Some of these tools are fitted on arms fitted to the feed hoppers of Crusher plants to break up any oversize rocks that have been missed at the quarry face or on demolition crushers to clear blockages. Normal practice is for the excavator or wheeled loader drivers to put oversize material to one side as it can damage truck bodies and jam the crusher. The oversize material is then broken up by a large hammer equipped machine at intervals and then sent to the crusher. This operation is often performed at intervals by external contractors or when normal production is shut down for maintenance.