more than one brass, and the construction is modified accordingly. I brush, shaped to fit the underside of the journal, whilst the lower Figs. 4 and 5 show an axle luv used for gouds wagons on the part consists of streamers of wick resting in the oil. The oil is Great Eastero railway, and they also illustrate the method of led to the brush by the capillary action of the streamers. The

reservoirs are filled with oil through the apertures P and 0. W

The bottom cap is held in position by the T-headed bolts Q, and Q, (fig. 5). By slackening the nuts and turning the T. heads fair with the sluis in the cap, the cap comes right away and the axle may be examined. A leather ring L is fitted as shown to prevent dust from entering the axle box.

Footsteps. --A bearing arranged to support the lower end of a vertical shaft is called a footstep, sometimes a pivot bearing -LA simple form of footstep is shown in fig. 6. A casting A,

designed so that it can be conveniently bolted to a foundation block, cross bcam, or bracket is bored out and fitted with a brass B, which is turned inside to carry the end of the shaft S. The whole vertical load on the shaft is carried by the footstep, so that it is important to arrange efficient lubricating apparatus.

Results of experiments made on a footstep, reported in Proc. - Inst. Meck. Eng., 1891, show that if a diametral groove be cut in the brass, as indicated at & (fig. 6), and if the oil is led to the centre of this gruove by a channel c communicating with the exterior, the rotation of the shaft draws in a plentiful supply of oil which radiates from the centre and makes its way

briselanod vertically between the shaft and the brass and finally overflows at the top of the brass. The overflowing oil may be led away and may

be re-introduced into the с

footsteps at c. The rota

tion of the shaft thus causes pad lubrication in general use for this kind of bearing. 'The a continuou circulation of main casting. A, is now uppermust, and is designed so that the oil through the footstep. upper part supports and constrains the spring buckle through One experiment from the which the load W is transmitted to the bearing, and the lower report mentioned above part insule is arranged to support the brass, B. The brass is may be quoted. A 3 in. juvinted freely with the main casting by means of a kemispherical shaft, revolving 128 times hump rosting in a corresponding recess in the casting. What par minute and supported may be called the cap, C, forins the lower part of the axle box, on a manganese bronze

bearing lubricated in the w

way explained above sus.

tained increasing loads
obsen until, at a load of 300

pounds per square inch of
the area of the end of the

FIC. 6.
shaft, it seized.

The mechanical details of a footstep may he varied for purposes of adjustment in a variety of ways similarly to the variations of a common bearing already explained.

Thorust Block Bearing. - In cases where a bearing is required to resist a longitudinal movement of the shah through it, as for "sample in the couse of the propeller shaft of a marine engine or a vertical shaft supporting a heavy load not carried on a footstep. the shit is provided with one or more collar which are grooved with corresponding recesses in the braves of the bearing A Beheralsketch of a thrust blok for a propeller shift is shown in tug 7. 'l bere are seven collars turned on the shaft and into the

FIG 4

molt C

De laat

Fro. 7. but instead of supporting a second brass it is formed into an oil circumferential grooves between them fit corresponding circunrezervoir in which is arranged a pad of cotton wick woven un aferential projections on the brasses, these projections being tin frame. The upper part of the pad is formed into a kind of formed in the case illustrated by means of hail rings which are

E

fitted into grooves turned in the brasses. This method of sometimes wholly, lined with a soft fusible metal, technically construction allows an individual ring to be replaced or adjusted known as white metal, which melts away before actual seizure if it should get hot. The total area of the rubbing surfaces should takes place, and thus saves the journal which is more expensive be proportioned so that the average load is not more than from 50 because it is generally formed on a large and expensive shaft. to 70 lb per sq. in. Arrangements are usually made for cooling a However perfectly the film fulfils its function, the work required thrust block with water in case of heating. The spindles of to overcome the viscous resistance of the film during the condrilling machines, boring machine spindles, turbine shafts may be tinuous rotation of the shaft appears as heat, and in consequence cited as examples of vertical shafts supported on one collar. the temperature of the bearing gradually rises until the rate at Experiments on the friction of a collar bearing have been made which heat is produced is equal to the rate at which it is radiated by the Research Committee of the Institution of Mechanical from the bearing. Hence in order that a journal may revolve Engineers (Proc. Inst. Mech. Eng., 1888).

