Working machine for hydraulic excavator
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydraulic excavator, such as a power shovel. More specifically, the invention relates to a hydraulic excavator apparatus including a boom, an arm, a bucket and a cylinder device, for pivotally driving the boom, the arm and bucket, in which the boom is formed with articulated first and second boom members. 
     2. Description of Related Art 
     Japanese Unexamined Patent Publication (Kokai) No. 64-66319 discloses a working machine of the type which the present invention is directed to. The disclosed construction of the working machine are illustrated in FIGS. 1 to 3. As can be seen, the tip end of a boom cylinder 1 is connected to a tip end of a first boom member 2a of first and second boom members 2a and 2b which form a boom 2. 
     This conventional apparatus operates in the following manner: 
     (1) First, while the tip end of a bucket 3 is engaged to a tip end portion of a frame 4, the boom cylinder 1 is expanded up to a maximum length, as shown in FIG. 2. This causes the first boom to be raised by a given amount. 
     (2) An arm cylinder 5 is then expanded and tends to cause pivoting of the arm 6 toward a folded position about an articulation point of the second boom 2b. At this time, since the bucket 3 is engaged with the frame 4, the second boom 2b is shifted forward. Therefore, the first boom 2a is raised further. At a position where the first boom 2a has been raised sufficiently, a pin 7c is inserted through pin holes 7a and 7b of the first and second booms 2a and 2b for fixing them in the predetermined fixed relationship and to restrict further relative angular displacement with respect to one another. 
     (3) Following this, a bucket cylinder 8 is operated to release the bucket 3 from the frame 4 and to establish an operating state, in which excavation can be performed by operating respective cylinders 1, 5 and 8. 
     In the above-mentioned conventional arrangement, it is necessary for a longer expansion length of the boom cylinder 1 during pivoting across a rising/falling dead point (on a line extending through a pivot point of the boom cylinder 1 and a pivot point of the first boom 2a) than when the boom is a fully raised excavating position. 
     Accordingly, at the operating position illustrated in FIG. 3, the boom cylinder 1 cannot assume a fully extended condition. Therefore, it is possible to cause erroneous operation wherein the boom cylinder is further expanded to place the boom 2 and the boom cylinder 1 at the rising/falling dead point. Under these conditions, the boom 2 is disabled and unable to be raised or lowered thus inhibiting excavating operation. 
     As an approach to this problem, it is possible to control the raising and lowering of the boom by means of a control system by sensing the boom position with a sensor. However, in general, it is difficult or even impossible to assuredly stop the boom at exactly the same point irrespective of the expansion speed (rate) of the boom cylinder. This makes the control by means of the control system ineffective and fails to prevent the above problem. In addition, using a sensor and a control system increases the complexity of the excavator resulting in reduced reliability and increased cost. 
     SUMMARY OF THE INVENTION 
     In view of the problems set forth above, it is an object of the present invention to provide a hydraulic excavator apparatus, which permits excavating operation at a maximum stroke of a boom cylinder and prevents dead-locking of a boom at the dead point. 
     Another object of the present invention is to provide an apparatus which will prevent rearward pivoting of the boom across the dead point irrespective of the stroke position of the boom cylinder during excavation and thus can eliminate any restriction in boom cylinder stroke. 
     In order to accomplish the above-mentioned and other objects of the present invention, an apparatus or operating mechanism as it will be referred to, includes a boom and boom cylinder in which the boom cylinder is cooperatively associated with the boom to establish a geometry which is such that during operation the axis of the boom cylinder will not pivot rearwardly across a dead point of the boom and so that the possibility of dead locking of the boom during operation can be successfully eliminated. 
     According to one aspect of the invention, an operating mechanism or apparatus for a hydraulic excavator comprises: 
     a boom comprising articulated first and second boom members: 
     an arm; 
     bucket; 
     boom cylinder for operating the boom; 
     an arm cylinder for operating the arm; 
     a bucket cylinder for operating the bucket; and 
     a holding means for holding the first and second boom members at a raised position; and 
     a pivot for the boom cylinder being arranged at a longitudinally front side of a pivot point of the first frame and an upper or tip end of the boom cylinder being connected to the second boom so that the expansion stroke of the boom cylinder maximizes at the maximally raised position of the boom while positioning the axis of the boom cylinder at the front side of a dead point of the boom. 
     According to another aspect of the invention, an operating mechanism or apparatus for a hydraulic excavator comprises: 
     a boom comprising articulated first and second boom members: 
     an arm; 
     a bucket; 
     a boom cylinder for operating the boom; 
     an arm cylinder for operating the arm; 
     a bucket cylinder for operating the bucket; and 
     a holding means for holding the first and second boom members at a raised position; and 
     a pivot for the boom cylinder being arranged at a longitudinally front side of a pivot point of the first frame, and an upper tip end of the boom cylinder being pivotally connected to the second boom in common to the arm cylinder so that the expansion stroke of the boom cylinder maximizes at the maximally raised position of the boom while positioning the axis of the boom cylinder at the front side of a dead point of the boom. 
     According to a further aspect of the invention, an operating mechanism for an excavator comprises: 
     a boom which carries an arm and a bucket assembly and which includes articulately connected first and second boom members, the first boom member being pivotally connected to an excavator frame at a base end and to a base end of the second boom member at an upper or tip end, the second boom member being pivotally connected to the arm and bucket assembly at a tip end; and 
     a boom cylinder for driving the boom and having a base end to be connected to the excavator frame and a tip end to be connected to the boom, the base end and the tip end of the boom. cylinder being arranged in such a geometrical relationship that the axis of the boom cylinder is constantly positioned at longitudinal front side of a dead point of the boom at working position irrespective of the stroke of the boom cylinder. 
