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Repair of centrifugal pump shaft2018-10-26 18:21:16

Repair of centrifugal pump shaft

Centrifugal water pump (referred to as "centrifugal pump") is a pumping machine that uses centrifugal blades of high-speed rotating impeller blades to drive water. The basic structure of the centrifugal pump consists of the pump body, impeller, pump shaft, bearing, sealing ring, stuffing box and so on. During the operation of the centrifugal pump, various failures will inevitably occur. The pump shaft not only supports all the components that are placed on the pump shaft, but also transmits torque through the pump shaft, which is a core component of the centrifugal pump. Accurate judgment of the failure of the pump shaft in time is an important means to ensure smooth operation of production.

2 main form and repair of pump shaft failure
2.1 journal hook, wear, eccentric wear
The pump pump shaft is generally made of high quality carbon steel after forging and cutting. For bearings of single-stage single-suction centrifugal pump shafts, rolling bearings are usually used. Due to the clearance fit between the pump shaft and the inner ring of the rolling bearing, there is a slight relative sliding between the pump shaft and the bearing. When the lubrication is poor, the lubricating oil film cannot be formed or the lubricating oil is contaminated with iron filings, sand particles, etc. The journal causes damage such as scratches or grinds, and is usually repaired by surfacing, chrome plating, thermal spraying, etc., and then processed into a standard diameter by car or grinding. In addition, along with the bending of the shaft, there is also a generation of a shaft diameter eccentric wear failure.
(1) Surfacing repair. For surfacing repair of pump shafts, cold surfacing is usually used. Before surfacing, the parts to be welded are polished to remove surface defects and oil rust in a certain range around them, and then repaired by cold surfacing. This cold surfacing repair does not cause deformation of the shaft diameter, and does not require annealing treatment. The wear resistance, heat resistance and corrosion resistance of the shaft diameter after repair can exceed the performance of the original shaft diameter. In order to ensure the repair welding strength and the uniformity of the weld overlay layer, the welding position is made to rotate vertically or horizontally. When surfacing, the front and back surfacing layers overlap at least 1/3.
(2) chrome repair. First use a wire brush or abrasive cloth to polish the parts that need chrome repair, remove the loose oxide on the surface, expose the base metal, and then measure and clean the size. The pump shaft is partially insulated and then chrome-plated, and then the original size and accuracy of the journal are restored by means of a car or a grinding. The thickness of the chrome layer should generally be greater than or equal to 0.3 mm.
(3) Thermal spray repair. Thermal spray repair consists of four steps: surface pretreatment, preheating, spraying, and post-spraying.
1) In order to ensure the quality of the sprayed layer, the surface of the journal with surface damage is first subjected to surface pretreatment. The steps of surface pretreatment are usually organic solvent cleaning, heating degreasing, threading, knurling, and electric drawing. Sometimes, in order to improve the bonding strength between the coating and the journal surface, a nickel-based material is first sprayed on the surface of the journal as a bonding underlayer.
2) Spraying material selected firstly according to factors such as pump shaft material, rotation speed and self-weight. The main considerations during spraying are pump shaft rotation speed, gun movement speed, paint spray distance, and spray gas pressure.
3) The journal after spraying should be treated as oil leakage, so that the lubricating oil can penetrate into the spraying layer more, and then the pump shaft is turned on the lathe to restore the size and accuracy requirements of the journal.

2.2 bending of the pump shaft
The main treatment methods for the bending repair of the pump shaft are cold straightening and hot straightening. When the pump shaft is bent and deformed, the amount of bending is first determined, and then the specific method of repairing the pump shaft is determined according to factors such as the material, diameter, length and bending amount of the pump shaft.
(1) Cold straightening method. The cold alignment of the pump shaft is commonly used for straightening the screw jack, straightening the press, and striking the straight bar. It is usually used in applications where the pump shaft is short and the amount of bending is small. When fixing, the shaft diameter or the force position should be protected with a certain thickness of cotton or copper pad to ensure that the contact surface is not damaged.
1) The screw jack is straightened. When the diameter of the pump shaft is d ≤ 55 mm, the screw jack can be used straight. When straightening, the first time the top pressure is straight, keep it for about 15 minutes; the second axis of the pump is slightly bent in the opposite direction of the original bend, and after 20 minutes, the pressure is released. If the axis of the pump shaft is visually flat, perform straightness detection. The accuracy of this method can reach 0.05-0.15 mm per meter.
2) The press is straightened. When the diameter of the pump shaft is d>55mm, it can be straightened by the press. During the straightening process, the pump shaft is convexly facing upward and fixed at both ends to prevent slipping out. When fixed, the fixed ends of the shaft diameter and the pressure point of the press are separated by a certain thickness of the yarn. Then, press the apex of the projection with a press and press down until the pump shaft is straightened.
3) The crowbar beats straight. When the diameter of the pump shaft is large, it can be handled by the method of tapping and straightening. When beating the pump shaft, the pump shaft should be concavely facing upward, and both ends should be pressed downward by the tensioning device, and then the pump shaft can be tapped with a 1-2 kg weight boring bar.
(2) Thermal alignment. The hot calibration of the pump shaft is commonly performed by local heating alignment, internal stress relaxation method alignment, and mechanical heating alignment. It is usually used in applications where the pump shaft is long and the amount of bending is large.
1) Local heating and straightening. For larger diameter pump shafts, local heating alignment is often used. When straightening, first place the pump shaft on the two support brackets, with the convex surface facing up, and wrap it with wet asbestos at the maximum bend of the pump shaft (the wet asbestos cloth has a rectangular notch along the direction of the pump shaft), and then use a blowtorch or Gas welding is hot. The heating temperature is about 100 ° C lower than the critical temperature of the pump shaft.
2) The internal stress is relaxed and straightened. For large diameter pump shafts, internal stress relaxation alignment is often used. When straightening, first place the pump shaft on the two support frames, with the convex surface facing up, use the induction coil to heat the bending position of the pump shaft, and apply a certain load under the convex surface. When the axis of the pump shaft is visually straight, the load is removed. The pump shaft after straightening should also be annealed.
3) Mechanical heating and straightening. For smaller diameter pump shafts, mechanical heating alignment is often used. When straightening, the pump shaft is first fixed on the two support frames, the convex surface is facing upward, and then the bending axial direction is pressed by the applied load, and then heated at the concave position to realize the alignment of the pump shaft.

2.3 shaft thread damage

If the pump shaft shaft thread damage is light, you can use the threaded socket for simple repair. For the pump shaft shaft thread with severe damage, first screw the screw shaft at the pump shaft, press the bushing, and then push the thread on the bushing. It can also be used to weld the shaft thread and then turn it out. Thread.

2.4 keyway pressure feed

The keyway mainly acts as a torque transmission on the pump shaft. The main failure mode of the pump shaft is pressure feed. If the keyway pressure feedback on the pump shaft is not serious, it can be polished with a trowel. When the keyway on the pump shaft is heavier, and the transmission power is not large, repair welding can be performed at the keyway position, and a new groove can be opened. A new shaft must be replaced when there is a serious keyway pressure feed on the pump shaft with a large transmission power.
The cause of the failure of the centrifugal pump may be multi-faceted. The pump shaft is a key component of the centrifugal pump. It is necessary to select the correct maintenance treatment according to the specific conditions of the fault, and repair and use it. However, in order to extend the service life of the pump shaft, attention must be paid to the maintenance of the centrifugal pump, while avoiding the overload phenomenon during start-up and shutdown, and improving the quality of the operators.