Ceramic Step Bearing in a Centrifugal Pump

The invention relates to bearings for the rotors of electrical motors or magnetic couplings driven by a motor, which have spherical magnetic air gaps between a driven rotor and a driving stator or driving outer magnetic ring. Such spherical motors or spherical magnetic couplings are becoming more and more important in modern pump design. The rotors of spherical motors or couplings are held in position by magnetic forces pressing the rotor against a stationary mounted ball. The impellers of pumps which form a unit with the driven rotor produce hydraulic thrust counteracting the magnetic thrust. When the electricity is switched off, the magnetic force of the driven stator ceases immediately. In contrast, the hydraulic thrust, caused by the difference of pressure between the suction side of the impeller and its backside, generated by the rotation, slowly decreases with decreasing rpm of the impeller. This results in these hydraulic forces tending to lift the impeller with the rotor away from the step bearing after switching the electricity off. The same change in the direction of the axial thrust occurs in magnetic couplings, if the hydraulic forces can exceed the magnetic forces during operation. Therefore it is necessary to support the rotor, not only in the direction of the magnetic forces during constant operation but also to prevent movement in the opposite direction by a ring-shaped, rotating annular member. Spherical bearings consisting of a cap and a ball made from metal are known. Metal balls can be brazed on the tip of a strut. Unfortunately, metal bearings have the tendency to seize when dry friction causes very small fractions of the surface to reach the melting temperatures of the metal causing the cap and the ball to fuse together. Cylindrical bearings produce a film of lubricating liquid which, by hydrodynamic principle, generate high dynamic forces preventing contact between the two members of the bearing. Spherical bearings do not build up such liquid film and therefore, run either dry or semi-dry. Consequently, the probability of seizing is much larger in spherical bearings than in cylindrical bearings. This is the season that metal balls have been replaced by balls made from ceramic materials such as aluminum oxide, circonium oxide or silicon carbide. The melting point of these oxides is so high that it will never be reached by heat of friction; the same applies to silicon carbide, which in addition sublimes and it has no melting point at all.

The disadvantage of ceramic balls is that the ball can be neither welded or brazed onto the strut. It is known to the art pressfit the ball into the hollow strut by pressing the ball into a hole slightly deeper than the radius of the ball, resulting in a useable surface smaller than a hemisphere. These bearings with ceramic balls act only as a step bearing because they can only bear the axial forces in one direction. This restricts their application to spherical rotors in applications where under no circumstances hydraulic forces will not exceed the magnetic forces under any circumstances.

The invention overcomes this restriction in the application of ceramic balls. It relates to spherical bearings having a second annular bearing member. In a first solution of the invention, the ball is encased in a partially spherical metal shell. The rim of the shell reaches slightly over the equator of the ball so that the ball is clamped by said rim. The outer surface of the shell follows the spherical contour over that region of the ball, and in cases where hydraulic thrust exceeds magnetic thrust, said second annular bearing member comes into contact with the shell on its spherical surface.

A second solution of the invention provides a circular groove in the ball with a distance from the equator wide enough to leave a sufficient gliding surface for said annular bearing member.

A third solution of the invention forms a stem on the ball leaving a sufficient gliding surface for said annular bearing, the stem being pressfitted into a hole of the strut.

A fourth variant of the invention shows a ball with a groove and a metal part fixed in a slot.

A fifth variant of the invention shows a ball with a hole through the ball and a screw to fix the ball on the strut, preferrably by means of a spring to dampen severe shock.

The invention shall be explained by embodiments of the foregoing characteristic features.

FIG. 1 is a cross section of a pump with a magnetic coupling.

FIG. 2 shows an enlarged design of the bearing with a shell.

FIG. 3 shows an alternative way to fasten the shell to a strut.

FIG. 4 shows a cross-section of a pump whereby the ball has a groove.

FIGS. 5 and 6 show designs of balls with a stem.

