Bearings Internal Combustion Engines

BEARINGS ; INTERNAL COMBUSTION ENGINES.) All oils change in viscosity with the application of heat though not all of them to the same degree at lower temperatures because of the nature of the crudes from which the mineral oils are made, as well as some of the methods of manufacture. The viscosity also increases with the increase of bearing pressure. The varying viscosity of the oils through the effect of heat and pressure is the most important item to be considered in making applications of the lubricants to practical operations. Every bearing requires a certain amount of time to work to a steady temperature above that of the surrounding atmosphere; after that it will remain in equilibrium for that speed and load condi tion, varying only with the temperature of the surrounding air. The lubricant, when cold, may be of exactly the proper viscosity to keep the surfaces apart. However, the heat generated within the oil itself through fluid friction may be sufficient to lower the viscosity so that actual striking of the high points of the surfaces will take place. It is necessary therefore to use a heavier oil than that theoretically required, depending upon the heat of operation to bring it to the desired viscosity for the machine condition. The frictional resistance of the machine will be high when starting, but the operation will be safe and free from wear at running temperatures. When the heat generated in a bearing equals the heat radiated, the body of the oil remains steady and the power required for the operation is likewise steady.

The manner of applying the lubricant is an influencing factor; intermittent lubrication, with its flooded and its dry periods, causes considerable losses. Supplying the lubricant to the film at frequent intervals by drop oilers can prevent most solid fric tion, but the heat is not carried away by the oil to the same extent as obtains in bath lubrication where the heat is conveyed to a reservoir where it can be dissipated over a larger surface.

Effect of Heating and Cooling.

In the case of marine turbine installations where but one oil is used in the circulating force-feed oiling system, it may be necessary to have certain parts of the equipment, such as the reduction gears, lubricated with a higher viscosity oil. This can be done by putting an oil cooler in a by-pass of the system ; the oil from this cooler would then be higher in viscosity than the main body of the oil. Heaters

and coolers can be put in the system for the purpose of keeping all of the oil at the required viscosity, or a heater can be put in a by-pass to secure as thin an oil as required for some par ticular high speed condition.

The Hot Bearing.

Undue heating in a bearing is caused by a film of lubricant interrupted by dirt or some unusual stress.

The high points of the bearing surfaces then strike and set up local heating. When this continues abrasion takes place, the oil becoming black on account of metal particles and finally charring.

As heating continues the film of lubricant becomes thinner, the metal expands and finally the film and surfaces rupture. This situation can be prevented by supplying large quantities of cool oil, which will carry the heat away, allowing the bearing condi tions to become normal. The circulating system, either for gravity or for force feed, does this automatically. As the oil heats it flows faster from the bearing and this carries more heat away. The increased flow of cool oil into the bearing reduces the tem perature of the metal and keeps the surfaces apart until they are back to normal when the oil feed slows up due to pipe friction induced by the higher viscosity of the cooler oil.

Reduction in Power.

When the lubricant in use in a spindle base is too high in viscosity, the machine starts with difficulty; the spindle will not turn its proper number of revolutions per minute because the belt slippage is excessive. When such an oil has reached its heat and viscosity equilibrium, the fluid friction and power consumption will still be too great and the temperature of the base will be high in consequence. When a condition such as this exists, changing the oil to one more suitable will cause a considerable reduction in the power necessary to operate the machine. At the same time the reduced resistance in the spindle base reduces its temperature and allows the spindle to rotate at a faster speed which may mean greater production. If a lubri cant that is too thin is used, the conditions are those approaching solid friction; excessive power is consumed, wear is continuous and the heat produced expands the metals until they seize.