Abstract:
The invention provides a method and apparatus for measuring the torque applied to the drum shaft of a hoist. By measuring the torque on the drum shaft, the force or tension on the fast line can be accurately determined. If the force or tension on the dead line is also measured, the forces on the fast line and dead line can be used to determine the force applied to the load. One embodiment of the invention uses a transmission coupled to the drum shaft as a moment arm. The transmission is coupled to a fixed point by a strain-sensing element located some distance from the center of the drum shaft. The distance between the center of the drum shaft and the point along the transmission where the strain-sensing element is mounted provides the moment arm for measuring the torque on the drum shaft. Another embodiment of the invention provides “C”-shaped side plates to support and mount the main bearings of the drum shaft. The cutout provided by the “C”-shape of the side plates allows the drum shaft, drum shaft bearings, and drum shaft bearing carriers to be passed from outside the side plates to inside the side plates without the need to remove components from the ends of the drum shaft. With the drum shaft in place, a plate or link installed to span the cutout of each side plate. The plate or link coupled to the side plate on each side of the cutout region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on provisional patent application Ser. No. 60/132,143, filed May 2, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to well drilling equipment and, more specifically, to a hoist or drawworks for well drilling. 
     BACKGROUND OF THE INVENTION 
     Well drilling involves the use of many large and heavy items, for example, drill collars, pipe, well casing, etc. To use these items effectively, the items must be lifted and moved. Because of the size and weight of these items a large tower, referred to as a derrick or mast, is erected. A block and tackle arrangement is installed at the top of the tower. Wire rope or cable is reeved or strung through the sheaves or pulleys of the block and tackle arrangement. 
     The block and tackle arrangement provides a mechanical advantage, allowing a relatively small force to be used to lift relatively heavy objects. However, this mechanical advantage involves a trade-off; the wire rope or cable is pulled a much longer distance than the distance that the load supported by the block and tackle arrangement moves. Also, the block and tackle arrangement introduces additional friction into the system, thereby reducing its efficiency. 
     Because of the long distance that the wire rope or cable must travel and the great weight involved, a hoist or drawworks is used. The hoist or drawworks has a drum for reeling the wire rope or cable in or out. The drum is mounted on a drum shaft. The drum shaft is coupled to a motor or prime mover through a transmission. The motor and transmission provide the force to rotate the drum and reel in the wire rope or cable. 
     The force provided by the motor and transmission needs to be sufficient to overcome the weight of the items being lifted, as well as any friction or other inefficiencies in the system. Since the motor and transmission have finite limits on the amount of force they can provide, and the wire rope or cable also has limits on the amount of force they can withstand, it is important to obtain an indication of the actual force present at the load. 
     Since the load may include a drill string extending a great distance into the well hole, numerous factors may contribute to the amount of force present at the load. When the load is static, the weight to the drill string and the traveling block of the block and tackle arrangement contribute to the force at the load. However, if, for example, the well hole is drilled so as to deviate from vertical, some of the weight of the drill string may be borne by the lower side of the angled region of the well hole. When the load is being raised or lowered, dynamic factors affect the force on the load. For example, friction between the drill string and the drill hole may increase the force needed to raise the load. Friction in the block and tackle arrangement may also increase the force needed to raise the load by effectively preventing some of the force applied by the hoist or drawworks from reaching the actual load. 
     To prevent damage to the equipment and to accurately control the forces being applied, techniques for measuring force are used. The end of the wire rope or cable opposite the hoist or drawworks as it comes from the block and tackle arrangement is referred to as a dead line. The dead line is anchored by a dead line anchor to a fixed location. The dead line anchor is provided with a force transducer to measure the force or tension on the dead line. However, because of friction in the block and tackle arrangement and energy needed to bend the wire rope or cable as it passes through the block and tackle arrangement, the amount of force or tension measured at the dead line does not, under dynamic conditions, accurately reflect the amount of force on the wire rope or cable leading from the block and tackle arrangement to the hoist or drawworks, which is referred to as the fast line. 
     The force or tension on the fast line is usually greater than the force or tension on the dead line when the load is being raised and less than the force or tension on the dead line when the load is being lowered. These differences are often approximately plus or minus 15 percent of the actual force on the load. The differences increase exponentially with the number of lines through the block and tackle arrangement or the number of sheaves or pulleys in the block and tackle arrangement. 
     The force on the load could be determined if the force or tension on both the fast line and the dead line were known. Unfortunately, while the force or tension on the dead line can be easily measured at the dead line anchor, the force or tension on the fast line is difficult to measure because of its motion. 
     Alternative approaches have been developed to measure the force on the load. Since friction in the block and tackle arrangement can be assumed to be fairly evenly distributed, the force on the crown block or middle line of the block and tackle arrangement can be measured. Since the middle line has the same number of sheaves or pulleys between it and the fast line as it has between it and the dead line, the frictional losses are approximately equally distributed on both sides and effectively cancel out each other. Unfortunately, this technique requires that the force transducer be located in the block and tackle arrangement, which is mounted at the top of the tower. Since the tower may be, for example, 200 feet high, the force transducer is relatively inaccessible, making it difficult to install and maintain. Also, the signals from the force transducer must be delivered down the tower to operators or equipment below. Communication of the signals is difficult to achieve accurately and reliably. 
     Another alternative approach is to install a pad-type strain gauge at one of the legs of the tower. The pad-type strain gauge senses indicative of force on the tower exerted by force on the load. This technique is difficult to implement because it requires integrating a strain gauge into the base of the tower, which is an immense and massive structure. As a result, installation and maintenance of the strain gauge is difficult. 
