MECHENG236 Hydraulics/Pneumatics Flashcards
What are fluid power systems?
Use a working fluid to transmit power and motion, no solid linkage
How is power transmitted in a fluid power system?
Conservation of mass, pascal’s law
Hydraulic inputs (Power sources)
Pumps, accumulators
Hydraulic outputs
Actuators: Linear, rotary
Hydraulic middle (control)
Valves: Rate, pressure, direction
Fluid power system advantages
Advantages
* Fluid easy to transport
* Components mechanically
simple
* Distribute pressure source
from actuator
* Low explosion risk*
* Variable Speed
* Pneumatics – air is
everywhere
Fluid power system disadvantages
Disadvantages
* Contamination
* Risk of leaks
* Gases are compressible
* Restricted forces
* Can be loud
Hydraulic advantages
Little energy
absorption by fluid
* Higher forces
Hydraulic disadvantages
- Oil may be
flammable - Heavy
components due to
pressure
Pneumatic advantages
- Fewer shock loads
- Lighter weight
- Air easily available
- Reliable
Pneumatic disadvantages
- Lower loads
- Energy stored in
fluid may be rapidly
released if leak
Are hydraulic and/or pneumatics compressible
Gas working fluid is compressible
Working fluid of hydraulics/pneumatics
Hydraulics: Liquid (water, oils)
Pneumatics: Gas (air)
Role of pumps
Add pressure to the flow, convert work to pressure
2 types of pumps
Positive displacement (PDP)
Dynamic
PDP pumps
Forced volume change
Pulsating, irregular flow
Very high pressures
Dynamic pumps
Momentum added
Smooth, fast flows
Lower pressures
Three types of positive displacement pumps
Gear
Vane
Piston
Gear pumps teeth relationship
- Fewer, bigger teeth = stronger,
but more
pulsating
Gear pumps adv/disadv
Advantages
* Low complexity, cost
* Compact
* Can handle high-viscosity fluids
Disadvantages
* Pressure and flow rate cannot be changed independently
* Abrasives wear teeth
* Sealing and losses
Vane pump adv/disadv
Advantages
* High pressures for low-viscosity fluids
* Good intake suction
Disadvantages
* Complex
* Pressure and flow rate cannot
be changed independently
* Not suitable for high viscosity
fluids
Types of piston pumps
Single Piston
Multi-piston
Swash Plate
Swash Plate adv/disadv
Advantages
* Large pressure range
* Pressure and flow rate can be independently controlled
* Wide viscosity range
Disadvantages
* Complex
* Tight tolerances
* Bulky
Accumulators
Store pressure, an energy reserve
Uses of accumulators
Emergency pressure supply
Smoothing peak loads
Types of actuators
Motor (continuous rotational motion)
Rotary and Semi-rotary (limited rotational motion)
Linear (hydraulic cylinder)
Rotary actuators compared to motors
Simpler
Cannot over-extend
Common rotary actuator
Vane-type
Semi-rotary actuators
Produce limited rotational motion from internal linear motion
Benefit of semi-rotary actuators over rotary actuators
Easier to stop and hold
Motor actuator adv/disadv
Adv: Large angle
range, can use
as pump
(some)
Disadv: More
complicated,
expensive
Rotary actuator adv/disadv
Adv: Simpler, cannot
over-extend
Disadv: Best for end-
to-end or
oscillatory
applications
Semi-rotary adv/disadv
Adv: Simpler, cannot
over-extend,
“stop-and-
hold”
Disadv: More
complicated
than rotary
Linear actuators retraction types
Single-acting: Hydraulic pressure acts on one side only
Double-acting: Hydraulic pressure acts on both sides of the piston
Double-ended linear actuators
Piston rod out both ends of casing
-equal surface area for extension and retraction
Cushioned linear actuators
Flow restricted on piston head
Increases back pressure near end of stroke
Reduces flow rate, slows piston
Prevents abrupt end to motion, less damage risk
What do pressure converters do? (also known as pressure intensifiers or pressure de-intensifiers)
Provide pressure change between input and output
- change proportional to area
Pressure converter use cases
Meeting high pressure when low pressure available
Can use dissimilar fluids
Limited by length of stroke
Increase pressure - reduce flow volume
What do control valves enable and control
More complicated, safer circuits
Control: Pressure, flow, direction
Purpose of pressure control
Enable excess pressure to be bled
Provide different pressures to different components
How are pressure control valves controlled
Springs
Pilot Pressure
Types of pressure relief valves
Single-stage relief valves (also known as direct acting or check valves)
-Max pressure set by spring compression
Variant - pilot (sample) pressure line pushes spool. Spring does not resist bulk flow (can be weaker, cheaper)
Multi-stage
Direct vs two stage valves
Direct: Simple, compact, low cost. Suffer hysteresis, losses: If weak spring starts to open at low pressures = hysteresis, losses. If strong spring: starts to open nearer target pressure, does not open far enough (hazard)
Two stage valves: Combine two valves, one sampling pilot, one on main line. More complex but less hysteresis. Important for narrow-band pressure response on sensitive components.
Pressure reducing valve
Reduce pressure to a set value
Differential pressure regulator
Reduce pressure by a set value
Proportioning pressure regulator
Reduce pressure by a fixed ratio
Pressure reducing valve operation
Maintains constant output pressure. Pilot on outflow. If pilot pressure exceeds spring-set target, spool shifts, blocking flow.
Differential pressure regulators operation
Maintain constant pressure difference. Pilot pressure on inflow and outflow. Use a spring to ensure constant pressure difference
Proportioning pressure regulator operation
Maintain constant pressure ratio
Pilot pressure on inflow and outflow
Uses area ratio on pilot pressures
How is flow controlled?
Ideally would vary flow rate at pump.
If cannot, or have branches, need flow control valve.
Introduce flow restriction, slow flow.
Combine with non-return valve, reduce flow only one way, free the other.
Types of direction control valves
Flow dividers - split flow into two or more parts, flow constrictors control proportions
Non-return (check) valves - prevent flow in one direction
Where are non-return valves often located?
Downstream of pumps (along with relief valves)
Counterbalance valves
Combines non-return valves and pressure control valve
-free flow in one direction, set back pressure the other
Multi-way directional control valve label meanings
4/2
4 connections (ports)
2 positions (ways)
Categories for controlling control valves
-Mechanical
-Pneumatic
-Hydraulic
-Electrical
Shape of filters and purpose
Cylindrical, maximise filter area, minimise pressure drop
Replace regularly
Debris enters through open vents, can damage components, clog pipes
Places to put filters
- Before reservoirs
- Before pump
- Before vent
Additional control components
- Provide human monitoring (gauges)
- Provide human control (switches)
Miscellaneous components
Pipes (rigid)
Hoses (flexible)
Fittings (binds hoses and pipes together)
Connectors (adapters, connect pipes and fittings to specific components)
