7.0. COMMON PROBLEMS FOR HYDRAULIC CYLINDERS
7.1. AIR IN OIL
Air in oil problem is found mostly in mobile hydraulics. These include three important effects such as jet cut effect, diesel effect and cavitation.
7.1.1. JET CUT EFFECT
Air can be soluble or insoluble in the media. There is molecular soluble air in all the hydraulic oils. Gas molecules are possibly mixed or bond to oil molecules. Depending on type of media, the rate of soluble air will vary. Oil pressure capability, viscosity and workability characteristics will not to be affected by soluble air.
Insoluble air will cause the oil to behave erratically especially at low pressures (approx 60 bar). For example, as media speed increases, air bubbles will be dispersed over varying distances.
If the pressurised media contains insoluble air, this air is pressurised and will find its way to the seal groove. Subsequently, these air bubbles will be freed when the pressure decreases which lead to the emission of tremendous energy brought about through air expansion. As a result, metal surfaces are affected and surface roughness is increased.
As a result of these explosions if the cracks on the sealing element are on the direction of the cylinder, these small channels cause a nozzle effect. As the speed of oil increases, it makes a jet effect on these nozzles and causes cuts on these parts. Meanwhile, oil molecules pass the seal at a high speed, penetrating behind and wearing the back of it. If there is too much insoluble air in the oil, this expansion might tear the seal out half. This type of damage will commonly occur with rubber fabric products which have been immersed in rubber solution. The reason being, these products contain too much porosity and air transferability other then homogenised rubber seals.
This type of damage can be stopped with increasing the dimensions of the groove at the design stage. The real reason for this damage is escaped pressurised air penetrating behind the seal rather than extrusion. Pressured air bubbles damage
the sealing element when they expand by being absorbed also by homogenised elastomer sealing elements. If these seals are removed for inspection, damage can be found on seals hydraulic sealing lips. The seal volume is expanded and material becomes softer.
Pressure shocks can be with short strokes in the hydraulic systems. Air bubbles in the system are loaded with high temperature energy. At the ideal gas formulation, pressure is to be a positive function of temperature and temperature will increase with pressure increase at the same time. If heated air bubbles are expanded, they start to wear the sealing element and tear pieces from it with high temperature and force. In this area, researches show that the temperature of these air bubbles could be far more than 200°C, even reach to 1000°C. This temperature changes depending on the size of the air bubbles before pressure, pressure, speed and load.
7.1.2. DIESEL EFFECT
Most serious damages in hydraulic cylinders originate from diesel effect explosions of the air in the oil. Speedily pressured air quickly reaches so high temperatures that it causes air and oil mixture to explode with fire. This situation is seen more often in cylinders that work against unsteady loads. While this explosion, pressure will cause an increase 5-6 times more then nominal working pressure. Consequently sealing, guidening elements and metal surfaces are damaged. This damage is observed as local burn and melting on sealing elements and thermoplastic parts.
Because of this reason some extra applications are needed to prevent air in oil in oil tanks, pumps valves and cylinders. Maintenance engineers should be sure about there is not any air left in the system on replacing the cylinder. Otherwise jet and diesel effects would damage the sealing element.
Hydraulic system is on danger when soluble air limit is exceeded on normal pressure. Even sometimes under limit saturation related vacuum might create air from system oil with evaporation and seal can be damaged (pls. see Cavitation part). If any problematic cylinder seals inspection is needed, inspection should be carried out with the seal designer and manufacturer at the same time while removing the seal from cylinder. Because replacing the sealing element would not solve the problem.
Pressured hydraulic media speed increases while transferring through a rod (for example valve). On Bernoulli Formula (Pst+Pdyn=constant),when speed related dynamic pressure increases static pressure drops and it can be continued to create vacuum. After that as a result, soluble air in the oil is evaporated like steam drops. This is called as Cavitation.
These steam drops explode while transferring to pressure point. If this explosion occurs on a sealing element or a metal surface, the force created by the explosion damages the surfaces. This is called jet erosion.
There is small possibility of cavitation in systems working with hydraulic oil, because steam pressure of oil is too low (1.5 - 2.5 Torr.) However, there is a possibility of cavitation with water processed systems, because water steam pressure is 0.3 BAR and the created force is enough to damage even metal surfaces.
Insoluble air in oil is very dangerous for hydraulic systems. How does the air come into being and how can we prevent it?
