MSHA - Code of Federal Regulations - 30 CFR 56.14112
30 CFR § 56.14112
Construction and maintenance of guards.
(a) Guards shall be constructed and maintained to--
(1) Withstand the vibration, shock, and wear to which they
will be subjected during normal operation; and
(2) Not create a hazard by their use.
(b) Guards shall be securely in place while machinery is being
operated, except when testing or making adjustments which cannot be performed
without removal of the guard.