The New Zealand Railways Magazine, Volume 4, Issue 12 (April 1, 1930)
The Lubrication of Bearings — Part IV. — Incorrect Structure Of Bearings
The introduction and distribution of oil within the bearing, the formation of the film and the effectiveness of the film in reducing friction are all dependent upon the materials and physical form of the bearing as resulting from design, workmanship, adjustment and wear.
Unsuitable materials—too hard or too soft, too granular or too fibrous—are sometimes, though rarely, the cause of bearing troubles. The subject of material selection is well understood by machine builders to-day.
Insufficient bearing area, resulting from faulty design, is rare in modern machines. It results in an excessive unit pressure on the bearing surface and enhances the importance of correct formation of the surfaces to produce an effective oil wedge. It may also require the use of a heavier-bodied oil in spite of the greater fluid-frictional losses which result.
Ineffective or insufficient oil inlets prevent complete lubrication. In a long bearing, two or more inlets are usually necessary. If the inlet enters the bearing clearance on the pressure side, it will be closed by the journal; so that oil cannot enter except by applying it under high pressure. This trouble often occurs where the oil holes are on the top of the bearing and there is a strong upward belt pull on the shaft. The location of the oil inlets should be changed to a low pressure area.
Uneven bearing surfaces, due to poor workmanship, result in a reduction of the true bearing area; making it impossible to maintain a strong supporting oil film. Wear may correct this fault or make it worse. The exact formation of the bearing surfaces by good workmanship is important.
Insufficient clearance, either from making the bearing and journal too close a fit or from too tight an adjustment, prevents oil distribution and film formation. Excessive clearance—resulting from inaccurate workmanship, wear, or loose adjustment—reduces the effective pressure of the oil film, and promotes oil waste.
The lack of a wedge-shaped clearance, in which the film pressure can be built up in order to support the load, is the cause of many bearing troubles. This occurs in bearings with small clearance, and is relieved by bevelling the sharp edges of the bearing parts and “easing away” the surfaces of the bearing adjacent to the bevel. This may be done by scraping or machining. Where grooves are employed the edges should always be rounded or eased away.
Incorrect grooving may permit the oil to escape from the end of the bearing or may destroy the film pressure at the point where pressure is required. Unnecessary grooves are harmful. Wear of chamfers and grooves may in time destroy their effectiveness or create sharp edges that cut away the film. Poor oil distribution often occurs in ring, chain, or collar oiled bearings, due to the lack of suitable channels to spread the oil over the full bearing surface.
Without the physical correctness of the bearing, perfection of lubrication is impossible. Especially in the cases where heavy loads must be sustained, much improvement of lubrication can be accomplished by paying attention to the correctness of oil inlets, bearing clearance and bevelled edges; and, where necessary, to the grooving and “easing away” of the surfaces to accomplish oil distribution and oil-wedge formation.
(2) Bearing pressures will be regarded as excessive whenever they are greater than intended in the design of the bearing, or greater than permissible in good practice. Overload on a machine is, therefore, an evident cause of excessive bearing pressures.
Incorrect alignment is a common cause of excessive bearing pressures. This may occur through the incorrect position of the bearing either horizontally or vertically; or the bearing may be twisted on its foundation. Correct alignment of bearings is the first essential of satisfactory installation and should not be neglected.
Heavy belt pull or unnecessarily tight driving chains result in excessive pressures and should be corrected in order to avoid trouble. Worn gears often result in heavy radial forces and vibratory loads on the bearing.
Excessive end thrust on a shaft, caused by expansion or contraction of a long lineshaft with changing temperature sometimes brings a heavy pressure on a thrust collar not intended for a heavy load, thus causing friction and heating.
Excessive pressures, such as have been described, often cause failure of the oil film, resulting in metallic contact, rapid wear and excessive frictional heat which may destroy the bearing. They should be overcome by mechanical correction wherever possible. When heavy pressures are unavoidable, the resultant troubles may be relieved by the use of a heavy-bodied oil. Heavy-bodied compounded oils, due to their superior adhesive qualities, are useful for extreme bearing pressures, provided the method of lubrication does not involve repeated use of the oil.
