SEAL AND SEAL PROBLEMS IN HYDRAULIC CYLINDERS
SEAL AND SEAL PROBLEMS IN HYDRAULIC CYLINDERS
SEAL AND SEAL PROBLEMS IN HYDRAULIC CYLINDERS
AIR IN OIL PROBLEM
Air in oil problem is very common especially in mobile hydraulics. The presence of air in the oil has three important effects. These; Jet cutting effect, Diesel effect and Cavitation.
JET CUT EFFECT
Air is present in the oil in dissolved or undissolved form. Molecularly dissolved air is present in all hydraulic oils. Gas molecules are either mixed with or attached to oil molecules. Depending on the type of fluid, the amount of air it can dissolve in is variable. Such dissolved air does not adversely affect the oil's compressibility, viscosity or sealant effectiveness.
The undissolved air in the oil causes the fluid to behave very differently, especially at low pressures (about 60 bar). For example, if the velocity of the fluid increases, the air in it is carried away in the form of bubbles.
If the pressurized fluid contains undissolved air, this air is trapped and finds its way to the seal housing. Then, when the pressure drops here, the compressed bubbles are released and expand with enormous energy. Not only the sealing element, but also the metal surfaces of the piston are adversely affected by this, and the surface roughness increases.
If the scratches on the sealing element as a result of these explosions are longitudinal, these capillary channels act as a nozzle. While the fluid is accelerating, it creates a jet effect in these nozzles and opens cuts in these areas. Meanwhile, the fluid particles pass through the gap rapidly, reach the back of the sealing element and wear the back surface of the sealing element. If there is a large amount of undissolved air in the fluid, this expansion can split the seal into two parts. This type of damage mostly occurs in sealing elements made of rubber-impregnated cloth. The reason for this is that its structure is more porous than a homogeneous rubber sealing element and its air permeability is higher.
This damage can be avoided by increasing the yield gap at the design stage. Because here it is not the flow that wears out the sealing element, but the compressed air escaping behind the sealing element. Compressed air bubbles also penetrate homogeneous elastomer sealing elements and wear out the sealing element when expanded. When this sealing is removed, it is generally seen that the wear is on the dynamic sealing lip surface of the sealing element. The volume of the sealing element has expanded and its material has softened.
In hydraulic systems, pressure shocks can also occur in short strokes and the air bubbles in the system are loaded with very high heat energy. In the ideal gas equation, pressure and temperature are directly proportional, and when the pressure increases, the temperature increases. When the heat-loaded air particles expand, they melt the sealing element surface with high temperature and tension force and break off pieces from it. Studies have shown that the temperature of these air bubbles is much higher than 200°C, and can even reach 1000°C. This temperature changes depending on the size of the air bubble before compression, pressure, speed and load.
DIESEL EFFECT
The most serious damages in hydraulic cylinders are caused by the explosion of the diesel effect of the air in the oil. The rapidly compressed air suddenly reaches such a high temperature that it causes the air-oil mixture in the environment to burn and explode. This is more common in cylinders operating against variable loads. During this explosion, the pressure in the explosion area causes an increase of 5 to 6 times the nominal working pressure. This causes damage to the bearing materials and metal surfaces, especially the sealing element. Damage to the sealing element and thermoplastic parts is seen as local burning and melting.
In conclusion, considering the damages caused by diesel impact, it is understood that controlling the amount of air in the oil is very important. For this reason, precautions should be taken to prevent air from entering the oil tank, pump, valves and cylinders. When a cylinder is being replaced or newly commissioned, it must be ensured that there is no air in it. Otherwise, the jet effect and diesel effect will damage the sealing element.
The system is in danger as soon as the air saturation point of the oil is exceeded at normal pressure. Even below the saturation point, the vacuum that will form in the system can separate the air from the oil by condensing and damage the sealing element (see Cation). When removing a damaged seal in a problem cylinder, it must be inspected with the piston's designer and seal manufacturer. Because the sealing element is not replaced with the new one.
CAVITATION IN HYDRAULIC SYSTEM
As a pressurized fluid passes through a throat, for example a valve, the fluid velocity increases. According to Bernoulli's equation, (Pst+Pdyn = constant) when dynamic pressure increases due to velocity, the decrease in static pressure may continue until a vacuum is formed. The result is to release the saturated air in the oil as vapor droplets. This phenomenon is called cation. When these vapor droplets pass through the throat and enter the pressure area, they explode. If this explosion happens on the seal or on a metal surface, the large forces from the explosion will degrade their surfaces. This condition is called jet erosion.