     In an actual embodiment, the connecting point between the tip end of the boom cylinder and the boom may be placed on the second boom member. Also, the connecting point between the base end of the boom cylinder and the excavator frame may be placed in front of a connecting point between the base end of the first boom member and the excavator frame. 
     Furthermore, the tip end of the boom cylinder may be connected to the second boom member in common with a base of an arm cylinder of the arm and bucket assembly. 
     According to a still further aspect of the invention, a working machine for an operating mechanism comprises: 
     a boom which carries an arm and a bucket assembly and which includes articulately connected first and second boom members, the first boom members being pivotally connected to an working vehicle frame at a base end and to a base end of the second boom member at a tip end, the second boom member being pivotally connected to the arm and bucket assembly at a tip end; and 
     a boom cylinder for driving the boom and having a base end to be connected to the working vehicle frame and a tip end to be connected to the boom, the base end and the tip end of the boom cylinder being arranged such a geometrical relationship that the axis of the boom cylinder is constantly positioned at longitudinal front side of an extension line extending through the base end of the boom cylinder and the base end of the first boom member, the boom at working position irrespective of the stroke of the boom cylinder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to be limitative to the present invention, but are for explanation and understanding only. 
     In the drawings: 
     FIG. 1 is a side elevation of a conventional excavator equipped with an operating mechanism of the type discussed in the opening paragraphs of this disclosure; 
     FIG. 2 is a side elevation of the conventional excavator of FIG. 1, illustrating an intermediate stage in the process of changing from a rest state to an operative or working state; 
     FIG. 3 is a side elevation of the conventional excavator of FIG. 1, which illustrates a boom in a maximally raised state; 
     FIG. 4 is a side elevation of an excavator employing the preferred embodiment of an operating mechanism according to the present invention; 
     FIGS. 5 and 6 are side elevations of the excavator of FIG. 4, but showing an intermediate state which occurs during preparation of the boom; 
     FIG. 7 is a side elevation of the excavator of FIG. 4, but showing at a state wherein the fixing of first and second boom members of the boom in fixed relationship, is completed; 
     FIG. 8 is a side elevation of the excavator of FIG. 4, with the boom at a maximally raised position; and 
     FIG. 9 is a side elevation of an excavator equipped with a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 4 to 9, the preferred embodiment of a hydraulic excavator employing a preferred construction of an operating mechanism according to the present invention, will be discussed. It should be noted that the same reference numerals used in FIGS. 1 to 3 illustrating the prior art are used to designate like elements. 
     Similarly to the prior art discussed above, a lower pivot point of a boom cylinder 1 is positioned at the longitudinally front side of a pivot point of a first boom 2a forming a part of a boom 2. However, the upper end of the boom cylinder 1 is pivotally connected to a second boom 2b instead of the first boom 2a as in the conventional construction. The first and second booms 2a and 2b are supported on a rotary platform 9 together with an operator&#39;s cabin 10. The rotary platform 9 is rotatably supported on a frame 4. 
     FIGS. 5 to 7 show the embodiment of a hydraulic excavator in the preparatory boom cylinder 1 expansion stage. As the expansion of the boom cylinder 1 occurs with the tip end of a bucket 3 maintained in engagement with the frame 4, the first boom 2a is initially pivoted upwardly from the position shown in FIG. 4 to the position shown in FIG. 5, with the attendant upward lifting the second boom 2b. Through further expansion of the boom cylinder 1, the first boom 2a assumes an essentially vertical position, as shown in FIG. 6. Under these conditions, the boom cylinder 1 is still expanding and does not reach the end of the expansion stroke. From this condition, by retracting the boom cylinder 1 while expanding an arm cylinder 5, the first and second booms 2a and 2b assume the positions illustrated in FIG. 7. It should be noted that FIG. 7 shows a position where the first and second booms 2a and 2b are adjoined by a pin 7c. 
     At this condition, by releasing the bucket 3 from the frame 4 via operation of the bucket cylinder 8, the excavator is rendered ready for operation. 
     FIG. 8 shows a condition where the boom 2 is placed at a maximally raised position. In this state, the boom cylinder 1 assumes a maximally expanded condition, i.e. the expansion stroke reaches an end position. In addition, under these conditions, the pivotal center of the boom cylinder 1 is positioned at the longitudinal front side of the dead point of the beam 2. 
     FIG. 9 shows another embodiment of the operating mechansim according to the present invention. In this embodiment, the tip end of the boom cylinder 1 is commonly pivoted to the pivot point of the arm cylinder 5 on the second boom 2b. 
     With the shown construction, the boom 2 is placed at the maximally raised position by expanding the boom cylinder 1 to the maximum expansion stroke. At this position, the axis of the boom cylinder 1 will never shift rearwardly across the dead point so that dead-locking of the boom 2 can never occur during operation of the excavator. 
     To supplement the above disclosure, commonly assigned co-pending U.S. patent application Ser. No. 07/476,447, entitled &#34;Working Machine for Construction Vehicles and Method of Operating the Same&#34;, is hereby incorporated by reference. 
     Therefore, according to the present invention, even when the boom cylinder is expanded up to the maximum stroke, the axis of the boom cylinder 1 can be maintained on the front side of the dead point in order to place the boom 2 at the maximally raised position. As a result, no restriction of the boom cylinder 1 expansion is required. Therefore, operative reliability can be improved without the need of special sensor or so forth thus assuring low cost. 
     Although the invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiments set out above, but to include all possible variants which can be encompassed by the features set out in the appended claims and the equivalents of those features.