1192

THE COLLIERY GUARDIAN.

Mat 29, 1914.

further warned the workmen that “ if the experience
of the business world should unhappily demonstrate
that unionism is symbolical of tyranny and treachery,
the position of the workmen will become hard
indeed.” These sentiments are reiterated by
Mr. Samuel Price, the Royal Commissioner who
enquired into the later strike, on behalf of the
Government.

It should not be forgotten that attempts were
successfully made to engage the active sympathies of
the British miner in what has plainly been shown to
be a high-handed cabal of the American union. All
of which proves how easy a dupe is the working man
in his collective capacity.

How many of those who habitu-
all3’ use the term " factor of
actor Of safety ” clearly understand its
Safety.	.	J . .

meaning? In mining operations
there are innumerable occasions
when ropes or machinery are subjected to severe
and unusual strains, and a blind confidence in
“ factors of safety ” may often prompt men to
take risks which would otherwise be rigorously
shunned. Let us, then, devote a column to the
consideration of what is the real significance of
the expression, “ factor of safety,” which, like
the blessed word Mesopotamia, imparts so
mystical a sense of comfortable assurance to the
user.

Some weeks back Mr. C. E. Stromeyer read
before the Staffordshire Iron and Steel Institute
a paper upon this subject, and his remarks are
well worthy of serious consideration by everyone
who employs mechanical aids to work. He points
out that the expression, “ factor of safety,” was
originally introduced for the purpose of inspiring
confidence. It is generally understood that a
machine or appliance such as, for example, a
haulage rope or a bridle chain, with a factor of
safety of say five, had been tested to five times
the maximum stress for which it was designed,
and in this general sense the term implies a
margin of safety which seems to be an efficient
safeguard even against occasional overloading.

On the Continent it is more usual to employ the
expression " permissible working load,” or some
such term, instead of “ factor of safety,” and
there is much to be said in favour of this practice.
The factor of safety in its relation to winding
ropes has recently been widely discussed in the
German technical Press, one of the main points
raised being the influence of depth, and, conse-
quently, of the increased weight of the rope itself
upon the normal factor of safety. Col. Blackett
alluded to the same subject when the general
regulations were before Lord Mersey. The regu-
lations provide that the strength of cappings, in
the case of haulage ropes, shall be based upon the
ultimate strength of the rope, whereas the owners
contended that the guiding factor should be the
maximum load, as in the case of winding ropes.
The distinction is subtle, but not immaterial.

The term "permissible working load” possesses
the advantage that a definite limit of safety is
guaranteed by the manufacturer’s tests. " Factor
of safety ” virtually means nothing at all. It is
a figure obtained in part by laboratory tests, in
part by mathematical calculation based upon
assured knowledge which is frequently wide of the
truth. As Mr. Stromeyer says, " the mathe-
matical formulae used by engineers are of the
crudest, and do not correctly determine the actual
stresses produced by external forces.” It is
generally assumed that the factor of safety in, for
example, a piece of metal, is the ratio of ultimate
stress to elastic limit. Thus if the ultimate break-
ing stress is 20 tons, and the elastic limit- is four
tons, we say that the factor of safety is five. But
it is not by any means an easy matter to deter-
mine either of these figures with accuracy. The
utmost that can be done is to take the mean of
a number of tests often widely divergent. Mr.
Stromeyer, indeed, goes so far as to say that in
certain cases the figure called " factor of safety ”

is made up of 50 per cent, of mathematical con-
venience and 58 per cent, of ignorance as to the
true value of the elastic limit. The truth is that
the elastic limit is a somewhat inconstant quan-
tity. As we now know from the results of
fatigue experiments, the elastic limit is reduced
by repeated stress, and this is a consideration
which possibly assumes great importance in con-
nection with such appliances as wire ropes.

