For the layman, the hydraulic cylinder is the best-known of the hydraulic components, The cylinder is an example seen in everyday life of how the effects of hydraulic forces are used to produce the most varied movements in a machine. for the specialist too, it is the cylinder which plays a dominating role because it is usually the cylinder which serves as the starting point for the design of the whole hydraulic system. The hydraulic motor converts hydraulic power into mechanical power in the form of rotational movement. The cylinder, on the other hand, converts the hydraulic power into linear movement. For this reason, it is also referred to as a hydraulic linear motor.
The task of generating linear movement (translation movement) is carried out by the cylinder with a minimum of structural outlay and with excellent power efficiency. Both force and velocity can be maintained constant throughout the whole of the piston stroke, or they can be varied at will it is in this respect that the cylinder differs essentially from purely mechanical devices.
The fact that the drive (pump) is located separate from the cylinder facilitates a larges degree of freedom in selecting the layout, with the result that optimum machine design can be achieved. The versatility of the hydraulic cylinders is even further increased by the variety of fastenings and mountings which are possible, and by its combination with levers and linkage.
What does hydraulic fluid do? The choice of the most suitable hydraulic fluid is to decisive importance for the faultless functioning, operational reliability, long service life and profitability of a hydraulic system.Most hydraulic systems will operate satisfactorily using a variety of fluids. These include multigrade engine oil, automatic transmission fluid and more conventional antiwear hydraulic oil. But which type of fluid is best for a particular application? While it is not possible to make one definitive recommendation that covers all types of hydraulic equipment in all applications, the following are some of the factors to consider when selecting a hydraulic fluid.
Multigrade or Monograde Viscosity is the single most important factor when selecting a hydraulic fluid. It doesn’t matter how good the other properties of the oil are if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system. In this situation, maximum component life will not be achieved. Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables. These include:
starting viscosity at minimum ambient temperature
maximum expected operating temperature, which is influenced by maximum ambient temperature
permissible and optimum viscosity range for the system’s components
Typical minimum permissible and optimum viscosity values for different types of hydraulic components are shown in Figure 1.
If the hydraulic system is required to operate in freezing temperatures in winter and tropical conditions in summer, then it’s likely that multigrade oil will be required to maintain viscosity within permissible limits across a wide operating temperature range. If fluid viscosity can be maintained in the optimum range, typically 25 to 36 centistokes, the overall efficiency of the hydraulic system is maximized (less input power is given up to heat). This means that under certain conditions, the use of a multigrade can reduce the power consumption of the hydraulic system. For mobile hydraulic equipment users this translates to reduced fuel consumption.
There are some concerns when using multigrade fluids in hydraulic systems. The viscosity index (VI) improvers used to make multigrade oils can have a negative effect on the air separation properties of the oil.1 This is not ideal, particularly in mobile hydraulic systems which typically have a relatively small reservoir with correspondingly poor deaeration characteristics. The high shear rates and turbulent flow conditions often present in hydraulic systems destroy the molecular bonds of the VI improvers over time resulting in loss of viscosity. When selecting a high VI or multigrade fluid, it is recommended that the hydraulic component manufacturers’ minimum permissible viscosity values be increased by 30 percent to compensate for VI improver sheardown. This adjustment reduces the maximum permissible operating temperature that would otherwise be allowable with the selected oil and thereby provides a margin of safety for viscosity loss through VI improver shearing.
If the hydraulic system has a narrow operating temperature range and it is possible to maintain optimum fluid viscosity using a monograde oil, it is recommended not to use a multigrade for the reasons stated above.
Detergent or No Detergent DIN 51524; HLP-D fluids are a class of antiwear hydraulic fluids that contain detergents and dispersants. The use of these fluids is approved by most major hydraulic component manufacturers. Detergent oils have the ability to emulsify water, and disperse and suspend other contaminants such as varnish and sludge. This keeps components free from deposits, however, it also means that contaminants do not settle out - they must be filtered out. These can be desirable properties in mobile hydraulic systems, which unlike industrial systems, have little opportunity for the settling and precipitation of contaminants at the reservoir, due to its relatively small volume.
