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Precautionary Measures, Continued
Figure 6-2.Typical ball-stop, swing-check valve.

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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CHAPTER  6 VALVES It  is  all  but  impossible  to  design  a  practical fluid   power   system   without   some   means   of controlling the volume and pressure of the fluid and  directing  the  flow  of  fluid  to  the  operating units. This is accomplished by the incorporation of different types of valves. A valve is defined as any  device  by  which  the  flow  of  fluid  may  be started, stopped, or regulated by a movable part that  opens  or  obstructs  passage.  As  applied in  fluid  power  systems,  valves  are  used  for controlling   the   flow,   the   pressure,   and   the direction  of  the  fluid  flow. Valves must be accurate in the control of fluid flow and pressure and the sequence of operation. Leakage  between  the  valve  element  and  the  valve seat   is   reduced   to   a   negligible   quantity   by precision-machined  surfaces,  resulting  in  carefully controlled  clearances.  This  is  one  of  the  very important reasons for minimizing contamination in  fluid  power  systems.  Contamination  causes valves  to  stick,  plugs  small  orifices,  and  causes abrasions  of  the  valve  seating  surfaces,  which results in leakage between the valve element and valve seat when the valve is in the closed position. Any  of  these  can  result  in  inefficient  operation or  complete  stoppage  of  the  equipment. Valves  may  be  controlled  manually,  electri- cally, pneumatically, mechanically, hydraulically, or  by  combinations  of  two  or  more  of  these methods.  Factors  that  determine  the  method  of control  include  the  purpose  of  the  valve,  the design and purpose of the system, the location of the valve within the system, and the availability of  the  source  of  power. The  different  types  of  valves  used  in  fluid power   systems,   their   classification,   and   their application  are  discussed  in  this  chapter. CLASSIFICATIONS Valves  are  classified  according  to  their  use: flow  control,  pressure  control,  and  directional control. Some valves have multiple functions that fall  into  more  than  one  classification. FLOW  CONTROL  VALVES Flow  control  valves  are  used  to  regulate  the flow  of  fluids  in  fluid-power  systems.  Control  of flow in fluid-power systems is important because the rate of movement of fluid-powered machines depends  on  the  rate  of  flow  of  the  pressurized fluid.  These  valves  may  be  manually,  hydrau- lically,  electrically,  or  pneumatically  operated. Some  of  the  different  types  of  flow  control valves are discussed in the following paragraphs. BALL  VALVES Ball  valves,  as  the  name  implies,  are  stop valves  that  use  a  ball  to  stop  or  start  a  flow  of fluid. The ball, shown in figure 6-1, performs the Figure 6-1.—Typical ball valve. 6-1







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