TENSILE LOAD TRANSMISSION WITH LOAD SENSOR

Abstract: A tensile load transmission that includes a load sensor for measuring the tensile load on the transmission. The transmission includes two elements (1, 5) which are mutually shape-bound with respect to tensile load. One of these elements includes a through-passing passageway and the other has a shaft which extends through said passageway and which also includes a head (7), said elements being mounted for relative rotation about the load direction. The head (7) and the first element (5) have mutually coacting parts between which pressure forces are transferred when the transmission is subjected to tensile load in the direction of said shaft. The load sensor has a central opening through which the shaft (2) extends.

Description: TENSILE LOAD TRANSMISSION WITH LOAD SENSOR The present invention relates to a tensile load transmission with load sensor, of the kind defined in the preamble of the independent Claim. It is desirable to be able to measure the load carried by lifting equipment for instance, for safety reasons among other things. The lifting equipment may, for instance, consist of a crane whose boom or jib extends in a horizontal plane from the crane chassis, wherewith the load carried by the crane introduces a turning moment that can topple the crane or other lifting equipment. It is also desirable to be able to prevent the strength limit of the equipment components (crane hook, wire/chain, etc.) from being exceeded. Earlier known load measuring means have poor precision, entail high costs, are of high complexity or are not sufficiently reliable. For example, it is highly desirable to be able to measure the load to be lifted by a crane, for instance a vehicle crane, in a simple and precise manner. When the crane includes hydraulic cylinders, the load can, of course, be measured indirectly by measuring the pressure of the hydraulic fluid. If the load is lifted by a wire cable, it is possible to detect the force required to deflect the cable laterally through a given distance between two axially spaced support points as a measurement of the tensile force in the cable. In practice, however, it is preferred to be able to measure the load as close as possible to the load attachment point. For instance, in the case of cranes it is desired to measure the load as close as possible to the hook. Crane hooks, however, are manufactured to fulfil far-reaching requirements with respect to load durability, and are manufactured in large series. However, it is not permitted to interfere with the structure of an accepted crane hook, for instance in order to incorporate a strain gauge therein, since this would set aside the bases on which the standard approval certificate of the hook was issued. Furthermore, it would be necessary to produce a series of crane hooks for each range of load concerned. The object of the present invention is to provide a load sensor that can be readily fitted to and removed from a standard load transmission such as a crane hook, so as to enable the transmission load to be measured accurately in operation, without changing the structure or the safety of the transmission. Another object of the invention is to provide a series of load sensors which have different response regions with one and the same size and which can be selectively built-into a transmission that shall be used with loads in corresponding regions. The inventive tensile load transmission can also be included in other constructions, such as in the wire rope of a forklift truck. These objects are achieved with a tensile load transmission that has the features set forth in the following independent Claim. Further embodiments of the invention are set forth in the accompanying dependent Claims. In one practical embodiment of the invention, the load sensor is based on an annular steel plate that is preferably rotational symmetrical and has a thickness which is much smaller than its outer diameter. The annular plate shall be disposed so as to extend in the plane normal to the load transmission direction, wherewith the opposite forces are transferred to the inner and outer edges of the annular plate, so as to cause it to twist. This twisting movement, which can be considered as bending of the annular plate in its axial plane, is measured with the aid of one or more strain gauges. The strain gauge may, for instance, consist of a gauge wire stuck to the annular plate so as to obtain a varying resistance that is essentially proportional to the elastic deformation of the annular plate and which is preferably proportional to the load applied. Conventional swivel-type hooks include a yoke which has a hole for receiving the hook pin. One end of the pin is threaded and carries a turret nut/?/ which transfers the load to the yoke around said hole through the medium of one or more washers. Lubricant is suitably applied between the washers and the yoke, primarily to prevent a rotating load from twisting the wire rope supporting the wire cable or wire rope carrying the yoke. An annular load sensor can be readily fitted to such a hook, simply by placing the sensor around the pin between the yoke and the turret nut. The strain gauge included in the sensor can then be connected to measuring equipment. This equipment may, in turn, be connected to a radio station which is carried by the hook and which functions to send load-related information to a radio unit managed by an operator located at a distance from said radio station. Alternatively, the sensed load may be shown on a display provided on the hook or transmitted by wire to a receiver. Another preferred use of the inventive load sensor is one of measuring the tensile load in a wire cable used to lift the forks