Specifications

Model 900 Tool Guide
Page O16
How a demolition tool cracks rock and concrete
When the hammer piston strikes the top of a demolition tool, it sends a compressive stress wave
down to the working end of the tool. Provided that the demolition tool is in contact with the rock or
concrete which requires breaking, it is this compressive stress wave which fractures the rock.
Then, immediately following the compressive wave, a tensile stress wave is formed due to the
hammer piston lifting from the top of the demolition tool.
The cycle of compressive and tensile stresses flowing down the tool is repeated for each hammer
blow. Obviously, anything that interferes with the strength of the compressive stress wave during
service, for example "blank firing" or bending of the demolition tool due to leverage, will result in
loss of breaker efficiency of up to 80% and possible failure of the tool itself.
Correct operating conditions
The continuous cycle of compressive and tensile stresses in the demolition tool, even under correct
operating conditions, creates fatigue stress in the tool which can lead to the fatigue failure of the
tool before it is worn out. Anything that interferes with the cycle of compressive and tensile stresses
will also increase the level of fatigue stress being applied to the demolition tool and, thus, increase
the risk of early fatigue failure of the tool.
The main cause of increased fatigue stress in a demolition tool is any form of side pressure during
service which creates bending. Thus, utilizing the tool as a lever, using an incorrect driving angle or
attempting to break ground using the pull of the excavator are all detrimental to the life of a demoli-
tion tool and should be avoided.
The hydraulic power available in the machine far exceeds the strength of a demolition tool
and if it is used incorrectly can "snap the tool like a pencil".
Demolition tool fatigue failure
Demolition tool fatigue failure will occur approximately 4 inches from either side of the chuck front
face or through the retainer pin flat. Another slightly less common failure area can fall approximate-
ly 8 inches from the working end, subject to the nature of use. The fracture face itself will normally
exhibit a semi-circular polished area with the remainder being of rougher appearance. The polished
semi-circular area is the fatigue area and generally started from a damage mark or other stress
initiated marks on the outside of the demolition tool and spread inward.
The fatigue area slowly widens until the stresses being applied to the demolition tool cause sudden
failure of the remaining section. Generally, the size of the fatigue area indicates the level of stress
applied to the tool, i.e., the smaller the fatigue area, the higher the stress level. However, it must be
borne in mind that once initiation of a fatigue crack has taken place, it requires a lower stress level
to cause it to grow.
Caution