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Soft Foot in Shaft Alignment
03-02-2008 
It is impossible to properly align a machine with a soft foot, but as many types of Soft Foot condition exist it is important to understand the possible causes.


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Soft Foot in Shaft Alignment
03-02-2008 

It is impossible to properly align a machine with a soft foot, but as many types of Soft Foot condition exist it is important to understand the possible causes.

A sophisticated laser shaft alignment system such as the Hamar S-660, S-670 or S-680 is able to reliably gauge the amount  and effect of soft foot problems in a machine before shaft alignment is attempted. Based on the most common cause of a soft foot condition being that the mounting feet are not coplanar, the system determines which foot needs to be shimmed and calculates shim thickness to eliminate the soft foot problem.

In simple terms this process is like packing up one leg of a 4 legged table to eliminate its wobble.

However, a soft foot result may also be caused by several other common errors in the machine, such as a bent foot condition. In this case, one or more of the machine feet may not be sitting parallel to the floor in addition to not being coplanar to the other feet.

If there is a significant bent foot problem, the soft foot condition detected by the laser shaft aligner will not be fully corrected by shimming the recommended foot. When this happens it is necessary to take a closer look at the condition of the machine feet. The result of  a bent mounting foot is that it is straightened elastically as the mounting bolt is tightened down, generating forces on the other feet and potentially inducing soft foot reading in the other machine feet when checked with the laser. When a bent foot is shimmed normally, the measured soft foot will not disappear.

The task to identify a bent mounting foot is to look at the parallelism on the gap under each single foot, which is often most effectively done with a feeler gauge check under each corner of the individual foot.

If a bent foot problem exists there are two possible solutions. The ideal solution is to re-machine the feet or the mounting base to be parallel across all mounting areas.

Another solution in the field is to build up stepped shims across the face of the non-parallel foot. This may be done by first shimming the gap to the lowest corner of the foot, then inserting a set of 4 to 6 shim steps progressively up to the highest corner of the foot. Each shim is cut down to support a smaller and smaller foot area, like a staircase.

Other possible causes of soft foot in machine shaft alignment include so called "Squishy Foot", where debris, grease or burrs prevent solid initial contact between the base, shims and foot surfaces,  and "Induced Soft Foot" which is potentially caused by extraneous forces on the machine.

In addition to these types of  soft foot, it is possible that a gap under one of the mounting feet may occur because the foot design is excessively flimsy or possibly damaged, and the rigidity of the machine structure is sufficient to resist movement as the foot is tightened down. This problem is a common result of poor mounting base design or poor construction techniques.

Base Design
It is essential that a machine base is designed to be sufficiently rigid to not bend under the machine weight or the pipe loads. Many bases are easily strong enough to support the operating loads but are not rigid enough to maintain a good alignment in field conditions, where a deflection of just a fraction of a millimetre may be sufficient to exceed soft foot tolerances.

Other common design errors include not making an allowance for the mounting pads to be machined flat after fabrication.

Construction problems include an allowed base machining operation not being done correctly with consideration of distortion due to internal welding stresses, that may require rough and finish machining operations separated by release and re-clamping of the frame after the roughing operation releases stress related distortion.

Similar errors may be also be induced when the machine frame is set down and grouted. For small frames, setting down the frame firmly on three points only prior to non-shrink grouting will often produce minimal distortion from its original state.

In summary, a high performance shaft laser alignment instrument is able to detect soft foot conditions and quantify the shimming correction required for a non coplanar foot. Soft foot correction should be done before every alignment, and will save hours of wasted alignment time and avoid machines being left in an unstable final aligned condition.

When a soft foot correction cycle does not produce good results, look carefully for other possible causes of the soft foot reading before simply adding to the shim stacks.

In the end, correctly resolving any measured soft foot problem is important in order to obtain good alignment results.

For more information go to the Laser Shaft Alignment page, call IS Tecnik on 02 9546 5676 or email us at:




 
 
 
 
 
 
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