Abstract:
A weight measurement method and apparatus for measuring and monitoring the weight load on a vehicle such as a tractor trailer rig. A load pin and bearing assembly mechanically couples the weight of a trailer and its payload to the leaf springs of a tractor trailer truck. The shackle pin is intersected by a longitudinal bore in which multiple strain gage sensors are mounted. A miniature signal processing unit is totally enclosed and shielded within the longitudinal bore and is electrically connected to the strain gage sensors. The signal processing unit develops weight signals that are communicated by conventional low voltage signal cabling to a load display unit in the tractor cab. An offset lubricant passage provides a means for lubricating the load pin bearings while preventing contact of the lubricant with the strain gages, internal wiring and signal conditioner components housed within the main longitudinal bore.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to method and apparatus for monitoring the weight load on a vehicle such as a truck, trailer or the like in which at least one strain gage sensor is internally mounted within a shackle pin that couples a trailer suspension bracket to an axle-mounted leaf spring for on-the-road, real time monitoring of dynamic as well as static load conditions. 
     Methods for weighing payloads are common on many types of vehicles, such as trucks, vans and other heavy payload vehicles, where weight distribution is an important factor. Operators of trucks driven over interstate highways must monitor the weight of the truck payload for several reasons. Rig operators must remain in compliance with the legal load limits to avoid paying substantial fines levied for weight violations. Also, a driver does not want to carry an excessive load that may damage the trailer, overload the tires and suspension, cause excessive wear on the engine, cause premature wear on the brakes, and reduce fuel efficiency. Additionally, a driver needs to know immediately if the payload has shifted so that he may take emergency measures to restore balance and secure the load before the trailer becomes unstable or unsafe. 
     Monitoring the payload carried by a tractor trailer can be a difficult task. A payload is often loaded at a remote site such as a gravel pit or logging operation, or other location where truck scales are not readily accessible. 
     Various devices have previously been used to measure and monitor the weight of a payload. For example, U.S. Pat. No. 5,811,738 (Boyovich et al.) entitled “Trunnion-Mounted Weight Measurement Apparatus” discloses a weight measurement apparatus for determining the weight of a load placed on a wheeled vehicle. The apparatus includes a shackle pin containing strain gages for measuring the stress caused by the load and is connected to the truck by replacing the truck&#39;s conventional trunnion coupling with a trunnion member containing the shackle pin and internally mounted strain gages. 
     U.S. Pat. No. 3,695,096 (Kutsay), entitled “Strain Detecting Cell,” discloses a strain load cell combined with a coupling pin or bolt. The body of the coupling pin is intersected by a longitudinal bore and two pairs of strain gages connected in a bridge circuit are mechanically attached to the internal bore sidewall surface. The longitudinal bore also serves as a lubrication passage through which lubrication is supplied to the trunnion bearings. The strain gage signals are routed through an electrical cable to an external processing unit that includes a signal conditioner that amplifies the low level signals and attenuates high frequency noise. 
     The Kutsay strain detecting cell illustrates certain performance limitations of conventional load measuring systems. The signal processing unit is remotely located from the load pin. Since the strain gage bridge circuit is carefully balanced for outputting a low voltage signal, the impedance of the connecting cable should be adjusted to provide an impedance match with the input of the signal conditioner. Consequently, calibrated cabling or a wireless transmitter/receiver system is required for connecting the strain gage sensor signal to the processor unit. Also, the lubricant present in the passage will contaminate the strain gage components, attacking the adhesive that bonds the sensors to the load pin sidewall. Such interference has been determined to be the cause of improper sensor attachment, producing irregular, distorted output signals. 
     Other weight measurement devices for transport vehicles are shown in U.S. Pat. No. 3,754,610 (Paelian et al.), entitled “Load Cell”; U.S. Pat. No. 4,102,031 (Reichow et al.), entitled “Method of Installing a Transducer on a Structural Member”; U.S. Pat. No. 5,402,689 (Grogan) entitled “Non-Thread Load Sensing Probe”; and U.S. Pat. No. 5,880,409 (Hartman) entitled “Onboard Weighing System for Truck Having Single Point Suspension.” 
     The data signals generated by a basic measuring device such as a strain gage cell or bridge circuit generally require processing or conditioning before being finally presented to the operator as a load indication. In installations on large vehicles such tractor trailer rigs, the load pin and load sensor circuit are remotely located from the signal conditioning amplifier and data display unit, which are typically installed in the operator cab for on-the-road, real time monitoring of dynamic as well as static load conditions. 
