Compressor Lubrication Systems
Compressor oil must reach the bearings, crankshaft, connecting rods, and other moving surfaces that require lubrication while the compressor is operating.
Different compressor designs distribute oil in different ways. Some use simple mechanical splash lubrication, while larger compressors may use an oil pump and internal oil passages to provide positive lubrication.
This lesson explains the basic lubrication systems described in the compressor material and introduces the pressure relationships used to determine net oil pressure.
What You Will Learn
By the end of this lesson you should be able to:
Describe how mechanical parts on the compressor shaft can distribute oil to internal compressor components.
Describe how compressor movement or an oil pump can force oil to bearings and other moving components.
Recognize the relationship between the compressor crankcase, oil-pump inlet, oil-pump outlet, bearings, and internal oil passages.
Recognize where each pressure is measured and how the two pressures are related.
Determine net oil pressure by subtracting suction or crankcase pressure from oil-pump discharge pressure.
Recognize how net oil pressure can be monitored and how excessive oil pressure can be limited.
Oil Must Reach the Moving Parts
In the previous lubrication lesson, we established that compressor oil provides lubrication, cooling, and sealing.
For the oil to perform those functions, it must be distributed to the internal components that need it.
Oil in the Crankcase
The compressor crankcase acts as an oil reservoir in many reciprocating compressor designs.
Oil at the Moving Parts
The lubrication system must move oil from the crankcase to bearings, crankshaft surfaces, connecting rods, and other components requiring lubrication.
Technician Point
Having oil in the compressor is not enough.
The oil must reach the parts that require lubrication while the compressor is operating.
Splash Lubrication
Splash lubrication is a simple mechanical method of distributing oil inside the compressor.
The source describes older compressors as using fingers attached to a shaft. As the shaft rotates, these fingers contact the oil and splash it upward onto the pistons and shaft. :contentReference[oaicite:1]{index=1}
The lower portion of the compressor contains the lubricating oil.
Mechanical fingers or similar components move through the oil.
The moving parts distribute oil onto internal compressor surfaces.
After lubricating the components, the oil drains back toward the oil reservoir.
Simple but Mechanical
Splash lubrication depends on compressor movement to physically distribute oil.
Forced Lubrication
The source also describes forced lubrication systems.
For smaller compressors of approximately 3 horsepower and below, the source states that crankshaft and piston movement can be used to force oil toward the bearings. It also notes that some systems combine this action with fingers on the shaft. :contentReference[oaicite:2]{index=2}
Mechanical Oil Movement
Compressor movement can create the force needed to distribute oil through the compressor.
Combination Systems
The source notes that some compressors combine forced lubrication with mechanical splash components.
Oil-Pump Lubrication
For larger compressor systems, the source describes an oil pump mounted at the rear bearing housing.
The pump draws oil from the compressor crankcase and forces it through machined internal passages to the compressor bearings and connecting rods. :contentReference[oaicite:3]{index=3}
Crankcase
Oil is stored in the compressor crankcase.
Oil Pump
The pump receives oil from the crankcase and increases its pressure.
Oil Passages
Pressurized oil moves through machined passages in the compressor.
Bearings and Connecting Rods
Oil reaches the internal surfaces requiring lubrication.
Technician Point
An oil pump does not simply indicate that oil exists in the compressor.
Its purpose is to create the pressure needed to move oil through the compressor lubrication passages.
Two Pressures Must Be Understood
When an oil pump is used, the technician must distinguish between the pressure entering the pump and the pressure leaving the pump.
Oil-Pump Intake Pressure
The source states that the oil-pump intake pressure is equal to compressor crankcase pressure.
Because the crankcase is on the low side of the compressor, this pressure corresponds to the low-side or suction pressure.
Oil-Pump Outlet Pressure
The oil pump increases the pressure of the oil above crankcase pressure.
The source describes the oil-pump outlet pressure as the combination of crankcase pressure plus the pressure developed by the oil pump.
The source directly identifies oil-pump intake pressure as equal to crankcase pressure and describes pump outlet pressure as the sum of crankcase pressure and pump pressure. :contentReference[oaicite:4]{index=4}
Net Oil Pressure
The important lubrication pressure is not simply the pressure measured at the outlet of the oil pump.
The compressor crankcase is already under refrigerant pressure. That existing pressure must be subtracted from the pump discharge pressure to determine how much pressure the oil pump is actually producing.
NET OIL PRESSURE
This formula is stated directly in the source material. :contentReference[oaicite:5]{index=5}
Do Not Confuse Total Oil Pressure With Net Oil Pressure
The pump outlet gauge may show a substantial pressure, but part of that pressure already existed in the compressor crankcase.
Net oil pressure tells you how much additional pressure the oil pump is actually producing.
