Modern Machine-Shop Practice Part 168

Fig. 2502.]

"FLUIDITY OF OILS.--Continuing my remarks on the thinness or fluidity of oils, I wish to call attention to an ingenious arrangement for testing the fluidity when subject to a slight increase of temperature, and also for detecting any tendency which they may have for combining with the oxygen of the atmosphere; this latter quality being advantageous in oils which are used to mix with paint, but which is a great evil when used for lubricating purposes. A piece of plate-gla.s.s placed at an angle is made warm to 200 Fahr. A drop of oil when placed on the upper end of this gla.s.s will flow down a few inches and thus indicate its fluidity when subjected to increase of temperature. Fig. 2499 shows a ready method I have designed for testing oil in this way. It consists of a tin box in which is fixed the gla.s.s, through which can be seen a thermometer. A graduated scale at the side of the box enables the track of the oil to be measured. The box has a door at the back which enables a copper vessel full of boiling water to be introduced; the box is lined with felt to prevent rapid radiation, and when the door is closed it will be seen that several experiments may be conducted before the apparatus becomes too cool for use. I think this a cleaner way than using a lamp for the purpose. The copper may also be used by itself for indicating the behavior of oil on copper when slightly warm in making it discolored or otherwise. As I have before stated, there are many oils which are good lubricants, but which become too thin when exposed to slight heat, and I do not hesitate to reiterate the statement, as I wish to have some influence on the future designs of bearing in this district. A correspondent writing to _Engineer_ from Queensland says that for six months in the year oil runs off the machinery like water and seems to have no lubricating power; he says that the thermometer registers in the summer 140 in the sun, and 110 in the shade. Great difficulty seems to have been experienced by him in keeping oil on the bearings; his experiments on locomotives show that it costs for lubricating a locomotive there about a halfpenny to three farthings a mile, according to the mixture used.

[Ill.u.s.tration: Fig. 2499.]

"INFLUENCE OF THE ATMOSPHERE ON OILS.--There are some oils which are excellent lubricants for the first few hours of use, but which have a low capacity for resisting the influence of the oxygen of the atmosphere upon them. The warm gla.s.s test may be used for indicating this weakness.

If after the test for fluidity the oil be permitted to remain on the gla.s.s any exhibition of a resinous or varnish quality may be observed.

Another test for this resinous or gummy quality is one which has been suggested to me by Mr. F. R. Wheeldon, of Bilston. He has made many experiments. He found that by permitting oil to remain on a Stapfer friction tester after one test which had been recorded, he tested again on the following day, without adding any fresh oil. This is a severe test, as the thermometer was made to indicate 200 Fahr. each time.

[Ill.u.s.tration: Fig. 2500.]

"LONGEVITY OF LUBRICANTS.--Supposing an oil to possess all the qualities which we think a good lubricant should have--that it has fluidity in season, and that it does not combine with the atmosphere and become varnish, that it does not become like water in summer and like mutton suet in winter, and is in most respects satisfactory. We then want to know its powers of endurance, its capacity to resist wear and tear--in other words, its longevity. A good test for longevity or durability of oil when subject to either heavy or light frictional pressure is one suggested by Mr. W. H. Hatcher, a very careful investigator, and chief of the Laboratory of Price's Patent Candle Company, who are extensive oil manufacturers. It consists in taking away the bottom step of the Stapfer tester and placing a small dish containing oil underneath the friction roller (as in Fig. 2500). This oil is carefully weighed before and after several hours' frictional wear and tear. The drawing (Fig.

2501) shows the application of this mode, which I have designed, for testing solid lubricants, such as lard and sulphur and other railway and steams.h.i.+p mixtures. It will be seen that the material is kept to its duty by the weighted lever, and its progress of diminution can be tested in its place by the scale-beam arrangement. When it is used with the pressure on the top step it is advisable to drive it at about 2,000 revolutions per minute; otherwise much time will be occupied in destroying a weighable quant.i.ty of oil. The large Stapfer tester (Fig.

2502) was designed a few months ago for this purpose for the Government railways of New South Wales, and it is also used by the Manchester, Sheffield, and Lincolns.h.i.+re, the Lancas.h.i.+re and Yorks.h.i.+re, and other railways. I have not been able to get any results of these tests in time for our subject on this occasion, but hope to do so at some future time.