of a forklift truck. Because the annular sensor is coupled coaxially in the load direction, the minimum of effort is required with respect to the design of the transmission into which the load sensor shall be incorporated. One requirement, however, is that the components between which the inventive load sensor is incorporated shall be designed to be shaped-bound in the load direction. Thus, if the load sensor should break down structurally or if it should be loaded far beyond its load limit, the construction will nevertheless not break down simply because the load sensor malfunctions. The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings . Fig. 1 is a schematic axially sectioned view of a crane hook provided with an inventive load sensor. Fig. 2 illustrates a preferred twisting pattern of a load sensor according to Fig. 1. Fig. 3 illustrates another embodiment of an annular load sensor. Fig. 4 is a side view of another load sensor construction. Fig. 5 is a view taken on the line V-V in Fig. 4. Fig. 6 is a cross-sectional view of another load sensor. Fig. 7 is a sectional view taken on the line VII-VII in Fig. 6. Fig. 8 illustrates another embodiment of an inventive load sensor. Fig. 9 illustrates a variant of the arrangement according to Figs. 4 and 5. Fig. 1 is a schematic illustration of a swivel-type crane hook. The crane hook includes a load carrying hook 1 that has a pin 2 which includes a threaded end-part 3. The pin 2 extends through an opening 4 in a yoke 5, the upper part of which (not shown) is coupled to a lifting cable, a lifting chain or some like device. A slide plate 6 is normally placed on the inside of the yoke 5 such as to rest against said yoke. The threaded end 3 of the pin 2 carries a so-called turret nut 7 which bears against a support plate 8. According to the invention, a load sensor 10 is fitted on the pin 2 for sensing the load transferred by the hook 1 to the yoke 5. The load sensor 10 has the form of a flat annular steel plate 11 which has a small thickness in the axial direction 42 of the hook and which extends in the plane normal to said direction, wherein the annular sensor 11 has a radial extension between its peripheral edges that is several times greater than its thickness. The inner peripheral part of the annular sensor 11 bears against the slide plate 6 through the medium of a support ring 12. The outer edge part of the annular load sensor 11 supports against the support plate 8 through the medium of a support ring 13. In turn, the slide plate 6 bears on a sleeve 9 which surrounds the pin 2 and which is positioned in the yoke opening 4. The sleeve 9 bears against a shoulder 21 on the pin 2. The height of the sleeve 9 is greater than the length of the opening 4. The yoke 5 is shape-bound axially between the shoulder 21 and the slide plate 6. The annular plate 11 is pre-stressed by tightening the nut 7, so that a positive reference signal will be obtained from the strain gauge 14 already at low loads L. The arrangement is such that the pre-tensioning of the plate 11 will not result in any change in the force that is transmitted between the hook 1 and the yoke 5. The sensor plate 11 will be twisted when the hook is subjected to tensile load, and the elastic deformation of said plate can be detected with the aid of a strain gauge. In the Fig. 1 embodiment, a strain gauge 14 is mounted on the cylindrical, outer circumferential surface of the annular plate 11 at a distance from the bending neutral plane of said plate. The strain gauge is thus able to sense the load on the lifting hook. The strain gauge 14 is connected by means of a conductor 15 to a monitoring device 16 which, in turn, is connected to a hook-mounted display or to a radio transmitter 17, for instance. Information concerning the magnitude of the load L can then be sent from the transmitter 17 to a receiver 18 for the benefit of the person operating the lifting equipment, said receiver 18 having, for instance, a display 19 which shows the current magnitude of the load. The receiver 18 may be integrated in communications equipment manipulated by the operator for remote control of the lifting equipment in which the lifting hook is included, for instance via radio. The measuring device 16 may also include a radio receiver which enables the operator to activate and deactivate said measuring equipment 16. It will be seen that the round construction, i.e. the lifting hook, has a so-called shape-bound connection between its main components in the load direction, so that the safety of the construction can be maintained even if the load sensor 10 should break down structurally. It will also be seen from Fig. 1 that the load sensor plate 11 can be given a small thickness and a large radial extension, so that even relatively small loads L will result in pronounced elastic bending/twisting of the sensor plate 11. This enables pronounced signals to be obtained from a strain gauge 14 connected to the sensor plate 11. In addition, the construction of the load sensor 10 enables a sensor of one and the same external size to be adapted for very different load ranges. For instance, load sensors may be fitted with sensor plates of much greater thickness when desiring to measure larger loads L. It will also be seen from Fig. 1 that the load sensor 10 can be readily fitted/removed/replaced. The strain gauge 14 may, of course, be placed in positions other than the illustrated position. For instance, the strain gauge 14 may be placed on the inner hole-defining edge of the sensor plate 11, or radially directed on one of the main surfaces of said plate 11. The design of the inner and outer rings 12, 13 that support the sensor plate 11 function to adjust the radial distance between the forces applied