After the anchor bolt horizontal and vertical forces are analyzed, it can be determined whether the bolts have sufficient resistance to withstand the bearing forces. If the vertical shaking forces exceed the vertical holding forces of the bolts, the grout joint will be placed in tension, which may cause an early failure. Many times an increase in the bolt torques can reduce the engine base to grout movement. The bolt tension stress can usually be increased to about 65 percent of yield stress. If the bolt torque is increased, it is desirable to use more exact methods for obtaining the torque than the torque wrench. If enough room is available, a hydraulic tensioner should be used. Other methods can be used such as the turn-of-the-nut method or extensionometers. A load cell can be used to calibrate the torque wrench to improve its accuracy.
If the shaking forces are too high for the bolts to withstand, even with increased torque, then an investigation should be made to determine if the shaking forces can be reduced. If the system has operated without repeated main bearing or crankshaft failures, the basic design of the frame should be adequate. Whenever the failures occur in the rods and compressor reciprocating parts, the problem can sometimes be traced to excessive relative vibrations. This can be reduced by increasing the holding forces ore reducing the dynamic forces. There are several ways to reduce the forces, such as adding balance weights, changing the location of the cylinders on the crankshaft, and changing to lighter reciprocating parts. The change that will offer the most improvement can usually be determined from a bearing force analysis.
If the friction force caused by the anchor bolts cannot withstand the shaking forces, it may be possible to reduce the relative vibrations caused by the slipping of the engine on the grout by adding external horizontal restraints ore outriggers. The basic concept involves drilling anchor bolt holes in the present foundation outboard of the engine base. An outrigger is then bolted down and possibly grouted-in to offer a structural member that is rigidly attached to the foundation.
