Telescopic boom extension device

A telescopic boom extension device includes a cylinder-boom coupling mechanism, an inter-boom fixing mechanism, and a driving mechanism that drives the foregoing two. The driving mechanism has a hydraulic supply part and a drive source generation part. The hydraulic supply part includes an AOH and is provided at a cylinder tube of a telescopic boom. The hydraulic supply part supplies operating oil selectively to actuators driving a B pin and a C pin. The drive source generation part includes an air hose and supplies compressed air to the AOH. The air hose is wound around a hose reel in an unrollable manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority from Japanese Patent Application No. 2015-019898, filed on Feb. 4, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a telescopic boom extension device for extending and retracting a telescopic boom mounted on a mobile crane.

Related Art

Mobile cranes such as a rough-terrain crane, for example, generally include a multistage telescopic boom. The telescopic boom is extended and retracted using a hydraulic cylinder in general. In particular, devices for extending and retracting a telescopic boom using a single double-acting hydraulic cylinder have been proposed (hereinafter, referred to as “extension device”) (for example, refer to JP 7-267584 A, JP No. 4612144, and JP No. 4709415).

The extension device is structured as described below.

The multistage telescopic boom includes bottom-stage and top-stage booms so called the base boom and the top boom, respectively, and one or more booms placed between the foregoing booms, which are so called intermediate booms. When the telescopic boom includes a plurality of intermediate booms, the intermediate boom adjacent to the top boom is referred to as a first intermediate boom, and the other intermediate boom adjacent to the first intermediate boom is referred to as a second intermediate boom while the other intermediate boom adjacent to the second intermediate boom is referred to as a third intermediate boom, and so forth. Each of the booms extends (slides forth) and retracts (slides back) relative to the adjacent boom and is kept by a boom fixing pin (hereinafter, referred to as “B pin”) in the fully-retracted state and the fully-extended state. In the telescopic boom, the top boom is extended first, sequentially followed by the intermediate booms.

At the extension device, one end (cylinder rod-side end) of the single hydraulic cylinder is coupled to the base end of the base boom. When the booms are in the fully-retracted state, the adjacent booms are coupled together by the B pins. First, a cylinder tube of the hydraulic cylinder is coupled to the top boom. The two are coupled by a cylinder fixing pin (hereinafter, referred to as “C pin”), and the B pin is removed from between the top boom and the first intermediate boom to allow the top boom to slide relative to the first intermediate boom. When the hydraulic cylinder extends in this state, the top boom extends relative to the first intermediate boom.

When the top boom enters in the fully-extended state relative to the first intermediate boom, the top boom is coupled again to the first intermediate boom by the B pin. The C pin is removed from between the top boom and the hydraulic cylinder to retract the hydraulic cylinder. Then, the hydraulic cylinder is coupled to the first intermediate boom by the C pin, and the B pin is removed from between the first intermediate boom and the second intermediate boom to extend the hydraulic cylinder in this state. Accordingly, the second intermediate boom extends relative to the third intermediate boom. In this manner, each of the booms extends sequentially relative to the adjacent boom, and the entire telescopic boom is finally in the fully-extended state. In the reversed manner, the telescopic boom is retracted.

At a conventional extension device, the B pins and the C pins are driven by a hydraulic actuator. The hydraulic actuator is placed in the vicinity of the cylinder tube of the hydraulic cylinder in general. Accordingly, a pressure oil (operating oil) serving as a drive source for the hydraulic actuator is supplied from a hydraulic pressure source (hydraulic pump) via a hydraulic pipe. As described above, each of the booms slides relative to the adjacent boom, and the pipe for supplying the operating oil is generally a hydraulic pressure hose with a hose reel.

SUMMARY OF THE INVENTION

The length of the telescopic boom varies depending on specifications for a mobile crane. In some cases, the distance from the hydraulic pressure source to the hydraulic actuator is very long. Meanwhile, the assumed environmental temperature during operation of the mobile crane ranges from −20° C. to 90° C. (Celsius). Under low-temperature environments in particular, a rise in the viscosity of the operating oil would cause a problem. That is, an increase in in the viscosity of the operating oil decreases the operating speeds of the B pins and the C pin, thus causing a decrease in responsiveness of the extension and retraction operations of the telescopic boom. This phenomenon is more prominent as the hydraulic pipe is longer.

