Maintenance of Spiral Classifier

Maintenance Content

Based on its scope and workload, maintenance can be classified as minor repair, medium repair, and major repair.

(1) Minor Repair. The main work contents include:

1. Inspecting and tightening all bolts.
2. Inspecting wear conditions at various locations.
3. Inspecting and replacing worn coupling parts and drive V-belts.
4. Cleaning and inspecting all rolling bearings; replacing damaged ones.
5. Replacing large and small blades that are severely worn.
6. Cleaning the reducer housing and replacing deteriorated lubricating oil.
7. Inspecting the sealing condition of the lower support assembly; addressing any abnormalities promptly.
8. Inspecting and welding/repairing the sand return chute and overflow chute; replacing the overflow screen.
9. Inspecting all water supply pipes and valves; repairing leaks or replacing valves.
10. Cleaning the motor, checking insulation and grounding conditions; adding or replacing oil in motor bearings.
11. Minor repair is performed once every 3 to 6 months, requiring 12 hours each time.

(2) Medium Repair. The main work contents include:

1. All contents of minor repair.
2. Replacing the lower support assembly.
3. Replacing rolling bearings at various locations (including motor bearings).
4. Cleaning and inspecting the main shaft sleeve.
5. Replacing severely worn spiral supports.
6. Medium repair is performed once every 6 to 12 months, requiring 1 day each time.

(3) Major Repair. The main work contents include:

1. All contents of medium repair.
2. Replacing the spiral main shaft or repairing the shaft by welding.
3. Replacing the tank body.
4. Replacing the main reducer.
5. Replacing the large and small transmission gears (spur gears, bevel gears).
6. Replacing the main shaft sleeve.
7. Major repair is performed once every 3 to 5 years, requiring 2 days each time.

11.1.7.2 Repair Methods

The details are as follows:

(1) Repair of a Broken Shaft. A spiral shaft is generally used for three years or longer. Under the combined effects of fatigue, torsion, vibration, corrosion, and welding stresses, the shaft is prone to fracture. For repair of a broken main shaft, the method is determined according to the condition of the fracture. For example, if the main shaft is completely broken, and the fracture ends are separated and deformed, the preferred repair method is to use an inner core tube combined with an outer arc patch plate. The specific steps are as follows:

1. Use a crane and manual chain hoists to align the two broken sections of the main shaft into a straight line. Then support both sections with two support stands, ensuring safety and stability.
2. Remove the large blades and supports within 600 mm on either side of the fracture. Clean the external scale and internal rust of the main shaft, and grind to a shine. Use gas cutting to remove the deformed portion at the fracture, and clean the cut surface thoroughly.
3. Measure the inner and outer diameters within 1 m of the fracture. Based on the measured dimensions, machine the inner core tube and outer arc patch plates. The outer diameter of the core tube should be slightly smaller than the inner diameter of the main shaft, ensuring both smooth insertion and a tight fit. The outer arc patch plates should also fit tightly against the outer circumference of the shaft. Generally, the inner core tube length is 800–1000 mm, the outer arc patch plate length is about 1000–1200 mm, and the outer plates are about 200 mm longer than the inner core tube to stagger the inner and outer welds and avoid the fracture area.
4. When assembling the inner core tube, first insert half of its length into one end of the fractured main shaft and tack weld it circumferentially from the inside. On the other end of the fractured shaft, about 100 mm from the end, open welding windows every 120° along the circumference. Each window should be approximately 300 mm × 50 mm, with rounded ends to reduce welding stress. Clean and grind the windows. Then insert this end over the core tube and begin alignment.
5. Use two 1 m long steel rulers to measure symmetrically at 8 points around the circumference. Repeatedly correct the alignment to ensure coaxiality. When the best result is achieved, tack weld the shaft sections in place.
6. Weld the fillet welds of the core tube at the opened windows. The weld height should equal the shaft wall thickness. During welding, prevent shaft movement or deformation to maintain the coaxiality achieved during alignment.
7. Ensure good weld quality at the fracture. After welding, grind the weld so that its height does not exceed the base metal surface.
8. When welding the outer arc patch plates, the weld should transition smoothly with the base metal, and the weld height should equal the thickness of the patch plates.
9. Reinstall the removed supports (with enlarged connection dimensions) together with the large and small blades in their original positions, ensuring the required dimensions are met and the installation complies with the spiral classifier installation specifications.
10. Repair quality requirements: The straightness deviation of the main shaft shall not exceed 0.5 mm/m, and the total straightness deviation over the entire length shall not exceed 1.5 mm. The center of the main shaft shall be aligned with the centers of the supports and spiral blades. The radial runout of the main shaft caused by center deviation and shaft bending shall not exceed 8 mm, and the axial runout shall not exceed 5 mm. The outer diameter of the spiral should be kept as straight as possible, with the concavity/convexity not exceeding 8 mm.

