Level luffing crane

Abstract

An articulated crane has an inner boom pivoted on a supporting base, an outer boom pivotally mounted on the inner boom, and a pair of hydraulic cylinder assemblies, one for moving each of the booms, respectively. The hydraulic cylinder assemblies may be interconnected when desired to coordinate their simultaneous movement, and they are so positioned with respect to the boom pivots that their coordinated movement causes the outer end of the outer boom to travel substantially horizontally.

Parent Case Text

This is a continuation-in-part of my copending application Ser. No. 779,403, filed Nov. 27, 1968 for Level Luffing Crane.

Description

This invention relates to hydraulically operated articulated cranes. As a type, these cranes have achieved commercial success because of their simplicity, flexibility in handling loads in a variety of positions, and economical price. Cranes of this type commonly employ a base member mounted on a turntable, an inner boom pivoted to the base member, and an outer boom pivoted to the inner boom. Separate hydraulic cylinder assemblies, one of each boom, are employed.

An important shortcoming of articulated cranes of this type is the difficulty of moving a load horizontally toward or away from the axis of the turntable, because both hydraulic cylinder assemblies must be operated at the same time, requiring very skillful operation of the manual control levers. Separate operation of each hydraulic cylinder assembly results in the load hook swinging in a vertical arc. Only by skillful operation of both controls at the same time is it possible to cause the load to move horizontally toward or away from the turntable axis. Such horizontal movement is known as "level luffing".

It is an important object of this invention to provide an improved hydraulically operated articulated crane that retains the advantages of simplicity, flexibility and price associated with the general design, but which in addition provides for automatic level luffing when desired.

Other and more detailed objects and advantages will appear hereinafter.

In the drawings:

FIG. 1 is a side elevation showing a preferred embodiment of my invention.

FIG. 2 is an end view thereof, partly broken away.

FIG. 3 is a side elevation showing a modified form of the invention.

FIG. 4 is a side elevation showing another modified form of the invention.

FIG. 5 is a side elevation showing still another modified form of the invention.

FIG. 6 is a side elevation of a somewhat similar device but which does not produce automatic level luffing.

FIG. 7 is a schematic diagram showing hydraulic connections for the devices of FIGS. 1 and 2.

FIG. 8 is a schematic diagram constituting a modification of a portion of FIG. 7.

Referring to the drawings, the base member 10 is carried on a stationary support 11 by means of a large horizontal bearing assembly 12. An inner boom 13 is pivotally mounted on the base member 10 and 14, and an outer boom 15 is pivotally connected to the inner boom at 16. The first hydraulic cylinder assembly 17 is operatively positioned between the base member 10 and the inner boom 13. Thus the outer shell of the assembly 17 is pivoted to the base member 10 at 18, and the piston rod end of the assembly 17 is pivotally connected at 19 to the bracket 20 fixed on the inner boom 13. Similarly, the hydraulic cylinder assembly 22 is operatively positioned between the inner boom 13 and the outer boom 15. The outer shell of the assembly 22 is pivoted at 23 to the bracket 20 and the piston rod end of the assembly is pivoted at 24 to the bracket 25 on the outer boom 15. From this description, it will be understood that actuation of the hydraulic cylinder assembly 17 serves to swing the inner boom 13 around the pivot 14, and actuation of hydraulic cylinder assembly 22 serves to swing the outer boom 15 around the pivot 16.

An hydraulically operated winch 27 of conventional design may be conveniently mounted on the inner boom 13 near the pivot 14. A cable 28 extending from the winch 27 passes over the pulley or sheave 29 which may be coaxial with the pivot 16. The cable then passes over the sheave 30 mounted at the swinging end of the outer boom 15, and this cable 28 pendantly supports the load hook 31. Operation of the hoist 27 to wind up the cable 28 serves to lift the load hook 31.

An hydraulic motor 33 mounted on the base member 10 is gear connected to the satisfactory support 11 in order to swing the entire crane assembly in either direction above the axis of the bearing 12.

