LUBRICANTS AND LUBRICATORS
FRICTION AND LUBRICATION
Since the earliest times a great many engineers have been seeking earnestly to discover some means for eliminating or at least reducing the friction in machinery to the lowest possible amount, while other engineers have been bending every energy to find some means to increase it for certain purposes. Thus it must be evident that friction has both good qualities and bad ones. It is true that friction has been called the highwayman of mechanical energy and many other names equally as reprehensible, and yet we could scarcely get along without it. But for friction, belt transmission of power would be impossible, trains could not run and men could not walk. For note what happens when the engine drivers strike a greased rail or a man tries to walk on glare ice. Then consider friction brakes and friction clutches and the many mechanical devices that depend upon friction. The engineers who deal in such goods study to increase friction, while those who build machinery aim to eliminate it from every joint and bearing surface.
Both those who aim to eliminate, and those who seek to increase friction, have been fairly successful in their efforts. The first by nice workmanship, by using the proper metals at the right points, and by greater care and skill in preparing lubricants that are adapted to the work; the latter also, strange as it may seem, by more accurate workmanship and by the careful selection and preparation of materials. As an illustration of the latter, it may be mentioned that a perfectly smooth, true clutch shoe acting on a smooth, though not lubricated, surface will grip better than if both were rough. A soft, pliable belt will grip a smooth pulley better than a hard belt will grip a rough one, and a leather belt will grip a wooden pulley better than it will an iron pulley, and a rubber or canvas belt will do better than leather.
This lesson, however, is not concerned so much with friction and how it may be increased, as it is with lubrication and the elimination of friction.
The general effect of friction in machinery is to cause heating of the adjacent parts, a fact well-known to all practical engineers. The reason is very evident. Wherever there is resistance to any force, there work must be done, for work is defined as the overcoming of resistance through distance. There is an exact relation between work and heat, and it is well known that a certain amount of work will produce a given quantity of heat; and, conversely, a given quantity of heat is capable of doing a certain amount of work. Whenever work is done on a bearing in overcoming friction, a certain amount of heat will be developed, depending upon the work done. If friction can be eliminated there will be no heating.
The object of lubrication is not only to reduce friction, but to carry away whatever excess of heat may have been generated. It prevents friction by forming a cushion between the bearing surfaces, which keeps the metals apart, and it dissipates the heat by running off from the bearing, as in the case of a pump oiling of bearings, or else vaporizes and carries away heat. Whenever steam forms it absorbs a great quantity of heat, a fact one can realize when he considers that the heat of the fuel fed to a furnace is absorbed and passes away with the steam. Likewise with any other liquid that vaporizes, it carries away a large quantity of heat, and it is so with the oil that vaporizes from a bearing. Of course, the vaporizing of the oil and the carrying away of heat, is not the primary function of a lubricant; it is only incidental, because to vaporize it must be heated, and this condition should not arise.
Good workmanship consisting of nicely fitted parts, smooth and well finished, help to eliminate friction provided the assembling is done accurately and all parts are in perfect alignment. This latter consideration, the proper alignment, is all-important for smooth, easy running, yet it is where much trouble arises, especially in threshing machinery. Not that the manufacturer does not do his work well, as a rule, although doubtless there are exceptions, but the man in the field is more often at fault by allowing his machinery to get in bad shape.
The selection of materials is another thing that has helped on toward the goal of perfection. Steel shafting working in brass or babbitt boxes, runs with much less friction than in iron boxes however well made, and here the quality of the brass or babbitt becomes another important factor.
To illustrate the great advancement made in reducing friction in engine building, the following is pertinent. A well-known authority says the internal friction of engines twenty years ago ran as high as fifteen or twenty per cent. On some recent tests of traction engines at the plant of one of the great thresher houses, the internal friction ran as low as three per cent and as high as eight under adverse conditions. Good stationary engines of large size run now-adays with as low as two per cent friction.
Now that we have discussed friction and lubrication in a general way, we will proceed to look into the matter of lubricants.
are derived from three principal sources, namely, animal, vegetable and mineral. Mineral lubricants are of fairly recent origin, having come into general use within the last thirty years. It may be interesting to know in this connection that the development of very high pressure of steam engines and of the gas engine was only made possible since petroleum oils and their compounds were discovered, but more of this later on.
