Page 115 - Machinery Component Maintenance

98 Machinery Component Maintenance and Repair

remove the fittings, rebore and retap the hole, and thereby use the same
injection site.
Pressure grouting sole plates seldom changes alignment. We attribute
this to the fact that excess epoxy is squeezed from beneath the sole plate
by the weight of the equipment. For purposes of illustration, assume the
equipment is being aligned with the aid of jack screws having hexagonal
heads and ten threads per in. One revolution of the screw would raise or
lower the equipment Vim; moving the screw one face would create a
change of l/6th this amount or 16/im. However, one face change on a jack
screw is scarcely detectable when measuring web deflections. Ncvcrthe-
less, the film thickness of epoxy under the sole plate should be far less
than 16/iooo. Thus, alignment should not be changed when equipment is
pressure-grouted .

Foundation Repairs*

Deciding what to do about cracked concrete foundations is no longer
the headache it was. Epoxy technology and application techniques have
evolved to where repairs are quite reliable. Equipment downtime is re-
duced from weeks or months to days. In some cases, repairs can even be
effected without a shutdown.

Types of Cracking
The tensile strength of concrete is only about 10 percent of its compres-
sive strength. Because of this weakness in tension, reinforcing steel is
embedded in concrete to carry the tensile loads. This addition of steel
inhibits one form of cracking and at the same time promotes another
form.
Concrete shrinks slightly on curing. Typical drying shrinkages of rein-
forced concrete structures are in the range of 0.02 to 0.03 percent. The
addition of reinforcing steel does not prevent shrinkage, but it does re-
strict the shrinkage. This shrinkage produces both tension in the concrete
and compression in the steel, and given enough steel, cracking of the
concrete can result. The resulting cracks are often referred to as curing
cracks. They are not considered serious because they are randomly lo-
cated and there is seldom any movement between adjacent segments.
They nevertheless provide a path for more rapid penetration of lubricat-
ing oil spilled later on the concrete. Such damage can easily be prevented
by sealing the concrete with the proper epoxy system after foundation
construction has been completed and before the equipment has been in-
stalled.