PRINCIPLES OF NAVAL ENGINEERING 



PILOT VALVE 

 OPERATING ROD 



ADJUSTABLE 

 TAPPET COLLAR 



ADJUSTABLE 

 TAPPET COLLAR 



-STAY ROD 



PUMP ROD 



38.100 



Figure 15-9.— Valve-operating gear of recipro- 

 cating pump. 



restricted to emergency feed pumps, fire and 

 bilge pumps, and fuel oil tank stripping and bilge 

 pumps. On auxiliary ships, reciprocating pumps 

 are still used for a number of services, includ- 

 ing auxiliary feed, standby fuel oil service, fuel 

 oil transfer, auxiliary circulating and conden- 

 sate, fire and bilge, ballast, and lube oil transfer. 



VARIABLE STROKE PUMPS.- Variable 

 stroke (also called variable displacement) pumps 

 are most commonly used on naval ships as part 

 of an electrohydraulic transmission for anchor 

 windlasses, cranes, winches, steering gear, and 

 other equipment. In these applications, the var- 

 iable stroke pump is sometimes referred to as 

 the A end and the hydraulic motor which is 

 driven by the A end is then called the B end. Var- 

 iable stroke pumps are also used on some ships 

 as in-port or cruising fuel oil service pumps. 



Although variable stroke pumps are often 

 classifed as rotary pumps, they are actually 

 reciprocating pumps of a special design. A rotary 

 motion is imparted to a cylinder barrel or cyl- 

 inder block in the pump by means of a constant- 

 speed electric motor; but the actual pumping is 

 done by a set ofpistons reciprocating inside cyl- 



indrical openings in the cylinder barrel or cy- 

 linder block. 



There are two general types of variable 

 stroke pumps in common use. In the axial-piston 

 type, the pistons are arranged parallel to each 

 other and to the pump shaft. In the radial-piston 

 type, the pistons are arranged radially from the 

 shaft. 



Figure 15-10 shows an exploded view of both 

 the pump end (A. end) and the hydraulic motor 

 (B^end) of an axial-piston type of variable stroke 

 unit. The pump usually has either seven or 

 nine2 single-acting pistons which are evenly 

 spaced around the cylinder barrel^ in the manner 

 shown in figure 15-10. 



The piston rods (sometimes called connecting 

 rods) make a ball-and-socket connection with a 

 piece called the socket ring. The socket ring 

 rides on a thrust bearing carried by a casting 

 called the tilting box or tilting block; thus the 

 socket ring, which revolves, is actually fitted 

 into the tilting box, which does not revolve. Fig- 

 ure 15-11 shows diagrammatically the arrange- 

 ment of the cylinder barrel, the socket ring, and 

 the tilting box. Although only one piston is shown 

 in this illustration, the others fit similarly into 

 the cylinder barrel and into the socket ring. 



Figure 15-12 illustrates diagrammatically 

 the manner in whichthe position of thetilting box 

 affects the position of the pistons. (Note that this 

 is not a continuous cross-sectional view, since 

 for illustrative purposes twopistons are shown.) 

 In order to understand how the pumping action 

 takes place, let us follow one piston as the cyl- 

 inder barrel and socket ring make one complete 

 revolution. When the tilting box is set perpendic- 

 ular to the shaft, as inpart Aof figure 15-12, the 

 piston does not move back and forth within its 

 cylindrical opening as the cylinder barrel and 

 socket ring revolve. Thus the piston is in the 

 same position with respect to its own cylindrical 

 opening when it is at the top position as it is when 

 the cylinder barrel has completed half a revolu- 

 tion and carried the piston to the bottom position. 

 Since the piston does not reciprocate, there is no 

 pumping action when the tilting box is in this 

 position even though the cylinder barrel and 

 socket ring are revolving. 



2 An uneven number of pistons is always used in order 

 to avoid pulsations in the discharge flow. 



3 Note that the term cylinder barrel actually refers to 

 a cylinder block which has cylindrical openings for all 

 of the pistons. 



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