The main concern with these fluids is that they have excellent water emulsifying ability, which means that if present, water is not separated out of the fluid. Water accelerates the aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation. Emulsified water can be turned into steam at highly loaded parts of the system. These problems can be avoided by maintaining water content below the oil’s saturation point at operating temperature.
Antiwear or No Antiwear The purpose of antiwear additives is to maintain lubrication under boundary conditions. The most common antiwear additive used in engine and hydraulic oil is zinc dialkyl dithiophosphate (ZDDP). The presence of ZDDP is not always seen as a positive, due to the fact that it can chemically break down and attack some metals, and reduce filterability. Stabilized ZDDP chemistry has largely overcome these shortcomings, making it an essential additive to the fluid used in any high-pressure, high-performance hydraulic system, such as those fitted with piston pumps and motors. A ZDDP concentration of at least 900 parts per million can be beneficial in mobile applications.
As far as hydraulic oil recommendations go, for commercial reasons relating to warranty, it is wise to follow the equipment manufacturer’s recommendations. However in some applications, the use of a different type of fluid to that originally specified by the equipment manufacturer may increase hydraulic system performance and reliability. Always discuss the application with a technical specialist from your oil supplier and the equipment manufacturer before switching to a different type of fluid.
The advantage which characterise hydraulics in its ability to transmit energy at high efficiency levels, and at the same time to infinitely control this energy have facilitated its application in practically all fields of mechanical engineering. the fact that linear movement can be generated through the use of hydraulic cylinders was also of great significance in this development. One of the major customers for hydraulics is the machine tool industry. Here, hydraulics are applied for clamping workpieces or tools, and for controlling forward speed and spindle drives on metal cutting machine tools. Hydraulic are also used for complete sequence controls, for instance on hydraulic presses. the construction equipment industry also applies hydraulics, to about some extent, for travelling and traversing drives as well as for all working function such as lifting, breaking ground, gripping, shifting etc. The vehicle industry provides a whole range of application : small and compact unit for truck liftgates and for axle-lift installations.Agricultural machinery is another field of particular interest for hydraulics. In the case of farm tractors, it is hydraulic which control the ploughing depth and provide the drive for auxiliary equipment and power assisted steering.The lifting and transport sector, with emphasis on the fork lift truck production, is another of the traditional areas of application for hydraulics, its importance is about the same as that of the agricultural machinery sector. All further uses of hydraulics belong under collective heading in of general mechanical engineering, a term which stands for a multiplicity of demanding and interesting applications.
Excellent Compact, reasonably priced components due to high operating pressures of up to approx. 400 bar. Most simple linear motor-cylinder
Good limitations though, due to max operating pressure of only 6 bar
Accuracy of motion (this can be improved through, in all system by positioning action
excellent because oil can hardly be compressed
Fair to good because air is compressible
efficiency
Fair to good, volumetric and frictional losses during primary and secondary energy conversion, as well as with the open and closed loop control of valves
Controllability
Excellent, by means of valves and variable displacement pumps servo valves for closed- loop techniques
Excellent by means of valves (for small to medium power)
Generation of linear movement
Extremely simple by means of cylinders
Extremely simple by means of cylinders
Signal linkage between hydraulic system and other system
pneumatics operation of directional- control valves
Criterium
Electrics/ electronics
Mechanics
Energy carrier
Electrons
Shafts, linkage, belts, chain, wheels etc.
Energy transmission
Electrically conductive material (cables etc.)
