All submersible motors are fitted with Hydrodynamic Self Adjusting Tilting Shoe Thrust Bearings to take the axial "down thrust" from the pump. These are a complex and clever part of a reliable submersible motor and probably not understood very well.
These thrust bearings will take a surprisingly heavy axial "down thrust" load and provided that they have been installed correctly, rated correctly, and looked after properly, they will provide many years of trouble free operation.
Motors also have an "up thrust" bearing to take thrusts - usually these occur at start up while the discharge pipe work is filling. The "up thrust" can be significant if pipe work is long between the pump and the surface and the Non Return Valve (NRV) has been drilled to allow the column to drain. While the column is filling there is a large momentum force upwards, and the head, acting down, is low, and the pump is quite likely to be operating beyond its Best Efficiency Point (BEP). The "up thrust" tends to be heavier with closed type impellers. On some pumps the thrust will reverse just after the BEP.
Pumps should have a means of accommodating temporary "up thrust" and the splined couplings between the pump and the motor should not be grub screwed.
The "up thrust" bearing in the motor is a fairly simple device, which consists of a resin/cloth fibre ring, which rubs against the underside of the Stainless steel Thrust Bearing Support plate. These bearings are not continuously rated and only take light loads.
Principle:The Hydrodynamic thrust bearing transmits the rotating shaft's axial thrust load to the frame of the motor, which is mechanically supported in the well.
The axial thrust load is transmitted through the bearing on a self-renewing film of lubricant, which is water in most of the submersible motors. The pressure in the fluid film supports the load without the thrust disc or pivot shoes making contact.
Theory:Due to viscosity, fluids tend to stick to surfaces, and in the case of this type of thrust bearings, they rely on the fluid sticking to the surface of the rotating thrust disc. The fluid is dragged circumferentially as well as being thrown outwards by the rotating disc. At the leading edge of the pivot shoes, a continuous fluid wedge will form. When the right conditions exist the wedge will produce sufficient lift to create a physical separation between the thrust bearing disc and the face of the pivot shoe. At this point a true fluid film (TFF) is said to exist.
TFF is essential for the successful operation of the thrust bearing. When the bearing is operating correctly there is no contact at all between the disc face and the face of the pivot shoes. The only time there is contact is when the motor is stopping or starting. This brief contact period means there should be only negligible wear between the faces - and no wear while the motor is operating.
The adjacent sketch shows how the pivot shoe tilts over and allows the fluid to be forced between the carbon thrust disc and the face of the pivot shoe by a combination of rotational drag and centrifugal force. The pivot point should be spherical which allows the shoes to rotate or pivot so that the fluid can form a wedge. The shoes need to be loosely constrained while still free to pivot.
The carbon thrust bearing drags the fluid around in a circular direction, but this fluid also experiences a centrifugal force, which is pulling the fluid towards the circumference of the bearing. The combination of these 2 forces means that the fluid is circulating and going outwards at the same time and it has been found that the peak pressure point is somewhere beyond the centre in the direction of rotation. Quite often this is assumed to be on the face of the pivot shoe approximately 75% across the face of the pivot shoe and 75% out towards the circumference. This is known as the 75/75 Rule in Thrust Bearing design and is where the bearing has peak loading, minimum film thickness, and high temperatures. If the thrust bearing only operates in one direction it is common to offset the pivot point so it is nearer to the 75/75 point.
Issues that need to be considered during the design of a hydrodynamic thrust bearing are maximum total loads, load per mm2 on the bearing surface, number of shoes, thrust disc surface speed, fluid viscosity, and maximum allowable fluid temperature. Hydrodynamic thrust bearings commonly have 6 or 8 pivot shoes because of the difficulty of ensuring that all the shoes share the load evenly. More shoes increase the likelihood of uneven loading because of variations in dimensions due to mechanical tolerances.
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