幻灯片 1 - Cal Poly

幻灯片 1 - Cal Poly

Fluid Power Engineering Week1/Lesson 2 Basics Fluid-power basics In this lesson we shall Look at the properties of hydraulic vs. pneumatic systems Explore standard symbology used in fluid-power drawings Learn several important components, which will allow us to build hydraulic circuits

Hydraulics vs. pneumatics Hydraulic fluid-power systems use a liquid, often an oil, as the working fluid in the system Pneumatic systems use, instead, air as the working fluid of the system Heres the big difference: Liquids are (relatively) incompressible, thus rigid Air is compressible, thus soft and spongy A way to think about hydraulic oil A useful way to think about hydraulic oil is to compare it

with a mechanical linkage Mechanical linkage for automotive application All the red, yellow, blue, green volumes are hydraulic oil Fluid-power linkage for aeronautical application

Oil, a flexible mechanical link The fluid volumes are like very flexible links They can fit tightly into a small space They can change direction They can go around corners Rigid mechanical links are hard to fit in a compact, complicated geometry They can get bent The joints can wear out The link must be in line with the line of action of the force But hydraulic power is not perfect

Despite all these positive characteristics, hydraulic power has its disadvantages too: Hydraulic seals can leak and spill fluid all over the place For hydraulic power, you need an entire oil supply and pumping system This has costs, takes up space, and has weight For mobile and airborne systems, space and weight are critical design parameters Hydraulic pumps can be noisy Oil can be a fire hazard Pneumatics instead Pneumatic systems have some advantages over hydraulic

systems: Air is free; i.e., you can just use air from the atmosphere Usually the compressor, which can be noisy, is located remotely to reduce noise The pressurized air is just piped in, like electricity is supplied from a remote generating station The air, once used to move something, can simply be exhausted to the atmosphere The return piping for oil systems can be eliminated Leaks dont create a mess But pneumatic power is not perfect

Despite all these positive characteristics, pneumatic power has its disadvantages too: Spongy actuation You are pushing with a spring Thus, there is a lack of precise motion and the mass And oscillates since airs viscosity is low, it tends to leak out of seals more readily than oil So seals have to grab tighter and this creates more friction/stiction

But pneumatic power is not perfect Low viscosity means tighter seals Unlike oil, air isnt self-lubricating Oils much higher specific heat makes it a better fluid for removing heat from a process than air And, very important, air contains a lot of energy of compression, so it is like a bomb waiting to go off Its useful to think of a pressurized volume of air as filled with tiny springs, all compressed Those springs want to expand, so any chance they get, they will take it Compressed air is dangerous

The energy in the air can be released suddenly The energy in a tire, for example, can kill or seriously injure a worker See https://www.youtube.com/watch?v=uQbKCd3ezrA The energy to compress oil, since it is incompressible, is negligible So if an oil system develops a leak, a little oil squirts out and the pressure is relieved This is precisely why natural gas pipelines are hydro-tested (brought up to pressure with water) before they are put into service transporting natural gas Further details about fluid-power symbology

Now lets look at some basic detail about the symbols used to show fluid-power circuits We have seen so far Arrow pointing out of device indicates that is a power producer Positive displacemen t pump

Further details about fluid-power symbology Reservoir Above bottom indicates suction from above bottom of reservoir Open top indicates nonpressurized reservoir On bottom

indicates suction from bottom of reservoir Two different kinds of reservoir supplies Further details about fluid-power symbology Hydraulic motor Arrows pointing inward indicate that device is a

power consumer Two arrows indicate that motor can run in forward or reverse Further details about fluid-power symbology Single rod

Cylinder A Cap end B Double rod A Rod end

B Cylinder ports always named A and B On single-rod cylinder fluid area on piston on cap end greater than fluid area on piston on rod end Further details about fluid-power symbology Valve Directional control valve (DCV) A DCV is the traffic cop in a circuit. It switches the flow from one path to another. A

B This valve has three positions or envelopes It also has 4 ports, A,B,P,T P T Thus, this is a 4-port, 3-position valve, in short a 4/3 valve

