Abstract:
A load cell assembly is provided utilizing a hydraulic load cell in combination with an electric load cell to eliminate undesired side loading and to provide two separate output signals indicative of a tensive load.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to force measuring devices and more particularly to a load cell assembly for measuring tensive loads. 
     2. Description of the Prior Art 
     It is at times desirable to determine the weight of a suspended load, such as the weight of a load carried by a crane or the weight carried by the bit of a drilling rig, and there have been attempts to measure such weights with a load cell inserted in series with the cable or other element by which the load is supported. Since the output of a load cell is affected by the angle at which the load is applied to the cell, a problem exists if there is any non-axial component of force on the cell. 
     It is in general an object of the invention to provide a new and improved load cell assembly for tensive loads. 
     Another object of the invention is to provide a load cell assembly of the above character utilizing a hydraulic load cell in combination with an electronic load cell. 
     Another object of the invention is to provide a load cell assembly of the above character in which the hydraulic load cell prevents side loading of the electronic load cell. 
     Another object of the invention is to provide a load cell assembly of the above character which provides two separate output signals corresponding to the load. 
     Additional objects and features of the invention will be apparent from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top planor view of one embodiment of a load cell assembly according to the invention. 
     FIG. 2 is a side elevational view of one embodiment of FIG. 1. 
     FIG. 3 is a cross sectional view taken along line 3--3 of FIG. 1. 
    
    
     SUMMARY OF THE INVENTION 
     The present invention provides a load cell assembly utilizing a hydraulic load cell in combination with an electric load cell to eliminate undesired side loading and to provide two separate output signals indicative of the load. The assembly includes two load receiving plates which tend to be drawn apart by a tensive load and two load cell plates connected to the load receiving plates in such manner that they tend to be drawn together by the tensive load. The two load cells are constrained between the load cell plates and deliver output signals corresponding to the compressive forces applied thereto. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The load cell assembly includes an upper load receiving member 10 and a lower load receiving member 11 to which a tensive load is applied in an axial direction, as indicated by arrows 12. The load receiving members comprise generally circular, radially extending plates 10a and 11a having axially disposed hooks or eyelets 10b and 11b on the outer sides thereof for connection to the load. These members are fabricated of a rigid material, such as steel, and in the embodiment illustrated, they are formed as unitary structures. Other solid materials also may be utilized. 
     The load receiving members are spaced axially apart, and radially extending load cell support plates 16, 17 are mounted between them. These plates are generally circular, and of the same diameter as load receiving plates 10a and 11a and they also are fabricated of a rigid material. Plate 16 is positioned toward the upper load receiving plate and rigidly connected to the lower load receiving plate by a plurality of peripherally spaced screws 18. These screws pass through complimentary clearance holes 11c in the lower plate and are threadedly received in the load cell plate. Spacer sleeves 19 are mounted on the screws and serve to hold the plates in a spaced, parallel relationship. 
     The load cell supporting plate 17 is positioned toward the lower load receiving plate 11a and is rigidly connected to the upper load receiving plate also by a plurality of peripherally spaced screws 21. These screws pass through complimentary clearance holes 10c in the plate 10a and are threadedly received in plate 17. Spacer sleeves 22 are mounted on screws 21 and serve to maintain the plates 10a and 17 in a spaced, parallel relationship. 
     Bolts 18 and spacers 19 pass through openings 17a in plate 17, and screws 21 and spacers 22 pass through openings 16a in plate 16. These openings are provided with bushings 23 and 24 which serve as guides and permit the two interconnected sets of plates to move axially of each other. Here, it can be noted that an axially applied tensive force tends to draw the load receiving plates 10a and 11a apart and tends to draw load cell supporting plates 16 and 17 together. 
     An electronic load cell 31 and a hydraulic load cell 32 are mounted between the plates 16 and 17. The load cell 31 is a conventional column-type strain gauge load cell having an electrical resistance dependent upon the compressive force applied thereto. This load cell is mounted on the plate 17 in a centrally disposed recess 17b formed therein. Electrical connections to load cell 31 are made by means of leads 31a. 
     The hydraulic load cell 32 includes a fluid chamber 16b formed in the load cell supporting plate 16. A radially extending flexible diaphragm 36 is mounted on the lower side of plate 16 and forms one wall of chamber 16b. The diaphragm is fabricated of a flexible material such as Viton, and it is secured to the plate 16 by an annular retaining ring 37 and a plurality of peripherally paced screws 38. A body of hydraulic fluid 39 is enclosed within the chamber, and a flow line 41 communicates with the chamber and can be connected to a suitable pressure responsive indicator. A piston 42 engages the outer surface of the diaphragm and the upper portion of the electronic load cell 31. This piston is formed with a centrally disposed recess 42a in which the upper portion of the electronic load cell is received. 
     Operation and use of the load cell assembly can be described briefly. It is assumed that a tensive load is applied to load receiving members 10 and 11, as indicated by the arrows 12 and that leads 31a and line 41 are connected to electrically and hydraulically actuated weight indicators, respectively. The tensive load tends to draw the plates 10a and 11a apart, thereby tending to draw the plates 16 and 17 together. This produces compressive forces on both load cells, as indicated by the arrows 46 and 47, and the load cells produce output signals corresponding to these forces. The flexible diaphragm of the hydraulic load cell permits the piston 42 to float and maintain the force on the electronic load cell in an axial direction at all times, with no side loading. 
     The invention has a number of important features and advantages. Undesired side loading of the electronic load cell is prevented by the hydraulic load cell, and the two load cells produce separate output signals which can be compared to check the operation of the system. If desired, one of the load cells can be used as a backup for the other in the event of a malfunction of either cell. 
     Although the invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that this is by way of illustration only and that the invention is not necessarily limited thereto, since alternative embodiments and operating techniques will become apparent to those skilled in the art in view of the disclosure. Accordingly, modifications are contemplated which can be made without departing from the spirit of the described invention.