Patent Publication Number: US-6209852-B1

Title: Removable chain hoist position encoder assembly

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a chain hoist position encoder assembly which may be removably attached externally on the casing of a chain hoist to track the movement of the load carried by the chain hoist relative to a fixed location. 
     2. Description of the Prior Art 
     Chain hoists are utilized in many different applications to raise and lower loads suspending from overhead supports. A chain hoist is comprised of a heavy-duty motor housed within a rugged casing and having at least one chain access opening in the casing. A chain may be suspended from an overhead support or from the chain hoist itself to carry a load. In either case the chain is routed around a chain drive gear located within the chain hoist casing. The chain drive gear within the casing is driven by the chair hoist motor. The slack portion of the chain, after passing around the drive gear within the casing, is routed back out through the chain opening and hangs from the chain hoist casing as a slack end having a length that varies with the position of the chain hoist casing relative to the overhead support or with the position of the load relative to the chain hoist casing. 
     The chain hoist motor, through the internal chain drive gear within the casing, pulls either the load or the motor casing vertically upward, or allows the load or chain motor casing to travel vertically downward. The travel of the chain hoist casing or the load vertically up and down is controlled by switches located remotely from the chain hoist casing and coupled to the chain hoist motor by means of an electrical control cable. One or more hooks that are attached to the chain motor casing suspend a load beneath the chain hoist casing. This load is raised and lowered, under the control of the chain hoist operator switches, by the upward and downward travel of the load or the chain hoist along the portion of the chain which is under tension and from which the chain hoist is suspended or by pulling chain in and playing chain out from the casing. One such conventional chain hoist is described in U.S. Pat. No. 2,991,976, while another is described in U.S. Pat. No. 3,960,362. 
     Chain hoists are utilized extensively and in widely differing applications. They are used in shops, factories, warehouses, shipyards, and numerous other types of commercial and industrial establishments. In many applications of commercially available chain hoists the position of the chain hoist motor and casing relative to the length of the suspended chain upon which it travels or the position of the chain which travels relative to it may be controlled merely by observing either the chain hoist itself, or the load suspended from it. Adjustments to the vertical position of the chain or chain hoist may be performed merely by providing manual inputs to the chain hoist control switches. Indeed, a simply manually operated control is sufficiently accurate for many, many chain hoist applications that do not require precise position control. 
     On the other hand, there are some applications in which precision control of the chain hoist is required. In the theatrical industry stage sets and props are often moved vertically utilizing general purpose chain hoists, but this movement must be controlled with great precision. For example, different portions of a stage prop may be moved vertically relative to the stage and relative to each other in a closely controlled and intricate sequence and at precise speeds in order to produce special theatrical effects. Precision control of general purpose chain hoists is often necessary in other applications as well. For example, precision control of a general purpose chain hoist may likewise be required at trade shows and expositions in order to create special effects or in order to move interdependent loads in a complex manner. Where precision control of a chain hoist is necessary, visual observation and corresponding adjustment utilizing manual controls is very inadequate and unacceptable. 
     To provide the necessary precision control for specific applications of general purpose chain hoists, various position-encoding systems have been devised. However, all of these prior position-encoding systems have involved modifications to the chain hoist within the structure of the chain hoist casing. For example, some conventional position-encoding systems for chain hoists have involved the installation of an optical or magnetic encoder within the casing of the chain hoist to sense the rotation of the chain hoist motor or the gear that engages the chain and which is driven by the chain hoist motor within the casing. The internally installed encoder provides corresponding electrical position output signals. 
     While such conventional position-encoding systems do provide the required positional information, they have significant disadvantages. For one thing, they can be installed within a chain hoist casing only by a person who has extensive knowledge of the internal operations of the components of a chain hoist. The services of such individuals are expensive and often are not readily available. 
     A further significant disadvantage of such conventional chain hoist encoder systems is that once the chain hoist casing is opened, the manufacturer&#39;s warranty for the chain hoist is voided. This is only reasonable since if a person without sufficient expertise attempts to install a position encoder within the casing of a chain hoist, connections can easily be made or broken that will cause permanent damage to the chain hoist and cause it to malfunction. Moreover, tampering with the internally protected components of a chain hoist by persons lacking sufficient expertise can result in alterations to the chain hoist that can cause vary hazardous malfunctions. This can lead to significant property damage and personal injury when the chain hoist is thereafter operated. 
