Overload Safety Device For Jib Cranes

Abstract

Overload safety device for jib cranes includes means for measuring a crane jib working radius, a multiple tapped potentiometer having a coil, input leads to the coil, and a slider, means for transferring the jib working radius measurement to an angular movement of the slider, means for supplying an independently adjustable voltage to each of the input leads of the potentiometer coil, load measuring means for producing an output voltage corresponding to a given load carried by the jib, means for comparing the output voltage of the potentiometer and the output voltage of the load measuring means, and means for releasing an overload signal when the output voltages of the potentiometer and the load measuring means are equal.

Description

Our invention relates to overload safety device for jib cranes, especially of the type having means for measuring the radius of the jib and the load carried by the jib.

In a heretofore known overload safety device of this type, the generally nonlinear relationship between permissible load and jib working radius, which is determined by the construction of the crane, is simulated by a cam disc which swivels with the jib. This nonlinear relationship between permissible load and jib working radius is hereinafter referred to as "cutoff characteristic" because of the characteristic curve which is produced by plotting the permissible or allowable load against the jib angle which determines the jib working radius.

Since a number of cutoff characteristics must often be taken into consideration for different operating conditions of a jib crane, for example in the case of a lattice mast crane having attachable jib segments, the heretofore known overload safety devices require a suitably shaped cam disc for each of the cutoff characteristics. This necessitates time-consuming exchange of the cam discs for each adjustment of the crane from one to the other operating condition and consequently from one to another cutoff characteristic, for example when inserting a jib segment in the jib of a lattice mast crane. Moreover, suitable storage means must be provided for the cam discs which, depending upon the number of cutoff characteristics associated with the particular crane, can be as many as 1,000 cam discs for one crane. Furthermore, by this repeated exchange of cam discs, they as well as their bearings are subject to excessive wear.

It is accordingly an object of our invention to provide overload safety device wherein a relatively good approximation of several cutoff characteristics for a crane is attainable in a comparatively simple manner without having to exchange any components of the device.

With the foregoing and other objects in view, we provide in accordance with our invention, overload safety device for jib cranes comprising means for measuring the working radius of the crane jib, a multiple tapped potentiometer including a coil, input leads to the coil, and a slider, means for transforming the jib working radius measurement to an angular movement of the slider, means for supplying an independently adjustable voltage to each of the input leads of the potentiometer coil, load measuring means for producing an output voltage corresponding to a given load carried by the jib, means for comparing the output voltage of the potentiometer and the output voltage of the load measuring means, and means for releasing an overload signal when the output voltages of the potentiometer and the load measuring means are equal.

In the safety device of our invention, the cutoff characteristics are simulated by a series of straight lines each representing one side of a sequence of polygons, the straight lines having slopes with varying positive and negative values and even having discontinuous slopes. The more output taps provided for the potentiometer, the better the simulated approximation of the cut-off characteristics of the safety device of our invention.

In accordance with our invention, the transition from one cutoff characteristic curve to another is effected by suitable adjustment of the voltages at the input taps without requiring the interchange or replacement of parts of the device.

According to a further feature of our invention, the measuring means for measuring the jib working radius comprise a device for measuring the length of the jib and a device for measuring the angle of the jib with respect to the horizontal, the length measuring device and the angle measuring device being adapted to issue respective output signals corresponding to the values measured, the output signals being combinable to a signal corresponding to the working radius of the jib, and each of the telescopically extended stages of the jib being coordinated with a separate group of voltages automatically impressable on the input taps of the potentiometer at the expansion of the telescopic jib, respectively at the start of a new telescopically extended stage.

In accordance with an additional feature of our invention, we provide means for combining the respective output signals of the length measuring and the angle measuring devices so as to obtain a value proportional to the product of the jib length and the sine of the jib angle with respect to the vertical. Consequently, further in accordance with our invention, we provide overload safety device wherein a sine generator is connected to the angle measuring device, and include electrical multiplying circuit means for multiplying the output signals of the length measuring device and the sine generator.

