Page 103 - Machinery Component Maintenance

86 Machinery Component Maintenance and Repair

It is believed that many foundation cracking problems and broken an-
chor bolts are caused by improper bolt installation. Anchor bolt sleeves
should be filled with a pliable material such as silicone rubber molding
compound. When sleeves are filled with a rigid material, and the equip-
ment is grouted without isolating the anchor bolts, extremely high
stresses are imposed on the foundation as the equipment grows from ther-
mal expansion. Growth on a 204 machine can be as much as 0.200 in.
when thermal equilibrium is reached. Anchor bolts near each end of the
equipment are placed under the greatest stress.

Outdoor Installations

Because epoxy grout and concrete absorb and dissipate heat rather
slowly, cyclic temperatures cause uneven thermal expansion or contrac-
tion. This unequal expansion produces unequal stresses. The weak link is
the tensile strength of the concrete, and cracking occurs in the corners
where stress risers exist (Figure 3-5). In Condition A, the system is in
thermal equilibrium and no stresses exist. During the cooling cycle
shown in Condition B, the grout surface contracts first. This causes a
thermal gradient within the grout which produces stress that promotes
edge lifting. As the temperature conditions are reversed during the heat-
ing cycle, as shown in Condition C, cracks have a tendency to close. As
the cycle is repeated, cracking progresses until it reaches a point under
the edge of the equipment where compressive loading exists. Because
tension is required for cracking, cracking cannot continue into an area
which is under compression. Thicker grout increases the tendency for
cracking until the cross-sectional dimensions of grout and shoulder are
about equal. After grout thickness equals or exceeds the shoulder width,

Heat Heat

Condition A Condition B Condition C
At thermal equilibrium During the cooling During the heating
cycle cycle

Figure 3-5. Stresses at foundation corners caused by cyclic temperature.