bubbles that normally occur as the fluid flows from the reservoir, through the system, and back to the reservoir. The air vent allows the air to be drawn in and pushed out of the reservoir by the ever-changing fluid level. An air filter is attached to the air vent to prevent drawing atmospheric dust into the system. Because it is essential that the fluid in the reservoir be kept at the correct level at all times, the sight gauge is provided to allow the normal fluid level to always be seen  The  baffle  plate  segregates  the  outlet  fluid  from the inlet. This allows the fluid time to dissipate air bubbles, contaminants to settle, and the return fluid to cool before it is picked up by the pump. The  proper  hydraulic  fluid  level  must  be maintained. In some systems low fluid level causes overheating because the fluid does not have enough time to cool in the reservoir before it gas back into the pump. Also, some systems will not work at all because the fluid is so low in the reservoir that air gets into the pump. Before  adding  hydraulic  fluid,  know  what  type  to use and make sure it is clean. Clean around the filler cap or tube so there is less chance that dirt can get into the  system. STRAINERS AND FILTERS Hydraulic systems have a strainer and one or more filters that remove the impurities that would eventually contaminate  the  hydraulic  fluid.  The  strainer  is normally located in the reservoir or in the inlet line to the pump. The filter is normally located so only a small amount of fluid is lost when the element is changed. The falter is equipped with a valve that allows the fluid to bypass  the  filter  element  should  it  become  clogged.  The falter element is usually of the paper cartridge, canister, or edge type and is similar to those used in engine lubrication   systems. Regular  filter  maintenance, performed by the mechanics, is necessary to prevent contaminated  fluid  from  being  recirculated  in  the system. HYDRAULIC PUMPS The hydraulic pump creates the flow of fluid within the hydraulic system. The pressure in a hydraulic system is caused by a restriction placed in the path of the fluid as it leaves the pump. Because of the resulting mechanical drive and positive displacement, the pump merely moves the fluid regardless of the restriction. When enough pressure is built up, movement of the restriction occurs or a relief valve placed in the system opens, allowing the fluid to return to the reservoir or the suction  side  of  the  pump. When the pump operates, hydraulic fluid is trapped between  the  gear  teeth  and  the  pump  housing  and  is carried to the outlet side of the pump. As the teeth mesh, a seal is freed by the mating surfaces that prevent the oil from leaking back to the inlet side of the pump. The sealing action causes the oil to be forced out of the pump and into the system. CONTROL VALVES Control valves are valves accessible to the operator for directing the flow of fluid within the system to operate the machine or its attachment. By skillful use of the control valves, the operator can regulate the speed and  operation  of  the  hydraulic  cylinders. NOTE:  Hydraulic  controls  should  be  operated smoothly  to  eliminate  the  jerking  motion  that  causes rapid  wear  and  failure  of  the  mechanical  parts  of  the machine. HYDRAULIC  CYLINDERS Hydraulic cylinders are used to transmit motion in relation to the volume of fluid directed into the cylinder. The force created by the cylinder is determined by the pressure of the fluid and the area of the piston contacted by the fluid. Thus the larger the piston, the more force generated. Hydraulic  cylinders  used  on  heavy  equipment  are either single- or double-acting cylinders. Single-Acting  Cylinders Single-acting cylinders, similar to the one shown in figure 4-11, view A, are used to exert force in only one direction.  This  means  the  weight  or  resistance  moved must be located so it causes the cylinder to return to its original position when pressure is relieved from the piston. A common use of this type of cylinder is in a hydraulic  jack. Double-Acting  Cylinders Double-acting cylinders are used on equipment where force is needed in two directions. Unlike the single-acting   cylinder,   the   double-acting   cylinder contains seals at both ends of the piston where the piston rod passes through the end of the cylinder. With the use of this cylinder, fluid can be directed to either side of the piston and cause the piston rod to extend or retract under 4 - 9