ALUMINUM IN CYCLE CONSTRUCTION—STRENGTH OF TUBES.
“We really thought that we were going to pass over a period of
three months without having to chronicle the discovery (?) of
a method of producing aluminum at a cost of not more than that
of first-class steel. The periodical inventor has appeared, and
this time he hails from Melrose, Mass., and his name is Washburn.
Next!!”—_Bicycling World._
Inventors do little harm in periodically making cheap aluminum or
increasing its strength without adding to its gravity, but when a
large corporation is started, as was done some months ago, with a
lot of money and aluminum medals issued, the same being made out of
copper, then the matter becomes serious. Probably, next to the hobby
of separating water and creating enormous power thereby, the aluminum
hobby holds undisputed sway. But as there really is something of
interest to cyclists and cycle makers in the subject, there seems a
need to touch upon it. Among the articles in the manufacture of which
aluminum can be satisfactorily used we find in the catalogue of a
well-known smelting firm mention made of bicycles, tricycles, etc. The
idea exists in the minds of many that a bicycle made from pure aluminum
would be a practical machine and much lighter than one of steel. This
notion arises from the fact that aluminum in the pure state has a
specific gravity of only 2.5, or about one-fourth the weight of steel.
Below we print a letter from the Cowles Smelting and Aluminum Company
on the subject.
“+Lockport, N.Y., U.S.A.+, August 20, 1888.
“+R. P. Scott+, Esq., Baltimore Md.:
+Dear Sir+,—Replying to your favor of August 16, you can obtain the
book on Aluminum, by Richards, from Philadelphia. Aluminum has a
great many uses in its commercial state, but a simple pure aluminum
casting has not sufficient strength to make it desirable for small
parts. If you could have it rolled or hammered to shape, so as to
make it rigid, it would become much more tenacious, but to secure
strength desired in bicycle parts, your castings would necessarily
be so large as to be ungainly, and we doubt if you would attain the
most desirable end,—viz., light weight. The alloys of copper and
aluminum are much better adapted to your requirements than the pure
metal could possibly be.
“Yours very truly,
“+The Cowles E. S. and Al. Co.+
“_Tucker_.”
It will be seen that the metal in its pure state lacks strength, and
can only be used in the arts to any extent when alloyed with copper
about in the proportion of nine of copper to one of aluminum. When
alloyed as above, it is about as heavy as steel.
AVERAGE ULTIMATE TENSILE STRENGTH OF METALS AND ALLOYS.
(_From Trautwine’s Engineer’s Pocket Book_, 1885.)
Pounds per
square inch.
Cast brass 23,000
Annealed brass wire 49,000
Cast copper 24,000
Annealed copper wire 32,000
Gun bronze of copper and tin cast 39,000
Average American cast iron 16,000
Good wrought iron 50,000
Best American wrought iron (exceptional) 76,100
Iron wire ropes 38,000
Malleable iron castings 48,000
Steel plates (rolled) 81,000
Cast steel average Bessemer ingots 63,000
+----------------------------------------------------------------------+
| ALUMINUM BRONZE. |
+---------+------+-----------------------+----------------------+------+
|Per cent.| | Tensile strength per | |Ingots|
| of |Grade.| square inch. | Elongation. | per |
|aluminum.| | | |pound.|
+---------+------+-----------------------+----------------------+------+
| | A 1 | 90,000 lbs. and over. | 0 to 5 per cent. |$0.45 |
| | A 2 | 75,000 to 90,000 lbs. |10 per cent. and over.| .40 |
| 10 | A 3 | 65,000 to 75,000 lbs. |25 ” ” | .37 |
| 7½ | B | 47,500 to 65,000 lbs. |20 ” ” | .33 |
| 5 | C | 35,000 to 47,500 lbs. |25 ” ” | .26 |
| 2½ | D | 27,500 to 35,000 lbs. |30 ” ” | .20 |
| 1½ | E | 20,000 to 27,500 lbs. |15 ” ” | .16 |
+---------+------+-----------------------+----------------------+------+
The specific gravity of the A grade is 7.56, that of steel
being 7.88. Its coefficient of expansion is small at ordinary
temperatures; its electrical conductivity is about 9, and with the
lower grades the expansion by heat, specific gravity and heat and
electrical conductivity increases the nearer the metal approaches
to pure copper. With more than eleven per cent. of aluminum the
bronze rapidly becomes brittle. In color, aluminum bronze of the C
and D grades is the nearest to gold of any known metal, the higher
grades being lighter in hue than the lower. The A grade melts at
about 1700° F., a little higher than ordinary bronze or brass.
Aluminum bronze shrinks about twice as much as brass.
In working aluminum I have found it to be a splendid substitute for
malleable iron, especially in many cases where the iron could not be
procured in time, or when it came so warped as to be unfit for use. I
have never been able, however, to get castings which would come quite
up to the strength claimed for it; the most satisfactory grade was
that of ten-per-cent. aluminum, which by the way is very hard to work,
especially in drilling. There is no doubt, however, that it can be
made to take the place of steel in many instances.
A knowledge of aluminum is a great boon to experimenters, as it will
probably come into quite general use with the manufacturer. The
ten-per-cent. aluminum finishes very handsomely, and in olden times
it would have been a splendid substitute for the brass hubs then so
common. As an antifriction metal it is unsurpassed by any of the
bronzes. It casts bright and sharp, but shrinks amazingly, although
not dangerously; at least I have never had a part of the casting drop
off, as in malleable it often does, and though the aluminum sometimes
leaves a great depression in the heavy part of the casting, it causes
no sponginess underneath. It can be readily bronzed or soldered.
