Upload
bruno-franklin
View
234
Download
0
Tags:
Embed Size (px)
Citation preview
HYDRAULICS & PNEUMATICS
Presented by: Dr. Abootorabi
Flow Control Valves
1
Flow Control Devices
Flow control devices produce the desired rate of actuator
operating speed by controlling the volume of fluid allowed
to reach the actuator.
Flow control devices can be divided into two general types:
Restrictor
Bypass
2
Flow Control Devices
Restrictor-type flow control valves limit the volume of
fluid through the valve.
Excess pump output is forced to return to the reservoir
through the system relief valve.
3
Flow Control Devices
Circuit containing a restrictor-type flow control valve
4
Flow Control Devices
Bypass type flow control valves use an integral control
port to return excess pump output to the reservoir.
The returned fluid is at a pressure less than system
relief valve pressure.
5
Flow Control Devices
Circuit containing a bypass-type flow control valve
6
Flow Control Devices
Conceptual operation of a flow control valve may be traced to a basic
orifice.
7
Flow Control Devices
The flow rate through a simple, sharp-edged orifice depends on:
Area of the orifice
Pressure difference between the inlet and outlet sides of the
orifice
Viscosity of the fluid, which varies with fluid temperature
8
Simplified formula:
Flow Control Devices
Flow control valves may be noncompensated or
compensated:
The flow rate through noncompensated valves varies
as the load or fluid viscosity changes
Compensated valves automatically adjust for fluid
pressure variations to produce a consistent flow rate
under varying load and temperature conditions
9
Flow Control Devices
Noncompensated and compensated flow control valves
may have:
Fixed flow rate
Adjustable flow rate
10
Flow Control Devices
The simplest restrictor-type flow control valve is a simple
orifice:
Basically a calibrated hole
Serves as a noncompensated, fixed-rate flow control
device
11
Flow Control Devices
A needle valve is the simplest restrictor-type,
noncompensated adjustable flow control device:
Consists of an orifice fitted with a tapered needle
machined on a threaded stem
Turning the threaded stem changes the effective
area of the orifice, which adjusts the flow rate
through the valve
12
Flow Control Devices
Basic adjustable flow control valve:
13
A simple needle valve
without check valve is
called also metering
valve.
Flow Control DevicesAdjustable restrictors:
14
Flow Control Devices
Adjustable restrictors:
15
Flow Control Devices
One-way flow control valve:
16
Flow Control Devices
When using a restrictor-type, noncompensated flow control
valve, actuator speed varies when system loads change.
Caused by the change in pressure drop across the control
valve, which varies the flow rate through the valve.
17
Flow Control Devices
A pressure compensator maintains a constant pressure
difference across the metering orifice of a flow control valve:
Senses pressure on the inlet and outlet sides of the orifice
These pressures generate forces that act on the end
surfaces of a sliding spool that is preloaded by a biasing
spring
18
Flow Control Devices
Force generated by the biasing spring establishes the
constant pressure difference across the orifice.
This constant pressure difference maintains constant fluid
flow through the valve even when system loads change.
19
Flow Control Devices
A basic pressure-compensated flow control valve
20
Flow Control Devices
Pressure compensator operation:
21
Flow Control Devices
Pressure compensator operation:
22
Flow Control Devices
Temperature compensation is necessary in flow control
devices if an accurate, consistent flow rate through a valve
is needed.
This is due to the fluid viscosity changes that occur as fluid
temperature changes.
23
Flow Control Devices
Temperature compensation is typically accomplished in flow
control devices by:
Specially designed, sharp edged orifice
Heat-sensitive metal rod that operates a needlelike
control device in the metering orifice of the valve
24
Flow Control DevicesTemperature compensation using sharp-edged orifice:
25
Flow Control Devices
Temperature compensation using a heat-sensitive metal rod:
26
Flow Control Devices
In a circuit using a restrictor-type, pressure-compensated
flow control valve:
Pressure drop across the internal flow-control device in
the valve remains constant, which produces a constant
flow rate through the valve
Actuator speed will not vary when system loads change
27
Flow Control Devices
In a circuit using a restrictor-type, temperature-
compensated flow control valve:
Internal flow-control device is adjusted for viscosity
variations that occur during fluid temperature changes
Flow remains constant as system operating
temperatures change
28
Flow Control DevicesCircuit containing restrictor-type, compensated flow control valve
29
Flow Control Devices
Bypass-type flow control valves:
Provide partly accurate flow to actuators
Direct any excess flow from the pump directly to the
reservoir through an integral port
30
Flow Control Devices
Typical bypass-type
flow control valve:
31
Flow Control Devices
The operating pressure of a system using a bypass-type flow
control valve is determined by the load on the actuator plus
the pressure needed to overcome the force of the biasing
spring.
