Breviary Technical Ceramics


      Joining and Bonding Techniques

 Example: Torque Transmission

If the medium being pumped is particularly aggressive, the metallic rotors of eccentric screw pumps are subject to high levels of mechanical wear. SiC rotors are significantly harder than metal rotors, and offer a service life up to 20 times longer.

Force is introduced to the rotor through a pin joint. Because the ceramic reacts sensitively to impact and tensile stress it is not helpful to make the rotor entirely out of ceramic. An appropriate jointing method is therefore required in order to use ceramic rotors and to exploit the benefits of ceramic material.

Gluing may be an appropriate jointing technique for such a case. When designing the joint for an eccentric screw it is necessary to bear the following points in mind:

  • the material of the (metal) head,
  • the size and type of the forces that occur,
  • the maximum temperature in application and
  • the surrounding medium.

Figure 205: A silicon carbide eccentric screw


The necessary surface preparation (e.g. degreasing and sand blasting), and treatment of the surfaces where relevant, depend on the silicon carbide used for the eccentric screw and the stainless-steel or hardened tool steel of the head. Some materials have passive surfaces that must be treated with bonding agents in order to ensure adequate adhesion between the glue and the jointed part.

Figure 206: Gluing location of the eccentric screw

The type and magnitude of the stresses that occur in operation will determine the design and dimensioning of the glued areas and the selection of the glue. Eccentric screws are primarily subjected to torsional stress, which means that gluing to the face is not effective. The force is therefore introduced around the circumference of a centralised head.

The sizes of the glued areas depend on the maximum forces. The moment (Nm) that can theoretically be transmitted can be calculated from the following formula:

where t is the adhesion strength of the glue, D is the diameter of the joint, and L the length of the glued region.

The stress on the joint, however, should not exceed 10 % - 30 % of the theoretical strength, depending on the glue in use.
The operating temperature is very important for the choice of glue. The maximum application temperatures of glues based on organic materials reach up to 400°C. If the temperature is too high the layer of glue will soften or decompose, and the joint will rapidly fail.
The effect of the surrounding medium is relevant to the long-term stability of the glued joint. The strength of some glues drops sharply if, for instance, moisture diffuses into the material.

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