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Operation of Hydraulic Components
Figure  2-22.An  automobile  brake  system.

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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The  output  piston  can  be  raised  higher  and maintained at this height if additional components are  installed  as  shown  in  figure  2-20.  In  this illustration the jack is designed so that it can be raised,  lowered,  or  held  at  a  constant  height. These results are attained by introducing a number of valves and also a reserve supply of fluid to be used in the system. Notice that this system contains the five basic components—the   reservoir;   cylinder   1,   which serves  as  a  pump;  valve  3,  which  serves  as  a directional control valve; cylinder 2, which serves as the actuating device; and lines to transmit the fluid  to  and  from  the  different  components.  In addition, this system contains two valves, 1 and 2, whose functions are explained in the following discussion. As  the  input  piston  is  raised  (fig.  2-20,  view A),  valve  1  is  closed  by  the  back  pressure  from the weight of the output piston. At the same time, valve 2 is opened by the head of the fluid in the reservoir. This forces fluid into cylinder 1. When the input piston is lowered (fig. 2-20, view B), a pressure  is  developed  in  cylinder  1.  When  this pressure exceeds the head in the reservoir, it closes valve 2. When it exceeds the back pressure from the output piston, it opens valve 1, forcing fluid into the pipeline. The pressure from cylinder 1 is Figure 2-20.—Hydraulic jack; (A) up stroke; (B) downstroke. thus transmitted into cylinder 2, where it acts to raise  the  output  piston  with  its  attached  lift platform. When the input piston is again raised, the  pressure  in  cylinder  1  drops  below  that  in cylinder 2, causing valve 1 to close. This prevents the  return  of  fluid  and  holds  the  output  piston with  its  attached  lift  platform  at  its  new  level. During this stroke, valve 2 opens again allowing a new supply of fluid into cylinder 1 for the next power  (downward)  stroke  of  the  input  piston. Thus, by repeated strokes of the input piston, the lift platform can be progressively raised. To lower the lift platform, valve 3 is opened, and the fluid from  cylinder  2  is  returned  to  the  reservoir. HYDRAULIC  BRAKES The  hydraulic  brake  system  used  in  the automobile  is  a  multiple  piston  system.  A  multiple piston system allows forces to be transmitted to two  or  more  pistons  in  the  manner  indicated  in figure 2-21. Note that the pressure set up by the force  applied  to  the  input  piston  (1)  is  transmitted undiminished  to  both  output  pistons  (2  and  3), and  that  the  resultant  force  on  each  piston  is proportional  to  its  area.  The  multiplication  of forces from the input piston to each output piston is the same as that explained earlier. The hydraulic brake system from the master cylinders   to   the   wheel   cylinders   on   most Figure 2-21.—Multiple piston system. 2-16







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