Crane warning system

Feb. 26, 1963 h. l. mason 3,079,080

CRANE WARNING SYSTEM Filed Feb, 12, 1960 ₂ sheets-Sheet 1

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Fl G. 2.

INVENTOR.

Henry Lea Mason

BY I iff r\

Attorney

Feb. 26, 1963

Filed Feb. 12. 1960

H. L. MASON

CRANE WARNING SYSTEM

3,079,080

Sheets-Sheet 2

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Fi g. 3.

INVENTOR.

Henry Lea Mason

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Attorney

United States Patent Office

3,079,080

Patented Feb. 26, 1963 angle is defined as the horizontal angle ψ between the axis of the moving part of the crane and that of the carrier, whether the latter be an automotive truck for road and field use, or locomotive type crane for use on railroad tracks or a floating crane installed on a marine hull.

Another object of this invention is to provide a crane warning system that, while it consists essentially of an analog computer responding to resolvers and a load-force transducer, will be simpler and less power-consuming than prior art systems.

Still another object of this invention is to provide a crane warning system that will be operable to indicate danger of overturn because of the counterweights when the boom is removed or the load is suddenly dropped as by hoist 15 cable failure.

A further object of my invention is to provide a crane warning system which will obviate the disadvantages of the prior art systems mentioned above.

Other and further objects and advantages of my inven20 tion will appear in conjunction with the following description and drawings wherein:

FIGURE 1 is a schematic side elevation of a typical automotive type crane disclosing the factors involved in the moment computation in accordance with the present 25 invention;

FIG. 2 is a schematic plan vew of the crane disclosing further factors used in moment computation; and

FIG. 3 shows the circuitry for my improved analog computer.

In the preliminary planning stages leading to my invention, two basic concepts were carefully considered and examined. The first of these concepts warns the operator at a fixed “percent tip,” i.e., fixed ratio between overturning moment and righting moment, computed for instantaneous variates of load L, vertical boom angle Θ and horizontal slewing angle ψ of FIGS. 1 and 2. The second concept limits the load L at all slewing angles ψ to 85% of that sufficient to cause overturn at any given radius^— i'-plo cos Θ—in the least stable direction, presumably at /=90° or 270°. This “least stable” requirement further provides “backward stability” for the unloaded crane by requiring a minimum force on specified sets of wheels. This latter requirement permits a relatively simple warning system but limits unnecessarily the loads which could be handled safely while working at slewing angles distant from the least stable position.

The second of the above concepts appeared to be productive of more fruitful results, hence my invention is based on the assumptions (1) that at any combination of slewing angle, boom length, and vertical boom angle, loads are permitted up to 85% tip for that combination and (2) that counterweights will not cause backward overturn even if load and boom are removed.

In view of the above, our further computations may be considered based on the following numerical values which are taken from a prototype crane, where 171=26,400 lbs. for cab plus counterweights 1^2=27,900 lbs. for carrier truck H^zb=4,025 lbs. for boom plus rigging g=4.7 feet, distance from axis of rotation of cab to center of gravity of weight of cab plus counterweights

71=4.43 feet, distance from axis of rotation of cab to center of gravity of truck /=2.5 feet, distance from axis of cab rotation to pivot point P of boom /1= 15.5 feet /2=5.167 feet 70 /^=/^=3.193 feet

No values are given for l₀, the length of the boom, and

3,079,08®

CRANE WARNING SYSTEM

Henry L. Mason, Chevy Chase, Md., assignor to the United States of America as represented by the Secre- g tary of the Navy

Filed Feb. 12, 1960, Ser. No. 8,468

Claims. (CL 235—151) (Granted under Title 35, U.S. Code (1952), sec. 266)

The invention described herein may be manufactured 10 and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to moment computing and indicating systems and more particularly to such systems as applied to certain types of material handling equipment such as cranes.

Heretofore the operation of certain types of material handling equipment under heavy load has been hazardous to both human life and property. One of the primary hazards of heavy load operation of such equipment is the danger of overturning or tipping over the equipment. This hazard is present in any equipment which is constructed to support a load to be lifted at a distance from the center of gravity of the equipment, such as a crane.

