Reasons and necessity for major overhauls of the main unit of oil-injected twin-screw air compressors
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Reasons and necessity for major overhauls of the main unit of oil-injected twin-screw air compressors. The structure of the main unit of an oil-injected twin-screw air compressor (also known as the compressor head) typically consists of a pair of intermeshing screws, a main unit housing, high-pressure end caps and low-pressure end caps. When a screw air compressor is operating normally, there is no contact between the screws, nor between the screws and the compressor housing, or between the screws and the high/low pressure ends. This is ensured by three main conditions: 1. The screws, the compressor housing, and the high/low pressure ends are all manufactured with high precision. This precision ensures that, after the host is assembled, there are appropriate gaps between the screws as well as between the screws and the host housing. The size of these gaps takes into account the deformation that may occur in the screws and the housing due to the high temperatures encountered during operation ； 2. The gap size between the screw and the high/low pressure end faces is ensured during screw assembly in accordance with technical requirements. During operation of the host, the screw does not experience any axial movement, while the allowable radial displacement of the screw is ensured by high-precision bearings ； 3. When the machine is operating normally, the oil film formed by the lubricating oil prevents direct contact between the screws. However, as the air compressor operates for an extended period of time, the bearings in the main unit will inevitably wear out, which leads to axial movement and increased radial displacement of the screws. This change results in alterations in the gaps between the screws, as well as between the screws and the main unit’s housing and its front and rear end surfaces. These variations in clearance are normal and acceptable throughout the bearing’s service life, and the resulting decline in the air compressor’s output volume and increase in the load on the main motor are also normal and acceptable. As the unit’s operating time approaches the scheduled major overhaul of the main engine, the lifespan of the engine bearings gradually approaches the maximum allowable limit. At this point, the axial and radial displacements of the screw gradually approach the maximum design allowable values; this change causes significant variations in the gaps between the screws, as well as between the screw and the main housing and its front and rear end faces. At this point, although the host is still safe to operate, it is time to consider planning a major overhaul of it. Because once the operating time of the unit exceeds the period required for major maintenance, bearing wear and the clearance between the components of the machine reach the limits permitted by its technical specifications; at this point, the machine is in an unsafe operating condition, and serious consequences can occur at any time. 4. The operating load on the machine increases, posing a risk to the main motor and electrical systems. Intense friction that may occur between the screws of the machine, between the screws and the front and rear end surfaces, and between the screws and the machine’s housing can lead to a sharp increase in the operating load of the machine. Additionally, severely worn bearings also generate high operating loads. As a result, the motor will operate under overload conditions, which can seriously threaten its safe operation. In severe cases, if the electrical protection devices of the air compressor unit are not sensitive enough or fail to function properly, it can also lead to motor burnout. 5. The exhaust volume of the air compressor will experience a significant decrease. An increase in the clearance between the host components leads to a significant reduction in the host’s efficiency; in other words, the exhaust volume of the air compressor decreases considerably, which has an impact on the normal production processes of the units that rely on this air supply. Especially for those users whose air compressor capacity is limited, as the output of the compressors decreases, and given that the demand for air in the systems using it remains relatively stable, the pressure of compressed air in the pipeline network drops significantly. This can result in the equipment in these systems failing to operate properly or not operating at all, thereby affecting the normal production of the facility or causing temporary shutdowns, which incurs losses for the enterprise. 6. The most serious consequence is the sudden \"lockup\" of the host. Once such a situation occurs, if the electrical protection system does not respond in time or fails to function properly, it can also cause severe damage to the main motor and the electrical system. Regarding the handling of a \"stuck\" main unit, on the one hand, the repair costs for major repairs are much higher than those for regular repairs; on the other hand, since the components of the main unit are damaged, its overall performance after repair will be lower compared to that of a main unit that has undergone a regular repair. If the damage to the host is severe to the point where it is no longer worth repairing or cannot be repaired at all, the only option is to discard it and replace it with a new one, resulting in the greatest possible loss. This is because the cost of a new host is usually around one-third of the cost of purchasing an entire air compressor unit, which is far higher than the cost of overhauling a host under normal circumstances. Under normal conditions, the overall technical performance of a host that has been overhauled is very similar to that of a new host! From the above analysis, it is clear that proper major maintenance of host equipment is not only a basic requirement for equipment maintenance, but also essential for companies to control their maintenance costs, avoid unnecessary financial losses, and ensure the smooth operation of their production processes! Therefore, it is not only necessary but also essential to carry out regular major overhauls of the air compressor unit in accordance with standards and schedules!
