The shaft inside a butterfly valve has two functions: to define the position of the disc inside the valve body, and to transfer the torque, which is needed to turn the disc, via the actuator. These are the main requirements which need to be taken into consideration during the design stage of the valve. In addition, the shaft material must exhibit sufficient strength to withstand the high operating temperatures that occur inside the valve.

Depending on valve design and application, it is possible to choose between different shaft materials with sufficient strength. The shaft of concentric, lined butterfly valves does not usu ally come into contact with the medium. The medium can only come into contact with the shaft when the liner is worn out or damaged. Therefore, the strength properties of the shaft are more significant than, for example, good chemical resistance. In eccentric valves, the shaft will be in direct contact with the medium, so that chemical resistance has to be taken into consideration in addition to mechanical stress.

Depending on the requirement's profile, it is very important to distinguish between product and application-specific properties.

In case the shaft needs to resist acid or rust corrosion, austenitic materials such as 1.4404 (AISI 316L) are used. If high operating tempera tures are an issue, heat-resistant, austenitic ma terials such as 1.4980 (ATM Grade 660) are the first choice. For high demands regarding torsion or bending resistance, materials such as 1.4542 (17-4 PH) or 1.4469/1.4462 (Duplex) are avail able.

At the interface between shaft and actuator, the shaft end design is based on EN ISO 5211. That means that either a square, a two-flat or a key connection can transfer the torque of the actuator to the shaft. The most common end of the shaft is a square, which enables the direct mounting of pneumatic and electric actuators, without having to use mounting brackets or couplings. Square shaft-end design is convenient for mounting actuators in North/South or East/ West configuration.

As mentioned in the chapter "Discs of butter fly valves: designs and materials", shafts can be cast in one piece with the disc. In that case, the shaft is always made from the same material as the valve disc. However, this is a good solution only if the torque requirements of the shaft are not excessive. For higher de mands, shafts need to be of a material that is higher in quality compared to the disc. In that case, the shaft and valve disc are produced sep arately from each other and connected with the disc in ways described before, such as square ends, pins or keys.