Hydraulic Actuators & Motors - Hydraulics - Airframes & Aircraft Systems #9
Summary
TLDRThis lesson delves into hydraulic actuators, essential for converting hydraulic pressure into mechanical movement in aircraft systems. It distinguishes between single-acting actuators, which move in one direction via hydraulic pressure and rely on springs for the return, and double-acting actuators, which operate in both directions hydraulically. The latter are further categorized into unbalanced, where the piston rod's presence creates a force imbalance, and balanced, which apply equal force on both piston sides. The script also explores hydraulic motors, their construction akin to multi-piston pumps, and their operation dependent on oil flow rate, with applications ranging from flap drives to power transfer units.
Takeaways
- βοΈ Hydraulic actuators are devices that convert hydraulic pressure into mechanical movement, and they come in various sizes and constructions to handle different operating loads.
- π§ Single-acting actuators operate hydraulically in one direction and use a spring to return, commonly used as locking devices like landing gear up-locks.
- π Double-acting unbalanced actuators are prevalent in aircraft systems, providing more force during extension due to the smaller area on the piston rod side.
- π― Double-acting balanced actuators allow equal force application on both sides of the piston, often used in applications like nose wheel steering and flight control systems.
- π A hydraulic lock occurs when fluid is trapped between the actuator piston and a valve, locking the actuator in position due to the incompressible nature of the fluid.
- π Hydraulic motors are rotary actuators that can drive components like screw jacks for flap drives or operate auxiliary systems like generators and pumps.
- π The speed of a hydraulic motor is typically dependent on the flow rate of oil into it, with a fixed swashplate angle in most designs.
- π οΈ Hydraulic motors can transfer power between hydraulic systems without fluid exchange, useful in scenarios like main pump failure in aircraft systems.
- π§ Flow control valves maintain a constant flow of fluid to components, ensuring they operate at a consistent speed, crucial for systems like trailing edge flaps.
- π§ The construction of a hydraulic motor is similar to a multi-piston pump, with pressure used to rotate the pistons around a usually fixed swashplate.
Q & A
What is the primary function of hydraulic actuators?
-Hydraulic actuators are used to convert hydraulic pressure into mechanical movement, which can be either linear or rotary motion.
What are the main components of a hydraulic actuator?
-The main components of a hydraulic actuator include an outer cylinder, a piston and seal assembly, and a piston rod or ram that passes through a seal and wiper ring assembly.
What are the three types of actuators used in aircraft systems?
-The three types of actuators used in aircraft systems are single-acting, double-acting unbalanced, and double-acting balanced.
How does a single-acting actuator operate?
-A single-acting actuator is hydraulically operated in one direction only, with the force to move it in the other direction provided by a spring, typically used as a locking device.
What is the difference between double-acting unbalanced and double-acting balanced actuators?
-Double-acting unbalanced actuators have a smaller area on the side with the piston rod, allowing more force during extension than retraction. Double-acting balanced actuators can apply equal force to both sides of the piston, achieved by having rods attached to both phases of the piston.
Why are double-acting unbalanced actuators more commonly found in aircraft systems?
-Double-acting unbalanced actuators are more commonly found in aircraft systems because they can apply more force during extension, which is often the direction that offers greater resistance, such as raising the landing gear.
What is a hydraulic lock and how is it formed?
-A hydraulic lock is a state where the actuator is unable to move because fluid is trapped between the actuator piston and a valve, preventing any movement even when an external load is applied.
What is the role of a hydraulic motor in an aircraft system?
-Hydraulic motors are a form of rotary actuator that can be used to drive components such as generators, pumps, or screw jacks, and can also transfer power between hydraulic systems without exchanging fluid.
How does the construction of a hydraulic motor differ from that of a multi-piston pump?
-While a hydraulic motor is generally similar in construction to a multi-piston pump, the swashplate angle in a hydraulic motor is usually fixed, making its speed dependent on the flow rate of oil into it.