At the operator's console 34 a plurality of manually operated control levers 35 are provided. One of these levers serves to operate the turntable motor 33, another operates the hydraulic cylinder assembly 17, another operates the hydraulic cylinder assembly 22, and the fourth lever hydraulically interconnects the hydraulic cylinder assemblies 17 and 22 to coordinate their simultaneous movement, so that the load-supporting hook 31 travels substantially horizontally toward or away from the turn-table axis. This fourth lever thus provides for automatic lever luffing. The hydraulic diagram of FIG. 7 shows how this hydraulic interconnection may be accomplished. Two hydraulic pumps 37 and 38 are driven from the same prime mover 39 mounted on the base member 10. The low volume pump 37 is connected through filter 40 to deliver hydraulic fluid under pressure to the valve 41 for the turntable motor and to the valve 42 for level luffing, described below. The high volume pump 38 delivers hydraulic fluid under pressure through the filter 43 to the valve 44 for the hydraulic cylinder assembly 17, and to the valve 45 for the hydraulic cylinder assembly 22, and to the valve 46 for the hydraulically operated hoist 27. Spring loaded relief valves 48 and 49 return hydraulic fluid to the central reservoir or sump, when the unit pressure exceeds the predetermined maximum. Other relief valves 50 and 51 are provided for the same purposes.

Each of the manually operable valves 41, 42, 44, 45 and 46 is shown in its intermediate blocking position in which no flow occurs to or from its respective work member. When the valve 44 is manually depressed, hydraulic fluid under pressure is delivered from line 53 to line 54, and line 55 to the rod end port 56 of the hydraulic cylinder assembly 17, thereby retracting the piston rod 17r. Hydraulic fluid in the lower portion of the assembly 17 passes out through the piston end port 57 and line 58 to the check valve 59. This check valve 59 is opened by pressure in the lines 55 and 60 from the pump 38, thereby allowing hydraulic fluid to pass from the line 58 through lines 63, 64 and 65 and back through valve 44. Similarly, manual movement of the valve 44 to an upper position connects line 53 with line 65 so that hydraulic fluid under pressure passes through lines 64 and 63, check valve 59, line 58 and port 57 to cause extension of the piston rod 17r. Hydraulic fluid in the rod end of the assembly 17 passes out through lines 55 and 54 and through the valve 44 to the line 66. The valve 44 is mechanically connected for operation by one of the console levers 35.

In a similar fashion, downward movement of the manually operable valve 45 delivers hydraulic fluid under pressure from line 66 through lines 68 and 69 to the rod end port 70 of the hydraulic cylinder assembly 22. Hydraulic fluid below the piston escapes through piston end port 71, check valve 72 (held open by pressure in line 73) and line 74 back to valve 45. Upward movement of the valve 45 reverses the flow to cause upward movement of the piston rod 22r.

The valve 46 may be moved in either direction to cause operation of the hoist 27 to wind up or pay out the cable 28. Similarly, the valve 41 may be moved in either direction to pressurize one of the lines 75 or 76 and cause return flow through the other line for the purpose of causing the turntable motor 33 to operate in either direction, as desired.

The level luffing valve 42 has three positions. In the center position illustrated in FIG. 7, no flow takes place through the valve. When the valve 42 is moved to the right, the pressurized line 77 from the pump 37 is connected to lines 78 and 55 leading to port 56. Back pressure in the line 77 acts through line 62 to cause the pilot valve 79 to connect line 63 to line 74 through interconnecting line 61. The back pressure in line 55 acts through line 60 to open check valve 59. Accordingly, hydraulic fluid passes from port 57 through lines 63 and 61, pilot valve 79 and line 74 into port 71. The hydraulic cylinder assemblies 17 and 22 are so proportioned that they require the same volume of hydraulic fluid during their respective level luffing strokes. Accordingly, hydraulic fluid is simply transferred from assembly 17 to assembly 22 so that, as the rod 17r retracts, the rod 22r is extended. The inner boom 13 and outer boom 15 move from the full line position "A" shown in FIG. 1 to the phantom line position shown at "B." Because of the transfer of hydraulic fluid during this operation, the path of the load-supporting hook 31 is shown by the line 81. While this path as shown by the line 81 is not precisely horizontal, it is near enough for commercial purposes and is referred to hereinafter as being substantially horizontal. In the fully retracted position shown at "B" in FIG. 1, the hydraulic cylinder assembly 17 is fully extended and the hydraulic cylinder assembly 22 is fully retracted. The intermediate position "C" illustrates independent action capability apart from level luffing between positions "A" and "B."

