GB2128760A - Load bearing apparatus having load measuring means - Google Patents

Abstract

Strain gauges 18 are attached to the piston rod 12 of fluid motor 10 which supports load-bearing boom 6 for sensing and measuring the state of compression of the rod 12, and hence the load supported by boom 6. Strain gauges 18 are mounted to the external portion of the rod 12 extending between the boom member 6 and the motor cylinder 14, and function to continuously measure the degree of compression in the rod. Further electrical means is disclosed for interpreting the output from the strain gauges as the weight of the load which the boom is lifting. By measuring the mechanical strain in the system instead of the fluid pressure in the cylinder variations in readings caused by the effects of external factors on the fluid are avoided. The piston rod provides a relatively unexposed location for the strain gauges. <IMAGE>

Description

SPECIFICATION Load bearing apparatus having load measuring means Et method therefor This invention relates to load sensing devices and method therefor for determining the magnitude of a load which is suspended from a remote end of a beam member.

There is known, load measuring devices which utilize beam deflection to measure the load which is suspended from the remote end of a boom. The deflection of a measuring beam is proportionate to the load which is suspended from the end of the primary boom, and in measuring this deflection a determination of the magnitude of a load can be made.

Typically, the assembly which is known consists of a boom which is vertically pivotal by operation of hydraulic cylinder which supports the boom throughout its range of pivotal motion. The boom comprises a portion of a hydraulic crane or the like, and provides block and tackle at a remote end for lifting loads.

The deflecting beam which is used for measuring the load, is mounted at the base of the boom and is secured so as to deflect proportionately to the magnitude of the load at the end of the primary boom. Such a structure is taught in US Patent 2319299.

There is further known, a means for detecting the magnitude of a load which is contained within a storage bin or container, consisting of a strain gauge which is mounted against the support legs of the container. The strain gauge is composed of specialty metals, and can detect strain forces introduced into the support legs of the container, to which it is attached, and electrically process information obtained thereby into a weight measurement. Such a strain gauge is manufactured by Kisler Morse Corporation and is commercially available.

Other known assemblies addressing the problem of measurement of a load suspended from a boom, utilize a dynamometer at the remote boom suspending end, which measures the load suspended therefrom. Such an apparatus is disclosed in US Patent 349001 5. A further known assembly utilizes a pulley arrangement around which the load bearing cable is routed, to measure the load suspended by the cable. US Patent 2030529 discloses such a device. Still further, another known assembly utilizes a load cell which measures directly the load on a hoist cable, and which is located at the load suspending end of the boom. US Patent 3756423 teaches such a structure. Still a further known assembly teaches placement of a pressure transducer within the hydraulic cylinder which supports the load bearing boom.The pressure senses fluid pressure within the lift cylinder, and translates such measurements into electrical signals which are then translated as weight magnitudes.

The present invention concerns a problem of providing load magnitudes sensing apparatus for a load bearing boom, which is reliable and immune from exposure to work place environments. Since as mentioned above, presently available known assemblies are mounted in external, exposed positions relative to the boom, they are susceptible to damage and harmful effects of deleterious work environments. Further, the present invention concerns the problem of providing a load measuring apparatus which is reliable and not affected by fluctuations in machine characteristics during operation of the equipment. Since, fluid viscosities change with operating temperatures, the load measuring devices which rely upon measuring fluid pressures are inherently susceptible to variations in readings due to viscosity fluctuations and other variables.

According to one aspect of the present invention, therefore, is a load measuring assembly as defined in this specification, comprising a strain gauge device which is attached externally to the piston rod supporting the load carrying boom. The strain gauge device functions to sense the compressive state of the piston rod in a continuous manner, and generates electrical analog signals which are transmitted to external data processing means for interpreting the strain measurement as load weight. The strain gauge therefore serves to measure the magnitude of the load on the end of the boom by measuring the proportionate strain introduced in the piston rod which supports the boom.

