GB2174173A

GB2174173A - Sheaves for cable-drawn transportation system

Info

Publication number: GB2174173A
Application number: GB08609183A
Authority: GB
Inventors: William H Gibson
Original assignee: Otis Elevator Co
Current assignee: Otis Elevator Co
Priority date: 1985-04-15
Filing date: 1986-04-15
Publication date: 1986-10-29
Also published as: CA1243295A; AU582839B2; DE3611258A1; GB8609183D0; FR2582372A1; AU5528386A; ZA862007B

Abstract

A sheave 18 for cooperating with a cable 12 is made from ultra- high-molecular-weight polyethylene (UHMWPE). The groove angle A 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. <IMAGE>

Description

SPECIFICATION Sheaves for Rope-Drawn Transportation System Technical Field of the Invention The invention relates to pulleys, or sheaves that cooperate with cables, such as in funicu lar-type transportation systems.

Background of the Invention Transportation systems are 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 Therefore, 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.

According to the first invention, sheaves are fabricated from ultra-high-molecular-weight polyethylene (UHMWPE) and the groove angle is reduced 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 of an example thereof.

Brief Description of the Drawings Figure 1 is a perspective view showing sheaves according to this invention in use in a cable-drawn transportation system; and Figure 2 is an edge-on view of a sheave, typical of the sheaves in Fig. 1.

Detailed Description Figure 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 SHUT TLE.

Figure 2 shows a sheave 18 which is essentially a disc having two sides 20, 22 and a circumferential edge 24. The edge 24 is not flat, but is formed as a groove 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.

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 the squat cylindrical disc from which the sheave may be machined.

The groove 24 is symmetrical from side-toside, forming two ears 30 and 32. At the apex of the groove it abruptly becomes approximately 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 tq 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 the groove. The higher the coefficient of friction, the 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.

The invention resides in the use of an ultrahigh-molecular-weight polyethylene (UHMWPE), such as HOSTALENtGUR, for the sheave material, in the selection of a groove angle, and in the unexpected advantages of this unique combination of features as applied to a cabledrawn vehicle. HOSTALEN"GUR is available from American Hoechst Corporation, Plastics Division, 289 North Main Street, Leominster, Massachusetts 01453, and is described in detail in Designing 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 defined as a linear homopolymer of polyethylene which may have an intrinsic viscosity of 16 or greater. 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 coefficent of friction.

Concerning the latter, the inventor has selected a sheave groove angle that is related to the coefficient of friction to prevent the cable from climbing 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. We have found that a groove angle of about 25 degrees provides a sheave of adequate strength and provides a safety factor of about 4.

As stated herein before, 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.

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 scope of the invention.

Claims

1. A sheave for supporting a movable cable under tension, characterized in that said sheave is made from ultra-high-molecularweight 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 UHMPE.

3. A sheave according to claim 1 or 2 wherein the coefficient of friction for the UHMWPE is of the order of the order 0.1 and the groove angle of the sheave is at least 6 degrees.

4. A sheave according to claim 1, 2, or 3 wherein the groove angle of the sheave is substantially 25 degrees.

5. A sheave for supporting a moving cable under tension, substantially as hereinbefore described with reference to the accompanying drawing.

6. A transportation system comprising a cable-drawn vehicle wherein the cable is supported by a plurality of sheaves as claimed in any preceding claim.