must be enough space to hold all the oil when the cylinders retract with some space to spare for expansion of hot oil.

An 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 by the ever- changing fluid level. A firmly secured filling strainer of fine mesh wire is always placed below the filler cap.

The sight gauge is provided so the normal fluid level can always be seen, as it is essential that the fluid in the reservoir be at the correct level. The baffle plate segregates the outlet fluid from the inlet fluid. Although not a total segregation, it does allow time to dissipate the air bubbles, lessen the fluid turbulence (contaminants settle out of nonturbulent fluid), and cool the return fluid somewhat before it is picked up by the pump.

Reservoirs used on CESE may vary considerably from that shown in figure 3-6; however, manufacturers retain many of the noted features as possible depending on design limits and use.

Strainers and Filters

Strainers are constructed of fine mesh wire screens or of screening elements, consisting of specially processed wire of varying thickness wrapped around metal frames. They do NOT provide as fine a screening action as filters, but they offer less resistance to flow and are used in pump suction lines where pressure drop must be kept to a minimum. If one strainer is not large enough to handle the supply of the pump. two or more strainers can be used in parallel.

The most common device installed in hydraulic systems to prevent foreign particles and contamination from remaining in the system are called filters. They may be located in the reservoir, in the return line, in the pressure line, or any other location in the system where the designer of the system decides they are needed to safeguard the system against impurities.

Filters are classified as full flow and partial flow. In the full-flow filter, all fluid that enters the unit passes through the filtering element, while in the partial-flow filter. only a portion of the fluid passes through the element.

Pumps

The purpose of a hydraulic pump is to supply a flow of fluid to a hydraulic system. The pump does not create system pressure, since only a resistance to the flow can create pressure. As the pump provides flow, it transmits a force to the fluid. As the fluid flow encounters resistance, this force is changed into pressure. Resistance to flow is the result of a resistance or obstruction in the path of flow. This restriction is normally the work accomplished by the hydraulic system, but can also be restrictions of lines, fittings, and valves within the system. Thus the load imposed on the system or action of a pressure-regulating device controls the pressure.

Pumps are rated according to their volumetric output and displacement. Volumetric output is the amount of fluid a pump can deliver to its outlet port in a certain period of time at a given speed. Volumetric output is usually expressed in gallons per minute (gpm). Since changes in pump speed affect volumetric output, some pumps are rated by their displacement. Pump displacement is the amount of fluid the pump can deliver per cycle. Since most pumps use a rotary drive, displacement is usually expressed in terms of cubic inches per revolution.

Many different methods are used to classify pumps. Terms, such as nonpositive displacement, positive displacement, fixed displacement, variable displacement, fixed delivery, variable delivery, constant volume, and others are used to describe pumps. The first two of these terms describe the fundamental division of pumps because all pumps are either nonpositive displacement or positive displacement. Basically pumps that discharge liquid in a continuous flow are referred to as nonpositive displacement, and those that discharge volumes separated by a period of no discharge are referred to as positive displacement.

Pumps may also be classified according to the specific design used to create the flow of fluid. Practically all-hydraulic pumps fall within three designs classifications - centrifugal, rotary, and reciprocating. Since the use of centrifugal pumps is limited, we will only discuss rotary and reciprocating.

ROTARY PUMPS. - All rotary pumps have rotating parts that trap the fluid at the inlet (suction) port and force it through the discharge port into the system. Gears (figs. 3-7, 3-8, and 3-9), screws (fig. 3-10), lobes (fig. 3-11), and vanes (fig. 3-12) are commonly used to move the fluid. Rotary pumps are positive displacement of the fixed displacement type.

Rotary pumps are designed with very small clearances between rotating parts and stationary parts