POLYETHYLENE SHEAVES FOR ROPE-DRAWN TRANSPORTATION SYSTEM

POULIES EN POLYETHYLENE POUR CABLES DE TRACTION

English Abstract

Polyethylene Sheaves for Rope-Drawn
Transportation System
Abstract
A sheave for cooperating with a cable is made
from ultra-high-molecular-weight polyethylene
(UHMWPE). The groove angle is selected according to
the coefficient of friction of the UHMWPE so as to
prevent the cable from climbing out of the sheave
groove. The low coefficient of friction of UHMWPE
makes possible a very shallow groove.

Claims

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Claims
1. A sheave for supporting a moving cable under
tension, characterized in that said sheave is made
from ultra-high-molecular-weight polyethylene
(UHMWPE).
2. A sheave according to claim 1 wherein the
sheave has an angled groove disposed about its
circumference for receiving the moving cable, and the
angle of the groove is selected according to the
coefficient of friction of the UHMWPE.
3. A sheave according to claim 2 wherein the
coefficient of friction for the UHMWPE is
approximately 0.1 and the groove angle is at least
approximately 6 degrees.
4. A sheave according to claim 3 wherein the
groove angle is approximately 25 degrees.
5. A transportation system comprising a cable-
drawn vehicle wherein the cable is supported by
sheaves as described in claim 1.

Description

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Description
Polyethylene Sheaves for Rope-Drawn
Transportation System
Technical Field of the Invention
The invention relates to pul~leys, or sheaves
that cooperate with cables, such as in funicular-type
transportation sys~tems.
.
Background of the Invention
Transportation systems àre known in~which a
vehicle is drawn by a cable. For runs of appreciable
length, the cable must be~supported every thirty
meters, or so, by grooved sheaves. Due to the twist
of the cable, and the tension thereon, there is a
tendency for the cable to climb out of the sheave
groove.
Steel sheaves are the norm. They are
inexpensive and durable,~and their smooth hard
surface reduces the tendency for the cable to climb
out of the groove. However, they impart a
significant amount of wear to the cable. Rubber, or
rubber-lined steel sheaves are expensive and less
durable than their steel counterparts, but they are
much easier on the cable.
Disclosure of the Invention
ThereEore, it is an object of this invention to
provide a sheave that is both inexpensive and
durable, and that tends to keep the cable in the
groove without wearing on the cable.
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According to the invention, sheaves are
fabricated from ultra-high-molecular-weight
polyethylene (UHMWPE) and the groove angle is
minimized according to the coefficient of friction of
the UHMWPE.
Other objects, features and advantages of the
invention will become more apparent in light of the
following description thereof.
Brief Description of the Drawings
Fig. 1 is a perspective view showing the sheaves
of this invention in use in a cable-drawn
transportation system; and
Fig. 2 is an edge-on view of a sheave, typical
of the sheaves in Fig. 1.
Detailed Description of the Invention
Fig. 1 shows a transportation system wherein a
car 10 is attached to a cable 12 and moved thereby
along a smooth track 14. ~The car rides on a cushion
of air as provided by hoverpads 16. Sheaves 18,
disposed every thirty meters, or so, along the track
accommodate cable sag. The transportation system of
Fig. 1 is representative of the OTIS SHUTTLE.
FigO 2 shows a sheave 18 which is essen~ially a
disc having two sides 20, 22 and a circumferential
edge 24. The edge 24 is not flat, but is formed to
cooperate with the cable 12. Since the sheave must
turn in response to cable motion, it is attached via
a suitable axle 26 journaled to suitable support
means 28 on the track.
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The outer edge 24 of the sheave is generally V-
shaped, concave towards the center of the sheave, and
forms an angle A with respect to an imaginary surface
28, which represents the side of a squat cylindrical
disc from which the sheave may be machined. The
groove 24 is symmetrical from side-to-side, forming
two ears 30 and 32. At the apex of the groove it
abruptly becomes appro~imately quarter-circular in a
region 34, with a radius corresponding to the radius
of the cable 12.
In the Otis Shuttle, cable tensions of 45,000
Newtons and velocities of 10 meters per second are
common. Furthermore, the track does not always
follow a straight and level course. Although the
sheaves may be inclined to provide both lateral and
vertical suport to the cable in inclines and curves~
the cable will still try to climb out of the sheave.
The coefficient of friction of the sheave and
the angle of the groove are key factors in
maintaining the cable in t`he groove. The higher the
coefficent of friction, thè~easier it is for the
cable to climb out of the groove. The shallower the
groove, the easier it is for the cable to climb out
of the groove. Therefore, on the one hand, it is
necessary to make the sheave groove as deep as
possible. However, on the other hand, the deeper the
groove, the flimsier the ears 30, 32 will become for
a sheave of a given size. There is no analogous
limit to reducing the coefficient of friction.
Simply stated, the slipperier the groove, the better.

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The invention resides in the use of an ultra-
high-molecular-weight polyethylene (UHMWPE), such as
HOSTALEN ~ GUR, for the sheave material, in the
selection of~a groove angle based on sheave material,
and in the unexpected advantages of this unique
combination of features as applied to a cable-drawn
vehicle. HOSTALEN ~ GUR is available from American
Hoechst Corporation, Plastics Division, 289 North
Main Street, Leominster, Massachusetts 01453, and is
described in detail in Desi~ning with the toughest of
all plastics, Plastics Design Forum, May/June 1978.
As applied to the sheave of this invention,
UHMWPE exhibits outstanding features. It is
extraordinarily tough, yet its paraffin-like surface
resists cable abrasion and is easy on the cable. It
is easily machined, and has a low coefficient of
friction. Concerning the latter, the inventor has
selected a sheave groove angle that is related to the
coefficient of frictio~ to minimize the tendency for
the cable to climb out~of the groove.
The coefficient of friction for HOSTALEN ~ GUR
is approximately 0.1. Therefore the slope, or
tangent, of the groove must also be at least 0.1 to
give a critical sheave groove angle "A" of
approximately 6 degrees. A safety factor of 4 would
dictate a groove angle of about 25 degrees.
As stated hereinbefore, the disadvantage of
increasing the groove angle is weakening the groove
ears. With UHMWPE, a much lower groove angle can be
employed which results in a stronger, smaller sheave.

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It should be understood that the invention has
been described with respect to a particular
embodiment thereof, and that various additions and
changes can be made thereto without departing from
the spirit and scope of the invention.
What is claimed is: