Fig. 102.—Jack Hammer Rock Drill.

Fig. 103.—Tripod Drill.

A type of drill heavier and larger than the jack hammer drill is shown in Fig. 103. It requires some form of support such as a tripod, or in tunnel work it can be braced against the roof or sides. Some data on steam and air drills are given in Table 56. The effect of the length of the transmission pipe, temperature of the outside air, pressure at the boiler or compressor, etc., will have a marked effect on the amount of steam or air to be delivered to the drill. Compressed air is affected more than steam by these outside factors, but it has an advantage in that as it loses in pressure it increases in volume so that the loss of power is not so marked. Gillette states:

We may assume that a cubic foot of steam will do practically the same work in a drill as a cubic foot of compressed air at the same pressure, because neither the steam nor the air acts expansively to any great extent in a drill cylinder, due to the late cut-off. This being so ... one pound of steam is equivalent to nearly 30 cubic feet of free air ... all at the same pressure of 75 pounds per square inch. If a drill consumes at the rate of 100 cubic feet of free air per minute ... it would therefore consume 240 pounds of steam (at 75 pounds pressure) per hour.... Where not more than three or four drills are to be operated, probably no power can equal compressed air generated by gasoline. It will require 12 horse-power to compress air for each drill, hence 1½ gallons of gasoline will be required per hour per drill while actually drilling.

TABLE 56
Data on Rock Drills
(From H. P. Gillette)
Diameter of cylinder in inches2¼2½2¾3⅛3¼3⅜
Length of stroke in inches566½6⅝6⅝7¼
Length of drill from end of crank to end of piston364350505052
Depth of hole drilled without change of bit, inches152024242424
Diameter of supply inlet. Standard pipe, inches¾¾¾111¼
Approximate strokes per minute with 60 pound pressure at the drill500450375350325300
Depth of vertical hole each machine will drill easily, feet6810141620
Diameter of holes drilled, inches¾ to 1½ as desired
Diameter of octagon steel, inches¾ to ⅞⅞ to 11 to 1⅛1⅛ to 1¼1⅛ to 1¼1¼ to 1⅜
Best size of boiler to give plenty of steam at high pressure, horse-power68891012
Best size of supply pipe to carry steam 100 to 200 feet, inches¾¾¾111¼
Weight of drill unmounted, with wrenches and fittings, hot boxed, pounds128190265315385390
Weight of tripod, without weights, not boxed, pounds80160160160210275
Weight of holding down weights, not boxed, pounds120270270285330375
Cubic feet of free air per minute required to run one drill at 100 pounds92104126146154160
For more than one drill, multiply the value in the above line by the following factors: For 2 drills, 1.8; 5 by 4.1; 10 by 7.1; 15 by 9.5; 20 by 11.7; 30 by 15.8; 40 by 21.4; 70 by 33.2.

Since gasoline air compressors are self regulating, when the drill is not using air very little gasoline is burned by the gasoline engine driving the compressor. A gasoline compressor possesses other very important economic advantages over a small steam-driven plant. First, there is the saving in wages of firemen and second, there is the saving in hauling and pumping of water and the hauling of fuel. The cost of gasoline is often less than the cost of coal for operating a small plant.

An electric drill[[89]] operated on the principle of the solenoid does away with motor, valves, pipes, vapor, freezing, and other difficulties attendant on the use of steam or air.

The rates of drilling in different classes of rock are shown in Table 57. Frequent changes of drills and relocation of tripods will materially reduce the performance of a drill, for as much as 45 minutes may be lost in making a new set up. In this the jack hammer drills show their advantage as no time is lost in a set up.