In reliability and maintenance, base-related vibration and alignment problems are a commonly encountered issue. These can include:
- Soft foot
- Bearing misalignment within
- machines, such as motors, pumps, compressors, etc.
- Seal and bearing failures (related to radial loading) due to tightening down the pump to a base that is not flat
- Hold-down bolts becoming loose
- Resonance problems
- Machine tool quality problems
Most of these problems are a direct result of a base not being installed flat. Let’s examine some of the reasons why bases are not installed to a flatness specification or how they get out of spec.
1. Specifications
Many industrial machine bases are specified to be level and flat. Pump installation instructions often state
“pump shall be level.” What is often not stated is how level the pump will be. American National Standards Institute (ANSI) pump specifications typically state pump bases shall be level to 0.005 inches per foot.
This is often measured with a precision machinist level. A typical one might be 12 inches long. This means the level can only measure the degree of level across 12 inches. A pump base could be 2 feet by 4 feet or more. Placing the level in only one or two places on the base plate is not sufficient to measure flatness across the entire base.
API610 specifications require a much tighter tolerance of 0.0018 inches per foot or 0.15 millimeters per meter.
The challenge is that while a level may be flat, it does not measure flatness. A base plate sitting horizontally may be considered level and flat, but if rotated vertically 45 degrees, it may still be flat, but it will no longer be level.
A true measure of flatness can only be done with specialized transits, lasers or precision jigs, which are designed to measure flatness and are what is specified. A base plate only needs to be flat in the places where machinery is bolted down to it. These should all be flat relative to each other.
2. Assumptions
Many machine base plates (often called sole plates) are carbon steel, stainless steel or composites. Often they are either not machined for flatness or only machined in the locations where hold-down bolts are located.
Depending on the application, the locations sole plates may be placed are:
On a concrete floor or foundation, which may not be flat
Over a dug-out location in the dirt
In this case, the sole plate is made flat by adjusting threaded lag anchors, which support the sole plate. Once the plate is prepped, the hole is often filled with concrete or grout, which should be vibrated to remove any voids or empty spaces, yielding a solid foundation. If flatness is critical, it should be rechecked after each of these operations and adjusted as needed.
Another challenge to consider is that while a base plate may be machined to within specification when it leaves the machine shop/manufacturer, the way it is transported, stored until use or field machined (such as drilling, tapping or welding) can change its degree of flatness.
3. Grout
Grout is a material used to fill voids underneath machine bases. It also should be designed to add mass to the base to minimize natural frequency problems in rotating machinery. Grout is specified and manufactured from many different types of materials, from something as simple as a cement mixture to modern nonshrink compounds.
With any of these, the possibility of the grout foundation shrinking or expanding is possible. The material and the form for
the material should be engineered to eliminate voids and to minimize the possibility of deforming the machine base as it cures. Also, sufficient time for curing should be allowed, and the machine base should be rechecked for flatness after curing time is complete.
4. Rechecking After Piping & Conduit Connection
Adding other components to the machines, such as piping, conduit, ductwork, piping supports and other accessory machine components can also have a negative effect on the degree of level of a machine base.
If a user does not have a precise method to measure flatness to the degree required, they are simply guessing as to whether this critical specification is achieved or not.