According to another aspect of the invention, a load measuring apparatus comprises a piston rod which supports a load carrying boom, and a strain gauge mounted to the external surface of the piston rod. The piston rod functions duly as a prime mover of the load carrying boom in a vertical plane, and as a component part of a load sensing apparatus.

Multiple strain gauges are attached to the piston rod, and the data produced by these multiple gauges are averaged to compensate for the effects of loading imbalance, wind, and other variables which can affect the accuracy of their independent readings.

According to a further aspect of the invention, a method for measuring the load suspended from a load bearing boom comprises the steps of: mounting at least one strain gauge to an external surface of the piston rod supporting the load bearing boom; connecting electrical means to the strain gauge for interpreting strain measurements in the piston rod as a magnitude of the load weight which is supported by the piston rod.

For a better understanding of the present invention reference will now be made by way of example to the accompanying drawings in which: Brief Description of the Accompanying Drawings Figure 1 is a side elevation view of assembled load bearing apparatus comprising a boom, a hydraulic motor supporting the boom, and a pair of strain gauges attached according to the teachings of the present invention.

Figure 2 is an exploded perspective view of the subject strain gauge, showing mounting screws exploded therefrom.

Figure 3 is a perspective view partially in section through the piston rod of the hydraulic motor proximate the strain gauges. A mounting templet is shown for accurately aligning the strain gauges on the piston rod.

Figure 4 is a perspective view partially in section of a piston rod, showing the strain gauges mounted thereto according to the teachings of the present invention.

Detailed Description of the Preferred Embodiment Referring first to Fig. 1, load bearing apparatus 2 is disclosed comprising an elongate boom member 4, having a load bearing end 6, an opposite end 8, which is pivotally connected to a crane chassis or the like. It will be appreciated that the boom 4 provides means (not shown) at the load bearing end 8, such as a block and tackle or platform, which functions to support a work load. Apparatus of this general type is found on crane units, aerial work platforms, etc. Load bearing apparatus 2 further comprises a fluid motor 10, consisting of a piston rod 1 2 and a hydraulic cylinder rod 14, of a type commonly used in the industry.Piston rod 1 2 has one end 1 6 fixedly connected to the boom member 4 intermediate the ends thereof, and the opposite end of the piston rod 1 6 extends into the hydraulic cylinder 14. It will be appreciated that the piston rod acts under the influence of hydraulic fluid to elevate and lower the boom 4 under operator control. A pair of strain gauges 18 are illustrated as mounted to the piston rod 1 2 proximate the point of attachment of the rod to the boom. Electrical leads 20 are shown connected to the strain gauges 18, and provide electrical input and output to the gauges.

With reference to Fig. 2, one of the strain guages 1 8 is shown in enlarged perspective view, as comprising generally a rectangular main body, having longitudinal support legs 22 depending therefrom. A pair of mounting screws 26 are used to attach the strain gauge 1 8 to the piston rod, and project through the appertures 20 and into the piston rod as will be explained below. The stain gauge 18 is of the general type available commercially and used within the industry primarily to monitor the volume in containers such as tanks, vats or silos. Specifically, strain gauges of the general type are used to determine the volume in a cylindrical container by sensing the compression of the support members of the container.Normally, one or more of the strain gauges are attached to each support member in order to average the loading effects, and to compensate for wind and thermal influences.

The construction of each strain gauge constitutes the bonding of two semi-conductor crystals together. Upon attachment of the strain gauge to a body, any compression of the parent body will also compress the strain gauge, causing resistant changes to occur in the crystals in a direct proportion to the degree of compression. Electrical packages are available which can interpret the electrical output from the strain gauges, responsive to the resistant changes, and interpret such resistant changes as load weight. The strain gauges are made from a material to match the thermal expansion coefficient of the material to which the strain gauges are to be attached.

A strain gauge of this general type is a Model 053 Microcell, manufactured by Kisler-Morse Corporation, Bellevue, Washington.