According to added features of our invention, we provide overload safety device for measuring a crane jib angle and length and to coordinate these measured values mechanically, wherein a rod is operatively connected to the length measuring device, the angle measuring device being in the form of a pendulum suspended by a pendulum bar from the jib, the rod being guided on the pendulum bar so that the distance between a fixed point on the axis of the pendulum bar and the rod is proportional to the working radius of the jib.

According to our invention and by mechanically coordinating the output signals of the length measuring device and of the angle measuring device, a value proportional to the working radius of the jib is obtainable. A right triangle is defined by the aforementioned rod, the pendulum bar and a measured line connecting the fixed point on the axis of the pendulum bar and the point of intersection of the rod with the pendulum bar, the angle between the jib and the lower chassis or cab of the crane or ground level being included between the hypotenuse and one of the sides of the right triangle. The measured distance between the fixed point on the pendulum bar axis and the point of intersection of the rod and the pendulum bar is automatically variable in proportion to the jib length by the operative connection between the rod and the length measuring device.

In accordance with another feature of the invention, the length measuring device, the angle measuring device and the device for producing the signal proportional to the jib working radius are provided in a common assembly.

According to an added feature of the invention, the rod extends perpendicularly to the pendulum and is guided thereon by means of a sliding sleeve; the value proportional to the jib working radius is measurable along the pendulum bar.

In accordance with concomitant features of our invention, we provide another embodiment of the overload safety device wherein the rod is guided by a parallelogram linkage parallel to the jib and angularly displaceable relative thereto. The angle defined by the jib with respect to the vertical is included, in this case, between the intersecting rod and the pendulum bar, and the value proportional to the jib working radius is measurable in direction perpendicular to the jib and between the rod and the suspension point of the pendulum bar, preferably beginning from a sliding socket mounted on the rod. According to a specific added feature of the invention, the rod is supported by a double sliding joint at the intersection thereof with the pendulum bar, the double sliding joint affording angular movement of the rod relative to the pendulum bar. Movement of the rod relative to the pendulum bar is obtainable with minimum mechanical resistance, according to another feature of our invention, by providing the parallelogram linkage with at least one counterpoise for balancing the weight of the rod and of the linkage. Consequently, the pendulum need then only overcome the frictional resistance in the double sliding joint and not the inertia of the rod and the suspension means therefor.

In accordance with other features of the invention and in another embodiment of the overload safety device, the parallelogram linkage is replaced by a guiding link formed with a groove extending parallel to the jib, the guiding link being supported for movement thereof perpendicularly to the jib and angularly with respect to the pendulum bar, the latter having a pin affixed thereto guidable in the groove.

According to another feature of the invention, we provide an indicating device for directly indicating the jib working radius, the indicating device being adapted to receive the measured value proportional to the jib working radius as obtainable in the aforementioned different embodiments of our invention, the indicating device being preferably an electrical device comprising a linear potentiometer having a slider adjustably driven to a given setting proportional to the value of the jib working radius.

In order to measure the same load value for a constant jib moment in the case of an overload safety device which is coordinated with a tilting fluid cylinder of a telescopic crane, we provide, in accordance with another feature of the invention, an electric compensation circuit for compensating any change in the load or force acting on the tilting cylinder due to swiveling of the jib.

According to additional features of our invention, the multitapped potentiometer of the overload safety device according to our invention is connected to an alternating current source, and has output terminals connected to a rectifier. The input terminals of the potentiometer are preferably connected through adjustable switches to an inductive voltage divider, each of the switches being connected to one of the input terminals, respectively, of the potentiometer and being manually or automatically adjustable for stepwise changing the voltage applied to the potentiometer by the inductive voltage divider.

