Danfoss Tips on Hydraulics User guide
Technical Information
Tips on Hydraulics
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Technical Information
Tips on Hydraulics
Revision history Table of revisions
Date Changed Rev
October 2019 1994 version scanned, then recreated 0101
2 | © Danfoss | October 2019 BC317769130475en-000101
Technical Information
Tips on Hydraulics
Contents
Introduction
Dimensioning
Selection of components
Selection of oil type
Checking the oil
Installation of system
Starting up and running-in
Maintenance
Fault location
Repair and testing
Size of pump.......................................................................................................................................................................................6
Sizes of pipes and hoses.................................................................................................................................................................6
Using the nomogram......................................................................................................................................................................7
Valves....................................................................................................................................................................................................7
Calculating on the tube diameter.............................................................................................................................................. 8
Calculation of piston velocity - oil flow.................................................................................................................................... 9
Calculation of cylinder force...................................................................................................................................................... 10
Table of cylinder power...............................................................................................................................................................11
The tank.............................................................................................................................................................................................12
Filters.................................................................................................................................................................................................. 13
Relative size of particles...............................................................................................................................................................14
Dimensions.......................................................................................................................................................................................15
Oil requirements.............................................................................................................................................................................16
Types of hydraulic-liquids...........................................................................................................................................................16
Oil types.............................................................................................................................................................................................16
Non-inflammable fluids...............................................................................................................................................................16
Additives........................................................................................................................................................................................... 16
Motor oil............................................................................................................................................................................................17
Viscosity classification system...................................................................................................................................................18
Water in the oil................................................................................................................................................................................19
Oil oxidation.....................................................................................................................................................................................19
The presence of water..................................................................................................................................................................19
Viscosity.............................................................................................................................................................................................19
The smell and appearance..........................................................................................................................................................19
Tables for conversing viscosity................................................................................................................................................. 20
Hydraulic oil temperature conditions oil life in %..............................................................................................................21
Rule 1..................................................................................................................................................................................................22
Fine filtration of the oil via a filling filter unit................................................................................................................. 23
Rule 2..................................................................................................................................................................................................24
Rule 3..................................................................................................................................................................................................24
Procedure for starting up............................................................................................................................................................25
Periodic inspection........................................................................................................................................................................27
The tank.............................................................................................................................................................................................27
The suction line...............................................................................................................................................................................27
The pump..........................................................................................................................................................................................27
The return line and return filter................................................................................................................................................ 27
Pump test..........................................................................................................................................................................................28
Fault location general...................................................................................................................................................................29
Hydraulic pumps............................................................................................................................................................................29
Hydraulic motors............................................................................................................................................................................30
Steering systems with OSPB - OSPC - OVP/OVR - OLS......................................................................................................31
Steering units OSPB - OSPC - OVP/OVR - OLS.................................................................................................................32
Steering systems with OSQA/B and OSPBX-LS....................................................................................................................34
Faults locations illustrations.......................................................................................................................................................35
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Tips on Hydraulics
Contents
Repair..................................................................................................................................................................................................36
Testing................................................................................................................................................................................................36
Symbols and Tables
Symbols............................................................................................................................................................................................. 37
Ratio factors..................................................................................................................................................................................... 37
Pump.................................................................................................................................................................................................. 38
Motor..................................................................................................................................................................................................38
Cylinder..............................................................................................................................................................................................39
Tube....................................................................................................................................................................................................39
Accumulator size............................................................................................................................................................................39
ISO/CETOP Symbols
Catalogues or leaflets available
Hydraulic components leaflets or catalogues.....................................................................................................................44
Service shops...................................................................................................................................................................................44
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Technical Information
Tips on Hydraulics
Introduction
Like so many other technical fields, hydraulics is both old and new at the same time.
Take waterwheels for example, people have been using them since before history was recorded. On the
other hand, the use of liquid under pressure to transfer force and also to control complicated movements
is relatively new and has undergone its most rapid development within the last 40-50 years, not least
because of the work that has been done in aeronautics.
Hydraulics and pneumatics are universal for the entire engineering industry and are amongst the three
most important media for the transference and control of force. The two other media are mechanical
transference for example via clutch pedals and gears) and electrical (for example via a generator).
"Flowing energy" is transferred and controlled through a medium under pressure - either air (pneumatic)
or liquid (hydraulic).
This form of energy has many exceptional advantages and is therefore often the most suitable form of
energy transference on land, sea or in the air.