The relief valve functions only when actuator loads are great
enough to increase system pressure above the cracking
pressure of the relief valve.32
Flow Control Devices
Operation of a bypass flow control valve during increasing or
decreasing load:
33
Flow Control Devices
Operation of a bypass flow control valve during steady load:
34
Flow Control Devices
Operation of a bypass flow control valve with stalled actuator:
35
Flow Control Devices
The bypass flow control design provides an efficient
operating flow control circuit
Pressure in the system is only as high as needed to
move the load and operate the valve compensator
This reduces system heat generation and energy
consumption
Care must be taken to accurately determine actuator
loads and the cracking pressure of the system relief
valve 36
Flow Control Devices
Priority and proportional divider valves are designed to
divide one fluid supply between two circuit
subsystems.
Many of these valves can also be used to combine the
flow from two different circuits.
37
Flow Control Devices
Priority divider valves provide flow to one port before
providing flow to a second port.
Often used in mobile equipment where pump output is
controlled by engine speed.
38
Flow Control Devices
Typical priority valve:
39
Flow Control DevicesCircuit containing a priority divider valve
40
Flow Control Devices
Proportional divider valve splits input port flow into two
proportional output flows.
Ratio between the output flows may be fixed or variable.
Ratio of 50-50 is most common.
41
Flow Control DevicesA typical use of proportional flow divider valve:
42
43
Design and Operation of Basic Flow-Related Circuits
Three basic flow control circuits are used in hydraulic
systems:
Meter in
Meter out
Bleed off
These basic circuits meet the varying flow-control demands
for systems with positive and negative loads.
44
Design and Operation of Basic Flow-Related Circuits
The meter-in flow control design places the flow control
valve between the pump and the inlet of the actuator:
Should only be used for positive loads
Cannot provide accurate control under a negative
load condition
The prime mover is always operating against the
maximum pressure setting of the system relief valve
45
Design and Operation of Basic Flow-Related Circuits
Basic meter-in circuit
46
Design and Operation of Basic Flow-Related Circuits
The meter-out flow control design places the flow control
valve between the actuator outlet and the reservoir:
Can provide accurate control for positive and
negative loads
The prime mover is always operating against the
maximum pressure setting of the system relief valve
47
Design and Operation of Basic Flow-Related Circuits
Basic meter-out circuit
48
Design and Operation of Basic Flow-Related Circuits
The bleed-off flow control design places the flow control
valve in a tee in the working line between the
directional control valve and the actuator inlet:
Outlet of the flow control is directly connected to
the reservoir
Measured flow is diverted to the reservoir while
remaining flow operates the actuator
The prime mover operates against a pressure only
high enough to move the load
49
Design and Operation of Basic Flow-Related Circuits
Basic bleed-off circuit
50
Design and Operation of Basic Flow-Related Circuits
Meter-in and meter-out circuits provide the most accurate
actuator speeds.
Both meter fluid flow delivered directly to or from the
actuator.
The meter-out circuit is the best method for negative loads
that may pull the actuator.
51
Design and Operation of Basic Flow-Related Circuits
The bleed-off flow control circuit is less accurate than
either the meter-in or meter-out system:
Flow is metered back to the reservoir while the
remaining pump output establishes actuator speed
The remaining flow can vary because of pump
efficiency and system leakage
Flow control valve metering accuracy under varying
load conditions is also a factor
52
Design and Operation of Basic Flow-Related Circuits
The bleed-off circuit is the most energy-efficient design.
This is due to the fact the prime mover operates at a
pressure only high enough to move the load.
The end.
53