As an expansion/contraction mechanism of a telescopic boom of a mobile crane, an expansion/contraction mechanism which telescopes a boom forming a telescopic boom, stage by stage, using a single telescopic cylinder (hydraulic cylinder) which is contained in the telescopic boom, is brought into practical use (and hereinafter, this expansion/contraction mechanism will be referred to as a “single-cylinder expansion/contraction mechanism”) A single-cylinder expansion/contraction mechanism has advantages in that a weight of a whole of an expansion/contraction mechanism can be reduced because of inclusion of a single telescopic cylinder, and that a lifting performance of a mobile crane can be improved (refer to Patent Literature 1, for example).
A typical configuration of a single-cylinder expansion/contraction mechanism includes boom fixing means, fixing-pin driving means, and cylinder-boom connecting means which are described below.
The boom fixing means is placed in each inner boom of adjacent booms. The boom fixing means includes a fixing pin (which will hereinafter be referred to as a “B in”) for fixing an inner boom and an outer boom. The boom fixing means moves a B pin back and forth relative to a fixing hole provided in an appropriate portion in an outer boom, to thereby fix or unfix an inner boom and an outer boom which are adjacent to each other (which will hereinafter be referred to as a “a pair of adjacent booms”). A telescopic boom which is extended by a single-cylinder expansion/contraction mechanism is kept being extended by the boom fixing means. The boom fixing means is essential means for a single-cylinder expansion/contraction mechanism.
The fixing-pin driving means is placed in a movable portion (which will hereinafter be referred to as a “telescopic-cylinder movable portion”) of a telescopic cylinder. The fixing-pin driving means acts on a B pin in an inner boom of a target pair of adjacent booms (a pair of booms including a boom being telescoped), to move a B pin back and forth. The fixing-pin driving means is used in shifting a state of a pair of adjacent booms from a fixed state to an unfixed state, or from an unfixed state to a fixed state. The fixing-pin driving means, like the boom fixing means, is indispensable for a single-cylinder expansion/contraction mechanism. The fixing-pin driving means (which will hereinafter be referred to as a “B-pin driving means”) includes a B-pin cylinder which drives a B pin back and forth. A B-pin cylinder requires a relatively large output though the B-pin cylinder should be placed in a small space of a telescopic-cylinder movable portion, and therefore, a B-pin cylinder includes a hydraulic cylinder.
The cylinder-boom connecting means is placed in a telescopic-cylinder movable portion. The cylinder-boom connecting means includes a connecting pin (which will hereinafter be referred to as a “C pin”) for connecting a telescopic-cylinder movable portion and a target boom (a boom being telescoped). The cylinder-boom connecting means moves a C pin back and forth relative to a connecting hole in a boom being telescoped, to thereby selectively connect or disconnect a telescopic-cylinder movable portion and a boom. The cylinder-boom connecting means is indispensable for a single-cylinder expansion/contraction mechanism which telescopes all booms using a single telescopic cylinder. The cylinder-boom connecting means includes C-pin driving means such as a C-pin cylinder which drives a C pin back and forth. A C-pin cylinder requires a relatively large output though a C-pin cylinder should be placed in a small space of a telescopic-cylinder movable portion, and therefore, a hydraulic cylinder is used also for a C-pin cylinder.
FIG. 13 is a view showing a conventional hydraulic circuit (which will hereinafter be referred to as a “B/C-pin-cylinder hydraulic circuit) for supplying a hydraulic pressure to a B-pin cylinder 5 and a C-pin cylinder 7 which are used in a single-cylinder expansion/contraction on mechanism.
In the single-cylinder expansion/contraction mechanism, the B-pin cylinder 5, the C-pin cylinder 7, and electromagnetic selector valves 1 and 9 are placed in a telescopic-cylinder movable portion 3.
The B-pin cylinder 5 which drives a B pin 4 is a single-acting hydraulic cylinder, and contains a spring 20 for a return therein. The B-pin cylinder 5 is driven upon supply of a hydraulic pressure via a single hydraulic pipeline 22.
The C-pin cylinder 7 which drives a C pin 8 is a single-acting hydraulic cylinder. A spring 21 which impels the C pin 8 functions as a spring for a return of the C-pin cylinder 7. The C-pin cylinder 7 is driven upon supply of a hydraulic pressure via a single hydraulic pipeline 23.
A hydraulic pressure is supplied from a telescopic-cylinder fixing-unit side 24 (a side where a base portion of a telescopic boom or a turntable of a crane is provided) to the telescopic-cylinder movable portion 3, while passing through a single long hydraulic hose 6 which is unreeled from, and reeled on, a hose reel 2 placed on the telescopic-cylinder fixing-unit side 24.
The electromagnetic selector valves 1 and 9 supply a hydraulic pressure which is supplied from the single hydraulic hose 6, to the hydraulic pipeline 22 for the B-pin cylinder 5 and the hydraulic pipeline 23 for the C-pin cylinder 7 while performing selecting. More specifically, the electromagnetic selector valve 1 selects either holding or on-holding of a hydraulic pressure which is supplied to the B-pin cylinder 5 or the C-pin cylinder 7. The electromagnetic selector valve 9 selects either supply of a hydraulic pressure to the B-pin cylinder 5 or supply of a hydraulic pressure to the C-pin cylinder 7. In a telescoping process of the single-cylinder expansion/contraction mechanism, the B-pin cylinder 5 and the C-pin cylinder 7 are sequentially driven.
In the above-described B/C-pin-cylinder hydraulic circuit, an increase of viscosity of a hydraulic working fluid at a low temperature results in an increase of pressure loss during passage through the long hydraulic hose 6, so that the B-pin cylinder 5 or the C-pin cylinder 7 operates slowly. This invites an operational delay of the B-pin driving means or C-pin driving means, and causes a fear that the single-cylinder expansion/contraction mechanism may be unable to properly operate. With regard to such a problem, it is possible to ensure operability at a low temperature by increasing an internal diameter of the hydraulic hose 6. However, an increase of an internal diameter of the hydraulic hose 6 results in an increase of a size and a weight of the hose reel 2, and thus, it is not preferable to provide an individual hydraulic-pressure supply system including the hydraulic hose 6 and the hose reel 2 for each of the B-pin cylinder 5 and the C-pin cylinder 7. For this reason, the conventional B/C-pin-cylinder hydraulic circuit employs a configuration in which only one hydraulic-pressure supply system for the telescopic-cylinder movable portion 3 is provided so as to be branched out by the electromagnetic selector valves 1 and 9 provided in the telescopic-cylinder movable portion 3.