     SUMMARY OF THE INVENTION 
     The present invention involves the provision of a position encoder assembly for a general purpose chain hoist that can be attached to and detached from the chain hoist casing, and which requires no internal connections within the chain hoist casing. Moreover, the position encoder assembly of the invention is readily removable and may be attached to and removed from the exterior of the chain hoist casing in merely a matter of seconds. Nevertheless, it provides highly accurate encoded position information that meets or exceeds the accuracy of position encoders that are internally wired within the casing of a chain hoist. 
     The present invention has significant advantages over conventional systems in that no particular knowledge of the internal operation of the chain hoist is required in order to properly mount the position encoder assembly of the invention on the casing of the chain hoist. Installation and removal may be performed by virtually any unskilled laborer. 
     A further very significant advantage of the present invention is that the position encoder assembly of the invention may be removably installed upon the casing of a chain hoist totally externally of the operating mechanism of the chain hoist. As a consequence, installation does not require the chain hoist casing to be opened, nor does it require any internal connections to the operating components of the chain hoist. As a result, the installation of the position encoder assembly of the invention on a chain hoist does not void or in any way affect the warranty provided by the chain hoist manufacturer. 
     A further advantage of the position encoder assembly of the invention is that all exposed components are highly rugged and durable and not readily susceptible to damage. Nevertheless, the position encoder assembly of the invention produces encoded position and direction signals which are highly precise and which may be provided as inputs to a computer-controlled system, thereby enabling a high degree of precision control of the operation of the chain hoist. 
     A further advantage of the removable position encoder assembly of the invention is that it is readily adaptable for installation on a wide variety of commercially available chain hoist equipment produced by different manufacturers. Chain hoists that are produced by different manufactures, and even different models of chain hoists produced by the same manufacturer, often have significant differences in their operating components and external configuration. However, the position encoder assembly of the present invention is installed and operated in such a way that it may be utilized with most commercially available, general purpose chain hoists. The only significant variable that must be taken into account is the link size and configuration of the chain utilized by the chain hoist upon which the position encoder assembly is installed. However, there are only a very limited number of different chain sizes and configurations with which conventional, general purpose chain hoists are utilized commercially. 
     In one broad aspect the present invention may be considered to be a removable position encoder assembly for attachment to a chain hoist that has a casing into which and with respect to which a chain travels. The removable position encoder assembly of the invention comprises: a position encoder assembly housing; a releasable connector that firmly couples the position encoder assembly housing externally on the chain hoist casing and holds the position encoder assembly housing in a fixed position relative to the chain hoist casing; a position encoder assembly chain gear rotatably mounted relative to the position encoder assembly housing so that the position encoder assembly chain gear engages the chain externally from the chain hoist casing as the chain travels relative to the chain hoist casing; a releasable clamp anchored to the position encoder assembly housing and engageable to maintain engagement of the position encoder assembly chain gear with the chain so that linear movement of the chain produces a proportional rotational movement of the position encoder assembly chain gear; and an encoder mounted on the position encoder assembly housing and which detects rotational movement of the position encoder assembly chain gear and provides output signals indicative of the extent and direction of rotational movement of the position encoder assembly chain gear relative to the chain hoist casing. 
     Preferably the releasable connector by means of which the position encoder assembly housing is attached to the chain hoist casing is formed of a ratchet strap. The belt of the ratchet strap passes through the position encoder assembly housing and about the chain hoist casing. A ratchet mechanism on the ratchet strap that includes a ratchet wheel and a pawl is cinched to from a loop that tightly grips the chain hoist casing therewithin. As a result, the position encoder assembly housing is releasably but tightly strapped against the outside surface of the chain hoist casing. 
     In another aspect the invention may be considered to be an position encoder assembly removably and externally attachable to a chain hoist having a casing with a chain that enters the casing. The position encoder assembly of the invention comprises: a position encoder assembly body; a releasable connector that is engageable to secure the position encoder assembly body to the chain hoist casing in a fixed disposition relative thereto and which is disengageable to permit removal of the position encoder assembly body from the chain hoist; a position encoder assembly chain gear mounted for rotation relative to the position encoder assembly body and engageable with the chain externally of the chain hoist casing; a releasable clamp mounted on the position encoder assembly body and which is operable to hold the chain in engagement with the position encoder assembly chain gear and alternatively to release the chain from the position encoder assembly chain gear; and an encoder mounted on the position encoder assembly body and which is responsive to rotation of the position encoder assembly chain gear to emit signals indicative of the direction and extent of rotation of the position encoder assembly chain gear relative to the position encoder assembly body. The linear movement of the chain produces a proportional rotational movement of the position encoder assembly chain gear. 
