Articulated boom including tensioning apparatus

An aerial lift comprising a frame, an articulated boom including a first and second boom portions, a workman support structure mounted on the upper end of the second boom portion for pivotal movement relative thereto about a generally horizontal axis, and a mechanism for leveling the workman support structure throughout the range of movement of the first and second boom portions, the leveling mechanism including first and second sections of elongate material extending through the boom portions, a hydraulic assembly connected to the second section and including a hydraulic cylinder, a piston dividing the cylinder into first and second chambers, and a piston rod having one end fixedly connected to the piston, and a hydraulic accumulator communicating with one of the chambers.

BACKGROUND OF THE INVENTION 
The invention relates to articulated booms, and more particularly to 
articulated booms including an arrangement for leveling the workman 
support bucket. 
A conventional articulated boom comprises a pair of elongated boom portions 
that are pivotally joined together. The lower end of one of the portions 
is pivotally mounted on a frame, and the upper end of the other portion 
supports a pivotally mounted bucket in which a workman rides. The boom 
also comprises means for leveling the bucket throughout the range of 
movement of the boom portions. This means typically includes a closed loop 
of flexible, elongate material, such as a cable or chain, which extends 
through the boom from the frame to the bucket and which is supported by 
pulleys or sprockets. A portion of this loop, referred to as the tension 
portion, resists downward tilting movement of the bucket and thus is 
maintained under tension by the force of gravity acting on the bucket. The 
remainder of the loop, called the holdback portion, is not maintained 
under tension by the bucket. 
Articulated booms are commonly mounted on trucks, and it is desirable to 
maintain tension in the loop so that the bouncing which inevitably occurs 
when the boom is being transported does not cause the loop to come off its 
pulleys or sprockets and does not cause damage to the loop itself or to 
other components within the boom. Maintaining the loop under constant 
tension also reduces elongation of the loop when it is loaded, such as 
when a workman enters the bucket. 
Houck U.S. Pat. No. 4,429,763 discloses an arrangement in which turnbuckles 
are provided for adjusting the tension in the loop. A disadvantage of 
using turnbuckles is that it is necessary to frequently tighten the 
turnbuckles in order to maintain tension. Another disadvantage of using 
turnbuckles is that the closed loop is typically enclosed within the boom 
portions and it is necessary to provide access to the interior of the boom 
portions in order to afford access to the turnbuckles. For example, in the 
Houck patent, access plates 13' and 14' are provided to afford access to 
the turnbuckles. Thus, it is necessary to remove the access plates each 
time the turnbuckles are tightened. 
Attention is directed to the following U.S. Pat. Nos. 
______________________________________ 
Johnson 4,081,055 Mar. 28, 1978 
Freudenthal, et al. 
4,512,436 Apr. 23, 1985 
Houck 4,429,763 Feb. 7, 1984 
Brendel 3,049,194 Aug. 14, 1962 
Geary 2,063,802 Dec. 8, 1936 
Gould 4,484,732 Nov. 27, 1984 
Pitts 2,323,352 Jul. 6, 1943 
Puryear, et al. 3,302,480 Feb. 7, 1967 
______________________________________ 
SUMMARY OF THE INVENTION 
The invention provides an aerial lift comprising a frame, and an 
articulated boom including lower and upper boom portions. The lower end of 
the lower boom portion is pivotally mounted on the frame, and the upper 
end of the upper boom portion has pivotally mounted thereon a workman 
support structure or bucket. The articulated boom also includes means for 
causing pivotal movement of the lower boom portion relative to the frame, 
and means for causing pivotal movement of the upper boom portion relative 
to the lower boom portion. These means preferably include hydraulic 
assemblies powered by a hydraulic pump. The boom also includes means for 
leveling the workman support bucket throughout the range of movement of 
the upper and lower boom portions. The leveling means includes tension and 
holdback sections of elongated material extending through the boom 
portions from the lower end of the lower boom portion to the workman 
support bucket. Preferably, the tension and holdback sections are part of 
a closed loop of elongate material. The holdback section includes first 
and second segments. 
The aerial lift also comprises means for maintaining tension in the 
holdback section. This means includes a hydraulic cylinder having opposite 
first and second ends, with the first end of the cylinder being connected 
to one of the segments of the holdback section, a piston dividing the 
cylinder into first and second chambers respectively adjacent the first 
and second ends of the cylinder, and a piston rod extending through the 
second chamber and having one end fixedly connected to the piston and an 
opposite end connected to the other segment of the holdback section. 
In the preferred embodiment, the means for maintaining tension further 
includes a hydraulic accumulator communicating with the second cylinder 
chamber. The accumulator, which is preferably located inside the 
articulated boom, automatically maintains the tension in the holdback 
section by pressurizing the second cylinder chamber. Therefore, it is not 
necessary to provide access to the interior of the boom in order to adjust 
the tension in the holdback section. 
In an alternative embodiment, the second cylinder chamber communicates, via 
a pressure reducing valve and a check valve, with the hydraulic pump 
rather than with a hydraulic accumulator. This arrangement causes the 
second cylinder chamber to be pressurized every time the pump is turned 
on. 