     Calibrated connecting cables conduct such information to the signal conditioning amplifier, which is usually located in the cab. The cable wiring, which may extend for several feet between the measuring bridge and the conditioning circuit, is subject to inductive pick-up of electromagnetic interference noise generated from various sources that tend to distort the low-level signal output from the measuring bridge, which is typically in the millivolt range. Moreover, the long length of cable wiring introduces unwanted impedances between the bridge circuit and the conditioning amplifier that can degrade the response time and transient overload recovery time of the indicating system. 
     Conventional strain gage load measuring systems have attempted to overcome these limitations by using a shielded, calibrated cable having a predetermined length and known impedance that is matched with the impedance of the measuring bridge and the conditioning amplifier. However, the calibrated cable is exposed to thermal cycling that causes impedance variations that affect the output of the measuring bridge. Since the cable is calibrated, it is not field-repairable; consequently, a damaged cable must be replaced by a new cable of the appropriate length that has been calibrated to match the particular load sensing circuit and signal conditioner installation on the damaged rig. After cable replacement, the overall system must be audited for accuracy and reliability. Consequently, there is considerable interest in improving such load measuring systems so that rig down-time and maintenance expenses can be reduced, while providing more accurate and reliable load measurements to the rig operator. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides improved weight measurement method and apparatus for sensing the weight load on a transport vehicle such as a tractor trailer rig. A load sensing transducer circuit is mounted internally of the load pin or shackle pin and senses the weight load imposed by a trailer and its payload on the leaf springs of the transport vehicle. The load signal produced by the transducer circuit is fed directly into the input of a signal conditioning processor that is also mounted internally of the shackle pin. The conditioned load signal is then transmitted via conventional non-calibrated low voltage signal cabling to a remote data display unit that can be monitored by the truck operator. 
     The body of the shackle pin is intersected by a longitudinal bore in which multiple strain gage sensors are mounted. A miniature signal processing unit includes a signal conditioner that is totally enclosed within the longitudinal bore and shielded by the metallic body of the surrounding load pin. The strain gage sensors are mechanically bonded by adhesive deposits to the internal bore sidewall of the load pin and are electrically coupled together and to the signal conditioner by internal wiring that is totally shielded by the metallic body of the load pin. 
     The strain gages develop a signal proportional to the weight of the trailer load for input to the signal conditioner. The load forces imposed on the shackle pin are amplified and filtered by the internal signal conditioner and are sent to the data display unit to calculate the total load borne by each wheel or axle. Preferably, the signal conditioner sends this data via conventional, non-calibrated low voltage signal cabling to a controller and display unit installed in the cab of the transport vehicle for real time monitoring by the rig operator. 
     The load pin is intersected by a radially offset, longitudinal bore forming an internal lubrication passageway that is isolated with respect to the longitudinal bore in which the strain gage sensors and signal processing unit are mounted. The internal lubrication passageway provides lubrication to a set of bearings located in a suspension bracket that couples the shackle pin and a leaf spring assembly to the trailer frame. 
     The radially offset lubrication passage is isolated from the longitudinal bore and the electronic signal conditioning components within the bore. Since the strain gages are bonded onto the internal bore sidewall of the pin by adhesive deposits, those deposits are vulnerable to attack by hydrocarbon compounds present in conventional lubrication grease. The offset lubricant passage provides a means for lubricating the bearings while preventing contact of the lubricant with the strain gages, internal wiring and signal conditioner components housed within the main longitudinal bore. 
     Since the lubrication passage is isolated from the main longitudinal bore passage, the lubrication passage can be pressurized with lubricant without pressurizing the electronic components within the main bore. Otherwise, the signal conditioner, strain gages and internal wiring would be exposed to high impulse lubrication pressure surges that could damage the components or possibly cause a discontinuity in the internal strain gage wiring as a result of bearing lubrication service operations performed during normal periodic maintenance of the vehicle. 
     The signal conditioner is internally mounted in the load pin and is closely coupled to the internally mounted strain gage sensing circuit by short, internal wiring conductors. Thus there is no need for a calibrated cable or radio transmission device to send the conditioned load sensor signals to the remote data display unit. Conventional, non-calibrated low voltage data transmission conductors are used to connect the conditioned output signals to the remote data display unit. Since there is no requirement for calibration or impedance matching, a damaged signal cable can be quickly repaired or replaced in the field with conventional low voltage signal cabling without significant rig down-time. Morever, different tractors can be attached to the trailer and operated with the installed load sensors and existing display equipment without calibration. 
