Sep . '78
CONSTRUCTION EQUIPMENT AND THEIR STRESS CRACKS
Equipment users often are plagued with items at earthmoving, construction, industrial, and materials handling equipment
that exhibits stress cracks in the structural members. Stress cracks are those cracks in metal members that are the result
of: (1) abnormal overloads, (2) fatigue from normal loads over a long time, and (3) any combination of (1) and (2). The
degree of stress cracking is governed by equipment use, time, and load variables. An important phase of PM servicing of
any item of equipment is the cleaning, inspections and repair, if necessary, of all structural members. Catching problem
areas in time prevents subsequent major repairs and equipment deadline. Due to the working nature of these lines of
equipment, a certain degree of abuse overloading, over speeding, and rough handling is bound to occur. The only way to
avoid abuse is not to operate the equipment.
Failures are the result of metal fatigue due to flexing, bending, twisting, vibration, shock, and distortion. The results show
up in the equipment structural metal members as weld cracks, metal cracks, and joint and fastener loosenings.
Repairs to minor components, and to members that are not part of the equipment's main structure; may be made without
special instructions in most cases. Just follow good shop and welding practices during repair and replacement. On the
other hand, repairs to major structural members must be made by an experienced expert mechanic one who can see the
requirements on the part to be repaired and who knows that his method, his workmanship, and his attention to small
details will do the job correctly. Care must be taken to see that the repairs do not contain stress risers areas of high
stress that can lead to another repeat failure or that just shift the next failure over a little bit. Repairs to plain carbon steel
are fairly easy. Repairs to high strength, low alloy (HSLA) steels, like SAE J410 and ASTM A242, especially need
attention to the details. Repairs to special heat-treated, truck frame type steels are the hardest of all. Exact procedures
cannot be covered here for making each type of repair. In order of preference, you should consult with the experts listed
below for equipment repairs:
- The equipment manufacturer's printed manuals and his service letters.
- The equipment manufacturer's dealer service personnel advice.
- The welder or fastener manufacturer's advice on the use of his equipment or product.
- General information available shops, library, schools, trade associations, etc.