To avoid such a problem, the capacity of the hydraulic pipe has been increased. Specifically, the hose reel is increased in diameter to reduce flow resistance and/or pressure loss of the operating oil. Taking this measure produces a certain effect (improvement in the operating speeds of the B pins and the C pin) but causes a new problem that the hose reel increases in size, resulting in significant increases in weight and costs. There has been a demand for the mobile crane to reduce the size and weight of auxiliary devices such as a hose reel as much as possible. This demand cannot be met with the larger-sized hose reel.

The present invention has been made under the abovementioned background. An object of the present invention is to provide a small, lightweight and low-cost telescopic boom extension device capable of smooth driving of the B pins and the C pin even under lower-temperature environments.

(1) A telescopic boom extension device according to an aspect of the present invention includes: a telescopic boom that includes a base boom, an intermediate boom inserted into the base boom, and a top boom inserted into the intermediate boom, one of the booms adjacent to each other being placed slidably relative to the other boom; a single extension cylinder that includes a cylinder tube and a cylinder rod and that is built into the telescopic boom along a longitudinal direction of the booms while the cylinder rod is coupled to the base boom; a cylinder-boom coupling mechanism that includes a first hydraulic actuator configured to engage selectively with the top boom or the intermediate boom to couple the engaged boom to the cylinder tube; an inter-boom fixing mechanism that includes a second hydraulic actuator configured to couple the adjacent booms to regulate relative sliding of the booms and to decouple specific coupled booms when necessary; and a driving mechanism configured to drive the cylinder-boom coupling mechanism and the inter-boom fixing mechanism. This driving mechanism includes: a hydraulic supply part that is provided at the cylinder tube to supply operating oil selectively to the first hydraulic actuator or the second hydraulic actuator; and a drive source generation part that includes a pneumatic supply part configured to supply a pneumatic pressure and that is configured to generate the operating oil under a predetermined pressure at the hydraulic supply part based on the pneumatic pressure.

According to this configuration, the hydraulic supply part configured to drive the cylinder-boom coupling mechanism and the inter-boom fixing mechanism is provided at the cylinder tube of the extension cylinder. Accordingly, the circuit length of the hydraulic supply part becomes very short as compared to the related art, and the reduction in operational responsiveness of the cylinder-boom coupling mechanism and the inter-boom fixing mechanism resulting from a change in the viscosity of the operating oil becomes very small. In addition, the drive source generation part supplies the pneumatic pressure to the hydraulic supply part, and a pneumatic pressure loss with a change in the environmental temperature is small even in a case where the distance between the pneumatic supply part and the hydraulic supply part is long. Thus, the operational responsiveness of the cylinder-boom coupling mechanism and the inter-boom fixing mechanism is not affected even in this case. Therefore, the pneumatic supply part does not need to be increased in size taking into account a pneumatic pressure loss, thereby achieving the lightweight and small part.

(2) It is preferred that the hydraulic supply part include an air over hydraulic booster (AOH), and that the drive source generation part include a pneumatic supply unit connected to the AOH.

According to this configuration, the AOH is employed to ensure that the operating oil under a required pressure (for example, 10 MPa) is supplied in an easy and reliable manner to the first hydraulic actuator or the second hydraulic actuator based on a low-pressure pneumatic source (for example, 1 MPa).

(3) It is preferred that the hydraulic supply part include a pair of the AOHs arranged symmetrically with respect to the cylinder tube.

According to this configuration, the AOHs can be made lightweight and small to facilitate the layout of the AOHs in the boom, and also enable even weight distribution in the boom.

(4) It is preferred that the pneumatic supply unit include: a pneumatic hose configured to connect the pneumatic source and the AOH; and a hose reel.

According to this configuration, a pneumatic unit that has been commonly used is employed in the pneumatic supply unit. Therefore, the pneumatic unit can be configured at low cost. In addition, the pressure loss of the compressed air supplied from the pneumatic unit is unlikely affected by the environmental temperature as described above, so that it is not necessary to employ a large-diameter pneumatic hose taking into account operations under low-temperature environments in particular. Thus, the pneumatic hose and hose reel can be reduced in weight and size.

