Patent Publication Number: US-6705179-B1

Title: Modified rope tensioner

Description:
TECHNICAL FIELD 
     The present invention relates to a rope tensioner which may be used to tension a rope operated switch assembly. Such an assembly may be used, particularly but not exclusively for controlling the power supply to kinetic machinery. 
     BACKGROUND ART 
     Rope operated switch assemblies are generally fitted in proximity to a machine or around any area which requires protection, and comprise two safety switches and a rope extending between the switches such that the electrical power supply may be turned off when the rope is pulled or slackened (e.g. if the rope is cut). In some applications, only one switch is provided, the end of the rope being connected to the single switch and the other end being connected to for example a spring secured to a fixed point. The rope is generally a plastics-coated metal wire or cable, and extends around the machine so that an operator can easily reach it from any position adjacent the machine. The term “rope” used hereinafter is intended to mean any elongate flexible element that is suitable for using in a rope operated switch assembly, for example, metal cable, cord formed of twisted elements or fibres such as wire, polymeric material, etc., which may optionally be coated with a plastics material. 
     The rope must be installed at the correct tension so that the machinery can be operated but so that a relatively small change in the tension of the rope will actuate the switch assembly. This is important because an injured operator may not be able to pull the rope with much force. 
     The installation of the rope at the correct tension is difficult to achieve. The rope must first be installed between the two switches, and is then pulled into an approximation of the correct tension. A turnbuckle is installed in the centre of the rope by cutting the rope and fixing the turnbuckle between the rope sections using thimbles and cable clamps. Once installed, the tension in the rope is set by adjusting eye bolts on the turnbuckle until the rope switches are pulled into a “run” position, that is, with safety contacts in the switches made. The turnbuckle then allows small alterations to be made to the tension of the rope to allow for expansion or contraction of the rope due to, for example, temperature differences. 
     Placement of the turnbuckle in the rope is a time-consuming and often difficult process, and can take up a large proportion of the total time taken to install the rope switch assembly. The setting of the correct tension in the rope is particularly difficult, as the switches must be continually checked to ensure that the switch mechanisms are in the correct position in order that the rope is not set at an incorrect tension. As the turnbuckle ideally is placed towards the centre of the rope to allow for even tensioning, the turnbuckle may be some distance from the switches. 
     In addition, the turnbuckle only allows small differences in tension in the rope to be corrected for. If the tension alters by more than can be dealt with using the turnbuckle, the slack or expansion must be dealt with by movement of the thimbles and cable clamps along the rope segments. A large alteration in tension can occur, for example, due to large temperature fluctuations in different seasons, especially on long runs of rope. 
     DISCLOSURE OF INVENTION 
     It is an object of the present invention to obviate or mitigate such disadvantages with prior art systems. 
     According to a first aspect of the present invention there is provided a rope operated switch assembly comprising a rope extending to at least one switch, the switch being actuable to switch between first and second conditions on a change in tension of the rope, and a tensioner for setting the tension of the rope, wherein the tensioner comprises a body, a rotatable member which engages the rope, means for rotating the member relative to the body to adjust tension in the rope by winding the rope around the member, and locking means to lock the member in position relative to the body. 
     The tensioner may be simply threaded or otherwise installed onto the rope during installation of the rope operated switch assembly, without the need to cut the rope. The installation is thus much simpler than with prior art switch assemblies. 
     The locking means preferably comprises a ratchet and pawl. Preferably, means displaceable relative to the body are provided to move the pawl out of engagement with the ratchet, thereby unlocking the shaft to release the tension in the rope. The displaceable means may comprise a screw that may be screwed into the body. 
     A plate is preferably provided to separate the tensioner into two compartments, the rope extending into one compartment, and the locking means being provided in the other compartment. This means that the rope does not become entangled in the locking means whilst the tensioner is being installed along the rope. 
     The tensioner is preferably provided adjacent the switch, meaning that the installation may be effected quickly as the operator can easily check the state of the adjacent switch as the tension in the rope is altered. 
     According to a second aspect of the present invention there is provided a method of installing a rope operated switch assembly comprising connecting the rope to at least one switch which is actuable to switch between first and second conditions on a change in tension of the rope, the rope being initially installed such that an approximation of the correct tension is achieved in the rope, and increasing the tension in the rope by means of a tensioner, wherein the tensioner comprises a body and a rotatable member which engages the rope, the tension in the rope being increased by rotating the rotatable member relative to the body such that the rope is wound around the member, and locking the rotatable member in position relative to the body after the rope has been tensioned. 
     The tension of the rope is preferably monitored as the tension is increased by viewing a tension indicator provided on the at least one switch. The tensioner is preferably placed close to an end of the rope, adjacent to the at least one switch, for ease of viewing of the tension indicator provided on the switch. 
     Unlocking means are preferably provided in the tensioner so that the tension of the rope may be released. 
