GB2156778A - Safe load indicator for a construction vehicle - Google Patents

Abstract

A safe load indicator for a hydraulically-operated construction vehicle comprises means for combining in a selected ratio, for example adding together, the hydraulic pressures in at least two of the operating rams of the vehicle, means for comparing the resulting value with a predetermined value and signal means to indicate that the resulting value is close to or exceeds the predetermined value, thereby indicating that an unsafe condition of the vehicle is approached or already exists. In the embodiment described an excavator comprises a main boom, a dipper arm and a bucket operated by respective rams. As shown shuttle valves 16,17 select the highest pressure from the full bare sides 10b, 12b, 14b of the three rams and that value is fed to the negative side of add/subtract unit 15. The pressures from the high pressure sides of the boom and dipper rams 10a, 12a are summed by the unit 15 and a "net" or working pressure determined by subtracting therefrom the pressure determined by the valves 16,17. This working pressure controls safe load indicator unit 19. <IMAGE>

Description

SPECIFICATION Safe load indicator and method This invention is a safe load indicator for a construction vehicle, more particularly for such a vehicle having a hydraulically-powered operating arm. It is of particular value as applied to a vehicle of this type in which the operating arm has at least two stages.

Construction vehicles designed to lift varying loads, for example conventional cranes or loaders or excavators, are liable to enter unstable conditions if a large load is lifted at an extended operating radius. With a simple crane having a single-stage jib, it is a relatively straightforward matter to generate a mechanical or electrical signal which reflects both the jib attitude and the load size and indicates when an unstable condition is being approached, either because the jib angle is being changed or because an excessive load is being lifted. However, when the operating arm has two or more stages, the number of variables is increased and their interrelationship becomes more complex, with the result that the generation of a warning of an unsta ble condition entails a complicated calculation.Systems for achieving this end are available but require a computer for carrying out the necessary calculations and are therefore very expensive.

It is therefore an object of the present invention to provide a method and device for detecting the approach of an unstable condition in a construction vehicle having a hydraulically-powered operating arm, which are simpler and therefore less expensive than hitherto-available devices.

According to the present invention, a method of detecting the approach of such an unstable condition comprises combining in a selected ratio the hydraulic pressures in at least two independent operating rams of the construction vehicle and comparing the resulting pressure value with a predetermined value. The device according to the present invention carries out these aforesaid operations and gives a warning indication when the value resulting from the combining of the pressures closely approaches and/or exceeds the predetermined value.

The method may be used, if desired, to monitor the operation of a simple single-stage operating arm and associated hydraulicallypowered lifting mechanism but it is of particular application, as indicated, to multi-stage systems, for example a two-stage operating arm carrying pivoted at its outer end an excavator bucket. Thus, in this latter system the pressures in the three rams which operate, respectively, the main boom, the dipper arm and the bucket angle are combined in a selected ratio and the resulting pressure value is then compared with a predetermined value representing the safe maximum laoding. This comparison may conveniently be carried out hydraulically such that, if the predetermined value is approached or exceeded, a suitable warning signal is generated electrically.

The ratios in which the measured pressure values are combined may be determined empirically for a given construction vehicle by measuring the values for various relative orientations of the components of the operating arm and combining those values in different ratios to determine their relative influence on stability. Thus the individual values may be expressed in graphical form and their differently combined values similarly shown. For many vehicles, the values may simply be combined in equal ratios. Thus the pressure in the ram operating the main boom may be added hydraulically to that in the dipper arm ram and then the pressure in the bucketoperating ram may be added to or substracted from the total, depending upon the orientation of the bucket at the relevant instant.

The invention will now be further described with reference to the accompanying drawings, in which: Figure 1 illustrates diagrammatically the arrangement of operating rams in a conventional two-stage excavator arm; and Figure 2 illustrates the hydraulic and associated electrical circuitry in a device according to the present invention for indicating safe/unsafe working of the excavator arm represented in Fig. 1.

Referring firstly to Fig. 1, the illustrated operating arm of an excavator comprises a main boom 3, pivoted to the vehicle body at 4, and a dipper arm 5, pivotally mounted at 6 on the outer end of the boom 3. The excavator bucket 7 is pivoted at the lower end 8 of the dipper arm 5. Raising of the boom 3 is effected by inward action of a piston 9 of a boom-operating ram 1 0. Extension of the dipper arm 5 outwards from the excavator, and associated lifting of dipper arm and bucket, are effected by inward action of a piston 11 of a dipper ram 1 2. Pivoting of the bucket about the point 8 is achieved by appropriate action of a piston 1 3 of a bucket ram 14.

