It is impossible to properly align a machine with Soft-Foot. 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 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 co-planar, 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 cafe table to eliminate its wobble. A good indication that the soft-foot error is of the simple and easily corrected kind is that two diagonally opposite legs will show no movement while the other two will both have similar errors.
The tricky things is that a soft-foot problem may also be caused by several other common errors in the machine, such as a bent-foot, squishy-foot or a dodgy-base problem.
In the case of a bent-foot, one or more of the machine feet may not be sitting parallel to the floor in addition to not being co-planar to the other feet. This results in the soft foot condition detected by the laser shaft aligner not being fully corrected by shimming the recommended foot. If this happens, it is necessary to take a closer look at the machine feet condition. What happens with a bent mounting foot is that it is straightened as the mounting bolt is tightened down, moving the machine from its aligned position. When a bent foot is shimmed up, the movement that occurs when it bends does not disappear.
The task to identify a bent mounting foot is to look at the parallelism of the gap under each single foot, usually with a feeler gauge check under each corner.
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.
In the field, a practical solution 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 trimmed down to support a smaller and smaller foot area, like a staircase.
Other possible causes of soft-foot problems in machine laser 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 movement caused by extraneous forces on the machine such as piping strain.
In addition to these types of soft foot, it is possible that movement occurs when a machine is tightened down because the foot design is excessively flimsy or the rigidity of the base structure is sufficient to resist movement as the foot is tightened down. This is a common result of poor mounting base design or construction techniques.
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, given that a deflection of only a fraction of a millimetre is sufficient to exceed the alignment tolerance.
Other common design errors include not making an allowance for the mounting pads to be machined flat after fabrication. Machine mounting pads must be machined surfaces!
Construction problems include distortion due to 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 (no more) prior to non-shrink grouting will often produce minimal distortion from its original state.
In summary, high performance shaft laser alignment instruments are able to detect soft foot conditions and display the shim correction required to fix a non coplanar foot. Soft foot checking and 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, it is necessary to look carefully for other possible causes of the soft foot reading before adding more to the shim stacks.
In the end, correctly resolving any soft foot problem is important in order to obtain good alignment results.
For more information go to our Laser Shaft Alignment page, or call ISTecnik on 02 9546 5676.