The Influence of Thermal Expansion
on the Design of Silicon Carbide Bearings

1.   Introduction

Ceramic slide bearings made of Silicon carbide (SiC) in sealless pumps have become accepted as a standard. The advantage of SiC over conventional material is that it has good corrosion- and high temperature resistance as well as a high hardness. All of this allows the use with hot and abrasive delivery media. From the design view there are difficulties in combining the physical very different materials ceramic and metal, particularly due to the different thermal expansion coefficients (17*10-6 1/K for stainless steal und 4*10-6 1/K for SiC) and due to the brittleness of the SiC, which only allows low tensile stresses.

Therefore in this paper the already existing design solutions will be researched and descript. The aim of this research is to list the available technical solutions in order to evaluate the quality of these solutions in the second part of the paper. In the first step of the search a patent search is carried out and in the second step internet resources and magazines are considered.

2.   Patent search

The patent search was carried out by means of DEPATISnet. DEPATISnet is provided by the “Deutsches Patent- und Markenamt” (DPMA) and it enables the online search for patent publications all over the world. The patents are sorted in categories in the database. These categories are listed in the International Patent Classification (IPC).

In this search all patents in the IPC category F16C 17/22 were considered. Additionally searches with the terms in the Table 1 were carried out in the category F16C 17/24. The meaning of the both considered categories are listed in Table 2. Further all patents which were cited in the found patents were considered.

Table 2: Considered IPC classes and its meaning

In the following chapter patents which are significant for this search are described.

2.1 Disk Springs (EP 0 771 957)

The invention of the KSB AG features that tensile loads on the bearing ring are avoided by a radial play between bearing bush and shaft. That’s why expansions of the shaft don’t act on the bearing ring. The radial centring of the bearing sleeve is realized over cone surfaces at the bearing sleeve and at two retainer rings over a large temperature range. The retainer rings exert pure compression stresses on the bearing sleeve. The different axial expansion of shaft, bearing sleeve and retainer rings, during a temperature rise, would lead with missing reconciliation to a gap becoming larger with rising temperature between bearing and retainer ring. This gap at the cone surfaces is compensated by spring elements, which are arranged between one of the retainer rings and an axial attachment. The spring element exercises an axial force on the retainer ring, which causes the retainer ring to shift on the shaft against the bearing sleeve. Thereby an actuated connection between the retainer rings and the bearing sleeve remains. The retainer ring connections exhibit close sliding fits, so that the bearing sleeve is centred by the cone surfaces in relation to the retainer rings and thus also in relation to the shaft. The reconciliation of the axial expansion can be defined by the dimensioning of the cone surfaces and the spring elements and is applicable over a great temperature range. As spring elements preferably disk springs are used.

1:    Representation of an execution of the invention as floating bearing

Figure 1 shows a possible design of the connection described above. Here two disk springs are used as spring elements. These push the retainer rings and the bearing sleeve against a socket serving as axial attachment. The outer bearing ring is pressed into the housing (lantern).

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Figure 2:    Representation of an execution of the invention as fixed bearing

Contrary to the sliding bearing arrangement represented in Figure 1, which ensures only a radial positioning of the shaft relative to the housing (moveable bearing), the bearing arrangement shown in two alternative designs in Figure 2 allows a radial and an axial positioning of the shaft (fixed bearing). This is made possible by the fact that the retainer rings have sliding surfaces on their axial sides.

The retainer rings represented in the top of Figure 2 have sliding bearing surfaces at the front, which cooperate with the axial surfaces of the bushes, and thus form an axial sliding bearing. The axial plays between the axial surfaces have to be designed in the kind that also at the maximum temperature arising during operation a shift of the retainer ring and the bearing is possible for the compensation of the thermal expansions. In order to be able to take up axial forces in both directions as well, the spring action of the spring element is dimensioned so that the force exerted by it is larger than the axial force which can be transferred.

2.2 Disk Springs 2 (EP 0 771 956)

In the KSB’s patent from 1996, the invention described in the previous chapter is extended to a complete arrangement of bearings.

Figure 3:    Representation of the complete bearing configuration

In Figure 3 an arrangement of bearings in accordance with this invention is depictured. Here both the inner bearing rings as well as the outer bearing rings are positioned on cone surfaces. Thereby both the different radial as well as the different axial expansions between shaft and inner rings respectively between the outer bearing rings and the housing can be compensated.

2.3 V-groove with circlip (DE 2 000 601)

A similar solution with cone surfaces as described in the two previous chapters is also the basis of the patent of Leyland Gas Turbines Limited out of the year 1970.
Here the cone surfaces, between which the inner bearing ring is wedged, are arranged so that they build a V-shaped groove, which opens radially outward. Due to this arrangement one of the cone surfaces can be implemented as part of the shaft. On the opposite side then however the cone surface must be part of a removable ring, in order to be able to install the bearing. Figure 4 shows a possible design of this invention.

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