According to another feature of the invention, we provide a transformer subjected to the full voltage of the voltage divider and connected to the load measuring device for energizing the latter.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in overload safety device for jib cranes, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is an electric circuit diagram of an overload safety device according to our invention for a conventional jib crane having a nontelescopic jib;

FIG. 2 is a plot diagram showing a simulation of a cutoff characteristic curve for the circuit of FIG. 1;

FIG. 3 is an electric circuit diagram of an overload safety device according to the invention for use with a telescopic jib crane;

FIG. 4 is a diagram of a compensation circuit connected to the load measuring device of the overload safety device shown in FIGS. 1 and 3;

FIG. 5 is a partly diagrammatic elevational view of an overload safety device for a telescopic jib crane wherein the output measuring signals of the jib length measuring device and the jib angle measuring device are mechanically combined by a rod extending perpendicular to the pendulum bar;

FIG. 6 is a view similar to FIG. 5 of another embodiment of the invention wherein the rod is guided parallel to the jib;

FIG. 7 is a plan view of the damping device of the pendulum according to FIG. 6;

FIG. 8 is a view similar to those of FIGS. 5 and 6 wherein the rod is replaced by a guiding link formed with a groove extending parallel to the jib.

Referring now to the drawings and first particularly to FIG. 1 thereof, there is shown an inductive voltage divider 1 connected to an alternating current source at input terminals a and b thereof and having a plurality of m output taps. The inductive voltage divider 1 cooperates with a potentiometer having n input taps T. Selective intermediate voltages can be taken from the inductive divider l through adjustable switches S.sub.1, S.sub.2 . . . S.sub.n.sub.-1 and S.sub.n and fed independently of one another to the individual input taps T.sub.1 . . . T.sub.n.sub.-1 and T.sub.n. The potentiometer 3 has a slider 4 which is electrically or mechanically coupled to a shaft of a nonillustrated device for measuring the jib angle of a jib crane. A rectifier 5, which is connected to the potentiometer 3, provides a rectified output U.sub.W of the potentiometer 3 which is variable in accordance with the size of the jib angle and with the voltages set or adjusted at the input taps of the potentiometer winding.

The inductive voltage divider l is connected in parallel to an inductive transmitter or transformer 6 having a voltage output which is fed to and rectified by a rectifier 7. A bridge circuit 8 having resistances consisting of strain gauges of a device 12 for measuring the load applied to a jib crane is supplied by feed lines with the rectified output voltage from the rectifier 7. The output voltage U.sub.K of the load measuring device 12 is applied through suitable leads to a location 9 at which it is compared to the rectified output voltage U.sub.W of the potentiometer 3. When the voltages U.sub.W and U.sub.K are equal, a relay 11 is energized by a null-balanced amplifier 10 to release an optical or acoustic overload signal or to automatically switch off the jib crane.

In FIG. 2 there is shown in a crane load-jib angle diagram, a sequence of polygons simulating the area below a representative cutoff characteristic. The jib angle measured along the abscissa of the diagram in FIG. 2 corresponds to the setting of the slider 4 of the potentiometer 3, the positions of the input taps T.sub.1 . . . T.sub.n.sub.-1 and T.sub.n corresponding to fixed points along the abscissa not necessarily equidistant from one another. The allowable load measured along the ordinate of the diagram in FIG. 2 is correlated to the voltage tapped from the inductive voltage divider 1. In order to adjust or vary this voltage, the switches S.sub.1 . . . S.sub.n can be adjusted to m positions whereby, respectively, the voltage applied to a given input tap T and therewith the value h of allowable load at this location can be adjusted independently of the voltages at the other input taps T.

In this manner, any arbitrary cutoff characteristic may be simulated by the upper sides of a sequence of polygons, the accuracy of the approximation being dependent on the number of input taps T.

The parts of FIG. 1 shown in FIG. 3 bear the same reference numerals and are not further described herein.