Referring to Fig. 3, the strain gauges 1 8 are mounted to opposite sides of the piston rod 12, and are located in appropriate position by an assembly templet fixture 28 having an arcuate interior profile 30 adapted to conform to the piston rod curvature. A pair of strain guages 1 8 are used to compensate for unbalanced loading, wind, and thermal effects, in order to enhance the accuracy of the reading. Upon location of the strain gauges 1 8 on opposite sides of the piston rod by the assistance of a templet fixture 28, a pair of holes are made in the piston rod by a drill and tap method. Thereafter, the screws 24 are inserted to securely attach the strain gauge 1 8 to the piston rod, and the templet fixture 28 is removed as illustrated in Fig. 4.

It will be appreciated by those skilled in the art that the piston rod 1 2 will be compressed by the loading of the boom 4 against the hydraulic fluid pressure exerted upon the piston rod by the hydraulic motor 1 0. The degree of compression of piston rod 1 2 will be directly proportional to the loading which is placed on the load bearing end 6 of the boom member 4. As the piston rod is compressed, the strain gauges 1 8 will sense the degree of compression, and produce an electrical analog voltage signal in proportion thereto. Typically available electronics packages can interpret the analog output from the strain gauges 18, and display continuously the weight which is being lifted by the boom member 4. The advantage of the present invention are that the strain gauges 1 8 can be retrofitted onto existing equipment since they are to be exter nally attached to the piston rod 1 2. Further, it will be appreciated by those skilled in the art that the strain gauges are located remote from the load bearing end of the boom, and in a relatively protected location to reduce the possibility of damage. Still further, it will be appreciated from Fig. 1 that the strain guages 18 function independently of the variations in fluid viscosities in the hydraulic motor 10 and relatively independent of ambient temperature variations. For this reason, reliable readings are produced by the present invention. Also, it should be appreciated that the load bearing apparatus with load means described above is considerably less expensive than alternative conventional means for determining the load which is supported by the boom member 4, since the sensor apparatus attaches externally.

The piston rod 12, also, performs a dual function as prime mover of the boom and as a component part of the load sensing apparatus.

The above describes the preferred embodiment of the present invention, but it should be appreciated that other embodiments; which will be appreciated by those skilled in the art, and which utilize teachings herein not set forth, are likewise intended to be within the scope and spirit of the present invention.

Claims (7)

1. A load bearing boom assembly comprising a boom member having a free end from which a load is suspended; a fluid motor for supporting the boom member, comprising a chambered body and a piston rod residing with the chambered body and having an end connected to the boom member; and a strain gauge mounted to an external surface of the piston rod for sensing the magnitude of the compressive stress within the piston rod.

2. An assembly according to Claim 1, wherein the boom member having a base end which is opposite the load suspending end, and which is pivotally anchored such that the boom member pivots about the base end in a vertical plane.

3. An assembly according to Claim 1, further comprising electrical circuitry for interpreting the magnitude of compressive stress within the piston rod as proportionate to the weight of the load suspended from the boom.

4. An assembly according to Claim 1, wherein the piston rod having an intermediate segment situated externally of the chambered body, the strain guage being mounted to the external piston rod segment.

5. An assembly according to Claim 1, wherein the piston rod moving reciprocally relative to the chambered body under influence of pressurized fluid within the chambered body, for pivoting the boom member about its base end.

6. A method of determining the load weight suspended from one end of a boom member comprising the steps of connecting a rod to the boom member such that substantially all of the load weight is transferred to place the rod in a state of compression; mounting a strain guage to an external surface of the rod for measuring the magnitude of compressive stress introduced into the rod by the load weight and producing an output responsive thereto; and connecting circuitry to the strain gauge for interpreting the output as the magnitude of the load weight suspended from the boom member.

7. A load bearing boom assembly substantially as hereinbefore described with reference to the drawings.