When it is desired to cause level luffing of the load-supporting hook 31 in the other direction, that is, away from the axis of the turntable, the valve 42 is moved to the left, as shown in FIG. 7. This serves to connect the pressure line 77 from the pump 37 to the line 81 and line 69 leading to port 70. This causes piston rod 22r to move downward. Hydraulic fluid passes out through port 71 and through check valve 72 which is held open by back pressure in the line 73. The hydraulic fluid then passes through the pilot valve 79 held by back pressure in lines 77 and 62 and then passes through line 61 and 63 to port 57 in the assembly 17. Accordingly, the piston rod 17r moves upward. The booms 13 and 15 move back from the position shown at "B" to the solid line position "A," all as shown in FIG. 1.

In the device shown in FIGS. 1 and 2, the dimensions of the cylinder assemblies 17 and 22 are chosen so that the total volume of oil discharged from cylinder assembly 22 is the same as that received by the cylinder assembly 17 (and vice versa) during the level luffing stroke. Also, at any given instant along the level luffing traverse, the unit pressure is substantially the same in these cylinder assemblies 17 and 22 because of the moment arm relationship of load and hydraulic cylinders. When they are interconnected relatively little additional pressure is required to cause a transfer of fluid from one cylinder assembly to the other and thus an articulating movement of the arms 13 and 15 results, which produces a level luffing of the load. Because the pressure and incremental volume released from one cylinder at any instant is so nearly equal to that expended in the other, the traversing movement is accomplished without the addition or release or work. Such movement can only be horizontal. Technically speaking, from physical principles, no work is performed on a load moving horizontally, neglecting friction and accelerations.

In order to achieve the level luffing operation, the angle X between the booms at the extended position is substantially less than 180.degree., and this angle decreases as the load is moved inward in the level luffing operation until the minimum angle Y is reached in the fully retracted position. In the form of the invention shown in FIGS. 1 and 2, the effective lever arm of the hydraulic cylinder 22 changes very little from the value L.sub.1 at the extended position "A" as compared to the value L.sub.2 at the retracted position "B." The hydraulic cylinder assembly 17 has its maximum lever arm L.sub.1 at the extended position "A" and this lever arm decreases to the minimum value L.sub.2 at the retracted position B." B" This relationship of the effective length of the lever arms of the hydraulic cylinder assemblies in the extended position "A" and the retracted position "B" produces the level luffing action, as shown by the following analysis:

To have level luffing, the arrangement and sizing of parts and cylinders must be such that the release of work from one cylinder assembly must equal that expended in the other.

In the arrangement of FIG. 1, when cylinders of equal volume are used, to obtain the equal work relationship the cylinder pressures must be equal to one another at any point in the transverse. If, at each increment of displacement along the traverse, the work is balanced, the traverse is level. If all other increments are balanced, then the total traverse is level. Expressed algebraically:

dv P.sub.17 = dv P.sub.22

Therefore

P.sub.17 = P.sub.22

where

dv = small volume of fluid transferred at some point during the level luffing traverse.

P.sub.17 = the pressure in cylinder 17

P.sub.22 = the pressure in cylinder 22 at that same point in the traverse.

To develop the necessary geometrical relationships to produce substantial level luffing, assume the pressures in both cylinder assemblies are equalized between them at either end of the traverse. The following derivation provides the relative change in cylinder moment arms that must transpire during a given traverse.

Taking the summation of moments about the pivot 16, when the booms are in the extended position:

Fd.sub.1 = P.sub.x A.sub.1 L.sub.1

where

F is the force or weight acting on the outer end of the boom 15;

d.sub.1 is the distance through which the force F acts;

P.sub.x is the unit pressure of fluid in the hydraulic cylinder 22 at extended position;

A.sub.1 is the area of the piston in the assembly 22 against which this hydraulic pressure acts;

L.sub.1 is the effective lever arm of the assembly 22 at extended position.