In the circuit of FIG. 3 there are shown diagrammatically a jib angle measuring device W and a jib length measuring device L, both devices being of conventional construction and well known in the art and therefore not described in detail herein, which actuate, through a circuit system generally designated by 20, a motor M which drives the slider 4 of the potentiometer 3.

A sine potentiometer R.sub.s is a function potentiometer having a number of input taps P.sub.1, . . . P.sub.r.sub.-1 and P.sub.r impressed with voltages from the inductive voltage divider 1 so that the output voltage U at the slider 23 of the sine potentiometer R.sub.s , which is driven by a shaft 22 of the jib angle measuring device W, is proportional to the sine of the jib angle measured by the device W. The voltage U is applied across a series connection of three resistances R.sub.1, R.sub.2 and R.sub.L. The resistance R.sub.1 represents the fundamental jib length and is constant, whereas the resistance R.sub.L is a potentiometer having a slider 24 that is driven by an output shaft of the jib length measuring device L. The output voltage U at the slider 24 is thereby proportional to the product l.sup.. sin .alpha., wherein l represents the jib length and .alpha. the jib angle. This voltage U is fed to the motor M which electromechanically drives the slider 4 of the potentiometer 3 and a slider 26 of a potentiometer R.sub.R.

The compensating voltage U.sub.r characteristic of the working jib radius is tapped from a constant, galvanically divided source voltage U.sub.c across a resistance R.sub.3 and through slider 26 serially connected thereto. The differential voltage between U and U.sub.r is fed through an amplifier to the motor M. The motor M drives the sliders 4 and 26 until U.sub.r equals U . The resistance R.sub.3 simulates the spacing between the center of rotation of the jib and the center of rotation of the crane. The resistances R.sub.2 and R.sub.4 serve for adapting to or matching the voltage U.sub.c.

In the embodiment of the invention illustrated in FIG. 3, the load measuring device 12 is coordinated with a tilting cylinder, having an upper end pivoted at the jib and a lower end at the lower chassis of the crane.

An electric compensating circuit for balancing a change in force acting on the tilting cylinder due to a change in the effective lever arm of the tilting cylinder is shown in FIG. 4. In this case, the voltage U.sub.K across the output leads of the load measuring device 12 is not directly compared with the voltage U.sub.W which corresponds to the working radius of the jib, but is rather subjected to various voltage drops across a resistance R having a tap S, whose position is set by the jib angle measuring device W in proportion to the angle of the jib, and across resistances R.sub.6, R.sub.7 and R.sub.8 so that an output voltage U'.sub.K dependent on the jib angle .alpha. is produced which has substantially the same characteristic curve as the characteristic curve of the change in length of the effective lever arm of the tilting cylinder when considered relative to the base location of the crane jib.

The amplitude of the function U'.sub.K is determined by the resistances R.sub.6, R.sub.7 and R.sub.8 and the location of the maximum voltages is determined especially by the resistances R.sub.7 and R.sub.8. This measure affords the maintenance of a constant datum or measured value U'.sub.K by employing a measurement transmitter or sensor which measures the load in the direction of the axis of the tilting cylinder, for a constant jib moment, independently of the angular position of the jib.

In FIG. 5, there is shown a jib 1' of a telescopic jib crane which is pivotable about a horizontally disposed pin 2' fixed on the lower chassis or cab 3' of a crane and is inclined at an angle .alpha. with respect to the horizontal. A load Q is suspended at the free end 4' of the jib 1'. The jib 1' carries a casing 6' at a nontelescoping portion 5' thereof for supporting a length measuring device in the form of a pulley 7' and for accommodating an angle measuring device in the form of a gravitational pendulum 8'. A cable 10' fixed to a rod projecting transversely from the free end 4' of the jib 1', the cable 10' extending parallel to the axis of the jib 1', is wound on the pulley 7' so that it is unwound therefrom and turns the pulley 7' when the jib 1 is telescoped. The number of rotations carried out by the pulley 7' as the cable 10' is being unwound is proportional to the jib length l.