The frictional roller is 6 inches in diameter, the pressure amounts to 1 cwt. on each step. As it takes a considerable time to wear away half a pound of solid lubricant, it may be advisable to measure by minutes instead of using the speed index. The speed should be at least 1,500 revolutions per minute. The Stapfer tester should be used in a room of equal temperature, and should not be subject to draughts of cold air, as it will be obvious these will interfere with the indications of the thermometer. A recent alteration in the Stapfer tester permits the quant.i.ty of oil used for testing to be measured with greater accuracy than before. A small oil-hole is made in the top step (see Fig. 2502 at _a_ and at _c_) in which is placed a gla.s.s tube. This only holds a few drops, and can be filled by simply dropping the oil in, holding the finger at the bottom to prevent it running away, and then place it in the hole. If a small needle lubricator be weighed and then filled with oil of a definite weight, and placed in this hole (see Fig. 2502 at _b_), oil may be tested for longevity and for its anti-frictional properties for a longer period than with the small tube. If oil be placed in this at the same time that oil is placed in the lubricators in the works and the oil tester be driven from the same shafting, permitting it to stop and start when the engine stops and starts, the effect of a week's work upon the weight of the oil may be seen; notice should be taken of the difference of the temperature between the thermometer on the instrument and the temperature of the atmosphere of the workshop.

"TESTING FOR SALTS AND ACIDS, ETC.--It will be obvious that however good as a lubricant an oil is, and however valuable its properties may be when examined, if it possesses any corrosive quality which will be injurious to the metals upon which it is placed, it will soon become detrimental to the machinery, and may also cease to be valuable as a lubricant. Mr. William Thomson, a.n.a.lytical chemist, of Manchester, read a paper on this subject at the British a.s.sociation at Glasgow, and he stated the results of elaborate experiments conducted by him to discover the influence of various oils of commerce upon bright strips of copper.

He permitted the copper strips to remain entirely covered by oil. He also conducted similar tests with half of the strip below the surface of the oil, and the other half exposed to the atmosphere, in order to see what influence the oil had, when the surface line touching the metal would, of course, be acted on by the atmosphere. After noticing the effect upon the brightness or dulness of the copper, he carefully tested the oils in order to detect the quant.i.ty of metal which had been dissolved. Mr. Thomson found the following oils dissolved the largest proportions of copper, leaving the surfaces of the copper slips bright--rape, linseed, sperm, raw cod-liver, Newfoundland cod, and common seal oils; and that the following dissolved much smaller proportions of copper, also leaving the slips bright--seal, whale, cod, shark, and East Indian fish oils; and that mineral oils seem to have no dissolving power on the copper, the only effect being a slight discoloration on the copper slip of a greyish color.

"SWISS WATCHMAKERS' TEST FOR FLUIDITY AND CAPACITY TO RESIST COLD.--It seems, according to the _Watchmaker and Jeweller_ (a monthly trade journal), that the plan I have described, and what may be called the warm gla.s.s test, seems to be looked upon with favor for testing oil in Switzerland. The degree of heat used for testing the fluidity of oil is 200 Fahr., and if this causes the oil to become a varnish two or three days after the test the oil is considered unfit for use. Another test is one to which I have not alluded, and that is, capacity to resist low temperatures. Oils are tried for their capability to withstand low temperature in the following manner: Fifteen parts of Glauber salts are put into a small gla.s.s vessel, a small bottle of oil to be tested is immersed into this; this done, a mixture of five parts of muriatic acid and five parts of cold water is placed over the salt. By means of a thermometer the temperature is indicated, and when it shows a very low temperature, the behavior of the oil, subject to this freezing mixture, may be observed and noted. Mr. Thomson, however, considers that this mixture is not so good or so cheap as ice alone, or a mixture of ice and common salt.

"BLOTTING-PAPER TEST.--It seems it is considered that the blotting-paper test for fluidity is more reliable, according to the writer of the article, than the inclined plane experiment. In order to use this test we must saturate the strip of blotting-paper with oil, and watch whether the drops fall off in pearls or have an inclination to spread out. The latter is a certain sign, the writer says, of a viscid oil. Although this may be considered viscid oil, and may not be valuable for watches, it may, however, be a good oil for heavier machinery."

The amount of friction between a journal and its bearing varies with the kind of metal of which the journal and bearing are composed; on the area of surface in contact in proportion to the load or pressure sustained by the bearing surfaces; on the nature or degree of the lubrication afforded; on the diameter of the journal in proportion to its length; on the manner in which the journal fits or beds to its bearing, and on the kind of motion, as whether the same be continuous, intermittent, rotatory, or reciprocating.

