If you’ve ever sat in a dinner chair, or a bar stool, and noticed that when you shift your weight around the chair does not sit flat, but rather wobbles as you move, you’ve witnessed a soft foot problem. “Soft foot” is a condition that results when one or more feet of a machine do not sit in the same plane as, are angled differently from, or are different lengths than, the rest of the machine’s feet. Soft foot conditions can also be induced by forces applied to the machine by fixtures that are attached to it. Soft foot is a term used to describe conditions that manifest themselves as machine frame distortion. If a machine’s feet are anything but level and coplanar with one another, distortion of the machine’s frame will result when the hold down bolts are tightened on a machine’s feet. This distortion applies forces to the machine’s bearings, which in turn transmit these forces to the rotor. This causes the rotor to bend, and, therefore, to transmit tell-tale vibration signatures back through the machine frame to be recorded and interpreted by a vibration analyst or maintenance technician. Soft foot conditions come in many varieties including inside/outside angle bent foot, high/low foot, pipe strain (i.e. induced soft foot), and loose feet; they can be detected and corrected using various methods (vibration analysis, dial gauges, feeler gauges, laser alignment systems); and effectively dealing with soft foot can pay dividends in time and money saved when compared with more costly repairs. This article will summarize various types of soft foot, how to detect and correct them, and the benefits of doing so.
The name often given to the soft foot condition that results when one or more feet/legs of a machine are shorter (longer) than the others is “high (low) foot.” A difference in foot height of .002 to .003 inches might not seem severe enough to warrant further action, but these are the usual standards by which a high foot condition is diagnosed. The simple action of tightening the hold down bolt of a foot that is .002 inches (2 “mils” henceforth) shorter than its neighbors can distort the machine’s frame. Such distortion exerts forces on its rotor that distort the rotor, which causes the machine to damage itself during operation. Fortunately, this condition can be easily diagnosed and remedied.
One way to diagnose a high (or low) foot condition is to use dial gauges. Place the dial gauge against the top of a machine foot and as close to its center as possible (follow the normal procedures for using dial gauges to ensure that your measurements are valid and precise). Then, loosen the hold-down bolt on that foot until the foot stops moving upward, recording the distance this foot moved upward on a simple diagram (see Figure 1 below). This diagram will come in handy during the diagnosis stage.
Figure 1: Example of how to record soft foot measurements (all measurements in mils).
Tighten that hold-down bolt, and proceed to measure the movement of the rest of the feet in this same manner, recording your findings in the diagram you created. Some laser alignment systems also have modes that allow you to take foot height measurements using the laser system instead of dial gauges. It only takes a little more time to diagnose angle bent foot conditions. To do this, use feeler gauges to measure beneath each of the four corners of the feet in order to get a feel for the angle the foot makes with the base. Record these measurements in the same diagram (see Figure 3).
Figure 2: Diagram representative of an outside angle bent foot condition (all measurements in mils).
Notice how the outside measurements on foot 1 show it to be much higher off the base than
the part of the foot toward the inside of the machine.
This diagram happens to represent an outside angle bent foot at position 1. By analyzing diagrams like these (and with some practice under your belt) you will be able to diagnose and correct for high and low-foot conditions.
In order to correct high, low, and angled bent foot conditions, we need only to decide the amount of shim necessary to place beneath each foot, and where to place it. For example, the condition in Figure 1 should be diagnosed as a high foot at positions 1 and 3. To correct this, we would place 5 mils of shim beneath feet 1 and 3 because each foot raises approximately 10 mils when its hold down bolt is loosened, and we want to balance out this effect so that each foot doesn’t move at all. To correct the outside angle bent foot condition in Figure 2, we would create a step shim to place beneath the foot at position 1.
We mentioned earlier that induced soft foot conditions can be caused by pipe strain, which is the force that any fixture attached to the machine exerts on the machine. Figure 3 shows a diagram that indicates an induced soft foot due to pipe strain.
Figure 3: Example of a soft foot condition induced by pipe strain (all measurements in mils)
We can see that the feet all exhibit the same behavior when their respective hold down bolts are loosened – they angle upward (away from the base) and toward the upper right of the diagram. Before you place shim beneath machine feet to alleviate this stress, realize that this is not an option! Doing so would create a misalignment condition. To correct this problem it is usually required that the fixture attached to the machine be changed in some way to alleviate the force it is exerting on the machine.
Loose feet, which encompass problems such as unstable or cracked foundations, cracked feet, eroded grouting, and loose hold-down bolts, can also be a source of soft foot. Many machines sit on inadequate foundations, which is a major contributing factor to this type of soft foot. A machine foundation should consist of a poured slab of concrete that is 3 to 5 times heavier than the machine itself, followed by a base plate (sole plate) on top of that, and finally grouting between the base plate and machine to act as a dampener. When a foundation is not adequate for the machine it supports, the condition of the foundation can deteriorate in the form of cracks. A foundation that is not heavy enough for its machine may also provide it with an insufficient damping platform. A cracked machine foot can have many causes, including oxidation and stress. Obviously, no amount of shimming can correct a cracked machine foot – it must be repaired in order to correct this type if soft foot. Fortunately for the technician, these problems can often be diagnosed by inspection. Where the problem may not be so apparent, phase analysis (which will not be discussed in detail here) can also be a useful diagnostic tool.
Simply placing your finger on the gap between a machine foot and its base plate can shed light on a machine problem. In doing so (and with a little practice), you will be able to notice that the foot is vibrating away from the base plate. This is indicative of either a loose hold-down bolt or the erosion of grouting between the two surfaces. Following this “finger analysis” you may do a visual inspection to notice whether or not the grouting has actually eroded from between the foot and base plate, or if the vibration is more likely due to a loose hold-down bolt. A visual inspection can also tell whether or not the foundation is cracked. While visual inspection can tell you a lot about obvious problems, phase analysis is usually needed to detect less obvious ones.
A phase analysis reveals the relative motion between two measurement points. Phase measurements can be easily taken with two accelerometers linked to a dual-channel data collector, and also with a tachometer and an accelerometer (although this setup is more time consuming). Ideally, we would like to see the entire machine vibrating at low amplitude, and all measurement points vibrating in phase with one another. However, when there is a phase difference between two measurement points (usually a 90 to 180-degree difference) this can indicate a problem. Taking phase measurements on a machine foot, its hold down bolt, a nearby position on the base plate, and on the base itself is a useful way to determine where the problem lies. For example, if the measurements are relatively in phase (to within 30 degrees) between the hold down bolt and the machine foot, and between the base plate and the base, but about 180 degrees out of phase between the machine foot and the base plate, then the problem lies between the foot and base plate. This is by far not the only use for phase analysis; it can also be used to diagnose other machine faults such as unbalance, misalignment, and bent shafts.
Correcting loose feet can be as simple as tightening hold down bolts, or as complicated as replacing an entire machine base. However, the money-saving benefits of correcting all types of soft foot are well worth the effort. Eliminating soft foot conditions removes machine frame distortion, which prevents a machine’s bearings from distorting the shape of the machine rotor. This all leads to increased machine life and productivity, and reduced long-term maintenance costs and down-time.
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