Methods and Procedures for Rotor Clearance Adjustment of Screw Compressors
This article mainly covers process screw compressors and dry oil-free screw compressors. The overhaul of components such as bearings and shaft seals is roughly the same as that of centrifugal compressors. The core of maintenance lies in adjusting and controlling the meshing clearance ratio between the male and female rotors by means of timing gears. The male and female rotors of a screw compressor are essentially a pair of intermeshing helical gears. Therefore, the timing gears and screw rotors can be regarded as an integral assembly. Based on the conjugate gear meshing theory for gear transmission, the meshing clearance ratio of screw rotors can be adjusted by changing the initial phase angle between the male and female rotors.
Clearance Design
When a screw compressor operates, the motor drives the rotors to rotate at high speed. The two end faces of the compressor casing are stationary components, so a certain end clearance must exist between them. As the power of the screw compressor increases, the required end clearance for adjustment also rises. Adjusting the discharge end clearance is a critical step during rotor assembly. Increasing the clearance will reduce the compressor displacement, while excessive reduction may cause friction between the rotors and the discharge end face. Therefore, the discharge end clearance of the screw compressor is determined by designers based on practical engineering experience.
Generally, the discharge end clearance is comprehensively determined by factors including compressor power, operating conditions, and materials of rotors and cylinder block. As long as the discharge end clearance varies within a specified range, normal operation of the compressor can be guaranteed.
Operating Characteristics of Rotors
The tooth surfaces of screw compressor rotors are used for gas compression. Multiple working chambers are formed by intermeshing tooth grooves of the male and female rotors. For process screw compressors and dry oil-free screw compressors, the male and female rotors do not make direct contact during operation; power transmission between the two rotors is realized via timing gears. Apart from satisfying conjugate meshing requirements and machinability, the end profile of screw rotors must feature a small leakage triangle, high area utilization factor and superior volumetric efficiency. Accordingly, the rotor profile of a screw compressor consists of multiple tangential curves including straight lines, cycloids, circular arcs, ellipses and parabolas. The end profile is the most critical element of the screw compressor rotor, and the quality of the rotor profile determines the overall performance of the screw compressor.
Theoretical Basis
The meshing clearance of process screw compressors and dry oil-free screw compressors is controlled by timing gears. The driving and driven timing gears share the same pitch circle with their corresponding male and female rotors. The driving gear is connected to the male rotor via keys, so they can be treated as an integrated gear assembly. However, the driven main gear ring and backlash gear ring are fitted on the hub. After the hub is key-connected to the female rotor, the two gear rings remain adjustable relative to the hub and female rotor.
For a specific gear pair with fixed center distance, the gear meshing backlash is determined by inherent gear parameters. Similarly, for the meshing of male and female screw rotors, the total rotor meshing clearance remains constant if the minor influence of bearing clearance on the center distance is ignored.
During maintenance and adjustment of the meshing clearance, keep the female rotor stationary and fine-tune the driven gear ring. Specifically, adjust the position of locating pins between the driven timing gear ring and the hub on the female rotor. This creates minor changes to the initial meshing phase angle of male and female rotors, thereby redistributing the leading side clearance and trailing side clearance at the rotor meshing nodes.
Implementation Procedures
Loosen the fastening bolts and locating pins between the driven gear ring and hub to ensure free movement. Bar the rotor to bring the male and female rotors into tight contact, and accurately measure the total meshing clearance between the rotors. Calculate the difference for the leading side clearance by comparing the target clearance value with the total clearance. Then bar the rotor and insert a feeler gauge with thickness equal or close to the calculated clearance difference into the meshing zone on the leading side of male and female rotors. Tap the screws on the timing gear with a copper bar during adjustment until the feeler gauge is fully clamped.
Rotate the driven gear ring to the position where it just meshes with the driving gear. If necessary, fully allocate gear backlash to one side of the meshing teeth, then tighten the fastening bolts between the driven gear ring and hub. Remove the feeler gauge between male and female rotors. The remaining clearance is the tooth surface meshing clearance. For safety, bar the rotor continuously and measure the meshing clearance at all rotor positions one by one with a feeler gauge for verification.
Owing to the relatively large total meshing clearance between male and female rotors of screw compressors, the adjustable range of the clearance ratio is wide. Normally, the clearance ratio should be controlled within 0.008 ~ 0.012 mm (0.8 ~ 1.2 wire). During high-speed operation, thermal deformation of tooth profiles and gear surface wear tend to reduce the leading side clearance and increase the trailing side clearance. To ensure long-term reliable operation, it is more reasonable to set a slightly larger leading side clearance and smaller trailing side clearance during maintenance, maintaining the ratio between leading side clearance and trailing side clearance at 1.0 ~ 1.2.