     Moreover, because impedance matching is not a limiting factor in the present load pin installation, signal delay and distortion are eliminated, thus overcoming a major limitation of conventional strain gage measuring systems that use calibrated cables. Because the load signals are pre-conditioned at the load pin, signal distortion, noise and impedance problems are avoided. The load monitoring system of the present invention responds immediately and accurately to transient load conditions. Therefore there is no lag time or load signal distortion experienced when measuring and indicating the weight of the load. Thus, it is possible to reliably sense gradual as well as rapid shifting overload conditions as they develop, thus providing an early warning of an impending dangerous load condition, allowing the operator to stop the transport vehicle and balance the load or take other corrective action at the onset of a load problem, before the trailer becomes unstable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawing is incorporated into and forms apart of the specification to illustrate the preferred embodiments of the present invention. Various advantages and features of the invention will be understood from the following detailed description taken in connection with the appended claims and with reference to the attached drawing figures in which: 
     FIG. 1 is a side elevation view of a tractor trailer transport rig including a cab, a tractor and a trailer on which the weight measurement apparatus of the present invention is mounted. 
     FIG. 2 is a perspective view of a load pin of the present invention having strain gage sensors and a signal processing unit mounted internally therein. 
     FIG. 3 is a perspective view of the load pin of the present invention installed in a shackle coupling bracket of a leaf spring suspension member. 
     FIG. 4 is a side elevation view, partly in section, of the load pin installation of FIG.  3 . 
     FIG. 5 is a side sectional view of the load pin installation showing the load pin with a strain gage sensor circuit and a signal conditioner, both internally mounted within a longitudinally extending main bore passage, and a radially offset lubrication passage that is isolated from the main bore passage. 
     FIG. 6 is a front elevation view of the load pin of the present invention, showing a cable connector disposed in the main bore passage of the load pin. 
     FIG. 7 is a rear elevation view of the load pin of the present invention, showing a lubrication fitting disposed in the radially offset lubrication passage of the load pin. 
     FIG. 8 is an electrical schematic diagram illustrating the electrical interconnection of the load sensor circuit, signal conditioner and data display unit of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the invention will now be described with reference to various examples of how the invention can best be made and used. Like reference numerals are used throughout the description and several views of the drawing to indicate like or corresponding parts. 
     Turning now to the drawing, and more specifically to FIG. 1, the weight sensing apparatus  10  of the present invention is shown by way of example installed on a conventional tractor trailer rig  12 . The weight measurement apparatus  10  could be installed the load bearing coupling structure of any wheeled or tracked vehicle such as a crane, dump truck, excavator, drag line, fork lift, caterpillar, aircraft or the like. It can also be used to good advantage in connection with fixed load lifting equipment used in various heavy industrial operations and manufacturing processes where load balancing is essential. 
     The tractor trailer  12  has an operator cab  14  mounted on a tractor that pulls a trailer  16  and a payload or load  18 . The trailer  16  has multiple axles  20  and wheels  22  mounted to the trailer  16  by multiple leaf spring suspension members  24 . A shackle pin  26  as shown in FIG. 2, also referred to as a load pin herein, senses the weight of the trailer  16  and its payload  18  on the axles  20 . A low voltage electrical signal cable  28  routes the load signals to a data display unit  30  in the tractor cab  14 . One or more cable connectors  32  serially connect multiple wiring conductors of the cable  28  from the load pin to the tractor cab  14 . 
     Referring now to FIG. 2, the load pin or shackle pin  26  of the present invention is shown in perspective view. The shackle pin  26  is formed from an elongated section of rod selected for its mechanical properties. Preferably, the shackle pin  26  is composed of steel. The shackle pin  26  has a generally cylindrical outer surface  34  with a lubrication end  36  and a signal connector end  38 . 
     Two bolt alignment notches  40  are formed in the outer surface  34  to position two bolts  42 , as shown in FIG.  3  and FIG. 4, for securing the shackle pin  26  to the suspension member  24 . Two intermediate hysteresis grooves  44  are formed in the outer surface  34  of the shackle pin  26 . The hysteresis grooves  44  are formed in a manner to concentrate the shear forces experienced by the shackle pin  26  at predetermined internal load sensing positions where strain gage sensors are located. 