     In still another aspect the invention may be considered to be an improvement in a chain hoist having a casing with a chain opening and a chain extending into the opening. The improvement of the invention comprises: a detachable position encoder assembly including a position encoder assembly frame removably attached to the exterior of the chain hoist casing and which includes a position encoder assembly gear mounted on the position encoder assembly frame for rotation relative thereto and engageable with the chain; a releasable clamp anchored to the position encoder assembly frame and operable to alternatively hold the chain in engagement with the position encoder chain gear and to release the chain from the position encoder chain gear; and an encoder mounted on the position encoder assembly frame and which detects rotation of the position encoder chain gear relative to the position encoder assembly frame and provides output signals indicative of the direction and extent of rotation of the position encoder chain gear relative to the position encoder assembly frame. 
     The invention may be described with greater clarity and particularity by reference to the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view illustrating a preferred embodiment of a position encoder assembly according to the invention releasably mounted on a conventional chain hoist. 
     FIG. 2 is a front elevational view illustrating the improved chain hoist of FIG. 1 with a load suspended therebeneath. 
     FIG. 3 is an enlarged front elevational view of the position encoder assembly shown in FIG.  2 . 
     FIG. 4 is a bottom plan view of the position encoder assembly of FIG. 2 shown with the cover of the housing thereof removed and with the manner of disengagement of the releasable clamp thereof illustrated. 
     FIG. 4A is an enlarged view of the upper portion of FIG. 4 showing the releasable clamp thereof moving to a disengaged position. 
     FIG. 5 is a front elevational view of the position encoder assembly of FIG. 2 shown with the releasable clamp thereof completely disengaged. 
     FIG. 6 is a sectional elevational view taken along the lines  6 — 6  of FIG.  4 . 
     FIG. 7 is a right-side elevational view of the position encoder assembly as shown in FIG.  3 . 
     FIG. 8 is a perspective view from the back side of the position encoder assembly of FIG. 3 shown with the housing cover removed. 
     FIG. 9 illustrates an alternative manner of mounting the chain hoist and position encoder assembly of FIG. 1 relative to a load and an overhead support. 
    
    
     DESCRIPTION OF THE EMBODIMENT 
     FIGS. 1 and 2 illustrate a dual sensor position encoder assembly  10  according to the invention removably mounted externally atop the casing  12  of a conventional, general purpose chain hoist  14 . The casing  12  of the chain hoist  14  is a rugged, durable, encompassing steel shell that encloses a chain hoist motor  16  and an internal chain hoist drive gear  18  therewithin. The chain hoist motor  16  and the internal chain hoist drive gear  18  are conventional and are illustrated in phantom in FIG.  2 . The chain host motor  16  is rigidly mounted within the casing  12  and the internal chain hoist drive gear  18  is fixed to the drive shaft of the motor  16 . 
     As best illustrated in FIG. 1, the chain hoist casing  12  has a rectangular chain access opening  20  defined therein. In the arrangement illustrated in FIGS. 1 and 2, the chain hoist  14  is suspended from an overhead support (not shown) in a conventional manner by a chain  22  having a multiplicity of chain links  24 . The links  24  are each formed as obloid steel rings linked together. Each of the links  24  resides in a plane oriented at right angles relative to the plane of orientation of the links  24  immediately adjacent thereto. 
     In the arrangement of FIGS. 1 and 2 the chain  22  extends downwardly from the overhead support and is engaged in an arcuate, semicircular loop around the internal chain hoist drive gear  18 . The vertically extending portion  26  of the chain  22  that is suspended from the overhead support is under tension due to the weight of the chain hoist  14  and the weight of the load  28  that is suspended therefrom by means of a hook  30  depending from the underside of the chain hoist casing  12 , as shown in FIG.  2 . The slack portion  32  of the chain  22  emanates from the chain access opening  20  in the chain hoist casing  12  and hangs in a loop which may be secured to the side of the chain hoist casing  12  by means of an eye-pad  34 , as illustrated in FIG.  2 . The structure of the chain motor  14 , the chain  22  from which it is suspended, the hook  30 , and the load  28  are entirely conventional in structure and arrangement. 