with a minimum resistance and without undue heating two Roller and Ball Bearings. If rollers are placed between two precautions must be taken: (1) means must be taken to ensure surfaces having relative tangential motion the frictional resistance that the film of oil is complete and never fails; and (2) arrangeto be overcome is the small resistance to rolling. The rollers movements must be made for controlling the temperature should it rise along with a velocity equal to one half the relative velocity of the too high. The various lubricating devices already explained surfaces. This way of reducing frictional resistance has been supply sufficient oil to form a partial film, since experiments have applied to all kinds of mechanical contrivances, including bearings shown that the friction of bearings lubricated in this way is akin for shafts, railway axle boxes, and axle boxes for tramcars. An to solid friction, thus indicating at least partial metallic contact. example of a roller bearing for a line shaft is illustrated in figs. 8 In order to supply enough oil to form and maintain a film with and 9. The main casting, A, and cap, C, bolted together, form a certainty the journal should be run in an oil bath, or oil should be spherical seating for the part of the bearing Ecorresponding to supplied to the bearing under pressure sufficient to force it in the brasses in a bearing of the usual type. Between the inside of between the surfaces against the load. A bearing to which forced the casting E and the journal are placed rollers held in posi- | lubrication and water cooling are applied is illustrated in fig. 10, tion relatively to one another by a " squirrel cage" casting, the which represents one of the bearings of a Westinghouse turbosection of the bars of which are clearly shown in the half sectional alternator installed at the power station of the Underground elevation, fig. 9. This squirrel cage ensures that the several axes Electric Railways Company of London at Lots Road, Chelsea. of the rollers keep parallel to the axis of the journal during the Oil fows under pressure from a tank rolling motion. The rollers are made of hard tool steel, and the on the top of a tower along a supply

yeen
helt pipe to the oil inlet 0, and after

passing through the bearing and
- performing its duty as a film it falls
Taway from each end of the journal

into the bottom of the main casting, A from which a pipe, E, conveys the

oil båck to the base of the tank tower

where it is cooled and finally pumped To back into the tank. There is thus a

continuous circulation of oil through

the bearing. The space C is for cooisurfaces of the journal and bearing between which they roll are ing water, in fact the bearing is water hardened.

jacketed and the jacket is connected Two rings of balls may be used instead of a single ring of to a supply pipe and a drain pipe so rollers, and the kind of ball bearing thus obtained is in general that a continuous circulation may be use principally in connexion with bicycles and motor cars (see maintained if desired. This bearing is 12 in. in diameter and BICYCLE). In ball bearings the load is concentrated at a few 48 in. long, and it carries a load of about 12-8 tons. The rise in points, the points where the balls touch the race, and in the roller temperature of the bearing under normal conditions of working bearing at a few lines, the lines of contact between the rollers and without water circulating in the jacket is approximately 38° F. the surfaces of the journal and bearing; consequently the load The speed of rotation is such that the surface velocity is about which bearings of this kind carry must not be great enough to 50 ft. per second. cause any indentation at the points or lines of contact. Both Forced lubrication in connexion with the bearings of highrollers and balls, and the paths on which they roll, therefore, are speed engines was introduced in 1890 by Messrs Belliss & Morcom, made of hard material; further, balls and rollers must al be Lid., under patents taken out in the name of A. C. Pain. It exactly the same size in an individual bearing in order to dis-should be understood that providing the film of oil in the bearing tribute the load between the points or lines of contact as uni- of an engine can be properly maintained a double-acting engine formly as possible. The finest workmanship is required therefore can be driven at a high speed without any knocking, and without to make good roller or good ball bearings.