(5) It is preferred that the AOH include an air cylinder having an air piston and an air tube, and that the air piston be slidable relative to the air tube without being biased in any direction.

Since the hydraulic supply part is provided at the cylinder tube as described above, the AOH generally constitutes a closed circuit as a hydraulic circuit. In such a closed circuit, when the environmental temperature changes to raise the pressure in the operating oil, for example, the air piston is in a freely movable state, so that a piston in the hydraulic cylinder pairing off with the air piston is easily displaceable. That is, making the air piston into the freely movable state would perform the function as if the hydraulic cylinder is provided with a reservoir tank. Therefore, it is not necessary to provide a separate reservoir tank at the AOH, thereby allowing the AOH structure and the hydraulic supply part to be reduced in weight and size.

The present invention provides a small, lightweight and low-cost telescopic boom extension device capable of smooth driving of the cylinder-boom coupling mechanism and the inter-boom fixing mechanism even under low-temperature environments.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will be described with reference as appropriate to the drawings. However, this embodiment is merely one mode of a telescopic boom extension device according to the present invention. As a matter of course, the embodiment can be modified without deviating from the gist of the present invention.

<Schematic Configuration and Features>

FIG. 1is an enlarged view of main components of a mobile crane (typically, a rough-terrain crane) employing a telescopic boom extension device10according to an embodiment of the present invention.

As illustrated in the drawing, the mobile crane includes a turning base11, and a telescopic boom13is supported on the turning base11via a derrick central shaft12. As described hereinafter in detail, the telescopic boom13includes a plurality of cylindrical booms that constitute a telescopic structure. The telescopic boom13is rotatable around the derrick central shaft12and performs a derricking action by extension and retraction of a derrick cylinder not illustrated. A single extension cylinder14is mounted in the telescopic boom13such that, as the extension cylinder14extends and retracts, the telescopic boom13extends and retracts longitudinally in a manner described, hereinafter.

FIG. 2is a schematic view showing a structure of the telescopic boom13according to the embodiment of the present invention.

As illustrated inFIGS. 1 and 2, a telescopic boom extension device (hereinafter, referred simply to as “extension device”)10includes: the telescopic boom13; the extension cylinder14that extends and retracts the telescopic boom13; a cylinder-boom coupling mechanism15that couples the extension cylinder14to a predetermined part of the telescopic boom13; an inter-boom fixing mechanism16that couples adjacent booms among a plurality of booms constituting the telescopic boom13; and a driving mechanism17(seeFIG. 1) that drives the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16.

FIG. 3is a schematic diagram showing a structure of the driving mechanism17according to the embodiment of the present invention.

The extension device10according to this embodiment is characterized by the structure of the driving mechanism17. As illustrated inFIGS. 1 and 3, the driving mechanism17includes a hydraulic supply part18and a drive source generation part19to be described hereinafter in detail. The drive source generation part19generates a predetermined hydraulic pressure at the hydraulic supply part18based on a pneumatic pressure. The hydraulic supply part18supplies the hydraulic pressure to the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16(seeFIG. 2) to activate the same in a manner described hereinafter. The drive source generation part19employs a pneumatic supply part41described hereinafter to feed compressed air to the hydraulic supply part18. Specifically, the driving mechanism17converts the pneumatic pressure to the hydraulic pressure to drive the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16. This produces the advantage that the entire driving mechanism17can be significantly reduced in weight and size.

<Operations of the Telescopic Boom>

As illustrated inFIG. 2, the telescopic boom13includes a base boom20, a top boom21, and four intermediate booms22to25between the base and top booms. The intermediate booms22to25will be called first, second, third and fourth intermediate booms22,23,24and25, respectively, in sequence from the intermediate boom adjacent to the top boom21. That is, in this embodiment, the telescopic boom13has a six-stage structure. The telescopic boom13is assembled such that the booms21to25slide in a longitudinal direction38from the base boom20, thereby constituting a telescopic structure as described above. However, the telescopic boom13does not have to be a six-stage telescopic boom, and there is no specific limitation on the number of intermediate booms.