     The position encoder assembly  10  of the invention, on the other hand, is a unique article of manufacture. The position encoder assembly  10  is formed with a position encoder assembly frame that is configured as a position encoder assembly body or housing  40 . The position encoder assembly housing  40  has a flat, front base plate  42  and a cover  44  shaped as a concave shell that encloses and protects two different optical encoders, sensors, or transducers  46  and  48 , illustrated in FIGS. 7 and 8. The housing cover  44  is attached to the front base plate  42  by machine screws that are secured through screw openings  43  in the base plate  42 . The position encoder assembly  10  also includes a releasable clamp indicated generally at  64  in the drawings. 
     The position encoder assembly housing  40  is removably attached to the exterior of the chain hoist casing  12  by means of a releasable connector in the form of a ratchet strap  50 . The ratchet strap  50  removably attaches the position encoder assembly housing or frame  40  to the exterior of the chain hoist casing  12 . The ratchet strap  50  includes a heavy-duty belt  52  which passes through a pair of slots  54 , closed at both ends and located near the lower edges on opposite sides of the housing shell  44 . The belt  52  of the ratchet strap  50  passes through the structure of the housing  40  and encircles the chain hoist casing  12 . The ratchet strap  50  has a conventional ratchet cinching mechanism  56  that engages the tail  58  of the belt  52  so that the ratchet strap  50  can be cinched firmly to form a loop that tightly grips the chain hoist casing  12  therewithin, as illustrated in FIGS. 1 and 2. The ratchet mechanism  56  may be released in a conventional manner, if desired, thereby releasing the tail end  58  of the belt  52 . The ratchet strap  50  thereby releasably holds the position encoder assembly housing  40  tightly against the chain hoist casing  12 , as best illustrated in FIGS. 1 and 2. The ratchet strap  50  allows the position encoder assembly  10  to be totally separated from or attached to the chain hoist  14  in a matter of seconds. 
     The position encoder assembly  10  includes a position encoder assembly chain gear  60 , best illustrated in FIGS. 4 and 4A, mounted on a gear shaft  62 . The outboard end of the gear shaft  62  is visible in FIGS. 1,  2 , and  3 . The position encoder assembly chain gear  60  is formed of steel and is rigidly secured to the cylindrical steel gear shaft  62  by means of a key, lock nut, set screw, or any other conventional means. 
     In addition to the flat base plate  42 , the housing  40  also includes a flat axle mounting plate  66  spaced from and held parallel to the base plate  42  by means of three hollow spacing sleeves  68 ,  70 , and  72 . The spacing sleeves  68 ,  70 , and  72  are interposed between aligned bolt openings in the axle mounting plate  66  and in the base plate  42 . Machine bolts  74  extend through the axle mounting plate  66 , through the spacing sleeves  68 ,  70 , and  72 , and through the base plate  42 . The machine bolts  74  have heads  73  that bear against the outside surface of the mounting plate  66  and clamping nuts  75  that bear against the back side of the base plate  42 , as best illustrated in FIGS. 4 and 4A. The axle mounting plate  66  is thereby rigidly held at a distance spaced from and parallel to the base plate  42 . The base plate  42  and the axle mounting plate  66  serve as bearing plates. 
     As illustrated in FIGS. 1,  6 , and  7 , an inner bearing assembly  76  is bolted to the outwardly facing surface of the base plate  42 . An outer bearing assembly  78  is bolted to the outwardly facing surface of the outer axle mounting plate  66  as shown in FIGS. 1-5 and  7 . The bearing assemblies  76  and  78  each include a ball bearing ring  80  that permits the position encoder assembly chain gear shaft  62  to rotate smoothly therewithin and relative to the fixed outer portions of the bearing assemblies  76  and  78 . The axle  62  is formed of a solid steel rod that carries the position encoder assembly chain gear  60  in rotation about a position encoder assembly chain gear axis of rotation, indicated at  82 , that is perpendicular to both the base plate  42  and the axle mounting plate  66 . One end of the position encoder assembly chain gear axle  62  terminates within the bearing ring  80  of the outer bearing ring assembly  78 , while the other end of the axle  62  extends in the opposite direction through the bearing ring assembly  76  and into a gearbox  84 , visible in FIGS. 7 and 8. 
