Extensible crane

Abstract

A crane with a telescoping boom assembly has a cable arrangement which multiplies the stroke of a hydraulic piston to extend one or more extensible boom sections while providing for automatic load leveling with an advantageous distribution of forces on the boom assembly when the boom is extended and elevated. A topping arrangement utilizes a piston operative in a substantially horizontal line. The crane is mounted on a turntable for swinging. The operations of hoisting, swinging, topping and telescoping can be independently and cooperatively controlled for full freedom of motion. Maintenance and repair are facilitated.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to extensible cranes, and more particularly to a crane with combined piston and cable telescoping means cooperating with swinging, topping and hoisting mechanisms.

2. Description of the Prior Art

Extensible cranes of the prior art have employed various arrangements for extending boom sections from a retracted position to an extended length. One common arrangement has used several interconnected pistons and cylinders, one piston for each boom section to be extended. Such an arrangement requires elongated pistons which are subjected to severe bending forces and are therefore unreliable and expensive to maintain.

Other prior art cranes have employed external cables to pull boom sections out to an extended position. Such arrangements are awkward and difficult to control.

Attempts have been made in the past to combine the advantages of piston actuation with the use of cables for extending a boom assembly. However, these arrangements have not gained general acceptance by industry.

U.S. Pat. No. 2,833,422 concerned a telescoping boom arrangement for use in conjunction with a digging implement or the like, and the patent was concerned with forcefully pushing and pulling the boom end carrying the digging element in a horizontal plane.

U.S. Pat. No. 2,919,036 related to a piston actuated extensible boom, and attempted to provide a reeving arrangement for a hoisting cable from a winch to a block which would compensate for extension of the boom to keep a load level.

U.S. Pat. Nos. 3,029,954, 3,308,967 and 3,465,899 related to cranes having cables running externally above their booms. U.S. Pat. No. 3,638,806 concerned a crane with tubular boom sections employing a cable running above the boom sections.

U.S. Pat. Nos. 3,047,160 and 3,282,411 related to arrangements for swinging and extending booms.

U.S. Pat. No. 3,307,713 related to a mast mounted telescoping boom having a piston mounted above a first boom section and cooperating with cables for extending further boom sections. The section extension cables run over sheaves angularly mounted within the boom sections.

U.S. Pat. No. 3,605,358 concerns a truck crane with a telescoping boom.

As evidenced by prior art references cited above, the topping or luffing of booms according to the prior art has usually been accomplished by pushing upward on a boom section, using rigidly mounted hydraulic assemblies. This type of arrangement does not provide the most efficient use of power for raising the boom. Very powerful pistons have been needed for elevation of large prior art booms.

To swing or slew booms, the prior art cranes have frequently relied upon rotating turntables with arrangements for braking through the turntable transmission mechanism. Such arrangements place large strains on the transmission, requiring frequent and expensive service.

In general, there have been few significant advances in the crane industry in recent years, despite the need for more effective and economical load handling.

SUMMARY OF THE INVENTION

The present invention overcomes numerous disadvantages inherent in prior art crane constructions while providing an economical telescoping crane having great flexibility of operation.

In accordance with the invention four basic crane functions of hoisting, swinging, luffing and telescoping are so coordinated that the crane of the invention can be effectively used for easy performance of material handling functions that are difficult, if at all possible, with conventional cranes. Thus, for example, it is possible to pick up a load with the crane of the invention extended at a generally horizontal attitude, then to lift the load while simultaneously retracting and elevating the boom assembly, to swing the load to the desired direction and to lower and again extend the boom assembly to place the load at its desired location, the transportation of the load being accomplished without inadvertently changing the load distance from the boom end. Such an operation which includes retraction and extension of the boom assembly while carrying a load permits easy operation in confined spaces such as in loading containers onto ships, or through narrow openings, etc.

A crane according to the invention may be mounted aboard a ship, on a railroad car, or some other wheeled vehicle, or in a permanent position on a fixed foundation.

Another advantage of the present invention is that very large loads can be handled. It is believed that cranes according to the present invention can be made to handle greater loads over greater distances than in prior art extensible crane arrangements.

In a preferred embodiment of the invention the crane proper is mounted on a turntable which is rotatable with respect to a platform pedestal or base. An anti-friction bearing is provided between the turntable and the platform and relative rotation is effected by means of a rack and pinion arrangement, preferably with the gear teeth cut in a race on the platform and the pinion gear coupled to a transmission on the turntable.