A contained liquid is one of the most versatile means of controlling and transferring force. It takes the
precise form of the walls that contain it and withstands its pressure. It can be divided into several streams
which, depending on their size, can perform work before being allowed to merge into one stream again
to perform still more work. It can be made to work fast in one part of a system and slowly in another.
No other medium combines the same degree of reliability, accuracy and flexibility while retaining the
capability of transferring maximum force with minimum volume and weight. The quality control with this
medium can be compared with the accuracy of an electronic micro-processor.
However, to achieve maximum utilization with highest efficiency and least possible operational stops, it
is very important that a hydraulic system be designed, manufactured, started and maintained absolutely
correctly. The special factors vital to the user (purchaser) must also be understood if operation in the field
is not to be plagued by stops and other disturbances.
Nearly all factory systems use "flowing energy" in production. More than half of all manufactured
products are based on this form of energy, and it is therefore of interest to all manufacturers, exporters,
purchasers, stockists, and repairers of production systems and machines, including agricultural machines
and machine pools, the village smithy and the automobile industry, shipping and aviation.
Clearly, the knowledge and experience of many designers, producers, repairers and owners (users) is
being outstripped by the dramatic development and rapid spread of hydraulics.
The purpose of this article is therefore not to try to provide patent solutions to all hydraulic problems, but
to help create an understanding of why problems arise and what steps can be taken to avoid them.
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Tips on Hydraulics
Dimensioning
Reliable sizing provides the most optimal selection of components.
It is obvious that if undersized components are used, they will not operate under overload. They will be
sensitive and become a frequent source of problems and complaints. More important still, in comparison
with a correctly sized component an oversize component will probably operate problem-free and
"effortlessly " for a very long time, but its original price will be too high.
If not able to carry out accurate calculations to obtain optimum conditions, the guidelines below are
worth following.
The first thing to establish is the maximum operating pressure required for the system since this is the
decisive factor in pump selection and, in turn, important as far as the size (output) of the prime mover
and the system price are concerned. The higher the operation pressure, the higher the price of many of
the components.
When the economic considerations have been made, particular types and sizes of operating cylinders,
motors, and steering units to be used in the system can be considered.
The pump size is found by adding the necessary amounts of oil (expressed in liters per minute) that can
be in use at the same time.
Consequently, the total is the amount the pump must be able to supply at the maximum intermittent
operating pressure (= pressure relief valve setting pressure).
Size of pump
The power applied to the pump must be found as a function of the pressure in bar, revolutions per
minute and flow in liters per minute, expressed in kW. The result can be used to find the size of motor
that will safely yield the necessary output. See the following example.
Hydraulic output N = pressure x flow, i.e. N = px Q
Example:
When calculating the necessary pump output (Pnec), account must be taken of the total pump efficiency,
(ղtot.) as stated in the catalog.
Example:
Sizes of pipes and hoses
The size depends on:
•
maximum system pressure
•
maximum oil flow
•
length of pipe system
•
environmental conditions
Pressure drop must be as small as possible. The greater the resistance in the system, the greater the
operational loss. It is important to avoid those factors which cause pressure drop such as, for example,
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Tips on Hydraulics
Dimensioning
the use of angled screwed connections. Where possible, these should be replaced by elbows. If long
lengths of pipe or high flow velocity are involved, then an increase in diameter up to the next size should
be considered.
Remember that the dimensions stated for the hydraulic pipes are the external diameters and wall
thicknesses. The internal diameters are equal to the external diameters minus 2x the wall thickness.
Remember that when the internal diameter is doubled the flow area of the pipe is quadrupled,
Now the oil capacity supplied per minute by the pump is known, along with the amount of oil the
individual components must have. The next stage is the dimensioning of pipes and hoses. This is also
very important as otherwise, generated cavitation (noise), heat generation, pressure drop and, in some
cases, bursting can occur.
There are many people who are frightened of this dimensioning as they associate it, incorrectly, with
difficult mathematical calculations. In actual fact, if the nomogram adjacent is followed when calculating
pipe dimensions, it is incredibly simple.