Accidents have frequently occurred when a heavily loaded crane boom has been elevated or lowered to a certain position relative to the earth. The weight on the crane boom is seldom known by the operator, consequently an overturning accident may occur should a certain com- 30 bination of load weight and boom length be reached or exceeded in excess of the moment created by the counterbalancing weight of the body of the crane.

Numerous attempts have been made to eliminate, or partially eliminate, the foregoing hazards of operation of 35 certain types of material handling equipment. A good example of a crane warning system, otherwise known as a moment computing and indicating system, is described in U.S. Patent No. 2,858,070, issued on October 28, 1958, to Leon Scharff. This patent provides an excellent resume 40 of the prior art.

As will be pointed out more fully hereinafter, my invention outlines a system for gauging crane stability which does not impose restrictions on the orientation of the crane counterweight, boom, and load relative to the supporting 45 truck. Instruments heretofore developed for this purpose have gauged stability relative to the least stable condition only; this is usually the configuration in which the horizontal projection of the crane boom is perpendicular to the edge of the fulcrum least distant from the machinery 50 table center of rotation.

Some of the prior art systems, which are essentially analog computers with angle-sensing resolvers and adding amplifiers responding to a load-force transducer, have been found to have the following disadvantages: (a) the 55 system is undesirably complicated and confusing to the crane operator; (Z>) the system power requirements are excessive since great numbers of vacuum tubes are used; (c) the system does not warn of incipient overturn in the counterweight direction; (z/) the inclusion of the effects 60 of load-swing, either fore-and-aft or sidewise, have been found to be unnecessary complications as such effects are generally negligible; (e) the inclusion of the effect of tilt of the carrier as might be due to uneven or soft ground or to flexure of the carrier frame, has also been found 65 unnecessary; (/) the inclusion of the stabilizing effect of outriggers has been found unnecessary; and such prior art systems employed a confusing number of indicators, all being part of the crane warning system.

The principal object of my invention, therefore, is to provide a crane warning system that will be operable for all angles of slew of the crane boom, where the slewing

3,079,080 it is assumed that 0, the vertical angle of elevation of the boom, never exceeds 90°.

For further purposes of our computation, let us propose a definition which is based on the second concept mentioned above. This definition is:

Tipping Load (without outriggers set). At any given radius for truck-mouted types of cranes, the tipping load with tires resting on a firm, level, and even supporting surface shall.be that load which overcomes the stability of the crane in the least stable direction to the extent that the tires opposite the load leave the supporting surface.

Since it is expected that the least stable direction is at ¢=90° or 270°, consider sidewise tipping of the entire crane structure due to load L about either of the two fulcrum lines Si Si or S₂ S₂ in FIG. 2. The net sidewise tipping moment, M_stli with load L suspended outside Si Si or S₂ S₂ will be:

M_sti=/iZ-[i sin ψ+1₀ cos 0 sin ψ—A] + W_B[i sin ¢+0.5 ί₀ cos t’ sin ¢-A] —W₂k—W₁[k.+g sin ¢) where sin ψ may be taken as an absolute numerical value without regard to sign. Tipping will occur when sin ψ, L, ίο cos 0, and load safety factor /_L become so large that Msti, ordinarily negative, becomes zero. Under our second concept, we take /_L=0.85, i.e., the lifting capacity in pounds shall not exceed 85 % of the tipping load “at any given radius in the least stable position.” We interpret this last phrase to mean the position in which sin ψ—±ϊ. However, the value of M_stt can readily be instrumented for values of sin ψ lying between 0 and ;±1. This would permit lifting heavier loads safely over wide sectors on each side of the 99° and 270° positions while retaining the restriction that the lifting capacity be 85% of the tipping load for the instantaneous position.

Consider next the possibility of tipping sidewise due to counterweight overhang beyond Sj Si or S₂ S₂, without outtriggers set and without load. The definition,: here, may be stated :

Backward Stability (without outriggers set). In applying the following values, the boom is to be at its recommended minimum radius (usually 10 feet), the crane is to be unloaded, the cooling system full and the fuel tank half full. Then, minimum load On all wheels on the side on which the boom is located, with the axis of the boom at right angles to the axis of the truck and using the minimum recommended length of boom, shall not be less than 15% of the total weight of the crane and truck in operating condition without load.