In this embodiment, the single extension cylinder14is built in the telescopic boom13. The extension cylinder14is a hydraulic double-acting cylinder, and the leading end portion of a cylinder rod39is coupled to the base end of the base boom20. The extension cylinder14is placed along the telescopic boom13in the longitudinal direction38, and a cylinder tube36is placed inside the top boom21in the state illustrated inFIG. 2. The operation to extend and retract the extension cylinder14causes the extension cylinder14to extend and retract in a manner described hereinafter.

FIG. 2illustrates the telescopic boom13in the fully-retracted state. In this state, the adjacent booms are constantly coupled together by the inter-boom fixing mechanism16.

FIG. 4is a vertical cross-sectional view of the telescopic boom13, andFIGS. 5A and 5Bare lateral cross-sectional views of the same, respectively.FIGS. 5A and 5Bare cross-sectional views of the telescopic boom13taken along V-V plane inFIG. 4. The drawings show schematically structures of the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16.

As illustrated inFIGS. 2, 4, and 5A and 5B, the inter-boom fixing mechanism16includes five pairs of boom fixing pins (hereinafter, referred to as “B pins”)26to30and a hydraulic cylinder31(equivalent to a “second hydraulic actuator” described in the claims) that drives the fixing pins26to30. The structure of the inter-boom fixing mechanism16is known. The B pins26penetrate through the top boom21and first intermediate boom22adjacent to each other to regulate the relative sliding of the two booms. As illustrated inFIGS. 2, 5A, and 5B, the B pins26are provided on the top boom21side and moves forward or backward relative to the first intermediate boom22to penetrate through the first intermediate boom22or separate from the first intermediate boom22. In the normal state, the B pins26are biased toward the first intermediate boom22by a spring not illustrated. The portions of the first intermediate boom22through which the B pins26penetrate are the base end portion and the leading end portion where bosses32and33are provided, and the B pins26are to be inserted into the bosses32and33(seeFIG. 2). The portions of the first intermediate boom22where the bosses32or33are provided are where the B pins26face when the top boom21is brought into the fully-retracted state or the fully-extended state relative to the first intermediate boom22. That is, the top boom21and the first intermediate boom22are coupled and fixed by the B pin26when the top boom21is in the fully-contracted state or the fully-extended state relative to the first intermediate boom22. As illustrated inFIGS. 5A and 5B, when the hydraulic cylinder31is activated, the B pins26are pulled out of the first intermediate boom22. Accordingly, the top boom21becomes slidable relative to the first intermediate boom22. The B pins27to30behave in the same manner as the B pins26.

As illustrated inFIGS. 2, 4, 5A, and 5B, the cylinder-boom coupling mechanism15includes cylinder coupling pins (hereinafter, referred to as “C pins”)34and a hydraulic cylinder35(equivalent to a “first hydraulic actuator” described in the claims) that drives the C pins34. The structure of the cylinder-boom coupling mechanism15is known. The C pins34are provided at the cylinder tube36side of the extension cylinder14, and is constantly fitted to the top boom21in the state illustrated inFIG. 2. As illustrated inFIGS. 5A and 5B, the hydraulic cylinder35includes a link mechanism40. When the hydraulic cylinder35is activated, the link mechanism40slides the C pins34in the right and left direction inFIGS. 5A and 5B. In the normal state, the C pin34is biased toward the top boom21by a spring not illustrated. Bosses37are provided at the base end portion of the top boom21, and the C pins34are fitted to the bosses37. When the hydraulic cylinder35is activated, the C pins34are pulled toward the extension cylinder14via the link mechanism40. When the C pins34are pulled out of the bosses37, the extension cylinder14is mechanically separated from the top boom21. That is, the extension cylinder14is coupled to the top boom21in the normal state, but the extension cylinder14becomes slidable relative to the telescopic boom13when the hydraulic cylinder35is activated. The bosses37are also provided at each of the base end portions of the intermediate booms22to25. The C pin34can be selectively coupled to the intermediate booms22to25in a manner described hereinafter.

FIG. 5Aillustrates the state in which the B pins26are pulled out of the first intermediate boom22and the C pins34are coupled to the top boom21.FIG. 5Billustrates the state in which the B pins26are coupled to the first intermediate boom22and the C pins34are pulled out of the top boom21.