Calculating Net Oil Pressure
The following is an instructional example using the formula from the source lesson.
70 PSIG
35 PSIG
70 − 35 = 35 PSI
What the Result Means
In this example, the oil pump is producing 35 PSI of pressure above the pressure already present in the compressor crankcase.
Net Oil Pressure Range in the Source Lesson
The compressor presentation states that net oil pressure will vary and gives a normal range of approximately 30–40 PSIG for the systems being discussed. :contentReference[oaicite:6]{index=6}
SOURCE LESSON REFERENCE
30–40 PSIG NET OIL PRESSURE
This range comes from the compressor material being studied and should not be treated as a universal specification for every compressor design.
Equipment Specifications Come First
Actual acceptable oil pressure depends on the compressor and manufacturer. The values in this lesson should be understood as the reference values given in the source presentation.
Oil-Pressure Controls
The source states that the oil-pressure switch operates from net oil pressure. :contentReference[oaicite:7]{index=7}
This is important because the control must determine whether the oil pump is actually creating enough pressure to circulate lubricant through the compressor.
Measure Oil-Pump Discharge Pressure
This is the pressure leaving the oil pump.
Measure Crankcase Pressure
This represents the pressure already acting on the oil entering the pump.
Determine the Difference
The difference between the two pressures is the net oil pressure.
Monitor Lubrication
The oil-pressure control uses this pressure difference to determine whether adequate oil pressure exists.
Oil-Pressure Bypass Valve
The source states that a bypass valve may be added where oil pressure becomes too high.
Its purpose is to keep oil pressure below approximately 60 PSIG in the systems described by the lesson.
The presentation also states that this bypass valve is not adjustable. :contentReference[oaicite:8]{index=8}
Normal Oil Pressure
The oil pump supplies the pressure required to circulate lubricant through the compressor.
Excessive Oil Pressure
A bypass valve can limit excessive pump pressure by allowing oil to bypass when pressure becomes too high.
Source Lesson Reference
Normal net oil pressure: approximately 30–40 PSIG
Bypass control point: below approximately 60 PSIG
Put the Oil Pressures Together
Oil is stored in the compressor crankcase.
The oil-pump inlet is exposed to compressor crankcase pressure.
The oil pump raises oil pressure above crankcase pressure.
Oil-pump discharge pressure includes both crankcase pressure and the additional pressure produced by the pump.
Net oil pressure is found by subtracting crankcase pressure from oil-pump discharge pressure.
The oil-pressure control responds to net oil pressure rather than simply to total oil-pump discharge pressure.
Compare the Lubrication Methods
How Oil Is Moved
Source Lesson Application
Mechanical fingers or moving components splash oil onto compressor parts.
Described as being used in older compressors.
Crankshaft and piston movement force oil toward bearings and other components.
Described for smaller compressors around 3 HP and below.
An oil pump draws oil from the crankcase and forces it through internal oil passages.
Described for larger compressors around 3 HP and above.
Can You Explain Compressor Lubrication Systems?
You should be able to answer these questions before continuing.
1. How does splash lubrication distribute oil inside a compressor?
2. What components does the source describe as being used to splash oil?
3. How is forced lubrication different from splash lubrication?
4. Where does an oil pump obtain its oil?
5. Where does the oil pump send the pressurized oil?
6. What pressure is present at the oil-pump inlet?
7. What does oil-pump discharge pressure contain?
8. What is the formula for net oil pressure?
9. Why must crankcase pressure be subtracted from oil-pump discharge pressure?
10. What net oil-pressure range is given in the source lesson?
11. What pressure does the oil-pressure switch respond to?
12. What is the purpose of the oil-pressure bypass valve described in the source lesson?
What You Should Have Learned
Splash lubrication distributes oil mechanically as compressor components move through the oil.
Forced lubrication moves oil toward compressor bearings and other internal components.
Larger compressor systems may use an oil pump to force lubricant through machined internal passages.
Oil-pump intake pressure is equal to compressor crankcase pressure in the source system.
Oil-pump discharge pressure contains crankcase pressure plus the additional pressure created by the oil pump.
Net oil pressure equals oil-pump discharge pressure minus suction or crankcase pressure.
The oil-pressure control operates from net oil pressure.
The source lesson gives approximately 30–40 PSIG as a normal net oil-pressure range for the compressors being discussed.
A bypass valve may be used to prevent excessive oil pressure.
Oil Pressure Is a Pressure Difference
The most important pressure concept in this lesson is that oil-pump discharge pressure by itself does not tell you how much lubrication pressure the pump is actually producing.
You must compare it with crankcase pressure.
Oil Pump Discharge Pressure − Crankcase Pressure = Net Oil Pressure
Understanding that relationship is essential when evaluating compressor lubrication and oil-pressure controls.