Bearing Cutting Procedures

Part 1 of 2

To the Steel Fabricator, Steel Erector or any construction trade, the category “Bearings” is comprised of many different products. Some, like elastomeric bearings are made to order and are not re-sized or adjusted in any way by the end fabricator or erector. For purposes of this article we will focus on the bearing materials that can be cut to size, drilled and shaped by the fabricator.

Note: For purposes of this discussion the terms Teflon® and PTFE are used interchangeably. PTFE is the generic name for Polytetrafluoroethylene. Teflon® is the DuPont/Chemours trademarked name for that product. The name Fluorogold® is fiberglass infused PTFE, and is trademarked by the Saint-Gobain Corporation.

Teflon® / PTFE.

Typically, white in color Teflon® or PTFE is a very easy to shape. Cutting to size can actually be done in the thinner sizes such as 1/16” and 1/8” with a utility knife. Thicker sizes can still be cut by hand, but will cut faster using any type of power saw. Holes can be drilled. Slots can be created by re- drilling, jig sawing or milling.

Important Note: Because Teflon® or PTFE is relatively soft and pliable it cuts easily so excessive heat is usually not an issue. If power tools are used the fabricator should bear in mind the melting point is a relatively low 620°F. If the material moves through the cutting area quickly heat will not be a problem. If this is not possible running the tool at a slower speed will help minimize the heat generation.

Teflon® can be lightly sanded to remove and burrs or edges. If sanded it is important the clean thoroughly to remove any abrasive particles or grain that may be left on the Teflon® surface.

It can be shipped in bulk or fabricated to specifications. While easy to shape and form, most of our customers still request Teflon® products be cut, shaped, drilled, etc. by The Steel Supply Company. Teflon® Slide Bearings / Fluorogold® Slide Bearings

Teflon® Slide Bearings and Fluorogold® Slide Bearings can be made in many sizes, thicknesses and from a

variety of materials. For simplicity when describing cutting methods we will use as an example the most common configuation, FC-1010-CS. This is 3/32” thick Fluorogold® bonded to a 1/8” carbon steel backing plate. The same techniques apply to different thicknesses and materials.

To see a review of some common Slide Bearing configurations, click here.

The bonding together of these two materials is a specific skill requiring the correct knowledge, tools, clamps and most of all an oven capable of heating the clamped assemblies until the bond between the Teflon® or Fluorogold® and steel backing plate reaches maximum strength. Figure 2 shows the finished bond between the 3/32” thick Fluorogold® and the 10 gauge backing plate.

Typically Slide Bearings are made in sheets 24” x 48”. Desired pieces are cut to size. Now that the Tefon® and carbon steel are bonded the cutting process is more complicated. The Teflon® presents very little resistence, but the steel requires saw cutting. For straight perimeter cuts use a band saw with a fine tooth blade running at a slower speed than would be used for regular hot rolled carbon steel.

The two things to guard against are;

1. As the steel plate is cut steel shavings will try to embed themselves on the Teflon® or Fluorogold® surface. If left there they become sharp abrasives on the sliding surface that will shorten the bearing life 2. The heat generated by the cutting can sufficiently raise the temperature of the steel enough to melt the epoxy bond holding the Teflon®. Any area where the bond is separated will decay rapidly once in service.

Important Note: Do not punch or shear slide bearings. The pressure and action of these tools can cause the Teflon® to delaminate.

Once the plate is cut to size holes can be drilled. It is important to drill through the Teflon® or Fluorogold® first, then through the steel. Again, this is to protect the Teflon® from delaminating.

With a Slide Bearing Assembly such as the one we are describing the lower member will typically have a hole and the upper member will have a slot to allow for movement. While any steel fabricator can drill the holes, the slots present a different problem. Shops without the proper machinery will often cut a slot by drilling two holes representing the outer edges of the slot and then cut the center of the slot out with a jig saw. While it will work the fabricator should consider the jigsaw uses a reciprocal action so at some point the blade can be pushing or pulling steel shavings into the Teflon® surface. If this method must be used we recommend running the jig saw at a slow speed and applying as little pressure as possible.