     Thus, a technique is needed to accurately determine the force on a load without the difficulties and disadvantages of the prior art techniques. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and apparatus for measuring the torque applied to the drum shaft of a hoist. By measuring the torque on the drum shaft, the force or tension on the fast line can be accurately determined. If the force or tension on the dead line is also measured, the forces on the fast line and dead line can be used to determine the force applied to the load. 
     One embodiment of the invention uses a transmission coupled to the drum shaft as a moment arm. The transmission is coupled to a fixed point by a strain-sensing element located some distance from the center of the drum shaft. The distance between the center of the drum shaft and the point along the transmission where the strain-sensing element is mounted provides the moment arm for measuring the torque on the drum shaft. 
     While the invention may be practiced with strain-sensing elements, such as electrical strain gauges, that can operate effectively without any substantial motion, other types of strain-sensing elements, such as hydraulic load cells, can also be used. Any movement of the transmission allowed by the strain-sensing element can be accommodated by a flexible gear tooth coupling between the motor or prime mover and the transmission. One example of such a flexible gear tooth coupling uses gears having spherically curved teeth to accommodate motion between the motor and transmission. Other techniques for accommodating motion between the motor and transmission may also be used. For example, elastomeric motor mounts could be used to mount the motor on its mounting surface. 
     Another embodiment of the invention provides “C”-shaped side plates to support and mount the main bearings of the drum shaft. The cutout provided by the “C”-shape of the side plates allows the drum shaft, drum shaft bearings, and drum shaft bearing carriers to be passed from outside the side plates to inside the side plates without the need to remove components from the ends of the drum shaft. Once the drum shaft and its bearing components are located within the cutout portions of the “C”-shaped side plates, the bearing carriers are bolted to the side plates so as to locate the drum shaft at the proper location relative to the side plates. 
     With the drum shaft in place, a plate or link installed to span the cutout of each side plate. The plate or link is coupled to the side plate on each side of the cutout region. For example, a link having an elongated “H”-shape may be used to span the gap of the cutout region. The ends of the link form a clevis-type arrangement, allowing a pin to be inserted through one side of the link, through the side plate, and through the other side of the link. A pin is inserted through each end of the link to couple each end of the link to the side plate on its respective side of the cutout region. 
     Using a pin, bolt, or other fastener of round cross section to connect the link to the side plate allows the link to pivot away from the cutout in the side plate when one of the fasteners is removed. Thus, the link serves as an easily releasable link to strengthen and stabilize the side plates while allowing easy access to the drum shaft and its bearing components for installation, removal, or maintenance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating a hoisting system having a crown block with two pulleys. 
     FIG. 2 is a schematic diagram illustrating a hoisting system having a crown block with three pulleys. 
     FIG. 3 is a schematic diagram illustrating one embodiment of the present invention. 
     FIG. 4 is a diagram illustrating an elevational view of one embodiment of the invention. 
     FIG. 5 is a diagram illustrating a perspective view of one embodiment of the invention. 
     FIG. 6 is a diagram illustrating a detailed front elevational view, a front elevational view, and a side elevational view of one embodiment of the invention. 
     FIG. 7 is a diagram illustrating a perspective view of one embodiment of the invention. 
     FIG. 8 is a flow diagram illustrating a process according to one embodiment of the invention. 
     FIG. 9 is a flow diagram illustrating a process according to the invention for removing a drum shaft from an end plate. 
     FIG. 10 is a flow diagram illustrating a process according to the invention for installing a drum shaft in an end plate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic diagram illustrating a hoisting system having a crown block with two pulleys. The hoisting system comprises a hoisting drum  101 , a hook  103 , a deadline anchor  102 , a cable, a traveling block, and a crown block. The crown block comprises pulleys  107  and  111 . The traveling block comprises pulley  109 . The cable passes through the pulleys, resulting in several parts of the cable. The part of the cable between the hoisting drum  101  and the crown block is referred to as the fast line  105 . Cable section  108  passes from pulley  107  to pulley  109 . Cable section  110  passes from pulley  109  to pulley  111 . The part of the cable between pulley  111  and deadline anchor  102  is referred to as the dead line  106 . A hook load  104  is supported from hook  103 . The hook load is understood to also include the weight of the traveling block as well as the weight actually hanging from hook  103 . 
     The crown block, traveling block, and the cable passing between the crown block and the traveling block constitute a block and tackle arrangement. Although the pulleys of the traveling block are typically coaxial, as are the pulleys of the crown block, the pulleys are more easily understood if depicted separately, as shown in the schematic diagram. The load on the fast line  105  when the hoisting drum  101  is in motion is referred to as the fast line load. The load pulling on the dead line anchor  102  is referred to as the dead line load. 
     The block and tackle arrangement provides a mechanical advantage, reducing the force required of hoisting drum  101  to lift hook load  104 . For example, the force applied to the fast line  105  to lift hook load  104  is approximately equal to the weight of hook load  104  divided by the number of lines strung between the crown block and the traveling block. In the example of FIG. 1, cable sections  108  and  110  are strung between the crown block and the traveling block. Thus, the hoisting drum  101  of FIG. 1 can lift hook load  104  by applying a force approximately equal to half the weight of hook load  104 . 
     Under static conditions, the hook load  104  will be supported by cable sections  108  and  110 , each of which will carry half the weight of the hook load  104 . The weight of the hook load  104  will also be distributed among fast line  105  and dead line  106  so that half of the weight of hook load  104  will be borne by fast line  105  and half of the weight of hook load  104  will be borne by dead line  106 . These relationships can be expressed mathematically. It is understood that force is equal to mass multiplied by acceleration. Thus, 
     
