Friday, January 6, 2012

Applications of friction: How an air lubrication works

1- Friction and brakes
            The brake lining of automobiles and industrial machinery are  commonly made from woven or moulded material incorporating the mineral asbestos. This has good frictional properties and much research has been carried out in the manufacturer's research laboratoris to produce a strong, hard-wearing material with freedom from fade. Fade is the name given to the loss of frictional force which occurswhen the brakes have been severe continuous operation. It results from slight chemical changes in the surface caused by the rise in temperature produced.
            Wise motorists do not place full reliance on brakes if the    linings get wet. Test trial drivers keep their feet lightly on the brake pedals for a short time after passing through deep water so that the resultant rise in temperature dries the linings and quickly restores their efficiency   ( very good idea ) .
2- Lubrication
            While friction is highly essential in some circumstances, it is a great nuisance in others. In a car engine, for example, oil under pressure is supplied continuously to all bearing surfaces. Failure of the oil supply will allow metal-to-metal contact and the resultant friction often raises the temperature and causes the bearings and pistons to "seize up".
Although liquid molecules attract one another they can interchange partners quite easily. The opposing force which one layer of liquid exerts an another is called viscosity. For liquid lubricants this is very much less than the frictional force most solids exert on one another. Some lubricating oil contain certain solid or liquid substances which attach themselves to the bearing surfaces to form a tenacious slippery coating. The possibility of metal-to-metal contact is thus greatly reduced, accompined by less wear and smoother running.

3- Air lubrication
As long ago  as 1854, the French scientist, Gustav Hirn, Suggested the use of air as a lubricant, but nearly a century was to pass before this idea was taken up seiously.
        Of recent years, research has been carried out at the National Engineering Laboratory at Kilbride in the development of air lubricated bearings and these are now in highly successful commercial production. They have a special application in machine tools, particularly grinding machines.
            When the final accurate finish is required on the component parts of a piece of machinery, it is customary to use a grinding wheel. Obviously, not the slightest wobble can be tolerated in the bearings. In the past oil lubricated bearings were used with very tight clearances, which had to be run for some hours before use so that they acquired a steady temperature. The consequent expansion took up any slackness and the bearings were run under conditions in which they just did not seize.
                When purified compressed air is employed as a lubricant, very tight clearances are unnecessary. Until oil which is partically incompressible, the air forms an elastic cushion which averages out spindle imperfections and gives truer rotational accuracy. The whole problem of friction and heating is eliminated with the further advantage that the outflow of air prevents the entry of grinding dust. The machines can be used immediately they are started up with the net advantage of shorter production time coupled with greater precision of finish on the workpiece.
        Further developments
        Other applications of air lubrication have been explored such as the hovercraft which travels over water or land on a cushion of air.

        Another example is the aerotrain. powered by aircraft engine, it sat astride an inverted T-shaped rail and rode on a cushion  of compressd air provided by powerful pumps.
        In ship holds and other confined spaces, heavy containers may easily be moved for stowage on hoverpads. These have flixible pleated skirt which takes up the air pumped in.