Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~79~
BOOM EXTENSION FOR CRANE
Background of the Invention
Field of Use
This invention relates generally to lattice type
boom extensions for crane boorns~
Description of the Prior ALt
Some cranes are constructed so that the length of
the crane boom can be extended to suit particular job needs
by temporary attachment of a boom extension to the
point end of the crane boom. Typically, such boom exten-
sions are lattice-type structures in order to keep weight
to a minimum. Furthermore, in the current state of the art,
the lattice boom extensions have often been designed as
cable-supported jibs wherein the top and bottom tubular
chord members had the same outside tube diameter and tube
wall thickness.
The dominant load on such cable supported lattice
structures is axial compression. Both chords bottom and
top are in compression. The bending moments resulting from
self-weight and eccentric tip support are usually small
compared to the direct axial load. These cable supported
jib structures act almost as a pure columns and the beam
load effects are relatively unimportant. Based on this mode
of loading the top and bottom chords are typically made to
the same outside diameter. However, as crane ~ooms and
extension booms are made longer and larger to suit
current construction industry needs, the equal size chord
jib design of the aforesaid character becomes inefficient
and unnecessarily heavy for long and highly-loaded extensions, and
improved boom extension are required.
Summary of the Invention
Applicant has discovered through study, experimentation
and tests that, unlike conventional cable-supported lattice type
boom extensions, a lattice type boom extension, unsupported by
cables and mounted on a crane boom which swings around (such as on
a mobile crane) is subjected to a combination of axial compression
and bending and, hence, is acting as a beam column and that load-
ing of this type is significantly different from that in cable-
supported structures.
The invention provides in a non-cable supported lattice
type boom extension for releasable attachment to the point end of
a crane boom, said boom extension comprising a base end, a point
-end, top, bottom and lateral sides, and a centerline extending
between said base end and said point end of said boom extension,
said centerline and said top and bottom sides being disposed at an
acute angle to vertical when said boom extension i5 supporting a
load whereby said bottom boom chord members of said boom extension
are subjected to greater axial compression than said top chord
members and to bending forces, in combination: a pair of tubular
top chord members and a pair of tubular bottom chord members on the
top and bottom sides, respectively, of said boom extension, the
chord members in each pair being laterally spaced apart from each
other and converging toward each other in proceeding from said base
end to said point end of said boom extension, the top chord member
and the bottom chord member on the same lateral side of said boom
extension converging toward each other in proceeding from said base
end to said point end of said boom extension, the chord members in
a respective pair being of the same size relative to each other as
regards outside diameter and wall thickness, each of said bottom
chord members being larger in outside diameter and having a greater
wall thickness than each of said top chord members, the longitudi-
nal centerlines of the top and bottom chord members on the same
lateral side of said boom extension lying in a common vertical
plane; cross braces interconnected between the two chord members
in each pair and between each top chord member and its respective
bottom chord member; means near the base end of said boom extension
for releasably connecting the base end of each chord member to the
point end of said crane boom; and a hoist line pulley rotatably
mounted near said point end of said boom extension.
A boom extension constructed and used in accordance with
the present invention offers several advantages over the prior art.
For example, in an actual embodiment of an extension on the order
of 42 feet long, tests showed that the weight saving was approxi-
mately 200 pounds. Considering that the material for the cord
tubing is quite expensive, the smaller weight more than adequately
off-sets the higher manufacturing cost resulting from fabricating
different lengths for top and bottom laces or cross-members.
Drawings
Figure 1 is a side elevational view of a mobile crane
showing its telescopic boom fully extended and provided with a
boom extension in accordance with the invention at its upper or
point end;
Figure 2 is a greatly enlarged side elevational view of
-4-
the extension shown in Figure 1 and also shows force-diagrams
associated therewith;
-4a-
_5_ li
Figure 3 is an enlarged top plan view (with
Portions broken awaY) of the boom extension of Fiqures 1
and 2;
Figure 4 is a side elevational view of the boom
extension of Fiqure 3;
Fiqure 5 is a cross-sectional view of the boom
extension taken on line 5-5 of Figure 4 on an enlarged scale;
Figure 6 is a cross-sectional view of the boom
extension taken on line 6-6 of Figure 4 on ar.enlarged scale; and
Figure 7 is a cross-sectional view of the boom
extension taken on line 7-7 of Figure 4 on an enlarged scale.
