Page 116 - Machinery Component Maintenance

Machinery Foundations and Grouting 99

Those cracks which appear as a result of externally applied dynamic
forces are characterized by continuous progression through the block and
by movement between the segments. Once movement begins between the
segments, it promotes further propagation through the entire block.
These constitute structural failures and are considered serious.
Movement of the block can place the equipment frame in tension. For-
tunately, fatigue of the grout bond to the equipment base usually occurs
before the casting itself fatigues and cracks. Failure of the grout bond,
which allows movement of the machine base, is easily detectable either
visually or by dial indicators. If such movement is allowed to continue
for long periods of time, crankshaft failure, failure of the main bearing
supports, or both can be the result.

Causes of Foundation Failure

Failures in concrete machinery foundations can be attributed to im-
proper or inadequate foundation design, poor construction practices and/
or poor operating conditions.
Criteria for the design of machinery foundations vary widely. In simple
analyses, mass is estimated by rules-of-thumb with allowance made for
local soil conditions, whereas highly sophisticated analyses use a wide
range of computer tools. In either case, it is common practice to place
reinforcing steel in the block in accordance with the same design princi-
ples that apply to reinforced concrete members in general. Hence, it is
common practice to place reinforcing steel in a symmetrical pattern
throughout the block. It would be prudent, however, to place reinforcing
steel with due consideration to source and direction of external forces ap-
plied to the foundation. Accordingly, the preponderance of reinforcing
steel in a reciprocating enginekompressor foundation should be placed in
the upper portion of the block in a direction perpendicular to the crank-
shaft. Preferential placement of steel in this location would reduce the
tendency for cleavage-type failures that sometimes begin at the top of the
foundation in the notch below the oil pan and extend through the block to
the mat below. These cracks are always parallel to the crankshaft and are
caused by forces set up by the power pistons or compressor stages.
Obtaining a good concrete foundation is not, however, the result of
good design only. Good construction practices must be followed as well.
After the reinforcing steel has been installed and the forms constructed,
the concrete is poured. Concrete must be properly prepared and poured
while the mixture is fresh. Ideally, the pour should be monolithic and
vibrated in the process to remove voids. Where the mass is quite large or
where ambient temperatures are at either extreme, curing temperature
control measures must be employed. Finally, moist curing conditions