21 is secured to the shaft 23 to rotate with it but a sufficient force can be used to displace the mating ring 21 along the shaft 30 accordingly the mating ring 21 is axially displaceable along the shaft 30 in axial direction x one or more seals 23 may be provided to seal off an interface between the mating ring 21 and the shaft 30 for instance the seal 23 may be an o ring a gasket etc made of a material capable of withstanding the pressures and temperatures in the apparatus moreover the material must be resistant to the nature of ambient fluids e g oil the mating ring 21 defines a shoulder 25 that is configured to receive thereon an annular seal 26 part of the annular seal assembly the annular seal 26 forms part of the dynamic seal interface of the magnetic seal system 20 as it will rotate with the mating ring 21 and the shaft 30 while rubbing against an annular magnet 27 part of an annular magnet assembly the annular seal 26 is therefore connected to the mating ring 21 to rotate with it in the illustrated embodiment the shoulder 25 provides a pair of abutment surfaces for the annular seal 26 to strengthen the connection between the annular seal 26 and the mating ring 21 and cause concurrent rotation other arrangements are contemplated such as an annular groove in the mating ring 21 to accommodate a portion of the annular seal 26.