TRUCK-MOUNTED MOBILE CRANES


Produced by:
Professional and Specialized Services,
Ministry of Labour,
Alert 10/0695
ISSN 1195-5228

HAZARD SUMMARY:

A construction accident investigated by Ministry of Labour 
officials involved the failure of a 22-ton truck-mounted mobile 
crane.

The failure appears to have begun at the inner bolts that secured 
the slewing ring (the gear assembly that rotates the boom 
assembly) to the crane's pedestal.  These bolts had been 
improperly torqued, which led to bolt failure and, eventually, to 
the complete failure of the boom assembly.

On truck-mounted cranes, failure to make sure that the torque 
values on bolts are equal and as prescribed in the manufacturer's 
specification can result in uneven distribution of stresses on 
the mounting bolts.  This can cause bolt failure, which will 
ultimately allow the crane assembly to come off the truck 
chassis.

REQUIRED ACTION:

-  Mobile cranes should be inspected to make sure that each 
   fastener is of the size and grade specified by the 
   manufacturer. 

-  The fasteners must be torqued, to the values set by the 
   manufacturer, with a torque wrench that is properly calibrated 
   at intervals not greater than those specified by the 
   manufacturer. 

-  Owners, suppliers and operators of mobile cranes must observe 
   the precautions outlined in the manufacturer's maintenance and 
   repair manual. 

For additional information, please contact the nearest office of 
the Ministry of Labour.