       
         
           F=M×A. 
         
       
     
     Weight refers to force on the mass of an object exerted by acceleration due to gravity. If the weight of hook load  104  is represented using the variable W, the other forces in the system can be expressed in terms of W. 
     The crown load is the force exerted on the crown block. The static crown load is the force on the crown block when the system is not in motion. The static crown load can be represented as follows: 
     
       
         Static crown load ( SCL )=fast line load+hook load+dead line load = W /2 +W+W /2=2 W.   
       
     
     FIG. 2 is a schematic diagram illustrating a hoisting system having a crown block having three pulleys. The hoisting system comprises a hoisting drum  201 , a hook  203 , a deadline anchor  202 , a cable, a traveling block, and a crown block. The crown block comprises pulleys  207 ,  211 , and  215 . The traveling block comprises pulleys  209  and  213 . The cable passes through the pulleys, resulting in several parts of the cable. The part of the cable between the hoisting drum  201  and the crown block is referred to as the fast line  205 . Cable section  208  passes from pulley  207  to pulley  209 . Cable section  210  passes from pulley  209  to pulley  211 . The part of the cable between pulley  211  pulley  213  is cable section  212 . The part of the cable between pulley  213  and pulley  215  is cable section  214 . The part of the cable between pulley  215  and deadline anchor  202  is referred to as the dead line  206 . A hook load  204  is supported from hook  203 . The hook load is understood to also include the weight of the traveling block as well as the weight actually hanging from hook  203 . 
     The crown block, traveling block, and the cable passing between the crown block and the traveling block constitute a block and tackle arrangement. Although the pulleys of the traveling block are typically coaxial, as are the pulleys of the crown block, the pulleys are more easily understood if depicted separately, as shown in the schematic diagram. The load on the fast line  205  when the hoisting drum  201  is in motion is referred to as the fast line load. The load pulling on the dead line anchor  202  is referred to as the dead line load. 
     The block and tackle arrangement provides a mechanical advantage, reducing the force required of hoisting drum  201  to lift hook load  204 . For example, the force applied to the fast line  205  to lift hook load  204  is approximately equal to the weight of hook load  204  divided by the number of lines strung between the crown block and the traveling block. In the example of FIG. 2, cable sections  208 , 210 ,  212 , and  214  are strung between the crown block and the traveling block. Thus, the hoisting drum  201  of FIG. 2 can lift hook load  204  by applying a force approximately equal to one-fourth the weight of hook load  204 . 
     Under static conditions, the hook load  204  will be supported by cable sections  208 ,  210 ,  212 , and  214 , each of which will carry one-fourth the weight of the hook load  204 . The weight on cable sections  208  and  214  will also be carried over pulleys  207  and  215  to fast line  205  and dead line  206 , respectively, so that one-fourth of the weight of hook load  204  will be borne by fast line  205  and one-fourth of the weight of hook load  204  will be borne by dead line  206 . 
     These relationships can be expressed mathematically. The static crown load can be represented as follows: 
     