perceptible wear of the rubbing surfaces. Fig. 11 shows that the Bearings for High Speeds and Forced Lubrication.-When the general arrangement of the bearings of a Belliss & Morcom shaft turns the metallic surfaces of the brass and the journal are engine arranged for forced lubrication. A small force-pump F, prevented from actual contact by a film of oil which is formed and driven from the eccentric strap X, delivers oil into the pipe P, maintained by the motion of the shaft and which sustains the along which it passes to A, the centre of the right-hand main pressure between the journal and the brass provided the surfaces bearing. There is a groove turned on the inside of the brass are accurately formed and the supply of oil is unlimited. This from which a slanting hole leads to B. The oil when it arrives film changes what would otherwise be the friction between | at A thus has two paths open to it, one to the right and left of metallic surfaces into a viscous resistance within the film itself. the groove through the bearing, the other along the slanting When through a limited supply of oil or imperfect lubrication hole to B. At B it divides again into two streams, one stream this film is imperfect or fails altogether and allows the journal to going upwards to the eccentric sheave, and a part continuing make metallic contact with the brass, the friction increases, and up the pipe Q to the eccentric pin. The second stream from B it may increase so much that the bearing rapidly becomes hot and follows the slanting hole in the crank shaft 10 C, where it is led may ultimately seize, that is to say the rubbing surfaces may to the big end journal through the pipe R to the crosshead pin, become stuck together. With the object of reducing

the friction and through the slanting hole to D, where it finds its way into the at the points of metallic contact and of confining the damage of a left main bearing. The oil forced through each bearing falls hot bearing to the easily renewable brass, the latter is partially, away to the right and to the left of the journal and drops into

Fig. 8.

Fig. 9.

Fig. 10.

the bottom of the engine framing, whence it is again fed to the pump through a strainer. The parts of an engine lubricated in this way must be entirely enclosed.

Oil Strainer

Valve A

by the simple formula put=constant=2, where the pressure in kilograms per square centimetre, and the temperature in degrees centigrade. If p is changed to pounds per square inch the constant in the expression is approximately 30. The expression is valid between limits of pressure 14 to 213 pounds per square inch, limits of temperature 30° to 100° C., and between limits of velocity 3 to 50 ft. per second.

Theory of Lubrication.-After the publication of Tower's experiments on journal friction Professor Osborne Reynolds showed (Phil. Trans., 1886, p. 157) that the facts observed in connexion with a journal lubricated by means of an oil bath could be explained by a theory based upon the general principles of the motion of a viscous fluid. It is first established as an essential part of the theory that the radius of the brass must be slightly greater than the radius of the journal as indicated in fig. 12, where J is the centre of the journal and I the centre of the brass. Given this difference of curvature and a sufficient supply of oil, the rotation of the journal produces and maintains an oil film between the rubbing surfaces, the circumferential extent of which depends upon the rate of the oil supply and the external load. With an unlimited supply of oil, that is with oil-bath lubrication, the film extends continuously to the extremities of the brass, unless such extension would lead to negative pressures and therefore to a discontinuity, in which case the film ends where the pressures in the film become negative. The

FIG. 12.

minimum distance between the journal and the brass occurs at the point H (fig. 12), on the off side of the point O where the line of action of the load cuts the surface of the journal. To the right and left of H the thickness of the film gradually increases, this being the condition that the oil-flow to and from the film may be automatically maintained. With an unlimited supply of oil the point H moves farther from O as the load increases until it reaches a maximum distance, and then it moves back again towards O as the load is further increased until a limiting load is reached at which the pressure in the film becomes negative at the boundaries of the film, when the boundaries recede from the edges of the brass as though the supply of oil were limited.