The pulley 7' is seated on a threaded spindle 11' disposed perpendicularly to the axis of the jib 1' and mounted rotatably and axially nondisplaceably in bearings 12' fixed within the casing 6'. A nut 13' is threadedly mounted on the spindle 11' and is nonrotatably guidable by a nonillustrated longitudinal guide, such as a conventional pin-and-slot mechanism, in the axial direction of the spindle 11'.

The gravitational pendulum 8' is suspended by a bar 15', according to the diagrammatic projection of FIG. 5, within the casing 6' at a suspension point 14' located on the axis of the spindle 11'. A rod 16' extending perpendicularly to the pendulum bar 15' is guided longitudinally along the pendulum bar 15' by a sliding sleeve 17' fixed to one end of the rod 16'. The rod 16' is supported at the other end thereof on a pin 18' outwardly projecting from the nut 13' so as to afford longitudinal and angular sliding of the rod 16'.

In the projection of FIG. 5, the pendulum bar 15', the rod 16' and the threaded spindle 11' form a right triangle. A line between the suspension point 14' of the pendulum 8' and the pin 18' forms the hypotenuse of the triangle, the length l' thereof being variable in proportion to the jib length l due to the movement of the nut 13' in the axial direction of the spindle 11'. Since the pendulum 8' and the bar 15' thereof always extend in a vertical and hence perpendicular direction with respect to the ground due to gravity action, the spindle 11', which is disposed perpendicularly to the jib axis, and the rod 16', which is guided in a direction perpendicular to the pendulum bar 15', define therebetween an angle .alpha. which is equal to and varies with the angle of inclination .alpha. of the jib 1'. Consequently the length a'=l' sin .alpha., which is proportional to the jib working radius a*=1 sin .alpha. may be measured off along the pendulum bar 15' between the suspension point 14' and the sliding sleeve 17'. This is effected by a cable 20', shown in dot-dash, which is fixed at one end thereof to the sliding sleeve 17', is looped about the suspension point 14' and is wound up on a pulley mounted on a drive shaft 19' which adjusts the setting of the slider 4 of the potentiometer 3.

In the embodiment of FIGS. 6 and 7, parts similar to those shown in the embodiment of FIG. 5 are identified by the same reference numerals.

As shown in FIG. 6, a gravitational pendulum 108 suspended by a pendulum bar at a suspension point 114 in a housing 6' guides a rod 116 by means of a conventional double-sliding joint 117 which permits longitudinal and angular movement of the rod 116. A line or cable 126, shown in broken line in FIG. 6, is fixed to the double-sliding joint 117 so that it may be wound on and unwound from a pulley mounted coaxially with the pulley 7', which with the cable 10' acts as a length measuring device in the same manner as in the embodiment of FIG. 5, the cable 126 being diverted about the suspension point 114 of the pendulum 108 so that it extends parallel to the pendulum bar 115 from the suspension point 114 to the double-sliding joint 117. The rod 116 is guided without inertia by a parallelogram linkage provided with counterpoises 120 and in a direction parallel to the axis of the jib 1'. At the location of the rod 116 that is the shortest distance from the suspension point 114, a sliding sleeve 121 is provided fixed to an endless line or cable 122, shown in dot-dash line in FIG. 6, which extends vertically and hence perpendicularly to the rod 116. The cable 122 is revolvably guided on two opposing pulleys 123 and 124, the latter being mounted on the drive shaft 19' which adjusts the setting of the slider 4 of the potentiometer 3.

In the case of the embodiment of FIG. 6, a right triangle is formed by the pendulum bar 115, the rod 116 and the right-hand run of the endless cable 122. The hypotenuse of this right triangle is the distance l" between the center of the double-sliding joint 117 and the suspension point 114, the distance l" being always maintained proportional to the jib length l by the cable 126 driven by the pulley 7'. The angle .alpha. is defined by the intersection of the pendulum bar 115 and the rod 116 so that a linear value a"=l" sin .alpha. proportional to the jib working radius a*=1 sin .alpha. may be measured along the cable 122 for driving the slider 4 of the potentiometer 3 to a given setting thereof.