Solving for P.sub.x : ##EQU1##

Taking the summation of moments about the pivot 14:

FD.sub.1 = P.sub.x A.sub.2 L.sub.1

where

P.sub.x is the unit pressure of the fluid in the assembly 17;

A.sub.2 is the area of the piston in the assembly 17 which is subjected to this hydraulic pressure;

L.sub.1 is the effective lever arm of the assembly 17.

Solving for P.sub.x : ##EQU2##

Equating (1) and (2), because the unit hydraulic pressure in each of the assemblies 22 and 17 is the same: ##EQU3##

Solving for D.sub.1 : ##EQU4##

Taking the summation of moments about the pivot 16 when the booms are in the retracted position:

Fd.sub.2 = P.sub.r A.sub.1 l.sub.2

where

P.sub.r is the unit pressure in the hydraulic cylinder assembly 22 at retracted position;

l.sub.2 is the effective lever arm of the assembly 22 in retracted position.

Solving for P.sub.r : ##EQU5##

Taking the summation of moments about the pivot 14:

FD.sub.2 = P.sub.r A.sub.2 L.sub.2

Solving for P.sub.r : ##EQU6##

Equating (4) and (5) because the unit hydraulic pressure is the same in each of the assemblies 22 and 17: ##EQU7##

Solving for D.sub.2 : ##EQU8##

From the foregoing: ##EQU9##

In FIG. 1, d.sub.1 is substantially the same as d.sub.2.

Accordingly: ##EQU10## and when l.sub.1 is the same as l.sub.2 as in FIG. 1 ##EQU11## Stated in other words, in order to achieve level luffing with the FIG. 1 device, and with hydraulic pressure in the assembly 22 substantially equal to that in the assembly 17 at each point along the level luffing traverse, the assembly 17 must be mounted so that its effective lever arm L.sub.1 at the extended position "A" of the booms is proportional to the overall horizontal reach D.sub.1 of the booms at extended position, while the effective lever arm L.sub.2 of the assembly 17 at retracted position "B" is proportional to the horizontal reach D.sub.2 of the booms at retracted position. Accordingly, the ratio of the effective lever arm of the second hydraulic cylinder assembly 22 to the effective lever arm of the first hydraulic cylinder assembly 17 increases as the booms move away from extended position toward retracted position.

In the device of FIGS. 1 and 2 but using cylinders of unequal displacement, the flow proportioning device 200-201 of FIG. 8 is interposed, to provide a related proportioning of fluid between cylinders 17 and 22.

Again, a relation of equal work transfer requires at any instant of level luffing that:

dv.sub.17 .times. P.sub.17 = dv.sub.22 .times. P.sub.22

where

dv.sub.17 = small volume of fluid transferred to or from cylinder 17 at an instant;

dv.sub.22 = small fluid volume transferred to or from cylinder 22 at the same instant;

P.sub.17 = fluid pressure within cylinder 17 at instant of fluid transfer;

P.sub.22 = fluid pressure within cylinder 22 at instant of fluid transfer.

But

V.sub.17 .noteq. (does not equal) V.sub.22

therefore

dv.sub.17 .noteq. dv.sub.22

But an equal number of volume increments are displaced in each cylinder during the total level luffing traverse.

Therefore

ndv.sub.17 = V.sub.17

and

ndv.sub.22 = V.sub.22

where

n = number of volume increments

and

ndv.sub.17 .sup.. P.sub.17 = ndv.sub.22 .sup.. P.sub.22

therefore

V.sub.17 .sup.. P.sub.17 = v.sub.22 .sup.. P.sub.22 (10)

where

V.sub.17 = volume cyl 17

V.sub.22 = volume cyl 22

Therefore, to have an equal work relation in each cylinder the product of the volume of one cylinder times its pressure at a given instant must equal the product of the volume of the other cylinder times its pressure at the same instant.

