A Drive Guards is part of Proper safety precautions start with the initial installation of the machine. Crushers, for example, are driven either by V-belts or by direct coupling should have a protective guard around it.
Crusher sheaves in particular are designed for maximum rim speeds. In these speeds are exceeded it is possible that the sheave could explode and cause severe injury or even death. 

Cyanide - NaCN dosage in Flotation Crystallography, Optical Properties and Cleavage Crane for Crusher Maintenance Identifying Lithium Minerals Mathematical Sizing of Grinding Mills Using Bond's Equation High pressure oxidation of refractory gold minerals Used Mining Equipment South Africa EXPERIMENTAL FLOTATION Sedimentary Rock – Limestone Composition & Structure Double-RolI Crushers The Effect of Lifter Height and Drop Height on Milling and Grinding Stope and Mine Control Plutonic, Coarse-grained Rocks Jadeite and Wollastite Composition, Crystallization & Structure Dodge Crusher