In a screw air compressor, the oil cut-off valve is an unassuming component. It controls the delivery rhythm of lubricating oil, and once it gets stuck, it often directly causes overheating failures. Many users wonder: how does an “oil control valve” relate to “temperature”? The core of the problem lies in this: a stuck oil cut-off valve cuts off or reduces the supply of lubricating oil. However, the lubricating oil of an air compressor is not only a “lubricant” but also a “coolant”. When both functions fail, overheating will inevitably occur.
I. Core Functions of the Oil Cut-off Valve
The lubricating oil of a screw air compressor serves two core purposes:
1. Lubrication and Friction Reduction
It forms an oil film on the surfaces of high-speed rotating components such as male and female rotors and bearings, preventing direct metal-to-metal friction.
2. Cooling and Temperature Control
A large amount of heat is generated during air compression (known as “adiabatic temperature rise” in the compression process). The lubricating oil absorbs this heat, then cools down through the oil cooler, maintaining the air compressor at a normal operating temperature of 60-100°C.
The oil cut-off valve acts as the “flow switch” for lubricating oil:
- When the compressor starts, it opens under pressure or solenoid valve control, delivering lubricating oil from the oil separator tank to the main unit (rotors and bearings) precisely.
- When the compressor stops, it closes to prevent lubricating oil from flowing back into the main unit or air inlet.
In simple terms, the oil cut-off valve is the only passage for lubricating oil delivery. If this passage is blocked, oil cannot reach the main unit.
II. A Stuck Oil Cut-off Valve: 3 Stages Leading to Overheating
An oil cut-off valve usually gets stuck in a “closed” or “semi-closed” state (caused by oil dirt/impurities sticking to the valve core, or spring aging preventing full opening). When this happens, the supply of lubricating oil is insufficient or cut off, leading to overheating through 3 key stages:
1. Stage 1: Lubrication Failure Causes Frictional Heat
Under normal conditions, rotors and bearings are separated by an oil film, resulting in low friction resistance and minimal heat generation. When the oil cut-off valve is stuck, the oil supply becomes insufficient, the oil film thins or even disappears, and direct metal friction occurs (e.g., between rotors, between rotors and the casing, and between bearing balls and inner/outer rings).
- For male and female rotors, which operate at high speeds (often 3,000-6,000 rpm), friction without an oil film can instantly raise local temperatures to over 200°C.
- When bearings lack oil, rolling friction turns into sliding friction, doubling heat generation. This quickly causes bearing overheating, which then spreads to the entire main unit.
2. Stage 2: Cooling Interruption Triggers “Heat Accumulation and Surge”
Air compression is a heat-generating process. For example, when atmospheric air is compressed to 0.8MPa, its temperature rises by 80-120°C. All this heat is carried away by lubricating oil: the oil absorbs the compression heat as it flows through the main unit, then cools down in the oil cooler (air-cooled or water-cooled) before cycling back to the main unit.When the oil cut-off valve is stuck, the circulation volume of lubricating oil decreases, and heat cannot be removed in time—causing compression heat to “accumulate” in the main unit:
- Heat that would normally be carried away by oil now only dissipates through the main unit’s casing, reducing efficiency by over 90%.
- Temperatures rise like a snowball: when the main unit’s temperature exceeds 100°C, the oil cooler lacks sufficient oil flow to dissipate heat. Temperatures continue to rise to 120°C or 150°C, eventually triggering the air compressor’s overheating protection and shutdown.
3. Stage 3: Local Dry Friction Causes Abnormal Overheating and Burnout
If the oil cut-off valve is completely stuck in the closed state, total oil cutoff occurs:
- Without an oil film on the rotor surfaces, high-speed rotation causes dry friction marks. Local temperatures can exceed 300°C, leading to rotor coating wear and metal deformation.
- Oil-starved bearings burn out quickly, with balls adhering to inner/outer rings. This not only produces loud abnormal noises but also disrupts the main unit’s rotation speed, further intensifying heat generation.
III. How to Determine if Overheating Is Caused by a Stuck Oil Cut-off Valve?
In addition to the thermometer showing overheating (usually triggering an alarm when exceeding 110°C), 3 typical signs can help confirm the cause:
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Sudden OverheatingTemperatures were normal before, but then rise rapidly (e.g., from 80°C to 120°C within 10 minutes) with no gradual process.
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Overheating Soon After StartupThe oil cut-off valve fails to open when the compressor starts, so lubricating oil does not reach the main unit. Overheating protection is triggered within a few minutes of startup.
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Traces Found During DisassemblyWhen the oil cut-off valve is disassembled, oil dirt or sludge is found sticking to the valve core, or the spring is deformed. During main unit inspection, no obvious oil film is seen on the rotors—even dry friction scratches may be present.
IV. Never Force Operation When Overheating Occurs Due to a Stuck Oil Cut-off Valve
Many users try restarting the compressor after noticing overheating. However, if the stuck oil cut-off valve is not fixed, this will lead to:
- Short-term impact: Overheating protection triggers shutdown, disrupting production.
- Long-term impact: Rotor wear and bearing ablation occur, with main unit overhaul costs reaching tens of thousands of yuan (accounting for 30%-50% of the air compressor’s total price).
- Extreme cases: High temperatures cause lubricating oil to carbonize, forming sludge that clogs oil passages. This triggers “chain failures” and may even result in total machine scrapping.
V. 3 Steps to Prevent Overheating from a Stuck Oil Cut-off Valve
1. Regularly Clean/Replace the Oil Cut-off Valve
During maintenance every 8,000-10,000 operating hours, disassemble the oil cut-off valve to clean oil dirt from the valve core, and blow through the oil passages with compressed air. If the valve core is worn or the spring is aged, replace the valve directly (the oil cut-off valve is low-cost, so repair is unnecessary).
2. Use Qualified Lubricating Oil
Inferior oil oxidizes easily, producing sludge that sticks to the valve core. Always use air compressor-specific oil (e.g., Kaishan’s dedicated screw air compressor oil) and replace it according to the recommended schedule.
3. Address Overheating Alarms Promptly
Once overheating occurs, first shut down the compressor and inspect the oil cut-off valve (prioritize this check), then inspect the oil cooler and temperature control valve. Do not conduct blind disassembly and repair of the main unit.
A stuck oil cut-off valve causes overheating essentially due to the simultaneous failure of the lubricating oil’s dual functions (lubrication and cooling). This is not an isolated oil control issue, but a temperature crisis with chain reactions.
Remember: The “oil” and “temperature” of a screw air compressor are always linked. If the oil cut-off valve—this “oil switch”—malfunctions, overheating will definitely follow. Timely inspection and troubleshooting are the keys to avoiding major failures.
Post time: Oct-29-2025
