THE selection of the proper type of wire rope for
each kind of service is most essential and should be the first
consideration of the wire rope user.
How to determine which construction and grade of
rope should be used to secure the most economical results in terms
of cost per foot-yard, or ton of material handled, is the subject
to which we shall direct your attention.
Granting wire rope is a complex machine, and a vital
unit in modern equipment which should not be misused if non-productive
work is to be kept to the minimum, its proper selection is of
great importance.
To meet the severe requirements of modern industry
in the diversified use of wire rope, the manufacturer must produce
many sizes and types or constructions of wire rope, each having
certain strength and characteristics. From this great variety
it is not difficult to select a wire rope of suitable size and
construction and made from the material best adapted to any proposed
service; provided, knowledge of the qualities of the ropes is
combined with knowledge of the conditions of use.
The first wire rope was made of iron and "iron
grade" is still used for certain purposes. Industrial developments,
however, have demanded wire rope of greater tensile strength to
meet requirements of increased production and exacting mechanical
conditions. As a result of these developments manufacturers produce,
principally, four grades of steel wire for fabrication into wire
rope, each differing in its physical characteristics.
These grades are usually referred to as cast steel,
extra strong cast steel, plow steel and improved plow steel. This
latter grade we term "Blue Center" steel wire rope --
a distinctive Roebling product. The order of listing is
indicative of the relative strengths, "Blue Center"
steel being the strongest. In these grades are to be found several
hundred different sizes and constructions of wire rope, making
it possible to select a proper size for practically any condition
of service.
Stress, Abrasion, Bending, Crushing and Corrosion
in many degrees and combinations are the principal prerequisites
of wire rope service, and must be carefully considered in the
selection of wire rope.
The rope must be of ample strength for the stress
requirements and the factor of safety must be chosen for the particular
service, since different installations require different factors
of safety.
In our previous discussion of wire rope constructions,
it was shown that there are proper ropes in the coarse laid group
to meet abrasive conditions; and in the group of flexible constructions.
others for conditions where bending is the major requirement.
Crushing, often erroneously identified as either
abrasion or bending, must be carefully analyzed and a construction
selected to suit this condition.
Corrosion is a very important factor which enters
into practically all rope service and exists not only by action
of moisture in the rope, but sometimes by carrying return or
stray electric current. However, this factror can be controlled
by the proper application of a suitable rope lubricant or protection
that will prevent the corrosion of the wires of the rope and the
deterioration of the hemp center.
Reserve strength, too, is an inherent characteristic
of wire rope which should certainly not be overlooked in determining
the proper rope to use.
It is not true that the lower grade or the cheapest
wire rope will prove to be the most economical, since analysis
and service records have proven that the proper quality and construction
of rope; even though the first cost be more, will prove most economical
and safest in the final results.
It should be appreciated that the service received
from wire rope on one installation cannot be correctly compared
with the service received from wire rope on another installation,
even though conditions appear to be the same as far as the usual
data is concerned.
On each installation there is some condition which
entirely governs the economy of rope life. It may be a different
characteristic in the operation of the engine or a difference
in some other part of the equipment or in the operating personnel.
Therefore, to make the necessary study and selection, each installation
must be considered individually.
To have an accurate check on the operation it is
apparent that records must be kept, but it should also be appreciated
that even the most carefully kept records will not tell the entire
story. For instance: the records will not show that the engineer
got up on the wrong side of the bed one morning and proceeded
to take it out on the engine, by opening the throttle and applying
the brakes with undue severity
Time-interval-of-service is another phase of the
subject that should not be used as a sole governing factor in
the selection of wire rope, or the retention in service of a certain
type or construction of rope. Records on this basis are very misleading.
Ropes handling greater average tonnage may show decreased
total service over the average. A rope hoisting greater tonnage
from lower levels will show reduced service over one hoisting
its tonnage from higher levels. Ropes working less than average
become affected by the corrosion factor. It is also true that
a rope working only a few days a month is not apt to receive the
same amount of attention as one which is working steadily.
To illustrate the value of service records in the
study of the selection of wire ropes, let us cite a mine installation
as typical.
It is really the number of trips made plus the distance traveled per trip and the loads handled, that should be considered-this is very hard to obtain.
The most satisfactory method for approximating this
is to keep a record of the tonnage of ore or coal or whatever
the product is, and the level from which the tonnage is hoisted.
In this record the tonnage from each level should be kept separate.
When it is considered that the lives of the miners,
as well as the product of the mine, depend upon the safe operation
of the Wire Hoisting Ropes, the comparatively short time required
to compile complete records would be well spent.
These records should show size; construction; quality
and make of wire rope; the mine and hoist where installed; the
date when installed; whether the rope is underwind or overwind;
condition of drum and sheave groove, at time of installation,
by template or other means of measuring; the date of cutting back
on the drum and at the attachment of the cage or skip: the dates
when lubricated and lubricant used, the date on which the wire
rope was turned and the tonnage handled up to that time (the same
data should be kept after turning); the date of removal from service
and the total tonnage.
The record should give the inspection reports on
the rope, showing the general condition and the worst broken-up
rope lay. Records should also show the changes in the main equipment
namely: regrooving head sheaves; replacing the sheaves or the
liners; or changes in speeds or loads. A careful analysis of this
data after several ropes have been run, would indicate whether
or not changes could be made, either in rope or operating practice,
which would produce more economical and safer service.
The size and character of many installations may
be, and frequently are, such as to make a close study as suggested
impracticable, and on initial installations there is no such opportunity
afforded, nor records available to the wire rope user.
Then, too, the average rope user is primarily interested
in the production of his plant or installation and cannot be expected
to know wire rope as does the manufacturer. On the other hand,
the manufacturer knowing thoroughly the physical and other properties
of the different grades and types or constructions, is no doubt
in a better position to make the best selection. It is absolutely
necessary, in order to recommend a rope of suitable size, of the
proper construction and made of the material best adapted to the
specific use to which it is to be put, that the manufacturer be
furnished with clear and complete data with reference to operating
conditions.
The manufacturers therefore through their engineering
department, are able to give wire rope users not only the benefit
of their knowledge of wire rope, but their wide experience and
investigations, based on records of many and varied installations.