From the equation (10) then the pressures are related ##EQU12## For convenience let ##EQU13## then

P.sub.17 = k P.sub.22

With this pressure relationship take moments at each end of the traverse to develop the desired geometric relationship:

Taking summation of moments about 16 in extended position: ##EQU14##

Taking summation of moments about 14 in extended position: ##EQU15## Equating (11) and (12) ##EQU16## Solving for D.sub.1 ##EQU17##

Taking summation of moments about 16 in retracted position: ##EQU18##

Taking summation of moments about 14 in retracted position: ##EQU19## Equating (14) and (15) ##EQU20## Solving for D.sub.2 ##EQU21## since d.sub.1 is substantially the same as d.sub.2, accordingly ##EQU22## Therefore in the case of a crane with cylinders of unequal volume the stroke and area relationships of the cylinders must be such that

V.sub.17 P.sub.17 = V.sub.22 P.sub.22

and the geometric relation of ##EQU23## must also exist.

If it is desired to refine the path of movement of the load-supporting hook 31 during the level luffing operation so that it more closely approaches an exact horizontal line, a cam-and-follower device may be employed for modifying the rate of transfer of hydraulic fluid between the assemblies 17 and 22. As shown in the drawings, this cam-and-follower device comprises a cam 84 fixed to the inner boom 13, together with a follower roller 85 held in contact with the surface of the cam 84. The follower roller 85 is mounted on the extending rod 86 of the hydraulic cylinder assembly 87 and the space below the piston is connected by line 87a to the line 74 to the pilot valve 79. In and out movement of the rod 86 under action of the follower roller 85 transfers hydraulic fluid into and out of the line containing the pilot valve 79, in accordance with the angular position of the inner boom 13. The varying volume of hydraulic fluid causes variations in the relative movement of the piston rods 17r and 22r to minimize deviation of the path of the load-supporting hook 31 from a true horizontal line.

In the modified form of the invention shown in FIG. 3 of the drawings, the parts are generally similar to those previously described, except that the outer boom 15a is provided with an extension member 100 telescopically mounted thereon. The stationary support 11a, turntable bearing 12a, base member 10a, inner boom 13a and pivotal connections 14a and 16a are all similar to corresponding parts previously described. The same is true of the hydraulic cylinder assemblies 17a and 22a and their respective pivotal connections 18a, 19a, and 23a, 24a.

A third hydraulic cylinder assembly 101 is provided for extending or retracting the extension member 100 with respect to the outer boom 15a. A sheave 102 is mounted at the projecting end of the extension 100 to receive a load-supporting cable, not shown.

The same hydraulic circuits previously described may be employed for providing independent operation of the hydraulic cylinder assemblies 17a and 22a, or for providing coordinated action thereof for accomplishing level luffing. The phantom line position at "A" in FIG. 3 shows the booms in retracted position, while the phantom line position at "B" shows the booms in a folded position for transport. In this latter position, the outer boom 15a underlies the position of the inner boom 13a.

It will be observed that the positions of the hydraulic cylinder assemblies 22a and 17a with respect to the booms 15a and 13a and with respect to the stationary support 10a are similar to those positions set forth in detail above with respect to the form of the invention shown in FIGS. 1 and 2.

The modified form of the invention shown in FIG. 4 has a base 10b pivotally supporting a first boom 13b at 14b, and a second boom 15b pivotally supported on the boom 13b at 16b. The first boom 13b is moved by the hydraulic cylinder assembly 17b and the second boom 15b is moved by the hydraulic cylinder assembly 22b. The hydraulic connections and the controls are the same as previously described. In this form of the invention, however, the hydraulic cylinder assembly 17b has a substantially constant effective lever arm L.sub.1 and L.sub.2, while the hydraulic cylinder assembly 22b has a short effective lever arm l.sub.1 in the extended position of the booms which increases to a long effective lever arm l.sub.2 in the retracted position. As shown by the mathematical analysis set forth in detail above: ##EQU24## and when L.sub.1 = L.sub.2 as in FIG. 4, ##EQU25##