     According to a third aspect of the present invention, there is provided a rope tensioner comprising a body defining a passage through which a rope can be inserted to project from each end of the passage, a member rotatable relative to the body and formed to engage a rope extending through the passage such that rotation of the member causes the rope to be wound around the member, and means for locking the member in position relative to the body to resist unwinding of the rope from the member. 
     The means for rotating the rotatable member around which the rope is wound may be a drive member mechanically coupled by a gear to the rotatable member. The drive member may be a worm gear meshed with a gear supported by the rotatable member. The locking means may be provided by providing a gear system with a mechanical advantage such that unwinding of the rope from the rotatable member is prevented unless the drive member is rotated to drive the rotatable member in a direction to unwind the rope. 
     The rotatable member is preferably a shaft defining an aperture aligned with openings in the tensioner body. 
     The locking means preferably comprises a ratchet and pawl assembly. 
     Preferably the tensioner comprises means for releasing the locking means. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a plan view of a rope operated switch assembly according to the present invention; 
     FIG. 2 is an exploded perspective view of a tensioner for use in the rope operated witch assembly of FIG. 1; 
     FIG. 3 is a perspective view of the base of the tensioner of FIG. 2; 
     FIG. 4 is a perspective view of the base of the tensioner of FIG. 3, with a cover plate; and 
     FIG. 5 shows perspective views of the tensioner from different viewpoints. 
     FIG. 6 is a perspective exploded view of a second embodiment of the present invention; 
     FIG. 7 is a view from above of a base component of the embodiment of FIG. 6 showing a spindle engaged by a worm drive; and 
     FIGS. 8 and 9 show the embodiment of FIG. 6 after removal of a cover and illustrating successive stages in the winding of a rope onto the spindle. 
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Referring to FIG. 1 of the accompanying drawings, there is illustrated a rope operated switch assembly comprising a first switch  1 , a second switch  2 , a rope  3  extending between the switches and a tensioner  4  installed along the rope. 
     The switches  1  and  2  are of any suitable design, and may be as described in WO-A-97/20334, for example. The switches are such that two tension thresholds are established. The first, lower threshold is such that unless the tension of the rope exceeds the first threshold the switch cuts off the supply of power required to enable the machinery to be operated. The second, higher threshold is such that unless the tension on the rope is less than the second threshold, the switch again acts to cut off the supply of power to the machinery. The switches  1 ,  2  each comprise a tension indicator  5 , an emergency stop button  6 , and a reset knob  7 . Each switch also comprises a tubular body extension  8  which receives a spring-loaded shaft  50  (shown in phantom) attached to a rotatable D-ring connector  9 . The switches are mounted such that the distance between the D-rings  9  is generally less than 75 meters. 
     The rope  3  is PVC coated steel cable, although any suitable rope may be used. The rope is attached between the switches  1 ,  2  by passing the rope around suitable thimbles looped through the rings  9  and clamping the rope ends in clamps  10 , in a known manner. The rope is supported along its length by means of a plurality of eye bolts  11  placed at distances of 2 to 3 meters apart along the machinery. 
     The tensioner  4  is further illustrated in FIGS. 2 to  5 . The tensioner comprises a base  12  and top  13 . The base  12  comprises four apertures  14  to allow screws  15  to be screwed into corresponding threaded apertures (not shown) on the top  13  to hold the tensioner together. A pawl  16  is mounted on a spindle  17 , and is acted on by a spring  18 . A threaded aperture  19  allows a screw  20  to be screwed into engagement with the pawl  16 , to push it against the biasing force of spring  18 . A circular wall  21  is provided in the base to form a recess, the wall  21  having a section  22  of reduced height over which the pawl extends. 
     Two further apertures  23  are provided in base  12 , one at either end thereof, to enable the tensioner to be threaded onto the rope  3 . 
     A rotatable spindle  24  is provided in the tensioner, having an aperture  25  therethrough, and having a ratchet-toothed wheel  26  provided at one end thereof. The aperture  25  may be aligned with apertures  23  as shown so that the rope may be pushed through one aperture  23 , the aperture  25 , and the other aperture  23 . A hexagonal recess  27  is provided in one end of the spindle  24  which extends through the top  13 , suitable to allow the spindle to be turned using an Allen key. The end of the spindle having the recess  27  is further provided with an arrow-shaped indicator to indicate when the aperture  25  is aligned with the apertures  23 . The ratchet-toothed wheel  26  sits in the circular recess created by wall  21  in the base  12 . 
     A cover plate  28  is provided between the base  12  and top  13 , having an aperture  29  in the centre thereof such that the spindle  24  may pass through the aperture leaving the toothed wheel  26  between the cover plate and the base  12  so that the aperture  25  is positioned between the cover plate and the top  13 . 
     To install the rope operated switch assembly, the tensioner  4  is threaded onto the rope  3 , and the rope is installed between switches  1  and  2 . The rope  3  is pulled to an approximation of the correct tension during installation. The cover plate  28  allows the rope to be easily threaded through the tensioner  4  without becoming entangled in the toothed wheel  26  and pawl  16 . 
