels or casters  24 ,  26 . The casters, preferably the rear casters  24 , are preferably provided with a brake that may be effectuated by a lever (not shown) to prevent the casters  24  from rotating and thus prevent the lift  10  from moving relative to the supporting surface. To assist an attendant in moving the lift  10 , a foot push pad  28  may be provided on the base  12  for application of pressure by the attendant&#39;s foot to move the base  12 . The mast  14  may also be provided with a handle  30  to aid the attendant in articulating the lift  10  in a desired direction. 
   In the exemplary embodiment, the legs  22  are supported for pivotal movement relative to the base seat panel  12  (i.e., along the lines A in  FIG. 1 ). Although this pivotal movement may be accomplished in any suitable manner, in the illustrated embodiment, the movement is accomplished via a leg articulation actuator comprised of, for example, a motor and gearbox arrangement, as generally indicated at  32  in  FIG. 2 . The motor and gearbox arrangement  32  is supported in fixed relation to the housing  20 , as is the pivotal support between the housing  20  and the legs  22 . A plate  34  is supported for rotation relative to some fixed point within the housing  20  by the motor and gearbox arrangement  32 . The legs  22  are supported for pivotal movement relative to the base  12  by pivotal connections  36 . Connected between the plate  34  and the pivotal connections  36  are actuation rods  38 ,  40 , which are displaceable in transverse directions relative to the housing  20  by rotational movement of the plate  34  via operation of the motor and gearbox arrangement  32 . Transverse displacement of the actuator rods  38 ,  40 , in turn, moves the legs  22  pivotally relative to the housing  20 . 
   Now with reference to  FIG. 3 , there is illustrated a lift actuator for raising and lowering the boom  16 . As shown in the drawing, the boom  16  is supported for vertical movement along a track  42  within the mast  14 . In the exemplary embodiment, the mast  14  has a forwardly presented, vertically extending opening or slot  44  through which extends a boom support  46 . The boom support  46  is carried by a carriage  48 . The carriage  48  is supported for vertical movement by a belt  50  that is threaded upwardly over an upper pulley  52  and then downwardly through the mast  14  to a lower pulley  54 , which forms a part of a motor and pulley arrangement  56 . 
   An exemplary embodiment of the motor and pulley arrangement  56  is shown in detail in  FIG. 4 . As shown in the drawings, the motor and pulley arrangement  56  includes a motor  58 , which drives the lower pulley  54 . The motor  58  is mounted to a pulley housing  60 , which houses the lower pulley  54 . The pulley housing  60 , in turn, is held in a fixed relation to the base housing  20 . A lower end of the belt  50  is attached to, and subject to be wound around the lower pulley  54 . 
   Now, with reference to  FIG. 5 , it can be seen that the belt  50  is threaded upwardly and over the upper pulley  52 , and then downwardly to the carriage  48 , where it is securely attached via, for example, a through pin  62 . The carriage  48  comprises a body  64 , to which is supported the boom support  46 . The carriage body  64  has one or more wheels, and in the particular embodiment shown, has four wheels  66 . The wheels  66  are subject to travel along one or more contact surfaces within the mast  14 . The mast  14  may be extruded, or otherwise formed, to include formations for receiving the carriage body  64  and tracks for engagement with the accompanying wheels  66 . 
   In operation, as the motor  58  is driven in a first direction, the lower pulley  54  gathers the belt  50 , which in turn pulls the belt  50  about the upper pulley  52 , wherein the belt  50  pulls the carriage  48  upwardly to raise the boom support  46  and, in turn, the boom  16 . Conversely, as the motor  58  is driven in a second direction, opposite to the first direction, the lower pulley  54  releases the belt  50 , which travels about the upper pulley  52 , allowing the mass of the occupant to pull the boom  16 , the boom support  46 , and the carriage  48  downwardly. 
   The leg articulation and lift actuators may be operated in any suitable manner. In the exemplary embodiment, a power supply, such as a DC battery and controller, together with suitable electronics, are supported within the base  12  and mast  14 , and further by a battery and controller housing  68 , which is attached to the outside the mast  14 . 
   Now, with reference to  FIG. 6 , a description of a load sensing safety device  70  will ensue. There is illustrated an exploded view of a pulley assembly. The assembly comprises a pair of laterally spaced support blocks  72 , at least one of which has a bore  73 , which extends downwardly, at least partially through the support blocks  72 , for receiving a return spring  74 , and an elongate hole  76 , which extends laterally through the support blocks  72 , for receiving a pulley shaft  78 . The bores  73  communicate with the holes  76  and the springs  74  are situated in the bores  73  beneath opposing ends of the pulley shaft  78  to apply an upwardly directed spring force against the pulley shaft  78 . The holes  76  and the pulley shaft  78  are sized to permit vertical movement of the pulley shaft  78  while substantially prohibiting horizontal movement in a direction perpendicular to the shaft so as to reduce the risk of having horizontal slop between the holes  76  and the shaft  78 . 
   Between the support blocks  72  there is carried by the shaft  78  the upper pulley  52 . The upper pulley  52  is supported for ease of movement relative to the shaft  78  by bearings, such as the needle bearings  80  diagrammatically shown. 
   Between one support block  72  and the upper pulley  52 , the shaft  78  further carries a switch plate  82 , which moves up and down with the shaft  78  as the shaft  78  moves vertically in the holes  76 . Mounted below the switch plate  82  to the same support block  72  is a mounting  84 , to which is mounted a switch  86 . In the exemplary embodiment, the switch  86  is a normally open switch, which is closed by contact with the switch plate  82  as the switch plate  82  moves downward with the pulley shaft  78 . 
   In operation, a downwardly directed force applied against the upper pulley  52  by the belt  50 , when the load of a patient is supported by the boom  16 , urges the pulley shaft  78  downwardly against the force of the springs  74 . The downward movement of the shaft  78 , in turn, causes the switch plate  82  to move downward into contact with the normally open switch  86  to cause the switch  86  to close. This completes the electrical circuit that supplies current to the motor  58  to drive the motor  58  in a desired direction to cause the lower pulley  54  to gather or release the belt  50 , which in turn raises or lowers the carriage  48 , the boom support  46  and the boom  16  to raise or lower the patient supported by the boom  16 . 
   Now, if the patient should come into contact with an underlying obstruction, continued downward movement of the boom  16  would relieve downwardly directed force applied against the upper pulley  52  by the belt  50 , due to a decrease in the load of the patient supported by the boom  16 . With insufficient force applied against the upper pulley  52 , and thus the pulley shaft  78 , the return springs  74  urge the pulley shaft  78  upwardly via the force of the springs  74 . The upward movement of the shaft  78 , in turn, causes the switch plate  82  to move upward out of contact with the normally open switch  86 . This allows the switch  86  to open, which breaks the electrical circuit that supplies current to the motor  58 , at least with regard to current that drives the motor  58  to further release to belt  50 . This, in turn, prevents the boom  16  from being further lowered, at least until the underlying obstruction is removed. 
   The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.