In order to use the nomogram, the first stage is to know the oil flow in liters per minute. After this it has to
be known whether the pipes and hoses in question are to be used as suction lines, pressure lines or
return lines. This is because there are some recommended velocities of oil flow available for these
categories. These values are as follows:
•
Suction line 0.5 - 1.5 m/s
•
Pressure line 3 - 10 m/s
•
Return line 2 - 5 m/s
Using the nomogram
Valves
Place a ruler over the two outer columns, that is, the known oil flow and the required speed for the pipe
type in question. Read off the nearest internal pipe diameter on the middle column. (See Calculating on
the tube diameter on page 8.)
Depending on the maximum pressure a decision can also be made as to whether to use light or heavy
hydraulic pipes and hoses. Here a large price difference is involved, especially with the associated fittings.
See table of pipe dimensions and maximum working pressure.
Valves are used in all hydraulic systems. In simple systems maybe only a pressure relief valve (safety
valve) and a single directional valve is used. Other systems might be more complicated and might involve
a large number and wide variety of electronically controlled proportional valves.
It is probably within valves that the choice of components is widest and where it is easy to use and waste
most money if wrong selections are made.
If in doubt, the suppliers of recognized makes of valves can be approached for advice on the selection. It
is important not to select a valve which is too small or too large in relation to flow. If it is too small, the
relative pressure drop will be too high, resulting in heat generation and possibly cavitation. If the valve is
too large it can result in poor regulating characteristics causing the cylinder to not operate smoothly or
the system to oscillate.
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Tips on Hydraulics
Dimensioning
Calculating on the tube diameter
Example:
•
•
•
Given volume = 50 I/min
Given speed = 1 m/s
Found diameter = 32 mm
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Tips on Hydraulics
Dimensioning
Calculation of piston velocity - oil flow
Example:
•
Given piston velocity = 0,03m/s
•
Given cylinder diameter = 112 mm
•
Found pump capacity = 18 l/min
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Tips on Hydraulics
Dimensioning
Calculation of cylinder force
Example:
•
Given pressure = 100 bar
•
Given cylinder diameter = 112 mm
•
Found force = 10.000 daN
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Dimensioning
Table of cylinder power
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Tips on Hydraulics
Selection of components
All hydraulic systems consist in principle of the same basic components, but just as with electronics, the
combinations are infinite and the range of components immense.
Which components are the most important in a system? Is it the:
•
cylinder or the motor that is going to perform the work?
•
liquid (oil) that transfers force to the motor or cylinder?
•
pipes and hoses that lead oil to motor and cylinder?
•
valves that control the oil flow paths?
•
pump that applies energy and movement to the oil?
•
motor that drives the pump?
•
filter that removes dirt from the oil?
•
oil cooler that ensures a suitable oil temperature?
•
tank that contains oil for the system?
The answer must be that specific demands are made on all these components and since none of them
can be allowed to fail, they must all be equally important. Therefore extreme care must be taken in all
stages of their creation, selection and application.
When a hydraulic diagram is being prepared, the designer must have quality in mind, including the
quality of the drawing itself, so that any errors in interpreting the drawing are avoided. It is a good idea
always to use the correct ISO/CETOP-symbols.
When the diagram is subsequently used in preparing parts lists and accurate component specifications,
sizing problems often occur. The designer is confronted with brightly colored brochures and catalogs
and, at first, all is confusion. The temptation is to revert to rule-of-thumb methods and "add a bit for
safety's sake", the result being a system which is either too expensive or is unstable and of poor quality.
All reputable hydraulic component manufacturers give real, usable values in their catalogs, not just
theoretical desired values. The technical data in Danfoss catalogs always represent average values
measured from a certain number of standard components. In addition to these data, the catalogs contain
a mass of useful and explanatory information on selection, installation and starting up of components,
together with a description of their functions. This information must, of course, be used as intended in
order to avoid overload, too high a wear rate, and con sequent oil overheating and to avoid an overdimensioned system with poor regulation at too high a price.
The tank
Let us look a little closer at an example system, starting with:
The tank, which has many functions, for example:
•
as a reservoir for the system oil
•
as a cooler
•
as a "coarse strainer", sedimentation of impurities
•
as an air and water separator
•
as a foundation for pumps etc.
The dimensions of the tank and its form are important and it should therefore be designed for its
purpose, the same as all other hydraulic components. Its location must also be taken into account so that
the sight glass, filters, filling cap, air filter, drain cock, etc. are easily accessible for daily inspection. If the
application is mobile, if there is no cooler built into the system, and provided the tank is located where air
circulation is good, the size of the tank can be fixed at approx. 3-4 times the capacity of the pump per
minute.
