HYDRAULIC PUMPS - HYDRONOUS

HYDRAULIC PUMPS

4.1 BASIC

            All hydraulic systems have one or more power-driven pumps and may have a hand pump as an additional source of power. Power-driven pumps are the primary source of energy, and may be either engine-driven or electric-motor driven. As a general rule, motor-driven pumps are installed for use in emergencies; that is, for operation of actuating units when the engine-driven pump is inoperative. Hand pumps are generally installed for testing purposes as well as for use in emergencies. In this section, the various types of pumps used in naval aircraft, both hand- and power-driven, are described and illustrated.

Hand Pumps Hand pumps are used in hydraulic systems to supply fluid under pressure to subsystems, such as the landing gear, flaps, canopy, and bomb-bay doors, and to charge brake accumulators.  Systems using hand pumps are classified as emergency systems. Most of these systems may be used effectively during preventive maintenance.

Figure- 4.1

4.2. Types of Hydraulic hand pump:

Figure- 4.2

4.2.1 SINGLE ACTING HYDRAULIC HAND PUMP:

            The   single-action pump provides flow during every other stroke, while the double-action provides flow during each stroke. Single-action pumps are frequently   used in hydraulic jacks. A double-action hand pump is illustrated in figure. This type of pump is used in some aircraft  hydraulic systems as a source of hydraulic power for emergencies, for testing certain subsystems during preventive maintenance inspections,  and for determining the causes of malfunctions in these subsystem. This pump (fig.  ) consists of a cylinder, a piston containing a built-in check valve (A), a piston  rod, an  operating handle, and a check valve (B) at the inlet port. 

When the piston is moved to the left, the force of the liquid in the outlet chamber and spring tension cause valve A to close. This movement causes the piston to force the liquid in the outlet chamber through the outlet port and into the system. 

Figure- 4.3

This same piston movement causes a low-pressure area in the inlet chamber. The difference in pressure between the inlet chamber and the liquid (at  atmospheric pressure)  in the reservoir acting on check valve B causes its spring to compress; thus, opening the check valve. This allows liquid to enter the inlet chamber. When the piston completes this stroke to the left, the inlet chamber is full of liquid. This eliminates the pressure difference between the inlet chamber and the reservoir, thereby allowing spring tension to close check valve B. When the piston is moved to the right, the force of the confined liquid in the inlet chamber acts on check valve A. This action compresses the spring and opens check valve A which allows the liquid to flow from the intake chamber to the outlet chamber. Because of the area occupied by the piston rod, the outlet chamber cannot contain all the liquid discharged from the inlet chamber. Since liquids do not compress, the extra liquid is forced out of the outlet port into the system.

4.2.2   DOUBLE ACTING HYDRAULIC HAND PUMP

Double-action type of hand pumps are used in hydraulic systems. Double action means that a flow of fluid is created on each stroke of the pump handle instead of every other stroke, as in the single-action type. There are several versions of the double-action hand pump, but all use the reciprocating piston principle, and operation is similar to the one shown in figure.

Figure- 4.4

This pump consists of a cylinder, a piston containing a built-in check valve (A), a piston rod, an operating handle, and a check valve (B) at the inlet port. When the piston is moved to the left in the illustration, check valve (A) closes and check valve (B) opens. 

Fluid from the reservoir then flows into the cylinder through inlet port (C).  When the piston is moved to the right, check valve (B) closes. The pressure created in the fluid then opens check valve (A), and fluid is admitted behind the piston. Because of the space occupied by the piston rod, there is room for only part of the fluid; therefore, the remainder is forced out port (D) into the pressure line. If the piston is again moved to the left, check valve (A) again closes. The fluid behind the piston is then forced through outlet port (D). At the same time, fluid from the reservoir flows into the cylinder through check valve (B). Thus, a pressure stroke is produced with each stroke of the pump handle. Hand pumps are examined frequently for leakage, general condition, and efficiency in operation. To check the operation of a hand pump, the following procedure is recommended:

 1.  Connect a direct-reading hydraulic pressure gauge into the emergency hand pump pressure line.

2.  Insert and lock the hand pump handle in the pump actuating socket.

3. Select an appropriate subsystem to operate, and place its selector valve in an operating position.

4.  Actuate the hand pump handle until the unit being operated has completed its movement. Check the pressure gauge for a drop in system pressure.

5. If a pressure drop is indicated, check the system for leakage before removing the pump for repair or replacement.

•           How Stuff Works                     This commercial website provides no technical  

                                                              explanations of technical topics.

•           www.wikipedia.com                Working of single acting and double acting cylinder.

•           www.wikipedia.com                Working of  single acting and double acting pumps.

•           www.festo.com                        This site is a good free source for information about the

                                                              calculations and designing of cylinder & pumps.

•           www.wikipedia.com                 Piston seals.

•           www.google.com                      Hydraulic Fluid.

Books Considered :

•           Strength of materials                 By  Dr. Sadhu Singh, Art. 12.9 Beam Column, pg. 566.

•           Machine Dersign                       By  R.S.Khurmi, Art.4.3,4.4,4.9  pg.88.