Referring to the friction as influenced by the nature of the metals in contact: the friction varies with the hardness of the metal; thus, with hard cast iron, there will, under equal conditions, be less friction than with soft cast iron. The friction is greater when the surfaces in contact are both of the same metal than when they are of different metals. Mr. Rankine summarizes General Morin's experiments on the friction of various bodies not lubricated as follows:--

GENERAL MORIN'S EXPERIMENTS ON FRICTION.

--------------------------------------+----------------+------------ | Angle of |Friction in Surfaces. | repose. |terms of the | | weight.

--------------------------------------+----------------+------------ | degrees. | Wood on wood, dry |14 to 26-1/2|.25 to .5 " " soaped |11-1/2 " 2 |.2 " .04 Metals on oak, dry |26-1/2 " 31 |.5 " .6 " " wet |13-1/2 " 14-1/2|.24 " .26 " " soapy |11-1/2 |.2 " elm, dry |11-1/2 " 14 |.2 " .25 Hemp on oak, dry |28 |.53 " " wet |18-1/2 |.33 Leather on oak |15 " 19-1/2|.27 " .38 " metals, dry |29-1/2 |.56 " " wet |20 |.36 " " greasy |13 |.23 " " oily | 8-1/2 |.15 Metals on metals, dry | 8-1/2 " 11-1/2|.15 " .2 " " wet |16-1/2 |.3 Smooth metal surfaces occasionally | 4 " 4-1/2|.07 " .08 greased | | " " " continuously | 3 |.05 greased | | " " " best results | 1-3/4 " 2 |.03 " .036 Bronze on lignum-vitae, constantly wet | 3(?) |.05(?) --------------------------------------+----------------+------------

"The 'angle of repose' given in the first column is the angle which a flat surface will make with the horizon when a weight placed upon it just ceases to move by gravity. The column of 'friction in terms of the weight' means the proportion of the weight which must be employed to draw the body by a string in order to overcome its friction, and the proportionate weight is sometimes called the _coefficient of friction_."[34]

[34] From Bourne's "Handbook of the Steam Engine."

In the following table are given some of the results obtained from Morin's experiments with unguents interposed.

------------------------------+-----------------+--------------------- Nature of surfaces in contact.| Coefficient of | | friction during | Kind of | motion. | unguent.

------------------------------+-----------------+--------------------- Bra.s.s upon bra.s.s | .058 | Olive oil.

Cast iron upon bra.s.s | .078 | "

" " " cast iron | .314 | Water.

Steel upon cast iron | .079 | Olive oil.

" " bra.s.s | .056 | Tallow or olive oil.

Wrought iron upon bra.s.s | .103 | Tallow.

" " " cast iron | .066 | Olive oil.

" " " wrought iron| .136 | "

Morin's experiments demonstrated that friction is always proportional to the pressure and independent of the area pressed in contact, providing that the pressure is not so great as to cause the surfaces to abrade in the manner or to the degree commonly known as cutting, which occurs when the area of bearing surface in proportion to the pressure is so small as to press out the lubricating material.

Now, between the degree of abrasion that is sufficient to cause a bearing to heat and the minimum, possibly lies a wide range that is very difficult of cla.s.sification, and that influences the friction of the bearing and journal. Under any given dimensions of journal area and any given pressure of the same to its bearing, the abrasion, and, therefore, the friction, will be less in proportion as the fit of the journal to its bearing extends over its whole area and with an equal pressure of contact. Under these conditions, and with a bearing area ample for the given pressure, the surfaces of a journal and bearing have a smooth, glossy appearance, with a surface as glossy as plate-gla.s.s.

This degree of perfection, however, is only occasionally reached in practice, because of imperfections in the fitting and lubrication.

Now, between this condition of glossy smoothness and the degree of abrasion known to practical men as _cutting_ lies, as already stated, a wide range of degrees of abrasion, and each of these has its own coefficient of friction. This may be readily proved by freely lubricating the bearings of a number of journals working under the usual conditions of practice and smearing the oil just as it pa.s.ses through the bearings upon a sheet of white note paper, when it will be found to contain fine particles of metal, the number and size of particles in a given quant.i.ty of the oil decreasing as the surfaces of the bearings are glossy, and increasing as those surfaces appear dull.

