Week 10 - Hydraulic Actuators Flashcards
Pounds per square inch (PSI)
A measurement of pressure in the Imperial system of measurement.
PSI is commonly used to measure the pressure of gases (pneumatic pressure) or liquids (hydraulic pressure).
In pneumatic and hydraulic pressure, PSI expresses the force exerted on its containing vessel by the two relative fluids.
In fluid pressure measurement, the use of PSI is generally relative to atmosphere.
Metrics pascal (Pa) = 6.8948 x PSI
1 Atmospheric Pressure (ATM) = 6.8046 x PSI
A hydraulic actuator…
Uses the power of the pressurised fluid.
The hydraulic actuator converts the hydraulic energy into the mechanical energy.
A hydraulic system can be controlled almost as accurately as an electric motor.
In many applications (particularly in small scale systems), electrical energy is the initial source of energy.
Main components of a hydraulic control system
Fluid (incompressible, good thermal properties)
Pump
Relief valve
Accumulator valve
Hydraulic actuator
Advantages of hydraulic systems
- Usually more powerful than electrical actuators
(Although electric motors can be equipped with gearboxes, high power and high ratio gearboxes tend to be very inefficient, expensive and hard to maintain)
- One hydraulic system can drive multiple units
- Linear motion does not need (relatively) complicated mechanisms (rotation is also easily achievable)
- Relatively easier maintenance procedure
- Do not need cooling (fluid itself is a conductor of heat)
- Self-lubricating
- Safe (relatively)
Disadvantages of hydraulic systems
- Lower efficiency
- Require a reservoir for fluid
- Non-linear in nature
- Risk of leaking
- Multiple actuator synchronisation can be very complex
- Tend to be more noisy
Applications of hydraulic actuators
- Vehicle steering and braking systems, active suspension systems
- Material handling devices
- Industrial mechanical manipulators such as hoists, industrial robots, rolling mills, heavy-duty presses
- Actuators for aircraft control surfaces (ailerons, rudders and elevators)
- Ship steering and control devices, excavators, actuators for opening and closing of bridge spans, tunnel boring machines, food processing machines
Tribology of hydraulic actuators
Reciprocating shaft seals are heavily used in aircraft engineering, where actuating hydraulic cylinders transform the fluid pressure’s energy into mechanical force.
A hydraulic pump
Converts mechanical power into hydraulic power
A hydraulic actuator
Converts hydraulic power into mechanical power
The objective of a hydraulic pump is…
to provide pressurised oil to a hydraulic actuator.
Three common types of hydraulic pumps are:
- Vane pump
- Gear pump
- Axial piston pump
The pump type used in a hydraulic control system is not very significant, except for the pump capacity, when considering the control functions of the system.
Cylinders
- Simple in construction
- Have the ability to withstand severe overloads
A typical cylinder consists of:
- Barrel
- End covers
- Piston
- Piston Rod
Two main design types:
- Single acting with a single port
- Double acting with a port at each end of the cylinder barrel
Cylinder (Single acting with a single port)
a. Power stroke in outward direction only
b. Return of piston is usually by means of a spring
c. Return can be achieved through the load
Cylinder (double acting with a port at each end of the cylinder barrel)
Occasionally required with a piston rod on both sides of the piston to ensure an equal thrust in both directions
Cylinder Force
- The cylinder consists of a piston with radius R moving in a bore
- The piston is connected to a rod of radius r which drives the load
- If pressure is applied to port X, the piston extends
- The force applied by a piston depends on the area and the applied pressure:
- Area A, for the extend stroke is A = pi * (R)^2
- For pressure p, applied to the port X, the extend force is F = ppi(R)^2
Cylinder Force II
If pressure, P, is applied to port Y, the piston retracts
- The annulus (ring) area A(a), is A(a) = pi(R^2-r^2) = A - pir^2
- The maximum retract force is F(r) = Ppi(R^2-r^2) = P(A-pir^2)
F(r) < F(e)