(2) Repair of a Partially Cracked Shaft (Not Fully Broken). If the main shaft is not fully broken and is discovered in time, with only a crack that does not completely encircle the circumference, a quick repair method can be used to minimize downtime and production losses. The specific steps are as follows:

1. Manually turn the shaft so that the cracked area on the outer surface faces upward. At this position, the crack will close due to the weight of the shaft.
2. Use a crane and manual chain hoists to lift and restrain the shaft sections on both sides of the crack. Support both sides with two support stands, ensuring stability and reliability.
3. Remove the supports and blades within 600 mm on either side of the crack. Clean external rust and scale from the main shaft, and grind to a shine.
4. Use two steel rulers to align the shaft and ensure coaxiality. Simultaneously adjust the crane lifting force and support stands to achieve the best alignment.
5. Use gas cutting to open a 45° bevel on both sides of the crack, reaching the shaft cavity. Use an angle grinder to clean the bevel, then weld it flat with electric welding. If the weld is higher than the shaft surface, grind it flat with an angle grinder.
6. Cut steel strips approximately 500–600 mm long, 50 mm wide, and of the same thickness as the shaft. Clean the cuts. Using the crack as the centerline, weld a steel strip every 50 mm around the circumference as outer patch plates. Weld the steel strips along their length direction to the shaft; do not weld the width direction. The weld should transition smoothly with the base metal, and the weld height should equal the thickness of the base metal.
7. After welding, reinstall the removed supports (with enlarged connection dimensions) together with the large and small blades, ensuring the dimensions meet the installation specifications.
8. The repair quality requirements are the same as above.

The above repair method can be performed directly on site. The repair time depends on the size of the spiral classifier, generally requiring 4–12 hours. It is convenient and fast, saving more than half the time compared to the first method. The disadvantage is a shorter service life after repair, typically 9–15 months. The user facility may decide which repair method to use or opt for complete replacement based on production schedules and the degree of shaft fracture.

(3) Repair of the Lower Bearing. The lower bearing of a classifier generally comes in two structural types: one is a rolling bearing support, and the other is a rubber bearing support. The main cause of damage to the rolling bearing support is seal leakage allowing ore slurry to enter, which damages the rolling bearing. Seal leakage is mainly due to loosening of packing wear and untimely injection of high-pressure grease. During repair, replace the rolling bearing, repeatedly and firmly compress the oil-impregnated asbestos packing, and fill all cavities inside the support with grease. During operation, grease should be injected frequently to maintain a certain positive pressure inside the support, preventing slurry intrusion. This requires a high sense of responsibility from the operating and maintenance personnel. Under normal conditions, the service life of the lower support assembly can reach one year, but due to assembly quality issues or untimely greasing, the service life is sometimes less than one month. Therefore, many user facilities have improved the support seal. A successful improvement is to replace the oil-impregnated asbestos packing with V-shaped sealing rings made of rubber or polyurethane. Multiple sealing rings stacked together replace the asbestos packing. Due to the good elasticity and wear resistance of rubber and polyurethane, the sealing performance is significantly improved compared to asbestos packing, stabilizing and extending the service life of the rolling bearing support.