1. System start-up, assembly or de-assembly can create air. Air has to be removed during a new pump, valve or piston assembly or when repair work is carried out for maintenance. For example, prior to the pumps starting a competent person should take the air out from air valves by turning the rod mil or cam by hand. In the same way, air should be taken out of pistons and hose or hose fittings have to be properly connected.
2. Loose fitting elements can cause air transfer to system. Using poor quality fitting materials is a very common cause. And on the other hand, fitting elements frequently become loosened while they are working under shock and vibrations (like heavy duty machines). Because of this reason they should be subject to periodic checking and if possible, chemical compounds that increase the sealing should be used.
3. Sometimes machine design factors cause air transfer to systems. The majority of machine designers work on minimal tank volume and pump positioning because of space limitations. Oil tank volume should be within the safety limit when oil is used by all the system and whilst the pump is operational. Moreover, the oil return outlet should not be positioned too close to the oil pump inlet or too high and create oil turbulence. Heavy duty seals should be used especially in the manufacturing of heavy machine pistons which work under shock and vibrations. Sealing element extrusion gap (groove) should be bigger and seal supported with guidening rings.
7.2. HYDRODYNAMIC PRESSURE PROBLEM
Hydrodynamic pressure is a common problem of hydraulic cylinders. The shortest definition of Hydrodynamic pressure is that the pressure in the space between the sealing element in hydraulic cylinders and the guidening element causes a type of permanent seal deformation by reaching a value far beyond the system pressure. Prior to the explanation, there is a list of key terminology provided for your convenience.
AB: inlet-outlet ports for hydraulic cylinder
C: Piston head
D: Cylinder cap
F: Guidening element
G: Rod seal
H: Piston Seal
S: Gap between rod and guidening element
In double acting hydraulic cylinder drawing as seen in figure 10, pressured media enters from A port and leads the movement of the rod to the right. At the same time pressured media fills the gap between rod and guidening element “S” and the gap in front of the rod seal. If pressured media is sent to B side and A side is made tank, rod starts to move left along the cylinder stroke length with pressure effect.
Meanwhile if tight tolerances have been applied in “S” gap, it is evident that much of the hydraulic media which remains on the front surface of the seal can not make a tank from the A line. In this application, at every stage the rate of media oil increases and the system starts to work like a pump and creates more pressure then normal. Consequently, the seal and possibly cylinder are damaged.
In figure 11 the rod section in the hydraulic cylinder design has been removed and the chart shows the increase of hydrodynamic pressure (ph). The result of this increase with repeated movement can be calculated with the empirical formula below.
As can be seen in the equation above, hydrodynamic pressure sliding speed is directly related to guidening length, dynamic viscosity of the media and gap between rod and guidening element.
V= Sliding speed (m/sn)
ly= Guidening length (m)
l= Dynamic viscosity (Pa.sn)
S= Gap between rod and guidening (mm)
Some hydrodynamic pressure advice and solutions are given in the paragraphs below. Reducing the three figures which are given in the numerator of the hydrodynamic formula (sliding speed, guidening length and dynamic viscosity) may seem like the solution but generally speaking, due to possible hydraulic design problems, we should not touch these figures. If the rate of the gap between the guidening element and the rod is increased, it can be seen that there will be a decrease in hydrodynamic pressure value proportional to the square root of that increase. As a result of “S” gap being widened, guidening element in the hydraulic cylinder will not function properly
and there will be more serious problems with the effect of perpendicular forces in the hydraulic cylinder.
Some of the parts that are used as guidening elements in hydraulic cylinders are as follows;
1. Phenolic resin (fibres) guides
2. Castled guides
3. Bronze guides
4. Polyacetal (POM) or polyamide (PA) guides
5. PTFE Guides
6. Special Teflon coated metallic rings.
Depending on the type of guidening element, it is evident that the solutions may vary. By opening helical grooves on cast iron or bronze guidening elements that are used in Figure 12, media accumulation that may occur on the front of sealing element can be foiled.
If it is not possible to create helical grooves, as seen in Figure 13, hydrodynamic pressure can be prevented by making a drain hole in rod seal housing.
If tape guides are used, as seen in Figure 14, the most important factor is that there must be a ‘k’ gap on the guidening elements. A gap should be left as shown in figure represented as ‘k’. Hydraulic media which remains on the front of the seal during the process should be returned to the system from this gap.
The minimum recommended ‘k’ gap values are as follows;
Hydrodynamic pressure can cause damage to hydraulic sealing elements and the hydraulic cylinder parts. Therefore, the build up of hydrodynamic pressure should not be permitted in hydraulic cylinder design and applications.