Traversing hoists – Having vertically swinging boom – Having means to swing boom vertically
Reexamination Certificate
2002-03-22
2003-01-21
Brahan, Thomas J. (Department: 3652)
Traversing hoists
Having vertically swinging boom
Having means to swing boom vertically
C212S239000, C052S116000, C052S690000
Reexamination Certificate
active
06508371
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a jib crane and, more specifically, to a jib crane which prevents a burden from being unexpectedly displaced upon dynamic lift off or laying-down of the burden.
BACKGROUND ART
FIG. 1
is a side view showing a jib crane in which reference numeral
1
denotes a traveling or stationary type (shown in the figure is the traveling type) support base; and
2
, a revolving frame revolvably mounted on the support base
1
via a revolving table
3
. The support base
1
and revolving frame
2
constitute a crane body
4
.
The revolving frame
2
in the crane body
4
has a front portion to which a jib
5
is pivoted for its luffing movement through a support pin
6
. Mounted on the revolving frame
2
is a luffing winch
7
which reels or unreels a luffing rope
8
which in turn is reeved on a sheave
10
a
at a top of an A-frame
9
on the revolving frame
2
, on a sheave
10
b
at a tip of the jib
5
and again on the sheave
10
a
and is fixed to the revolving frame
2
. The luffing winch
7
reels or unreels the luffing rope
8
to cause luffing motion of the jib
5
.
Also mounted on the revolving frame
2
is a hoisting winch
11
which reels or unreels a lifting rope
12
which in turn is reeved on a sheave
13
at the top of the A-frame
9
, between the sheave
13
and a sheave
14
(lifting point) at the tip of the jib
5
and between the sheave
14
and a sheave
16
of a hook block
15
. The lifting rope
12
is wound at its end around a luffing drum (not shown), which cooperates with the luffing winch
7
, in a direction opposite to that of the latter. Driving the hoisting winch
11
causes a burden
17
suspended from the hook block
15
to be lifted up or down.
The lifting rope
12
is unreeled by the luffing drum when the luffing rope
8
is reeled by the luffing winch
7
to raise the jib
5
, and is reeled by the luffing drum when the raised jib
5
is lowered into substantially horizontal, thereby providing level luffing of the burden
17
without changing its height. Number of times of reeving the lifting rope
12
between the sheave at the lifting point
14
and the sheave
13
at the upper end of the A-frame
9
is, for example, doubled against number of times of reeving the lifting rope
12
between the sheave at the lifting point
14
and the sheave
16
of the hook block
15
, which prevents load applied by the lifting rope
12
from acting as resistance to the luffing motion of the jib
5
to facilitate the luffing motion of the jib
5
and enable smooth level luffing of the burden
17
.
FIG. 1
shows the jib
5
of the jib crane in its most raised position (with a luffing angle &thgr; of the jib
5
to horizontal plane being maximum). In this state, the jib crane can lift up the burden
17
of maximum load (or load rating). When the jib
5
is lowered to substantially horizontal as shown in
FIG. 3
, load of the burden
17
liftable is decreased in terms of increased moment load.
FIG. 2
shows a typically known A-frame
9
mounted on the revolving frame
2
and comprising a front frame
9
a
with rigidity and a rear frame
9
b
with a smaller cross-sectional area and acting as a tension bar. Also in
FIG. 1
, the front frame
9
a
is a structure with rigidity and the rear frame
9
b
is a tension bar with a smaller cross-sectional area.
In the jib crane of
FIGS. 1 and 2
and when the jib
5
is substantially horizontal, the front and rear frames
9
a
and
9
b
of the A-frame
9
are subjected to compression and tensile loads, respectively. When the jib
5
is raised to lift up the burden
17
of maximum load (or load rating), both the front and rear frames
9
a
and
9
b
are subjected to large tensile load T.
The above-mentioned conventional jib crane generally has a following problem. Shown in
FIG. 1
by the solid lines is the jib crane with the jib
5
being raised without the burden
17
. When the burden
17
of maximum load is lifted up from this state, the crane body
4
and jib
5
are deflected and tilted forward as indicated by chain double-dashed lines due to the heavy load. In other words, the tip of the jib
5
is bent downward and the support base
1
of the crane body
4
is deflected forward and the revolving table
3
is deflected forward.
When the burden of maximum load is lifted up, extremely large tensile load T acts on both the front and rear frames
9
a
and
9
b
of the A-frame
9
as shown in
FIG. 2
; the rear frame
9
b
of the conventional A-frame
9
, which is used as a tension bar and has a smaller cross-sectional area, is lengthened by the tensile load T, resulting in deflection and forward tilting of the entire A-frame
9
as indicated by chain double-dashed lines.
The above-mentioned forward deflections of the crane body
4
, jib
5
and A-frame
9
are greatest when the burden
17
of maximum load is lifted up with the jib
5
being raised. When the jib
5
approaches horizontal, a forward displacement distance of the tip of the jib
5
is decreased in connection with reduced load of the burden
17
liftable and the luffing angle &thgr; of the jib
5
from horizontal plane.
As mentioned above, when the burden
17
of maximum load is lifted up by the jib crane, the crane body
4
, jib
5
and A-frame
9
are deflected and tilted forward so that the lifting point
14
at the tip of the jib
5
is displaced forward by a forward displacement distance +X as shown in FIG.
1
. This causes the burden
17
to be displaced forward, by the forward displacement distance +X, from a position originally expected.
As a result, in the jib crane of FIG.
1
and upon dynamic lift off of the burden
17
of maximum load on the ground with the hook block
15
being aligned to a gravity center of the burden
17
, the crane body
4
, jib
5
and A-frame
9
are tilted forward as shown by the chain double-dashed lines as mentioned above and the burden
17
is thrown forward by the forward displacement distance +X, resulting in a problem of the burden
17
being swung back and forth.
When the burden
17
of maximum load lifted by the jib crane as indicated by the chain double-dashed lines in
FIG. 1
is laid down into a predetermined position, the load of the burden
17
is relieved the very moment the burden
17
contacts the installation position, which causes the forward tilted crane body
4
to be raised up as shown by the solid lines. As a result, the burden
17
is unexpectedly drawn back by the forward displacement distance +X.
Thus, since the burden
17
is displaced when it is dynamically lifted off the ground or laid down, collision of the burden
17
with any nearby structure or other problems may occur. In a case where the burden
17
such as a steel block is lifted up, moved and positioned for placement on an object to be welded, the steel block is displaced the very moment it is placed on the object to be welded, which results in difficulties in accurately positioning the block and causes a problem of a long time being required for the positioning work.
SUMMARY OF THE INVENTION
An object of the invention is to provide a jib crane wherein cross-sectional areas of upper and lower beam members constituting a jib are determined depending upon overhang eccentric lengths of the upper and lower beam members such that, when a burden of maximum load is lifted up, an upper portion of the jib is recurved toward a crane body and thus a forward displacement distance of a tip of the jib due to forward tilting of the crane body is counterbalanced with a backward displacement distance due to the recurvature of the jib toward the crane body, thereby preventing unexpected displacement of the burden when the burden is dynamically lifted off a ground or laid down by the jib crane.
A further object of the invention is to provide a jib crane wherein cross-sectional areas of front and rear frames constituting an A-frame are determined such that, when a burden of maximum load is lifted up, the front frame is lengthened to displace backward an upper end of the A-frame and thus a forward displac
Brahan Thomas J.
Ishi Kawajima-Harima Jukogyo Kabushiki Kaisha
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