Roots blowers are easily affected by their surrounding environment in real-world working conditions, leading to malfunctions. Therefore, analyzing and handling Roots blower malfunctions is crucial. After consulting numerous materials and discussing with dozens of colleagues, I have summarized the common causes and solutions for Roots blower malfunctions.

 

1. Working Principle of Roots Blowers

A Roots blower is a positive displacement rotary gas dynamic machine with two involute rotors. The main and driven shafts use gears to make the two rotors rotate at the same speed in opposite directions, completing the intake and exhaust process.

As shown in the figure, When the left rotor rotates clockwise, the right rotor rotates counterclockwise. Gas is drawn in from the inlet, moves with the rotating working chamber, and is finally discharged from the outlet at the top.

Between the two rotors, and between the rotors and the casing and side cover, it is necessary to ensure that they do not collide with each other, and that the efficiency is not affected by excessive clearance. A small gap is maintained between the two rotors during operation to minimize the return of discharged gas to the intake chamber.

Its characteristic is that the output air volume is proportional to the rotational speed, and the air volume does not change significantly when the blower outlet pressure changes. Roots blowers themselves cannot compress gas; the pressure increase relies on the back pressure of the exhaust system.

 

2. Roots Blower Malfunction Phenomena

Based on repeated maintenance summaries and discussions, the main causes of unit failure include: Broken shafts, bearing damage, large unit vibration, rotor seizure, and shaft seal leakage etc. The main malfunctions that increase the frequency of maintenance are: Rotor sticking and friction.

 

3. Analysis of Roots Blower Malfunction Causes

Friction between the rotor and the casing or between rotors is a common malfunction of Roots blowers, and if this malfunction occurs during operation, it will be accompanied by severe vibration and noise. Roots blowers often experience rotor sticking after prolonged shutdown. The main causes of this malfunction are summarized into the following categories:

►Inappropriate rotor lobe/lobe clearances

Inappropriate rotor lobe/lobe clearances, with changes in lobe/lobe clearances during operation. Collisions may occur between the two rotors at low speeds, causing friction or even seizure between the rotors.

If this malfunction occurs during operation, it will cause collisions between the two rotors or between the rotor and the casing, producing loud impact sounds; increased vibration, even causing foundation vibration; and a rapid increase in temperature at the friction points, even causing the casing to become hot and red.

 

►Bearing damage

Poor blower operating conditions, damaged oil seals, and excessive assembly clearances can all lead to bearing damage , which causes increased temperature, and the bearing housing temperature will also increase. In severe cases, the bearing housing may deform.

Especially when the inner and outer rings of the bearing are seized, the bearing housing and side cover plate are subjected to a large additional force, and the temperature will also be very high. Uneven heating of the bearing housing and side cover plate can cause deformation and cracking of the side cover plate in severe cases.

 

►Shaft wear, resulting in excessive clearance between the shaft and the inner ring of the bearing, and bearing housing wear resulting in excessive clearance between the bearing housing and the outer ring of the bearing, can both cause rotor seizure or friction. In particular, for belt-driven blowers, the main shaft is subjected to a larger unilateral force due to the belt tension, easily causing unilateral wear of the bearing housing on the belt side of the main shaft.

 

►Gear wear

Gear wear increases the clearance on the gear side, causing changes in the position of the two rotors, reducing the lobe clearance and increasing the lobe clearance between the driving rotor and the driven rotor, leading to collisions and friction between the two rotors.

 

►Rotor or casing deformation

If the rotor or casing is deformed, the clearance between the rotor outer diameter and the casing will be too small, causing rotor friction and seizure.

 

►Small clearance between rotor end face and side cover plate

If If the clearance between the rotor and the side cover plate is too small, foreign objects are squeezed between the rotor and the side cover plate; or after wear of the axial positioning bearing, the axial displacement of the rotor increases, all causing friction between the rotor and the side cover plate, causing rotor seizure.

 

4. Roots Blower Malfunction Handling Methods

Adjusting lobe/lobe clearances

Use a feeler gauge to check the rotor lobe/lobe clearance values and ensure they are within the standard range. Check the condition of the adjusting cap and bolts to ensure their integrity.