FIGS. 7 shows a design with a ball having a hole.

FIGS. 8a and 8b show a ball with an axial groove.

FIG. 9 shows a ball with a hole forming a slight undercut.

FIG. 10 shows a ball screwed to a strut.

FIG. 11a and 11b show a variant of FIG. 10 with a nut in a hole.

FIGS. 12a and 12b show a variant of FIG. 10 with a nut positioned in a slot.

FIGS. 13a and 13b show a variant of FIG. 10 with a nut in a hole.

FIG. 14 shows a design with a ball being springloaded by a spring washer.

FIG. 15 shows a similar design as a FIG. 14 with a screw type spring.

FIG. 1 illustrates a cross section through a centrifugal pump with a housing 1, having an outlet opening 2 and inlet opening 3. The duct 4 leads fluid to the eye region 5 of the impeller 6. The impeller 6 with the veins 7 and the convex magnet 8 forms a rotating unit. This rotating unit is driven by the concave magnet 9 mounted on the shaft 10 of the motor 11. Between the convex magnet 8 and the concave magnet 9 lies the separation wall 12 which separates the wet compartment 13 from the dry compartment 14. The rotating impeller-magnet unit 6, 7, 8 is attracted by magnetic forces in the direction of the arrow 15. The step bearing is formed by the rotating cap 16 and the stationary ball 17. The cap is part of the rotating impeller-magnet unit. The ball is fixed on strut 18 which forms a part of the separation wall. Annular bearing member 19 made from slightly elastic material prevents movement of the rotating unit in the direction opposite to arrow 15. Instead of a rotating concave magnet 9 a stationary stator with windings can also produce a rotating magnetic field. The convex magnet 8 will then be substituted by a convex armature.

FIG. 2 shows an enlargement of the region encircled by circle II in FIG. 1. The ball 17 is encased by a shell 20. The rim 21 of said shell slightly overlaps the equator 22. The spherical ring portion 23 of the ball 17 forms the gliding surface with the concave region of the cap 24. The shell 20 has a spherical surface 25 forming a narrow clearance 26 with the annular bearing member 19. If the rotating unit 6, 7, 8 starts to move in the direction of arrow 15, the annular bearing member 19 gets in sliding contact with the spherical outer surface of the shell 20. The shell 20 has a protrusion 28 which is fixed to the strut by snap action or brazing. Alternatively the protrusion 28 itself could form the strut.

FIG. 3 shows a different design whereby the shell 30 forms a semi-spherical body. This body is fixed to the strut 18' by brazing material 29.

FIG. 4 shows a spherical pump motor unit with inlet opening 40, outlet opening 43 forming together with space 42 the wet part of the pump motor unit. Magnetically permeable separation wall 44 encases together with the flange portion 45 and the pump housing 46 said wet compartment. The impeller 47 with veins 48 and armature 49 form a rotating unit. The hub portion 50 of the impeller 47 contains the bearing cap 51. This cap and stationary ball 52 make up the step bearing. The ball 52 has a circular groove 53. The rim 54 of the stationary mounted shell 55 is slightly bent inward entering groove 53. The shell 55 is mounted on a strut 57. The cap 51 reaches beyond the equator 56.

FIG. 5 shows the end portion of strut 58, consisting of a hollow body made of stainless steel sheet metal. Said strut 58 has a collar 59 which defines the seat of the annular axial surface 60 of the ball 61 produced by grinding. The inner part of the ground portion forms a stem 62. The inner diameter 63 of the strut is slightly larger than the stem 62, except in the region of the annular shoulder 64. This shoulder 64 holds the stem 62 by pressfit. The periphery of the collar 59 has a spherical configuration, matching the sphere of the ball 61. The larger diameter of said collar 59 is identical with the diameter of the ball 61 or preferably slightly smaller so that the surface 60 has a slight axial distance from the equator 66. The smaller diameter of the collar 59 matches the inner diameter of the annular member 19 shown in FIGS. 1 and 2. The method to produce the device consists of the following steps:

