Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGPUMP ROD-STAY RODPILOT VALVEOPERATING RODADJUSTABLETAPPET COLLARADJUSTABLETAPPET COLLAR38.100Figure 15-9.— Valve-operating gear of reciprocatingpump.restricted to emergency feed pumps, fire andbilge pumps, and fuel oil tank stripping and bilgepumps. On auxiliary ships, reciprocating pumpsare still used for a number of services, includingauxiliary feed, standby fuel oil service, fueloil transfer, auxiliary circulating and condensate,fire and bilge, ballast, and lube oil transfer.VARIABLE STROKE PUMPS.- Variablestroke (also called variable displacement) pumpsare most commonly used on naval ships as partof an electrohydraulic transmission for anchorwindlasses, cranes, winches, steering gear, andother equipment. In these applications, the variablestroke pump is sometimes referred to asthe A end and the hydraulic motor which isdriven by the A end is then called the B end. Variablestroke pumps are also used on some shipsas in-port or cruising fuel oil service pumps.Although variable stroke pumps are oftenclassifed as rotary pumps, they are actuallyreciprocating pumps of a special design. A rotarymotion is imparted to a cylinder barrel or cylinderblock in the pump by means of a constantspeedelectric motor; but the actual pumping isdone by a set ofpistons reciprocating inside cylindricalopenings in the cylinder barrel or cylinderblock.There are two general types of variablestroke pumps in common use. In the axial-pistontype, the pistons are arranged parallel to eachother and to the pump shaft. In the radial-pistontype, the pistons are arranged radially from theshaft.Figure 15-10 shows an exploded view of boththe pump end (A. end) and the hydraulic motor(B^end) of an axial-piston type of variable strokeunit. The pump usually has either seven ornine2 single-acting pistons which are evenlyspaced around the cylinder barrel^ in the mannershown in figure 15-10.The piston rods (sometimes called connectingrods) make a ball-and-socket connection with apiece called the socket ring. The socket ringrides on a thrust bearing carried by a castingcalled the tilting box or tilting block; thus thesocket ring, which revolves, is actually fittedinto the tilting box, which does not revolve. Figure15-11 shows diagrammatically the arrangementof the cylinder barrel, the socket ring, andthe tilting box. Although only one piston is shownin this illustration, the others fit similarly intothe cylinder barrel and into the socket ring.Figure 15-12 illustrates diagrammaticallythe manner in whichthe position of thetilting boxaffects the position of the pistons. (Note that thisis not a continuous cross-sectional view, sincefor illustrative purposes twopistons are shown.)In order to understand how the pumping actiontakes place, let us follow one piston as the cylinderbarrel and socket ring make one completerevolution. When the tilting box is set perpendicularto the shaft, as inpart Aof figure 15-12, thepiston does not move back and forth within itscylindrical opening as the cylinder barrel andsocket ring revolve. Thus the piston is in thesame position with respect to its own cylindricalopening when it is at the top position as it is whenthe cylinder barrel has completed half a revolutionand carried the piston to the bottom position.Since the piston does not reciprocate, there is nopumping action when the tilting box is in thisposition even though the cylinder barrel andsocket ring are revolving.2 An uneven number of pistons is always used in orderto avoid pulsations in the discharge flow.3 Note that the term cylinder barrel actually refers toa cylinder block which has cylindrical openings for allof the pistons.400