Lack of lubrication can be categorised under two main headings. They are:
- Lack of lubrication
- Lack of oil
Lack of lubrication:
Lack of lubrication is where the oil either cannot lubricate the bearings due to the oil being degraded, or when the lubricant cannot reach the bearings due to blockages or loss of oil.
- Oil way pick up points, filters etc. can be blocked by system debris. This will cause bearings to overheat, dry out and cause scoring and seizures.
- Degraded oil is caused by the compressor operating in an overheated condition. Oil in this condition will lose its ability to support bearing loads, and be unable to sufficiently lubricate and cool the bearing surfaces. This will cause bearing and cylinder scoring and bearing, con rod seizures. This can be seen where the bearing surfaces are discoloured by over heating of the rubbing surfaces.
- Oil which has been saturated by liquid migration also will cause scoring and bearing seizures as liquid tends to wash the lubricant out of bearings. This will be seen where the bearing surfaces have softer metallic particles welded to the crankshaft, journals etc.
Loss of oil:
Loss of oil is where the compressor oil has been pumped from the machine and has not returned in sufficient quantity to satisfy the amount its needs to function correctly.
Loss of oil can be easily caused by:
- The Compressor short cycling on its control stat
- Oil traps in the pipe work (incorrect system design)
- Loss of gas charge
- Pipe work design (incorrect pipe work velocity, risers etc.)
- Too much oil in transit around the system
- Insufficient oil charge in the system (long pipe runs)
Too many compressor starts can cause the compressor oil to leave the machine, as on start up the crankcase pressure drops, which causes refrigerant in the oil to “fl ash” which causes the oil to foam and be lifted through the compressor and pumped out with the discharge gas. Unless the system continues to operate for 5-10 minutes oil will be left in the system. When short cycling continues, the oil left in the machine can become critical.
Oil traps can prevent oil from circulating around the system and returning to the compressor. Traps on the evaporator outlet connection and on risers will assist in returning oil if properly designed. Systems where long pipe runs exist must be checked for proper oil control, and may need a proper oil control system fitted to maintain the proper level in the machine at all times. (Fit an Oil separator, Reservoir, and Oil Float unit). An extra oil charge will be required in the system.
Preventative measures required to prevent oil loss:
- Check oil level under all conditions and make sure there is always a level of ½ oil sight glass
- Remove any traps that could cause oil return prevention
- If short cycling is the problem, increase the control stat differential or set up a minimum run time to increase oil return
- Top up oil level during the commissioning stage
- Fit oil separators on all systems that operate below minus 10°C
Typical damage to compressor parts:
- Scored cylinder walls and pistons
- Seized up and broken con rods
- Seized or worn-out main bearings
- Stator to Rotor contact due to worn main bearing
- Heat discolouration to worn and damaged parts
- Low or nil oil quantity in compressor shell
- Metallic particles around the compressor and in the sump
Remember:
Good Clean Liquid free lubricating Oil will give the machine a Long & Reliable life Oil leaving the machine must = oil returning to the machine at all times. Oil is not only a lubricant, but also acts as a coolant to the hot friction bearing surfaces in the machine.