Adjustment of screw compressor meshing clearance is the most critical step throughout maintenance, which directly determines whether the compressor can run normally. It is a highly difficult operation that often requires repeated adjustment to meet specifications. Different from conventional adjustment methods, this technique adopts reverse thinking: preset the initial rotor phase angle, adjust the meshing clearance between driven and driving gear rings, and fix them with fastening bolts. Under normal circumstances, this method can directly achieve qualified rotor meshing clearance, reduce repeated clearance measurement and gear ring adjustment work, and effectively shorten the maintenance cycle.
Clearance Design
When a screw compressor operates, the motor drives the rotors to rotate at high speed. The two end faces of the compressor casing are stationary components, so a certain end clearance must exist between them. As the power of the screw compressor increases, the required end clearance for adjustment also rises. Adjusting the discharge end clearance is a critical step during rotor assembly. Increasing the clearance will reduce the compressor displacement, while excessive reduction may cause friction between the rotors and the discharge end face. Therefore, the discharge end clearance of the screw compressor is determined by designers based on practical engineering experience.
Generally, the discharge end clearance is comprehensively determined by factors including compressor power, operating conditions, and materials of rotors and cylinder block. As long as the discharge end clearance varies within a specified range, normal operation of the compressor can be guaranteed.
Operating Characteristics of Rotors
The tooth surfaces of screw compressor rotors are used for gas compression. Multiple working chambers are formed by intermeshing tooth grooves of the male and female rotors. For process screw compressors and dry oil-free screw compressors, the male and female rotors do not make direct contact during operation; power transmission between the two rotors is realized via timing gears. Apart from satisfying conjugate meshing requirements and machinability, the end profile of screw rotors must feature a small leakage triangle, high area utilization factor and superior volumetric efficiency. Accordingly, the rotor profile of a screw compressor consists of multiple tangential curves including straight lines, cycloids, circular arcs, ellipses and parabolas. The end profile is the most critical element of the screw compressor rotor, and the quality of the rotor profile determines the overall performance of the screw compressor.
Theoretical Basis
The meshing clearance of process screw compressors and dry oil-free screw compressors is controlled by timing gears. The driving and driven timing gears share the same pitch circle with their corresponding male and female rotors. The driving gear is connected to the male rotor via keys, so they can be treated as an integrated gear assembly. However, the driven main gear ring and backlash gear ring are fitted on the hub. After the hub is key-connected to the female rotor, the two gear rings remain adjustable relative to the hub and female rotor.
For a specific gear pair with fixed center distance, the gear meshing backlash is determined by inherent gear parameters. Similarly, for the meshing of male and female screw rotors, the total rotor meshing clearance remains constant if the minor influence of bearing clearance on the center distance is ignored.
During maintenance and adjustment of the meshing clearance, keep the female rotor stationary and fine-tune the driven gear ring. Specifically, adjust the position of locating pins between the driven timing gear ring and the hub on the female rotor. This creates minor changes to the initial meshing phase angle of male and female rotors, thereby redistributing the leading side clearance and trailing side clearance at the rotor meshing nodes.
Implementation Procedures
Loosen the fastening bolts and locating pins between the driven gear ring and hub to ensure free movement. Bar the rotor to bring the male and female rotors into tight contact, and accurately measure the total meshing clearance between the rotors. Calculate the difference for the leading side clearance by comparing the target clearance value with the total clearance. Then bar the rotor and insert a feeler gauge with thickness equal or close to the calculated clearance difference into the meshing zone on the leading side of male and female rotors. Tap the screws on the timing gear with a copper bar during adjustment until the feeler gauge is fully clamped.
Rotate the driven gear ring to the position where it just meshes with the driving gear. If necessary, fully allocate gear backlash to one side of the meshing teeth, then tighten the fastening bolts between the driven gear ring and hub. Remove the feeler gauge between male and female rotors. The remaining clearance is the tooth surface meshing clearance. For safety, bar the rotor continuously and measure the meshing clearance at all rotor positions one by one with a feeler gauge for verification.
Owing to the relatively large total meshing clearance between male and female rotors of screw compressors, the adjustable range of the clearance ratio is wide. Normally, the clearance ratio should be controlled within 0.008 ~ 0.012 mm (0.8 ~ 1.2 wire). During high-speed operation, thermal deformation of tooth profiles and gear surface wear tend to reduce the leading side clearance and increase the trailing side clearance. To ensure long-term reliable operation, it is more reasonable to set a slightly larger leading side clearance and smaller trailing side clearance during maintenance, maintaining the ratio between leading side clearance and trailing side clearance at 1.0 ~ 1.2.
Adjustment of screw compressor meshing clearance is the most critical step throughout maintenance, which directly determines whether the compressor can run normally. It is a highly difficult operation that often requires repeated adjustment to meet specifications. Different from conventional adjustment methods, this technique adopts reverse thinking: preset the initial rotor phase angle, adjust the meshing clearance between driven and driving gear rings, and fix them with fastening bolts. Under normal circumstances, this method can directly achieve qualified rotor meshing clearance, reduce repeated clearance measurement and gear ring adjustment work, and effectively shorten the maintenance cycle.
