What is Soft Foot?

“When one or two machine feet are not in the same plane as the other feet.” Soft foot is a major problem. One that can make a 30 minute alignment take hours or even days sometimes so it is a big source of frustration in the alignment process.

You can think about the analogy of the table (Fig. 1).

Fig. 1 Table placed on a floor

If you see a table in a restaurant or in your office cabin sometimes, when you sit down at a table and one leg is too short or may be two legs are short and this table turns to rock back and forth (Fig. 2).

Fig. 1 Short leg of Table

We fix them with coasters or napkins or whatever you need to do but the table wobbles. Electric motor does exact same thing, it turns to wobble back and forth and when it does, the shaft center line ends up in different places.

Types of Soft Foot:

There are two types of soft foot exist basically.

·        Parallel soft foot

·        Angular soft foot

Fig. 3 Types of Soft Foot

It can occur either as a parallel soft foot, an angular soft foot, of as a combination of both (Fig. 3). In parallel soft foot where distance between the bottom of the foot than the top at the base is the same all over across this motor foot. And in angular soft foot, it is a taper or wedge shape or slope underneath. You can have on of these or you can have both of these as a combination. 

How does soft foot affect alignment?

Even when you have a good alignment, once you tighten the bolts, everything can change, your alignment gets much worse. One thing is very common observed that your alignment tool tell you to add shims and you do that and tighten everything down and then it says remove them and you go up and down again and again and that’s always caused by soft foot problem. The reason is as you tighten the foundation bolts down, the machine’s shaft centerline can move into different positions and this make sure alignment measurements unrepeatable.

Some people try to just tighten the bolts too much to get rid of the problem which may cause more problems, and could even bend or warp the feet of your machine. Bent feet can cause bearing misalignment, or an eccentric stator in motors, resulting in higher motor temperature and vibration so too much tightening the bolts is not the way to fix the soft foot.

Fixing and minimizing soft foot:

You should minimize the soft foot before you ever begin the alignment, use shims to find loose foot or soft foot (Fig. 4). Shims actually serve for two purposes:

·        To change the machine’s vertical shaft centerline to raise it or lower it

·        To correct the soft foot with it

Fig. 4 Checking loose foot or soft foot

Before attempting to fix the soft foot:

·        If possible, the base of the machine should be clean, flat, and free from burs, rust, paint, slag. Anything that gets between at the bottom of the motor foot and the top of the base is the shim

·        Shims should also be straight and clean, free of burrs

·        Pre-cut stainless steel shims are preferred as they do not compress very much.

Identifying Soft Foot:

There are three mechanical measurements method to check for identifying soft foot:

·        Use a feeler gauge to check under the foot if the bolt is loose

·        Use a dial indicator at the feet of the motor while loosening and tightening the bolt (Fig. 5)

·        Use a dial indicator at the top of the coupling similar to above mentioned method (Fig. 6)

·        Also you can also use your laser alignment tool to check for soft foot, provided it has a soft foot application.

Fig. 5 Checking loose foot or soft foot with dial indicator

Fig. 6 Checking loose foot or soft foot with dial indicator

Question arises many times: when should soft foot be corrected; before or after aligning the machines? The answer is both! You should minimize the soft foot before the alignment begins, but you should also be aware of the possibility of the soft foot occurring at any time during the alignment process. You can suspect a soft foot during alignment when one of these things takes place:

·        The alignment measurements are not relatively repeatable

·        The numbers change dramatically as the bolts are tightened

·        When the shims move even after the bolts are tightened. Obviously the shims is not carrying its part of the load if you can move around them underneath the foot after the bolts are tightened.

Some troubleshooting question that may arise that does a particular bolt require more rotation than others to get it tight? In other words, you may tight one corner of only 10° to 15° of rotation during final tightening while going to the next corner and you rotate it a quarter turn (90°), so that is a good sign of soft foot because it requires a lot of rotation.

If soft foot is minimized and controlled before the alignment begins, you will

·        Save time

·        Improve repeatability in measurements

·        Improve your confidence in the tool

You learned to trust it a lot more if the numbers act like they are supposed to. If you expect zero change or very little change loose bolts and tight bolts and you get that, you will trust your tool a lot more.

FAQ’s:

1.     What do you recommend for the maximum amount of shims before using plates to take up the gap?

Ans: Shims are come up in many sizes up to 1/8th in. unless you make an order. Usually no more than 4 shims are recommended per foot if it possible. Sometimes it is not possible and you put up a 5th one. But if you got really big gaps; half of an inch or 3/8th or something like that, you really should either make some plate to fill in those and then use pre-cut standard shims for remainder.

2.     How much soft foot is acceptable?

Ans: One should correct or minimize soft foot to 2 mills or less. The more you minimize the soft foot, the less change you get in your values during alignment when the bolts are loose or when the bolts are tight.

3.     What do you recommend the maximum amount of shims before using plates?

Ans: 8th of an inch (1/8th of inch). If you need of an quarter, cut some 2/8th inch of a plate, it is recommended to cut on a milling machine which should be de-burred and flat as possible.

4.     Should we have equal shims placed in each foot before checking for soft foot?

Ans: It would be great if you have only one shim added before alignment on both in-board and out-board side foots.

5.     How do you check for soft foot on a bigger machines? (30 ton generator, turbo machines)

Ans: Huge machines usually contain no soft foot because of their huge mass pushing it downward side, but go around with feeler gauge on each corner. At least 2 to 3 edges of one foot and try to minimize the soft foot.

6.     What is the best way to come back angular soft foot?

Ans: Some people don’t like it but you should have to cut a step shim / partial shim or use one of the plastic shims that are meant to take up angular soft foot.

7.     Do you have any recommendation for hot soft foot measurements?

Ans: The machine actually grows in all directions as it is heats up (vertical, horizontal and in axial direction). It does not affect soft foot unless it get some metallurgical change even in cold and in hot condition substantially.