Limit switches can be advantageously provided to prevent excessive rotation. Braking can be suitably effected through a pair of band brakes mounted on the relatively fixed platform for braking the turntable.

Luffing or topping of the boom assembly is achieved by means of a novel hydraulic cylinder piston and linkage which provides great effective force at the crane positions at which force is most needed; that is, when the boom assembly is horizontal or near horizontal. The cylinder and piston have a substantially horizontal orientation, but are pivoted at one end for some upward tilting as the boom assembly moves to its greatest angle of elevation. A novel linkage between the piston and the rear end of the boom assembly converts the substantially horizontal piston motion into a raising motion to elevate the boom assembly. By using a horizontal piston orientation according to the invention it has been possible to provide a self-aligning and floating arrangement which avoids the stresses imposed on the rigidly mounted hydraulic topping mechanisms of existing cranes.

The telescoping function by which one, two or more boom sections are extended from a first, or shipper, boom, utilizes a single, relatively short, hydraulic piston and cylinder assembly (or two parallel mounted piston and cylinder assemblies for heavy duty use), rather than a series of separate pistons for each boom section as in some prior art extensible cranes.

A cable arrangement effectively doubles the stroke of the piston for each extensible boom section, so that if there are three boom sections, a piston stroke of 10 feet will project a second section or center boom a distance of twenty feet, and will project a third section or extension boom a distance of 40 feet.

For extending the center boom, gear racks mounted on the shipper boom and center boom cooperate with a pinion mounted in a crosshead on the piston to double the effective length of the piston stroke. A closed cable loop reeved on sheaves within the center boom and secured to the inner end of the extension boom is effective to project the extensible boom with respect to the center boom as the center boom is itself projected outward by the piston through its rack and pinion movement.

The above-described telescoping mechanism permits the use of a relatively short piston and cylinder which can be operated with great force.

The hoisting tackle includes a line reeved to cooperate with the boom telescoping system so that the load ordinarily automatically remains a fixed distance from a head sheave on the outer end of the extension boom regardless of the extent to which the boom is extended, and even while the boom is being extended or retracted. The hoisting line can thus preferably be reeved over a sheave near the outer end of the shipper boom, back through the shipper boom to a sheave mounted near the inner end of the center boom, and thence through the center and extension booms to the tackle at the outer boom end. This arrangement automatically compensates for any boom extension. In some assemblies, two lines can be advantageously used, one at each side of the boom assembly. An alternative cable arrangement using a movable sheave is especially effective in certain preferred embodiments of the invention.

The invention also provides for the effective utilization of the tensile strength of the hoisting cable to relieve the boom sections of a large part of the load thereon when the booms are extended and elevated.

A sensing arrangement can be effectively provided at the head sheave at the boom end to prevent pulling the hoisting tackle against the boom end. Instead of the complex and unreliable electrical and mechanical arrangements which have been proposed and sometimes used in the prior art, the safety mechanism of the present invention uses a simple and effective wire that follows the path of the hoisting cable itself from a sensing bar or ring near the head sheave back to a control at the hoisting winch.

These and other new and advantageous features of the crane of the invention will be more fully understood from the following detailed description of a preferred embodiment of the invention when taken in conjunction with the accompanying drawing

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view from the side of a crane according to the invention with a three section boom elevated and extended.

FIG. 2 is a side view similar to that of FIG. 1 with the boom lowered and retracted.

FIG. 3 is a side view similar to those of FIGS. 1 and 2 with the boom lowered but extended.

FIG. 4 is a view from above, of the crane in the position shown in FIG. 3.

FIG. 5 is a side plan view of a three section boom according to the invention in retracted condition with internal parts shown by shadow lines.

FIG. 6 is a view of the boom of FIG. 5 in an extended condition.

FIG. 7 is a view similar to that of FIG. 5 and showing a two-section boom.

FIG. 8 is a view of the boom of FIG. 7 in extended condition.

FIG. 9 is a view from above of a topping mechanism of the crane according to the invention.

FIG. 10 is a side view of the topping mechanism of FIG. 9 with an elevated position shown in shadow lines.

FIG. 11 is a view in section of the mechanism of FIG. 10 taken along the line 11--11 and looking in the direction of the arrows.

FIG. 12 is a view similar to that of FIG. 5 and showing a safety mechanism.

