Shaft couplings are used to couple two adjacent sections of shafting together. The most typical use for a shaft coupling is to couple a solid gearbox output shaft to the mixer shaft (Many smaller mixers use "hollow quill" gearboxes where the end of the mixer shaft is machined to a close tolerance to fit into the gearbox quill, and therefore do not require a shaft coupling). Couplings are also required when the overall shaft length is greater than can be safely transported and/or handled during installation. The two most common types of shaft couplings are the rigid flanged coupling and the rigid split coupling (Flexible couplings are rarely used on mixer output shafts, and are more often referred to regarding high speed motor couplings).