     The releasable clamp  64  is formed of a semicylindrical aluminum clamp block  87 , a hinge assembly  90 , a latch assembly  92 , a catch  94 , and a pair of Teflon® guide guides  96  that are bolted to the flat, inwardly facing surface of the semicylindrical clamp block  87 . Together the clamp block  87  and Teflon® guides  96  form a guide block  88  that is hinged on one end by the hinge assembly  90  to the base plate  42 . The latch assembly  92  at the other end of the guide block  88  is adapted to engage the catch  94 , which is anchored by screws to the outer periphery of the bearing assembly  78 , which in turn is anchored to the axle mounting plate  66 . 
     The guide block  88  resides in a position adjacent to the position encoder assembly chain gear  60  and the latch mechanism  92  is engaged with the catch  94  when the clamp  64  is engaged to maintain engagement of the position encoder assembly chain gear  60  with the chain  22 . As illustrated in FIGS. 4,  4 A, and  5 , the latch assembly  92  can be disengaged from the catch  94 . The guide block  88  can then be moved to a position in which its end bearing the latch assembly  92  that lies opposite the end fastened to the hinge  90  is moved to a position remote from the position encoder assembly chain gear  60 . In this disengaged position, depicted in FIGS. 4A and 5, the releasable clamp  64  releases the chain  22  from engagement with the position encoder assembly chain gear  60 . 
     The hinge assembly  90  includes a pair of hinge arms  98  and  100  which are respectively anchored to hinge mounting rods  102  and  104 . The hinge mounting rods  102  and  104  are formed as the legs of a U-shaped structure. The legs forming the hinge mounting rods  102  and  104  are pass through openings in the walls  105  of a hinge cup  106  that are below the level of base plate  42 , as viewed in FIGS. 4 and 4A. The hinge cup  106  has mounting flanges  107  secured by bolts  108  to the base plate  42 , as illustrated in FIG.  5 . 
     At their opposite ends, the hinge arms  98  and  100  are rotatably coupled to hinge pins  110  and  112 , respectively, which are secured to the lower extremities of the walls of a channel-shaped mounting bracket  114 . The channel-shaped mounting bracket  114  is set into a longitudinal channel  116  defined in the outer, convex, generally cylindrical wall of the guide block  88 , as best illustrated in FIG.  7 . Machine bolts  118  anchor the channel-shaped mounting bracket  114  in the channel-shaped slot  116  in the guide block  88 . 
     A wire coil spring (not visible) is disposed about the hinge pin  110 . The ends of this coil spring act against the hinge arms  98  and  100 , tending to force them apart from each other. This spring action serves to urge the guide block  88  toward the fully engaged position of FIG. 4 from the position of FIG. 4A, once it has been rotated inwardly toward the position encoder assembly chain gear  60  beyond the position depicted in FIG.  4 A. This spring thereby aids in asserting pressure so that the chain links  24  fully engage the position encoder assembly chain gear  60  as the chain  22  travels relative to the chain hoist  14 . 
     The latch assembly  92  includes a latch mounting bracket  120  having a pair of mutually parallel, mounting ears  122  projecting outwardly away from the top end of the clamp block  87 , as best depicted in FIG.  5 . The latch assembly  92  also includes a pair of generally flat latch members  124  and  126  which are coupled together in telescopic engagement. At one of its ends the latch member  124  has a mounting sleeve  128  that is wrapped about the transversely extending spine portion of a very stiff latch mounting spring  130 . The transverse, linear spine portion of the latch mounting spring  130  extends transversely beyond the mounting sleeve  128  through openings near the back of the latch mounting bracket ears  122 , as viewed in FIGS. 4 and 4A. The latch mounting spring  130  has coiled loops  132  at the ends of its transversely extending linear spine portion about which the mounting sleeve  128  is wrapped, and latch mounting spring ends  134  which are inserted into openings  135  in the latch mounting bracket ears  122  nearest the catch  94 . The openings in the mounting ears  122  of the latch mounting bracket  120  through which the wire of the latch mounting spring  130  passes are not circular. Rather, the openings that receive the spine of the spring  130  and also the openings  135  that receive the spring ends  134  are elongated within the structure of the mounting ears  122 . This permits the portions of the latch mounting spring  130  passing therethrough to move slightly toward and away from the axis of rotation  82  of the position encoder assembly gear shaft  62 . This feature provides a slight degree of flexure of the latch mounting sleeve  128  toward and away from the axis of rotation  63 . 
