The derriCk itself consists of four uprights (8), known as 'legs,' braced by horizontal girts (10) and diagonally-placed braces (11). Its size is designated by the size of the floor and the height, a 20 by 74 derrick having a floor 20 ft. square and being 74 ft. high, and this size has been used probably more than any other. It is rarely that the floor is made less than 20 ft. square, and the heavier types are 22 and 24-ft.; while the heights in recent practice are going more towards 84 ft. for wells using standard tools and 106, 114, and even 124 ft. with those using the rotary method.
The legs are built up by placing 2 by 10 and 2 by 12 planks trough-shaped, with each side taking the direction of one side of the derrick. Ordinarily one set of these, with an extra set for the first 18 ft., give enough strength ; heavier derricks are supplied with two sets known as doublers, the entire length with a third set at the lower 18 feet. Besides the usual braces shown in Fig. 34, derricks requiring additional strength are 'sway-braced' by adding another set of girts on the outside of the legs opposite every other set of inside girts, and placing long braces between the outside girts (49) The construction of the rig starts with placing the mud-sills (28) and the main-sill (27) (Fig. 35), and the derrick foundations are then set so that the derrick floor is even with these, except when a rotary derrick is being fitted for standard tool work, when of course the mud-sill and main-sill are placed to conform with the position of the derrick as it was erected for the rotary work. The derrick is next run up, heavily nailed and surmounted with the crown (7), the water table (6), the bumpers (5) and the crown block (4), and the latter faced with hard wood bearings for the sheave-wheels on which run the various ropes.
Next are put up the jack-post (38), the bull-wheel posts (23), the bull-wheels (12), the calf-wheel (39), and engine foundation (41). The sampson-post (25), walking-beam (24) and band-wheel (31) are not erected until the bull-wheels may be used for pulling them into place. Finally the sand reel (43) and friction pulley (44) are built in, having been left till the last because they must be placed so that the friction pulley runs true with the band wheel. Rough 1 by 12 lumber is used for the engine and belt houses and for the lower portion of the derrick if it also is to be housed. Cor rugated iron for this purpose is a trifle more expensive, but the lessened construction cost, the diminished danger of fire and the better protection of the belt make the added expense well worth while and it is finding an increased use. A plank-walk connects the
engine house with the derrick and a casing rack, of 6 by 6 or 8 by 8 timbers, is built beside the walk for the purpose of holding casing, tubing and such equipment as cement tanks at the time the well is being cemented.
The well is not drilled exactly in the centre of the square floor- . space, but is started either 8 or 9 ft. from the front side, towards the engine house, leaving either 11 or 12 ft. between it and the opposite side in a 20-ft. floor.
Derrick Lumber List.
The following lumber lists are typical of the lumber required for derricks using the different methods and for drill ing shallow or deep wells. The details of construction vary greatly in minor particulars but those cited here are in common use and well suited to the class of drilling for which they are designed.
The wheels for use when the cable-tool method is being fol lowed are about the same size in all the styles of derricks. The material for a 10-ft. band-wheel is as follows : Motive power passes by belt from the engine-pulley to the band wheel, and from the band-wheel it is transmitted to the various moving parts. This wheel is 10 ft. diameter, built of lumber and runs on a crank-shaft, supported by boxes on the jack-posts. Fig ure 37 illustrates the crank-shaft carrying, from left to right, the crank used for actuating the walking-beam, a box, the band-wheel flanges, the second jack-post box, the clutch-sprocket and clutch. The sprocket carries the chain which drives the calf wheel and is not fastened to the shaft but turns only when the clutch, which is keyed to the shaft, has been thrown over so that it meshes with an opening in the sprocket. On the clutch side of the band-wheel, there is built either one or two or 7 ft. grooved wood tug-pulley circles, on which run the bull-ropes that drive the bull-wheels.
The sand-reel is a drum on which is wound the sand-line that carries the sand-pump, or bailer, in and out of the hole. It is turned by means of a friction pulley (44) pressed against the band-wheel by pulling the reach-rod (32) and the swing-lever (33) ; its speed is retarded by swinging the friction-pulley back and forcing it to bear against the back-brake (35). The reels are made with either single or double drums. For deep-hole work the latter are now almost universally used, one drum serving to hold that portion of the line not being used. It passes from the sand reel up on the outside of the derrick, over the sand-line sheave (1) and down inside the derrick.