Insulated boom structure for telescoping aerial lift

The invention provides improved telescopic boom structure for aerial lifts, including the provision of a lower boom structure outer end portion made of electrical insulating material, which together with other provisions, accomplishes the essential elimination of electric shock hazard to personnel without incurring any of the disadvantages of significantly increased expense, reduced effective working area due to reduced range of boom extension, or increased stowed length of the boom structure.

FIELD OF THE INVENTION 
This invention concerns improved insulated boom structure for telescoping 
aerial lifts. 
DESCRIPTION OF THE PRIOR ART 
Aerial lifts of the type in which a personnel carrier bucket is mounted at 
the outer end of a telescoping boom are commonly used for work in areas 
where electric power lines are present, with the result that personnel may 
be exposed to the hazard of an electric shock that could occur due to the 
completion of an electric circuit through the body of a person occupying 
the bucket and via the telescoping boom mechanism to ground. This problem 
has been addressed in the prior art of which I am aware by making the 
upper or outermost section of the boom, to which the bucket is mounted, of 
insulating material such as fiber glass. This arrangement, however, does 
not provide adequate shock protection because the next outermost or lower 
boom section is metal and the upper boom section can be retracted to 
positions near enough to the next outermost or lower boom section to be 
reached by a person occupying the bucket and furthermore, safe leakage 
current levels can be exceeded in the presence of high voltage. 
It has been suggested that the inward travel of the upper boom section 
could be limited by a stop means such that safe leakage current levels 
would not be exceeded in the presence of high voltage and a person 
occupying the bucket could never reach the metal lower boom section. If 
this is done, however, the range of the boom extension is accordingly 
limited and the effective working area is reduced, and also the stowed 
length of the boom structure is increased. 
The objective to this invention is to provide an improved telescopic boom 
structure for aerial lifts such that the electric shock hazard to 
personnel is essentially eliminated without incurring any of the 
disadvantages of significantly increased expense, reduced effective 
working area due to reduced range of boom extension, or increased stowed 
length of the boom structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 there is shown an aerial lift which incorporates improved 
telescoping boom structure 11 in accordance with a preferred embodiment of 
the invention. The improved telescoping boom structure 11 is mounted in a 
conventional manner to a support arm structure 13 for pivoting movement in 
a generally vertical plane so as to provide elevational movement for the 
telescoping boom structure 11. Conventional means (not shown) are provided 
to power and control the elevational movement of the telescoping boom 
structure 11. The support arm structure 13 is mounted for rotational 
movement in a conventional manner on a pedestal structure 15 which in turn 
is adapted for mounting on a transport vehicle (not shown). The support 
arm structure 13 is powered and controlled by conventional means (not 
shown) so as to provide swinging movement for the telescoping boom 
structure 11. 
The telescoping boom structure 11 is made up of a lower boom section 17 and 
an upper boom section 19. The upper boom section is adapted for mounting a 
personnel carrier bucket 21 in a conventional manner. 
As can best be seen in FIG. 2, the lower boom section 17 is made up of a 
main portion 23 which is made of metal and an outer end portion 25 which 
is made of an electric insulating material, preferably fiber glass. The 
entire length of the upper boom section 19 is made of an electric 
insulating material, preferably fiber glass. The upper boom section 19 is 
mounted for extending and retracting movement within the lower boom 
section 17 and is supported within the lower boom section 17 by 
conventional bearing means 27, 29. Conventional means, shown as a 
hydraulic cylinder 31 with a reciprocable output rod 33, is provided to 
power the upper boom section 19 for extending and retracting movement. 
Shown schematically at the outer end portion of the upper boom section 19 
is a portion of conventional means 35 for mounting the personnel carrier 
bucket 21. 
The means 31, 33 for powering the upper boom section is generally 
electrically conductive, and has an electrically conductive upper 
extremity 37 which, when the upper boom structure is fully retracted, is 
near the upper extremity 39 of the electrically conductive lower boom 
structure main portion 23. The lower boom structure outer end portion 25, 
which is made of electric insulating material, when the upper boom 
structure is fully retracted, extends beyond the electrically conductive 
upper extremities 37, 39 for a distance that is sufficient to insure that 
a person occupying the bucket could not reach the metal portion of the 
lower boom section and that safe leakage current levels will not be 
exceeded in the presence of high voltage. The effect of providing a lower 
boom structure outer end portion 25 made of insulating material is to move 
the upper extremity 39 of the metal portion of the lower boom structure 
inward, permitting the insulating outer end portion to be a structural 
substitute for the length of the metal portion that was moved inward. 
Tests have shown that this distance should be at least 36 inches, and it 
has been found in practice that 42 inches is a desirable distance. 
It should be apparent in view of the foregoing that the invention provides 
improved telescopic boom structure for aerial lifts such that the electric 
shock hazard to personnel is essentially eliminated without incurring any 
of the disadvantages of significantly increased expense, reduced effective 
working area due to reduced range of boom extension, or increased stowed 
length of the boom structure. 
The foregoing disclosure and the showings made in the drawings are merely 
illustrative of the principles of this invention and are not to be 
interpreted in a limiting sense.