Many factors affect the service life of rolling bearing supports, making the service cycle difficult to predict. Generally, they are disassembled and inspected during monthly scheduled maintenance. If slurry ingress has caused bearing damage, they are replaced, and the repair cost is high. Some user facilities have converted rolling bearing supports to rubber or polyurethane bearing supports, adding a steel bushing to the lower support stub shaft. The service life of the support can reach 3–6 months, and the steel bushing can last 8–12 months. The service cycle is easier to control. Once the life cycle pattern is understood, scheduled maintenance can be effectively managed, with regular replacement of the support and steel bushing. This requires less responsibility from operating and maintenance personnel, and the repair cost is lower.

Another simpler repair and modification method is to disassemble the rolling bearing support assembly, leaving the flange connecting to the hollow shaft and the stub shaft unchanged. Remove the rolling bearing, seal sleeve, packing, and front/rear packing glands inside the support, keeping only the support housing. Using old conveyor belt material, roll a rubber belt roll with a width equal to the axial width of the housing, an outer diameter slightly smaller than the housing inner diameter, and an inner diameter slightly larger than the outer diameter of the stub shaft. Insert this roll into the housing and onto the stub shaft, thus creating a simple rubber belt lower bearing. When rolling the belt, note that the rolling direction should be opposite to the rotation direction of the stub shaft, so that when the spiral shaft rotates, the belt is tightly pressed against the inner surface of the support housing. The belt roll can last for more than 3 months. This method is simple and feasible, requires no additional machined parts, has low investment (using waste conveyor belt), is low-cost, stable, reliable, and can be used long-term.

(4) Repair of the Transmission Part. The transmission part of a single spiral classifier generally consists of an electric motor, a pair of belt pulleys, a reducer, and a cylindrical gear pair. The transmission part of a double spiral classifier generally consists of an electric motor, a coupling, a reducer, a pair of cylindrical gears, and two pairs of bevel gears. The main operational faults of the transmission part include the following: First, excessive clearance or breakage of the coupling pin bushings due to wear, causing unstable reducer operation, which affects the normal meshing of the cylindrical gears and in turn the stability of the bevel gear pair. The wear of the pin bushings should be regularly inspected, and their replacement cycle should be determined so that timely replacement can be made, avoiding impact on the smooth operation of the entire transmission system. Second, severe wear of cylindrical or bevel gears causing vibration and affecting the stability of the entire transmission mechanism. Gears should be replaced promptly according to gear wear replacement standards. Third, severe wear of the main shaft sleeve causes the main shaft to sink, and the meshing clearance of the cylindrical or bevel gear pair no longer meets specification requirements, resulting in unstable transmission. In this case, the main shaft sleeve should be repaired or replaced promptly, and the meshing clearances of all gears should be adjusted to meet the specified requirements.

(5) Repair of the Metal Tank Body. The metal tank body includes the water tank, the sand return chute, and the overflow chute. During long-term operation, the tank body is prone to wear, deformation, and local leakage. Among these, the liners and overflow grates are easily worn parts. During maintenance, in addition to replacing or welding the severely worn parts mentioned above, attention should also be paid to checking the lower truss and foundation of the tank body for deformation, cracks, or other phenomena. Truss deformation should be reinforced promptly, and foundation cracks should be dealt with promptly. The water tank is a major component of the spiral classifier. During maintenance at various stages, wear and deformation conditions should be checked and addressed promptly. The sand return chute and overflow chute are generally made of 10–12 mm steel plates. Due to slurry flow and corrosion, they wear quickly, typically lasting only 4–6 months. To extend their service life, wear-resistant liners (made of cast stone or waste rubber conveyor belt) should be installed on the bottom and side walls of the chutes.