     The latch mounting spring  130  and the hinge spring wound about the hinge pin  110  are quite strong and stiff so that, when the latch assembly  92  is engaged with the catch  94 , as illustrated in FIGS. 2 and 3, the guide block  88  is held firmly in position relative to the base plate  42  and relative to the axle mounting plate  66 . Nevertheless, the latch mounting spring  130  does provide a very slight yielding connection of the guide block  88  relative to the axle mounting plate  66  to prevent any bent or misshaped link  24  of the chain  22  from becoming jammed as it passes between the position encoder assembly chain gear  60  and the guide block  88 . This avoids a condition in which the chain  22  could jam and severely damage the position encoder assembly  10 . 
     The latch member  124  is shaped as a flat plate having a circular opening therein and longitudinal edges that curve around to form channels that receive the outer edges of the slightly narrower latch plate  126 . The latch member  126  is thereby reciprocally removable in a telescopic manner within the confines of the channels formed at the outer edges of the latch member  124 . 
     The latch member  126  has at its distal extremity a transversely extending curved hook  136  that is configured to engage a corresponding, oppositely disposed hook  138  on the latch  94 . In its flat portion the latch member  126  has an oblong cam slot  140  through which the narrow neck of a clamping pin  142  passes. The cam slot  140  is not parallel to but is inclined relative to the alignment of the mounting sleeve  128 . The head of the clamping pin  142  is slightly larger than its neck and slides against the face of the flat portion of the latch member  126 . At its other end the neck of the clamping pin  142  extends into a disc-shaped latch tightening turret  144 . As best illustrated in FIGS. 3 and 5, the pin  142  is eccentrically mounted relative to the center of the latch tightening turret  144 . The latch tightening turret  144  may be rotated about its center by twisting the ears of a butterfly handle  146 . 
     The chain guide  88  of the releasable clamp  64  is mounted to the base plate  42  of the position encoder assembly housing or frame  40  by the hinge assembly  90 . When engaged and tightened, as shown in FIG. 3, the latch mechanism  92  draws the chain guide  88  toward the position encoder assembly chain gear  60  to thereby press the chain  22  toward the position encoder assembly chain gear  60  to enhance engagement of the chain with the position encoder assembly chain gear  60 . 
     The cam interaction between the eccentrically mounted clamping pin  142  and the cam slot  140  as controlled by the latch tightening turret  144  and the butterfly handle  146  serves as a tightening mechanism for drawing the latch assembly  92  toward the catch  94  when the latch assembly  92  is engaged with the catch  94 . 
     When the hooks  136  and  138  of the latch assembly  92  and the catch  94 , respectively, are aligned with each other as illustrated in phantom in FIG. 4, clockwise twisting of the butterfly handle  146 , as viewed in FIG. 3, causes the latch tightening turret  144  to rotate, thereby carrying the eccentrically mounted cam pin  142  in the oblong cam slot  140  from a position proximate the latch  94  to a position remote therefrom, as illustrated in FIG.  3 . This eccentric cam action has the effect of pulling the latching member  124  toward the position encoder assembly chain gear axis of rotation  82 , thereby increasing the grip between the hooks  136  and  138  and enhancing the force with which the guide block  88  bears radially inwardly toward the axis of rotation  82 . The tightening mechanism thereby enhances the force with which the guide block  88  presses the chain  22  toward the axis of rotation  82  of the position encoder assembly chain gear  60  once the hook  136  of the latch assembly  92  is engaged with the catch  94 . 
     When the latch tightening mechanism has been engaged in this manner, the guide block  88  forces the links  124  of the chain  22  tightly against the corresponding pockets defined in the surface of the position encoder assembly chain gear  60 , thereby ensuring complete engagement of the chain  22  against the position encoder assembly chain gear  60 . This tight engagement between the chain  22  and the position encoder assembly chain gear  60  ensures that each incremental longitudinal movement of the chain  22  relative to the chain hoist  14  produces a corresponding, proportional rotational movement of the position encoder assembly chain gear  60  and the gear shaft  62  to which it is attached. Nevertheless, due to the actions of the hinge spring disposed about the hinge pin  110  and the latch mounting spring  130 , the links  24  of the chain  22  cannot become jammed in between the guide block  88  and the position encoder assembly chain gear  60 . 