       
         Static crown load ( SCL )=fast line load+hook load+dead line load  =W /4 +W+W /4=3/2 ×W   
       
     
     In general, under static conditions, 
     
       
         fast line load= W/N,   
       
     
     and 
     
       
         dead line load= W/N,   
       
     
     where N is the number of lines strung between the traveling block and the crown block. Thus, for N lines, the static crown load is as follows: 
     
       
           SCL=W/N+W+W/N=W  (1+(2/ N )) =W  (( N +2) /N ). 
       
     
     Under dynamic conditions, i.e., when the line is moving, the dynamic crown loading is as follows: 
     
       
         dynamic crown load=fast line load+hook load+deadline load, 
       
     
     where the fast line load is now magnified as a result of the effects of pulley efficiency due to movement of lines. 
     In a block-and-tackle system where the cable is strung over a number of pulleys, the line pull exerted by the hoisting drum is gradually reduced toward the deadline due to losses caused by friction in the pulleys and in the bending of the cable around the pulleys. The efficiency of the hoisting system is further reduced by internal friction in the cable and by hole friction (friction in the well hole). 
     FIG. 3 is a schematic diagram illustrating one embodiment of the present invention. The system of FIG. 3 comprises hoisting drum  301 , deadline anchor  302 , hook  303 , hook load  304 , a crown block, and a traveling block. The crown block comprises pulleys  307 ,  311 ,  315 , and  319 . The pulleys of the crown block are preferably mounted coaxially about axis  320 , although the pulleys may be mounted non-coaxially as an alternative. The traveling block comprises pulleys  309 ,  313 , and  317 . The pulleys of the traveling block are preferably mounted coaxially about axis  321 , although the pulleys may be mounted non-coaxially as an alternative. A block and tackle arrangement comprises the crown block, the traveling block, and a cable. The cable runs from hoist drum  301  to the crown block. The cable then alternates between the crown block and the traveling block, according to the number of pulleys used in the system, with the crown block having one more pulley than the number of pulleys in the traveling block. The cable runs from the crown block to the dead line anchor  302 . 
     The cable can be considered as having a number of sections. The fast line  305  runs from the hoisting drum  301  to the crown block pulley  307 . Cable section  308  is between crown block pulley  307  and traveling block pulley  309 . Cable section  310  is between traveling block pulley  309  and crown block pulley  311 . Cable section  312  is between crown block pulley  311  and traveling block pulley  313 . Cable section  314  is between traveling block pulley  313  and crown block pulley  315 . Cable section  316  is between crown block pulley  315  and traveling block pulley  317 . Cable section  318  is between traveling block pulley  317  and crown block pulley  319 . Dead line  306  is between crown block pulley  319  and dead line anchor  302 . Dead line anchor comprises cable clamp  333 , which securely holds the cable. A free end  334  of the cable extends from the cable clamp  333 . The free end  334  may include new cable on a cable spool for future use in the system. 
     Hoisting drum  301  is part of a hoist that comprises, in addition to hoisting drum  301 , transmission  323 , motor  324 , load link  327 , pins  328  and  329 , and base  326 . Motor  324  provides rotational motion about axis  325 . Transmission  323  comprises gears, clutches, and brakes to transfer the rotational motion from motor  324  to hoist drum  301 , which rotates about axis  322 . The transmission  323  extends away from axis  322  and provides a moment arm. 
     Either or both of pins  328  and  329  may comprise a strain gauge pin to measure strain resulting from a load on the pin. Any suitable strain gauge pin, for example an electrical or hydraulic strain gauge pin, may be used. In the example of an electrical strain gauge pin, an electrical strain gauge is embedded in or attached to a mechanical part, such as a pin. A line  330  from the strain gauge pin is used to carry the signal from the strain gauge pin to appropriate instrumentation, for example a gauge, a display, a monitor, or a controller. 
     Torque present on hoist drum  301  is transferred through a shaft at axis  322  to transmission  323 . Motor  324  is flexibly coupled to transmission  323  to allow some motion of transmission  323  relative to motor  324 . For example, a flexible gear tooth coupling, such as a spherical curved tooth coupling, may be used to couple motor  324  to transmission  323 . Alternatively, motor  324  may be flexibly mounted to base  326 , for example with elastomeric motor mounts to allow some motion of motor  324  relative to base  326 . 
     Since transmission  323  is coupled to hoist drum  301 , torque on hoist drum  301  tends to cause rotational force on transmission  323 . Transmission  323  is mounted to base  326  via load link  327  and pins  328  and  329 . Pin  328  is attached to transmission  323  at a point some distance D from axis  322 . Torque is a force exerted over a distance, determined by multiplying the force times the distance. Mathematically, this relationship is expressed as follows: 
     
       
         
           T=F×D. 
         
