Safety device for a cable hoist

A safety device for a platform cable hoist within a latticework tower including a plurality of skid members which are flexed inwardly when there is tension from a lift cable. Brake members are secured to the skid members and, in the event the cable fails, the skid members deflect outwardly so that the brake members, which have hook portions at their upper ends, engage transverse braces of the tower. Between the hook portions and the skid members, the brake members are formed with an energy absorbing portion which absorbs kinetic energy from the falling platform to slow and stop its descent.

BACKGROUND OF THE INVENTION 
This invention relates to a cable hoist for a platform contained within a 
latticework tower and, more particularly, to a safety device for such a 
hoist which is effective to stop the free fall of the platform in the 
event of a cable break or other failure. 
Cellular telephone base stations typically have an electronics assembly 
mounted where it is readily accessible to a technician and one or more 
antennas mounted on an elevated structure to increase the line-of-sight 
range of the base station. Recently, there has been developed a smaller 
cell site, called a microcell, to cover "hot spots" and "dead spots". The 
microcell has less power and provides fewer channels than a "normal" cell 
site and was designed for a smaller coverage area. However, for some 
applications it would be advantageous to increase the coverage area of the 
microcell. Increased coverage area could be achieved by installing a more 
powerful radio frequency amplifier. However, the size of the box 
containing the microcell is too small to accommodate the more powerful 
amplifier and dissipate the additional heat generated thereby. 
The increased coverage area could also be achieved by radiating from a 
taller tower, but if the cell site is at the base of the tower, 
significant losses occur in the cabling between the cell site and the 
antennas. In any event, in the microcell the antenna may be integrated 
with the electronics in the same box. Accordingly, it would be 
advantageous to locate the microcell at the top of the tower, since 
changing the elevation of the microcell from twenty feet to one hundred 
feet would increase the coverage area by a factor of about four. However, 
active electronics on the top of a tower need maintenance, so that the 
electronics either has to be lowered to a technician or the technician has 
to be able to raise and lower the electronics. This has been done in the 
past by using a cable and a winch with pulleys at the top of the tower. 
In the event that the hoist cable breaks or otherwise fails to hold the 
microcell, the microcell will go into a free fall condition. Accordingly, 
it would be desirable to provide a safety device for stopping the fall of 
a microcell under such conditions. 
SUMMARY OF THE INVENTION 
For cost reasons, the tower of choice would be a guyed latticework tower. 
One standard such tower is a triangular latticework structure measuring 
approximately three feet along each side. This structure is just large 
enough to fit a microcell within the confines of the tower. The advantages 
of putting the microcell within the tower are: 
If the microcell were to fall, it would be confined within the tower. 
The center of gravity of the microcell can be located very near the center 
of the tower, reducing distortions on the tower. 
The "superstructure" for supporting the winch and pulley arrangement that 
lifts and lowers the microcell can be supported across members of the 
tower, rather than cantilevered off the edge, resulting in a less 
expensive installation. 
The microcell can be constrained from "wobbling" as it moves up and down 
the tower by means of guides that are positioned against the ribs of the 
tower. If the microcell were supported external to the tower, added 
hardware would be needed to keep the microcell stable, thereby increasing 
the cost of the installation. 
According to the present invention, there is provided a safety device for a 
platform cable hoist for a platform within a latticework tower. The tower 
includes at least three vertically oriented members and a plurality of 
transverse braces interconnecting adjacent vertically oriented members. 
The hoist includes a lift cable having a first end adapted to be secured 
to the platform and a second end coupled to a winch. The safety device 
comprises a plurality of vertically oriented skid members secured to the 
platform. Each of the skid members is positioned between a respective 
adjacent pair of vertically oriented tower members and has a generally 
planar surface adapted to slidingly contact the transverse braces 
interconnecting the respective adjacent pair of vertically oriented 
members as the platform is raised and lowered. Each of the skid members 
has an upper portion extending above the platform. A plurality of cable 
connection members are each secured to a respective skid member upper 
portion and a plurality of connector cables are each secured at a first 
end to a respective cable connection member and at a second end to the 
lift cable. A plurality of brake members are each attached to a respective 
skid member upper portion between the platform and the respective cable 
connection member. Each of the brake members includes a hook portion 
extending outwardly and downwardly. The hook portion is sized to fit over 
a transverse brace of the tower. Each brake member also includes an energy 
absorbing portion coupling the hook portion to the respective skid member 
upper portion. Accordingly, when tension is applied by the lift cable 
through the connector cable, the skid member upper portions are flexed 
inwardly so that the brake member hook portions clear the transverse 
braces of the tower during movement of the platform. When tension is 
released, as by a cable break, the skid member upper portions move 
outwardly so that the brake member hook portions engage respective 
transverse braces as the platform falls, with the brake member energy 
absorbing portions gradually slowing the rate of descent of the platform 
until it comes to a halt. 
In accordance with an aspect of this invention, each of the brake members 
is formed unitarily from a planar sheet of metal with the energy absorbing 
portion being formed with corrugations extending generally horizontally. 