The best method of creating slots we have found is to mill the slot from one end to the other. A good milling machine and the correct diameter end mill will cut the slot in one pass, very accurately. It will still create shavings but done slowly and carefully the circular action of the end mill will not embed them in the Teflon® surface.

Water jet cutting can be employed to fabricate slide bearings. It does required a skilled water jet operator who is experienced in cutting multi-layer products. (See Thermal Break cutting below or in part. 4) The extra consideration when cutting bonded Slide Bearings on a water jet table is when cut face up the jet stream cuts through the Teflon® or Fluorogold® then hits the steel plate a flays to the sides. This causes a debonding of the two. To prevent this cut with the steel side up.

Creating the recess or “Lip” in the Teflon® is easily the most difficult step in fabricating Slide Bearings.

Click here to see an example of a recessed lip Fluorogold® Slide Bearing.

The recess is desireable in cases where the welding required to hold the Slide Bearing in place would cause the steel backing plate to heat up and melt the bonding epoxy.

Click here to learn more about Slide Bearing Welding, Delamination and Installation.

To create this set back lip, the Slide Bearing is cut to size and the Teflon® or Fluorogold® is cut away. Keep in mind how strong the bond is, making this is a difficult and time consuming process. We have seen Fabricators attempt to cut back a lip in a Slide Bearing using everything from a utility knife, a hammer and chisel, a 1/4” grinding wheel and an abrasive flap disc. The fact is, Teflon® can withstand rough treatment. Even more so with the very durable Fluorogold®. Regardless of the effort the result is usually rough at best, and very time consuming. Fig. 3 shows an attempt that overheated the steel backing plate and melted the epoxy bond.

The best way we have found to create the desired recess is to mill away the unwanted Teflon®. This requires a vertical milling machine. The workpiece must be securely in place and perfectly level. It is important to remove all of the Teflon® or Fluorogold®. For this reason removing several thousanths of the steel backing plate is acceptable. The milling should be done at a low RPM rate and slow advance rate, or IPM (Inches per minute.) Ridges, burrs and strands of the material may be present when done. These can be cleaned up with a utility knife, a dremel or they can even be sanded. Figure 4 shows a Slide Bearing correctly and thoroughly recessed. It is hard to see in the image, but to assure the Fluorogold was completely removed the milling machine was set to take .002” of the steel backing plate.

As mentioned above any particles from metal shavings or grit from any abrasive used should be cleaned thoroughly from the sliding surface. The absence of these particles will assure maximum life of the Slide Bearing.

Rubber Bearing Pads

Commercial Grade Neoprene

Rubber does not respond well to saw blades and drill bits that are designed to cut wood or metal. One method that helps the workability of the material is to freeze it. This can be done with dry ice or and actual freezer. While it is not really freezing, the material firms up and becomes more workable.

Commercial Grade Neoprene Rubber Bearing Pads follow cutting techniques similar to Teflon®. When working with thin material a utility knife will suffice for the outer dimensions and a drill can be used for the holes. When drilling expect the finished hole to be a smaller diameter than the drill bit. Hand cutting applies up to approximately 1/4” thick bearings. As the material gets thicker a saw or power knife, also known as a “Slitter” will be required. The flexibility of the Neoprene requires tools with blades specifically for cutting rubber. For this reason most customers default this work to The Steel Supply Company. Figure 5 shows fabricated rubber blocks 21” x 8” and 4” thick. The holes shown are 1-5/8” diameter, drilled 2” deep. The remaining 2” depth is drilled 13/16” diameter on center. This allows for a countersunk 3/4” bolt and washer for attaching the rubber blocks and still not interfere with the profile. To accomplish this 4 layers of 1” thick neoprene were cut to size and bonded under heat and mild pressure. Figure 6 shows a side view where the layers can be clearly seen. The holes were drilled after the bonds had fully cured. This eliminated any mis-alignment of the holes while the bearings were being bonded.

