Fluid Power Systems (ME353)

Fall 2012

Lecture 3

Transmitting the power to Hydraulic Motor

Fluid Storage and Distribution

Reservoirs, Conductors, and Connectors

Reservoir

The primary purpose of the reservoir is to hold

the system fluid not currently in use in the

system

Other important functions of the reservoir are:

– Remove heat

– Separate solid particles

– Release air from fluid

– Separate water from fluid

The typical hydraulic system reservoir is a rectangular, covered steel tank

The tank is typically fitted with:

– Pump inlet line

– System fluid return line

– Drain line

– Filler cap

– Air breather

– Fluid-level indicator

Typical reservoir construction

Baffles are used in the interior of reservoirs to direct flow to maximize

the distance the fluid must travel between the return line and the pump inlet

line

– Slows the movement of the fluid

– Increases cooling

– Increases separation of solid particles, air, and water

Baffles are typically included in a reservoir

L-shaped and overhead reservoir designs may

be used in systems where positive pressure is

needed on the pump inlet line

L-shaped reservoir Overhead reservoir

As a general rule, the capacity of the reservoir

should be three times the rated flow of the

pump

– Fixed installations may be higher

– Mobile applications, where weight and space are

factors, may be less

Reservoir Capacity

Conductors The proper selection, installation, and maintenance of

conductors is critical to efficient hydraulic system

operation

Pipe, tubing, and flexible hose are considered basic

conductors in a hydraulic system

Conductors must have:

– Adequate strength to withstand high system pressures

– Low flow resistance to assure low energy loss during system

operation

– A design that allows economic installation and low

maintenance

Conductors must not only withstand normal system operating

pressure, but also hydraulic shock pressures

Shock pressures result from kinetic energy in the system when:

– Directional control valves are shifted to reverse the

movement of a load or heavy machine member

– Actuators encounter sudden load changes

Low flow resistance requires a conductor with an inside

diameter large enough to allow the needed volume of fluid to

move through the line within recommended fluid velocities

Restrictions may cause turbulent flow

Flow resistance in a system results from resistance to fluid flow

caused by:

– Viscosity of the fluid (friction between flow layers)

– Turbulence in the fluid stream (turbulence and eddies for

high Re numbers)

– Surface of the conductor (roughness)

– Bends and fittings in the lines (secondary losses)

– Orifices in components (secondary losses)

Fluid flow resistance resulting from fluid movement through

conductors and other system components:

– Lowers the work output of a system

– Produces heat, which may cause operating problems

Future maintenance must be carefully considered when

designing and installing hydraulic system conductors to assure

minimal difficulty in removing components for service

Pipe in a hydraulic system should be:

– Seamless, black pipe

– ANSI schedule rating of 40, 80, or 160, depending on the

maximum pressure expected in the system

Various nominal pipe dimensions

Schedule number indicates wall thickness

Tubing: – Is a relatively thin-walled, semi rigid conductor

– Can be bent and shaped into lines that provide good flow

characteristics with a minimum of visual clutter

Tubing can be bent, unlike pipe

The size of tubing is indicated by the actual

outside diameter

– Inside diameter varies according to wall thickness

– Most tubing is manufactured to the specifications of

a standardizing organization such as ANSI or SAE

Hose

is a flexible conductor made up of:

– Inner tube to conduct the fluid

– Middle layer of reinforcing material for strength

– Outer protective coating to withstand abrasion and abuse

Hose provides flexibility

Hose with braided-wire middle layers

Hose with spiral-wound-wire middle layers

A wide variety of hoses are available, often making selection

difficult

Standardizing organizations provide guidelines that help

compare various products on the market

Fittings A wide variety of fittings are available to assist in attaching

conductors to system components such as:

– Reservoir

– Pump

– Valves

– Actuators

Careful selection of fittings can:

– Help control construction costs

– Improve system efficiency

– Allow easier system maintenance

Dry seal standard pipe threads should be used on pipe fittings,

rather than standard pipe threads, to assure a tight thread seal

that will not leak under high system pressure

Standard pipe threads Dry seal standard pipe

threads

Fittings with pipe threads or straight threads sealed with an O-

ring or a metal compression washer are typically used to attach

tube and hose to hydraulic components

Tubing is attached to fittings by flaring the tube, compression,

soldering, or brazing

Hose attached to an actuator with elbow fitting and adapter with

pipe threads

Hydraulic systems typically use a 37° flare angle

Compression fittings use an O-ring, ferrule,

sleeve, or patented configuration to provide the

seal

Soldered or brazed fittings usually use split

flanges to mount the fitting to the component

Metal hose-end fittings are used to connect flexible hose to the

components of the system

– Available in both permanently attached or reusable styles

– Attached to the components using pipe threads, flare fittings,

or split flange fittings

Reusable hose-end fittings are either:

– Screw-together type

– Clamp type

These fittings can be assembled to the hose using basic hand

tools, which allows field assembly

Permanently attached hose-end fittings are crimped or swaged

onto the hose

Special equipment is required to install this type of fitting on a

hose

Crimped hose-end

fittings Swaged hose-end fitting

A wide variety of adapter fittings are available

for making proper connections between

conductors and system components:

– Transitions from one thread type to another

– Elbows at 45° and 90°

– Swivels that allow component movement

Analysis of Circuit and System Operation

Properly selecting a conductor requires an

examination of not only the hydraulic

system, but also the mechanisms operated

by the system

Factors that must be considered are:

– Pressure requirements

– Flow requirements

– Vibration

– Required movements of machine members

When selecting a conductor for a system, consider pressure:

– Normal system operating pressure

– Shock pressures

Flow velocity must be carefully considered when selecting a conductor for a

system

– Pump inlet line average fluid velocity should not exceed 1.2 m/s

– Working line fluid velocity should not exceed 6 m/s

Low pump inlet velocity prevents excessively low pressure in the line

– Too low of a pressure can lead to pump cavitation

– Cavitation can cause serious pump damage

Excessively high working line velocity will produce turbulent flow

– Turbulent flow can cause high flow resistance

– High resistance results in increased system operating temperatures

Very high and very low temperatures can adversely affect the

service life of a hose

Continuous system operation at or above recommended

temperatures can cause damage

Data sheets are available from the conductor manufacturer

– Good source of information

– Include a full range of information about a conductor:

• Construction details

• Typical applications

• Technical data related to pressure, flow capacity, and

temperature tolerance

Conductor Installation

When installing pipe and tubing, it is important to have the

correct lengths

– Should not be distorted

– Should not be placed under tension

– Distortion and tension can result in material fatigue and lead

to part failure

When installing tubing, the number of fittings in a system can

be reduced by bending the tube where possible (Hand tools and

power equipment are available to produce accurate bends)

Long lengths of pipe and tubing should be supported by

brackets or clamps to secure the conductor (This will reduce

fatigue caused by conductor weight or system vibration)

System pressure changes will change the length and diameter of

a hose

– Allow slack in the hose

– Allow adequate hose-to-member clearances

Allow slack in hose when

it is installed

When hose is installed,

ensure it is not twisted

When assembling hydraulic system conductors and their

associated fittings and adapters, the result should:

– Be a neat and uncluttered conductor layout

– Efficiently distribute fluid throughout the system

A clean and efficient conductor

installation

Conductors and Transmission line

Calculations

Pressure loss (friction) estimated using Bernoulli's equation.

Designers are interested in :

a) Avg. flow req.

b) Peak flow req.

c) Pressure req.

d) Acceptable energy loss

e) Temperature req.

Rule of thumb – Design for average velocity of 4.5 m/s for tubing and piping.

Pressure loss in Straight Pipes

f depends on Reynolds's number (Re)

Re < 2000 laminar

Re >3000 turbulent use moody chart to get f according to Re, ε/d

velocityaverage v

density mass fluid ρ

diameter internal d

length L

factorfriction f

drop Pressure ΔP

2

ρv

d

Lf ΔP

2

ρvd Re

Re

64f

Moody Chart includes the following regions (equations):

Tubing Bends- Pressure loss in bends must be corrected for radius and angle of bend as, Where,

KB = resistance coefficient for 900 bends c = correction factor for bend angle Kvalves = resistance coefficient for valves

These coefficients can be obtained from the following figures.

2

ρvcK

dfΔP

2

B

ValvesK