       
     
     Thus, torque exerted on hoist drum  301  results in a force on load link  327  and pins  328  and  329  equal to the torque T divided by the distance D. The force on the strain gauge pin is measured and, given a known distance D, provides a measurement of the torque T on hoist drum  301 . 
     The measurement of torque T on hoist drum  301  is meaningful because it relates to the tension or force on fast line  305 . As fast line  305  is wound or unwound, it meets hoist drum  301  tangentially at a radial distance R from the axis  322  of hoist drum  301 . Since force is applied to fast line  305  as a result of the influence of motor  324  and hook load  304 , the application of the force of the fast line load over the radial distance R produces torque on hoist drum  301 . Since the moment arm of transmission  323  and the strain gauge pin used to mount transmission  323  provide a technique for measuring the torque on hoist drum  301 , the tension or force on fast line  305  can readily be measured. 
     As fast line  305  is reeled in and wound around hoist drum  301 , fast line  305  is wound spirally across the surface of hoist drum  301  from one end of the drum to the other end, at which point the direction of the spiral reverses and fast line  305  is wound spirally in the opposite direction over the first layer of fast line  305 . Since the first layer of fast line  305  is then between the fast line  305  being wound and the surface of hoist drum  301 , the radial distance R from the center of the hoist drum  301  increases slightly. If the ratio of the thickness of fast line  305  to the diameter of hoist drum  301  is small enough, the difference in radial distance R may be negligible and may be ignored. However, if the ratio of the thickness of fast line  305  to the diameter of hoist drum  301  is large enough to influence the measurement, the change in radical distance R can be measured and taken into account in the calculation. 
     For example, an optical beam or a series of optical beams may be used to determine the number of layers of cable on the hoist drum. The optical beams may be oriented across the drum at several different radial distances. As the number of layers of cable on the hoist drum increases, the beams are progressively occluded. For each layer of cable on the hoist drum, the radial distance R can be increased accordingly. Alternatively, a mechanical sensor or sensors, such as a lever connected to a switch can be used to count the number of layers of cable on the hoist drum. Several levers may be employed to contact the cable at different layers around the hoist drum. Alternatively, an ultrasonic transducer or optical sensor may be used to project an ultrasonic or optical beam radially toward the surface of hoist drum  301  to measure the distance from the transducer or sensor to the hoist drum  301 . As cable builds up on hoist drum  301 , the distance is reduced and radial distance R is adjusted accordingly. Alternatively, magnetic or proximity sensors may be used to detect the build-up of cable around the hoist drum  301 . Alternatively, a roller or other measurement device may be used to measure the movement of fast line  305  as it winds or unwinds from hoist drum  301 . By keeping track of the amount of fast line  305 , wound on hoist drum  301 , the number of layers of cable, and thus the radial distance R, can be determined. To further increase reliability, several of these techniques may be used in conjunction with one another. In one embodiment of the invention, it is preferred to have only three or four layers of cable around hoist drum  301  at any time. Alternatively, embodiments with any number of layers of cable around hoist drum  301  may also be practiced. 
     Dead line anchor  302  comprises a dead line drum  331 , an arm  332 , cable clamp  333 , linkage  335 , load cell  336 , and load cell line  337 . Dead line anchor  302  provides a measurement of the dead line load by sending a signal through load cell line  337 . The signal from dead line anchor  302  can be transmitted to appropriate instrumentation, for example the instrumentation that also receives the signal from line  330 . The signals representative of fast line load and dead line load can be processed to provide information regarding the hook load  304  and the efficiency of the block and tackle arrangement. 
     For hoisting operations, an expression for the efficiency of the block and tackle arrangement can be provided. Let 
     EF=block and tackle efficiency factor 
     K=pulley and line efficiency per pulley 
     N=number of lines strung to traveling block 
     FL=fast line tension 
     DL=dead line tension 
     Starting with a hoisting fast line pull of FL, the friction from the first block pulley reduces the line pull in the first traveling line from FL to P 1 , where P 1  is given by the following expression: 
     
       
         
           P 
           1 
           =FL×K. 
         