The animal oils include the oils of animals and fishes. The most important are lard oil, tallow and neat’s-foot oil. The various fish oils such as whale oil, sperm oil, black fish oil and porpoise oil while valuable are of secondary importance.
Lard is the best all around animal lubricant and is well adapted to medium and light machinery. It is the best oil known for use on dies for cutting bolt threads or pipe threads and is also used on lathe tools where a very fine, smooth cut is desired.
Before petroleum oils were developed tallow was used as the principal steam cylinder oil. It worked quite successfully, too, for the pressures then in vogue but it would not be suitable for the high pressures used today because it would decompose with the heat. It contains a certain amount of free acid too that is objectionable in that it attacks the metal that it comes in contact with. Most oils contain some acid and leading authorities on the subject say that three per cent or more present in an oil should condemn it for lubricating purposes.
Neat’s-foot oil is derived from the hoofs and bones of cattle. It is a fairly good lubricant for machinery but its principal use is as a belt dressing for leather belts and for harnesses. Only a little should be applied to the leather, just what can be absorbed. It acts as a softener and as a preservative. If separator belts were cleaned and given a dressing with neat’s-foot oil at the close of each season’s run before being put away they would last a good deal longer.
Black fish oil has been used quite largely for guns and sewing machines but in recent years it has been largely displaced by the cheaper paraffin oils derived from petroleum.
Sperm oil is very expensive and is used very little except possibly for watches and similar delicate mechanism.
Vegetable oils are expressed from the seeds and fruits of plants. Those most largely manufactured are cotton seed oil, olive oil, linseed oil and rape seed oil.
Olive oil is probably the best of the vegetable oils for machinery but it is too expensive for general use.
Linseed oil which is made from flax seed is absolutely worthless as a lubricant because it very quickly becomes hard and gummy. This property which unfits it for use as a lubricant, however, is the very thing which gives it especial value in mixing paints. It dries readily and in doing so forms a thin rubber-like coat over the painted surface.
Rape seed oil is made on the continent of Europe and is said to be used in preparing the better grades of hard oils.
The mineral oils are all made from petroleum or from rock rich in petroleum products. After the lighter oils have been distilled off the residue is raised to a higher temperature and the different grades of lubricating oils are obtained. These oils vary from the light paraffin oils to the heavier grades of engine oils.
In addition to the oils above mentioned we have the following dry or solid lubricants, namely, graphite, mica, sulfur and soapstone. Of these four, graphite is the most widely known and is the best. It is excellent to use on a roughened bearing when mixed with grease. It is also excellent for a hot bearing when used in the same way. Mica is also sometimes used mixed with grease, but it is not as good as graphite. While these solid lubricants are good to smooth the inequalities in a bearing and for use on packing, they do not take the place of oil or grease. In all cases they should be used sparingly.
In addition to the lubricants noted above there is another large class which comes under the head of greases or hard oils. These have as a base either animal, vegetable or mineral oils. The best grade of hard oil is made chemically by boiling either animal or vegetable oils in lime water thus making a sort of insoluble soap. Soap is made in the same way by using caustic potash instead of lime. Mineral hard oil has a paraffin base and is not quite as good a lubricant as the hard oils made from animal or vegetable oils. However, it is much cheaper and for many classes of work it is quite satisfactory notwithstanding the fact that it requires a larger quantity than the former for the same class of bearings. In general it may be said that greases are not as good lubricants as liquid oils, a fact which has been proven by many careful scientific tests. Its ease of application, however, and its cleanliness more than compensate for the slight difference in lubricating qualities, especially for such places as the crank pin, cross head pin and other moving parts of machinery. The various axle greases also come under the head of hard oils and are valuable for the purpose for which they are intended.
The value of a lubricant is said to be largely dependent upon the number of greasy particles it contains and upon its viscosity. A viscous oil is one that is syrupy in consistency. Thick, heavy oils are generally the most viscous but this is not always true because sometimes oils are adulterated to improve their viscosity. Oil treated in this way is of course a very inferior lubricant. An analysis of some of these oils show that they contain a considerable amount of gelatin, a substance having absolutely no lubricating value.