     An annular channel or lubrication groove  46  is formed in the outer surface  34  of the shackle pin  26 . A lubricant, preferably grease, exits the shackle pin  26  through a grease port  48  and flows into and around the lubrication groove  46 . 
     A first collar  50  is disposed between the outer surface  34  and the connector end  38 . An annulus  52  is formed between the collar  50  and a female cable connector  54 . The connector  54  includes multiple female sockets  55  for receiving male pins of a mating signal connector for making external electrical connections to a signal conditioner enclosed within the shackle pin  26 . 
     A second collar  56  is positioned between the outer surface  34  and the lubrication end  36 . An end plate  58  is formed between the collar  56  and a plug or cover plate  60  and a lubrication fitting  62 , preferably a one-way check valve commonly known as a zert fitting. The cover plate  60  and the zert fitting  62  are physically separated from one another by the end plate  58 . 
     Preferably, the shackle pin  26  is constructed of schedule E4340 steel per AMS-2301 with a heat treated and hardened case. The shackle pin is preferably rated at a load capacity of 5,000 pounds and can accommodate overloads of 300% without damage and overloads of 500% without structural failure. 
     Referring now to FIG. 3, a suspension member  24  with leaf springs  64  is shown in perspective view. A suspension member  24  utilizing leaf springs  64  is a common form of suspension system. Leaf spring suspension systems can be used on any vehicle, such as a truck, van or other heavy payload vehicle, where sensitivity to mechanical vibration is an important factor. Leaf springs  64  resiliently support the trailer relative to the axle and carry loads on the axle  20  and aft to frame-mounted supporting devices. The leaf spring  64  of FIG. 3, is by way of example, designed for use on the tractor trailer rig  12 . 
     In a large vehicle that includes two or more axles  20 , such as the tractor trailer rig  12 , the central portion of the series of leaf springs  64  is secured to the trailer  16  by a trunnion shaft  66  mounted to a trunnion bracket  68  by two U-shaped bolts  70 . Additionally, a clamp secures the leaf springs together. 
     The leaf springs  64  are pivotally connected at one end to a front bracket  74  in a manner such that the leaf spring  64  is connected at another end to a rear bracket  76 . Preferably, the connection at the rear bracket  76  of the suspension member  24  has a double rotatable configuration, that is, two parallel axes of rotation. The double rotatable configuration aids in preventing buckling of the leaf spring  64 . Buckling of the leaf spring  64  results from the axle  20  moving relative to the trailer  16  as the leaf spring  64  deflects due to changes in its horizontal length. 
     Two front bracket links or front bracket shackles  78  are attached at their top ends to the front bracket  74  and are pivotally attached at their bottom ends to the leaf springs  64 . 
     Similarly, two rear bracket links or rear bracket shackles  80  are pivotally attached at their top ends to the rear bracket  76  and are pivotally attached at their bottom ends to the leaf springs  64 . In this manner, the leaf spring assembly is still pivotally attached to the axle, but also may still move in the fore and aft directions relative to the frame of the vehicle to help prevent buckling of the leaf springs  64 . 
     The leaf springs  64  are secured to both the front bracket shackles  78  and the rear bracket shackles  80  with shackle pins  82  and shackle pins  26  of the present invention. A system of caps and pinch bolts  84  secure the shackle pins  82  and  26  to the front bracket shackles  78  and rear bracket shackles  80 . In particular, the front end of a leaf spring assembly  64  is pivotally connected to the front bracket  74  using a shackle pin  26  rotatably attached to the front end of the leaf spring  64  and to the front bracket  74 . The shackle pin  26  is secured in place using a fitted cap  84  that is attached to one end of the shackle pin  26  such that lateral movement of the shackle pin  26  relative to the leaf spring  64  and the front bracket  74  is prevented. 
     The rear end of the leaf spring assembly  64  is connected in a double rotatable configuration to the rear bracket  80  using two shackle pins  82  and  26 , two shackles  80 , four pinch bolts and four caps  84 . One shackle pin  26  is rotatably attached to the rear end of the leaf spring  64  and one shackle pin  26  is rotatably attached to the rear bracket  76 . 