Directional control valve The A and B ports lead to the A/B cylinder ports P stands for pressure This is from the pump

A B P T T stands for tank This leads to the reservoir Directional control valve

Shifted into the left position We call this PA,B-T Directional control valve Shifted into the right position We call this PB,A-T Directional control valve In the center position A

B P T The pressure source is blocked off, the tank too, and the cylinder is locked Directional control valve Springs center the valve into the central position when it is not actuated

This zig-zag line always represents a spring DCV Solenoid-actuated One way to shift the valve is with solenoids DCV Hand lever-actuated Another way is with hand levers DCV Pilot-actuated Yet another way to shift the valve is with pilot pressure Dotted

line indicates a pilot line The pilot pressure comes from some other part of the circuit DCV Air pilot-actuated Sometimes the pilot source is pneumatic Hollow symbols

indicate air, solid symbols oil So pilot air is sometimes used to shift a hydraulic valve A few more components Filter A few more components DCV Check valve Flow seats ball on

orifice Flow blocked this way A check valve is the hydraulic analog of an electrical diode But in this direction, flow lifts ball off seat Flow passes through Pressure-relief valve (PRV) PRV protects the circuit from over-pressure The spring deflects the internal flow path from the

main flow channel But if the pressure builds too high at the inlet, this pilot pressure deflects the internal passage of flow so the fluid can flow through the valve This is a 2/1 valve, i.e. two ports but one

envelope but the valve can change positions within that one envelope Flow restriction - orifice A flow restriction has a symbol that is very clear Cut-away view of orifice ISO standard

symbol for orifice This represents any restricted flow path, like an orifice Variable flow restriction needle valve A needle valve can change the size of the orifice opening In general, an arrow means something is adjustable

Unit calculations for fluid power It is especially important to perform unit calculations clearly and thoroughly when analyzing or designing fluid-power systems. This is critical when working in the English measurement system. But it is also de rigueur in the SI system. Unit calculations done correctly often will save the day, that is point to the fact that you have left something out or have made a mistake in your calculations. Also important in performing these calculations is to make them checkable . Checkability is an underrated quality, but it shouldnt be. If you cannot go back and check your calculations, you may as well not do them. There is a methodology for performing unit calculations that I highly recommend, because it promotes and preserves this quality of checkability. Its best demonstrated by an example.

In the SI system the sort of common units are N (newtons), m (meters), and sec (seconds). But it is common to give flow rates in lpm (liters/minute), pressures in bar, rotational speeds in rpm (revolutions per minute), pump or motor displacement in cm 3, etc. So often a unit calculation will involve converting quantities given in some of these other units into the standard N, m, sec, performing the calculation, then converting back into noncommon units. Unit calculations example The theoretical flow rate of a pump is the volume swept by a gear tooth, vane, or piston per revolution multiplied by the pump speed. For example, if the volume swept per revolution is 6 cm3 and the speed is 3000 rpm, then with with

cancels cm 3 rev = 6 3000 rev min ( m

100 cm 3 ) min m3 = 0.003 60 sec sec

cancels cancels with Note how the units are cancelled with each other. The strike-out pattern for each pair of units is unique. That is, the strike-out pattern is not just one, uniform pattern. Thus the designer or analyst can go back to his/her work, even after a good deal of time, and understand how the calculation was done. Calculation methods for fluid power

Also, since fluid-power calculations can be long and laborious, it is my recommendation to perform them in a spreadsheet, so that if later an error is detected, it can be fixed easily. This avoids having to perform a series of error-prone calculations more than once. Also, since there are a lot of conversions required in fluidpower calculations, I recommend the use of a calculator with built-in unit conversions. A good one that I have found as an app for an Android phone is TechCalc+, which cost just $1.00. The picture at right shows the conversion of bar to MPa. Also remember the importance of checkability. Do not perform calculations that are not clear and that cannot be checked. Outside learning To better understand this subject matter, view the following videos Dont forget to turn the closed-captioning on to be able to understand better the

details of the lectures Watch: Hydraulic Schematics Hydraulic Symbols for Beginners Hydraulic circuit symbol explanation End of Week 1/Lesson 2 34

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