Good oil should, as previously stated, contain not to exceed three per cent of free acid, in fact, a perfectly neutral oil, one containing no acid at all, is the best. Oil should not dry out and become gummy. It should not contain any grit or dirt or any other foreign substance, neither should it become rancid or bad smelling when exposed to the air for a considerable length of time. In addition to all this, the oil should be adapted to the work in hand. Heavy oil for a large heavy bearing and a light thin oil for light machinery, is the general rule. Guns, cream separators and sewing machines require a light oil which would be totally unsuited for the bearings around a grain separator or the main bearings of an engine.
Cylinder oils are in a class by themselves. Here we must have an oil that has considerable body and which will stand a high degree of temperature without decomposing or breaking up. In other words it must have a high flash point. The flash point of an oil is the temperature at which a vapor is given off that will take fire.
The usual method of making a flash test is to put some of the oil in an iron cup, then place this in a tin dish with dry sand in the bottom and place over a fire. A thermometer that will read to 500 or 600 degrees Fahrenheit should be placed in the oil and after the temperature reaches say 300 degrees, pass a lighted match over the top of the cup every time the temperature goes up ten or fifteen degrees. When the vapor takes fire the thermometer shows the flash point of the oil.
Steam at one hundred pounds (per square inch) gauge pressure has a temperature of 337 degrees and at one hundred fifty pounds (per square inch) the temperature is 361 degrees. Consequently the oil used for cylinder lubricating purposes should have a flash point considerably higher than these temperatures. For ordinary steam engine practice a flash point of 400 degrees is considered about right. For gas engine practice where the cylinder temperatures are very much higher, a still higher flash point is desirable.
An oil suitable for steam engine cylinders is generally unsuited to gas engines. Steam cylinder oil is what is known as compounded oil and is made by mixing either an animal or a vegetable oil with a mineral oil. The compound thus partakes of the nature of both. A pure mineral oil will not form an emulsion with water and hence will not adhere to a moist surface. For this reason it is useless for steam cylinders. It has, however, a high flash point which the animal oils do not possess. The compound of the two forms an oil that possesses the characteristics of both and makes a very much better lubricant than either alone.
A cylinder oil when adapted to the pressure carried will atomize or break up into a very fine spray (not a vapor) when caught in the current of steam and becomes intimately mixed with it, thus finding its way to every part that the steam reaches and producing perfect lubrication. If oil of too low a flash point is used, it will be decomposed by the steam forming a gas and its power of lubrication is completely lost. On the other hand, a very heavy oil of too high flash test may not be even atomized, in which case it simply flows down along the inside of the steam pipe and forms a pool in the steam chest. Here again lubrication is a failure. In case a rather heavy oil is used and difficulty is experienced in getting good lubrication it is well to take off the steam chest cover and observe if there is oil in the bottom of the chest. If there is, it is evidence that the oil is too heavy and a change should be made to one somewhat lighter.
Less trouble will generally be experienced with cylinder lubrication if care is taken to see that the small delivery pipe of the lubricator projects well into the steam pipe where the drop of oil will be caught in the current of steam. If the drop simply discharges along the side of the pipe it is much more apt to run down without becoming atomized.
Lack of lubrication in a steam engine cylinder can be recognized by a groaning noise in the cylinder and by the jerking of the valve. If not well lubricated, the valve and valve seat are pretty sure to become scored and likewise the cylinder and piston rings. No general rule can be given as to just the amount of oil to use. This depends upon the workmanship throughout on the engine, whether there is much water in the steam, and whether the engine is new or has been run for some time. If the cylinder is bored smoothly and the workmanship is first class, less oil will be required than where the work is done in an indifferent manner.
