First and foremost, you should be using high quality, precut slotted stainless steel shims. Cutting your own shims by hand out of cheaper rolls of carbon steel or brass shim stock may save you money in materials, but will prove far more expensive overall for several reasons. For one thing, you will be able to cut only the thinner thicknesses with scissors or shears, whereas thicker thicknesses (over 0.004") will require using an acetylene torch or a saw, which is labor intensive and presents several safety concerns. After you have cut your shims by hand, it is essential to deburr them carefully with a ball peen hammer and file. All of this will cost you the most valuable commodity of all: time. Moreover, the end result will be fewer available shim thicknesses than you would otherwise have with precut stainless steel shims, resulting in less precise alignments. In addition, if you are cutting shims by hand, there is a far higher risk of minor cuts, requiring a visit to the nurse for a bandage, with the attendant's loss of time and safety-reporting paperwork this would entail.
It is important to select your brand of precut stainless steel shims carefully, as they vary widely in quality and tolerances. It is extremely important that the shim be of consistent quality, completely even in its thickness throughout, accurate in its thickness, flat and burr free. Also, it should have no hazardous sharp edges. The metallurgy of the shim is also important to guarantee its hardness and corrosion resistance. Only the best quality precut stainless steel shims offer all these features. An excellent shim will always save you time and money in the long run. Let's see why:
Advantages Of Precut Stainless Steel Shims
Precut stainless steel shims are:
Precut: You don't have to hand cut them, saving you much time.
Burr free: This means greater precision in the thickness.
Accurate in their thickness (up to 0.025").
Even thickness throughout.
Flat and undistorted.
Convenient: Steel carrying cases make transporting shims easy and help to keep them neat and organized.
Versatile: More thicknesses and sizes to choose from means you will use fewer shims to achieve the desired thickness, thereby saving both time and money in materials.
Safe: The safety tab allows for safe manipulation of each shim. Never allow your fingers to get under a machine foot!
Some of these points require further attention, as they have a direct impact on your best practices regime. Let's examine a few of these.
Number of Shims
You should try to never use more than three (or four at most) precut shims under one machine foot, except in exceptional circumstances. The reason for this is too many shims under a machine foot leads to increased risk of exceeding your permissible soft foot tolerance. You can calculate approximately from a quarter to half a thousandths of an inch (0.00025-0.0005") movement from compression for each air space under the foot. If you have three shims, you will have four air spaces as follows: Between the underside of the foot and the first shim; between the first and second shim; between the second and third shim; and between the third shim and mounting surface. This means three shims will inevitably produce about 1 mil (0.001") worth of movement every time you tighten or loosen the anchor bolt.
This effect is usually due to a slight lack of coplanarity between the machine's feet and base, and in some cases, slight bowing of the shims. If you add to this the effect of surface contaminants present on both faces of each shim (finger oils, dust, grease, etc.), the movement can be greater. This is why a tolerance of 2 mils for soft foot is considered standard. In addition, stainless steel can be expected to compress about half a percent of its overall thickness under load, so you can expect a 100 mil shim stack to yield another half thousandths under load. (Contrast this with some brass alloys that can yield as much as six percent!)
You may think that limiting your shimming to just three shims under a machine foot is unrealistic and unreasonable, however, it is not. Good machine installation practices dictate that a good rough alignment between the machines should not necessitate ever having more than 100 mils worth of shims under any machine foot to achieve final alignment within tolerance. Good quality, precut stainless steel shims come in 13 thicknesses, ranging from 1 mil to 120 mils. These are: 0.001", 0.002", 0.003", 0.004", 0.005", 0.010", 0.015", 0.020", 0.025", 0.050", 0.075", 0.100" and 0.120". With these 13 thicknesses, you can achieve any desired shim thickness, from 1 to 150 thousandths, with never more than three shims.
Examples:
24 = 20 + 4
69 = 50 + 15 + 4
97 = 75 + 20 + 2
149 = 120 + 25 + 4
These examples show that you will save money with precut shims because the greater variety in available thicknesses means you almost never need to use more than two, or at the most three, shims under one machine foot. Over time, this adds up to a lot of saved shims. As part of this best practice, it is imperative that you always maintain your shim cases fully stocked with all thicknesses, otherwise shim consumption may balloon. For instance, if you run out of fifteens, you will find yourself using a ten and a five, or three fives, which quickly escalates your shim costs and increases the risk of violating the rule of using no more than three shims under a foot.