Conversion from or into mechanical energy
Generators,batteries, Electric (E) motors, magnets,solenoids, linear (induction) motor
Most important characteristic quantities
Voltage V, electric current i
Force,torque, velocity, rotational speed
Power efficiency
Fair to good Weight coefficient of E-motors is approx. 10 times that of HY motors. Electric switch, though,has considerable advantages compared to directional control valve
Good because energy conversion is not needed. Limitations become apparent when high demands are placed upon controllability
Accuracy of motion (this can be improved through , in all system by positioning action
Differs considerably on the one hand hysteresis and slippage, on the other hand synchronous motors and stepping motors
Excellent due to positive mechanical connection (index, toothed gearing, etc)
efficiency
Good. As long as electricity is available as primary energy
Good. Due to their being no energy conversion, mechanical frictional losses
Controllability
For small power: excellent,for larger powers fair to good By means of switches,relays ,semiconductors,variable speed motors and variable resistors etc.
Fair to good Through gearing and lever system etc.
Generation of linear movement
Slightly more complicated By means of linear motors
Simple by means of crank mechanism and spindle etc.
Signal linkage between hydraulic system and other system
Electromagnetic control of valves (solenoids,proportional valves),limits switches and pressure switches
Drive. Or drive take- off,on pumps, hydraulic motors, cylinders operation of valves through cams and contours
Hydraulics ( which stems from the Greek word “Hydor” = water ) is, from the scientific view point, the science of liquids at rest and in motion (i.e. hydrostatics and Hydrodynamics). When reference is made in the mechanical engineering, automotive, and aircraft industries to hydraulic. Then one is speaking of the practical application of this branch of physics in the areas of power transmission and closed and open loop control techniques.
In contrast to hydrodynamics converters, turbo clutches and coupling etc, in which the kinetic energy of accelerated fluids is utilized to transmit power, one refers to hydrostatic system when considering the remainder of hydraulics as a whole. Although the laws of hydrodynamics apply here equally as well in connection with motion, flow resistance and the effects upon pressure of changes in the cross-sectional area of a restriction.
COMPARISON OF SYSTEM
Hydraulics stands side by side with mechanics, electrics, electronics, and pneumatics as a mean of transmitting power and as medium for use in open and closed loop controls. In some cases these system are indirect competition with each other. On the other hand, they complement each other in some respects and are often combined. For instance, it is quite normal to apply hydraulics for the energy flow in a system whereas, on the other hand, the signal flow is taken care of electrically.
The correct decision regarding the application of a particular system ( or combination of system ) presupposes a thorough knowledge of the characteristic involved, as well as of the advantages and disadvantages. The following table contain a comparison of the above named systems taking into account a number of different criteria. Considering examples of application, for which all the marginal conditions are defined, a firm decision can usually be reached. In border line cases, cost, operating reliability, service, know-how, and other criteria which are impossible to allow for in overall comparison, are in the facts which finally lead to a decision.
Hydraulics have become absolutely indispensable in the fields of modern-day drive and control techniques. hydraulics is supremely capable of controlling linear and rotating movement, both as regards very large forces, and precision of movement path and speed. The information presented in this blog is orientated to conditions and examples drawn from everyday practice. basic theory though is also deal with. for this reason this blog is ideally suited to booth engineers and technicians who are actually involved in this sphere of works, as well as to those who at present are merely studying it. In addition to a variety of instruction related to applications of engineering, physical principles are described as well as the design of individual pieces of equipment and complete hydraulic system.
This blog is so arranged that orientation is a simple matter. All hydraulic equipment and system are arranged clearly and systematically.In addition, didactic consideration were taken into account in order that private study of the material is made easier. The emphasis of this blog applies to the description of the component and this application,whereby the pictorial and graphic representation is in the forefront.Those subjects, such as ''cartridge valves'' and ''proportional valves'' which have lately been the subject of so much discussion are deal with in supplementary chapters. The application of hydraulics on agricultural tractors has also been allocated a special section. This blog is intended to clearly demonstrate the function, effects and basics of hydraulics. it is to be regarded as both an informative as well as a reference work, and thus as a reliable guide and partner. The fact remains, though, that nothing is perfect and there is nothing which cannot benefit from slight improvements here and there. For this reason, therefore, the technical writers responsible for this blog would welcome any suggestion in this respect.
