The Lubrication of Bearings.
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Contributed by an Engineer-in-Charge.
The efficient lubrication of the bearings of a steam wagon or tractor is a very important item, and demands careful and constant attention on the part of the driver. He must see that every bearing is getting just sufficient oil to maintain it in good working condition, for the evils arising from neglect will result in hot bearings, and brasses; journals will be cut and scored, and, in all probability, they will seize. Once these conditions exist there is endless trouble ahead, as a bearing which has become hot and scored, cannot again be relied upon until it has been taken apart and re-scraped. The three conditions under which bearings can be maintained in thorough working order are :—(1) good fitting and adjustment : (2) the choice of a suitable lubricant ; (3) the method of applying it. The employment of suitable metals in contact and the size and length of the bearings play an equally important part. Gun-metal and phosphor_bronze are most generally used for bearings, but bearings which are lined with white-metal are also much in use. Journals are usually of forged steel, as this metal is capable of taking a high polish, and there is less risk of its seizing in the bearing. It is essential that the brasses of a bearing should be in contact with the journal throughout its whole length, and, unless this condition is met, the bearing cannot be relied upon for any length of time on account of its tendency to become overheated; owing to the fact that the rate of expansion for gunmetal is greater than that for iron or steel, the higher the temperature of the bearing becomes, the tighter the brasses nip" the shaft. If the temperature continues to rise, the bearings become so tight that they are capable of bringing the engine up and, in all probability, this may lead to a serious breakdown.
One reason for a bearing becoming overheated may be that it has become too slack and allows such freedom to the shaft that it knocks against the brasses; the heat is generated by reason of the rapid succession of the blows, as is the case when a piece of iron is given several blows with a hand hammer: a rise in temperature is easily noticed. A slack bearing also allows the oil to run out and makes matters worse.
Another cause of the overheating of bearings is the presence of grit in them ; this may find its way to the bearing either from the road, or by way of the oil feeder. The presence of gr't causes particles of brass or steel to become detached and these, getting between the surfaces of the shaft and the bearing, very soon cause both to become scored.
In the event of a bearing becoming hot, it is advisable to stop immediately and slacken back the brasses ; if it is very hot, allow it to cool down gradually, but under no circumstances pour cold water over it until it has cooled down sufficiently for the hand to be laid upon it ; cold water can then be squeezed over it from a piece of waste, until it is cool enough to handle. The bearing should then be taken apart and carefilly examined for cuts or scoring, and, if such are found in it, any cut. tings should be carefully wiped out and the brasses and journal should be examined all over for any rough places ; these must be rubbed off with a piece of emery cloth wrapped round a file, or eased off with a scraper. The oil holes and channels should be examined before replacing the bearing to make sure the oil has free access, and it should be seen that the lubricator is clean and is working properly. After it has been overheated, special attention must he given to a bearing for a few days. Blacklead is often put into bearings, or mixed with the oil, with the idea of keeping them cool ; in many cases it gives the desired results, although great care must be exercised to make sure there is no grit present. The blacklead must be of the best quality, otherwise as soon as it loses its lustre it is useless as a lubricant and may do more harm than good.
The class of oil used for different bearings is of the greatest importance. Many men take it for granted that, as long as it is oil of some description, it is good enough for lubricating purposes; if it is capable of being burnt in the lamps at night, its value will rise 50 per cent, in the estimation of such men. There are many grades of oils : the thick oils, or oils having much
viscosity " "body," and the thin oils which have less "body." The conditions under which the different bearings work must be taken into consideration when deciding upon the kind of lubricant to be used. The object of a lubricant is to form a thin film of oil between the shaft and bearing and practically to keep them separated from one another, and so reduce the friction produced by the rubbing of the parts in contact. It is apparent that, for a bearing heavily loaded, a thin oil will not answer the desired purpose, because•it is too easily squeezed out from between the surfaces.
There are three kinds of oil in use, and these are :—Animal, Vegetable, and Mineral; for use as lubricants they are usually compounded with the object of neutralising the objectionable qualities, and obtaining the good qualities, of the constituent parts. Animal oils develop fatty acids, which corrode and pi; the journals ; vegetable oils oxidise rapidly at a high temperature, and this causes gumming of the bearing ; whilst mineral oils have a low "flash point," i.e. they fire, or flash, at a low temperature. The effect of heat onall oils is to reduce their "body" or "viscosity," so that the oil selected must be suited to the temperature. A thicker oil would be more suitable for an engine placed over the boiler than for one that was placed under the wagon and running in Oil. The oil used in the cylin. ders is always a thick oil having a high flash point and free from acids, so that it will not decompose and cause gumming as this, in a cylinder, would make the piston rings 'set" and become inoperative.
