Baseplate Design

In “Baseplate Installation” we saw that baseplates are considerably more than a slab of steel bolted to the floor. Because they are critical in the alignment of machinery, care must be taken to design and install baseplates correctly.

A list of items the designer must know before designing a baseplate might include:

• The footprint of the machine
• The machine weight and dynamic loads

Once the weight and dynamic loads are known, the Structural Engineer can determine how many and what size anchor bolts must be used to secure the baseplate to the floor. With this knowledge, the designer can determine a rough thickness for the baseplate.

Without undertaking a rigorous analysis of the bending of plates, the following method may be used. If you know the size of the AB, you can look up the thickness of the nut and washer. The nut must be below the surface of the baseplate, in a counterbored hole. The thickness of the baseplate can be determined by doubling the thickness of the nut, and providing an allowance for the washer and for machining.

The top surface of the baseplate must be smooth and flat. A surface finish of 125 microinches is usually sufficient. If the top surface was not machined, there would be mill scale present, and it would be impossible to accurately shim the machine during alignment. If the machined surface is too rough, with deep machining grooves, over the course of time it is conceivable that brinnelling of the ridges could occur, with a subsequent settling and misalignment of the machine.

The diameter of the AB holes should exceed the diameter of the AB by about 1/8”, to permit some “wiggle room” for the anchor bolt.

For a large baseplate, a threaded hole for the jackscrews must be located within about 2” of each of the AB holes. It is important that there be no deflection of the baseplate. Therefore, the jackscrews must be placed at each AB hole, and close to them.

Grout holes should also be provided on large baseplates, to facilitate the placing of grout. Sometimes, if there is a large side force on the equipment that must be transferred to the foundation, shear blocks may be welded to the bottom of the baseplate. Provisions must be made to ensure that there are no air pockets created in the grout between such shear blocks. This can be accomplished by placing vent holes or grout holes in these locations.

Finally, as an aid in setting the baseplate, it is sometimes desirable to machine alignment lines across the center of the top surface of the baseplate. The millwrights can use these lines to accurately shoot in the baseplate prior to locking down the AB nuts.

Proper design and installation of baseplates leads to successful machinery installations. A good bond between the baseplate and the foundation minimizes vibrations. A flat, smooth surface minimizes alignment problems. All of this adds up to lower maintenance, longer life, and pride in a well-executed job.