Levelling, Grouting and Anchoring a Machine Base

Epoxy versus cementitious grout, anchor bolt stretch length and embedment, levelling methods and why the installation is part of the machine's structural loop.

A machine base is only as good as what it sits on. The most carefully designed, stress-relieved, precisely machined structure will underperform on a bad installation, and the installation is usually treated as someone else’s problem.

The installation is part of the structure

The loop that determines accuracy runs from the tool, through the spindle, the column, the ways, the base, the levelling elements, the grout and the foundation, and back to the workpiece. Every element is in series. A soft element anywhere dominates.

Levelling screws are small contact points. Left ungrouted, the entire load of the machine passes through a handful of them, the foundation mass takes no part in the dynamics, and the structure is far more compliant than the designer assumed. That is why it matters who decides the support scheme, and why it should be the designer rather than the installer. It is also why the FEA should model the real support condition rather than fully fixed feet.

What grout is for

Grout fills the space between the base and the foundation, and it does four jobs:

• Load transfer. It converts point contact into continuous bearing over the full footprint.
• Alignment. It holds the levelled position permanently so the setting does not drift.
• Mass coupling. Base, grout and foundation vibrate as one mass rather than as a machine bouncing on springs.
• Preload retention. It supports the anchor system so bolt tension stays where it was set.

That third point is the one that gets underestimated. An ungrouted machine has only its own mass to resist excitation. A grouted one has the foundation block as well.

Epoxy or cementitious

Epoxy grout has become the default for precision and vibrating equipment. It offers high resistance to compressive, tensile and shear loads and bonds more strongly to both concrete and steel than cementitious grout. It holds alignment well over time and resists oil and coolant.

The trade-offs are real: it costs more, it has a limited working time once mixed, it is temperature sensitive during placement and cure, and it is exothermic in thick pours. A badly planned epoxy pour is worse than a well-planned cementitious one.

Cementitious grout is cheaper and entirely adequate for static, moderately loaded equipment. It is more forgiving to place. Choose shrinkage-compensated formulations: ordinary sand-cement mixes shrink and debond, which defeats the purpose.

Either way the surface preparation decides the outcome. Concrete must be chipped back to sound aggregate, cleaned and free of laitance, oil and dust. Steel must be clean and, for epoxy, free of paint and primer where the bond is expected. Most grout failures are bond failures and most bond failures are preparation failures.

Anchor bolts

Anchor bolts hold the machine down and keep the assembly in compression. Published practice for equipment anchors is consistent about the geometry:

• Stretch length at least 12 bolt diameters. The bolt needs free length to behave as a spring. A bolt that cannot stretch cannot hold preload through thermal cycling and will loosen.
• Embedment at least 10 bolt diameters into sound concrete.
• Sleeve the upper portion, with a sleeve diameter around twice the bolt diameter, so the stretch length is genuinely free and not bonded into the grout.
• Preload deliberately. Tension high enough that the foundation stays in compression under the worst cyclic load, so the joint never opens and the bolt never sees the full load range.

Bolts that are short, fully bonded or hand-tightened are the most common anchoring fault, and they present as a machine that goes out of level a few months after commissioning.

Levelling methods

Levelling screws in the base, bearing on plates or chocks. Simple and adjustable. Adequate for lighter machines, and should normally be grouted afterwards on anything precise.

Wedge mounts. Adjustable, reusable, good for machines that may be relocated, and they keep the load spread better than a screw point.

Sole plates. A machined plate grouted to the foundation first, levelled carefully, with the machine then set on it. More work, and the best result for large or precise equipment because the levelling and the machine setting are separated.

Vibration isolators. A different intent: they deliberately decouple the machine from the floor, either to protect it from outside disturbance or to protect the building from the machine. They are very compliant by design and will dominate the low-frequency behaviour, so a machine on isolators must be analysed that way.

A workable sequence

1. Cure and prepare the foundation properly. Chip to sound concrete, clean thoroughly.
2. Set anchor bolts with correct embedment, sleeved for free stretch length.
3. Set the base on levelling elements and level it, checking in two directions.
4. Let it settle, then recheck. A large structure moves after it is first set down.
5. Form and pour grout, respecting the working time and pour thickness limits.
6. Cure fully before applying any load.
7. Torque anchors to the specified preload in a defined sequence.
8. Recheck level and record the readings.
9. Recheck again after a period of operation and thermal cycling.

Steps four, eight and nine are the ones that get dropped under schedule pressure and the ones that show up as complaints months later.

What the base designer should provide

• Levelling point locations, chosen by the designer, not the installer.
• Load per point, so the foundation and the elements can be sized.
• Anchor bolt positions, sizes and required preload.
• Grout type and pour thickness.
• The permitted sequence, and any features that must not be loaded during setting.
• Where level is to be measured and to what tolerance, under the conditions set out in measuring machine base flatness.

Supply those and the installation becomes repeatable rather than a matter of who turns up.

See also: the levelling and grouting reference, with support schemes and a drawing checklist.

Common questions

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