race from falling out when the whole bearing is lifted away.

The manufacture of ball bearings entails a very high class of workmanship, as already mentioned in the beginning, to ensure precision, and durability. Both balls and races must be hardened to enable them to retain their form; the balls are of cast steel, and can

Fig. 58.-Thrust Bearings.

Upper Fig., thrust for drilling machine pillar. Lower Fig., ditto for crane hook.

be now made within very close limits, both in regard to sphericity, and uniformity of size. Limits of from 3000 to 10000 inch are worked

to.

The balls are prepared first either in the form of forgings, stamped from bar in dies, or are turned up from bar in automatic machines,

or they are cut as short cylindrical pieces from bar, and then rolled into spheres, in any of which forms they are then ready for grinding. This is not done in a single stage, but roughing and finishing operations are essential. The grinding is usually done across the face of a wheel, to which the balls are presented in quantity, and passed across, rolling as they go, in a groove in the holder. This is repeated until the balls are reduced sufficiently in size to make their escape into a receptacle. After this preliminary roughing, hardening is done, very often in an automatic furnace, through which they pass, being heated meanwhile, and are then shot into a bath of oil. Tempering succeeds the hardening. The finish grinding is performed in other machines, which embody provision for automatically controlling the finished size of the balls. Polishing, and final inspection complete the process.

The manufacture of the races is a question of grinding in the concluding stages, after hardening, so that any inaccuracies introduced by this process are eliminated. It will be apparent therefore that the perfection of the ball bearing is due to the employment of grinding machinery, without which commercial production would not be feasible.

The table on page 65 gives particulars of bearings having balls ranging from in. to 1-in. diameter, with safe working loads at various speeds, from the practice of the Auto-Machinery Co., Ltd.

Ball Bolt.-A bolt having its head, and sometimes the nut of globular form, to fit in semispherical recesses, which allows of slight swaying or swivelling motion occurring. The bottom faces of nuts alone are frequently globular, to allow the screw or bolt to accommodate itself to varying angles, a familiar instance of which happens in compasses and calipers, and in various types of joints. See Bolt.

Ball Breaker. A ball suspended from a crane, and used to break up pig iron and scrap with. The ball usually weighs from half a ton to a ton, and when hoisted to a suitable height, is let fall by the release of a lever catch, pulled by a cord.

Ball Cage. The cage which confines the ball of a ball-cock in proximity to its seating.

To permit of the insertion of the ball the cage is made in two parts screwed together, the cage proper above and the seating below. See Fig. 59.

Ball Casting.-Governor and other balls which have to be turned and polished free from specks are best cast in the manner shown in Fig. 60. The metal is run through a skimming chamber, having a riser coming up from it. The centrifugal action generated by the rotation of the metal in its passage through the chamber buoys the lighter matters to the top of the chamber, whence they collect in the riser.

Ball-Cock, or Ball Valve. This has two meanings. One that in which the cock or valve is closed by a floating hollow ball of copper at the end of a lever, as in cisterns.

Fig. 60.-Ball Casting.

The other denotes the ball of brass or gun metal employed in check, and other valves in place of disc lift valves (see Fig. 59). The superiority of the ball over the latter lies in the constant change of position of the ball around its axis, by which equal wear is ensured.

A particular application of a ball-valve is embodied in some water- gauges. In these there is a ball in the lower portion below the gauge glass. In the event of the glass breaking, the outward rush of steam drives the ball up against a seating in the passage way, and shuts off the steam.

Ball Counterweight.-A weight made to grip the lower end of a crane chain, just above the hook, to overhaul the chain when it is not strained by a load.

Baller. The workman who attends to the balling or reheating furnace in an iron works.

Ball Grip. A term sometimes used to denote the self-adjusting curved grip pieces for test specimens. See Spherical Grips.

Ball Handle.-A type of balanced handle which is much used for actuating machine tool slides, the feature of which is the balancing, by forming a large ball at one end to make up for the weight of the handle at the other. When it is fitted with two outstanding handles, the knobs are of equal sizes. The manufacturing methods of producing ball handles have greatly cheapened them, since either die forging can be followed, or form-turning in turret lathes or forming machines, employing tools shaped to the profile of the handle. The importance of having a handle balanced is felt in fine work, where an unbalanced type causes screws and adjustments to alter, when vibration shakes the heavier end of the handle downwards, causing partial rotation to occur.

Balling Furnace.-A forge furnace of reverberatory type for piles of malleable iron, in which they are raised to a welding heat preparatory to the working of the same under the hammer, or in the merchant, or mill rolls.

These furnaces are built of cast-iron plates lined with fire-bricks. The bottom bricks are covered with sand, rammed on while moist. There is a double slope on the bed; from the working door at the side to the back, and from the fire-bridge next the grate to the chimney stack at the other end. The cinder from the hearth flows from the bridge away to the stack end, and out at the tap hole situated at the lowest part of the slope. The working door is at one side, situated almost centrally with the hearth. The stoking door is about in the centre of the grate, and the brick fire-bridge reaches to within about 14 inches of the roof.

The reheating of the piles occupies from half an hour to an hour, during which period they are moved about to promote equal heating. The oxidation of the iron at and near the welding heat causes scale to fall on the bed, which combining with the silicious sand, produces the slag or cinder which flows towards the tap hole.

Balling up. See Ball.

Ballistic Galvanometer.-See Galvanometer.

Ballistic Pendulum. - An instrument used in class-room demonstrations to illustrate and measure the momentum produced by a horizontal blow. In its essential form it was invented by Robins in 1742, and remained in use until about 1840, when the Chronoscope was invented by Wheatstone.

The ballistic pendulum was essentially a pendulum with a massive wooden bob, to which was fixed a steel ribbon passing through a clamp set to the pressure desired. A bullet fired against the wooden bob set the pendulum in motion, and the length of the arc described was recorded by the ribbon, whence, the conditions and masses being known, the velocity was calculated. Count Rumford suspended the gun on the pendulum. About 1840, in experiments carried out by the French Government, the weight of the apparatus was greatly increased to suit heavy shot, the receiving pendulum being made of nearly 6 tons weight, to receive the impact of 50-lb. balls. But since the invention of the Chronoscope, operated electrically, interest in the old ballistic apparatus has nearly ceased.

Ball Mill. A crushing mill employing balls as the means of trituration. Some of these have large heavy balls revolving in a pan, the lower part of which is an annulus of concave shape, between which and the balls, the crushing takes place. The balls are revolved from a vertical shaft driven by bevel wheels. The dust is taken away through a shoot in the bottom of the pan.

More ball mills are made of cylindrical form, the axis lying horizontally. They are closed barrels, with a door for the insertion and removal of the material, and containing a large number of small balls of different diameters, which are thrown about by the rotation of the drum. The balls may be of iron, case hardened, or of steel. These are for both dry and wet grinding.

Details of construction vary to suit different classes and quantities of materials. Among these are included argillaceous earth, blast furnace slag, bones, charcoal, coal, coke, copper ore, emery, ferro-manganese, glass, graphite,