Lack of oil
Lack of oil is one of the compressor faults that are easy to identify. When the air compressor is out of oil, the amount of oil in the crankcase is small or even no lubricating oil. The air compressor is a special air pump. When a large amount of refrigerant gas is discharged, it also entrains a small part of lubricating oil (called running oil or running oil). It is unavoidable to run oil in an air compressor, but the speed of oil run is different. The operation time of the air compressor without oil return is more than ten minutes. If the lubricating oil discharged from the air compressor does not come back, the compressor will run out of oil. Generally speaking, there are two ways for air compressor to return oil, one is to return oil from the oil separator, and the other is to return oil from the air return pipe.When there is an internal oil return problem, it can be observed that the oil level of the compressor continues to drop until the oil pressure safety device operates. After the compressor was stopped, the oil level in the crankcase quickly recovered. The root cause of the internal oil return problem is cylinder leakage, and the worn piston assembly should be replaced in time.
The oil pressure safety device will automatically stop when there is no oil to protect the compressor from damage. Hermetic compressors (including rotors and scroll compressors) and air-cooled compressors without sight glass and oil pressure safety devices, there are no obvious symptoms when there is oil shortage, and will not stop, the air compressor is unknowingly Wear damage. Air compressor noise, vibration or excessive current may be related to lack of oil. Accurate judgment of air compressor and system operating conditions is very important. Too low ambient temperature may cause failure of some oil pressure safety devices, which will cause compressor wear.
The abrasion caused by lack of oil in air compressors is generally more uniform. If there is little or no lubricating oil, there will be violent friction on the bearing surface and the temperature will rise rapidly within a few seconds. If the power of the motor is large enough, the crankshaft will continue to rotate, and the surface of the crankshaft and the bearing will be worn or scratched, otherwise the crankshaft will be locked by the bearing and stop rotating. The reciprocating movement of the piston in the cylinder is the same. Lack of oil will cause wear or scratches. In severe cases, the piston will be stuck in the cylinder and cannot move.
The direct cause of wear is insufficient lubrication. Lack of oil will definitely cause insufficient lubrication, but insufficient lubrication is not necessarily caused by lack of oil. Insufficient lubrication can also be caused by the following three reasons: the lubricating oil cannot reach the bearing surface; the lubricating oil has reached the bearing surface, but the viscosity is too small to form an oil film of sufficient thickness; the lubricating oil has reached the bearing surface, but it decomposes due to overheating If it falls, it will not be able to provide lubrication.
Blockage of oil suction network or oil supply pipeline, oil pump failure, etc. will affect the delivery of lubricating oil, and the lubricating oil cannot reach the friction surface far away from the oil pump. The oil suction net and the oil pump are normal, but bearing wear, excessive clearance, etc. cause oil leakage and low oil pressure. The friction surface away from the oil pump will not get lubricating oil, causing wear and scratches.
Liquid flood back is a common system problem, and a major hazard of liquid flood back lies in diluting the lubricating oil. After the diluted lubricating oil reaches the friction surface, the viscosity is low and a protective oil film of sufficient thickness cannot be formed, which will cause wear over time. When the amount of liquid return is relatively large, the lubricating oil will be very thin, not only can not play a lubricating effect, but also dissolves and washes the original oil film, causing the refrigerant to wash out.
Due to various reasons (including the compressor start-up phase), the friction surface temperature that does not get lubricating oil will rise rapidly, and the lubricating oil will begin to decompose after exceeding 175°C. "Insufficient lubrication-friction-high surface temperature-oil decomposition" is a typical vicious circle. Many vicious accidents, including connecting rods and piston jams, are related to this vicious circle.
Insufficient lubrication and oil shortage can be seen in the disassembled air compressor. Oil shortage generally manifests as large area, relatively uniform surface damage and high temperature, while insufficient lubrication is more abrasion, scratches and high temperature in some specific parts, such as the bearing surface away from the oil pump.
When the piston moves up and down, the load of the piston pin is alternated between the upper and lower parts of the bearing surface, which allows the lubricating oil to be evenly brushed across the piston pin and provides sufficient lubrication. If the exhaust valve plate is bent or broken, or the compressor is compared with the high pressure for a long time, it will cause insufficient lubrication and wear on one side of the piston pin, and the pores will be large. If the piston pin has a sloshing gap, the piston will be thrown out at the top dead center and hit the valve plate and the valve plate, resulting in an impact sound. Therefore, when replacing the valve, check the wear of the piston pin.
conclusion and suggestion
Lack of oil will cause serious lack of lubrication. The root cause of the lack of oil is not the amount and slowness of the air compressor, but the poor oil return of the system. The installation of an oil separator can quickly return the oil and extend the air compressor's non-return operation period. The design of the evaporator and the return gas pipeline should take the oil return into consideration. Maintenance measures such as avoiding frequent starting, timing defrosting, timely replenishment of refrigerant, and timely replacement of worn piston components also help to return oil.
Liquid return and refrigerant migration will dilute the lubricating oil, which is not conducive to the formation of oil film; the failure of the oil pump and the blockage of the oil circuit will affect the oil supply and oil pressure, resulting in lack of oil on the friction surface; the high temperature of the friction surface will promote the decomposition of the lubricant and make the lubricant oil Loss of lubrication ability. Insufficient lubrication caused by these three problems often causes damage to the air compressor.
The root cause of the lack of oil lies in the system. Therefore, only replacing the air compressor or some accessories cannot fundamentally solve the problem of oil shortage
The above is the editor's personal views on the lack of oil and insufficient lubrication of the air compressor. I hope it will be helpful to you when choosing a screw air compressor.