In systems working with hydraulic oil, the possibility of cavitation is very low, because the vapor pressure of the oil is very low (1.5-2.5 torr). It can be enough to abrade even surfaces.
CONCLUSION
The presence of undissolved air in the oil is a great danger to hydraulic systems. Why is there air in oil? How can we prevent it?
1. Air is generated in the system during commissioning, disassembly and assembly. When a pump, valve or piston is newly connected to the system, or when it is disassembled for malfunction or maintenance, air must be removed. For example; The pumps should be operated after the air is removed from the air vent plugs by manually turning from the motor shaft or the pulley, because the air should be discharged from the pistons and the pipe or hose connections should be made properly.
2. Loose fittings cause air to enter the system. The use of poor quality fasteners is an important factor. In addition, in systems with shock loads and vibrations (for example, construction machines), fasteners often loosen. For this reason, frequent checks should be made, and if possible, chemical combiners that increase sealing should be used in the connections of such machines.
3. Design factors may cause air to enter the oil. Many machine designers work at minimum dimensions in hydraulic tank volumes and pump placement due to space problems. The volume of the oil tank, all users max.
When using oil, it should provide the required safe oil level for the pump's suction. In addition, oil return to the tank should not be from high, fast and in a way that will shake the oil, and it should not be done close to the pump suction. Heavy-duty sealing elements should be used, especially in the manufacture of certain pistons that will be exposed to shock impacts and vibration, such as construction equipment, and the leakage space of the sealing element should be left excessive, and the sealing element should be supported with a bearing ring.
HYDRODYNAMIC PRESSURE PROBLEM
One of the problems we frequently encounter in hydraulic cylinders is hydrodynamic pressure. The shortest definition of hydrodynamic pressure; It is when the pressure in the space between the sealing elements in the hydraulic cylinders and the bearing element reaches a value far above the system pressure, causing permanent deformation of the sealing element. Before we describe the formation of hydrodynamic pressure, we would like to define some terms that we will use.
Some of the solution suggestions related to hydrodynamic pressure suggestions are given below. Although reducing the numerator values (sliding speed, bearing length, dynamic viscosity) given in the hydrodynamic pressure formula seems to be a solution, these values should not be altered mostly due to hydraulic system design. If the amount of space between the bearing element in the denominator and the rod is enlarged, it is seen that there will be a decrease in the hydrodynamic pressure value in direct proportion to the square of it. As a result of enlarging the “S” space, it will not be able to act as a bearing element in the hydraulic cylinder and much more serious problems may occur with the effect of vertical forces on the axis in the hydraulic cylinder.
Parts used as bearing elements in hydraulic cylinders are given below;
1. Phenol resin (Fiber) bearings
2. Peak bearings
3. Bronze bearings
4. Polyacetal (POM) or Polyamide (PA) bearings 5. Teflon added bearings (Bronze, carbon etc.)
6. Special Teflon added metallic rings
It is obvious that the solutions differ depending on the type of bearing element. Fluid accumulation that may occur on the front surface of the sealing element can be prevented by opening helical channels on the cast iron or bronze bearing element used in Figure 12.
CONCLUSION
Hydrodynamic pressure may cause the hydraulic cylinder parts to become unusable together with the hydraulic sealing element. Therefore, hydrodynamic pressure formation should not be allowed in the design and application of hydraulic cylinders.
SEALING ELEMENTS STORAGE CONDITIONS
Environment, Humidity and Temperature
Rubber, plastic and polyurethane products should be stored in a cool and dry place. Storage temperature should be around 15°C and should not exceed 25°C; relative humidity should be less than 65%. It is necessary to prevent the contact of the sealing elements with air while being packed.
Light and Ultraviolet Rays
It is important to protect products against direct sunlight and strong artificial light with high UV levels.
Oxygen and Ozone
No ozone-generating devices such as electric motors or high-voltage devices should be located in the storage area.
Deformation
Rubber products should not be subjected to tension, pressure or bending. Placing the product flat to avoid crushing protects it from stretching and reduces the possibility of deformation.
Contact with Oils and Hydraulic Fluids
Contact with liquids and semi-solid materials, especially solvents such as oils or greases, should be avoided.
Contact with Metals
Metals such as manganese, iron and copper or copper alloys can damage rubber. Therefore, it is also necessary to avoid contact with dangerous metals or chemicals.