The order of value to resist wear is generally considered in practice to be as follows:--

1st in value, hardened steel running on hardened steel.

2nd (and by some considered equal to the first when the pressure per square inch of area is light), cast iron either upon cast iron, hardened wrought iron, or hardened steel.

3rd, under light duty cast iron upon wrought iron or steel not hardened.

4th, wrought iron upon hard composition or bra.s.s.

5th, wrought iron upon some anti-friction metal, as Babbitt metal.

Cast iron appears to be an exception to the general rule, that the harder the metal the greater the resistance to wear, because cast iron is softer in its texture and easier to cut with steel tools than steel or wrought iron, but in some situations it is far more durable than hardened steel; thus when surrounded by steam it will wear better than will any other metal. Thus, for instance, experience has demonstrated that piston-rings of cast iron will wear smoother, better, and equally as long as those of steel, and longer than those of either wrought iron or bra.s.s, whether the cylinder in which it works be composed of bra.s.s, steel, wrought iron, or cast iron--the latter being the more noteworthy, since two surfaces of the same metal do not, as a rule, wear or work well together. So also slide-valves of bra.s.s are not found to wear so long or so smoothly as those of cast iron, let the metal of which the seating is composed be whatever it may; while, on the other hand, a cast-iron slide-valve will wear longer of itself, and cause less wear to its seat, if the latter is of cast iron, than if of steel, wrought iron, or bra.s.s. The duty in each of these cases is light; the pressure on the cast iron, in the first instance cited, probably never exceeding a pressure of ten pounds per inch, while in the latter case two hundred pounds per square inch of area is probably the extreme limit under which slide-valves work; and what the result under much heavier pressures would be is entirely problematical.

Cast iron in bearings or boxes is found to work exceedingly smoothly and well under light duty, provided the lubrication is perfect and the surfaces can be kept practically free from grit and dust. The reason of this is that cast iron forms a hard surface skin when rubbed under a light pressure, and so long as the pressure is not sufficient to abrade this hard skin, it will wear bright and very smooth, becoming so hard that a sharp file or a sc.r.a.per made as hard as fire and water will make it will scarcely cut the skin referred to. Thus in making cast-iron and wrought-iron surface plates or planometers, we may rub two such plates of cast iron together under moderate pressure for an indefinite length of time, and the tops of the sc.r.a.per marks will become bright and smooth, but will not wear off; while if we rub one of cast iron and one of wrought iron, or two of wrought iron, well together, the wrought-iron surfaces will abrade so that the protruding sc.r.a.per marks will entirely disappear, while the slight amount of lubrication placed between such surfaces to prevent them from cutting will become, in consequence of the presence of the wrought iron, thick and of a dark blue color, and will cling to the surfaces, so that after a time it becomes difficult to move the one surface upon the other. If, however, the surfaces are pressed together sufficiently to abrade the hard skin from the cast iron, a rapid cutting immediately takes place, which is very difficult to remove.

To obtain the best results from cast-iron bearings the bedding of the journal to the bearing must be full and perfect, and the surfaces bright and smooth, in which case it will wear better than hardened steel, unless it be very heavily loaded.

Again, a cast-iron surface will hold the lubricating oil better than either steel, wrought iron, or bra.s.s of any kind. Indeed, if a cast-iron surface be made very true and smooth so that it is polished and no marks are visible upon its surface, it will take _much patient_ rubbing and cleaning with a _dry clean rag_ to remove the oil entirely, whereas other metals will clean comparatively easy. In testing this matter upon surface plates the author has found that the only safe method, and by far the quickest, of removing the oil from cast iron is an application of alcohol or spirits of turpentine, because the oil will enter and to some extent soak into the pores of cast iron and gradually work out again as it is continuously wiped, so that if apparently quite clean (after having been oiled and wiped) a short period of rest will cause oil to again be present to some extent upon the surface.

As a general rule motion in a continuous direction causes more wear under equal conditions than does a reciprocating one, because when a revolving surface commences to abrade, the particles of metal being cut are forced into and add themselves, in a great measure, to the particles performing the cutting, increasing its size and the strain of contact of the surfaces, causing them to cut deeper and deeper until at least an entire revolution has been made, when the severed particles of metal release themselves, and are for the most part forced into the grooves made by the cutting.