What is the purpose of a flow control valve in a hydraulic system?
-A flow control valve in a hydraulic system maintains a constant flow of fluid to a particular component, allowing it to operate at a constant speed, such as in a trailing edge flap system.
Outlines
π§ Hydraulic Actuators and Their Applications
This paragraph introduces hydraulic actuators, which are devices that transform hydraulic pressure into mechanical movement. It explains the basic construction of an actuator, which includes an outer cylinder, a piston and seal assembly, and a piston rod. The paragraph further distinguishes between three types of actuators: single-acting, double-acting unbalanced, and double-acting balanced. Single-acting actuators are hydraulically operated in one direction and rely on a spring for the return motion, typically used as locking devices. Double-acting unbalanced actuators are more common in aircraft and operate in both directions but have a difference in force due to the piston rod's presence, leading to more force during extension. Lastly, double-acting balanced actuators can apply equal force on both sides of the piston, often used in systems like nose wheel steering. The paragraph also discusses the concept of hydraulic lock, where the actuator is held in position when fluid is trapped between the piston and a valve, preventing movement despite external loads.
π Power Transfer Units and Hydraulic Motors
The second paragraph delves into power transfer units and hydraulic motors. It describes a scenario where a hydraulic motor from one system (yellow) can pressurize another system (green), which is particularly useful in the event of a main pump failure. The text emphasizes that this power transfer does not involve fluid exchange, keeping the systems separate. The construction of a hydraulic motor is likened to a multi-piston pump, with the key difference being the fixed swashplate angle, which means the motor's speed is dependent on the oil flow rate. The paragraph also explains the function of a flow control valve, which maintains a constant flow rate to ensure components operate at a steady speed. An example of this is in a trailing edge flap system, where the flow control valve helps regulate the flaps' movement. The paragraph concludes by summarizing the functions of hydraulic actuators and motors, highlighting the hydraulic lock's role in immobilizing the actuator when necessary.
Mindmap
Keywords
π‘Hydraulic Actuators
π‘Piston and Seal Assembly
π‘Piston Rod or Ram
π‘Single-Acting Actuator
π‘Double-Acting Unbalanced Actuator
π‘Double-Acting Balanced Actuator
π‘Hydraulic Lock
π‘Hydraulic Motors
π‘Flow Control Valve
π‘Torque Tube
Highlights
Hydraulic actuators convert hydraulic pressure into mechanical movement.
Actuators are used for linear or rotary motion and vary in size and construction.
All actuators consist of an outer cylinder with a sliding piston and seal assembly.
Piston rod or ram passes through a seal and wiper ring assembly at the cylinder end.
Three types of actuators: single-acting, double-acting unbalanced, and double-acting balanced.
Single-acting actuators operate hydraulically in one direction and use a spring for the other.
Double-acting unbalanced actuators are common in aircraft systems and operate in both directions.
Double-acting balanced actuators apply equal force to both sides of the piston.
Hydraulic lock is formed when fluid is trapped between the actuator piston and a valve.
Hydraulic motors are rotary actuators and can be connected to operate screw jacks.
Hydraulic motors can drive generators or pumps and transfer power between systems.
The speed of a hydraulic motor depends on the flow rate of oil into it.
Flow control valves maintain a constant flow of fluid for constant speed operation.
Hydraulic actuators can be hydraulically locked, preventing movement regardless of external load.
Hydraulic motors are similar to multi piston pumps with a fixed swashplate angle.
Transcripts
In this lesson we will examine the components which are used to convert
hydraulic pressure into mechanical movement.
Hydraulic actuators or jacks are used to convert fluid flow into linear or rotary
motion they vary in size and construction depending on the operating
loads.
But all consist of an outer cylinder within which slides a piston and seal assembly
attached to the piston is a piston rod or ram which passes through a seal and
wiper ring assembly fitted into the end of the cylinder.
Three types of actuator are used for different purposes in an aircraft system.
They are single-acting,
double-acting unbalanced,
and double-acting balanced.