The further modified form of the invention shown in FIG. 5 has a base 10c pivotally supporting a first boom 13c at 14c, and a second boom 15c pivotally supported on the boom 13c at 16c. The first boom 13c is moved by the hydraulic cylinder assembly 17c and the second boom 15c is moved by the hydraulic cylinder assembly 22c. The hydraulic connections and the controls are the same as previously described. In this form of the invention, however, neither of the hydraulic cylinder assemblies 17c or 22c has a substantially constant effective lever arm. Thus, the effective lever arm L.sub.1 of the assembly 17c decreases to L.sub.2 as the booms are moved from extended position to retracted position, while the effective lever arm of the assembly 22c l.sub.1 increases to l.sub.2 during the same movement of the booms. Thus, one effective lever arm increases as the other decreases, and vice versa. So long as the relationship of the parts is such that the relationship of equation (8) apply, substantial level luffing is achieved.

However, as equation (8) indicates, substantial level luffing action is not achieved if the boom-operating hydraulic cylinder assemblies both have substantially constant effective lever arms, that is, when L.sub.1 = L.sub.2, and l.sub.2 = l.sub.1. Moreover, level luffing is not achieved in other designs in which one or both of the hydraulic cylinder assemblies have effective lever arms which change from extended position to retracted position, unless the ratio of the effective lever arm of the second hydraulic cylinder assembly to that of the first hydraulic cylinder assembly increases as the booms move away from extended position toward retracted position. This is shown in the diagram of FIG. 6. In this construction, the hydraulic cylinder assembly 22d operating the second boom 15c has an effective lever arm which decreases from l.sub.1 when extended to l.sub.2 when retracted. Also, the hydraulic cylinder assembly 17d operating the first boom 13d has an effective lever arm which decreases from L.sub.1 when extended to L.sub.2 when retracted. The arched curved line 81d which shows the motion of the outer end of the second boom 15d is substantially higher at the center than it is at either end. Substantial level luffing action is not obtained because the required ratio of effective lever arms is not achieved.

The modification of the hydraulic diagram, as shown in FIG. 8, is for use when the hydraulic cylinder assemblies 17 and 22 are dimensioned so that they displace different volumes of oil during the level luffing stroke. This may be desirable in some cases for best economical use of materials. In such case, means are provided for proportioning the rate of flow of oil to produce the desired coordination of movement of the piston rods. Thus, two positive displacement pumps 200 and 201 are employed, mechanically connected by shaft 202. Hydraulic fluid leaving cylinder assembly 22 through port 71 during the level luffing stroke passes through valve 79 and is metered through pump 200 to tank. This causes pump 201 to draw oil from the tank and discharge it through check valve 59 into the cylinder 17 through port 57. The capacity of the pumps are proportional to the displacement volumes of the cylinders; if the cylinder 22 has twice the displacement volume as cylinder 17 during the level luffing stroke, then pump 200 is proportioned to have twice the capacity of the pump 201. In a similar fashion, when pump 201 meters hydraulic fluid from cylinder 17, the shaft-connected pump 200 draws fluid from the tank and discharges it through check valve 72 into cylinder 22 through port 71.

Having fully described my invention, it is to be understood that I am not to be limited to the details herein set forth but that my invention is of the full scope of the appended claims.

Claims

I claim:

1. An articulated crane, comprising in combination: a base member, an inner boom, pivot means mounting said inner boom on the base member, an outer boom pivotally mounted on the inner boom, means including a first extensible hydraulic cylinder assembly for independently swinging said inner boom with respect to said base member in an arc on both sides of the vertical position, said assembly defining a first effective lever arm with respect to said pivot means, means including a second extensible hydraulic cylinder assembly for independently swinging said outer boom with respect to said inner boom, said second assembly defining a second effective lever arm with respect to the pivotal mounting of the outer boom, said booms being movable in a traverse from an extended position in which the inner boom inclines upward on one side of the vertical and the outer boom extends further to the same side thereof and the angle between the booms being less than 180 degrees, to a retracted position in which the inner boom inclines upward on the other side of the vertical and the outer boom extends to the first mentioned side thereof, the positions of the booms, hydraulic cylinder assemblies, and pivotal connections being such that ##EQU26## where: D.sub.1 = horizontal reach of the booms when extended