(3) A bearing may become heated as a result of the temperature of the surrounding atmosphere or objects, or it may become hot because the frictional heat generated in the bearing has no adequate means of dissipation. On the other hand cold weather conditions or refrigeration may greatly reduce the temperature of the bearing. When these high or low temperatures are considered in selecting the oil, bearing troubles from these causes can be avoided. Unforeseen changes in these conditions, however, may cause trouble.
Hot surroundings tend to raise the temperature of a bearing to a fixed limit. The reduction of oil body by increased temperature is allowed for by the use of heavy-bodied oil.
Insufficient heat radiation from the bearing, due to a confined or non-ventilated location, may cause the accumulation of heat to such an extent that the high temperature of the bearing reduces the body of the oil to a dangerous degree. Proper ventilation should be provided for bearings, especially those of high-speed machines. When the circulation system is employed for lubrication, a large quantity of oil circulated through the bearing prevents heat accumulation where, with another system of lubrication, heat radiation might be insufficient to keep the bearing temperature moderate. In some cases bearing-housings or brasses are cored out and connected to a water-circulation system for cooling.
Cold surroundings sometimes reduce the temperature of the bearing below that at which the oil in use will flow, causing difficulty in starting a cold machine. Frictional heat generally raises the temperature to a point where the page 51 oil will flow and produce lubrication, but it is quite possible that injury may occur to the bearing before oil flow is established.
Where the bearings are expected to operate under very low temperature conditions, it is advantageous to design them in such a way that a supply of lubricant is held adjacent to the journal by waste packing. The first frictional heat generated on starting will then act upon the supply of oil in the packing adjacent to the journal. The transmission of heat throughout the bearing then gradually liquefies the entire mass of oil, so that the supply is maintained.
In some machines hot water or steam is circulated through water jackets while starting, being shut off later when frictional heat becomes sufficient to maintain the desired temperature. Depending on the type of machine and the method of oil application, it is often necessary to use an oil having a sufficiently low cold test to avoid congealing, even under the lowest temperature at which it has to operate.
Contamination of the Lubricant.
(4) Carelessness in handling lubricants and their application by methods which permit contamination, are responsible for many bearing troubles. From the moment that the lubricants are received in barrels or tank cars they should be guarded against dirt, grit, water, and any possibility of mixing with other oils. Storage tanks into which barrels are emptied by the user should be covered thoroughly and marked plainly with the name and purpose of the oil. In distributing the oil throughout the plant, open containers should be avoided, and care should be taken that each container is used for only one kind of oil, to prevent mixing.
Hand application of oil or grease is responsible for many impurities entering the bearing. The danger from this source is reduced somewhat by the use of closed oil cans and self-closing oil-hole covers on the bearings. The various types of automatic lubricating appliances are adapted more or less to the exclusion of foreign matter from the bearing. Bottle oilers and wick feed oilers are particularly desirable from this standpoint.
Circulation and splash lubrication systems are sensitive to the presence of foreign matter, since extreme purity of the lubricant is necessary in order that it may be able to maintain long-continued service. A frequent source of trouble is the accumulation of water in the circulation system which may be caused by leakage from water cooling systems or even condensation of water vapour from the atmosphere.
(To be concluded.)page 52
Empire Farmers’ Party Tour The South Island.
(Rly. Publicity photos)
(1) and (3) Empire Farmers’ Party leaving Pelorus Sounds (Marlborough) by launch; (2) fishing in the Sounds—catching a blue cod; (4) and (5) luncheon at Mr. R. D. Fell's home. Pelorus Sounds; (6) inspecting Merino rams at Amberley (Canterbury); (7) a scene in Nikau Bay, Pelorus Sounds; (8) and (9) arrival and welcome at Amberley Domain; (10) at Amberley Railway Station; (11) Mr. E. C. Bacon (England), replying to the address of welcome at Amberley; (12) Mr. A. V. Allen (left), manager of the South African Party; (13) and (14) members of the party partaking of afternoon tea, at Amberley.