A single-acting actuator is one that is hydraulically operated in
one direction only. The force to move it in the other direction is provided by a
spring. This type of actuator is normally used
as a locking device. The lock being engaged by spring pressure and released
by hydraulic pressure. A typical application would be a landing
gear up-lock.
This type of actuator has more fluid in the cylinder when the ram is extended
then what it is retracted.
The surplus fluid is stored in the reservoir when it is not being used.
Double-acting unbalanced actuators at the time most commonly found in aircraft
systems. These are operated hydraulically in both
directions.
Because of the presence of the piston rod. The piston face to which the rod is
attached has a smaller area on which the fluid can act than the other face.
As we know force is equal to pressure multiplied by area.
So more force can be applied during extension of the piston rod
then during retraction.
If equal pressure is supplied to both sides of an actuator of this type
because of the difference between the areas on the two sides of the piston the
ram will extend.
Normally the operation which offers the greater resistance is carried out in the
direction in which the piston rod extends.
For example in the case of landing gear the rod will extend to raise the gear
and retract to lower it.
A double-acting balanced actuator is one in which equal force can be applied to
both sides of the piston this is achieved by having rods attached to both
phases of the piston. It is often used in applications such as nose wheel steering
and hydraulically powered flying control systems.
Either one or both ends of the piston rod may be connected to a mechanism
or in many instances the rod is connected to the airframe and the body
moves to operate the service.
When fluid is trapped between the piston of the actuator and a non-return valve
or a selector valve at its neutral position a hydraulic lock is set to be
formed. Because the fluid is incompressible and
is unable to flow through the system the piston cannot move even if an external
load is applied to it and is therefore locked in its position.
In our example the gear lever has been selected to off the weight of the gear
is now being held by a hydraulic lock
hydraulic motors are a form of rotary actuator and has sometimes connected
through gearing to operate screw jacks on such components as trailing edge flap
drives illustrated here is the Beach 747
trailing edge flap drive motor it is mounted directly onto a gearbox
which in turn transmits the drive to the flaps through a torque tube
hydraulic motors can also be used to drive generators or pumps in some
aircraft they are used for driving a hydraulic pump unit thus enabling power
to be transferred from one hydraulic system to another without transferring
fluid in the setup shown here the hydraulic
motor is driven from the yellow system and the pump is pressurizing the green
system so this particular power transfer unit
PT you would operate in the event of a main pump failure in the green system
note that there is no exchange of fluid so the two systems remain separate
the construction of a hydro motor is generally similar to the
construction of a multi piston pump with the pressure which is being fed into the
Pistons being used to rotate them around the swashplate
however the swashplate angle is usually fixed
therefore the speed of a hydraulic motor is usually dependent on the flow rate of
oil into it
a hydraulic motor will often have a flow control valve in the supplied line to it
the purpose of this valve is to maintain a constant flow of fluid to a particular
component allowing it to operate at a constant speed that just will be
required in a trailing edge flap system a simplified flow control valve is shown
here
it has a piston with a number of holes in it through which fluid can freely
flow
the piston controls the position of a variable orifice valve
inlet pressure is felt on one side of the piston
with a spring pushing on the other
if the inlet pressure increases
then the flow rate through the valve will tend to increase
however the increased pressure will be felt on the piston which will move the
valve towards closed
that's reducing the flow rate
the opposite will happen if the pressure reduces the spring will push the piston
to the right the valve will open allowing an increased flow
thus for variations in Inlet pressure the flow rate through the valve will
remain constant
to summarize them hydraulic actuators can be single acting double acting
balanced or double acting unbalanced irrespective of what type of actuator it
is if fluid is trapped between the actuator piston and a valve the actuator
will be hydraulically locked and unable to move
a hydraulic motor is generally similar in construction to a multi piston pump
however the swashplate angle is usually fixed consequently the speed of a
hydraulic motor is usually dependent on the flow rate of oil into it
you
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