The corresponding general expression M'_s6e for sidewise tipping, in the absence of load and due to the counterweight positions in the vicinity of either 90° or 270°, is: Jtf'stc— Wilg sin Ψ—£] — Wjc ~W_s[i sin ¢+0.5l₀ cos Θ sin ¢+10

The corresponding minimum wheel loading, for some •factor /w applied to the sum of Wx and W₂, may be expressed as. a sidewise restraining moment, M_sr:

M_sr=/_w[lFi+IF₂]2A

By the Backward Stability definition above, the quantity M_5tc=M'_Etc+M_sr must be kept negative, with sin 1 and f_w=0.15. For the prototype crane with boom vertical,

M_sr=52,500 ft.-lb.

M'_stc=39,600— 89,000—2,300=-72,400 ft.-lb.

This shows that the counterweighting will not be dangerous even under these conditions of operation, i.e., with boom vertical and with no load to couterbalance the crane counterweights.

However, it may be necessary at times to remove the boom completely and there is no danger if Wx stands inside pr between the fulcrum lines Si Sj—S₂ S₂, i.e., if g sin ψ is less than k. When g sin ψ=1·: for the prototype crane with neither boom nor load,

M'_ste=+39,600-89,000=-49,000 ft.-lb.

In this case, the value of M_st0 has passed the required safety margin with the specified counterweighting, so that a warning signal will be necessary. This need is made more important by the fact that a new operator coming on the job has no guarantee that the counterweights have not been changed. Provision will be made in my invention for automatic readjustment of the warning system for the case where removal of the boom or reduction of the load causes the value of M_st0 to become dangerous. This will be done without interposing switches, with their potential unreliability, in the normal operating channels for the large forces L, W_t and W₂.

Consider now the danger of tipping about the aft axle A A. The aft tipping moment, M_ati, due to load, L, is:

Λ(βΐι=/χΛΙ’ cos ¢+18 cos 9 cos ψ—/₂], +JF_B[i cos ¢+0.5 l₀ cos 0 cos ψ—j₂]

M_ati=/irL[z cos ψ+ΐο cos S cos ¢-^]

This condition is most dangerous for ψ=0, i.e., when the boom is extending rearwardly perpendicular to the aft axle AA, and a warning signal seems advisable when the net negative M_att becomes numerically small. The danger decreases as L is swung inward (with increasing ψ and (?) and vanishes when i’+ lo cos 0 cos ¢=/2 as L swings inside, i.e., forward, Of line A A. In the prototype crane, g is less than j₂, so there is little danger from the counterweighting. For other models in which we might find g greater than j₂> the simplest procedure would be to instrument a condition for backward stability by requiring a minimum load on the front wheels.

Lastly, consider the danger of tipping about the front or fore axle FF. The forward tipping moment due to load L, Mfti, with cos ψ understood as an absolute value without regard to sign, is:

Miti=cos ¢+1₀ cos 0 cos ¢-/1] +W_B[i cos ¢+0.5 i₀ cos ff cos ¢-/1] — W^h—nl—Wilg cos ¢+/1]

This is most dangerous for ¢=180° and a warning signal may be needed when the net negative M_itl becomes numerically small. However, if /χ is approximately equal to 3/₂, as in the prototype crane, there is little danger. Also, the danger of tipping decreases as L is swung inward, i.e., rearward of FF, and vanishes when i+lo COS 0 COS ¢=/1 as L swings inside or rearward of the line FF. There is no danger from the counterweighting because g is less than /i and this condition is likely to be so for most cranes.

We now see that overturn or tipping might occur (1) about Si Si or Sa Sa due either to L or to Wx and (2) about A A or FF due to L. Overturn moment about a single wheel, where these fulcrum lines intersect, is smaller and, this behavior being physically unlikely, will be neglected. Because it seems probable that an experienced crane operator will instinctively move his controls toward safety, regardless of the direction of the tipping tendency, it seems unnecessary to supply a compound visual indicator for his use. Such an indicator would only distract his attention from the loading area which he should be watching. An audible signal is much to be preferred. However, a group of simple visual indicators, might well be provided for setting up the system and checking its performance.