Requirements: The lobe clearance accounts for 1/3 of the total clearance, but not less than 0.15 mm; the lobe clearance accounts for 2/3 of the total clearance, and must be more than 1.5 times the gear backlash.

In the case of engaging lobes, the engaging surface of the active rotor tends to approach the passive rotor; in the case of separating lobes, the engaging surface of the active rotor tends to move away from the passive rotor. If the clearance between the rotor's engaging and separating lobes is inappropriate, the circumferential relative angle between the gear sleeve and hub or the gear and shaft can be adjusted.

Adjusting the rotor's separating and engaging lobes is a crucial part of Roots blower maintenance, directly determining the quality of the maintenance. Different types of Roots blowers have different adjustment methods. The shaft and gear are tapered keyless connection In the case of a tapered keyless connection, the relative circumferential position of the shaft and gear is generally adjusted; if the shaft and gear are keyed connection then the relative position of the driven gear and hub is generally adjusted.

During normal operation of the blower, the clearances change gradually due to gear wear, with the engaging lobe clearance tending to decrease and the separating lobe clearance tending to increase. When adjusting the working clearance between the two impellers, the engaging lobe clearance should be appropriately increased in advance; generally, the engaging lobe clearance is twice the separating lobe clearance.

When adjusting the impeller clearance, first rotate the blades to a 45° angle with the horizontal direction. At this time, the clearance between the two impellers is the separating or engaging lobe clearance.

During on-site adjustment, feeler gauges of the same thickness are generally fixed between the two rotors, and the two rotors are fixed. Then, the active gear is installed, and the mark of the driven gear is aligned with the mark of the active gear and pressed onto the shaft. The gear retaining ring, stop washer, and tightening nut are installed sequentially. After adjustment, the actual size of the separating and engaging lobe clearances is measured with a feeler gauge. If it is not appropriate, readjust until it meets the standard.

 

Bearing replacement

Check the side cover plate for deformation and cracks. If the deformation is not serious, the high points can be ground; if the deformation is serious, the side cover plate should be replaced; if there are cracks, welding can be performed.

Check the bearing seat and shaft diameter for wear. If the shaft is worn, welding is generally performed; if the bearing seat is worn, a separate bearing seat is generally replaced with a new one and not repaired; if the bearing seat and side cover plate are integrated, the bearing seat is welded to meet the technical requirements.

Check the tooth side clearance and replace the gear

Use red lead to check the contact area of the gear meshing part. Methods for checking the tooth side clearance include the dial gauge method, feeler gauge method, and lead wire method.

Standard requirements: Side clearance 0.10~0.18mm, meshing position in the middle of the tooth surface, tooth surface contact area not less than 50% along the tooth height direction, and not less than 70% along the tooth width direction. If the test results do not meet the requirements, the gear needs to be replaced.

Measure the clearance between the rotor and the casing with a feeler gauge

The standard range of the clearance between the rotor and the casing is 0.3~0.6 mm. During on-site maintenance, the clearance at 6 points is generally measured, the upper and lower and horizontal directions of each rotor, as shown in the figure.

Small deformation can be ground; large deformation or cracks between the rotor and the casing should be scrapped in time.

Clean the rotor end face and side cover plate

Use the feeler gauge method to check the axial clearance between the rotor end face and the casing. Positioning end (gear side) clearance d=0.1~0.2 mm, non-positioning end (pulley side) clearance c=0.3~0.5 mm, the standard for total clearance is 0.4~0.7mm.

Different types of Roots blowers have different end clearances under different operating conditions. During on-site maintenance, adjust according to their own maintenance standards. If the clearance on one side is inappropriate, it can be adjusted by adding or subtracting bearing adjustment shims; if the total clearance is inappropriate, it can be adjusted by adding or subtracting adjustment shims on the side cover plate joint surface, but the shim thickness generally does not exceed 1mm. Use the push shaft method to measure the axial runout of the rotor. The runout standard is 0.05~0.10mm. If it exceeds 1.5 times the maximum standard value, replace the bearing.

 

In summary, we can see that blower failures are caused by a combination of multiple complex factors. However, as long as the maintenance personnel strictly follow the maintenance procedures to confirm each maintenance data and determine whether it meets the maintenance standards, avoiding repeated maintenance, the fault handling time can be greatly shortened, ensuring safe, stable, and long-cycle operation of the equipment.