Load on bearings.-The distribution of pressure over the film of lubricant separating the rubbing surfaces of a bearing is variable, being greatest at a point near but not at the crown of the brass, and falling away to zero in all directions towards the boundaries of the film. It is usual in practice to ignore this variation of pressure through the film, and to indicate the severity with which the bearing is loaded by stating the load per square inch of the rubbing surfaces projected on to the diametral plane of the journal. Thus the projected area of the surfaces of a journal 6 in. in diameter Oil Reef and 8 in. long is 48 sq. in., and if the total load carried by the bearing is FIG. 11. 20,000 pounds, the In the mathematical development of the theory it is first necessary bearing would be said to carry a load of 417 pounds per square to define the coefficient of viscosity. This is done as follows:-If inch. When a shaft rotates in a bearing continuously in one two parallel surfaces AB, CD are separated by a viscous film, and if direction the load per square inch with which it is safe to load whilst CD is fixed AB moves in a tangential direction with velocity the bearing in order to avoid undue heating is much less than if U, the surface of the film in contact with CD clings to it and remains the motion is intermittent. A table of a few values of the bearing at rest, whilst the lower surface of the film clings to and moves with the surface AB. At intermediate points in the film the tangential loads used in practice is given in the article LUBRICANTS. Bearing Friction-If W is the total load on a bearing, and if is motion of the fluid will vary uniformly from zero to U, and the the coefficient of friction between the rubbing surfaces, the tangential tangential resistance will be F-Uh, where a is the couthcient of resistance to turning is expressed by the product W. If is the viscosity and h is the thickness of the film. With this definition of a viscous fluid, the following equation is established, giving the relative velocity of the rubbing surfaces, the work done per second viscosity and from the general equations representing the stress in against friction is We foot pounds. This quantity of work is conrelations between p, the pressure at any point in the film, the thickverted into heat, and the heat produced per second is thereforeness of the film at a point x measured round the circumference of the W/778 British Thermal Units. The coefficient is a variable quantity, and bearing in mind that a properly lubricated journal is journal in the direction of relative motion, and U the relative tanseparated from its supporting brass by a film of lubricant it might gential velocity of the surfaces, be expected that would have values characteristic of the coefficient of friction between two metallic surfaces, merging into the characteristics properly belonging to fluid friction, according as the oil film varied from an imperfect to a perfect condition, that is, according as the lubrication is partial or complete, completeness being attained by the use of an oil bath or by some method of forced lubrication. This expectation is entirely borne out by experimental researches. Beauchamp Tower ( Report on Friction Experiments," Proc. Inst. Mech. Eng., November 1883) found that when oil was supplied to a bearing by means of a pad the coefficient of friction was approximately constant with the value of 1/100, thus following the law of solid friction; but when the journal was lubricated by means of an oil bath the coefficient of friction varied nearly inversely as the load on the bearing, thus making W=constant. The tangential resistance in this case is characteristic of fluid friction since it is independent of the pressure. Tower's experiments were carried out at a nearly constant temperature. The later experiments of O. Lasche (Zaisch. Verein deutsche Ingenieure, 1902, 46, pp. 1881 et seq) show how depends upon the temperature. Lasche's main results with regard to the variation of a are briefly:-W is a con-. stant quantity, thus confirming Tower's earlier experiments; is practically independent of the relative velocity of the rubbing surfaces within the limits of 3 to 50 ft. per second; and the product uf is constant, being the temperature of the bearing. Writing for the load per unit of projected area of the bearing, Lasche found that the result of the experiments could be expressed

£ (hdf) -6μUdh

(1)

The thickness

In this equation all the quantities are independent of the co-ordinate
parallel to the axis of the journal, and U' is constant.
of the film is some function of x, and for a journal Professor
Reynolds takes the form,
h=a[1+c sin(@— ~)).
in which the various quantities have the significance indicated in
fig. 12. Reducing and integrating equation (1) with this value of h
it becomes

(2)

being the value of for which the pressure is a maximum. In order to integrate this the right-hand side is expanded into a trigenometrical series, the values of the coefficients are computed, and the integration is effected term by term. If, as suggested by Professor J. Perry, the value of h is taken to be k➡ho+ax2, where is the minimum thickness of the film, the equation reduces to the form C

6 U

(3) and this can be integrated. The process of reduction from the form (1) to the form (3) with the latter value of h, is shown in full in The Calculus for Engineers by Professor Perry (p. 331), and also the final solution of equation (3), giving the pressure in terms of s.