Aluminum bronze drawn into wire will make very good spokes, and it has
been used for this purpose to some extent in England. All tendency to
rust is obviated, and it saves all nickeling; it resists corrosion
sufficiently well to dispense with any covering, but it does not look
as well as a nickel finish. No better authority on the subject can be
had than that of the Cowles Catalogue; useful information also can
be gathered from “Richards’s Aluminum,” and “Thurston’s Material of
Engineering.” The last-named treatise speaks on the subject as follows:
“The alloys of aluminum are very valuable. Its remarkable
lightness, combined with its strength, makes it useful as a
constituent of those alloys in which strength and lightness are the
needed qualities. It has a pleasant metallic ring when struck, and
confers a beautiful tone when introduced into bellmetal.
“Aluminum may be added to bronzes and brasses with good results.
The alloys (copper ninety per cent., aluminum ten per cent.) may be
worked cold or hot like wrought iron, but not welded. Its tenacity
is sometimes nearly one hundred thousand pounds per square inch.
Its specific gravity is 7.7. In compression this alloy has been
found capable of sustaining a little more than in tension,—one
hundred and thirty thousand pounds per square inch (nine thousand
one hundred and thirty nine kilos per square millimetre),—and its
ductility and toughness were such that it did not even crack when
distorted by this load. It is so ductile and malleable that it can
be drawn down under the hammer to the fineness of a cambric needle.
“It works well, casts well, holds a fine surface under the tool
and when exposed to the weather, _and it is in every respect
considered the best bronze yet known_. Its _high cost alone has
prevented its extensive use in the arts_. These alloys are very
uniform in character and work regularly and smoothly. Even one per
cent. of aluminum added to copper causes a considerable increase in
ductility and fusibility, and enables it to be used satisfactorily
in making castings. Two per cent. gives a mixture used for castings
which are to be worked with a chisel. It is softened by sudden
cooling from a red heat. Its coefficient of expansion is small at
ordinary temperatures.
“It has great elasticity when made into springs; it has been found
useful for watches, and has the decided advantage over steel of
being little liable to oxidization. Kettles of aluminum bronze are
used in making fruit syrups and preserves. Steel containing but
.08 per cent. of aluminum is said to be greatly improved by its
presence.”
Aluminum bronze, such as would be required for cycle castings, costs
from thirty to fifty cents per pound, according to quality and
quantity. A valuable alloy of aluminum and iron has recently been made,
by which it is maintained that wrought-iron castings are possible. The
factory is, I believe, at Worcester, Mass. In our endeavor to learn
more upon the subject we have been referred to the United States Mitis
Co., No. 26 Broadway, New York, which company has the exclusive right
in this country to make Mitis castings, or of granting permission to
those who desire to make these castings themselves.
STRENGTH OF TUBES.
Metal in the form of tubes resists all strain liable to occur in cycle
work better than in any other form. In regard to strain of compression,
we find, in “Wood’s Resistance of Materials,” the following summary:
“Experiments heretofore made do not indicate a specific law of
resistance to buckling, but the following general facts appear to
be established: The resistance of buckling is always less than that
of crushing, and is nearly independent of the length. Cylindrical
tubes are strongest, and next in order are square tubes, and then
+-------+
the rectangular. Rectangular tubes | | are not so strong as
+-------+
+---+---+
tubes of this form | | |.”
+---+---+
There is, however, very little direct crushing strain on the tubes in a
cycle; it is almost entirely a strain of flexure or bending; hence this
is the only interesting feature pertaining to the subject in cycling
work.
Since a tube is stronger than a solid bar, for same weight the
intuitive idea is to make the tube as large as possible, and the
mathematical demonstration which we append shows this to be correct,
generally speaking.
Let _R_ equal the strain per square inch of cross-section of the tube
at the point farthest removed from the neutral axis at the instant of
rupture.
[Illustration: Tube sections.
Fig. 1
Fig. 2]
Suppose Fig. 1 to represent the half of the tube, and that you are
trying to bend it down at the ends. The particles towards the top
will be pulled apart, while those at the bottom are crowded together;
somewhere between the top and bottom the particles are neither pulled
apart nor crowded together. Were the tube solid, the line of these
particles would be the neutral axis. In the tube an imaginary line
through the centre of the hole does not vary much from said axis. Now
_R_π
the moment of rupture = ——————(_r_ₑ⁴ − _r_ᵢ⁴), where _r_ₑ and _r_ᵢ (Fig.
4 _r_ₑ
_R_π
2) are the exterior and interior radii; ———— is a constant, which
4
we will call _K_, whence we can write moment of
rupture = _K_(_r_ₑ² − _r_ᵢ²) (_r_ₑ² + _r_ᵢ²) ÷ _r_ₑ. Here the factor
(_r_ₑ² − _r_ᵢ²) is proportional to the area of the annular
cross-section and is constant, while the other factor,
_r_ᵢ
(_r_ₑ² + _r_ᵢ²) ÷ _r_ₑ or, _r_ₑ + ———— _r_ᵢ, though less than 2_r_ₑ,
_r_ₑ
gets nearer and nearer to 2_r_ₑ as _r_ₑ gets large and _r_ᵢ approaches
_r_ₑ.
Therefore we have, that in resistance to flexure the tube should be as
large in diameter as practicable, which means that it must be as thin
as possible. This result is only modified in practice by the necessity
of guarding against dinging and also against imperfections in the
steel. A surface crack will ruin a very thin tube which otherwise may
be harmless in a thicker, but it is safe to say that it is best to use
reasonably large thin tubes.
Oval tubes are of an advantage only when the direction of the strain is
positively known and when it invariably occurs in that direction. Since
the tube finds its greatest limit of general resistance in cylindrical
form, to alter that form must necessarily weaken it more in one
direction than it strengthens it in another.