２４は左後縦板２０の前部上端側に設けられた左側の前上フランジで、該前上フランジ２４は、平板状の板材を用いて形成され、その長さ寸法は、図９、図１０に示す如く左後縦板２０の前部２０Ａの前端側と左前縦板１４の後端側と左，右方向で重合せた重合部分、即ち両者の接合部２２を上側から覆う短尺な長さに設定されている。前上フランジ２４は、左後縦板２０の前端側と左前縦板１４の後端側との重合部分（接合部２２）の上端を上側から覆った状態で溶接により接合されている。これにより、図１０に示すように断面Ｉ字形のビーム構造が、センタ底板１３、左前縦板１４、左後縦板２０および前上フランジ２４により形成されている。
24 is a left front upper flange provided on the upper end side of the front part of the left rear vertical plate 20, and the front upper flange 24 is formed using a flat plate material, and its length dimension is shown in FIGS. front side and rear side and the left of the left front vertical plate 14 of the front 20A of the left rear vertical plate 20 as shown in 10, a short length for covering the overlapping portion obtained by polymerization in the right direction, that is, the joint portion 22 therebetween from above It is set to length. The front upper flange 24 is joined by welding in a state where the upper end of the overlapping portion (joining portion 22) between the front end side of the left rear vertical plate 20 and the rear end side of the left front vertical plate 14 is covered from above. Thus, as shown in FIG. 10, a beam structure having an I-shaped cross section is formed by the center bottom plate 13, the left front vertical plate 14, the left rear vertical plate 20, and the front upper flange 24.
２９は左，右の後縦板２０，２１間に接合して設けられた横板で、該横板２９は、下端側がテール底板１９の前端部に接合され、左，右方向の両端側が後縦板２０，２１に接合されることにより、テール底板１９と後縦板２０，２１とを互いに一体化するように連結している。横板２９は、後述の後横板３０よりも図２に示す寸法Ｌ分だけ短尺な平板材を用いて形成されている。横板２９の左側端部は、左後縦板２０の前部２０Ａ側に接合されている。
Reference numeral 29 denotes a horizontal plate provided between the left and right rear vertical plates 20 and 21. The horizontal plate 29 is joined at the lower end side to the front end portion of the tail bottom plate 19 and at the left and right ends at the rear side. By joining to the vertical plates 20 and 21, the tail bottom plate 19 and the rear vertical plates 20 and 21 are connected so as to be integrated with each other. Transverse plate 29 is formed with a transverse plate by a dimension L min shown in Figure 2 than 30 short long flat plate material after below. The left end portion of the horizontal plate 29 is joined to the front portion 20 </ b> A side of the left rear vertical plate 20.
このとき、左後縦板２０は、前部２０Ａ側と後部２０Ｂ側とを右後縦板２１と平行に前，後方向に延びるように形成し、前，後方向の中間部には、左外側へと寸法Ｌ（図２参照）分だけ張出すように折曲げられた折曲げ部２０Ｃを予め形成しておく。また、前部２０Ａの上端側には、左前縦板１４の後部上端側に向けて突出するように短尺な前上フランジ２４を溶接により接合しておく。一方、右後縦板２１の上端側には、その全長にわたって延びるように長尺な上フランジ２５を溶接により接合し、上フランジ２５の前端側も右前縦板１５の後部上端側に向けて突出するよう配置する。
At this time, the left rear vertical plate 20 is formed so that the front portion 20A side and the rear portion 20B side extend in the front and rear directions in parallel with the right rear vertical plate 21, and the left and right middle portions have a left A bent portion 20C is formed in advance so as to be bent outward by a dimension L (see FIG. 2). Further, the upper end of the front 20A is a short length of the front upper flange 24 so as to protrude toward the rear upper side of the left front vertical plate 14 keep joined by welding. On the other hand, a long upper flange 25 is joined to the upper end side of the right rear vertical plate 21 by welding so as to extend over its entire length, and the front end side of the upper flange 25 also protrudes toward the rear upper end side of the right front vertical plate 15. Arrange to do.
この場合、左前縦板１４の後端側と左後縦板２０の前端側とを、図９、図１０に示す如く左，右方向で互いに重合せた状態で溶接による接合部２２を形成することができる。そして、短尺な前上フランジ２４を両者の重合せ部分に上側から被せるように接合して設けることにより、断面Ｉ字形のビーム構造を形成することができる。このため、左前縦板１４の後端側と左後縦板２０の前端側との繋ぎ目となる接合部２２における曲げ剛性を高めることができ、左後縦板２０の板厚を最小限の厚さに抑えることができる。
In this case, the joining portion 22 is formed by welding in such a manner that the rear end side of the left front vertical plate 14 and the front end side of the left rear vertical plate 20 are overlapped with each other in the left and right directions as shown in FIGS. be able to. By providing joined so as to cover from above the superimposed portion of the short length of the front upper flange 24 both can form a beam structure of the cross section I-shaped. For this reason, it is possible to increase the bending rigidity at the joint portion 22 that is a joint between the rear end side of the left front vertical plate 14 and the front end side of the left rear vertical plate 20, and minimize the thickness of the left rear vertical plate 20. The thickness can be suppressed.
この場合、左後縦板２０は、右後縦板２１のように全長にわたる上フランジ２５を設けていないため、前述の如き旋回フレーム１１としての強度を確保する上で、例えば左，右の前縦板１４，１５よりも厚い板材で形成せざるを得ない。しかし、左後縦板２０の上端側には、前述の如き短尺な前上フランジ２４とウエイト取付部２６を構成する後上フランジ２８とを設ける構成としている。
In this case, the left rear vertical plate 20 is not provided with the upper flange 25 extending over the entire length unlike the right rear vertical plate 21, so that, for example, the left and right front plates can be secured in order to secure the strength as the revolving frame 11 as described above. It must be formed of a plate material thicker than the vertical plates 14 and 15. However, the upper end of the left rear vertical plate 20, has a configuration provided an upper flange 28 after configuring the short length of the front upper flange 24 and the weight mounting portion 26 such as described above.