FIG. 13 shows the boom assembly of FIG. 12 with the safety mechanism in operation

FIG. 14 is similar to FIG. 13 except that the safety mechanism is in its normal inoperative state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An extensible crane having a telescoping boom assembly according to the invention is shown generally in FIGS. 1-4 in which details have been ommitted for the sake of simplicity. Thus FIG. 1 shows the crane generally indicated by reference numeral 10 to comprise three boom sections 11, 12 and 13, a turntable 14, a counterweight W, and means 15 for rotating the turntable 14 with respect to a base (not shown). When reference is made hereafter to a three section boom assembly the first and largest boom section 11 will be referred to as a shipper boom, the section 12 will be called a center boom and the section 13 will be called an extension boom.

The extending and elevating motions of the boom assembly of the crane 10 can be understood by comparison of FIGS. 1-4, wherein FIG. 2 shows the boom sections retracted and lowered to a generally horizontal position; FIGS. 3 and 4 show the sections 11, 12 and 13 extended but not elevated and FIG. 1 shows the crane with the boom assembly extended and elevated. When it is understood that the crane is also rotatable about its base, all of the movements of rotation (swinging), elevation (topping), extension (telescoping) and hoisting being concurrently and/or independently operable, it will be appreciated that the crane of the invention is highly versatile and adaptable to a wide variety of uses. For example, the crane of the invention is particularly well adapted to use on board a ship, where weight and space limitations are important considerations.

EXTENSION

Reference is now made to FIGS. 5 and 6 showing the boom assembly of a crane according to the invention having three boom sections. In FIGS. 5 and 6 the shipper boom 11 and the center boom 12 are shown as being generally hollow, rectangular and box-like in cross-section, whereas the extension boom 13 is of generally rectangular cage construction for lightness in weight, but it should be understood that any or all of the boom sections could be of either box-like or cage construction if desired. The boom section 13 is sized to fit telescopingly and slidably with the center boom 12 and the center boom 12 is sized to fit telescopingly and slidably within the shipper boom 11. Suitable bearings must, of course, be provided for smooth sliding motion of the sections during extension and retraction.

The shipper boom 11 has a pair of rearwardly and downwardly extending ears for pivoting connection to an elevating linkage to be described hereinafter. A piston and cable arrangement is utilized to telescopically extend and retract the boom assembly whereby a piston having a stroke about one-fourth the length of the maximum boom extension serves to project the center and extension booms to their fully extended length. Thus if the piston moves through a ten foot stroke, the center boom will be extended by a length of 20 feet and the extension boom will be extended by a distance of 40 feet.

As shown in FIG. 5 and 6, the shipper boom 11 carries an internally mounted hydraulic cylinder and boom extending piston assembly 17. A piston rod 18 mounted for sliding motion in and out of the cylinder carries a gear 19 at its forward end to cooperate with upper and lower racks 20 and 21 fixedly mounted on the center boom 12 and the shipper boom 11 at the lower portions thereof. Preferably there will be two sets of racks 20 and 21, one set at each side of the boom assembly to distribute stresses evenly. In an arrangement with two laterally spaced sets of racks 20, 21 there would be two cooperating gears or pinions 19, one gear 19 cooperating for rack and pinion drive with each set of racks 20 and 21. Two such gears 19 can, as will be understood by those acquainted with mechanical apparatus, be carried by a transversely extending crosshead mounted at the forward end of single centrally located piston rod 18. The hydraulic cylinder assembly 17 can suitably be equipped with an over-center balanced valve mounted on the lower side of the cylinder with a pilot line connected to a false load piping arrangement to compensate for the low pressure which would be required to extend the boom in the horizontal direction as in FIGS. 2-4, or the possible negative pressure when the boom is positioned below horizontal.

The telescoping spur gear and rack can, for example, advantageously comprise two 19-tooth spur gears and 12 rack sections, six of said rack sections being bolted to the underside of the center boom and six other rack sections mounted on a frame that is bolted to the underside of the shipper boom. To remove the hydraulic cylinder assembly 17, 18, the rack frame can be lowered to unmesh the gears.

It will be understood that the rack and pinion movement effectively doubles the piston stroke of the rod 18 in extending and retracting the center boom 12. Instead of the prior art technique of using another piston to extend the boom 13, which is troublesome since such a piston would have to move with the center boom 12, a cable arrangement is used.

Inside the center boom 12, a pair of laterally spaced cables 25, (one such cable being shown in FIGS. 5 and 6) form closed loops running over sheaves 26 and 27. Both ends of the cable 25 are secured to the shipper boom 11 near the forward end of the shipper boom at 28 and 28' and the sheaves 26 and 27 are mounted respectively within the center boom 12 near the rear and front ends thereof. At a point 29, a central portion of the cable 25 is secured to the rear end of the extension boom. Accordingly, when the center boom 12 is advanced by the piston 18 through the rack and pinion arrangement 19, 20, 21, the cable 25 travels around the sheaves in a clockwise direction and the point 29 advances toward the right as shown in FIGS. 5 and 6. This serves to advance the extension boom at twice the speed of the center boom.