to the sensor plate 11 from the hook 1 and the yoke 5 respectively. As will be seen in Fig. 1, the supports rings 12, 13 are received in recesses in the main surface of the sensor plate 11. The purpose of this arrangement is to minimise the deviation in the radial distance between the load application points when the sensor plate 11 is bent/twisted in the load range in question. It can be assumed that the support rings 12, 13 are stationary and that there is a certain radial clearance between said rings and the sensor plate 11. The change in the radial distance between the load application points can be minimised by placing said application points at a determined axial distance apart, which can be chosen by the person skilled in this art such as to obtain this minimisation. Fig. 2 illustrates a simple explanation model in which it is assumed that a plane P extending parallel with the sensor plate coincides with the radially inner load application point when the sensor plate is not subjected to load. A line K : extends between the load application points in the non-loaded state of the sensor plate and defines an angle to the plane P. When the sensor plate is subjected to maximum load, the line will extend K' t such as to define the same angle α but on the other side of the plane P. If it is assumed that the radially inner load application point is stationary, it will be seen that the radially outer load application point will first move radially outwards until half the maximum load has been reached, and then move back radially inwards. This minimises the radial change in distance between the load application points to the illustrated deviation Δ. Fig. 3 illustrates a load sensor plate 11 which includes two radially separated recesses for a support ring 13. A sensor plate 11 according to Fig. 3 can therefore be used for two mutually different load ranges, since it allows selection of the radial distance between the load application points. It will be obvious to the person skilled in this art that a strain gauge can be placed in many different positions on a ring plate that is imparted bending deformation in any axial plane whatsoever or in all axial planes, as described above. However, essentially the same effect can be obtained with another sensor-plate support arrangement. In the Fig. 5 embodiment, for instance, the annular sensor plate 11 is supported by two mutually intersecting elongated elements which bear against two diameters that are spaced 90° apart. Deformation of the sensor plate 11 will thus be manifested in bending of the plate in the circumferential direction, and consequently the strain gauges 14 are fitted in said circumferential direction. Figs. 4 and 5 thus show two elongated elements 17, 18 for the transfer of load to the sensor plate 11. The elongated elements 17, 18 mutually intersect at the axis of the sensor plate 11. Fig. 9 illustrates a sensor plate 11 which can be considered as being formed from an annular, rotational-symmetrical flat metal blank that includes a central, cylindrical hole. The sensor plate has four deformation regions 110 of reduced material thickness that are spaced at equal angles apart. These deformation regions have a constant width along their radial length extension and a constant thickness over their respective areas. The regions 110 lying between said deformation regions form load distribution plates. The deformation regions 110 experience bending when the adjacent load distribution plates 111 move axially in response to being subjected to load by the elongated elements 17, 18 in the arrangement shown in Figs. 4 and 5. This bending will be approximately the same along the deformation regions 110 and can be measured with the aid of strain gauges 14 attached to the main surfaces of said deformation regions so as to measure said deformation generally in the circumferential direction of the sensor plate or more specifically in the tangential direction of said plate. The precision of the load measuring process can be facilitated by pre-tensioning the sensor plate 11 with a nominal minimal load that will cause the strain gauge to produce a readable output signal. Figs. 6 and 7 illustrate an alternative embodiment in which the load sensing plate 11 is replaced with a single bar 11' which receives load on one main surface in a central region along its length, and which receives the oppositely acting load on the other main surface at respective ends thereof, so that bending of the bar 11' will constitute a measurement of the transmitted load. The embodiment also includes end clamps 22 which hold together the clamping bars 17, 18 by means of which the loads are transmitted to the load sensor bar 11'. The end clamps 22 form a pre-tensioning mechanism of the kind discussed in the aforegoing. Finally, it will be seen from Fig. 8 that the elastically deformable elements of the load sensor may alternate in design between a single bendable bar and a single rotational- symmetrical cylindrical annular plate. The Fig. 8 embodiment includes a hub which is provided with a central opening and from which mutually equidistant arms extend radially outwards, wherewith strain gauges can be orientated radially on the arms, on the inner defining edge of said hub opening, or on the outer edges of respective arms. Bending deformation/twisting deformation of the sensor plate can be measured representatively with strain gauges or corresponding devices in many different positions. The strain gauges 14 may consist of wire strain gauges although it will be understood that their functional equivalence may alternatively be used. According to one important feature of the invention, the annular load sensor is incorporated coaxially in the force flow direction.

