The bull-wheels (Fig. 40) are built on a 16-in. bull-wheel shaft (48) supported at each end by the bull-wheel posts (23). The line car rying the tools used for drilling is wound on this shaft and passes up inside the derrick and over the crown-pulley (3). The wheels are of wood, 8 ft. diameter, and the one in line with the grooved circle on the band-wheel (Fig. 36) is similarly grooved in order to carry the bull-rope for power transmission. This wheel is known as the bull-wheel tug-pulley and has two such circles when two bull-ropes are used. The rim• of the wheel at the other end of the shaft is surrounded with an iron brake-band, to retard the speed of the tools when being lowered into the hole and at other times to prevent the wheels from moving.
The calf-wheel (Fig. 41) is a comparatively recent innovation for handling casing without having to disengage the drilling-line from the tools for that purpose. It has a single wheel, placed at one end of a shaft that is supported by two posts (29), and, like the bull-wheel, is controlled by a brake-band. When first used it was driven from the band-wheel by ropes, as is still done with the bull wheels, but this has now been almost entirely discarded in favor of the more positive chain drive, the chain running from the clutch sprocket on the band-wheel shaft-to-an rim attached to the calf-wheel (Fig. 42). The calf-line' passes from the calf wheel shaft over ope of the casing-pulleys (2), and thence back and forth between these and a snatch-block. Ordinarily there are seven lines between the latter and the casing-pulleys, but when the weight to be sustained in taking heavy pulls on casing demands nine lines instead of seven, a fifth casing-pulley is inserted between the usual crown-block \and an additional parallel piece of timber placed on the bumpers.
The crank shown at -end of the main shaft in Fig. 37 turns with the band-wheel and by its off-set imparts the up-and down motion to the walking-beam by means of a wrist-pin passed through one of the holes and the opening in the pitman (26). The length of the movement or sweep of the beam depends upon which of these holes is used, within limits of about 2 to 5 feet. The one nearest the shaft is known as hole, the next succeeding as the second hole, and so on. The first hole-is-r,arely used in drilling but is one employed iti pumping.
All the metal parts used in the conslruction f a derrick with the exception of the nails, sand-reeliand\ guy wire, are known collectively as the 'rig irons,',:a-tiP,daig'nate4 by the size of the crank-shaft that carries' the band-wheel: Rig irOn.s of the 4-in.
and 5-in, sizes are used only for;fairly light work anTh.the 6-in.
commonly employed for heavier diity. Recently iron's have been tried with marked success where the conditions are such as to require unusually heavy tools and equipment.
Rig,.Iron List.
1, 7%-ft. Shaft with crank, wrist pin, set of 36-in. band wheel flanges and bolts, collars and keys, and clutch sprocket.
1, Sprocket tug-rim for calf-wheel. . 1, Jack-post box and cap.
1, Calf-wheel box and cap. 4, Turnbuckle rods.
2, Jack-post rods.
1, Jack-post plate.
4, Eye-bolts.
4, Double-end bolts.
1, Set center irons and bolts, for walking-beam.
1, Set bull-wheel—gudgeon, and brake-band.
1, Set calf-wheel gudgeons.
1, Brake-band for calf-wheel.
1, Walking-beam stirrup.
1, Crown pulley.
1, Sand-line sheave.
4, Casing-line pulleys.
55, feet of sprocket chain, for calf-wheel drive.
With the increase in the size and weight of equipment has come the introduction of iron and steel for many parts formerly made exclusively of wood. The wood pitman, bull-wheel shaft. calf-wheel, and crown-block may all be replaced with metal forms of greater strength and durability. Usually when the severe duty of drilling a well is over, and it has to the metal parts are replaced with the cheaper wood construction and moved to a new drilling-well.
Engines and Boilers. The well-drilling engine is a remarkably efficient piece of machinery when its low cost, the service required of it and the treatment it receives are taken into account. The construction is simple. It has a single cylinder, a simple slide valve, and link reversing gear of the locomotive type. The length of stroke is almost invariably 12 in., the cylinder diameters ranging from 8 to 12 inches. In the eastern United States 9 by 12 and in the west 10 by 12 where the duty is heavier, are the sizes most commonly used for cable-tool work.. The 12 by 12 size is frequently required for rotary equipments. The engine is installed so that the pulley wheel lines with the band-wheel, and while the crank-shaft carries a fly-wheel at the other end, yet the constant pull on the belt pulley tending to work the shaft out of alignment has led to the introduction of an outboard-bearing (Fig. 47) that provides an outside supporting box for the shaft. The weight of the flywheel may be varied by the use of removable rings or balances fastened to it with bolts to suit the duty on the engine. Balances are usually added to steady the motion as the depth of a drilling-well increases. Pumping wells run at a low speed and the balances tend to maintain it at a uniform rate and prevent the engine from stalling on centre.