During operation, centrifugal water pumps experience significant axial forces due to uneven liquid pressure distribution on both sides of the impeller. If these axial forces are not effectively balanced, they can lead to pump shaft misalignment, bearing overheating, mechanical seal failure, and even frictional collisions between the impeller and pump casing, seriously threatening equipment safety and service life. Therefore, the design of the axial force balancing device is a core aspect of centrifugal water pump structural optimization. Its core objective is to counteract the axial force through mechanical structure while reducing wear on the balancing device itself and extending equipment maintenance cycles.
The balancing disc is one of the most widely used axial force balancing devices in centrifugal water pumps, especially suitable for multi-stage pumps. Its working principle involves installing an axially movable balancing disc at the end of the pump shaft. The left side of the balancing disc connects to the high-pressure zone at the outlet of the final impeller, while the right side connects to the low-pressure zone at the pump inlet via a return pipe. When the axial force pushes the pump shaft to the right, the gap between the balancing disc and the pump body decreases, increasing the liquid pressure in the high-pressure zone and generating a balancing force opposite to the axial force. Conversely, when the axial force decreases, the gap increases, and the balancing force weakens accordingly. This dynamic adjustment mechanism allows the balance disc to automatically adapt to changes in axial force under different operating conditions. However, during operation, friction occurs due to relative sliding between the balance disc and the balance ring, leading to wear and increased leakage over long-term use. To reduce wear, modern designs often use high-hardness, low-friction materials (such as hard alloys or ceramics) to manufacture the balance disc and balance ring, and coat the contact surfaces with a wear-resistant coating. Simultaneously, optimized gap design reduces contact pressure.
The balance drum is another common axial force balancing device. Its structure is similar to the balance disc, but its working principle differs. The balance drum consists of a cylindrical drum mounted on the pump shaft, forming a narrow radial gap between the drum and the pump body. High-pressure liquid flows through this gap into the balance chamber and then returns to the pump inlet through the balance pipe. Because the pressure in the balance chamber is slightly higher than the inlet pressure, a pressure difference is generated on both sides of the balance drum, creating a balancing force opposite to the axial force. The advantages of the balance drum are its simple structure, stable balancing force, and the absence of axial gap, avoiding the leakage problems caused by gap changes in the balance disc. However, the radial clearance of the balancing drum is small, making it sensitive to impurities in the liquid and prone to clogging or particle abrasion, leading to balancing failure. Therefore, in practical applications, a filter is often installed before the balancing drum, and the drum body and pump body are made of wear-resistant materials to reduce the risk of wear.
Double-suction impellers achieve natural axial force balance by changing the impeller structure. Its core design is to configure the impeller with symmetrical water inlets on both sides, ensuring uniform liquid pressure distribution on both sides of the impeller, thereby eliminating axial force. The advantage of double-suction impellers is that they do not require additional balancing devices, have a simple structure, and extremely low wear, making them particularly suitable for high-flow, low-head centrifugal water pumps. However, the manufacturing process of double-suction impellers requires higher precision, ensuring the symmetry of the water inlet channels on both sides; otherwise, uneven pressure distribution will regenerate axial force. Furthermore, the pump body structure of double-suction impellers is relatively complex and costly, therefore they are mostly used in scenarios with extremely high reliability requirements.
Balancing holes and balancing pipes are commonly used axial force balancing methods in single-stage centrifugal water pumps. Balancing holes, achieved by drilling multiple small holes in the impeller's rear cover plate, allow some of the high-pressure liquid to flow back to the impeller inlet, reducing the pressure on the rear cover plate side and thus decreasing axial force. Balancing pipes, on the other hand, use dedicated piping to guide the high-pressure liquid in the rear pump chamber to the pump inlet, achieving a similar effect. The advantages of these two methods are their simple structure and low cost; however, liquid backflow can reduce pump efficiency, and they cannot completely eliminate axial force, requiring the use of thrust bearings. To reduce wear, the design of the balancing holes and pipes needs to optimize the hole and pipe diameters to avoid excessive liquid velocity that could cause cavitation or erosion wear.
While thrust bearings do not directly balance axial force, as an auxiliary component of the axial force balancing system, their performance is crucial for reducing mechanical wear. Thrust bearings prevent pump shaft movement by bearing residual axial force, protecting the balancing device and other components. Modern centrifugal water pumps often use angular contact ball bearings or thrust self-aligning roller bearings, whose high load-bearing capacity and self-aligning properties effectively disperse axial force and reduce localized wear. Furthermore, the design of the lubrication system (such as oil lubrication or grease lubrication) significantly affects bearing life, requiring the selection of appropriate lubrication methods and cycles based on operating conditions.
The axial force balancing device of the centrifugal water pump, through structures such as a balancing disc, balancing drum, double-suction impeller, balancing holes, and balancing pipes, combined with the auxiliary effect of thrust bearings, effectively counteracts axial force and significantly reduces mechanical wear. In the future, with the development of materials science and fluid mechanics, new balancing devices (such as magnetic balancing and hydraulic self-balancing) will further optimize the performance of the centrifugal water pump, driving its evolution towards high efficiency, low wear, and long service life.
