1. Fault Overview
A certain batch was manufactured in February 1994, and one of the air conditioning seawater pumps malfunctioned continuously after prolonged use. It was replaced in February 2009.
The new air conditioning seawater pump is a vertical centrifugal pump, model RSV100-300A, manufactured in December 2006, and its performance indicators are consistent with the original pump.
The brand new water pump, with performance indicators consistent with the original pump, has been tested and debugged by the manufacturer and should operate well; But after nearly half a year of operation, the mechanical shaft seal has been continuously leaking water. In September 2009, a large amount of water leakage was replaced, and two months later in November 2009, another large amount of water leakage occurred. Upon dismantling and inspection, it was found that:
The entire lower bearing 6307 falls onto the upper plane of the positioning groove of the water pump cover;
The lower inner diameter of the bearing seat has a 7-8mm wide wear ring from the lower edge upwards;
Mechanical shaft seal, static ring broken, dynamic ring plane damaged by 1 small piece;
The pressure cover on the water pump and the groove at the bottom of the hole where the shaft seal static ring is installed are worn out;
The pump shaft has obvious eccentric wear at the shaft seal.
2. Cause analysis
Analyzing the fault phenomenon, the main cause is that the entire lower bearing 6307 fell onto the upper plane of the positioning groove of the water pump cover, while the other four items were caused by it.
The entire bearing 6307 falls onto the upper plane of the positioning groove of the water pump cover, with the following three necessary conditions.
(1) Improper fit between bearing 6307 and the shaft and bearing seat
Bearing 6307, the inner ring should have an interference fit with the shaft, and the outer ring should also have an interference fit with the bearing seat. It slid and fell, proving that the interference fit between its inner ring and shaft, as well as between its outer ring and bearing seat, was insufficient, causing it to slide due to its own gravity and vibration. This is either a design error or a machining defect.
(2) Lack of positioning measures for bearings
If bearing 6307 has positioning measures, such as pressing the inner ring of the bearing onto the shaft and/or pressing the outer ring of the bearing onto the bearing seat, even if the interference fit between the bearing and the shaft and bearing seat fails, it will not fall off completely. The lack of positioning measures for bearing 6307 is clearly a design defect.
(3) There is space for bearing 6307 to fall off
The structure of the upper end of the bearing and water pump cover is shown in Figure 1.
Figure 1: Bearing and Upper Cover of Water Pump
Radial measurement shows that the diameter of the positioning groove on the upper cover of the water pump is 90mm, which is larger than the outer diameter of the bearing by 80mm.
Vertically, the measured distance between a-b is 40mm, while the height of bearing 6307 is 21mm, with a sliding space of 19mm.
But 'having a drop space' is not a defect, it is just a condition for the bearing to fall onto the flat surface of the positioning groove of the water pump cover.
The pump meets three necessary conditions, so the entire bearing 6307 falls onto the upper plane of the positioning groove of the water pump cover.
In summary, the cause of the malfunction was the improper fit between bearing 6307 and the shaft and bearing seat, as well as the lack of positioning measures. As a result, bearing 6307 fell off completely, causing the pump shaft to wear unevenly, the inner grinding chamber of the water pump cover to be damaged, and the mechanical shaft seal to be damaged, resulting in water leakage.
3. Corrective measures
The design errors and processing defects of the equipment are almost impossible to correct on board, and only relevant parts and components can be purchased and replaced after they arrive on board.
At that time, the spare parts were not complete, only the shaft seal and bearing 6307 were replaced. To prevent bearing 6307 from falling off, a 0.05mm shim is added to the outer ring of bearing 6307, and an old 6208 bearing (with a height of exactly 19mm) is placed underneath it. The method is to cut the 6208 bearing into two half circles from the middle, polish it, and insert it into the bearing seat between the lower plane of the 6307 bearing and the upper plane of the positioning groove of the water pump upper cover. Then, the two half circles are welded together to form a bracket with a diameter of 80mm. A metal gasket is added to the lower end to tightly press against the outer ring of the 6307 bearing, temporarily maintaining its use. At the same time, purchase relevant spare parts and provide feedback to the manufacturer about the pump malfunction to remind them to monitor the quality. It is recommended to add a 2.5 × 2.5mm snap spring groove below the bearing 6307 on the inner wall of the bearing seat and equip it with a snap spring to prevent the bearing from sliding down.
In December 2009, the ship arrived in Shanghai and all the requested spare parts were delivered to the ship. In February 2010, due to severe water leakage of the water pump, the pump shaft, two upper and lower bearings, bearing seat, mechanical shaft seal, water pump upper cover and other components were replaced.
There is no responsibility to ensure the quality of procurement on board, but there is a responsibility for acceptance. Before replacing the spare parts, the pump shaft and bearings were measured to confirm a 0.06mm interference fit between the bearings and the bearing seat; At the same time, it was found that the manufacturer has accepted our suggestion and added a 2.5 × 2.5mm retaining groove and corresponding retaining spring on the inner wall of the bearing seat below the 6307 bearing.
After updating the spare parts, the pump operated normally.
2024 August 5th Week VAFEM Product Recommendation:
A linear-motion bearing or linear slide is a bearing designed to provide free motion in one direction. There are many different types of linear motion bearings. Motorized linear slides such as machine slides, X-Y tables, roller tables and some dovetail slides are bearings moved by drive mechanisms. Not all linear slides are motorized, and non-motorized dovetail slides, ball bearing slides and roller slides provide low-friction linear movement for equipment powered by inertia or by hand. All linear slides provide linear motion based on bearings, whether they are ball bearings, dovetail bearings, linear roller bearings, magnetic or fluid bearings. X-Y tables, linear stages, machine slides and other advanced slides use linear motion bearings to provide movement along both X and Y multiple axis.
2024-08-31