Figure 1
If you absolutely must shim your machine up by more than 150 thousandths, then go ahead and use four shims; however, if you must shim up 0.250" or more, then have your machine shop make you a chock (a chock is a shim that is 250 mils or more in thickness) and make sure this chock is carefully milled flat and coplanar on both faces. Then use three or fewer shims, normally on top of the chock, to complete the alignment.
Ascertaining Precision Of Shims
Always measure the thickness of any shim 0.050" or thicker with a micrometer. While certain brands of shims are of excellent quality and evenness in their thickness throughout, like all commercial precut shims, they are only guaranteed accurate in their marked thicknesses from 0.001" to 0.025". Thicker thicknesses are always nominal (just as the steel mill rolled the sheet) and therefore, should be always miked. For instance 0.075" shims may mike out at 0.078" and 0.100" shims may actually measure 0.104" thick. This does not matter, so long as you know it! Thus, make sure a 1" micrometer is a standard part of your alignment toolkit.
Marking And Quality Of Shims
Always look for precut stainless steel 304 shims whose thickness and size are indelibly etched upon them. This is a sign of a best quality shim. Avoid using cheaper brands that only ink their shims, or worse yet, punch stamp them with the thickness marking. Punch stamped shims is a sign that they are not flat, resulting in a leaf spring effect under the machine's feet that contributes to a "squishy foot" soft foot.
Some lesser quality shims usually have raised knife edges (burrs) along their edges because they are not tumbled after being stamped out of the sheet and the dies used to stamp them out are not sharpened often enough. This also results in a shim of inaccurate thickness, which will contribute to or even cause a squishy soft foot; it also constitutes a significant safety hazard for the millwright who is not using safety gloves.
In the end, cheaper shims always end up costing you much more than good quality shims, given the increased labor costs resulting from the necessity of making repeated adjustments of the machine's position and the greater downtime that results from the shims' inaccuracies. Not to be overlooked is the lost time consequences of minor cuts caused by poor quality shims that can be very significant not just in terms of the time required to treat the injuries, but also from all the consequent paperwork required to comply with your internal safety incident reporting requirements.
Shimming Technique
Always sandwich your thinner shims between thicker ones to protect them. In fact, if you have to shim 26 thousandths, choose to use a 20 and two 3s, rather than a 25 and a 1 alone.
Always insert your shims until you feel them touch and then withdraw them slightly. This way you know for sure that you are not letting the slot of the shim get caught in the threads of the anchor bolt.
Always handle your shims by grasping them by the safety tab (see Figure 1); never let your fingers get under a machine's foot while the machine is being pried up or lifted!
Shim Sizes
Precut stainless steel shims come in several standard industry sizes, as pioneered by Lawton Industries many years ago: These are:
Size A: 2" × 2" with a 5/8" slot
Size B: 3" × 3" with a 13/16" slot
Size C: 4" × 4" with a 1 1/4" slot
Size D: 6" × 5" with a 1 5/8" slot
Size G: 7" × 7" with a 1 3/4" slot
Size H: 8" × 8" with a 2 1/4" slot
Lawton has published a chart of horsepower ranges and motor frame numbers associated with the different sizes of shims. See Tables 1 and 2
Alternatively, choose your shim size more by the slot size than by the foot's surface area. It is a myth that you have to support the entire surface area of the machine foot with the shim. You only need to support the load zone around the anchor bolt. See Figure 2 for an illustration of the load zone and the area that should be supported by the shims. Most machine feet are made much larger in surface area than what is strictly necessary to support the mass of the machine and its operational load stresses. So, if a machine foot is larger than a given size of a shim, but the shim adequately supports the load zone around the hole in the foot, you do not have to worry if the outer edges of the foot overhang the shim a bit- that is perfectly okay.
Figure 2: Load zone to be supported
The load zone (depicted in red) is defined as a cone approximately 45 degrees from the edge of the flat washer under the anchor bolt through the thickness of the machine foot. As long as your topmost shim supports the surface area defined by the cross-sectional cut of this cone under the foot, you are adequately supporting the load zone of the machine's foot and do not need to support any excess surface area under the foot.
Figure 3 shows an example of how not to shim a machine: Too many shims were used and the shims do not support the load zone of the machine's foot around the anchor bolt. Moreover, the Size ‘A' shim used on top of the shim stack is too small for a motor of this size. When the anchor bolt of this foot is tightened, the foot will be stressed, distorting the machine's frame and altering the internal alignment of the bearing bores, as well as affecting the air gap between rotor and stator. Note too that the motor was painted in situ, covering the shims and jackscrews in paint and also allowing paint to go under the foot. These are all bad practices.