     Referring now to FIG.  3  and FIG. 4, as previously described, each shackle pin  26  in the cap and pinch bolt mounting apparatus  84  has one semi-circular bolt alignment notch  40  near each end of the shackle pin  26 , wherein the axis of each bolt alignment notch  40  is substantially perpendicular to the longitudinal axis of the shackle pin  26 . 
     Two front bracket shackles  78  are used to link one end of the shackle pin  26  to the corresponding end of the shackle pin  26 . Each front bracket shackle  78  has a central body portion  88  and end  90 ; each end  90  includes a pair of arms  92  forming a cradle for holding a bushing  94 . The arms  92  do not contact each other when fully engaged around the bushing  94 , thus allowing adjustment of the compression force applied to the bushing. 
     Each end of the shackle is intersected by a bolt hole  96  passing through both arms in a direction perpendicular to the cradle. The diameter of the cradle is adjusted by tightening a threaded bolt  42  that passes through the bolt holes  96  in the arms  92 . Additionally, the bolt hole  96  is oriented such that a bolt  42  passing through the cradle arms  92  partially enters the cradle space enclosed by the arms  92 . 
     Similarly, two rear bracket shackles  80  also support link one end of the shackle pin  26 . Each rear bracket shackle  80  has a central body portion  98  and two ends  100 ; each end comprises two arms  102  forming a cradle  104 . The arms  102  do not completely close when fully engaged around the bushing  94 , so that the compression force applied to the bushing can be adjusted during installation and replacement. 
     Each end  100  of the shackle  80  also has a bolt hole  106  passing through both arms  102  in a direction perpendicular to the cradle  104  such that the diameter of the cradle can be reduced, as described above. 
     When assembled, the shackle pin  26  is aligned through the cradle such that a bolt  42  threaded through the bolt hole  106  in the arms  102  of the shackle  80  is aligned with the bolt alignment notch  40  near the end of the shackle pin  26 , and the bolt  42  is tightened to form an interface between the shackle pin  26  and the shackle  80 . 
     In this manner, the shackle pin  26  is prevented from rotating relative to the shackle  80  and is prevented from moving laterally relative to the shackle  80 . Finally, a cap  84  is placed on the end of each shackle pin  26  to further secure the shackle pin  26  in place. 
     The shackle pin  26  of the present invention can be inserted into any bracket of any suspension member. For example, the shackle pin can be placed in the rear wheel suspension members  24  of the trailer, as shown in FIG. 1, the front wheel suspension members of the trailer (not shown), or on any of the suspension members of the tractor cab (not shown). 
     Referring now to FIG. 4, the shackle pin  26  of the present invention is fitted into the front shackle bracket  78  that is fixed to the leaf springs  64  and trunnion shaft  66  that is projecting externally from the trunnion bracket  68 . The front shackle bracket  78  is being described by way of example. Similarly, the shackle pin  26  could be installed in the rear shackle bracket  80 . 
     The bushing  94 , in the form of a hollow cylindrical sleeve, is compressed and held securely by the cradle arms  92  of each bracket  78 , thus forming an annulus  95  around the shackle pin  26 . Needle roller bearings  108  are retained in the annulus  95  between the shackle pin and the bushing  94  and engage the shackle pin  26  on the outer cylindrical surface  34  between the external grease groove  46  and the hysteresis grooves  44 . 
     Each bushing  94  is approximately 4.00 inches long and has outside diameter of approximately 2.00 inches and inside diameter of approximately 1.30 inches. The size of the bushing  94  varies in length and diameter according to the equipment specifications of various trucks and manufacturers. The dimensions given here are typical. 
     The inner and outer surfaces of the bushing  94  are precisely machined and highly polished. Both surfaces are machined to a 63 to 125 micron standard scale smooth finish. All surfaces for bearing contact are machined to a roundness tolerance of 0.0030 inch. 
     For aligning and retaining engagement with the shackle pin  26  and retaining bolts  42 , the shackle pin  26  has bolt alignment notches  40  whose principal surfaces are polished to the same degree as the other bearing contact surfaces. Herein, it is polished to a 63 to 125 micron standard scale finish and is machined to a roundness tolerance of 0.005 inches to provide a very smooth and round surface. 
     The needle bearings  108  are disposed on opposite ends of the bushing  94  so as to equally transmit the loading from the leaf springs  64  to the shackle pin  26 . Preferably, the bearings  108  are separated by a cylindrical spacer  110  that is made of bearing steel. However, the spacer  110  could be made of any material sufficiently rigid to maintain the bearings in their proper positions that also has a thermal expansion coefficient sufficiently close to that of bearing steel so as not to induce binding or warping of the bearings during thermal expansion and contraction. 