Then we have the influence of the law of pro-
bability. . There are innumerable small causes
which may influence elastic limit one way or the
other to a slight degree. These may generally
tend to neutralise each other, but under certain
conditions they may be cumulative. Thus if there
are 10 of these minute influences at work, each
capable of influencing the elastic limit either
positively or negatively to a slight extent, say one
ton, this will give 210 or 1,024 possible combina-
tions of chances, giving a total range of 10 tons
in the break down point, and there is one chance
in 1,024 that all the influences will be adverse.
Similiarly we may calculate other chances, with
the result that there is always a possible ’ error,
and also a probable average error, in the experi-
mental determinations upon which the ‘‘ factor
of safety ” figure is based.

Let us take another case of the greatest impor-
tance in mining operations, viz., the difference
between static and dynamic stress, that is,
between dead loads and live loads. In the text-
books it is usually shown that a load which is
applied suddenly results in a stress which is twice
as great as that which a gradually increasing load
will produce. Mr. Stromeyer believes that this
is not true in all cases, and that suddenly applied
loads often produce a much greater stress than
double the static effect. We are here apparently
in a state of profound ignorance, and have been
too ready to deduce a general law from insufficient
data. This question came recently into promin-
ence in Mr. W. Walker’s report on an accident
at Bufford Colliery, due to the failure of a spring-
hook under a sudden jerk. The factor of safety
as shown by testing was about 4-3, and it was
admitted to have been insufficient under the cir-
cumstances of dynamic stress. It was, in fact,
recommended in this report that 10 should be a
minimum factor of safety in connection with all
ropes, chains, hooks, etc., used in such work as
sinking operations. Even this margin is quite
uncertain when questions of momentum have to
be considered. Very similar conditions prevail in
connection with the strength of winding ropes and
more particularly of cage cappings. Breaking
strains in such cases, as found by testing under a
gradually increasing load, give but little informa-
tion as to the results of a violent jerk, such as
may readily be experienced in practical use.

The above examples go to show how mythical
the "factor of safety” figure may be. Permis-
sible safe load would seem to be a far more trust-
worthy guide in practice; but even in this case the
term has only a limited application on account of
changes due to the ageing of materials in use. A
steel rope may start from the factory with a high
breaking limit, but this becomes continually lower
in use, partly as a result of continued and repeated
stresses inducing fatigue, partly by reason of
deterioration of the material. The latter factor is
especially serious in the case of certain metals
and alloys entering now somewhat largely into the
construction of machinery of all kinds. Mr.
Stromeyer remarks that the risk of ageing is so
great that some engineers are becoming shy of
. using certain nickel-steel alloys and some brasses
•and bronzes, because these sometimes become
absolutely rotten after a time, even although they
may be perfectly satisfactory when new. There
is much yet that remains to be investigated in
the mysterious diseases to which some metals
seem to be liable.

Although we have called attention to some of
the defects that may vitiate ‘ ‘ factor of safety
calculations, and to the uncertainty in which this
term is often involved, it is not to be concluded

that no value at all is to be placed upon such
figures. Engineers have, of course, long known
what degree of faith should be placed in values so
computed. The danger arises from those who
take "factor of safety” too seriously, and are
thereby led to try and get too much out of
machinery, or to misuse appliances, and especially
to delay necessary renewals. We have a lively
recollection of travelling up a step incline in a
German pit, where we were literally hanging on a
rope whose appearance suggested the last stages
of senile decay, and we were scarcely encouraged
by the assertion of our worthy guide that a new
rope was actually ready to be installed. Even in
this case, however, the " factor of safety ” proved
to be sufficient for the work in hand, possibly
because the thousand-and-one chances computed
by the laws of probability were not on this occasion
all adverse. We can scarcely refrain from the
belief, however, that not a few accidents arise from
working too near the limiting conditions, with an
insufficient margin to allow of possibilities which
cannot be mathematically defined.

LABOUR AND WAGES.
South Wales and Monmouthshire.

Speaking at Aberdare on Friday, Mr. W. Brace, M.P.,
president of the South Wales Federation, said that if the
employers, in their contracts, did not provide for the men’s
demands in regard to the Minimum Wage Act, there was a
battle royal in sight. The men were determined that better
wages and safety must come before dividends.