The pendulum 108 is clamped to prevent undesired vibrations. For this purpose a cylindrical weight 127 is carried at the lower end of the pendulum bar 115, as seen in FIG. 6, a ball bearing 128 being seated on the cylindrical weight 127. The outer race of the ball bearing 128 is guidingly received between two parallel tracks 129 having a curvature corresponding to the curved travel path of the pendulum 108 when set in oscillation. The tracks 129 are connected at one end thereof by a hinge 130 and at the other end thereof by a tension spring 131 (FIG. 7). When the jib 1' is swiveled about its pivot axis 2', a solenoid 132 located opposite the tension spring 131 is suitably energized by an alternating current of appropriate frequency so as to attract and thereby withdraw the upper track 127, as viewed in FIG. 7, away from the ball bearing 128. Thus, a stepwise, vibration-free rolling movement of the ball bearing 128 at the lower track, as viewed in FIG. 7, is afforded.

FIG. 8 shows another embodiment of our invention wherein parts like those shown in the embodiments of FIGS. 5 and 6 are denoted by the same reference numerals and serve similar functions.

In FIG. 8, the gravitational pendulum 108, which is suspended at the point 114 in the housing 6', guides a pin 134 which is displaceable in the direction of the pendulum bar 115. One end of a line or cable 126, shown as a broken line in FIG. 8, is fixed to the pin 134. The cable 126 may be wound up on or unwound from a pulley coaxial to the pulley 7' which with the cable 10' form a length measuring device, in the same manner as in the embodiment of FIG. 6, the cable 126 being diverted by the suspension point 114 of the pendulum 108 so that the length of the cable 126 extending between the suspension point 114 and the pin 134 is parallel to the axis of the pendulum bar 115. The pin 134 is guided in a groove 135 formed in a link or crank 133 which is axially displaceable in longitudinal bearings secured in the housing 6'. The groove 135 is elongated and extends in a direction parallel to the jib 1' and perpendicular to the main axis of the guiding link 133. In the embodiment of FIG. 8, a right triangle is formed by the pendulum bar 115, a line parallel to the main longitudinal axis of the guiding link 133 and crossing the center of the pin 134, and a line parallel to the axis of the jib 1' and crossing the suspension point 114.

The hypotenuse of the right triangle thus defined in FIG. 8 is measured by the distance l'" between the suspension point 114 and the pin 134, the distance l'" being always maintained proportional to the jib length l by the line or cable 126 driven by the pulley 7' of the length measuring device L.

The jib angle .alpha. is defined by the intersection of the pendulum bar 115 and the line parallel to the jib axis and passing through the suspension point 114 so that the linear value a'"=l'" sin .alpha. proportional to the jib working radius a*=l sin .alpha. is measured by a toothed rack 137 which is secured to the guiding link 133. The toothed rack 137 mates with a pinion 138 which drives the shaft 19' for adjusting or setting the slider 4 of the potentiometer 3.

The assembly shown in the circuit diagram of FIGS. 1 and 3 and employable with any of the embodiments shown in FIGS. 5, 6 and 8 has the following advantages: An inductive voltage divider having a plurality of input taps is able to be manufactured at less expense than an assembly having a similar function but operated with direct current. Due to the inductive coupling of the supplies of the load measuring device and the potentiometer, a simple comparison of output voltages is afforded for a galvanic division. Stabilization of the input or supply voltages is unnecessary in this case. Should the overload safety device of our invention, however, be operated with alternating current in all parts thereof, a phase-sensitive rectification is required for comparison of the output voltages of the potentiometer and load measuring device. Relatively long lines have often had to be installed between load measuring device, potentiometer output terminals and the location of the voltage comparator. With alternating current operation, the measuring circuit would be undesirably affected by the changes in the line capacities which periodically vary for example due to temperature effects. Furthermore, screened cables would have to be used. With the output voltage of the potentiometer and the input voltage of the load measuring device being rectified, however, then conventional unshielded lines can be used, the effect of temperature change thereon and varying length thereof having no influence on the measuring circuit.

