In mechanical trades, most of us have been taught that most installed machines must be installed level and the bases must be flat. But contrary to what many of us think, level and flat are two completely different terms. Level simply means that the machine is parallel to earth, or horizontal. Flat means that the base, and the feet which mount upon it, are in the same plane.

For most machines, the degree of level is relative. I have installed pumps which were required to be level, but were leveled with a torpedo level. Not exactly the most accurate of leveling devices. The main reason a pump needs to be level is usually just to make sure that oil gets to both bearings, and so that gravity doesn’t tend to move the shaft downhill.

If a machine is a few tenths of a degree out of level, it usually is not harmful. But if a machine is installed on a base that is not flat, some problems may occur:

  • Shaft alignment will be difficult, due to soft foot. But this can be corrected by shimming.
  • Machine cases can be warped when tightening the foot bolts down. This can cause changes in bearing clearances, gear mesh clearances, even deforming the stators in electric motors. But this can also be corrected by shimming the feet so that this distortion does not occur while tightening.
  • The machine feet can be flat (in the same plane) while the foot bolts are loose, but change when they are tight.

So, keep this simple rule in mind: Most machines can be run slightly out of level, but they cannot be run “out of flatness.”

I can bolt a baseplate to the floor, and it can be both level and flat. I can bolt it to a wall, and it will no longer be level, but it will still be flat.

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  1. Vishnu Gandhi on September 10, 2019 at 10:31 am

    For 10 MW Gas Turbine Skid manufacturer specified “The top surface of the support frame at the location of 3 skid mounting pads shall be installed flat within 0.03”/feet (2.5mm/m) and coplanar to 0.12” (3mm) to the other pad locations.”

    Installer interpreted that he can use a one meter machinist level and 2.5 mm filler gauge. Which lead to very uneven mounting surface. What is right way to measure flatness 2.5mm/m

  2. Stan Riddle on September 10, 2019 at 11:07 am

    A machinist’s level only measures level, not flatness. If the machinist’s level can contact all three mounting pads, it could measure the degree of level between the three pads. If the level is being used as a precision straightedge, then, in conjunction with a 2.5 mm feeler gauge, it could be used to flatness, but only as it relates to the surfaces in which it can contact at one time.

    An accurate way would be to use an optical micrometer or transit, or a laser tool such as our NXA Ultimate, or other laser tools, which are designed to measure flatness and coplanarity.

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