The cables 25 can suitably be strong wire ropes welded or otherwise fixedly connected at the points 28, 28' to the wall structure of the center boom 12. To equalize pulling tension in both cable loops 25, a bearing arrangement can be used to carry the sheaves of the respective cables at opposite ends of a bar mounted transversely accross the boom section and journalled for limited angular movement about its midpoint.

An extensible boom assembly having two boom sections instead of the three sections already described is shown in FIGS. 7 and 8, wherein the boom section 41 generally resembles the shipper boom 11 of FIGS. 5 and 6, and the forward boom section 42 is constructed similarly to the center boom 12 described with respect to FIGS. 5 and 6, with pivot ears 46 and box-like cross-section. The two boom section arrangement has no cable extension arrangement, the boom section 42 being extensible and retractible by means of a cylinder 47, piston rod 48, gear 49 and racks 50 and 51 generally corresponding to the elements 17, 18, 19, 20 and 21 described in the three boom section assembly. There are certain important differences between the three section and two section boom assemblies which will be apparent from the following description of the cable reeving arrangements for hoisting a load in the different embodiments.

HOISTING

As shown in FIGS. 5-8, a hoisting winch generally indicated by the reference numeral 60 is mounted at the rearward end of the shipper boom 11. The winch 60 is preferably hydraulically powered and should have its own built-in holding brake. The hoisting drum 61 of the winch sized to hold enough windings of cable or rope for all operations of the crane. In a typical embodiment a crane according to the invention with a full boom extension of about 60 feet could have a hoisting drum with a bare diameter of 16 inches for 3/4 hoisting rope. Such a drum can have a capacity of 400 feet or more of 3/4 inch wire rope.

In the embodiment of FIGS. 5 and 6 it will be seen that the hoisting rope 62 passes above the outside upper part of the shipper boom 11 to a reversing sheave 63 mounted in a housing atop the forward end of the shipper boom. After passing over the reversing sheave 63 the rope 62 is carried back to pass around an idler sheave 64 mounted within the rearward end of the center boom, where the rope 62 again reverses direction to pass forward within the center boom 12 and through the extension boom 13 to a further idler sheave 65 near the forward end of the extension boom 13 and thence to a head sheave 66. A load hoisting hook is shown at 67.

It is an important feature of the hoisting arrangement of the present invention that the reeving of the hoisting rope is so arranged that the distance between the hook 67 and the center of the head sheave 66 does not change during extension and retraction motions of the boom sections unless the winch 60 is in operation. This automatic load levelling is effected in the triple boom arrangement of FIGS. 5 and 6 by the fact that the length of the portion of the hoisting rope 62 taken up in its passage back to and forward from the idler sheave 64 automatically compensates for the amount of extension of the center boom 12 and extension boom 13.

A similar load-levelling arrangement is effectively provided in the two-boom section assembly of FIGS. 7 and 8 by the employment of a sheave 74 that is mounted for longitudinal motion within and with respect to the boom section 41. In FIGS. 7 and 8, as in the three-section boom arrangement of FIGS. 5 and 6, there is shown a hydraulic winch 70 with its drum 71 and a hoisting rope 72 extending to and over a reversing sheave 73. From the reversing sheave 73 the hoisting rope 72 passes back to and around the movable sheave 74 and thence to the idler sheave 75 and the head sheave 76 at the forward end of the boom 42. Unlike the idler sheave 64 shown in FIGS. 5 and 6, the sheave 74 is not fixed in position at the rearward end of the boom section 42, but is free to move forward with respect to the boom 41 at a slower rate than the boom section 42. Comparison of FIGS. 7 and 8 will show that as the boom section 42 moves from its retracted position in FIG. 7 to its extended position in FIG. 8, the sheave 74 moves forward only about half as far as the boom section 42, thus compensating by taking up less of the length of the hoisting rope 72 when the boom assembly is extended.

