TOPIC 1: Hydraulic and Pneumatic Systems Flashcards
Pascal’s law states what?
This if there is an enclosed vessel full of liquid and you apply a force to a piston in the vessel to raise the pressure, this increase in pressure will be the same anywhere in the system
State the definition of force?
Is energy exerted or brought to bear and is the cause of motion or change (caused by hydraulic or pneumatic action or by a spring) (expressed in pounds, grams or kilograms)
State the definition of pressure?
Is when a force is applied over a given area (expressed in pounds per square inch, PSI or grams per square inch g/cm2 )
State the definition of work and how is it calculated?
- Any time a force causes an object to move (expressed in foot-pounds lb/f or in kilogram-metres kg/m)
- Work = force x distance
How is power calculated?
FORCEXDISTANCE/TIME
How is mechanical advantage defined?
The ratio of output force to the input force applied to a mechanism
What are the two major advantages hydraulic systems has over other types of mechanical systems?
- Ease with which force can be transmitted over large distances and into and out of sealed compartments
- The large gain in mechanical advantage made possible by varying the size of pistons
What functions does the reservoir conduct in regards to the hydraulic system?
- ensure a good supply of fluid to the pump
- cater for ram displacement
- allow for thermal expansion
- provide a reserve of fluid to allow for small leakage
- provide a means of filling and checking the system
- allow a space for trapped air to settle out (in certain systems only)
- provide a means of cooling the fluid when the pump is not under demand
- provide an emergency supply of fluid to a hand pump (in certain systems only).
What are the two types of reservoirs used in aircraft hydraulic systems?
- Non-pressurised
- Pressurised
What are the components of non-pressurised reservoirs (brief description of each)?
- Sight glass = visual identification
- Filler cap = prevent fluid loss or the ingress of contaminants and a means of topping up fluid
- Vent = allows reservoir to breathe
- Filter = provides initial fluid filtration
- Return line = allows fluid to return, angled into a “swirl can”
- Power pump outlet = connected to standpipe at bottom of reservoir
- Hand pump outlet (emergency system) = located at lowest point
Why are reservoirs pressurised?
- Ensure a positive feed of hydraulic fluid to the pumps at all times during high altitude flight
- Prevent the fluid from foaming, thus preventing cavitation
How is a desired pressure range within the reservoir maintained?
- By containing a relief valve that stops the reservoir over-pressurising
What are the two methods used to pressurise reservoirs? Give brief description
- Bleed air from the aircrafts pneumatic system - relief valve fitted to relive excessive pressure if pressure reg fails
Hydraulic pressure - quantity of fluid is indicated by amount the piston protrudes from the body of the reservoir
Single acting and double acting are what?
Types of hand pumps
What is the basic operation of a single acting pump?
When handle is moved toward the right the piston movement creates a low-pressure condition and draws fluid from the reservoir through the check valve into the cylinder. (vice versa for handle moving left)
What is the basic operation of a double acting pump?
- Piston moved right increasing pressure in chamber to the right of the piston closing the inlet port check valve
- As pressure of the chamber diminished with the piston movement, fluid forces the piston check valve open and fluid flows through the piston to the next chamber
What power driven pump is driven by the engine via an accessory gearbox?
Engine driven pump
Why are electric motor driven pumps used?
- Installed for emergencies (i.e. when EDP is inoperative)
- In some aircrafts they are used to power auxiliary hydraulic systems or to supplement the normal hydraulic power supply
What are the four power driven pump categories?
- Gear type, vane type, gerotor type and multi-piston type
What power driven pump consists of steel gears, one driven by either the engine or other power unit and the other meshes with the drive gear?
Gear pumps
What power driven pump consists of a rotor drive shaft and eccentric rotor with sliding vanes?
Vane pump
What is the basic operation of a vane pump?
- Space between vanes and case fills with oil as it passes the inlet
- Then is carried towards the outlet
- As space near outlet diminishes oil is forced out of the pump
- Downstream resistance to flow will determine oil pressure
What power driven pump does the gear ride inside the rotor which rotates freely within the housing?
Gerotor
What is the basic operation of the gerotor pump?
Turned by the accessory drive from the engine and as it turns it rotates the even tooth outer gear rotor
What is a fixed delivery multiple-piston pump?
- Deliver a fixed amount of fluid at any given number of revolutions per minute
- Consists of a drive shaft where pistons are attached, a cylinder block where pistons are inserted and a stationary valving surface
What is a variable delivery multiple-piston pump?
- Designed to vary the volume of fluid being delivered at a given number of revolutions per minute
- Same basic construction of delivery pump and is designed so the alignment of the rotational axis of the cylinder block can be varied
What are the types of filter elements in hydraulic systems? Give brief description
- Paper - constructed from specially treated cellulose paper, where fluid flows from outside through the element and then out
- Sintered metal - comprised of composed metal particles that have undergone a sintering process, where fluid flows through the sides into the center and out
- Stainless steel - are a one piece wove wire mesh, where the operate same as paper but are reused rather then thrown out
What does the filter bowl assembly consist of?
- Filter head
- Filter case/bowl
- Filter element
- By-pass valve
What is the different purposes of accumulators?
- Primary purpose = store hydraulic fluid under pressure
- Act as a cushion or shock absorber
- Store enough under pressure for emergencies
- Supplement the hydraulic pump output peak load conditions
What does a piston type accumulator consist of?
A metal cylinder, a flowing piston, an air or nitrogen charging valve and a hydraulic fluid port
What is a diaphragm accumulator constructed of?
- A metal sphere, a synthetic rubber diaphragm, an air or nitrogen valve and a hydraulic fluid port
What is a flow control vale?
Used to control the flow of a fluid within the system (i.e., water tap, closed = no flow etc.)
How can valves by controlled?
- By the pilot directly
- Electrically or electronically
- Mechanically
- Hydraulically
- Pneumatically
- A combination of two or more of these methods
What is a check valve?
Is a non-return valve that allows fluids to flow one direction only