A principal feature of the invention is the provision of means for 
automatically maintaining tension in the holdback section. With known 
turnbuckle arrangements, tension is decreased by stretching or elongation 
of the elongate material in the holdback section. With the disclosed 
arrangement, the hydraulic accumulator both reduces stretching of the 
elongate material and also maintains the desired tension in the holdback 
section regardless of stretching of the elongate material. 
Other principal features and advantages of the invention will become 
apparent to those skilled in the art upon review of the following detailed 
description, claims and drawings.

Before one embodiment of the invention is explained in detail, it is to be 
understood that the invention is not limited in its application to the 
details of construction and the arrangements of components set forth in 
the following description or illustrated in the drawings. The invention is 
capable of other embodiments and of being practiced or being carried out 
in various ways. Also, it is to be understood that the phraseology and 
terminology used herein is for the purpose of description and should not 
be regarded as limiting. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
An apparatus 10 embodying the invention is illustrated in the drawings. As 
best shown in FIG. 1, the apparatus 10 comprises a vehicle or truck 12, 
and an aerial lift 14 mounted on the truck 12. More specifically, the 
truck 12 has mounted on the rear end thereof a conventional turntable 16, 
and the aerial lift 14 is mounted on the turntable 16 for pivotal movement 
relative to the truck 12 about a generally vertical axis. 
The aerial lift 14 comprises a frame 18 mounted on the turntable 16, and an 
articulated boom 20 including a first or lower boom portion 22 having an 
upper end, and a lower end pivotally mounted on the frame 18 for pivotal 
movement relative thereto about a generally horizontal axis 24. The 
articulated boom 20 also includes a second or upper boom portion 26 having 
an upper end, and a lower end mounted on the upper end of the lower boom 
portion 22 for pivotal movement relative thereto about a generally 
horizontal axis 28. The boom portions 22 and 26 include respective 
housings 30 and 32, preferably made of steel and fiber reinforced plastic, 
which house the internal components of the boom 20. The boom 20 further 
includes a workman support structure or bucket 34 mounted on the upper end 
of the upper boom portion 26 for pivotal movement relative thereto about a 
generally horizontal axis 36. 
The articulated boom 20 further includes means for causing pivotal movement 
of the lower boom portion 22 relative to the frame 18, and means for 
causing pivotal movement of the upper boom portion 26 relative to the 
lower boom portion 22. While various suitable means can be employed, in 
the preferred embodiment, the means for causing pivotal movement of the 
lower boom portion 22 includes (see FIG. 1) a hydraulic assembly 38 
connected between the frame 18 and the lower boom portion 22, and the 
means for causing pivotal movement of the upper boom portion 26 includes a 
hydraulic assembly 40. As best shown in FIG. 2, the assembly 40 includes a 
cylinder and piston assembly 42 mounted within the lower boom portion 
housing 30, and a flexible, elongate member 44 which extends from the 
cylinder and piston assembly 42, which passes over a pulley 46 mounted for 
pivotal movement on the axis 28, and which is connected to the upper boom 
portion 26. This arrangement is conventional and will not be described in 
greater detail. 
The hydraulic assemblies 38 and 40 are powered by a hydraulic pump 41 
communicating with a reservoir 43. As shown schematically in FIG. 4, the 
pump 41 communicates with the hydraulic assemblies 38 and 40 via 
respective control valves 150 and 152. This arrangement is also 
conventional. 
The articulated boom 20 also includes means for leveling the workman 
support bucket 34 throughout the range of pivotal movement of the boom 
portions 22 and 26. While various suitable leveling means can be used, in 
the illustrated construction, the leveling means includes a closed loop 47 
of flexible, elongate material extending through the boom portions 22 and 
26 and within the housings 30 and 32 from the lower end of the lower boom 
portion 22 to the workman support bucket 34. More particularly, the 
leveling means includes a first or lower sprocket 48 fixedly connected to 
the frame 18 and centered on the axis 24, and a second or upper sprocket 
50 pivotally mounted on the upper end of the upper boom portion 26. The 
upper sprocket 50 engages, via a single gear reduction 52, a sprocket 54 
which is pivotally mounted on the end of the upper boom portion 26 for 
rotation about the axis 36, and which is fixedly connected to the workman 
support bucket 34. Thus, pivotal movement of the upper sprocket 50 causes 
pivotal movement of the sprocket 54 and pivotal movement of the bucket 34. 
The leveling means also includes idler sprockets 56 mounted in the lower 
end of the lower boom portion 22, and pulleys 58, 59 and 61 mounted in the 
upper end of the lower boom portion 22 and in the lower end of the upper 
boom portion 26. This arrangement is conventional and will not be 
described in greater detail. 