An engine that primes requires a large amount of oil because the water in the steam washes the oil out. In a case of this kind the difficulty may be due to bad water or it may be due to lack of steam space in the boiler. If bad water is the cause the boiler will foam, but if the trouble lies in the proportioning of the boiler, priming may be looked for instead of foaming. Whenever an engine primes badly the cylinder needs a large amount of oil at once and for this purpose a small hand oil pump is desirable. Some traction engines are regularly equipped in this way. Where the water is strongly alkaline it is always well to have an oil pump as a part of the regular equipment. A new engine always requires more oil than one that has been run a considerable time because no matter how carefully the work may have been done the cylinder, the piston and valve are considerably rougher than they will be after having run some time, provided of course, the engine has been well cared for. In the same way, and for the same reason, the other bearings of a new engine require more oil than one that has been run for some time and there is much more danger of heating.
Some engines require only four or five drops of oil per minute and some require twenty or thirty, depending upon the conditions above described, consequently no general rule can be laid down covering cylinder lubrication.
For lubricating gas engine cylinders a pure mineral oil is used. Here the metal is dry and the heat is intense. At the moment of explosion the temperature ranges between 2,000 and 3,000 degrees. The cylinder walls are generally jacketed either with water or oil, which keeps the temperature of the metal from becoming dangerous. Nevertheless, the heat is considerable and it requires an oil of from 400 to 600 degrees flash point. A mineral oil has the property of spreading over a dry surface and it can be procured with any flash point desired. For water cooled engines where the jacket water does not get very hot, a pure mineral oil from 400 to 450 degrees flash point will give good satisfaction and will generally give even better satisfaction than the more expensive kinds showing a higher test. With oil cooled or air cooled gas engines which run hotter, a higher test oil should be used.
Oil that burns on the gas engine piston always causes trouble by forming a carbonaceous substance that causes the piston rings to stick and also gets between the valves and valve seats, causing trouble.
Oil that is used on the main bearings of a traction engine should be able to withstand the heat at that point without evaporating. The temperature of the main bearings is almost the same as that of the boiler to which they are attached and if an oil is used that evaporates quickly, that which remains becomes thick and gummy and useless as a lubricant. A good test for an oil to be used for the main bearings is to drop a little oil on the boiler near them and observe if it burns quickly, if so it is not suitable.
In conclusion, the writer wishes to advise buying the oil from a reputable dealer and when a brand is found that is suitable for the work in hand stick to it. It is very easy to adulterate oils and only an expert in oils can tell much about them, and then only after a careful test and analysis. There are a number of firms which make good lubricants and quite a good many others that make a cheap article that is practically worthless. Since the good working as well as the life of the machinery depends so largely upon lubrication, too great care can not be exercised in the selection of the lubricants.
There are a great many different styles and forms of lubricators used on machinery. At first sight one might think it would be impossible to make a systematic classification. A little thought, however, will make it evident that there are two primary classes, namely, bearing lubricators and steam lubricators. The former are used on the bearings of all classes of machinery, the latter for the cylinders and valves of steam engines. The different lubricators used on bearings naturally fall into the following classes: plain lubricators, sight-feed oilers, grease cups (both plain and automatic) and pumps.
Figure 59 shows one of the simplest forms of plain lubricators, or wick oiler, as it is generally called. It is a plain brass cup having a central tube extending well up toward the cover, A few strands of candle wicking are pushed down into the tube, leaving a coil on the outside in the oil. The oil rises in the wick by capillary attraction just as it does in a lamp wick, and flows down the tube to the bearing. The only method of regulation is to add more strands of wicking to increase the flow or take out some to reduce it. Close regulation is, of course, out of the question. This form of lubricator is not used on traction engines at the present time.
A sectional view of an ordinary sight feed oiler is made the subject of illustration in Figure 60. The oil is contained in a glass reservoir, which has ground ends and is made oil tight by means of gaskets at the top and bottom, The cover is screwed down on a central tube extending up through the reservoir. The bottom of this tube is provided with a valve seat for a needle valve. This needle valve extends up through the tube and is provided with a cross bar or eccentric lever wherewith it may be raised from its seat. It is held down normally by a small spiral spring in the upper part of the tube. A thumb-nut at the top may be raised or lowered, thus increasing or decreasing the distance the needle valve moves from its seat and so regulating the flow of oil. A sight feed glass below the reservoir shows how much oil is being delivered. If oil rises in this glass it shows that the opening below is obstructed and needs cleaning out.