Mr. T. Bichards, M.P., secretary of the Federation,
declared at Varteg on Friday, that the old sliding scale was
ineffectual, the Conciliation Board was fast becoming
obsolete, and a better scheme would have to be devised.

Upon the question of amalgamating the enginemen's and
stokers’ organisation with the Miners’ Federation, the
executive council of the former have ordered yet another
ballot of the members.

Cardiff coal trimmers will take a holiday on Whit-Tuesday.

Alleging that gas had accumulated in the workings, about
800 men refused on Tuesday to descend the No. 3 South
Griffin Colliery, Blaina.

A special meeting of the Western Valley miners was held
at Abertillery on Monday, when the Celynen men applied
for leave to tender notice, upon the ground of victimisation.
Approval was given, and the matter now goes to the federa-
tion executive.

North of England.

The latest published minutes of the joint committee meet-
ing of the Northumberland miners and coal owners state
that Mr. Edward Shortt, K.C., M.P., has been elected
chairman for the ensuing 12 months.

Contrary to expectation, the Northumberland Miners’
Association Council held a 5th session on Thursday of last
week, when that portion of the agenda which they had
previously decided to deal with six months hence was dis-
cussed. The following resolution from Ashington was
defeated by 54 votes to 15	“ That our association repre-

sentatives ’meet the Coal Owners’ Association representatives
with a view of getting an allowance (such to be shown
separately on the pay checks) instead of colliery houses.” A
rather drastic resolution from New Delavel that “ we again
ask the owners for the restoration of the short shifts obtained
by the night workers prior to the passing of the Eight Hours
Act; and should our request be refused we ballot our,mem-
bers on a stoppage of work to enforce our demands,” was
defeated by 35 votes to 20, the council agreeing that the
shifts should be restored, but opposing balloting on the ques-
tion of a strike, as the funds of the association were not
large enough to support a strike of the members. By 42
votes to 21 the council recommended that the association
retain their scholarships at Buskin College, Oxford, Ashing-
ton’s resolution that they be transferred to the Central
Labour College, London, being lost. Old Delavel moved the
following resolution, which was unanimously carried :—
“ That we seek to- obtain much better arrangements for
heating and ventilation of heapsteads and screens, and
other matters adding to the comfort of the surface workers,
notably eating houses, commonly called ‘ bait cabins.’ ” The
Blucher Pit resolution, “ that we endeavour to get inserted
in the Coal Mines Act a clause to restrict shot firing opera-
tions to night time, or some time when there is least number
of workers underground,” was carried by a substantial
majority. The county will vote upon the resolutions after
recommendations of the council have been published.

The dispute with the miners in the Slatey seam at the
Alice Colliery, in Dearham, has been settled with the
owners. The managers finally agreed to continue the work-
ing of the seam, providing the men accepted a reduction of
2d. per ton if not trailing 50 yds., and Id. per ton if trailing
50 yds. A penny per ton to be returned to those suffering
a reduction of 2d. as the trail increased to 50 yds. The
miners agreed to accept the terms, and -work is proceeding.

A resolution, pledging themselves to support, financially
and morally, the Ellington miners in the attitude they have
taken up on the question of fines, has been passed by men
employed at Ashington, Woodhorn, Linton, and Ellington
collieries, Northumberland.

Federated Area.

Mr. William Johnson, M.P., general secretary, addressed
a well-attended meeting of the Birch Coppice Lodge of the
Warwickshire Miners’ Association at the Central Schools,
Polesworth, on Sunday. He said negotiations for increasing
the wages of surfacemen who manipulated coal had come to
splitting a penny. He believed they would settle it on
Thursday. It would mean 3d. to 6d. a day advance, and it
was to come into operation on June 1. The advance would
be for the members of the Warwickshire Miners’ Associa-
tion only. Alluding- to the North Warwickshire Miners’
Association, Mr. Johnson urged the men not to be led away
when they were told that men were joining by hundreds.

The executive, committee of the Yorkshire Miners’ Asso-
ciation on Tuesday concluded a two-days’ meeting. It was