D.sub.2 = horizontal reach of the booms when retracted

L.sub.1 = effective lever arm of said first hydraulic cylinder assembly extended

L.sub.2 = effective lever arm of said first hydraulic cylinder assembly retracted

l.sub.1 = effective lever arm of said second hydraulic cylinder assembly extended

l.sub.2 = effective lever arm of said second hydraulic cylinder assembly retracted

the cylinder assemblies being sized according to the relation

P.sub.1 V.sub.1 .congruent. P.sub.2 V.sub.2

where:

P.sub.1 = unit pressure in first hydraulic cylinder assembly at a given instant

P.sub.2 = unit pressure in second hydraulic cylinder assembly at the same instant

V.sub.1 = displacement volume of the first hydraulic cylinder assembly during said traverse

V.sub.2 = displacement volume of the second hydraulic cylinder assembly during said traverse

and hydraulic means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous action to cause the outer end of the outer boom to move substantially horizontally and above the level of said pivot means.

2. The device set forth in claim 1 in which l.sub.2 = l.sub.1.

3. The device set forth in claim 1 in which the means interconnecting the hydraulic cylinder assemblies is such that as one retracts the other extends, and vice versa.

4. An articulated crane, comprising in combination: a base member, an inner boom, pivot means mounting said inner boom on the base member, an outer boom pivotally mounted on the inner boom, means including a first extensible hydraulic cylinder assembly for independently swinging said inner boom with respect to said base member in an arc on both sides of the vertical position, said assembly defining a first effective lever arm with respect to said pivot means, means including a second extensible hydraulic cylinder assembly for independently swinging said outer boom with respect to said inner boom, said assembly defining a second effective lever arm with respect to the pivotal mounting of the outer boom, said booms being movable in a traverse from an extended position in which the inner boom inclines upward on one side of the vertical and the outer boom extends further to the same side thereof and the angle between the booms being less than 180.degree., to a retracted position in which the inner boom inclines upward on the other side of the vertical and the outer boom extends to the first mentioned side thereof, the ratio of the effective lever arm of the second hydraulic cylinder assembly to the effective lever arm of the first hydraulic cylinder assembly increasing as the booms move along the traverse away from extended position toward retracted position, and means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous action so that one retracts as the other extends, and vice versa, to cause the outer end of the outer boom to move substantially horizontally and above the level of said pivot means.

5. The combination set forth in claim 4 in which the hydraulic cylinder assemblies are proportioned so that they require the same volume of hydraulic fluid to complete their full strokes, respectively.

6. The combination set forth in claim 4 in which each hydraulic cylinder assembly has a piston end, and wherein said hydraulic means includes a valve-controlled passage connecting the piston ends of both hydraulic cylinder assemblies, for transfer of hydraulic fluid from one to the other.

7. The combination set forth in claim 4 in which an extension boom is telescopically mounted on said outer boom, and sheave means at one end of said extension boom.

8. The combination set forth in claim 7 in which full retraction of the second hydraulic cylinder assembly serves to swing the outer boom to a folded position under the inner boom.

9. The combination set forth in claim 4 in which

P.sub.1 V.sub.1 .congruent. P.sub.2 V.sub.2

where

P.sub.1 = unit pressure in first hydraulic cylinder assembly at a given instant;

P.sub.2 = unit pressure in second hydraulic cylinder assembly at the same instant;

V.sub.1 = displacement volume of the first hydraulic cylinder assembly during said traverse;

V.sub.2 = displacement volume of the second hydraulic cylinder assembly during said traverse.

10. An articulated crane, comprising in combination: a base member, an inner boom, pivot means mounting said inner boom on the base member, an outer boom pivotally mounted on the inner boom, means including a first extensible hydraulic cylinder assembly for swinging said inner boom with respect to said base member in an arc on both sides of the vertical position, means including a second extensible hydraulic cylinder assembly for independently swinging said outer boom with respect to said inner boom, said booms being movable between an extended position and a retracted position, in extended position the inner boom including upward on one side of the vertical and the outer boom extending further to the same side thereof, and the angle between the booms being less than 180.degree., in retracted position the inner boom inclining upward on the other side of the vertical and the outer boom extending to the first mentioned side thereof, the effective lever arm of the first hydraulic cylinder assembly remaining substantially constant, and the effective lever arm of the second hydraulic cylinder assembly increasing as the booms move away from extended position toward retracted position, and hydraulic means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous action to cause the outer end of the outer boom to move substantially horizontally and above the level of said pivot means.

