Ove Isaksson, Ph.D. Department of Machine elements, Luleå University of Technology, Luleå, ove.isaksson@mt.luth.se Roger Tuomas, Ph.D. Student Department of Machine elements, Luleå University of Technology, Luleå, tuomas@mt.luth.se Lubrication of bearings in refrigerating machines ABSTRACT The bearings in a modern refrigeration screw compressor are lubricated with a mixture of oil and refrigerant. However, little or no published bearing life data is available for the new generation non-chlorinated refrigerants. The work presented in this report concerns the development of a measuring technique and experimental equipment for bearing life studies. The equipment is intended to provide data about bearings lubricated with mixtures of oil/refrigerant for use by compressor designers. Bearing life is affected by the working lubricant’s ability to form a film to separate the contact surfaces. To provide a sufficient film thickness in an elastohydrodynamic lubricated (EHL) contact, the lubricant’s viscosity, η , and pressure-viscosity coefficient, α , both play an important role. The film thickness in an EHL contact lubricated with an oil/refrigerant mixture with increasing amounts of refrigerant and different load ratios have been measured experimentally. The lubricant mixture tested consisted of a VG68 polyolester refrigeration oil, Solest 68, with R-134a refrigerant. To measure the film thickness, an on- line capacitance method, SKF´s Lubcheck was used. The amount of refrigerant in the oil was increased until the lubricating film broke down and asperity contact occurred. The refrigerant’s influence on the rheological properties of the oil was studied in a high pressure Höppler viscometer. R-134a, R-32, R-410a and R-22 refrigerants mixed with a VG68 polyolester oil, Mobil 68 Arctic, were investigated. A test procedure has been developed to determine the refrigerant concentration at which metal-to-metal contact occurs. The work showed that film formation in contacts lubricated with oil/refrigerant mixtures is more sensitive to load than other investigations have indicated. The results also showed that run-in behaviour appears in bearings used in refrigeration applications and that the viscosity and pressure-viscosity coefficient decrease with increasing dilution by the refrigerant. resulting in the oil becoming diluted by the INTRODUCTION refrigerant. Bearings in a screw compressor are used to provide The lubricated bearings in a refrigeration screw accurate radial and axial positioning of the rotors. compressor are often assigned to the This makes it possible to design compressors with elastohydrodynamic lubrication (EHL) regime, i.e. small clearances, which reduces leakage and thus the surfaces are separated by a lubricating film and increases the efficiency. Commonly used bearing the lubricated surfaces deform elastically. types are single row angular contact bearings and cylindrical roller bearings; although other types To maintain the thermodynamic efficiency of the such as deep groove ball bearings, four-point refrigeration system, the refrigerant is separated contact ball bearings, needle and taper roller from the oil before it continues through the bearings and nowadays also the CARB bearing refrigeration cycle. However, whilst it is relatively are used. easy to remove oil from the refrigerant, it is far more difficult to remove any remaining refrigerant The lubricant used in a compressor is expected to from the oil. Fore this reason, the oil can become be able to provide a lubricating film thick enough to diluted by the refrigerant in concentrations of up to prevent contact between asperities on the 30-40 %. Wardle et al [1] discovered that mixtures interacting surfaces. The lubricant also transports containing less than 75% oil by weight will not wear particles out of the compressor and helps cool sustain an oil film in rolling element bearings and the machine elements. In the screw compressor, the are therefore unsuitable for lubrication purposes. lubricant also plays a significant role in reducing the temperature of the compressed gas and Refrigerants and refrigeration oils providing a seal between the rotors. Oil is injected into the bearings and between the rotors, which CFC´s (Halogenated hydrocarbons, Freons ), have brings the oil into contact with the refrigerant, been used as refrigerants since the 1930´s. CFC´s