In the preferred embodiment, the closed loop 47 includes a first or lower 
chain 60 extending over the lower sprocket 48, a second or upper chain 62 
extending over the upper sprocket 50, a first length 64 of elongate 
material extending through the boom portions 22 and 26 from one end of the 
lower chain 60 to one end of the upper chain 62, and a second length 66 of 
elongate material extending through the boom portions 22 and 26 from 
adjacent the opposite end of the lower chain 60 to the opposite end of the 
upper chain 62. As shown in FIG. 2, the first length 64 more particularly 
includes an insulator rod 68 connected to the lower end of the lower chain 
60, an insulator rod 70 connected to the lower end of the upper chain 62, 
and a cable 72 connecting the insulator rods 68 and 70 and reeved over 
pulleys 58 and 61. The second length 66 includes an insulator rod 80 
connected to the upper end of the lower chain 60, an insulator rod 82 
connected to the upper end of the upper chain 62, and a cable 84 
connecting the rods 80 and 82 and reeved over pulleys 59 and 61. The 
insulator rods prevent conduction of electricity through the articulated 
boom 20. 
The closed loop 47 can be viewed as including a first section 92 comprising 
the lower end of the lower chain 60, the first length 64, and the lower 
end of the upper chain 62, and a second section 94 comprising the upper 
end of the lower chain 60, the second length 66, and the upper end of the 
upper chain 62. As can be appreciated by viewing FIG. 2, tension is 
maintained in the first section 92 by the force of gravity acting on the 
workman support bucket 34. The first section 92 is accordingly referred to 
as the tension section. The second section 94, which is referred to as the 
holdback section, is not maintained under tension by the force of gravity 
acting on the workman support bucket 34. 
The aerial lift 14 also comprises means for maintaining tension in the 
closed loop 47, and preferably in the second or holdback section 94 of the 
closed loop 47. This means includes a hydraulic assembly 96 connected to 
the holdback section 94. In the preferred embodiment, the assembly 96 is 
interposed in the holdback section 94 and includes a hydraulic cylinder 98 
having first and second or right and left ends, with the right end being 
connected to the insulator rod 80. The hydraulic assembly 96 also includes 
a piston 100 dividing the cylinder 98 into first and second or right and 
left chambers 102 and 104, respectively, and a piston rod 106 extending 
through the left chamber 104 and having one end fixedly connected to the 
piston 100 and an opposite end connected to the upper end of the lower 
chain 60. The right cylinder chamber 102 communicates with the atmosphere 
via a vent 103. 
In the preferred embodiment, the tension maintaining means also includes 
(see FIG. 3) a hydraulic accumulator 108 communicating with the second or 
left cylinder chamber 104. Preferably, the hydraulic accumulator 108 is 
located inside the articulated boom 20, i.e., inside the housings 30 and 
32, and communicates with the cylinder 98 via a conduit 110. 
As best shown in FIG. 3, the hydraulic accumulator 108 includes a housing 
112, and a flexible diaphragm 114 separating the housing 112 into a fluid 
chamber 116 and a gas chamber 118. The fluid chamber 116 communicates with 
the cylinder 98 via the conduit 110. The gas chamber 118 communicates with 
a source (not shown) of gas under pressure. As is known in the art, the 
gas in the gas chamber 118 is maintained at a desired pressure, and this 
maintains the fluid in the fluid chamber 116 at the desired pressure. 
Because the fluid chamber 116 communicates with the left cylinder chamber 
104 via the conduit 110, the fluid in the left cylinder chamber 104 is 
also maintained at the desired pressure. Thus, the fluid in the left 
cylinder chamber 104 exerts on the hydraulic assembly 96 a force which 
tends to contract the hydraulic assembly 96 and which is equal to the 
fluid pressure times the area of the piston 100 within the left cylinder 
chamber 104. This force creates tension in the closed loop 47. 
Since the hydraulic accumulator 108 maintains the fluid in the left 
cylinder chamber 104 at the desired pressure, the tension in the closed 
loop 47 is not affected by elongation of the loop 47. The hydraulic 
assembly 96 will simply contract to take up any slack. Once the hydraulic 
accumulator 108 is charged, it is not necessary to gain access to the 
interior of the boom 20 in order to adjust the tension in the holdback 
section 94. The tension is automatically maintained by the gas in the 
hydraulic accumulator 108. 
It should be understood that while in the illustrated construction the 
leveling means includes a closed loop of elongate material, in alternative 
constructions the leveling means can include two separate lengths of 
elongate material. 
In an alternative embodiment of the invention, which is shown in FIG. 4, 
the left cylinder chamber 104 communicates with the pump 41 rather than 
with a hydraulic accumulator. More particularly, the chamber 104 
communicates with the pump 41 via a pair of pressure reducing or relief 
valves 154 and 156, and via a check valve 158 which permits fluid flow to 
the chamber 104 and prevents fluid flow from the chamber 104. Therefore, 
the chamber 104 is pressurized to the desired pressure (set by the valves 
154 and 158 each time the pump 41 is turned on. 
In another alternative embodiment (not shown), the pump 41 can be connected 
to the fluid chamber 116 of the accumulator 108, so that the fluid chamber 
116 is pressurized every time the pump 41 is turned on. 
Other features and advantages of the invention are set forth in the 
following claims.