Tl NNKLS TIIKiirUH HAKD KOCK 103 



boring-machines can be mounted on the rack-bar, and can be 

 adjusted in any reasonable position. 



The boring-machine performs the double function of con- 

 tinually pressing the drill into the rock by means of a hollow 

 ram (1), and of imparting to the drill and ram a uniform rotary 

 motion. This rotary motion is given by a twin cylinder single- 

 acting hydraulic motor (J?), the two pistons, of 2$ ins. stroke, 

 acting reciprocally as valves. The cranks are fixed at an angle 

 of 90 to each other on the shaft, which carries a worm, gearing 

 with a worm-wheel ((>) mounted upon the shell (.R) of the 



Pio. 57. General Details of the Brandt RoUry Drill* Employed at the SImplon Tunnel. 



hollow ram (1), and this shell in turn engages the ram by a 

 long feather, leaving it free to slide axially to or from the face 

 of the rock. The average speed of the motor is 150 revolutions 

 to 200 revolutions per minute, the maximum speed being 800 

 revolutions per minute. The loss of power between the worm 

 and worm-wheel is only 15 % at the most; the worm being of 

 hardened steel and the wheel of gun-metal, the two surfaces in 

 contact acquire a high degree of polish, resulting in little wear- 

 ing or heating. Taking into consideration all other sources of 

 loss, 70 % of the total power is utilized. The pressure on the 