Turning now to Fig. 2, it will be seen that the pressure on the high-pressure side 1 0a of the boom ram and from the high-pressure side 1 2a of the dipper ram are both fed to a unit 1 5 in which the various pressures are added and/or substracted hydraulically. The pressures from the full-bore sides 1 4b and 1 2b of the bucket ram and dipper ram respectively are fed to a shuttle valve 16, of which the output pressure equals the higher of the two pressures input to the valve. That output pressure, and the pressure from the full-bore side lOb of the boom ram, are fed to a second shuttle valve 17, which again gives an output pressure equal to the higher input pressure.Thus the two shuttle valves 1 6 and 1 7 together select the highest pressure from the full-bore sides of the three operating rams and that value is fed to the negative side of the unit 15.

The "net" pressure which represents the output of the unit 1 5 is the working pressure of the excavator arm and is determined in the unit by summing the high-pressure values from the boom and dipper rams and subtracting the pressure determined by the shuttle valves 1 6 and 1 7. This working pressure may be expressed in the form of an electrical signal generated by a linear voltage differential transducer incorporated in the unit 1 5 and is displayed on the scale of a manometer 18 in the safe load indicator unit 1 9.

This same working pressure controls the operation of two pressure switches 20 and 21. With the excavator working well within its safe limit, both switches 20 and 21 are at rest and an indicator light 22 shows "normal" operation. As the working pressure rises to exceed a predetermined value set by the operating pressure of the switch 20, the latter switch is activated and an indicator light 23 is energised, signifying the "full load" condition of the ecavators. If the working pressures rises still further, the switch 21 is activated, an indicator light 24 warns of "overload" condition and an audible alarm 25 is sounded.

As is well known to operators of excavator machines, when the bucket is digging in firm ground, extending of the bucket ram may induce very high loads and hence high pressures in the boom ram. This high pressure is not, in this case, an indication of potential instability of the machine. The above-described pressure circuitry ensures that such boom ram high pressure is compensated for and thereby any erroneous "overload" warning is avoided.

Another common occurrence in excavator operation is the bursting of hydraulic hose lines due to such induced high pressures. The result of such bursting is usually the immediate collapse of the operating arm, with potentially dangerous consequences. In the illustrated embodiment of the present invention, such collapse is avoided by the provision of pilot-operated non-return valves 26 and 27, immediately adjacent to the high-loaded sides of the boom ram and dipper ram respectively.

By means of these safety valves, if a hydraulic failure occurs in the high-pressure line, the ram is immediately locked in its position at the moment of failure. These same valves also prevent "creeping" of the rams due to minor leakage of pressure under load.

Claims (11)

1. A method of detecting the approach of an unstable condition in a construction vehicle having a hydraulically-powered operating arm, comprising combining in a selected ratio the hydraulic pressures in at least two independent operating rams of the vehicle and comparing the resulting pressure value with a predetermined value.

2. A method as claimed in claim 1, wherein said hydraulic pressures are added together.

3. A method as claimed in either of claims 1 and 2, wherein the vehicle is an excavator having a two-stage operating arm carrying pivoted at its outer end an excavator bucket, and wherein the pressures in hydraulic rams operating said two stages and said bucket pivoting are combined as aforesaid.

4. A method as claimed in any of claims 1 to 3, wherein the combining and/or the comparing of said pressures is carried out hydraulically.

5. A method as claimed in any of claims 1 to 4, wherein a warning signal is generated when the pressure value resulting from said combining closely approaches and/or exceeds said predetermined value.

6. A method of detecting the approach of an unstable condition in a construction vehicle having a hydraulically-powered operating arm, said method being substantially as hereinbefore described with reference to the accompanying drawings.

7. A safe load indicator for a construction vehicle having a hydraulically-powered operating arm, said safe load indicator comprising means for combining in a selected ratio the hydraulic pressures in at least two independent operating rams of the vehicle, means for comparing the resulting pressure value with a predetermined value and signal means operable to indicate that said resulting value is close to or exceeds said predetermined value.

8. A safe load indicator as claimed in claim 7, wherein said combining means is a device for hydraulically adding and/or subtracting pressures.

9. A safe load indicator as claimed in either of claims 7 and 8, wherein said comparing means is a pressure-responsive switch operating a visual and/or audible alarm.

10. A safe load indicator for an excavator vehicle having a two-stage operating arm carrying pivoted at its outer end an excavator bucket and wherein said respective arm stages and said bucket pivoting are operated by independent hydraulic rams, which safe load indicator comprises means for combining in selected ratios the pressures in said rams, at least one pressure switch set to operate when said combined pressure value approaches or exceeds a predetermined value and signalgenerating means operated by said switch.

11. A safe load indicator for a construction vehicle, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.