Fluid is carried in cylinder to outlet and forced  out as piston is pulled back in by shaft flange To outlet Cylinder block From inlet Shaft  Shaft flange  Fluid pressure at inlet generates thrust on pistons Piston thrust on shaft flange generates torque on output shaft FWD 12-35 •  The type motion (rotary or reciprocating), if any.  •  The metal finish and the clearance between adjacent  parts. •  The type fluid used in the system. •  Pressure at which the system operates. Seals are used to  prevent fluid from passing a certain point,  and to keep air  and dirt out of the system in which they are  used. The increased use of hydraulics and pneumatics in  aircraft systems has created a need for packings and gaskets  of varying chara cteristics and design to meet the many  variations of oper ating speeds and temperatures to which they  are subjected. No one style or type of seal is satisfactory for  all installations. Some of the reasons for this are:  displacement hydraulic pump. The two units, hydraulic pump  and hydraulic motor, are connected via a single drive shaft  so that power can be transferred between the two systems.  Depending on the direction of power transfer, each unit in  turn works either as a motor or a pump.  [Figure 12-57] The HMDG is a servo-controlled variable displacement  motor integrated with an AC generator. The HMDG is  designed to maintain a desired output frequency of 400 Hz.  In case of an electrical failure, the HMDG could provide an  alternative source of electrical power. pump assembly is generally installed on the inner surface of  a door installed in the fuselage. The door is hinged, allowing  the assembly to be extended into the slipstream by pulling a  manual release in the flight deck. In some aircraft, the RAT  automatically deploys when the main hydraulic pressure  system fails, and/or electrical system malfunction occurs.  [Figure 12-56] The PTU is able to transfer power but not fluid. It transfers  power between two hydraulic systems. Different types  of PTUs are in use; some can only transfer power in one  direction while others can transfer power both ways. Some  PTUs have a fixed displacement, while others use a variable  Hydraulic Motor-Driven Generator (HMDG) Seals Power Transfer Unit (PTU) Figure 12-55.  Bent axis pistion motor. Figure 12-56.  Ram air turbine.