The extent of the forward motion of the sheave 74 upon extension of the boom assembly is controlled by a cable 80 attached at one end at 81 to a point on the rear end of the boom 42. The cable 80 passes over a sheave 82, back through the boom section 41 and over sheaves 83 and 84 to the sheave 74 to which the other end of the cable 80 is secured. Thus the cable 80 is played out over the sheave 82 as the boom section 42 is extended forward and the sheave 74 attached to the other end of the cable 80 is allowed to advance at a rate half as fast as the speed at which the boom 41 moves forward. This arrangement automatically keeps the hook 77 at a constant distance from the center of the head sheave 76 when the winch 70 is not in operation.

ELEVATION

The elevation or topping of the boom assembly in a crane according to the invention can best be understood with reference to FIGS. 9 - 11 of the drawing.

In FIGS. 9-11 the lower end of the shipper boom 11 of the boom assembly is shown mounted for pivotal topping motion about an axis 90 provided by a rod 91 firmly journalled in a support fixture provided by fixed members 92 mounted on the platform 14 as shown in FIGS. 1 and 11. Power for the topping movement is provided by a hydraulic cylinder 93 mounted in a generally horizontal position beneath the lower end of the boom section 11. The boom elevating piston 94, movable upon hydraulic actuation in and out of the cylinder 93, carries a crosshead 95 linking the piston 94 to both sides of the boom section 11 through a linkage which converts generally horizontal movement of the piston 94 into elevating motion of the boom assembly about the axis 90. The arrangement of the topping linkage according to the invention causes the piston 94 to exert its greatest topping effort when the boom is horizontal, that is, when such effort is most needed. An over-center balanced valve is preferably mounted on the lower end of the cylinder 93 with a pilot line connection from the line supplying fluid to the piston end of the cylinder, requiring power for lowering the boom and thus acting as a safety device in case the fluid power line to the cylinder should be accedentally severed.

Referring to FIGS. 9 and 10 it will be seen that the crosshead 95 is pivotally connected at its ends 96 to rearwardly extending links 97 which are in turn pivotally jointed at 100 to short linking pieces 101. The linking pieces 101 are themselves pivotally secured to the ears 16 at the lowermost end of the boom section 11 by means of strong pins 102. Through this linkage, forward, generally horizontal, motion of the piston 94 and crosshead 95 is converted to downward and forward motion of the ears 16 of the shipper boom 11, and consequent topping movement of the boom assembly.

Besides the pivotal connection to the linking piece 101, each link 97 has an upwardly projecting portion 103, pivotally joined through a pin 104 to a generally L-shaped member 105. Each of the L-shaped members 105, at its end remote from the pin 104, is joined pivotally to the adjacent fixed member 92 by a pin 106. The pivotal connections provided by the pins 106 are substantially directly above the axis 90 about which the boom section 11 pivots in its topping motion. The L-shaped members 105 accordingly constrain the links 97 and thereby the piston 94 to move in a generally horizontal direction. The shadow lines of FIG. 10 show the boom elevated through a substantial angle with respect to the generally horizontal orientation shown in solid lines. It will be noted that the piston 94 is only slightly raised from the horizontal with the boom in its elevated position.

It will be understood by those acquainted with the mechanical arts that by using a piston moving in a generally horizontal direction, power is very effectively controlable. The weight of the piston is not an important consideration. Actually, when the boom assembly according to the invention is elevated through a very substantial arc, say of 70.degree. or 80.degree. above the horizontal, the piston will tend to move slightly upward out of the horizontal direction, and a pivotal connection is indicated at 110 at the rear of the cylinder 93 to permit such minor pivoting motion as may be required. But through most of the useful range of elevating and lowering motion of the crane, the cylinder 93 and piston 94 operate in a substantially horizontal orientation.

In some applications it may be desirable to operate a crane with its boom extending somewhat downwardly in an orientation below horizontal. It will be readily understood that suitable adjustments can be made in the linking mechanism of FIGS. 9 - 11 to keep the cylinder 93 at or near horizontal through the most widely used range of degrees of elevation of the crane boom.

SAFETY DEVICE

Yet another feature of the crane of the invention is a novel safety arrangement illustrated in FIGS. 12-14 for keeping hoisting tackle from being damaged or jammed by forceful abutment against the boom end or head sheave. For this purpose a flexible wire 120 can be led along a path corresponding in length and direction to the path followed by the hoisting cable from an on/off switch 121 near the winch to a safety ring 122 above the hoisting tackle. Thus when the hoisting tackle contacts the ring 122, the wire 120 will mechanically carry a force to the switch to stop the winch. Such a safety system is more readily applicable to a crane of the type described, with its mainly internal arrangement of the hoisting cable 62, than to other types of cranes, which in the past have utilized complicated and unreliable electrical safety systems.