Accordingly, when a brake member hook portion engages a transverse brace 
as the platform falls, flattening of the corrugations occurs to absorb the 
kinetic energy of the falling platform. 
In accordance with another aspect of this invention, each of the brake 
members comprises a generally rectilinear sheet having one end formed as 
the hook portion and a region between the hook portion and the end 
opposite the one end being formed as the energy absorbing portion. That 
region has a plurality of apertures arrayed along a substantially vertical 
line. Each of the brake members also comprises a pin member secured to the 
respective skid member upper portion. The pin member extends through the 
aperture closest to the one end. Accordingly, when a brake member hook 
portion engages a transverse brace as the platform falls, the respective 
skid member moves downwardly relative to the sheet and the pin member 
applies a load to the material of the sheet between the aperture through 
which it extends and the next lower aperture, thereby deforming the 
material. If the applied load is sufficient, the material is sheared. This 
process absorbs the kinetic energy of the falling platform and continues 
down the sheet from aperture to aperture to slow the falling platform.

DETAILED DESCRIPTION 
Referring now to the drawings, FIG. 1 shows a portion of a tower, 
designated generally by the reference numeral 10, in which is installed a 
safety device for a cable hoist constructed according to the present 
invention. Illustratively, the tower 10 is a three-sided latticework tower 
having three vertically oriented members 12, 14, 16 which are 
interconnected by a plurality of transverse braces 18. Although the tower 
10 is shown as being triangular, other multi-sided towers or a circular 
tower can be utilized when practicing the present invention. In all cases, 
the transverse braces would interconnect adjacent vertically oriented 
members of the tower, so that the interior of the tower is open. 
The microcell 20 is secured between a top plate 22 and a bottom plate 24, 
together making up a generally polygonal platform which is raised and 
lowered. The vertices of the plates 22, 24 are each adjacent a respective 
one of the vertically oriented members 12, 14, 16. Secured to the plates 
22, 24, as by welding or the like, are a plurality of skid members 26, 28, 
30. Each of the skid members 26, 28, 30 is vertically oriented and is 
positioned between a respective adjacent pair of vertically oriented tower 
members 12, 14, 16. Each of the skid members 26, 28, 30 has a generally 
planar outwardly facing surface adapted to slidingly contact the braces 18 
as the platform is raised and lowered. Further, each of the skid members 
26, 28, 30 has a respective upper portion 32, 34, 36 extending above the 
top plate 22. Near the top of each upper portion 32, 34, 36, is secured a 
respective cable connection member 38, 40, 42, illustratively an eyelet 
plate. 
The platform including the microcell 20 is adapted to be raised and lowered 
along the tower 10 by a cable hoist including a lift cable 44 having a 
first end 46 secured to the platform and a second end coupled to a motor 
(not shown). Secured to the first end 46 of the cable 44 are a plurality 
of connector cables 48, 50, 52. Each of the connector cables 48, 50, 52 
has its first end secured to a respective one of the cable connection 
members 38, 40, 42 and its second end to the first end 46 of the cable 44, 
illustratively by means of the ring 54. 
The present invention provides a "brake" for the microcell platform in the 
event that the cable 44 breaks or otherwise fails to hold the platform. 
Specifically, a plurality of brake members are each attached to a 
respective skid member upper portion 32, 34, 36 between the platform and 
the respective cable connection member 38, 40, 42. Each brake member 
includes a hook portion extending outwardly and downwardly and sized to 
fit over a transverse brace 18 of the tower 10, and an energy absorbing 
portion coupling the hook portion to the respective skid member upper 
portion. 
In a first embodiment of the present invention, as shown in FIGS. 1-4, each 
of the brake members 56 is formed unitarily from a planar sheet of metal 
with the energy absorbing portion 58 being formed with corrugations 
extending generally horizontally and with the hook portion 60 extending 
outwardly from the skid member upper portion 32. Each of the skid member 
upper portions is a generally planar sheet of metal and, according to the 
first embodiment, each of the brake members 56 is formed from an elongated 
three-sided rectilinear segment cut from the respective skid member upper 
portion sheet, with the uncut fourth side of the segment attaching each 
brake member to its respective skid member upper portion. 
FIG. 4 shows two of the skid members 26, 28 secured to the cable 44. For 
clarity, only two of the skid members have been illustrated. As the cable 
44 lifts the microcell platform, the upper portions 32, 34 of the skid 
members 26, 28 flex inwardly, so that the hook portions 60 of the brake 
members 56 clear the transverse braces 18 of the tower 10. The weight (W) 
of the microcell platform is balanced by the tension (T) on the cables 48, 
50, 52 to the skid members 26, 28, 30. Thus, W=3 T sin .theta.. Simple 
trigometric relations yield the horizontal component of the tension 
(T.sub.x) to be T.sub.x =W/3 tan .theta.. Thus, the horizontal component 
of the tension increases to infinity as the angle .theta. approaches 
0.degree.. Accordingly, by changing the angle .theta., any desired force 
to flex the skid member upper portions 32, 34, 36 can be obtained. 