       
     
     Similarly, the line pull in the second traveling line will be reduced to P 2 , where P 2  is given by the following expression: 
     
       
         
           P 
           2 
           =P 
           1 
           ×K 
         
       
     
     or 
     
       
         
           P 
           2 
           =FL×K 
           2 
         
       
     
     Similarly, 
     
       
         
           P 
           N 
           =FL×K 
           N 
         
       
     
     If N is the number of lines supporting the hook load W, then              W   =                  P   1     +     P   2     +     P   3     +   …   +     P   N                   =                  FL   ×   K     +     FL   ×     K   2       +     FL   ×     K   3       +   …   +     FL   ×     K   N                     =                FL                   (     K   +     K   2     +     K   3     +   …   +     K   N       )                                    
     The terms in brackets form a geometric series, the sum of which is given by 
     
       
         ( K (1 −K   N )/(1 −K ) 
       
     
     Hence 
     
       
           W= ( FL×K (1 −K   N ))/(1 −K ) 
       
     
     or 
     
       
           FL= ( W (1 −K ))/( K (1 −K   N ) 
       
     
     In the absence of friction, 
     
       
         
           FL=P 
           1 
           =P 
           2 
           = . . . =P 
           N 
         
       
     
     and hook load W is given by 
       W=P   AV   ×N   
     or 
     
       
         
           P 
           AV 
           =W/N 
         
       
     
     where P AV =average line pull on block and tackle arrangement. Hence the efficiency factor (EF) of the hoisting system is the ratio of P AV  to FL, i.e., 
     
       
         
           EF=P 
           AV 
           /FL 
         
       
     
     
       
           EF= ( K (1 −K   N ))/( N (1 ×K )) 
       
     
     The efficiency factor and fast line load during lowering can be expressed as follows: 
     
       
         ( EF ) Lowering =( NK   N (1 −K ))/(1 −K   N ) 
       
     
     
       
         ( FL ) Lowering =( WK   −N (1 −K ))/(1 −K   N ) 
       
     
     The hook load HL is given by 
     
       
           HL=W= weight of drill string (or casing) in mud+weight of traveling block, hook, etc. 
       
     
     The hook load is supported by N lines, and, in the absence of friction, the fast line load FL is given by 
     
       
           FL= hook load/number of lines supporting the hook load =HL/N   
       
     
     Owing to friction, the fast line load required to hoist the hook load is increased by a factor equal to the efficiency factor. Thus, 
     
       
           FL=HL/ ( N×EF ) 
       
     
     Under static conditions, the dead line load is given by HL/N. During motion, the effects of pulley friction must be considered and the dead line load is given by 
     
       
           DL= ( HL×K   N )/( N×EF ). 
       
