Impellers: Set Screwed, Keyed or Welded?
Mixing impellers used on most small scale industrial mixers depend upon set screws for assembly to the mixer shaft. This convenient method is ideal where frequent adjustment in position is desirable and where the forces created during application are small enough that set screws are reliable. Laboratory stirrers and portable mixers with shaft diameters up to 5/8” feature set screws as the sole method of impeller attachment. With larger models, heavier impellers and higher forces additional means are provided for securing impellers. Mixers with shaft diameters from ¾” through 1 ¼” have machined flats on the shaft to provide more engagement with the set screws and to prevent against rotational slipping of the impeller.
Our larger mixers with shaft diameters 1 ½” and greater incorporate a jib key. This L-shaped key is recessed into a shaft keyway. The impeller hub with matching keyway slides down over the vertical portion of the jib key and is supported below by the horizontal portion of the jib key that extends beyond the shaft diameter. Set screws are also used with one of them tightened onto the key. This design prevents rotation of the impeller and prevents heavier impellers from falling off when the set screws are loosened.
Welding of the impeller to the shaft is most often the solution when small crevices are unacceptable due to a sanitary process. The impeller is permanently located on the shaft via a continuous weld of the same alloy. These welds are generally ground and polished to a sufficiently large radius to promote cleaning.