5.4.2 Pinned roots
Multifork pinned roots are generally easier to manufacture than fir-tree roots, since the machined faces are plain with no undercutting, and the pin or rivet holes are simply drilled and reamed.
The enormous force on moving blade roots, particularly in the LP stages ensures rigidity in any type of root fixing at synchronous speed but, contrary to the fir-tree root, the multifork pinned root provides a rigid fixing when the machine is stationary. This enables the zero speed vibration characteristic of the blades to be confirmed with confidence and ensures a smooth unstepped spectrum of characteristics, as the machine is run up. Replacement of multiforked blades, however, involves further reaming of the securing pin holes. This is awkward in-situ and can only be carried out a limited number of times.