When the extension cylinder14extends in the state ofFIG. 5A, the top boom21slides together with the cylinder tube36of the extension cylinder14leftward in the direction of arrow38relative to the first intermediate boom22as illustrated inFIG. 2. When the extension cylinder14extends up to the position of the first intermediate boom22where the B pins26face the bosses33, the hydraulic cylinder31is deactivated, and the B pins26return to the first intermediate boom22side because of the spring and fit to the bosses33. Accordingly, the first intermediate boom22and the top boom21are fixed to each other while the first intermediate boom22is in the fully-extended state relative to the top boom21. Then, as illustrated inFIG. 5B, the hydraulic cylinder35is activated to decouple the C pins34from the top boom21via the link mechanism40. That is, the C pins34are pulled out of the bosses37of the top boom21. When the extension cylinder14retracts in that state, only the cylinder tube36moves toward the base end of the base boom20(rightward inFIG. 2).

Meanwhile, the hydraulic cylinder35remains activated to keep the C pins34in the state ofFIG. 5B. When the extension cylinder14retracts to move the C pins34down to the position of the bosses37provided at the first intermediate boom22, the retraction of the extension cylinder14is stopped while the hydraulic cylinder35is deactivated, and the C pins34are coupled to the bosses37of the first intermediate boom22as illustrated inFIG. 5A. Subsequently, when the first intermediate boom22is to be extended, the same action as in the case of the top boom21is performed to extend the second intermediate boom23. Similarly, the second, the third, and the fourth intermediate booms23,24, and25are extended in sequence. When the telescopic boom13is to be retracted, the foregoing actions are reversely performed.

<Drive Circuit of the Extension Device>

FIG. 6is a circuit system diagram of the driving mechanism17.

The driving mechanism17drives the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16as described above. As illustrated inFIG. 6, the driving mechanism17according to this embodiment includes the hydraulic supply part18and the drive source generation part19, and the drive source generation part19operates with compressed air as a working fluid. That is, the driving mechanism17is a hydraulic-pneumatic composite system.

The hydraulic supply part18includes electromagnetic switching valves47and48, check valves49and50, and a pair of air over hydraulic boosters (AOHs)51. These components are connected to the hydraulic cylinders31and35. The boom fixing pins26to30and the cylinder coupling pins34are driven by the hydraulic cylinder31and the hydraulic cylinder35as described above. The hydraulic supply part18constitutes a so-called closed circuit together with the hydraulic cylinders31and35, which is provided at the cylinder tube36of the extension cylinder14. Each of the AOHs51has a pneumatic input port52and a hydraulic output port53, and outputs from the hydraulic output port53a predetermined hydraulic pressure corresponding to the pneumatic pressure input into the pneumatic input port52.

In this embodiment, each of the AOHs51includes an input cylinder66(equivalent to an “air tube” described in the claims), an air piston67, an output cylinder68, and a hydraulic piston69. The pneumatic input port52is provided at the input cylinder66, and the hydraulic output port53is provided at the output cylinder68. The air piston67and the hydraulic piston69are coupled together by a main shaft70and slide in an integrated manner. In this embodiment, the air piston67is held in a freely movable state within the input cylinder66. Specifically, the air piston67is held only by frictional force generated between the air piston67and the hydraulic piston69in the input cylinder66. That is, the air piston67is in the freely movable state and is not biased in any direction within the input cylinder66. The advantage of the air piston67being movable without any biasing force will be described hereinafter.

The drive source generation part19includes: a pneumatic supply part41including a pneumatic supply unit54; and a control valve unit55.

The pneumatic supply unit54includes a quick release valve56, an air hose57, and a hose reel58. The quick release valve56has an input port59and an output port60. The output port60is connected to the pneumatic input ports52of the AOHs51. The air hose57is cut into a predetermined length and wound around the hose reel58in an unrollable manner. In this embodiment, the hose reel58is attached to the back part of the turning base11as illustrated inFIGS. 1 and 3. The length of the air hose57is set as appropriate, and in this embodiment, the length of the air hose57corresponds to the stroke of the extension cylinder14. The pneumatic supply part41includes a pneumatic source not illustrated. The pneumatic source may be an air tank included in the mobile crane, for example. The pressure of the pneumatic source is 1 MPa, for example.