Description of a Preferred Embodiment
Figure 1 is a side elevational view of a mobile
crane 10 having a lower section, including a chassis 11 on
which ground engaging wheels 12, outriggers 13 and an oper-
ator's cab 14 are mounted, and having a rotatable upper
section including a machinery housing 16 on which an oper-
ator's cab 17, a multisection telescopic boom 18, boom hoist
cylinder 19, and a cable winch 20 are mounted.
Figure 1 shows the outriggers 13 extended so as
to raise mobile crane 10 off its wheels and also shows boom
hoist cylinder 19 extended to raise boom 18. Telescopic
boom 18 is also shown as fully extended.
Figures 1 and 2 show that boom 18 is provided at
its uppermost or point ena with a boom head 22 of conven-
tional construction which is understood to comprise two
laterally spaced apart side plates, such as plate 23, between
which a pair of rotatable or hoist line cable pulleys 24
I -6- ~1
and 25 aré rotatably mounted. Each plate 23 is understood
to be provided with two support members or pins, not shown,
which are releasably engageable with clevises, hereinafter
identified, on the base end of a boom extension 26 in accord-
ance with the invention, which is shown in Figs. 1 and 2 asmounted on the point end of boom 18. Boom extension 26 is
provided at its point end with an extension head 27 having
a rotatable cable or hoist line pulley 28 around which a
cable 30 from winch 20 is reeved, such cable being provided
with a load-handling hook 31.
As Figures 1-7 show, extension 26 is of the lattice
type which comprises four sides ~namely, a top side 31, a
bottom side 32, a right side 33 and a left side 34) and
which tapers to reduced dimensions, proceeding from its base
end to its point end. Extension 26 comprises four elongated
tubular chord members designated 35, 36, 37 and 38 which
are interconnected by lacing or cross braces hereinafter
described. The cord members 35 and 36, herein designated
as top chord members are arranged in laterally spaced apart
side-by-side converging relationship. The chord members 37
and 38, herein designated as bottom chord members, are also
arranged in laterally spaced apart side-by-side converging
relationship. The center lines of the bottomchord m ~ ers 37
and 38 lie in a vertical plane directly below the center
lines of the top chord members 35 and 36, respectively.
The top chord members 35 and 36 are interconnected with each
other by cross braces which are welded therebetween and
arranged in a conventional manner i.e., some cross braces
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being arranged at right angle~ to the chord members and
others at acute angles, such as the braces 40 and 40A
respectively. The bottom chord members 37 and 38 are inter-
connected with each other by welded cross braces (see Figs. 3,
5, 6, 7) such as 41 and 41A, respectively. Each top chord
member 35, 36 is interconnected with a corresponding bottom
chord member 37, 38, respectively, by welded cross braces
(see Figs. 4, 5, 6, 7) such as 42 and 42A. As Figures 5,
6, 7 best show, each top chord member 35, 36 is also inter-
connected at intervals therealong by internal braces such
as 43, 44, respectively, with an opposite side bottom chord
memJ~er 38, 37, respectively. Gusset plates 50 and 51 are
provided at the base and of appropriate configuration and
size point ends, respectively, of boom extension 26 to afford
necessary strength and rigidity. As hereinbefore mentioned,
four clevises 55, 56, 57 (not visible) and 58 are provided
at the base end of 3~oom extension 26, being connection as
by welding at the base ends of the chord members 35, 36, 37,
38, respectively.
As Figures 2 and 4 s~w, the largest unsupp~rte chord length
along any bottom ch~rd rn~er 37, 38 may be on the order of about 1096
of the total length of the boom extension 26, depending on the si e of
the bottam chord. UnsuP~orted chord len~th is that distance alon~ a
member 3? or 38 between the connection ~oints of ~ cross braces (such
as 42 and 42A, for exam~le~. The actual formula for the unsupported
chord length is KL ~ 50.
In accordance with the invention, the top chord
members 35, 36 have the same outside diameter and wall
thickness as each other. Sin~larly, the ~ttom chord members 37, 38
have the same outside diameter and wall thickness as each other.