Claims

We claim:

1. Overload safety device for jib cranes comprising means for measuring a crane jib working radius, a multiple tapped potentiometer having a coil, input leads to said coil, and a slider displaceable along said coil, means operatively connecting said jib working radius measuring means and said potentiometer for transforming a jib working radius measurement to an angular movement of said slider, means for supplying independently adjustable voltages corresponding to the loads permissible for the actual jib working radii to the input leads of said potentiometer coil, load measuring means for producing an output voltage corresponding to a given load carried by the jib, means for comparing respective output voltages of said potentiometer and said load measuring means, and means responsive to said output voltage comparing means for releasing an overload signal when the compared output voltages of said potentiometer and said load measuring means are equal.

2. Safety device according to claim 1 for use with a telescopic jib crane, wherein said jib working radius measuring means comprises a jib length measuring device, a jib angle measuring device and means for combining output signals corresponding to the respective measurements of said measuring devices into a signal corresponding to the value of the working radius of the jib, and voltage source means adapted to provide a separate set of voltages coordinated with each telescopically extended stage of the telescopic jib, said voltage source means being operatively connected to said potentiometer for automatically applying a respective set of voltages to the input leads of said potentiometer at the beginning of each telescoping step as the telescopic jib is being extended.

3. Safety device according to claim 2 wherein said jib angle measuring device is operatively connected to a sine generator, and including multiplying circuit means for multiplying output signals of said length measuring device and said sine generator.

4. Overload safety device for use with telescopic jib cranes comprising means for measuring a crane jib working radius, a multiple tapped potentiometer having a coil, input leads to said coil, and a slider displaceable along said coil, means operatively connecting said jib working radius measuring means and said potentiometer for transforming a jib working radius measurement to an angular movement of said slider, means for supplying an independently adjustable voltage to the input leads of said potentiometer coil, load measuring means for producing an output voltage corresponding to a given load carried by the jib, means for comparing respective output voltages of said potentiometer and said load measuring means, and means responsive to said output voltage comparing means for releasing an overload signal when the compared output voltages of said potentiometer and said load measuring means are equal, said jib working radius measuring means comprising a jib length measuring device, a jib angle measuring device and means for combining output signals corresponding to the respective measurements of said measuring devices into a signal corresponding to the value of the working radius of the jib, and voltage source means adapted to provide a separate set of voltages coordinated with each telescopically extended stage of the telescopic jib, said voltage source means being operatively connected to said potentiometer for automatically applying a respective set of voltages to the input leads of said potentiometer at the beginning of each telescoping step as the telescopic jib is being extended, and a rod operatively connected to said length measuring device, said angle measuring device being in the form of a pendulum having a pendulum bar and suspended thereby from the jib, said rod being disposed transversely to and guidable along the axis of said pendulum bar so that a distance between a fixed point on the axis of said pendulum bar and said rod is proportional to the working radius of the jib.

5. Safety device according to claim 4 including a sliding sleeve carried by said rod for guiding the same along the axis of said pendulum bar, said rod being disposed perpendicularly to the axis of said pendulum bar.

6. Safety device according to claim 5 including a rotatable threaded spindle, fixed against axial displacement, extending perpendicularly to the jib, said spindle being operatively connected to said length measuring device and being rotatably driven thereby, a nut threaded on said spindle and being guidable longitudinally relative to said spindle as said spindle is rotated, said rod being supported on said nut so as to be longitudinally and angularly displaceable relative thereto.

7. Safety device according to claim 5 including a line fixed to said sliding sleeve and extending parallel to said pendulum bar, and means cooperating with said line for driving the slider of said potentiometer.