Should any doubt exist as to the quality of the oil, it may easily be tested for the amount of acid present, by mixing a small quantity of the oil with an equal amount of a saturated solution of caustic soda. The mixture should be bottled and, after being thoroughly shaken, it should be put to one side for a day or two. If, at the end of that time, it is found that the oil has absorbed all the caustic solution and has become a hard mass, that grade, of oil is useless for lubricating purposes. For heavy bearings, such as axle boxes, thick grease is more suitable, and there are several kinds of this class of lubricant to be found on the market. For such a purpose, grease is better than oil, and, if mixed with a little palm oil and blacklead, it makes a good mixture for use on gear wheels. All oils and greases should be obtained from reputable firms which make a speciality of lubricants, and, although it may cost a little more when obtained from such a source, it is far cheaper in the long run. Cheap oils and greases, obtained from obscure and irresponsible dealers, are always unsatisfactory, and their use leads not infrequently to serious trouble due to the overheating or gumming up of a journal in its bearing. The proper supply of oil to a bearing requires careful and constant attention oa the part of the driver ; it is surprising to find the amount of carelessness or ignorance some men exhibit. I have seen men " slush " the engine over with oil while it was running, and, should any oil reach the bearing, well and good ; the excess was allowed to trickle down off the engine on to the road.
The practice of having the brasses open at the top and bottom along their length, to allow for taking up," is very wasteful with, regard to the oil, as it can thus run out more easily than if liners were fitted to them; the latter are easily taken out and filed back, and their presence lessens the risk of the bearing being tightened up too much. The oil channels cut in the brasses, as shown in Fig, 1, should not extend to the outside by kin., and they should be nicely rounded on the edges; the hole in the top half of the brass must be well countersunk to prevent the oil from running away between the cap and brass. The lubricator used with this type of bearing is usually of the simplest kind, viz., the syphon" lubricator, shown in Fig. '2. This is generally found on crosshead brasses, guide bars, eccentric straps, etc. The oil is gradually fed to the bearing by means of stranas of worsted, the free ends of which are immersed in oil; all the strands are attached by means of a twisted wire which passes through a central tube to the bearing. The end of the twisted wire must be below the lowest level of the oil in the cup, otherwise the syphon action will stop before the cup is emptied. The worsted should be taken out occasionally, and scalded; all dirt or sediment should be removed from the cup. The chief disadvantage attending the use of this type of lubricator is the necessity for removing the worsted when the engine is stopped.
Fig. 3 shows a " needle " lubricator, such as is commonly used on. connecting-rod big-ends, and main bearings. In this lubricator, a piece of steel wire is loosely fitted in the central hole and the lower end of thus wire is alloweri to rest on the journal. The oil is fed down gradually by reason of the vibration set up in the wire. If properly fitted, the wire will cease to deliver the oil as soon as the engine is stopped.
Where thick grease is employed as the lubricant, a "screwdown," or spring-loaded, greaser should be used. Fig. 4 shows one of the latter type. It will be noticed that the cup is fitLed with a plunger and a rod, which passes through the screwed cover; a helical spring forces the plunger down, thus driving the grease to the bearing. The quantity of grease delivered may be adjusted for each particular bearing by means of tne small cone (A) which is placed at the bottom of the cup. This may be drilled out to any required diameter. It is advisable occasionally to disconnect all bearings (even if they have been running well for some time) and thoroughly to examine them, and, if necessary, to clean out the lubricator and the oil channels, and fill up with fresh oil or grease. Anyhard places can be eased off with a steel scraper ; any parts of the journal that feel at all rough should be rubbed down with a piece of oil stone. The "adjustment of bearings" is of vital importance if they are to run well for any length of time ; and the driver is unknown, who has not, at some time or another, had.a hot bearing. For every cause there is a remedy and, so long as this cause exists, a bearing will continue to give trouble.
[An article on the "Adjustment of Bearings" appeared in "THE COMMERCIAL MOTOR" of the 28th of July, 1906.—ED.]