     The tension in the rope  3  is then increased by turning the spindle  24  using an Allen key. This causes the rope extending through the spindle  24  to be wound about the spindle from both sides, thereby increasing the tension on the rope. The tension is maintained on the rope due to the ratchet and pawl preventing the spindle from turning in the wrong direction. The tension of the rope  3  is monitored by means of the tension indicators  5  provided on switches  1 ,  2 . The tensioner  4  may be installed on the rope near to one of the switches so that it is easy for the operator to monitor the tension of the rope. 
     The tension may be released if necessary by tightening the screw  20 , which pushes on the pawl  16  to release it from the ratchet-toothed wheel  26 . The rope can then be pulled to release some of it from the tensioner, and the screw  20  can be unscrewed to re-set the ratchet and pawl. The tension in the rope may then be increased again by turning spindle  24 . 
     If the tension in the rope alters over time, for example owing to temperature variations, friction and wearing of the rope caused by mis-aligned eye-bolts, etc, the tension may be simply re-set by either turning the spindle to tighten the rope, or by releasing the pawl to unlock the tensioner. 
     To operate the machinery, the tension of the rope is adjusted so that switches  1  and  2  are in the “run” position between the two tension thresholds. If the tension on the rope is then increased (i.e. by an operator pulling the rope), or decreased (i.e. if the rope is cut), the switches  1  and  2  are tripped, and the power to the machine is cut, preventing it from operating. 
     It should be appreciated that various modifications to the exemplary embodiment may be made. For example, the spindle may be held in position by a locking mechanism other than a ratchet and pawl. 
     The tensioner may be attached to only one end of the rope and connected to a switch by, for example, a hook passed through the ring  9 . In this instance, the end of the rope that is connected to the tensioner would have to be securely attached to the spindle. 
     Although in the illustrated embodiment the tensioner is attached to the rope before the rope is connected to the or each switch, the tensioner could be attached to the rope after it has been installed, to allow for retro-fitting of existing rope operated switch assemblies. For example, the tensioner could comprise hooked attachment means that may be used to connect the tensioner to the rope, the rope becoming engaged in a separately rotatable member in the tensioner body. 
     Referring now to FIG. 6, this illustrates an alternative embodiment of the invention. Whereas in the case of the embodiment of the invention illustrated in FIGS. 1 to  5  the spindle  24  is rotated by inserting a suitable tool into a recess provided in one end of the spindle, in the embodiment of FIG. 6 the rope is tensioned by rotating a spindle using a worm drive gear arrangement. 
     Referring in detail to FIG. 6, the illustrated embodiment of the invention comprises a base  30 , a top  31 , and an intermediate plate or rope confining partition  32  which is sandwiched between the base and top, the three components being secured together by screws  33  to form a body defining an open-ended passage  51  receiving the rope. A spindle  34  supports gear teeth  35 , the spindle being received within a socket  36  moulded into the base  30  and extending through an opening  37  in the plate  32 . The spindle  34  defines a hole  52  (see FIG. 8) through which a rope  38  to be tensioned is inserted. 
     A worm drive  39  aligned with an aperture  40  in the base has a flanged end  41  which is received in a socket  42  defined by the base. The worm  39  is retained between the base  30  and plate  32  and engages the gear  35 . A tool may be inserted through the opening  40  to engage in a socket  41  defined in the end of the worm  39  to enable the rotation of the worm about its axis, such rotation causing the spindle  34  to rotate about its axis as a result of the interengagement of the worm  39  and the gear  35 . Thus the rope  38  can be caused to wind around the spindle  34 . 
     FIG. 7 shows the base  30 , spindle  34 .and worm drive  39  before the plate  32  is mounted on the base. FIGS. 8 and 9 show the embodiment of FIG. 6 after removal of the top cover  31  and insertion of a rope. FIG. 8 shows the rope wound around the spindle  34  after the spindle has been turned through slightly more than 90°, and FIG. 9 shows the rope after further rotation of the spindle. It will be appreciated that the manner in which the rope is wound around the spindle as shown in FIGS. 8 and 9 is the same as the manner in which a rope is wound around the spindle  24  in the embodiment of FIGS. 1 to  5 . 
     In contrast to the embodiment of FIGS. 1 to  5 , in the case of the embodiment of FIG. 6 tension in the rope can be closely controlled as a rotation through 360° of the worm  39  causes a relatively smaller rotation of the spindle  34 . Furthermore, the mechanical advantage provided by the gearing system can be such that it is unnecessary positively to lock the worm  39  in a position to which it has been rotated. This means that the installer can increase or decrease the rope tension by simple rotation of a tool inserted into the socket  41 , enabling very fine adjustment to the rope tension. Additional locking means (not shown) may however be provided to positively lock the worm and spindle  5  in the positions to which they have been rotated.