The control valve unit55includes pressure control valves (pressure reducing valve61and relief valve62) and an electromagnetic switching valve63. The pneumatic source is connected to an input port64of the pressure reducing valve61, and the electromagnetic switching valve63is connected to an output port65of the same. The relief valve62is provided between the pressure reducing valve61and the electromagnetic switching valve63.

As described above, to extend the telescopic boom13, the B pins26to30and the C pins34are operated. This operation is performed in a manner described below.

When the top boom21extends in the state ofFIG. 2, the drive source generation part19feeds compressed air to the hydraulic supply part18. Specifically, the electromagnetic switching valve63is switched (the symbols are switched inFIG. 6) to feed the compressed air to the air hose57. The air hose57is wound around the hose reel58, but the compressed air is sent through the air hose57to the quick release valve56. The compressed air activates the quick release valve56and reaches the AOHs51.

The electromagnetic switching valves47and48are switched together with the electromagnetic switching valve63(the symbols are exchanged inFIG. 6). With a supply of compressed air, each of the AOHs51generates a predetermined hydraulic pressure (for example, 10 MPa). That is, each of the AOHs51feeds a high-pressure operating oil from the hydraulic output port53. The operating oil is supplied to the hydraulic cylinder31through the check valve49and the electromagnetic switching valve48. The hydraulic cylinder31operates to remove the B pins26from the first boom22. At this point in time, the excitation of the electromagnetic switching valve63is canceled (the symbol returns to the state illustrated in FIG.6), and the supply of the compressed air is shut off. Even when the supply of the compressed air is shut off as described above, the electromagnetic switching valve47and the check valve49keep the pressure in the hydraulic cylinder31. As the extension cylinder14extends in this state, the top boom21extends.

When the top boom21is in the fully-extended state, the extension cylinder14stops. Accordingly, the excitation of the electromagnetic switching valves47and48is canceled (the symbols return to the states illustrated inFIG. 6). Thus, the operating oil supplied to the hydraulic cylinder31returns to the output cylinders68of the AOHs51through the check valve50and the electromagnetic switching valve47. The B pins26fit to the bosses33to couple again the top boom21and the first intermediate boom22.

As described above, the air pistons67of the AOHs51are held in the freely movable state within the input cylinders66. Thus, when the operating oil returns to the output cylinders68, the hydraulic pistons69and the air pistons67slide together. The air in the air pistons67is fed to the quick release valve56and is discharged (released to the atmosphere) from the quick release valve56.

Subsequently, the electromagnetic switching valve63is switched (the symbol is switched inFIG. 6), and the compressed air is fed to the air hose57. That is, the compressed air is fed from the drive source generation part19to the hydraulic supply part18. In the same manner as described above, the compressed air is fed through the air hose57to the quick release valve56and reaches the AOHs51. The AOHs51feed the operating oil under a predetermined pressure from the hydraulic output ports53.

The electromagnetic switching valve63and the electromagnetic switching valve47are switched (the symbols are switched in the drawing). The operating oil is supplied to the hydraulic cylinder35through the check valve49and the electromagnetic switching valve48. The hydraulic cylinder35operates to remove the C pins34from the top boom21. At this point in time, the excitation of the electromagnetic switching valve63is canceled and the supply of the compressed air is shut off. Even when the supply of the compressed air is shut off as described above, the electromagnetic switching valve47and the check valve49keep the pressure in the hydraulic cylinder35. In this state, as the extension cylinder14retracts (seeFIG. 2), the top boom21remains held in the fully-extended state by the first intermediate boom22, and only the cylinder tube36slides toward the base end portion of the first intermediate boom22.

When the extension cylinder14retracts and the C pins34move to the position of the bosses37of the first intermediate boom22, the extension cylinder14stops. Accordingly, the excitation of the electromagnetic switching valve47is cancelled. The operating oil supplied to the hydraulic cylinder35returns to the output cylinders68of the AOHs51through the electromagnetic switching valves48and47. As a result, the C pins34fit to the bosses37and the extension cylinder14is coupled to the first intermediate boom22. When the operating oil returns to the output cylinder68, the air pistons67of the AOHs51are held in the freely movable state within the input cylinders66, and the hydraulic pistons69and the air pistons67slide together. The air in the air pistons67is fed to the quick release valve56and is discharged (released to the atmosphere) from the quick release valve56.

