PLYMSTOCK 



6203 



PNEUMATIC APPLIANCES 



Plymstock. Village of Devon- 

 shire, England. It stands on Ply- 

 mouth Sound, 3 m. from Plymouth. 

 Here is the Perpendicular church of 

 S. Mary and All Saints, restored in 

 the 19th century ; it has a Norman 

 font, and some memorials to the 

 Harris family. 



Ply illy mmon. Mountain trf 

 Wales. It stands on the borders of 

 Cardiganshire and Montgomery- 

 shire, and has three summits, the 

 highest being 2,465 ft. high. The 

 word is said to be a corruption of 

 a Celtic term for five rivers, as five 

 rivers rise hereon Wye, Severn, 

 Rheidol, Ystwyth, and Llynfnant. 

 On and around the mountain are a 

 number of bogs. 



Plywood. Name given to 

 boards built up of plies or thin 

 veneers of wood. These are ce- 

 mented or glued together with the 

 grain of each ply placed at right 

 angles to that of the ply on either 

 side of it. The special advantages 

 of plywood, in comparison with 

 solid timber, are its extreme light- 

 ness, great strength, the excep- 

 tional widths to be obtained with- 

 out a joint often up to 84 ins. 

 and the counteracting of the 

 natural tendency of timber to 

 warp and split. 



In the first operation of the 

 manufacture of plywood, a log is 

 revolved in a specially constructed 

 lathe, against a horizontally placed 

 knife, which cuts a continuous 

 " peeling " from the surface of the 

 log. This peeling, or ply, is then 

 cut into the particular size de- 

 sired, and after drying, two or 

 more plies, according to the final 

 thickness desired, are cemented 

 together under pressure. For special 

 reasons more than one variety of 

 wood may be used to form the 

 finished article. 



Plywood is used for all kinds of 

 purposes, chief among which are 

 wall and ceiling panelling, furni- 

 ture and chair seat manufacture, 

 and box-making, and in the con- 

 struction of aeroplanes. Although 

 the records of its origin are 

 somewhat obscure, it is reported 

 to have been used in 2-ply form 

 by the ancient Egyptians, for 

 mummy cases. See Joinery ; 

 Timber. 



Pneumatic Appliances. Name 

 given to that class of machine or 

 instrument which depends largely 

 upon compressed air for power. 

 Pipes conveying it need not be 

 protected against cold, as it cannot 

 freeze or condense under ordinary 

 conditions. If it escape it is per- 

 fectly inoffensive. Owing to its 

 expansibility it may be employed 

 in engines, which differ only in detail 

 from steam engines. In fact, many 

 forms of motors will operate as well 



Plynlymmon, North Wales. Distant view of the mountain 



with air as with steam, though per- 

 haps not so economically. 



AIR COMPRESSORS. Air is com- 

 pressed in one, two, three, or four 

 stages, according to the final pres- 

 sure which is required. The com- 

 pound, or two-stage compressor, is 

 suitable for pressures up to 100 Ib. 

 per sq. in. As a rule isothermal 

 compression is used, i.e. the air is 

 kept cooled to as near atmospheric 

 temperature as possible by cold 

 water circulated through jackets 

 outside the cylinders, and in coolers 

 through which the air passes be- 

 tween any two stages of compres- 

 sion. The efficiency of the com- 

 pressor depends largely on the 

 efficiency of the cooling, as a given 

 weight of heated air requires more 

 power to compress it ; or, to state 

 the matter differently, the work 

 put into it is partly lost by the 

 subsequent cooling and loss of 

 pressure in the container. 



Fig. 1 represents diagrammatic- 

 ally a four-stage compressor. Two 

 compressing cylinders are used. 

 The upper sides of pistons P 1 and 

 P 2 are exposed fully, whereas the 

 lower, owing to the pressure of the 

 large plungers, have a much 

 smaller annular face. Air enters 

 by valve A into space W, is com- 

 pressed during the upward stroke 

 of P 1 , and passes out through 

 valve B to the first cooler ; while 

 air already compressed is drawn 

 into space X. On the downward 

 stroke of P 1 the air in X undergoes 

 the second stage of compression, 

 and is forced out into the second 



Relief Valve 

 <on !?< Cooler 



cooler. The process is repeated 

 in space Y, and again in Z. C, F, 

 and G are the requisite valves. 



Cooling means a great loss of 

 energy, which in extreme cases 

 may reach 65-70 p.c. A consider- 

 able proportion 40-50 p.c. of 

 this loss can be made good, how- 

 ever, by re-heating the air just 

 before use, as is done in compressed- 

 air locomotives and torpedoes. 

 The latter are provided with a 

 small heater, which comes into 

 action automatically and burns 

 liquid fuel in contact with the air. 

 The result is a gain' in power of 

 100 p.c., of 22 p.c, in maximum 

 speed, and of 70-100 p.c. in useful 

 range. (See Compressed Air.) 



PNEUMATIC HAMMERS. The pneu- 

 matic hammer is a heavy piston 

 moved to and fro by compressed 

 air inside a cylinder, furnished 

 with a handle and a thumb valve. 

 At the end of an outward stroke it 

 strikes against a loose piece, which 

 transmits the blow directly, or 

 through a chisel, caulker, or other 

 tool fitted into it. There are two 

 classes of hammers, the small 

 short-stroke, which works at tre- 

 mendous speed and makes 2,000- 

 12,000 light blows a minute, and 

 the larger long-stroke, giving 800- 

 1,200 heavy blows in the same 

 time. Short-stroke hammers are 

 valveless, the hammer itself open- 

 ing and closing the ports. A long- 

 stroke hammer usually has an 

 automatic valve, to admit and 

 release air from the ends of the 

 hammer alternately. Short-stroke 

 hammers are invaluable for chip- 

 ping the surfaces of castings and 

 other metal-work. Riveting is 



Fig. 1 



AirOuUet 



Pneumatic Appliances. Diagram illustrating construction of a four-stage 

 air compressor. See text 