Viblon®, Sorbtex® and Fiber Reinforced Rubber

Viblon® and Sortex® are fiber re-inforced nitrile rubber pads. This type of material is sold under a variety of names but the fundament principle is consistent. Very fine fabric mesh which is layered and impregnated with a low viscosity rubber compound. It’s purpose is to provide a rubber pad that has the cushioning benefits of rubber with the added benefit of lateral strength derived from the fabric mesh.

The structure is important to understand for anyone considering cutting this material for the first time. The fabric mesh is typically an 8 oz. cottonpolyester that is treated with mold and mildew inhibitors to prevent premature decay in exterior environments. This makes each strand more tenacious to cut than simple cotton. Also important is the layering. A 1” thick Viblon® or Sorbtex® sheet will have approximately 64 layers of this fiber mesh. In the Steel Fabricating and Steel Erecting industries the thicknesses we most commonly see are 1/16” thick for Dielectric Pads, and 1/8” and 1/4” thick for vibration dampening and underlayments. For cutting purposes;

1/16” thick = 4 layers of cotton fiber mesh 1/8” thick = 8 layers of cotton fiber mesh 1/4” thick = 16 layers of cotton fiber mesh

Fig. 7 shows a top view that illustrates the density of the fabric used in Viblon®. The weave shows approximately 36 threads per inch. Figure 8 is a side view of the same 1/4” thick bearing showing 16 fiber layers. These photos show this is a densely packed rubber and fiber bearing.

This material can be cut with a utility knife but the difficulty and resistence can be better anticipated considering the layers of mesh.

For perimeter and straight cuts a bandsaw will work. It is best done with a coarse tooth blade, approx 10 tooth per inch, and run at a high speed. Advancing the material slowly allows the teeth to cut the fibers neatly.

A far more effective method of cutting Viblon® or Sorbtex® is a more sophisticated machine called a Flash Cutter that utilizes a series of knife blades following a CNC program. In addition to much faster cutting speed, it has all the same advantages as a Plasma or Laser table in that the material does not move. The cutting head does. This allows for much greater accuracy, but more importantantly it does not need to be manned continuously.

Summarizing Viblon® or Sorbtex® cutting if a small quantity is required a fabrication shop can undertake the project using utility knives and band saws. If the project requires any quantity of bearings it is most economical to enlist a company with machinery specific to that work.

Thermal Break Bearings Unlike any of the other bearings mentioned in this discussion Thermal Break Bearings are built to handle static loads only. Their function is to interrupt “Thermal Bridging”, or the transfer of energy through steel members. T hey should be considered struct ural components and will usually be found between beams that connect through the building envelope , i.e. walls ( Fig. 9 ) or under base plates. (Fig. 10 ).

As shown in these two illustrations thermal break material requires very high compressive strength. Fig. 11 is a typical Thermal Break Bearing . This piece is 12” x 10” x 1” thick and has a compressive strength of 48 ksi. To achieve that strength the bearing is made of a dense fiber reinforced resin. For the steel fabricator attempting to cut this material the composition and density are essential to understand.

The density of Thermal Break material makes it much harder to cut than other bearing materials. If cut with a high speed rotational saw blade or drilled the hardness and speed of the blade/bit will generate enough heat to destabilize the bearing. As the heat diffuses through the bearing material it causes “Radicalization”, or a separation of the fibers and resins. The result is a lowering of the compressive strength and a compromise in the ability to act as a structural component.

To properly fabricate bearings from Thermal Break stock material all dimensions and holes should be done by a Water Jet Cutter. This is a pressure stream of water carrying very fine garnet silicate which acts as the abrasive. The water, the propelant, has the side benefit of providing an endless stream of coolant and prohibits any heat from developing on the work surface. Immediately after cutting the workpiece surface is actually cool to the touch.

The cutting process goes beyond the simple propulsion of water carrying an abrasive. To properly fabricate Thermal Break bearings the operator needs to understand the subject material and the effects of media type, hardness, size and shape, water stream width and pressure, which can range from 30ksi to 90ksi. With all the variables to consider and the critical nature of the finished product in the structure it is important to have these bearings done by an experienced Thermal Break operator who can fabricate accurately and maintain the integrity of the bearing.

Figure 12 shows a side view of a bearing that has been properly cut.