As shown in FIGS. 12--, the ring 122 is flexibly supported by a wire 123 haging from a mechanical fastener or the like 124 in the very forwardmost end of the extension boom 13. The inner, or lower end of the ring hangs from the end of the wire 120. Thus when the block, shown at 66 approaches too close to the end of the boom 13, the ring 122 is tipped, the wire 120 is loosened, and the switch 121, which is suitably spring-biased, is released to turn off the winch. The pulleys or sheaves carrying the wire 120 can be suitably mounted adjacent to, and optionally on the same axis as, the sheaves for the hoisting cable 63, 64 and 65. Accordingly similar reference numerals 63a, 64a, and 65a designate the pulleys carrying the wire 120.

The ring 122 could be an annulus, or could be horseshoe shaped or of some other form, but must be free to move upon contact by the block 126 or other tackle.

OPERATION

Besides the aforementioned flexibility of the crane of the invention which permits topping and extending of the boom assembly to be acutated independently and/or concurrently, it will be understood that by mounting the power cylinder 93 on the platform 14, it is also possible to slew or swing the crane independently of, or concurrently with, the other crane operations. Such an arragement provides for simplicity of power transmission and weight reduction in comparison to prior art cranes.

The effective distribution of load forces in a crane according to the present invention is yet another advantage provided by the cooperating piston and cable arrangement for boom extension. Referring again to FIGS. 5 and 6 it will be seen that the cables 25 pass through the upper portions of the boom when the boom is viewed as horizontal, whereas the piston rod 18 is positioned at the lower portion of the boom. When the boom is topped, all of the weight of the boom and whatever load is being carried is not borne by the piston alone, but some of this load is borne by the cable 25 as tension forces. Thus the distribution of forces tends to be self-centering within the boom assembly, with the result that lighter weight construction is possible.

The several embodiments of the crane of the invention will suggest numerous modifications and adaptations within the spirit and scope of the present invention to those acquainted with the art. What has been disclosed is a new and versatile crane.

Claims

What is claimed is:

1. A crane comprising means for hoisting a load including a winch and a hoisting cable; a telescoping boom assembly carrying said hoisting cable on sheaves for compensation of free cable length upon extension and retraction, said boom assembly including a first boom and a second boom extensible from said first boom by means of a boom extending piston in a cylinder carried by said first boom, said boom extending piston carrying a pinion for cooperation with gear racks on said first and second booms to double the extension distance produced by motion of said boom extending piston; means for rotating said boom assembly with respect to a base to swing a load; means including a generally horizontally mounted boom elevating piston and cylinder assembly, the cylinder of said boom elevating assembly being pivotally mounted for limited motion with respect to the horizontal upon elevation of the boom, a linkage operatively coupling a forward end of the boom elevating piston to the boom for conversion of substantially horizontal piston motion to topping movement of the boom, said linkage comprising a crosshead carried by the boom elevating piston, a pair of generally rearwardly extending arms carried by said crosshead, a relatively short linkage pivotally connected to a rear end of each of said arms and to a part of the boom which moves down as the boom is elevated, and a relatively long, generally L-shaped link pivotally connected near the rear end of each said arm and to a fixed frame element; said hoisting, telescoping, swinging and elevating means being concurrently and independently operable for transporting a load by any combination of hoisting, telescoping, elevating and swinging.

2. The crane according to claim 1 and wherein said boom assembly includes a third boom projectable from said second boom by means of a cable loop carried by said second boom, said cable loop cooperating with said piston and cylinder carried by the first boom for concurrent extension of said second and third booms.

3. The crane of claim 1 wherein said hoisting cable is carried by a sheave movable longitudinally with respect to said first boom from a first position near the inner end of said second boom when said second boom is in a retracted state to a second position longitudinally spaced from said first position by a distance equal to about half the distance by which the second boom is extended.

4. The crane of claim 1 and including hoisting tackle on said hoisting cable and a safety device limiting the motion of said tackle toward a head sheave at the outer end of said boom assembly, said safety device including a sensing member located adjacent said head sheave for sensing the presence of said tackle at a predetermined position, said sensing member being mechanically connected to switch for turning off said winch upon sensing the presence of said tackle at said point.

5. The crane according to claim 4 wherein said switch includes a spring biased switch lever normally in a switched-on position.

6. The crane of claim 4 wherein said sensing member is connected to said switch by a flexible wire, said wire following a path substantially parallel to the path of the hoisting cable.

7. The crane of claim 1 wherein said boom assembly is movable through an angle of about 90.degree. from a lowered to an elevated position.