Steel Manufacturing Blog: Keeping it Steel

Shims – Steel vs. Plastic

Posted on Mon, Dec 14, 2015 @ 10:57 AM

Often our Steel Fabricating customers require shims and choose them without considering material options. Slotted Steel Shims are the default selection. Typically stamped from grade A-36 or grade 50 steel they offer tremendous compressive strength.

We have isolated a stock shim below for the purposes of this review. Whether stock or made to order, The Steel Supply company can provide any shim required for you project in either carbon steel, stainless steel, aluminum or plastic.

The data below is taken from a standard A-36 Coil Steel Mill Certification;

Tensile Strength - 58,000 psi
Yield Strength - 36,000 psi
Compressive Strength - 22,000 psi

To translate into the actual strength we can use the very common 3” x 4” Slotted Steel Shim. The surface area is 12 sq. inches, less the slot. To keep this discussion simple we’ll say the slot is 1” wide and 3” deep, with square corners.

steel shimNote: The slot is almost never 1” wide. Ideally the slot is 1/16” wider than the bolt. Our stock slotted steel shims come with slots 9/16” wide, 13/16” wide, or 1-1/16” wide. This accommodates the most common bolts. Also, the slot has rounded corners as shown in the photo.

For a list of steel shims in stock, click here.

So the 12 sq. inch steel shim has a surface area of 9 sq. inches when the slot is accounted for. Apply the compressive strength shown above of 22,000 psi and the shim will withstand 198,000 lbs. of compressive strength before “permanent deflection” occurs.

Okay, that’s a great shim, but is that much strength necessary and is it adding any value to the strength of the structure.

Look at the same shim made with Polypropylene Plastic via injection molding. This plastic has a compressive strength between 4,500 and 6,000 psi. Multiplied by the 9 sq. inches we are using in this example and this shim will have a compressive strength of 40,500 and 54,000 psi.

In actual testing the samples from the batch used produced the results below. The variation between the numbers above and below are the result in varying thicknesses. As the shim gets thicker the compressive value where the plastic shim reaches the point of permanent deflection decreases. Also, the numbers above were derived from our assumed slot size. The plastic slotted shims listed below have a slot size of 7/8” wide x 3” long with rounded corners. This leaves a shim surface area greater than we calculated.

                                   Plasticshim.pngCompressive Strength

1/32” thick = 127,000 lbs.
                  1/16” thick =  95,000 lbs.
                  1/8”   thick =   75,000 lbs.
                  1/4”   thick =   70,000 lbs.
                  3/8”   thick =   42,250 lbs.
                  1/2”   thick = 30,000 lbs.

Note; These are total compression results. Not psi.

If Plastic Slotted Shims can be used the benefits are significantly lower costs and they eliminate the need to galvanize. Polypropylene Plastic Shims do not rust and are resistant to most oils and chemicals.

For a list of Slotted Polypropylene Plastic Shims click here.

For a list of Solid Plastic Shims (no slot) click here.

To see the full compression tests click here.

Tags: Shims