However, the wall thickness of a bottom chord member is greater th~n
that of a top chord me~ber7 as is the outside tube diameter. The purpose
of this disfference is to reduce weight and cost of the jib 26 ~ithout
reducing strength and load handling ability. More specifically, in an
actual symetrical em~xbnent of the invention which was built and tested,
the b~om e~tension 26, fab~icated of c~ drawn, heat treated alloy
100,000 P.S.I. yield type steel chord members, and cross m~s of low
carbon high yie]d electric resistance welded, 55,000 P.S.I. yield, had
the followqng dim~ns~ns:
length. . ~ . . . . . . . . . . . . 42 feet
base width. . . . . . . . . . . . . 3 feet
base height . . . . . . . . . . . . 4 feet, 9 inches
point width . . . . . . . . . . . . 1 foot, 4 inches
point height. . . . . . . . . . . . 0 foot, 10.56 inches
bottom chord O~D. . . . . . . . . 3.25 inches
bottom chord wall thickness . . . . .188 inches
top chord O.D. . . . . . . . . . . 2.5 inches
top chord wall thickness. . . . . . .156 inches
maximum unsupported chord length
been ~race attachment poi~ts. . . 46 inches
As the elementary two-plane force diagram F in
Figure 2 shows, with its longitudinal center line the boom
extension 26 was disposed at an angle oC of 15 from the
vertical. A load of 30,000 pounds was disposed on hook 31,
which imposed a load of 15,000 pounds on load line 30 between
winch 20 and pulley 28. Boom deflection, extension jib
deflection and bending in lace or brace ioints is neglected
in diagram F; nevertheless, the diagram illustrates the
basic force and stress relationships between the top chords
35, 36 and the bottom chords 37, 38, respectively.
1~7~C3~2
The following simplified e~lculations, and a m~re
sophisticated study made by finite element analysis in addition to
testing confirmed the advantages of using diff~rent size top and ~ottcm
chords in accordance with applicant's invention or discovery.
Calculations for Boom ~xtension
Force on Top Chords:
1) Force Due to Vertical Load
MR2 = = 57.18 x 15,000 +48.12 x Rl + 24.06 x OOS 15
(30,000) - 504 x Sin 15 (30,000)
~ = 2'43858i238-= 49,012~
2) Force Due to Side Load
MR4 = 0 = .03 x 30,000 x 504 - 36 R3
R3 = R4 = 12,600 (Compr. R.H; Tension L.H)
Force on I~p R.H. Chord Force on Top L.H. Chord
49,012 12,600 = 18,206# 49,012 + 12,600 = 30,806# (Tensin)
(Tension) HIG~EST
Force on Bottom Chords:
.... . .
MR1 = 9.06 x 15,000 - 24.06 x Cos 15 (30,000)
- 504 x Sin 15 (30,000) = 48.12 R2
R2 = 4474649 = 92 989~
Force on Bottom L.H. Chord ~orce on Bottom R.H. Chord
92,989 _ 12,200 = 40,195 92,989 + ___~ _ = 52,795#
(oompression) (aompression)
HIG~ES~
Allowable compressive stress for bottom chord based on unsup~orted ch~rd
length and yield strength of material (Yield = ~00 K.S.I.)
L = 46 in K = 1.0 h = 1. 0847
1,~.7g~2 I`
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= liOo8x446 = 42.4
Allowable Stress Fa = 45 350 K.S.I.
Allowable Tensile S1,^ess :EOL Top Chords
Fa = .6 x 100 - 60.00 K.S.I.
S Assumed Chord Material and Mechanical Properties:
Bottom Chord: 3.25 in O.D. x .188 Wall
A = 1.8930 in2 I = 2.1228 in4
S = 1.3063 in3 ~ = 1.0847
Top Chord: 2.50.D x .156 Wall
A - 1.1505 in2 I = .7934 in4
S = .6437 in3 ~ = .8304
Nomenclature
Rl; R2 ~ Vertical Reactions.
R3; R4 - Side Reactions.
MR2 ~ Moment About Reaction Point R2
MR4 - Moment About Reaction Point R4
L - Unsupporteci Chord Length
K - Effective Length Factor
- Radius of Gyration
KL Slenderness Ratio Determining~the Allowable Compressive
~ Stress Usually Restricted to = 50 for Boom Extension
Design
Fa ~ Allowable Stress (Either Tension or Compression)
A - Area of Chord Section
I - Moment of Inertia of Chord Section
25 S - Section Modulus of Chord Section