     
     Because of the non-ideal efficiency of a practical block and tackle arrangement, the fast line load and the dead line load deviate from the values they would otherwise have in an ideal system. The fast line load is often higher than it would be in an ideal system, and the dead line load is often lower than it would be in an ideal system. By processing signals from line  330  and load cell line  337 , accurate values for various parameters can be obtained. For example, the actual hook load can be determined. Changes in tension during acceleration or deceleration of the load can be measured even if the changes in tension are of short duration or a transient nature. The invention may also be used to measure the real torque on the brake, which can be used to assess the condition of the machine. For example, changes in real torque over time may be used to determine the amount of wear on the brake. This measurement can be used to signal a warning when the brakes reach a given level of wear. Other conditions, such as anomalies in the bearings, clutch, or motor can also be detected and warning or other indication given. 
     FIG. 4 is a diagram illustrating an elevational view of one embodiment of the invention. The embodiment of FIG. 4 comprises a cable  401 , which wraps around a hoist drum having an axis  402 . The hoist drum rotates about a drum shaft, which also rotates about axis  402 . The drum shaft is coupled to transmission  403 . Transmission  403  comprises gears, a clutch, and a brake. The clutch is mounted coaxially with axis  404 . The brake is mounted coaxially with axis  402 . Other brake and clutch configurations relative to transmission  403  may also be used. Transmission  403  is coupled to motor  406  using a flexible coupling technique along axis  405 . Elastomeric motor mounts  411  may also be used to provide a flexible relationship. The gears of transmission  403  transfer rotational motion from motor  406  to the hoisting drum, which provides linear motion to cable  401 . The linear motion of cable  401  allows cable  401  to be wound on or unwound from the hoisting drum. 
     Since transmission  403  is coupled to the drum shaft, but is only flexibly coupled to base  410  through motor  406 , torque on the hoist drum induces a corresponding rotational force on transmission  403 . Although the housing of transmission  403  need not be coupled to the drum shaft, friction in the gears, brake, and clutch of the transmission, as well as torque from motor  406  result in rotational force being applied to the housing of transmission  403 . To keep transmission  403  from moving excessively about axis  402 , load link  407  and pins  408  and  409  couple transmission  403  to base  410 . Either of pins  408  and  409  may be provided with a strain gauge pin to measure the force exerted on load link  407  by the torque about axis  402 . 
     FIG. 5 is a diagram illustrating a perspective view of one embodiment of the invention. The embodiment of FIG. 5 comprises fast line  501 , hoist drum  502 , transmission  503 , brake and clutch housing  504 , motor  505 , blower  506 , end plate  507 , end plate link  509 , pins  510  and  511 , front panel  512 , transmission  513 , brake and clutch housing  514 , motor  515 , and blower  516 . End plate  507  defines gap  508 . End plate link  509  spans gap  508 . Pins  510  and  511  mount end plate link  509  to end plate  507 . 
     To provide increased torque, power, and versatility, the embodiment illustrated in FIG. 5 provides two motors to rotate hoist drum  502 . Blowers  506  and  516  provide forced-air cooling of motors  505  and  515 , respectively. Other motor cooling techniques may also be used. Motors  505  and  515  provide rotational motion through transmissions  503  and  513 , respectively, to hoist drum  502 . Hoist drum  502  converts the rotational motion to linear motion of fast line  501 . 
     Brake and clutch housing  504  covers and protects the brake and clutch assemblies coupled to transmissions  503  and  513 , respectively. End plate  507  and the corresponding end plate on the opposite side of hoist drum  502  support the drum shaft upon which hoist drum  502  rotates. Front cover  512  covers and protects hoist drum  502  and the portion of fast line  501  wound around hoist drum  502 . 
     FIG. 6 is a diagram illustrating a detailed front elevational view, a front elevational view, and a side elevational view of one embodiment of the invention. The embodiment of FIG. 6 comprises end plate  601 , bearing carrier  602 , bearing  603 , drum shaft  604 , end plate link  606 , pins  607  and  608 , and cover  609 . End plate  601  defines gap  605 , which extends from the region in which bearing carrier  602  is mounted to the edge of end plate  601 . End plate link  606  spans gap  605 . Cover  609  covers and protects gap  605 . 
     Gap  605  is wide enough to allow bearing carrier  602  to pass through gap  605 . Thus, mounting of drum shaft  604  and its bearings is greatly simplified. To mount drum shaft  604  in end plate  601 , either of pins  607  and  608  is removed, allowing end plate link  606  to swing out of gap  605 . Alternatively both pins  607  and  608  may be removed, allowing end plate link  606  to be removed entirely. Cover  609  is removed. 
     Bearings  603  and bearing carrier  602  are mounted around drum shaft  604 . Shaft  604  with bearings  603  and bearing carrier  602  is moved from a position outside of end plate  601 , through; gap  605 , to the desired location within end plate  601 . Bearing carrier  602  is connected to end plate  601 , for example with mounting bolts. Cover  609  is installed and end plate link  606  is installed using pins  607  and  608 . 
     End plate link  606  bears tensile forces exerted on end plate  601  by fast line  610 . For example, weight on the hook results in a hook load that also loads the fast line  610 . The tension on the fast line  610  exerts an upward force on drum shaft  604 , which pushes upward on the upper portion of end plate  601 . The upward force on the upper portion of end plate  601  would tend to spread gap  605 . However, end plate link  606  and pins  607  and  608  resist the force, reducing the stress on end plate  601  and maintaining dimensional stability of end plate  601 . 
     One embodiment of end plate link  606  is such that end plate link has an elongated “H” shape. The ends of the “H” form a clevis structure that supports pins  607  and  608  on both sides of end plate  601 , thereby greatly reducing the shear stresses on pins  607  and  608 . Other configurations of end plate link  606  may also be used. 
