After 30 years of research, an engineering team in Japan has developed a hybrid-type submersible bearing that prevents burnouts during vertical pump dry-starts, exploits the elasticity of the synthetic rubber to level the pressure during typical operation, and ensures stable bearing behavior by conferring vibration control while supporting the rotating shafts.
Using polytetrafluoroethylene (PTFE) strips as slide members and synthetic soft rubber for cushioning between the slide elements and metal shell (or the base plates), the hybrid bearing can be used for dry-start operation of vertical pumps without applying lubricating water from the outside prior to pump operation.
Advantages of Adopting a Dry-Start Bearing
A wet-start vertical pump system requires that water be injected from outside the pump into shaft protection tubes at the top of the column pipes before operation. In most cases, the water is pumped up automatically after a fixed time to avoid wasting the feed water pump power or the water from the tap, which is usually called self-feed water. A dry-start pump does not require lubrication and is less prone to environmental damage from crevice corrosion in joint parts where seawater remains. Because the stainless steel shafts are exposed directly to the pump main flow, pitting corrosion—prone to occur in low-flow-velocity or stagnant regions—is reduced.
Structure of Hybrid Bearings
Three molds have been developed to produce three types of bearings, each suitable for a different range and scale of application. These include full-molded, segmental and barrel type bearings.
Bearings are basically composed of four layers: PTFE strips as slide elements, synthetic rubber for cushioning, base plates as the backing-plates and a metal shell that serves as the holder. The full-molded bearing is used almost exclusively for vertical pumps, making a simple, three-layered structure as shown in Figure 1.
Creating submersible bearings with materials that have low friction coefficients has been a top priority for submersible bearing manufacturers. Figure 2 shows friction coefficients in tap water of different bearing materials (PTFE, polyether ether ketone [PEEK] and polyurethane in hybrid structure with rubber) used in dry-start vertical pumps and rubber bearings used in wet-start pumps. Friction coefficients were obtained using identically structured bearings to match test conditions.
The graph plots one of the outcomes obtained by changing the bearing loads from 0.25 to 1.0 mega-Pascals (MPa) at four stages. Results show that all bearing materials have excellent friction coefficients.
Effects of Synthetic Rubber
Friction coefficients obtained using the two test bearings during wear resistance testing indicate that the test bearing with the persistently soft rubber layer (72 Shore A hardness) has a lower friction coefficient than the bearing with the rubber layer turned into ebonite (80 Shore D hardness). This suggests that the rubber layer may prevent sharp rises in the local pressure on the bearing conferred by the shaft deflection. The rubber seems to keep pressure low overall and limit the solid contact friction areas.
The free surfaces of the rubber made by or among the PTFE strips may improve the elastic effect compared with the bearings without free surfaces facing the shaft, as with a the bearing with a monolithic ring-like structure of metal and resin.1 The balance between the number of grooves and the size of the area in which the water film formed to lower the friction coefficients is important. If the number of grooves is increased to enhance the elasticity of the rubber, the size of the water film area will decrease and invite the larger friction coefficients and vice versa.
Because the pump shafts of the vertical pumps are suspended on the center of the column pipes, the bearing load by the shaft weight is comparatively small, which is typical with vertical pumps. This reduces the importance of self-alignment, but another problem may emerge.
Adhesive & Abrasive Wear Resistance
Wear resistance related to adhesive wear and the abrasive wear of the slide members is an important factor for submerged bearings from the viewpoint of tribology. Figure 3 shows the results of an adhesive wear test on two pieces of same-sized bearings. One was the PTFE and rubber hybrid bearing, while the other contained abundant sulfur and was vulcanized to harden the rubber into ebonite with the hardness of 80 Shore D.
Figure 3 plots the coordinating friction data according to the wear amount after confirming the friction coefficients through a series of tests performed concurrently for the pure wear test and the measurement of the friction coefficients. The wear amount of the PTFE/ebonite hybrid bearing is displayed as a ratio, while the wear amount of the PTFE/rubber hybrid is assumed to be 1.0. The graph indicates that the wear of the original bearing with the soft rubber layer is about one-half of the wear amount of the bearing with a rubber layer transformed into ebonite.