     As will be appreciated by one having ordinary skill in the art, any method for assuring proper longitudinal alignment of the bearings  108  that does not create thermal expansion or contraction problems is acceptable. 
     In the preferred embodiment, all relative motion within the bushing  94  occurs at the bearing  108  and shackle bracket  78  interface and bearing  108  and shackle pin interface  26 . This prevents the cable  28  from becoming twisted or damaged. 
     In keeping with the design criterion of the present invention the bearings  108  are preferably press fitted into the bushing  94 . In the preferred embodiment, the bearing assemblies  108  are maintained in their positions abutting the spacer  110  or other separating device by a permanent adhesive which bonds the bearing races to the bearing housing inner surface. Other types of retaining devices, such as lock-rings, may be used instead. 
     The precision grinding and smooth surfaces on the shackle pin  26  are chosen such that the spacing between the bearings  108  and the outer pin bearing surface  34  does not exceed 0.002 inch so that there is not a large gap or space to cause a vertical pounding of the bearing elements along rough roads. 
     The dimensions of the cylindrical bushing  94  are selected to yield an inside diameter substantially larger than the shackle pin diameter to provide an annular reservoir space  112  between the annular grease groove  46  and the bushing  94  to hold grease or lubricant. 
     Referring now to FIG. 5, the shackle pin  26  is intersected along its length by a longitudinal main bore  114 . Installed within the longitudinal bore is a strain gage bridge circuit  116  and a signal conditioner  118 . The bridge circuit  116  includes four strain gages  116 A,  116 B,  116 C and  116 D, or other sensor transducer devices connected in a conventional Wheatstone bridge arrangement. 
     Referring to FIG. 5, FIG.  6  and FIG. 7, the main bore  114  extends longitudinally from the connector end  38  of the shackle pin  26  to the lubrication end  36  of the shackle pin  26 . The cable connector  54  is positioned at the junction of the longitudinal bore  114  and the connector end  38  and the cover plate  60  seals the main bore  114  at the junction of the longitudinal bore  114  and the lubrication end  36 . 
     Referring again to FIG. 5, the strain gage bridge circuit  116  is arranged in longitudinally spaced pairs of stain gages  116 A,  116 C and  116 B,  116 D preferably placed in close alignment with the hysteresis grooves  44 . The hysteresis grooves  44  are positioned and configured to concentrate the shear forces experienced by the load pin  26 . Preferably, the strain gages of each bridge pair  116 A,  116 C and  116 B,  116 D are stacked overlapping one another and are bonded to the bore sidewall surface  114  in proximate alignment with the hysteresis grooves  44 , respectively. The leaf springs  64  deflect in response to the payload  18  on the trailer  16 . The stress or strain on the shackle pin  26  changes with the bending of the leaf springs  64 . The strain gage bridge circuit  116  detect and react to deformations in the hysteresis grooves  44  when the leaf springs  64  are subjected to the weight of the load  18 . 
     The strain gage pairs  116 A,  116 C and  116 B,  116 D are electrically coupled together and to the signal conditioner by signal wiring  120  that is color coded brown(+excitation), white(+signal),blue(−signal),black(−excitation)and grey(N/C). The strain gage bridge circuit  116  produces an output voltage signal that is directly proportional to the weight of the trailer load, typically providing an output level of about 0.6 millivolts per volt of excitation, yielding output signals in the range of 3.0-9.0 volts DC or A.C. in response to excitation in the range of 5-15 volts D.C. or A.C. 
     The shear forces experienced by the shackle pin  26  at each hysteresis groove  44  are sensed by the strain gages  116 A,  116 C and  116 B,  116 D generate load signals to the input of the signal conditioner  118 , which in turn produces an output signal proportional to the total load borne by each wheel  22 . The signal conditioner  118  outputs this signal to a data display unit  30  in the tractor cab  14  of the tractor trailer rig  12 . The signal conditioner  118  could be an analog scaling circuit with an internal DC amplifier, noise filter and wave shaping features energized by an internal lithium battery power supply, or it could be a digital signal processor including an analog-to-digital converter with comparable features. 