In the event of failure of the cable 44, the cable tension is lost and the 
skid member upper portions 32, 34, 36 spring back to the vertical, 
unflexed, position. Then, as the microcell platform falls downwardly, the 
hook portions 60 engage the nearest transverse braces 18 on the tower 10. 
As the microcell platform continues to fall, its kinetic energy is 
absorbed by the energy required to flatten the corrugations 58. 
The design parameters must be carefully balanced for this design to be 
effective. The length and cross section of the skid members 26, 28, 30 
must be stiff enough to resist flexing too easily, must be flexible enough 
to bend clear of the braces 18 when loaded with the weight of the 
microcell platform, and must stay springy under load (i.e., must not 
undergo plastic deformation). Also, the angle of the connector cables 48, 
50, 52 must be carefully controlled. If the connector cables 48, 50, 52 
form too shallow an angle, the horizontal loads can get very high, even 
for a modest lifting load. At the other extreme, if the connector cables 
48, 50, 52 are almost vertical, there will be insufficient horizontal 
loads to deflect the skid member upper portions 32, 34, 36. 
FIGS. 5-7 illustrate a second embodiment of a brake member 62 according to 
this invention. In this embodiment, the brake member 62 is formed from a 
generally rectilinear sheet separately from each skid member. One end of 
the sheet is formed into the hook portion 64 of the brake member 62 and 
the region between the hook portion 64 and the opposite end 66 is formed 
as the energy absorbing portion 68. The lateral edges of the brake member 
62 adjacent the energy absorbing portion 68 are folded back away from the 
hook portion 64 to form opposed channels 70 for receiving the respective 
skid member upper portion. 
The energy absorbing portion 68 is formed with a plurality of apertures 72 
arrayed along a substantially vertical line. Each of the apertures is 
substantially rectangular and has a notch 74 located substantially 
centrally within the aperture 72 and extending toward the next lower 
aperture 72. All of the notches 74 lie along the substantially vertical 
line. A pin member 76, illustratively a headed bolt, extends through the 
aperture 72 which is closest to the hook portion 64 and is secured to the 
skid member upper portion 32, as by the nut 78. 
In the event of failure of the cable 44, if the microcell platform falls, 
the hook portion 64 engages a transverse brace 18 to stop the motion of 
the brake member 62. However, the microcell platform continues to descend 
and, as the skid member 26 continues to descend, the bolt 64 applies a 
large load to the crumple region 80 between the uppermost aperture 72 and 
the next lower aperture. This causes the crumple region 80 to deform. If 
the load is sufficient, the load is concentrated by the notch 74 and the 
crumple region 80 shears, dropping the bolt into the next lower aperture 
72. The deformation and shearing of the crumple region 80 absorbs kinetic 
energy from the falling microcell platform, thereby slowing its fall. The 
aforedescribed process continues until all of the kinetic energy of the 
falling microcell platform is absorbed. 
In an experiment, the brake member 62 was formed from a 0.090" thick sheet 
of aluminum. Each of the apertures 72 was 2" wide and 0.60" high and each 
crumple zone 80 was 0.267" high. In the experiment, a cart filled with one 
hundred pounds of concrete was put on a ramp that allowed the equivalent 
of a twenty inch vertical drop. During the experiment, four crumple 
regions 80 sheared, a fifth crumple region 80 was deformed but not broken, 
and a sixth crumple region 80 was untouched. 
As previously described, the dimensional parameters of the safety device 
must be selected carefully. For example, if the platform and microcell 
weigh several hundred pounds, the skid member upper portions 32, 34, 36 
become very long in order to avoid plastic deformation for any reasonable 
skid cross section. To avoid this problem, as shown in FIG. 8, the lift 
cable 44 may be extended to fasten to the retainer 82 secured to the top 
plate 22. In this configuration, a further retainer 84 is rigidly fastened 
to a point on the cable 44 so that the three connector cables 48, 50 and 
52 are geometrically positioned to provide the appropriate amount of 
deflection to the skid member upper portions 32, 34, 36. This 
configuration displaces most of the lifting load from the skid member 
upper portions 32, 34, 36 to the top plate 22. Thus, the dimensions of the 
skid member upper portions 32, 34, 36 can be selected based only on their 
need to deflect away from the tower transverse braces 18. Alternatively, a 
separate cable section could be provided between the ring 54 (FIG. 1) and 
the retainer 84. 
Accordingly, there has been disclosed an improved safety device for a cable 
hoist. While illustrative embodiments of the present invention have been 
disclosed herein, it is understood that various modifications and 
adaptations to the disclosed embodiments are possible. Thus, while a 
triangular latticework tower has been described, it will be appreciated 
that, provided there are transverse braces which can be "hooked" by the 
inventive safety device, any shape tower can accommodate the inventive 
safety device. It is therefore intended that this invention be limited 
only by the scope of the appended claims.