In the same manner, the second to fourth intermediate booms23to25are extended. In addition, as the extension cylinder14retracts, the hydraulic supply part18and the drive source generation part19operate in the same manner. In this embodiment, since the control valve unit55includes the pressure control valves (pressure reducing valve61and relief valve62), the compressed air under an appropriate pressure according to the load is supplied from the pneumatic source to the drive source generation part19.

FIG. 7is a cross-sectional view of the top boom21.

In this embodiment, the hydraulic supply part18includes the two AOHs51. The AOHs51are arranged in the vicinity of the cylinder tube36of the extension cylinder14as illustrated inFIG. 7. These AOHs51are radially symmetric (bilaterally symmetric inFIG. 7) with respect to a virtual plane71including the center axis of the extension cylinder14. The operational advantage of arranging the two AOHs51symmetrically will be described hereinafter.

<Operations and Effects of the Extension Device According to This Embodiment>

According to the extension device10in this embodiment, since the hydraulic supply part18is provided at the cylinder tube36of the extension cylinder14, the distance between the hydraulic supply part18and the hydraulic cylinders31and35is very short. That is, the circuit length in the hydraulic system of the driving mechanism17is much shorter than the related art, and the operational responsiveness of the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16do not decrease significantly with variations in the viscosity of the operating oil. In addition, the hydraulic supply part18generates a predetermined hydraulic pressure based on the compressed air supplied from the drive source generation part19. Thus, even in a case where the circuit length is long in the pneumatic system of the driving mechanism17as in this embodiment, the pressure loss of the air with changes in environmental temperature is small. The operational responsiveness of the cylinder-boom coupling mechanism15and the inter-boom fixing mechanism16is not affected even in this case.

Therefore, the pneumatic supply part41in this embodiment does not need to be increased in size taking into account the pressure loss of the air but can be designed to be lightweight and small. That is, the air hose57can be decreased in diameter and the hose reel58can be designed to be compact, and thus they can be significantly small in weight as compared to the related art. As a result, the space for placement of auxiliary devices at the periphery of the turning base11can be wider to improve the degree of freedom in layout of the hose reel58. In particular, as illustrated inFIG. 1, the hose reel58can be arranged above the turning base11, for example, in the vicinity of the derrick central shaft12included in the telescopic boom13.

In this embodiment, since the hydraulic supply part18includes the AOHs51, the pressure in the pneumatic source is kept small, whereas the pressure of the operating oil supplied to the hydraulic cylinders31and35becomes large. That is, the hydraulic pressure necessary for operating the hydraulic cylinders31and35can be easily obtained.

In this embodiment, the pair of AOHs51is provided. Accordingly, the load on each of the AOHs51to generate the necessary hydraulic pressure is reduced, and the AOHs51can be made compact and laid out between the cylinder tube36and the inner wall of the top boom21as in this embodiment. In addition, the AOHs51are arranged symmetrically with respect to the cylinder tube36to produce the advantage that the weight distribution in the telescopic boom13is uniform.

In particular, in this embodiment, the AOHs51constitute a closed circuit as a hydraulic circuit, and the air pistons67of the AOHs51are arranged in the freely movable state within the input cylinders66. For example, when the pressure of the operating oil in the hydraulic supply part18increases with a change in environmental temperature, since the air pistons67are in the freely movable state, the hydraulic pistons pairing with the air pistons67are easily displaced. That is, arranging the air pistons67in the freely movable state achieves the same function as the case where the output cylinders68are provided with reservoir tanks. Therefore, there is no need to provide separate reservoir tanks in the AOHs51. As a result, it is possible to simplify the structure of the AOHs51and reduce the size and weight of the hydraulic supply part18.

<Variation of This Embodiment>

In this embodiment, the pair of AOHs51is employed. Alternatively, a single AOH may be employed. In addition, the air tank included in the pneumatic supply part41also acts as a brake air tank. Alternatively, separate air tanks or other pneumatic sources may be provided for the AOHs51. In this embodiment, the pressure of the compressed air supplied to the pneumatic supply unit54is set to 1 MPa, but is not limited to this value. The pressure of the pneumatic source can be set as appropriate as far as the outputs of the AOHs51are 10 MPa.