In reciprocating surfaces, when any part commences to cut, the edge of the protruding cutting part is abraded by the return stroke; which fact is clearly demonstrated in either fitting or grinding in the plugs of c.o.c.ks, in which operation it is found absolutely necessary to revolve the plugs back and forth, to prevent the cutting which inevitably and invariably takes place if the plug is revolved in a continuous direction. Furthermore, when a surface revolves in a continuous direction, any grit that may lodge in a speck, hollow spot, or soft place in the metal, will cut a groove and not easily work its way out, as is demonstrated in polis.h.i.+ng work in a lathe; for be the polis.h.i.+ng material as fine as it may, it will not polish so smoothly unless kept in rapid motion back and forth. Grain emery used upon a side face, such as the radial face of a cylinder cover, will lodge in any small hollow spots in the metal and cut grooves, unless the polis.h.i.+ng stick be moved rapidly back and forth between the centre and the outer diameter. If a revolving surface abrades so much as to seize and come to a standstill, it will be found very difficult to force it forward, while it will be comparatively easy to move it backward, which will not only release the particles of metal already severed from the main body, and permit them to lodge in the grooves due to the cutting, but will also dislodge the projecting particles which are performing the cutting, so that a few reciprocating movements and ample lubrication will, in most cases, stop the cutting and wash out the particles already cut from the surfaces of the metal.

In determining the metals to be used for a journal and bearing it is preferable to use the softer metal, or that which will wear the most, in the position in which it can be the most easily and cheaply replaced, which is usually in the bearing rather than in the journal; and since two metals of a different kind run better together than two of the same kind, the bearing is usually of a different kind of metal from that composing the journal. It may be stated, however, that under _light duty_ cast iron will wear upon cast iron better than wrought iron or bra.s.s upon cast iron (for reasons which have already been stated), especially if the bearing area be broad and the lubrication ample and perfect.

To facilitate the removal of the bearings, bra.s.ses or boxes are provided, but in the case of small journals, as, say, of about 3 inches and less in diameter, the duty being light, the lubrication ample and equally distributed, and the journals an easy working fit when new, it is found that solid cast-iron boxes will last for a great length of time without sensible wear.

In some cases cast-iron boxes are cast with a receptacle for some soft metal, such as the various compound metals known under the general name of Babbitt metal.

Babbitt metal is composed of tin, antimony, and copper, mixed in varying proportions. A good mixture for general use where the duty is light is composed of 50 parts tin, 5 parts antimony, and 1 part copper. A harder composition, sometimes termed white metal, is composed of tin 96 parts, copper 4 parts, and antimony 8 parts. This mixture is especially suitable for journal boxes or bearings. It is mixed as follows: Twelve parts of copper are first melted, and then 36 parts of tin are added; 24 parts of antimony are put in, and then 36 parts of tin, the temperature being lowered as soon as the copper is melted in order not to oxidize the tin and antimony; the surface of the bath being protected from contact with the air. The alloy thus made is subsequently remelted in the proportion of 50 parts of alloy to 100 tin.

For bra.s.s bearings or boxes a mixture of 64 parts copper, 8 parts tin, and 1 part zinc is found to answer well; but for bearings not requiring so hard a metal, the quant.i.ty of zinc is increased, and that of the tin diminished.

[Ill.u.s.tration: Fig. 2503.]

Bearings or boxes that are to be babbitted are usually cast as in Fig.

2503, there being a rib at A, B, and C, forming a cavity at D, into which the melted metal is poured. The ribs (in new boxes) are sometimes bored out, or for rougher work may be chipped and filed out to fit the shaft, and hold it in line; and to prevent the ribs A, B, &c., from bearing and cutting the shaft, a piece of pasteboard is laid on ribs A and B, thus confining the journal bearing to the babbitt. The best method is to pour the bearing and then rivet the babbitt well into the cavity D, which is made wider at the bottom, to prevent the babbitt from coming loose, and then bore out the bearing in the usual manner.

The princ.i.p.al advantage of a babbitted bearing is the ease with which it can be renewed, and the fact that the metal will soon bed itself to the journal. This is of great advantage in the case of solid bearings in the framing of fast-running machines, and in situations where it would be awkward or difficult to take bra.s.ses or bushes out to fit them, or align them to the shaft, which in many cases would also require to be taken out to remove the bra.s.ses. On the other hand, any particles of grit that may find ingress to babbitted boxes are apt to become bedded into the babbitt metal and cut or grind away the journal.