     The components of the position encoder assembly  10  that are protected by the cover  44  of the housing  40  are illustrated in FIGS. 7 and 8. As shown in those drawing figures, the gear shaft  62  extends into the speed reducing gearbox  84 . There are two outputs from the gearbox  84 . The first output appears as an output drive  150  that is axially aligned with the position encoder assembly gear shaft  62  along the axis of rotation  82 . There is no speed alteration between the input of the gear shaft  62  and the output shaft  150 . The output shaft  150  is coupled in a conventional manner to the encoder or sensor  46 , which is a precision optical incremental encoder. The precision optical incremental encoder  46  completes one revolution for no more than about every fourth link  24  of the chain  22  that passes by and rotates the position encoder assembly chain gear  60 . 
     The output of the precision optical incremental encoder  46  from the position encoder assembly  10  appears on wires in a cable  154 , illustrated in FIG.  1 . Since the encoder disc of the precision optical incremental encoder  46  performs a complete 360° rotation for each four links  24  of the chain  22  that pass between the position encoder assembly gear  60  and the guide block  88 , the output on cable  154  is highly precise. This output is utilized as an input to a computer to control the operation of the chain hoist  12 . The electrical connections from the computer to the chain hoist  12  are conventional and are not illustrated in the drawing figures. 
     The speed reducing gearbox  84  also produces an output indicated at  152  in FIG. 7 that is substantially reduced in speed from the input speed of the gear shaft  62 . More specifically, the output  152  is at a speed reduction of 100:1. The sensor or encoder  48  is an absolute electronic encoder with a battery backup. The absolute encoder  48  rotates at a speed which is but a small fraction of the rate of rotation of the precision encoder  46 . 
     The outputs of the absolute encoder  48  are carried on electrical wires in the cable  154 , illustrated in FIG.  1 . The absolute encoder  48  is geared down by the gears in the gear speed reduction box  84  so that the encoder  48  will rotate an angular distance of less than 360° for fifty feet of travel of the chain  22 . Since most applications use chains  22  that are sixty feet in length, the position output of the absolute encoder  48  represents an absolute position to a computer connected thereto. In some applications where one hundred foot chains are utilized, the speed reduction of the gearbox  84  should be even greater, such as at a ratio of 200:1. 
     FIG. 9 illustrates an alternative arrangement in which the position encoder assembly  10  may be employed with the chain hoist  14 . In this arrangement the chain hoist  14  is inverted from the position depicted in FIGS. 1 and 2, and is held suspended by the hook  30  from an overhead support. The chain  22  is connected by means of a hook  158  to the load  28 . The chain  22  passes upwardly from the hook  158  and is engaged with the position encoder assembly chain gear  60 . The chain  22  is held engaged with the gear  60  by the guide block  88  by means of the latching assembly  92  as previously described. The portion of the chain under tension passes upwardly through the position encoder assembly  10  and into the access opening  22  of the chain hoist housing  12  and around the gear  18  located therewithin. The chain hoist motor  16  pulls the portion  26  of the chain in tension upwardly, or plays it out, as controlled in a conventional manner. The slack portion  32  of the chain  22  hangs from the chain hoist casing  12  in a slack loop as illustrated. The spacing sleeves  68 ,  70 , and  72  of the position encoder assembly  10  guide the slack portion of the chain  22  away from the chain gear  60  of the position encoder assembly  10 , so that it cannot become fouled in the precision encoder assembly chain gear  60 . 
     Undoubtedly, numerous variations and modifications of the invention will become readily apparent to those familiar with chain hoists and position encoder assemblies. For example, magnetic encoders could be substituted for the optical encoders  46  and  48 . Also, the system need not necessarily employ dual sensors  46  and  48 , but could employ either a precision encoder  46 , an absolute encoder such as the absolute encoder  48  that is operated at a speed significantly reduced from the speed of rotation of the precision encoder assembly chain gear  60 , or some output at an intermediate speed of rotation. 
     Also, while the releasable clamp  64  illustrated represents a preferred embodiment of a device for ensuring engagement between the position encoder assembly chain gear  60  and the chain  22 , other types of releasable clamp mechanisms may also be utilized. In addition, different types of releasable connectors  50  may be substituted for the ratchet clamp illustrated in order to firmly, but removably, attach the position encoder assembly  10  to the casing  12  of the chain hoist  14 . For example, the housing  40  could be provided with magnets that tightly hold the housing  40  of the position encoder assembly  10  in position against the casing  12  of the chain hoist  14 . Accordingly, the scope of the invention should not be construed as limited to the specific embodiments depicted and described.