Text Common hydraulic failures(questions and answers) · Can you mention any common hydraulic failures? Are you familiar with them? · Have you ever fixed a car? What are your impressions? Think a little and prepare a short story about your experience or about your friend or a relative. Words and phrases:
сontamination загрязнение; механические примеси clearance (радиальный) зазор; просвет; промежуток abrasion абразия; стирание, износ, шлифовка scoring количественная оценка, задирание to slide скользить, axial осевой, продольный cylinder barrel блок цилиндров valve plate пластина клапана, клапанная доска lubricating film смазочная плёнка rod seal уплотнение штока (цилиндра), штабиковый спай to exceed превосходить aeration аэрация, насыщение кислородом to convert превращать, изменять to dissipate рассеивать viscosity вязкость, коэффициент вязкости inherent собственный, присущий, свойственный capacity вместимость, основная характеристика umbilical занимающий центральное положение, округленный
· Translate the forms of the verbs in bold type say what functions do they play in the sentences of the text given below: air is drawn, it explodes when pressurized, the volume of fluid being demanded by the rod side of the cylinder, most rod seals are designed keep high-pressure fluid in, it can be expressed as, to achieve stable fluid temperature, an efficiency of 80% needs to be capable of dissipating a heat load, there are two ways to solve overheating problems, decreasing this heat load to an acceptable level, by replacing the pressure, the most economical solution was to install additional cooling capacity.
Common hydraulic failures ü How does contamination cause this type of damage to a hydraulic pump? When hydraulic fluid is contaminated with hard particles that are the same size as the clearance between two lubricated surfaces, a process known as three-body abrasion occurs. Three-body abrasion results in scoring and heavy wear of sliding surfaces.
ü What other explanations are there for this type of damage? In axial piston designs, the cylinder barrel is hydrostatically loaded against the valve plate. The higher the operating pressure, the higher the hydrostatic force holding the cylinder barrel and valve plate in contact. However, if operating pressure exceeds design limits or if the valve plate is not in proper contact with the cylinder barrel, the cylinder barrel separates from the valve plate. Once separation occurs, the lubricating film is lost, the two surfaces come into contact and a process known as two-body abrasion occurs.
ü What is the 'diesel effect'? The diesel effect occurs in a hydraulic cylinder when air is drawn past the rod seals, mixes with the hydraulic fluid and explodes when pressurized.
ü How does this affect a hydraulic cylinder? When a double-acting hydraulic cylinder retracts under the weight of its load, the volume of fluid being demanded by the rod side of the cylinder can exceed the volume of fluid being supplied by the pump. When this happens, a negative pressure develops in the rod side of the hydraulic cylinder, which usually results in air being drawn into the cylinder past its rod seals. This occurs because most rod seals are designed keep high-pressure fluid in and are not designed to keep air out. The result of this is aeration – the mixing of air with the hydraulic fluid. Aeration causes damage through loss of lubrication and overheating, and when a mixture of air and oil is compressed it can explode, damaging the hydraulic cylinder and burning its seals.
ü Why do hydraulic systems overheat? Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat. A hydraulic system's heat load is equal to the total power lost (PL) through inefficiencies and can be expressed as: PLtotal = PLpump + PLvalves + PLplumbing + PLactuators If the total input power lost to heat is greater than the heat dissipated, the hydraulic system will eventually overheat.
ü Hydraulic fluid temperature – how hot is 'too hot'? Hydraulic fluid temperatures above 180°F (82°C) damage most seal compounds and accelerate degradation of the oil. While the operation of any hydraulic system at temperatures above 180°F should be avoided, fluid temperature is too high when viscosity falls below the optimum value for the hydraulic system's components. This can occur well below 180°F, depending on the fluid's viscosity grade.
ü Maintaining stable hydraulic fluid temperature To achieve stable fluid temperature, a hydraulic system's capacity to dissipate heat must exceed its inherent heat load. For example, a system with continuous input power of 100 kW and an efficiency of 80% needs to be capable of dissipating a heat load of at least 20 kW. It's important to note that an increase in heat load or a reduction in a hydraulic system's capacity to dissipate heat will alter the balance between heat load and dissipation.
ü Beat the heat There are two ways to solve overheating problems in hydraulic systems: decrease heat load; or increase heat dissipation. Decreasing heat load is always the preferred option because it increases the efficiency of the hydraulic system. For example, the heat load of the umbilical alone is nearly 30% of available input power, a figure that would normally be considered unacceptable. Decreasing this heat load to an acceptable level would have involved reducing the pressure drop, by replacing the pressure and return lines in the umbilical with larger diameter hoses. The cost of doing this for what was a temporary installation meant that, in this case, the most economical solution was to install additional cooling capacity in the circuit. (from http://www.hydraulicsupermarket.com/technical.html)
· Answer the following questions in your own words: 1. What are the typical hydraulic failures? 2. What happens if the cylinder barrel separates from the valve plate? 3. What aeration is? 4. How can you prevent hydraulic systems from overheating? 5. Why hot hydraulic fluids cause damage? 6. Is it necessary to maintain stable hydraulic fluid temperature? Why?
· Look at the constructions given below. What is peculiar about these constructions? Read the sentences which may give you a hint:
I have just had my car washed. I washed my car two hours ago.
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