     FIG. 7 is a diagram illustrating a perspective view of one embodiment of the invention. The embodiment of FIG. 7 comprises fast line  701 , hoist drum  702 , transmission  703 , motor  705 , blower  706 , end plate  707 , end plate link  709 , transmission  713 , brake and clutch housing  714 , motor  715 , blower  716 , motor shaft  717 , motor gear  718 , primary clutch gear  719 , secondary clutch gear  720 , clutch  721 , drum shaft gear  722 , brake  723 , drum shaft  724 , bearing carrier  726 , bearing  727 , flexible coupling shaft  728 , motor mounts  729 , load link  730 , blower motor  731  blower filter  732 , electrical junction box  733 , blower motor  734 , and blower filter  735 . End plate  707  defines gap  708 . 
     This embodiment provides two motors (motors  705  and  715 ) to provide rotational motion. The rotational motion is coupled through transmissions  703  and  713  to drum shaft  724 . Rotation of drum shaft  724  provides rotation of drum  702 , which reels in or reels out fast line  701 . While the motors  705  and  715  are used to reel in fast line  701 , fast line  701  may be reeled out without the use of motors  705  and  715 . The influence of gravity on the hook load may be used as the urgent force to reel out fast line  701 . Alternatively, motors  705  and  715  may assist in the reeling out process. 
     Motors  705  and  715  are cooled by blowers  706  and  716 , respectively. Blowers  706  and  716  are powered by motors  731  and  734 , respectively. The air provided to blowers  706  and  716  is filtered by air filters  732  and  735 , respectively. Electrical power is provided to motors  731  and  734 , as well as motors  705  and  715  through electrical junction box  733 . Motor  705  is mounted on motor mounts  729 . Flexible shaft coupling may flex to allow some rotation of transmission  703  about drum shaft  724 . Motor gear  718  and primary clutch gear  719  may be provided with teeth that are cut to accommodate motion of motor shaft  717  relative to the axis of primary clutch gear  719 , allowing some rotation of transmission  703  about drum shaft  724 . Depending on the type of strain gauge used with load link  730 , transmission  703  may rotate somewhat under the influence of torque on drum shaft  724 . Preferably, strain gauges that allow measurement of force on load link  730  with no or little motion of transmission  703  are used. 
     Clutch  721  employs dual coaxial shafts to provide separate shafts for primary clutch gear  719  and secondary clutch gear  720 . Clutch  721  is preferably an alternating plate disc clutch. 
     Brake  723  is preferably an alternating plate disc brake assembly operated by air or spring pressure. Brake  723  may be provided with water cooling or other cooling techniques. 
     FIG. 8 is a flow diagram illustrating a process according to one embodiment of the invention. The process begins in step  801 . In step  802 , the fast line load is measured using a force-sensitive load link coupled to a transmission, and the dead line load is measured using a dead line anchor. In step  803 , the fast line load and dead line load measurements are processed. Differences between the fast line load and dead line load may be used to calculate the hook load. Fluctuations in the fast line load and dead line load may be analyzed. For example, changes in hook load resulting from changes in downhill pressure may be observed, providing indication of well kicks and other factors affecting the condition of a well. Long term variations in fast line load and dead line load may be stored and analyzed to determine changes in the condition of the machine. These change in the condition of the machine may be used to schedule events, such as replacement of brakes, clutches, slipping the cable to replace worn cable, lubricating the pulleys and other mechanical components, etc. 
     In step  804 , output indications and/or warnings are provided. These include indications and warnings of hook load, changes in tension, condition of the machine, etc. These indications may be stored for later use and comparison or may be presented immediately. Warnings may be set to trigger at certain levels of certain parameters or when certain combinations of parameter values or ranges occur. After step  804 , the process returns to step  802 . 
     FIG. 9 is a flow diagram illustrating a process according to the invention for removing a drum shaft from an end plate. The process begins in step  901 . In step  902 , the cover is removed. Included in this step is the removal of any covers or panels that block removal of the drum shaft. In step  903 , one or more pins in the end plate link are removed. In step,  904 , the end plate link is rotated about one of its pins, or, if all pins have been removed, the end plate link is removed. In step  905 , the bearing carrier is disconnected from the end plate. This may, for example, involve unbolting the bearing carrier from the end plate. In step  906 , the drum shaft is moved out of the end plate through the gap in the end plate. In step  907 , the process ends. 
     FIG. 10 is a flow diagram illustrating a process according to the invention for installing a drum shaft in an end plate. The process begins in step  1001 . In step  1002 , the drum shaft is moved into the end plate through the gap. In step  1003 , the bearing carrier is connected to the end plate. This may involve bolting the bearing carrier to the end plate. Other techniques for attaching the bearing carrier to the end plate may also be used. In step  1004 , the end plate link is rotated or replaced. If one of the pins is already installed in the end plate link, the end plate link; is rotated about that pin into its installed position. If none of the pins have been installed in the end plate link, the end plate link is replaced into its installed position. In step  1005 , any remaining pins are installed in the end plate link. In step  1006 , the cover is installed. This step includes installing any covers or panels or moving them to their final installed positions. In step  1007 , the process ends. 
     While the above description contains many specific features of the invention, these should not be construed as limitations on the scope of the invention, but rather as one exemplary embodiment thereof. Many other variations are possible. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.