(a) by a grinding process, a recess on one hemisphere of the ball 61 is produced, by which process a stem 62 is formed, its length being shorter than the radius of the ball,

(b) a strut 58 in the form of a body of rotation with a hollow cylindrical end portion is produced by a deep drawing process, whereby the inner diameter of said end portion is slightly smaller than the diameter of the stem 62,

(c) the rim of said end portion is bent outwardly forming a flange 67a,

(d) the inner diameter of said end portion is slightly increased at both sides of an annular shoulder 64,

(e) the stem 62 of the ball 61 is driven through said shoulder 64 until the axial surface of the flange 67a sits on the flat annular surface 60 of the ball 61,

(f) by tool machining for removal of chips, the outer surface 59 of the collar 67a becomes spherically shaped whereby the sphere matches the sphere of the ball 61.

FIG. 6 shows the same ball configuration, but between the strut 58a and the ball 61a a body of rotation 68 is arranged, which is held by pressfit in a hole of the strut 58a. The stem 62a of the ball 61a is held by pressfit in the hole 64a of the body of rotation 68.

FIG. 7 shows a ball 71 with a blind end bore 77. The base around the blind end bore 77 rests on the upper arm 74 of the strut 72. A screw 73 is hollow and is used as fixture. The outer diameter of said screw 73 is slightly larger than the diameter of the hole. This results in high friction forces. The screw 73 is preferrably lubricated with a viscous lubricant. After mounting the lubricant evaporates under heat treatment. It is also anticipated to use a two component resin such as epoxy which hardens and provides high shear forces. To prevent contact between the epoxy and the liquid to be conveyed an O-ring 75 will be placed into a groove.

FIG. 8a shows a ball 91 with an axial annular groove 97. The end portion 93 of the strut 92 is formed as a hexagon providing high friction on the surface of the stem 98.

FIG. 8b shows a cross section through plane C--C in FIG. 8a.

FIG. 9 shows a ball 111 with a blind end bore having a slight undercut, its blind end bore in the region 117 being wider than the opening 119. The hole will be filled with hot metal. The metal body 118 will then be brazed to the end of the strut 112.

FIG. 10 shows a ball with a bore hole and a widened reset for the screw head. The strut 81 has a conical end portion with an opening angle equal or smaller than the complement angle to the angle 82 which matches the angle of the tangents 83. The screw 66 fits into the screw hole 85 and the ball is pressed against the rim 84.

FIG. 11a shows a variation of the design in FIG. 10, whereby a nut 88a is pressed into a hole 100 to hold the screw 86b.

FIG. 11b shows the view of the strut from above.

FIG. 12a shows a strut with a slot 87, its width matching the width of the nut 88, holding the screw 86a.

FIG. 12b shows a cross section along the line A--A in FIG. 12a.

FIG. 13a shows a similar design whereby the nut 88b is placed inside a hole 89. The screw head rests on an annular plane 113.

FIG. 13b shows a cross section through line B--B in FIG. 13a.

FIG. 14 shows a cross section through a strut 115 with a tapering end portion 116. The ball 117 rests on the rim 118. The ball is pressed against this rim 118 by the forces of a prestressed spring washer 119, which forces are transmitted to the ball 117 by a screw 120 and the nut 121. The direction of rotation to fasten said screw 120 is identical to the direction of rotation of the bearing cap 122 which is also held in position by the bearing ring 123.

In case of exorbitant forces acting on the ball 117, as may happen if a pump is dropped, the ball moves away from the rim 118 and the spring washer 119 gives way. Immediately thereafter the ball snaps back into the operational position and will be centered by the rim 118.

FIG. 15 shows a similar design as in FIG. 14, suitable for cylindrical struts 125 which have a tapering rim portion 126. The spring 127 is formed as a screw type spring, supported by three lashes 128, which are bent inwardly.