The necessity and procedures for major overhauls of screw air compressor units. When a screw air compressor unit is in operation, the screws are theoretically not in contact with each other, nor are they in contact with the compressor housing or its front and rear ends. There are three reasons for this: ① The oil film formed during normal operation of the compressor prevents direct contact between the screws; however, during the short period immediately after the compressor starts up or stops, when the oil film has not yet been established or is of poor quality, there may be some direct contact between the screws； ② The machining precision of the screw and the main housing ensures that an appropriate gap exists between them after the assembly of the main unit; this gap size takes into account the difference in deformation amounts of the screw and the housing when the main unit operates at high temperatures ； ③ The clearance between the screw and the front and rear end faces is ensured during screw assembly in accordance with technical specifications; this clearance size also takes into account the difference in deformation of the screw relative to those end faces when the main unit operates at high temperatures. Meanwhile, both the radial and axial loads generated by compression are borne by the positioning bearings at both ends of the main unit, so the screw does not experience any axial displacement, thereby further ensuring the clearance at the end faces. As the air compressor operates for an extended period of time, the main bearings inevitably wear out, which leads to increased axial and radial movement of the screws. This change results in alterations in the gaps between the screws, as well as between the screws and the main housing and its front and rear end surfaces. Such variations in the gap are normal and acceptable throughout the bearing’s service life, and the resulting decrease in the air compressor’s output volume as well as the increased load on the drive motor are also normal and acceptable. However, when the bearing wear and the clearance between the main components exceed the allowable limits, serious consequences may occur. Firstly, friction occurs between the screw inside the host and its front and rear end surfaces, as well as between the screw and the host housing. This leads to a sharp increase in the motor’s load; in the worst cases, the screw can get stuck or even the entire host may be damaged. If the motor protection system does not respond promptly or fails to function properly, it may result in the motor burning out. Secondly, a significant decline in the air production volume of the air compressor may affect the normal production of the units that use this air. It is evident that a major overhaul of the host is necessary, and indeed essential. II. Determination of when major maintenance is required for the compressor. Strictly speaking, the time at which major maintenance should be carried out on the compressor should be based on the expiration of the bearing’s service life. However, it is difficult to find accurate indicators to determine whether the bearing’s service life is about to expire or has already expired. Therefore, well-known international compressor manufacturers have established a standard based on their nearly century-long experience: major maintenance should be performed after the compressor has run for 20,000 hours or after 4 years. In addition to the above performance standards, experience also allows for a more accurate assessment of the host’s condition based on the following aspects: 1) The sounds emitted by the host during operation, especially any abnormal noises from the bearings ； 2) Difference between the motor’s operating current, especially the no-load operating current, and the standard value ； 3) Vibration of each bearing in the machine head. Comprehensive testing tools are necessary; by comparing the test results with technical data, it is possible to determine effectively whether major repairs to the main unit are required. III. Main tasks of the main engine overhaul 1) Disassemble and pull out or press out the corresponding gears (or pulleys and other transmission components) as well as bearings ； 2) Clean the screw, shaft, and gap adjustment shims; grind and repair the damaged surfaces of the shaft, screw, and stator ； 3) Press in the bearings, adjust the clearances, and test each clearance ； 4) Trial operation, running-in adjustment. For oil-free screw compressors, it is not advisable for users to carry out maintenance on them by themselves ； For the main unit of an injection screw compressor, the following points should generally be taken into consideration: 1. The bearings must be replaced as a complete set ； 2. Check the rotor profile to determine if corrections are needed ； 3. Inspection of the intake and exhaust bearing housings – is correction required? ； 4. Assembly requirements for the host unit; it seems that the host unit cannot be opened if these requirements are not met ； 5. Generally, the clearance at the intake side is 0.35–0.50 mm ； The clearance at the exhaust end is 0.08-0.15mm ； Since it is a repair, it should be as large as possible within the allowable range.