Two arrangements are shown below. The arrangement to the left, is preferred as this increases the
cooling effect as much as possible.
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Tips on Hydraulics
Selection of components
The tank functioning as a cooler - two arrangements
To increase the ability of the tank to separate dirt and water, the bottom must be slightly inclined
(deepest end opposite the inlet/outlet end). An ordinary cock (without handle) is fitted so that impurities
can easily be drained off. Increased separation of the air that is always present in the oil can be obtained
by fitting an inclined coarse metal strainer (approx. 25-50 mesh/inch) by the return line.
Both suction and return pipes must be cut diagonally. The ends of the pipes must be located 2-4 times
the pipe diameter above the bottom of the tank, partly to avoid foaming at the return line, and partly to
prevent air from being drawn into the suction line, especially when the vehicle/vessel heels over to one
side. With regard to the annual "spring-clean", the tank must have large removable covers, either in the
sides, in the top, or in the ends, in order to give easy access for cleaning. If filters are installed, they must
be located above the tank oil level and must be easy to replace without significant spillage. That is to say,
it must be possible to place a drip tray under the filter inserts.
Since tanks are made of steel plate, rust is inevitable (even below the oil level, because oil contains both
water and oxygen) and it is therefore advisable to surface-treat the inside. If the tank is to be painted,
thorough cleaning and degreasing is necessary before primer and top coats are applied. The paint used
must, of course, be resistant to hot hydraulic oil.
If the cooling effect from the tank and other hydraulic components is insufficient in order to keep oil
temperature down to an acceptable maximum an oil cooler must be fitted. Most suppliers prescribe 90°C
as an absolute maximum partly because of lifetime of rubber parts partly because of alterations of
tolerances and possibly bad lubrication. Today quite often electronic devices are fitted directly onto the
hot hydraulic components. In consideration of the electronics a reduction of the maximum oil
temperature to under 80°C must be aimed at.
Filters
The degree of filtering and filter size are based on so many different criteria, that generalization is seldom
possible. The most important factors to be considered are as follows:
Operational environment: How serious would the consequences be if the system failed because of dirt?
Oil quantity: Would there be a few liters or several hundred liters in the system? Is it an expensive or a
cheap oil?
Operational stop: What would it cost per hour/day if the system came to a standstill? How important is
this factor?
Dirt sensitivity: How dirt-sensitive are the components? What degree of filtering is recommended by the
component suppliers?
Filter types: Are suction filters, pressure filters or return filters to be used, or a combination of these with or
without magnets? Is exclusive full-flow filtering involved, or will there also be bypass filtering through
fine filters? Which type of dirt indicators are to be chosen, visual, mechanical or electrical?
Air filtration: Air must be filtered to the same degree as the finest filter in the system. Otherwise too much
dirt can enter the tank with the air. If there are large differential or plunger cylinders in the system, the
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Tips on Hydraulics
Selection of components
Relative size of particles
tank breathes in/pushes out large amounts of air. Therefore the size of the air filter must be on the large
side. Remember that dirt particles visible to the naked eye (larger than 40 µm) are as a rule, less
dangerous than those that cannot be seen. It is often the hard particles of 5-25 µm, corresponding to
normal hydraulic component tolerances, that are the most dangerous.
The naked eye is unable to see objects smaller than 40 µm.
For normal operation the degree of filtration for hydraulic products can generally be divided into the
categories below:
Motors: 25, µm nominal - degree of contamination 20/16 (see ISO 4406) for return filter, or combined with
a magnetic insert if a coarser filter is used, e.g. 40 µm.
Steering units: For systems having an efficient air filter and operating in clean surroundings, 25 µm
nominal is adequate. If this is not the case 10 µm absolute - to 19/16 must be fitted Filters can be either
pressure or return filters.
Proportional valves: In most cases, 25 µm nominal for return filter is adequate, but in systems especially
subjected to contamination, a pressure filter is recommended to ensure operational reliability.
Radial piston pumps: In open as well as closed systems:
Suction filter: 100 µm nominal or finer, but not finer than 40 µm nominal.
Return filter: 20 µm absolute or 10 µm nominal - 19/16.
Filters should be fitted with a dirt indicator so that operating conditions can be kept under observation.
This is especially important with suction filters to avoid impermissible pressure drop in the suction line
and consequent cavitation. The pressure in the suction line must not be less than 0.8 bar absolute.
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