Figure 3: How not to shim a machine
If the existing standard shim sizes cannot properly fit the load zone of a given machine foot, or you cannot accommodate the anchor bolt(s), consider having custom shims manufactured in the exact shapes and sizes that you require. The best shim manufacturers offer this customization and can make you shims in any desired material (SS-304, Monel, etc.) at a lower cost and with higher precision than you usually will be able to have them made for in-house.
Unusual Circumstances: Step Shimming
Sometimes, a machine will have feet that are significantly angled with respect to the contact surface of the base plate or sole plate supporting them. This can be caused when the feet are accidentally bent, or when the machine is "rolled" to accommodate a horizontal misalignment problem (definitely not a best practice!) Tightening the anchor bolts under these conditions would force the feet flat to the base, thereby distorting the machine and increasing the radial load on the bearings. This affects air gap clearances between internal components and places undue stress on the feet or other machine parts. How do you solve this problem? The best way, of course, would be to re-machine the base plate, sole plate, or undersides of the machine feet in such a way as to eliminate the lack of coplanarity. However, this may not be practical or economical to do. This leaves us with the only other possible solution: step shimming.
Step shimming requires several thinner shims to be carefully inserted between the machine's foot and its support surface in such a way that they are offset from one another in step fashion. This fills the tapered gap between the underside of the foot and its support plane as evenly as possible, as illustrated in Figure 4. This may require you to use more than three shims overall, but as with everything, there is always an exception to the rule and here the benefits of doing this outweigh the disadvantages. Although not elegant, the step shimming solution is expedient, easy and economical, and will prevent a much more serious machine frame distortion problem, allowing your machines to run satisfactorily until your next major outage when you can schedule the time and resources to fix the problem properly in a more permanent manner. It goes without saying that if the time and resources are readily available, you should consider re-machining the undersides of the feet or the contact surfaces of the base plate, or making a tapered "dutchman" shim for the foot. Keep in mind, too, that the "bent" foot may be symptomatic of a larger problem with the machine, such as internal damage if it resulted from the machine being dropped in transport or installation. If the angularity between the foot and the base is too large, consider replacing the entire machine or the base, as may be.
Figure 4: End view of step shimmed machine foot
While we are on the topic of angled feet or irregular surfaces, let us address the topic of "soft" shims for a moment. Soft shims that mold themselves to the uneven opening under the feet are not recommended; they allow the machine's casing to distort when the feet are tightened before they set and harden, thereby resulting in an undesirable, strained condition of the machine's frame. In other words, you are creating a shim fitted to the distorted condition of the machine, rather than to its undistorted, stress-free configuration. Soft shims may seem like an expedient solution, but we strongly discourage their use.
Calculating a Shim Step
To step shim correctly, choose a shim thickness that will allow you to stack no more than four shims to fill the uneven air gap between the underside of the foot and its support surface. Place these shims between the underside of the foot and any shims already present for the alignment of the machine as evenly as possible. To calculate the ideal shim thickness, measure the largest air gap with an inside micrometer and divide this gap by five. This will reveal the ideal thickness of shim to use for the four steps. For instance, if you must fill a 20 thousandths tapered air gap, dividing this gap by five yields an ideal step shim thickness of 4 mils. Use four, 4 thousandths shims to fill the tapered air gap as evenly as possible.
Step shimming works well since the angles involved are sufficiently small enough to fall well within what is known as the ‘swedge' angle for the coefficient of friction of the materials involved. This means as you tighten the anchor bolts and apply load to the shims, they will tend to remain in place rather than "squirt out" like watermelon seeds. Once the step shimming task is complete and the alignment of the machines has been rechecked with all anchor bolts tight, do not neglect to trim off the excess part of the shims protruding out from under the edge of the machine's foot to prevent possible injury.
In conclusion, best practices in shimming for machinery alignment and soft foot correction involve some care and attention to detail, the use of high quality, burr free, precut stainless steel shims and good shimming technique. Choose your shim supplier with care and keep your shim boxes fully stocked with all available thicknesses, in all the sizes needed to adequately support the load zones of the machines in your plant.
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Alan Luedeking is Vice President of Ludeca, Inc. in Doral, FL. He has 30+ years experience in machinery shaft alignment and training and holds an ISO level I Vibration Analyst Certificate. Besides his work, Alan enjoys spending time with his family and numismatics. www.ludeca.com
