30 CFR § 57.14206
Securing movable parts.
(a) When moving mobile equipment between workplaces, booms, forks,
buckets, beds, and similar movable parts of the equipment shall be positioned
in the travel mode and, if required for safe travel, mechanically
secured.
(b) When mobile equipment is unattended or not in use, dippers,
buckets and scraper blades shall be lowered to the ground. Other movable
parts, such as booms, shall be mechanically secured or positioned to prevent
movement which would create a hazard to persons.