Figure 61 shows an automatic grease cup. It is provided with a plunger resting upon the grease, which is pressed down by means of a strong spiral spring between the plunger and the cover. The plunger spindle is threaded and extends up through the cover. A thumb-nut at this point enables the operator to compress the spring and lift the plunger off from the grease. When working, the thumb-nut is unscrewed until it is free from the cover. When the machine is stopped the nut should be screwed down until the plunger is lifted off from the grease. A screw in the spindle of the oiler below the reservoir is provided with an opening the same size as that in the spindle. This opening is parallel with the slot in the head of the screw. By turning this screw so that the opening in the spindle is reduced, it is easy to govern the flow of oil to the bearing.
Cylinder lubrication may be divided into the following classes, namely, water displacement lubricators, hydrostatic lubricators, and mechanical lubricators or pumps.
The first one of these to be described is the water displacement lubricator, which is shown in Figure 62. It consists of a brass reservoir having a central tube open at the top, which reaches up almost to the cover. Below the reservoir a valve B is provided which shuts the opening between the steam space and the reservoir. A sight feed glass may be inserted below the reservoir and generally is, although some oilers of this sort, like the one illustrated, are made blind.
The principle of operation is as follows. When the valve B is opened, steam from the steam chest rises through the central tube, condenses by meeting the cold tube and cover and the water being heavier than the oil flows down the outside of the tube to the bottom of the reservoir. If the reservoir is full, an equal amount of oil will be displaced and, having no other place of egress, will flow down the tube to the steam chest. This operation continues until the reservoir is filled with water and emptied of oil. Without the central tube a lubricator of this kind would be unable to work as long as steam pressure was on.
The hydrostatic lubricator, one form of which is illustrated in Figure 63, depends for its operation upon the weight of a column of water acting upon the oil and forcing it out into the steam main. The lubricator shown has two connections to the steam main, one at the top, the other at the side. To put the lubricator in operation, first fill the reservoir completely full of oil. Then open the valve in the delivery pipe (not shown in the figure) and the one on the back side of the upper part of the gauge glass (also not shown in the figure) and allow the sight feed glass to fill with water. Now open the valve in the upper connection and regulate the feed of oil with the valve below the sight feed glass. The action is as follows. Steam flows down the pipe above the reservoir and condenses both in the steam pipe and in the globular brass condenser bulb just above the reservoir. The water resulting from condensation flows up the pipe P to the bottom of the reservoir and displaces an equal amount of oil which enters the pipe S, at the top of the reservoir and flows down and then up into the sight feed glass. Here the drop is liberated and rises through the water and thence flows across through pipe T to the steam main where it is caught in the current of steam and whirled along toward the cylinder. When the lubricator is working, the upper pipe is filled with water. The steam pressure at the lower connection is the same as at the upper. Thus the pressures due to the steam are balanced, but the weight of the column of water in the condenser pipe acting upon the oil pushes it out into the steam main. The longer the condenser pipe is the greater will be the pressure on the oil.
In single connection lubricators there is a loop above for a condenser pipe and the height of the column of water which moves the oil is measured from the top of the loop to the bottom of the reservoir.
Care should be taken in connecting up hydrostatic lubricators that all joints be tight. A leak anywhere may offset the weight of the column of water and make it fail to work. The cause of the glass getting dirty is often due to running the oil entirely out of the reservoir, then if the steam pressure is very high the last of the oil may become heated to such an extent that the drops burst when they come up into the sight feed glass. The fouling of the glass may happen if the delivery jet in the glass becomes roughened or bruised, causing the drop to strike the glass instead of rising straight through the water.
- The Thresher’s Guide, Volume I, 1910 pages 89-99
Being a reprint from the Threshers’ School Of Modern Methods of the American Thresherman
Published By The American Thresherman, Madison, Wisconsin
Copyright 1910 By The American Thresherman
Prepared By Professor P.S. Rose, North Dakota Agricultural College
This book courtesy of Brian Szafranski