786 HYDKAULICS AND ITS APPLICATIONS 



hour is brought to rest with a pressure of 580 Ibs. per square inch, giving 

 an uniform resistance of 52 tons. An automatic valve is fitted to a pipe 

 connecting the two ends of the cylinder so that this may open if the 

 pressure becomes excessive. As an additional precaution against the 

 production of a dangerous cylinder pressure the links supporting the 

 whole stop are designed so as to break under an excessive load. 



It will be noted in Fig. 394 that the piston rod passes through both 

 ends of the cylinder, the reason being that where the rod is in compression 

 during the working stroke, if no tail rod is fitted the "displacement of rod 

 plus piston increases throughout the stroke, and if the cylinder were 

 originally full of liquid no motion of the piston would be possible. The 

 difficulty can be avoided by the provision of an air space inside the 

 cylinder of volume slightly greater than that of the portion of the rod 

 entering the cylinder during the stroke. Owing to this air space, how- 

 ever, the piston during the first portion of its stroke is engaged in 

 compressing air, with the result that the pressure rises more slowly and 

 the necessary maximum pressure for a given weight and velocity is 

 increased by about 15 per cent. Where the piston rod is in tension 

 during the working stroke the above difficulty is entirely obviated. The 

 cylinder is filled with liquid with the piston run in, and the resistance is 

 now increased by the formation of a partial vacuum behind the piston on 

 its outward stroke. 



In a more recent type of compression buffer stop without tail rod, 1 

 the difficulty is overcome by an escape valve set to blow off at 50 Ibs. per 

 square inch, at the front end of the cylinder. A volume of water equal to 

 the displacement of the piston rod is then discharged through this valve to 

 waste per working stroke. The piston is returned automatically to the 

 front of the cylinder by means of pressure water from the town's mains 

 which is supplied to the cylinder through a small back-pressure valve. 

 This does away with all necessity for springs or counterbalance weights 

 and gives a very neat and compact arrangement, the only drawback being 

 due to the fact that town's water is used and that effective lubrication 

 of the working surfaces must be relied upon to prevent corrosion. In 

 this buffer stop the connecting passages are formed by two longitudinal 

 taper grooves cut in the body of the cylinder 1J inches wide and varying 

 in depth from inches at the front to zero at the back end of the 

 cylinder, while in addition there are twelve -inch holes bored through 

 the body of the piston, which is 12 inches diameter and has a travel of 



1 By the Hydraulic Engineering Company, of Chester. 