were initially used because they were considered to bearing material. All bearing materials used today are better than those used in 1977 so a 2 ≥ 1, and a 3 be harmless to humans. However, Molina and Rowland [2] showed in1974 that refrigerants that is an operating parameter and is mostly dependant contain chlorine were a key factor in the destruction on the lubrication of the bearing. If the bearing is of ozone in the upper atmosphere. In time, this lead running at normal contamination levels the value of to industry developing more environmentally a 3 is equal to κ (kappa); the ratio between the actual friendly refrigerants, such as hydroflourocarbons, viscosity of the lubricant, ν , and the minimum which did not cause damage to the ozone layer. viscosity, ν 1 , needed to separate the surfaces. The first of these new refrigerants, R-134a, was ν κ = (Eq 3) ν introduced as early as the beginning of 1980. 1 However this refrigerant did not become a The rated life L 10 will decrease linearly with commercial product until 1990 because of problems increasing load in a logarithmic plot. The with miscibility with lubricating oil’s in the adjustments factors, a 1 , a 2 and a 3 do not change the compressor. To help overcome this problem, new slope of this line. A complement to the bearing life oil families were developed such as models was developed by Wuttkowski and polyalphaglycol (PAG), polyolesters (POE) and Ioannides [5] and is shown below. polyalphaolefin (PAO). There is still a considerable amount of development p C = ⋅ (Eq 4) L a a to be undertaken since, these new oils have shown 1 nna SKF P high wear rates on the machine elements in the compressors when diluted with the new generation This new adjustment factor, a SKF considers the of refrigerants. Jacobson [3] demonstrated lubricating film ( κ ), level of contamination ( η c ) and experimentally that roller bearings lubricated with the fatigue load (P u ). For this adjustment factor, a chlorinated R-22 refrigerant have lower wear rates complex relationship of the above mentioned than the new R-134a under the same running parameters are given in SKF’s general bearing ( ) conditions. One explanation is that chlorine in the catalogue [6] as a function of η P P for c u old chlorinated refrigerants, acts as an anti-wear different values of κ and depends on the bearing additive in the bearing. family. Problems associated with high concentrations of BEARING LIFE THEORY refrigerant in the oil have lead SKF to refine their Lundberg and Palmgren [4] developed a method for bearing life theory for bearings operating in a calculating bearing life, which is shown below. refrigeration environment [7]: p C 0 , 72 = (Eq 1) L α ν ⋅ 10 ν α P min adj eral (Eq 5) κ = = ν 3 ν ⋅ 1 1 adj The formula was standardized by ISO in 1962. In In this equation, the actual viscosity is adjusted by the formula, L 10 is the statistic nominal life, in the ratio of the pressure-viscosity coefficients of the million of revolutions, which 90 % of the bearings actual lubricant, α , and a reference value α mineral , for will survive. C is the dynamic load capability and P is the equivalent dynamic bearing load. The pure mineral oil. The pressure-viscosity coefficient describes the viscosity change with pressure. ν 1 is exponent p varies depending on the type of bearing; p = 3 for ball bearings and p = 10/3 for roller adjusted by a factor 3 for HFC refrigerants and a bearings. The theory assumes that the probability of factor of 2 for HCFC (hydrochloroflourocarbons). a given volume element surviving N stress cycles This new value of κ is then used to determine the and then failing, is proportional to its size and is a correction factor a SKF in SKF´s bearing life model. function of its location as well as the number of cycles. RHEOLOGY OF OIL/REFRIGERANT MIXTURES This formula is useful in many applications, but in The chemical composition and viscosity data for a order to include other parameters that affect bearing given oil are seldom available from the oil life, a modified life theory was developed and manufactures. It is even harder to find viscosity and standardized by ISO in 1977: pressure-viscosity coefficient data for different = ⋅ ⋅ ⋅ (Eq 2) L na a a a L types of oil/refrigerant mixtures. Jonsson and Lilje 1 2 3 10 [8] developed an empirical model that can be used where L na is the modified life in millions of to predict the pressure-viscosity coefficients of revolutions, a 1 is used if another reliability than 90 mixtures of polyolesters and R-134a based on the % is required for the bearing, a 2 describes the amount of branched acids in the oil.
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