Tipo de estudio y diseño de investigación

With this type of system, operating pressure is maintained in that part of the system which leads to the selector valves, and some method is used to prevent over-loading the pump. In systems which employ a fixed volume pump (constant delivery) an automatic cut-out valve is fitted, to divert pump output to the reservoir when pressure has built up to normal operating pressure. In other systems a variable volume pump (constant pressure) is used, delivery being reduced as pressure increases, whilst in some simple light aircraft systems, operation of an electrically- driven pump is controlled by a pressure-operated switch. A simple closed system is illustrated in Figure 2.6.

RESERVOIRS

A reservoir provides both storage space for the system fluid, and sufficient air space to allow for any variations of fluid in the system which may be caused by:

jack (actuator) ram displacement, since the capacity of the jack is less when contracted ¾

than extended.

thermal expansion, since the volume of oil increases with temperature. ¾

it provides a head of fluid for the pump. ¾

it compensates for small leaks. ¾

Most reservoirs are pressurised, to provide a positive fluid pressure at the pump inlet, and to prevent air bubbles from forming in the fluid at high altitude. The fluid level will vary according to:

the position of the jacks. ¾

whether the accumulators are charged. ¾

temperature. ¾

Air pressure is normally supplied from the compressor section of the engine, or the cabin pressurisation system.

Refer to Figure. 2.7.

A reservoir also contains a relief valve, to prevent over pressurisation; connections for suction pipes to the pumps, and return pipes from the system; a contents transmitter unit and a filler cap; and, in some cases, a temperature sensing probe. In systems which are fitted with a hand pump, the main pumps draw fluid through a stack pipe in the reservoir. This ensures that, if fluid is lost from that part of the system supplying the main pumps, or supplied solely by the main pumps, a reserve of fluid for the hand pump would still be available.

FILTERS

Filters are fitted in both suction and pressure lines i.e. both sides of the pump and sometimes in the return line to the reservoir; a suction filter to protect the pump, and a pressure filter to ensure the cleanliness of fluid during use. They remove foreign particles from the fluid, and protect the seals and working surfaces in the components. In addition, individual components often have a small filter fitted to the inlet connection, and constant pressure pumps will have a “case drain filter” to help monitor pump condition.

Some filters are fitted with a device which senses the pressure differential across the filter element, and releases a visual indicator, in the form of a button or illuminates a warning lamp, when the pressure differential increases as a result of the filter becoming clogged. False indication of element clogging, as a result of high fluid viscosity at low temperature, is prevented by a bi- metal spring which inhibits indicator button movement at low temperatures.

Other filters are fitted with a relief valve, which allows unfiltered fluid to pass to the system when the element becomes clogged; this type of filter element must be changed at regular intervals. Paper filter elements are usually discarded when removed, but elements of wire cloth may usually be cleaned. Cleaning by an ultrasonic process is normally recommended, but if a new or cleaned element is not available when the element becomes due for check, the old element may be cleaned in trichloroethane as a temporary measure.

PUMPS

Draw oil from the reservoir and deliver a supply of fluid to the system. Pumps may be: hand operated

¾

engine driven ¾

electric motor driven ¾

pneumatically (air turbine motor) (ATM) ¾

e)ram air turbine (HYDRAT or RAT) ¾

hydraulically (Hyd. motor driving a hyd. pump) Known as a Power Transfer Unit or ¾

PTU.

In most cases the ATM, RAT or PTU is used to provide an alternate supply as part of the redundancy provision for the safe operation of the aircraft.

Hand Pumps may be the only source of power in a small, light hydraulic system, but in larger aircraft are employed:

to allow ground servicing to take place without the need for engine running. ¾

so that lines and joints can be pressure tested. ¾

so that cargo doors etc., can be operated without power. ¾

The hand pump is usually a double acting pump (delivers oil on both strokes) in a very compact body. It incorporates non-return valves, and a relief valve which can be set to relieve at any required pressure, typically this is about 10% above normal system pressure. Refer to Figure

2.9.

Engine driven pumps (EDP) or electrically driven pumps may be classified as follows: Constant Delivery (Fixed Volume) Type Pump

¾ . This pump supplies fluid at a con-

stant rate and therefore needs an automatic cut-out or relief valve to return the fluid to the reservoir when the jacks have reached the end of their travel, and when the system is not operating, it requires an idling circuit. The pump gives a large flow at small pres- sure and is usually a single or double stage gear pump.

Figure 1.10. A Spur Gear Type Oil Pump.

Constant Pressure (Variable Volume) Pump

¾ . This pump supplies fluid at a variable

volume and controls its own pressure, this type of pump is typically fitted in modern aircraft whose systems operate at 3,000-4,000 psi.. The cylinder block and drive shaft are co-axial and rotate carrying the pistons with them which slide up and down in the cylinder block. The pistons are attached to shoes which rotate against a stationary yoke, and the angle between the yoke and cylinder block is varied to increase or decrease piston stroke thus increasing or decreasing pump output.

Figures 2.11 and 2.12 shows the operation of the pump. When pressure in the system is low, as

would be the case following selection of a service, spring pressure on the control piston turns the yoke to its maximum angle, and the pistons are at full stroke, delivering maximum output to the system. When the actuator has completed its stroke, pressure builds up until the control piston moves the yoke to the minimum stroke position; in this position a small flow through the pump is maintained, to lubricate the working parts, overcome internal leakage and dissipate heat. On some pumps a solenoid-operated depressurising valve (off load valve) is used to block delivery to the system, and to off- load the pump. System pressure is maintained and the pump output falls to 50 - 200 psi approx allowing oil to circulate, lubricating and cooling the pump. The solenoid is energised when the pump is off-loaded.

Figure 1.11 Constant Pressure Pump at Maximum Stroke Figure 2.11: Constant pressure pump at maximum stroke.