11. An articulated crane, comprising in combination: a base member, an inner boom, pivot means mounting said inner boom on the base member, an outer boom pivotally mounted on the inner boom, means including a first extensible hydraulic cylinder assembly for independently swinging said inner boom with respect to said base member in an arc on both sides of the vertical position, said assembly defining a first effective lever arm with respect to said pivot means, means including a second extensible hydraulic cylinder assembly for independently swinging said outer boom with respect to said inner boom, said second assembly defining a second effective lever arm with respect to the pivotal mounting of the outer boom, said booms being movable between an extended position and a retracted position, in extended position the inner boom inclining upward on one side of the vertical and the outer boom extending further to the same side thereof, and the angle between the booms being less than 180.degree., in retracted position the inner boom inclining upward on the other side of the vertical and the outer boom extending to the first mentioned side thereof, the effective lever arm of the second hydraulic cylinder assembly remaining substantially constant, and the effective lever arm of the first hydraulic cylinder assembly decreasing as the booms move away from extended position toward retracted position, and hydraulic means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous action to cause the outer end of the outer boom to move substantially horizontally and above the level of said pivot means.

12. An articulated crane, comprising in combination: a base member, an inner boom, pivot means mounting said inner boom on the base member, an outer boom pivotally mounted on the inner boom, means including a first extensible hydraulic cylinder assembly for swinging said inner boom with respect to said base member in an arc on both sides of the vertical position, means including a second extensible hydraulic cylinder assembly for independently swinging said outer boom with respect to said inner boom, said booms being movable between an extended position and a retracted position, in extended position the inner boom inclining upward on one side of the vertical and the outer boom extending further to the same side thereof, and the angle between the booms being less than 180.degree., in retracted position the inner boom inclining upward on the other side of the vertical and the outer boom extending to the first mentioned side thereof, the effective lever arm of the second hydraulic cylinder assembly progressively increasing, and the effective lever arm of the first hydraulic cylinder assembly progressively decreasing as the booms move away from extended position toward retracted position, and hydraulic means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous action to cause the outer end of the outer boom to move substantially horizontally and above the level of said pivot means.

13. An articulated crane, comprising in combination: a base member, an inner boom, pivot means mounting said inner boom on the base member, an outer boom pivotally mounted on the inner boom, means including a first extensible hydraulic cylinder assembly for independently swinging said inner boom with respect to said base member in an arc on both sides of the vertical position, said assembly defining a first effective lever arm with respect to said pivot means, means including a second extensible hydraulic cylinder assembly for independently swinging said outer boom with respect to said inner boom, said second assembly defining a second effective lever arm with respect to the pivotal mounting of the outer boom, said booms being movable in a traverse between an extended position and a retracted position, in extended position the inner boom inclining upward on one side of the vertical and the outer boom extending further to the same side thereof, and the angle between the booms being less than 180.degree., in retracted position the inner boom inclining upward on the other side of the vertical and the outer boom extending to the first mentioned side thereof, and the angle between the booms being less than at extended position, the ratio of the effective lever arm of the second hydraulic cylinder assembly to the effective lever arm of the first hydraulic cylinder assembly increasing as the booms move away from extended position toward retracted position, whereby substantially equal pressures are induced in said hydraulic cylinder assemblies at each point along said traverse, and means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous action to cause the outer end of the outer boom to move substantially horizontally and above the level of said pivot means.

14. The combination set forth in claim 13 in which the hydraulic cylinder assemblies displace different volumes of fluid in their respective strokes between extended position and retracted position.

15. The combination set forth in claim 14 in which two positive displacement pumps are provided, one connected to each hydraulic cylinder assembly, respectively, the pumps being mechanically connected for dependent rotation.