     To install the strain gage bridge circuit  116  and signal conditioner  118  into the shackle pin  26 , the main bore passage  114  is formed by drilling and machining the shackle pin  26  from the connector end  38  to the lubrication end  36 . The strain gages of the bridge circuit  116  along with the signal wiring  120  are then positioned within the bore  114 . After the strain gages have been bonded to the bore sidewall in alignment with the hysteresis grooves, the signal conditioner is inserted into the open annulus  52  of the connector end  38  for attachment to the cable connector  54 . Preferably, the signal conditioner  118  is preassembled and electrically attached to the cable connector, and the combination is inserted and installed as a unit afer the bridge circuit wiring has been completed. 
     Once the strain gage bridge circuit  116 , signal conditioner  118  and wiring  120  are installed, the bore  114  may be filled with potting material to protect the strain gages  116  and wiring  120  from environmental hazards such as chemicals, dirt and moisture. Typically, a protective gel, wax or polyurethane is applied directly to the strain gages to seal them from moisture, and polysulfide is injected into the main bore passage  114  through a fill port  122  to completely fill in the passage. Other suitable potting materials are well known to those skilled in the art and may be used as well. Finally, the shackle pin  26  is capped and sealed at the connector end  38  with the cable connector  54  that couples the signal conditioner to the cable  28 . The fill port  122  is then capped at the zert lubrication end  36  with the plug  60 . 
     Referring now to FIG.  5  and FIG. 7, a lubrication passage  122  extends substantially parallel to and radially offset from the longitudinal main bore  114 . The lubrication passage  122  is isolated with respect to the longitudinal main bore  114 . The lubrication passage  122  extends longitudinally between the lubrication end  36  and the grease port  48 . A lubricant, preferably grease, is inserted under high pressure into the lubrication passage through the zert fitting  62 , and is discharged into the grease reservoir annulus  46  through the grease port  48 . 
     The lubrication passage  112  is formed by a centrally oriented 0.25 inch diameter longitudinal duct running parallel to the longitudinal bore  114  such that it conducts lubricant through the grease port  48  into the annular grease reservoir  46 . In the preferred embodiment grease is the lubricant of choice, but dry or liquid lubricants may be substituted, depending on the load application conditions. 
     The radially offset lubricant passage  122  is formed by drilling a radially offset hole into the shackle pin  26  from the zert end  36  of the shackle pin  26  to the external grease groove  46  of the shackle pin  26 . A zert fitting  62  is threaded into the hole on the zert end  36 . A bore  48  perpendicular to the lubricant passage is drilled through the shackle pin from the external grease groove  46  to the lubricant passage  122 . Lubricant is injected into the lubricant passage through a nipple N on the zert fitting  62 . 
     For precision operation over extended service intervals, the bearings  108  must remain lubricated. The lubrication passage  122  and reservoir  112  provide lubricant storage of for the bearings  108 . Lubricant is discharged through the grease port  48 , fills the annular reservoir space  112  within the shackle bracket  78  and lubricates the bearings  108 . 
     The radially offset lubricant passage  122  is isolated from the longitudinal bore  114  and the electronic components within the bore by the load pin body  124 . Thus, the lubricant passage  122  provides a source of pressurized lubricant for the bearings  108  without damaging or interfering with the electronic components housed within the longitudinal bore  114 . 
     Referring to FIG.  6  and FIG. 8 where an electrical schematic diagram illustrating the electrical interconnection of the components of the preferred embodiment of the invention is shown. The signal conditioner  118  can be either analog or digital. In the preferred embodiment, the signal conditioner  118  is digital. 
     The strain gage transducers  116 A,  116 B,  116  and  116 D are preferably 350 ohm sensing transducers. The output of the strain gage bridge  116  is coupled to the signal conditioner  118  which preferably has a five pin connection and digital output of up to 16 bit wide measurement resolution. Preferably, a two-wire duplex cable type RS-485 is used. A typical digital signal conditioner  118  will have the following five electrical connections: brown (positive excitation), white (positive signal), blue (negative signal), black (negative excitation) and grey (no connection). 
     The connector  54  is a nickel plated brass, five pin female electrical connector for engaging a five pin male connector that interfaces with the five conductor connector cable  28 . 
     Although the invention has been described with reference to certain exemplary arrangements, it is to be understood that the forms of the invention shown and described are to be treated as preferred embodiments. Various changes, substitutions and modifications can be realized without departing from the spirit and scope of the invention as defined by the appended claims.