Screw air compressor is widely used, and regular maintenance is the premise to ensure its normal operation.

The main engine is the core part of the air compressor, which has been operating at a high speed for a long time. Similar to most operating machines, it must undergo preventive overhaul after a certain period of time or service life. The overhaul period is usually 30000-40000h (4-5 years).

01 Overhaul opportunity of screw host

After a period of operation, the screw host will have the following problems:

1. The clearance increases

① The distance between the male and female rotors of the main engine increases. The direct consequence is that the leakage (i.e. back leakage) of the compressor increases during compression, and the volume of compressed air discharged from the machine decreases. In terms of efficiency, the compression efficiency of the compressor is reduced.

② The increase of the clearance between the male and female rotors and the rear end cover and bearing will mainly affect the sealing and compression efficiency of the compressor, and will also have a great impact on the service life of the male and female rotors, because many mechanical failures are initially caused by the clearance between the bearing and the main shaft.

2. Increased wear

As we all know, as long as the machinery is running, there is wear. Normally, the wear will be reduced a lot because of the lubrication of lubricating fluid. However, the wear is slowly increasing when the machinery is running at a high speed for a long time.

Take Sweden SKF bearing as an example: under almost impossible conditions (such as long-term idling, clean lubricating fluid, good cooling system, and ambient temperature and humidity), it can only be guaranteed for several years. As far as the compressor main engine is concerned, there is wear on other parts besides the bearing. If proper preventive measures are not taken for minor wear, it is easy to increase wear and damage other major components.

3. Component deformation

The main engine of the air compressor is the core compression component of the compressor. The internal components of the air compressor are in a high temperature and high pressure environment for a long time. In addition, the high-speed operation will inevitably lead to the deformation of the components over a long period of time. At the same time, the deformation is also caused by the use of environmental factors, and the air in the general use environment of the air compressor will have dust and impurities. If these small solid substances accumulate into large solid blocks after entering the machine, it may cause the deformation of the yin and yang rotors, increase the gap, and even cause the main machine to be stuck seriously.

4. Component damage

The vulnerable parts of the main engine of the air compressor mainly include: various gaskets, sealing rings, shaft oil seals, etc., especially the shaft oil seals, which have been in the environment of high temperature, high pressure and high-speed operation for a long time, and their service life ratio has been greatly shortened. The direct consequence of the damage is that the compressor leaks oil. If it is not found in time, the male and female rotors and bearings will be burned due to lack of lubrication.

02 Disassembly and assembly of compressor head

1. Disassembly process and steps

① Tools and instruments preparation, including special support, puller, jack, hand tools (wrench, copper rod, hammer, pliers, etc.), measuring instruments (vernier caliper, depth gauge, lead wire, etc.), oxygen and acetylene, bearing heater, lifting equipment, etc.

② Remove the LP end cover (bearing seat) and HP end cover.

③ Key data measurement before disassembly:

• The clearance at the low pressure end (the clearance between the female and male rotor end faces and the front housing end faces) shall be measured with a feeler gauge;

• High pressure end clearance (clearance between female and male rotor end faces and rear housing end faces) shall be measured with feeler gauge;

• The bearing clearance of male and female rotors at the high pressure end shall be measured by pressing lead wire method.

④ Take out the main shaft drive gear, key and shaft sleeve by surface heating.

⑤ Remove and pull out the main engine housing, the low pressure end bearing of the male and female rotors and their glands.

⑥ Dismantle and pull out the HP end housing (bearing seat), male and female rotor bearings and their gland and inner ring gaskets.

⑦ Clean and check the main shaft, rotor, bearing and inner housing for damage.

⑧ Repair of damaged components.

2. Data measurement record before disassembly

The clearance between the screw and the front and rear end faces is a technical parameter that should be guaranteed when assembling the screw. The clearance size has taken into account the expansion deformation of the screw and the front and rear end faces when the host is operating at high temperature. At the same time, the radial load and axial load of the screw generated by compression are borne by the positioning bearings at both ends of the host. At the beginning of operation, the screw has no axial displacement. However, after a certain period of operation, the main engine bearings will inevitably be worn, resulting in axial and radial displacement of the screw, which will change the clearance between the screw and the screw, between the screw and the main engine housing, and between the front and rear end faces. Therefore, these assembly clearances shall ensure the normal operation of the main engine for a long time.

3. Reassembly process and steps

      ① Let the male and female rotors and fix them after meshing, and put on the exhaust end cover.

      ② Put in the gasket of the inner ring of the bearing, heat the bearing and perform shrink fitting, and install the bearing gland and locking bolts.

      ③ Lift the rear cover to make the male and female rotors fall freely, and measure the total clearance of the exhaust end.

      ④ If the total clearance of the exhaust end is unqualified, re-install the bearing and adjust the number of inner ring gaskets until the clearance is at the required value.

      ⑤ Install the exhaust end cover, and adjust the bearing clearance of the male and female rotor exhaust end through the thickness of the cover gasket. The exhaust end clearance during operation is the difference between the total exhaust end clearance and the bearing clearance.

      ⑥ Reassemble the rotor casing of the main engine, the male and female rotor bearings and the cover at the intake end in sequence.

      ⑦ Adjust the air intake end clearance through the thickness of the air intake end cover gasket.

      ⑧ Heat the bushing and install it on the main shaft (to the bottom), install the drive gear key, and heat the gear for shrink fitting.

     ⑨ Install the gear gland and locking bolts, apply thread fastening glue on the locking bolts, and tighten the bolts.

     ⑩ Pay attention to the tightening force of the bearing gland bolts and each bolt of the main engine during reassembly of the main engine.

     ⑪ Rotate the main engine to make sure that the main engine rotates flexibly.