8. Safety device according to claim 4 including parallelogram linkage means connected to said rod for guiding the same in direction parallel to the jib and for angularly displacing the same at said pendulum bar.

9. Safety device according to claim 8 including a double sliding joint at said pendulum bar, said rod being guidable by said double-sliding joint at said pendulum bar so as to be angularly displaceable relative to said pendulum bar.

10. Safety device according to claim 9 including a line fixed to said double sliding joint and means cooperating with said length measuring device for winding up said line.

11. Safety device according to claim 10 including a sliding sleeve carried by said rod, another line fixed to said sliding sleeve and extending perpendicularly to said rod, and means cooperating with said other line for driving the slider of said potentiometer.

12. Safety device according to claim 8 including at least one counterpoise carried by said parallelogram linkage means for balancing the same and said rod.

13. Overload safety device for use with telescopic jib cranes comprising means for measuring a crane jib working radius, a multiple tapped potentiometer having a coil, input leads to said coil, and a slider displaceable along said coil, means operatively connecting said jib working radius measuring means and said potentiometer for transforming a jib working radius measurement to an angular movement of said slider, means for supplying an independently adjustable voltage to the input leads of said potentiometer coil, load measuring means for producing an output voltage corresponding to a given load carried by the jib, means for comparing respective output voltages of said potentiometer and said load measuring means, and means responsive to said output voltage comparing means for releasing an overload signal when the compared output voltages of said potentiometer and said load measuring means are equal, said jib working radius measuring means comprising a jib length measuring device, a jib angle measuring device and means for combining output signals corresponding to the respective measurements of said measuring devices into a signal corresponding to the value of the working radius of the jib, and voltage source means adapted to provide a separate set of voltages coordinated with each telescopically extended stage of the telescopic jib, said voltage source means being operatively connected to said potentiometer for automatically applying a respective set of voltages to the input leads of said potentiometer at the beginning of each telescoping step as the telescopic jib is being extended, and said angle measuring device comprising a pendulum having a pendulum bar and being suspended thereby from the jib, and including a guide link operatively connected to said length measuring device and formed with a groove extending parallel to the jib, said guide link being mounted so as to be movable in direction perpendicularly to the jib and angularly to said pendulum bar, said pendulum bar having a pin projecting therefrom and guidingly received in said groove.

14. Safety device according to claim 13, including a line fixed to said pin, and means cooperating with said length measuring device for winding up said line.

15. Safety device according to claim 13 wherein said guiding link carries a toothed rack, and a pinion operatively connected to the slider of said potentiometer for driving the same is in meshing engagement with said toothed rack.

16. Safety device according to claim 1 including an indicating device operatively connected to said means for measuring the jib working radius for receiving therefrom a signal corresponding to the value of said working radius.

17. Safety device according to claim 16 wherein said indicating device comprises a linear potentiometer having a slider adjustable in accordance with the value of said jib working radius.

18. Safety device according to claim 4 including damping means for damping movement of said pendulum.

19. Safety device according to claim 1 wherein said voltage supplied by said supplying means to the input leads of said potentiometer coil is alternating voltage, and including a rectifier connected to said potentiometer for rectifying said voltage.

20. Safety device according to claim 19, wherein said voltage supplying means comprises an inductive voltage divider, having a plurality of switches connected thereto and respectively to each of the input taps of said potentiometer, said switches being adjustable for varying the voltage supplied by said voltage divider to said potentiometer.

21. Safety device according to claim 20 wherein the primary coil of a transformer is connected across the entire voltage divider so as to be subjected to the total voltage applied to said voltage divider, the secondary coil of said transformer being operatively connected to input leads of said load measuring device.

22. Safety device according to claim 1 for a jib crane having a fluid cylinder for tilting the jib, including means compensating for variation in force acting upon said cylinder due to swiveling of the jib.