Claims: CLAIMS 1. A tensile load transmission that includes a load sensor for measuring the tensile load of said transmission, wherein the transmission includes two elements (1, 5) which are mutually shape-connected with respect to tensile load and of which one has a through-passing passageway and the other has a shaft that extends through said passageway and includes a head (7), wherein said elements are mounted for relative rotation about the direction of said load, and wherein said head (7) and said first element (5) have mutually coacting parts between which pressure force is transferred when the transmission is subjected to load in the direction of said shaft, characterised in that the load sensor includes a central opening, and in that the shaft (2) extends through the opening in said load sensor (10), wherein said load sensor includes a springy, elastic body (11, 11') which is adapted to be deformed elastically by said load (11), wherein said body (11, 11') is provided with at least one strain gauge (14) which delivers a signal that is representative of the magnitude of said load (L), wherein said body (11, 11') is generally disc-shaped having a main plane which extends generally in a plane normal to the intended load direction of said body, wherein the thickness of the disc in its intended load direction is substantially smaller than the extension of said disc in its main plane, and wherein disc parts that are mutually separated in said main plane are arranged to experience substantial relative displacement in the intended load direction of said disc when the disc is subjected to load in this direction. 2. A tensile load transmission according to Claim 1, characterised in that the load (11) is intended to be applied in a central region of one main surface of the disc, and in that the load is intended to be applied to the peripheral region of the other main surface of said disc. 3. A tensile load transmission according to Claim 1 or 2, characterised in that the strain gauge (14) is adapted to measure elongation of the body (11, 11') in an axial plane. 4. A tensile load transmission according to any one of Claims 1-3, characterised in that the body (11, 11') has the form of a plate which is adapted to be twisted towards and away from a conical shape when subjected to load. 5. A tensile load transmission according to Claim 1, characterised in that the load is intended to be applied to one main surface of the disc essentially in a plane that extends in the axial plane of the disc; and in that the load is intended to be applied to the other main surface of said disc in an axial plane that intersects the first mentioned axial plane at generally right angles thereto; and in that the sensor includes a strain gauge (14) which is adapted to measure local bending of the disc in its circumferential or tangential direction. 6. A tensile load transmission according to any one of Claims 1-5, characterised in that the transmission is a swivel lifting hook. 7. A tensile load transmission according to any one of Claims 1-4 and Claim 6, characterised in that the sensor includes two rigid annular discs through which said shaft extends and between which said body (11, 11') is positioned; and in that one of said rigid discs bears axially on said shaft and the other of said discs bears against said head. 8. A tensile load transmission according to Claim 7, characterised in that said head has the form of a nut carried by a threaded part of said shaft. 9. A tensile load transmission according to Claim 7 or 8, characterised in that the transmission includes a nut and a thread which coacts with said nut for pre-tensioning of the body (11, 11' ).

Inventors: HAMMARQVIST PETER, HAMMARQVIST, PETER
Applicants: HAMMARQVIST PETER, HAMMARQVIST, PETER
Application year: 1999
Main classification: G01G19
IPC codes: B66C1, G01G19, G01L1