Simplicity and reliability should be the controlling characteristics for a crane warning system. To minimize the number of components necessary to compute all of the above tipping moments, it will be desirable to form inter3,079,080 mediate sums wherever possible. It has been helpful to reformulate the overturn or tipping moment equations as follows, so that for each situation the safe operating limit is reached when —M approaches zero:

Af_sti= [/_LL+0.5P7_B]i sin ψ+0.5)Τ_Β! sin ψ +l₀ cos 0[f_LZ.+O.5U^z_B] sin ψ—Wig sin ψ

-[^L+Ws+Wi+W^k

W_B[i sin V+0.5 l₀ cos Θ sin ψΊ+Wig sin ψ -LW_B+Wi+W₂1k+f_w[Wi+W212k cos ψ+O.SWzi cos ψ + ¾ cos 0[/i/+O.5I+] cos ψ—Wig cos ψ

- [/ιΛ+^β+^ι+ W₂lh-W₂n

M_itl=[/_LL+0.5JF_B]z cos ψ+0.5Ι7_Β! cos ψ +¾ cos ¢[/++0.51+] cos ψ—Wig cos ψ + [/++ Ws+Wi+WJh+Wtin

Using for intermediate sums the notations:

^=1++1++1++/^ $2=/ιΛ1ο+0·51+1ο ^3= [/++1+++¾ cos e—Wig these moment equations reduce to:

M_iti=Siji—W₂n—S_s cos ψ M_aii=Sij₂-]-W₂n—S_a cos ψ M_sii=Sik—S₃ sin ψ

Af_stc=SiA[l—2/+1+¾ sin ψ

FIG. 3 shows a block diagram of a warning system based on these reduced equations and using a minimum number of conventional components from the analog computer art. It is assumed in that figure that amplifiers have a gain of —1; that resolvers have a gain of +1; that voltage dividers represent factors less than unity; and that all components have sufficient adjustability to accommodate a range of material handling equipment including, but not necessarily limited to, power cranes and power shovels. Conceivably, certain types of firefighting equipment, tree surgery equipment, mobile concrete conveyers for high buildings and other similar equipment could be equipped with such a warning system.

Conventional analog computer components, either A.C. or D.C., may be used and only the load transducer might be considered a special item. Amplifiers may contain vacuum tubes, transistors or magnetic amplifier elements. Resolvers may be inductive or resistive. Voltage dividing resistors may be either wire-wound or carbon-film type. No manual switching is required and automatic switching is arranged for maximum reliability. Two variable resistors are used for representing the value of l₀ in order to avoid loading effects on the voltage dividers.

In normal operation, both visual indicator and audible signal are simultaneously effective for M_fH, M_ati, and M_stl, where danger comes from large values of L. On the other hand, where L is small, the danger of overturn comes from the counterweight, and M_stc should govern with L acting as a stabilizing effect. The transition takes place automatically via coil and normally open magnetic relay contacts 1, according to the value V set on the voltage dividing resistor. For any lower value of L, including zero and not excluding the possibility of 1+=0, the relay 1 closes and the warning system operates under M_st₀. Contactor 2, normally closed, represents a switch in the bearing for the boom foot; when open it removes the stabilizing effect of the boom.

Although the present invention has been disclosed in association with an automotive crane, it is to be understood that the inventive concept is not so limited. The inventive concept disclosed herein may be advantageously employed in any installation or equipment in which a computation of moments is desired. Differently expressed, it may be stated further that the present invention can be employed to advantage in connection with any equipment which is subject to variable loads at various angles between a portion of the equipment and the vertical and its various horizontal angles between the said portion of equipment and the generally longitudinal axis through 5 the supporting structure of said portion of equipment.

Further, while the invention has been particularly described in connection with a warning system for material handling equipment whereby the operator thereof may be warned of an incipient overturn condition for all posi10 tions of the load handling equipment, including the weights of the loads, it will be apparent to those skilled in the art that further embodiments and modifications may be made without departing from the scope and spirit of the invention as defined by the claims appended hereto.