See below for disclaimer on the trademarks regarding Teflon® and FLUOROGOLD®.
For this discussion Teflon® shall be referred to by its industrial name PTFE.
Most frequently our Steel Fabricating customers are called on to use PTFE (Teflon®) in the basic Slide Bearing configuration. The actual product in a Slide Bearing should be a glass fiber reinforced PTFE known as FLUOROGOLD®.
As engineering and environmental standards have evolved there is also an increasing demand for PTFE in other uses in steel construction.
- Gaskets - Typically on carbon steel to carbon steel connections gaskets are not required. When connecting dis-similar metals a gasket will often be used to eliminate galvanic corrosion. In the photo to the right PTFE inserts were used to separate carbon steel from aluminum. (Note white pad beneath the clip angle)
- Electrical Insulation - PTFE is an excellent electrical insulator. Standard measurements indicate PTFE can insulate 500 volts per thousandth of an inch. Note again the drawing to the right. We were unable to verify with the engineer why Teflon® was required in this application, but these beams are supporting the ground floor of a laboratory that uses a lot of radiology and medical imaging equipment. Excluding any considerations for insulation against electrical current, more common insulations would be a high durometer commercial grade neoprene or Korolath HDPE plastic, both of which are supplied by The Steel Supply Company.
Bearing Pads - For light loads standard white (virgin) PTFE can be used as a bearing pad. For heavier loads the design usually calls for a Teflon® Bearing made with FLUOROGOLD® which provides much greater dimension strength.
Thermal Break - PTFE is also an excellent insulator to prevent thermal bridging. Thermal bridging is the path energy would follow to move from a heated environment into a colder environment. Uses such as these are becoming increasing more common as designers and engineers adapt to LEEDS standards and other environmentally conservative practices.
PTFE has a number of characteristics that are very attractive to engineers;
- Very low co-efficient of friction
- Corrosion Resistance
- Chemically Inert ( i.e. PTFE is unaffected by almost all acids, caustics and solvents)
- Excellent machine-ability
- Temperature feasibility ; PTFE can have operating temperatures from - 100° F to 500° F
- Good abrasion resistance
At times a drawing will call for the Fluorogold® or Teflon® to have a dimpled surface. This would be a case where an addition lubricant will be applied. The dimples serve to collect the lubricant and distribute it sparingly as the surfaces move. Otherwise two perfectly flat surfaces would very quickly squeeze the lubricant out. Situations like this are usually applications subject to "Stick-Slip" conditions that are produced by sudden movement. An example would be a Slide Bearing being utilized on the mounting of an electrical generator. In building construction the anticipated movements are not sudden enough that Stick-Slip will be a factor.
To maximize the value of a Slide Bearing it is important the surfaces of the upper and lower member be as parallel as possible. The drawing to the right shows in exaggerated form what occurs when the bearing surfaces are not parallel. The red arrows indicate where stress is created. The Teflon® or FLUOROGOLD® can absorb some of the deflection. As well, in steel construction there is more than enough strength to offset and overcome the stress. At the least, this condition will cause accelerated wear and most likely premature failure.
Of course Steel Fabricating, and especially Erecting are subject to field conditions and physical limitations. While it may be theoretically impossible to align the bearing surfaces perfectly, care should be taken to see they are as straight as possible.
Note: Teflon® is a registered trademark of the DuPont Corporation. In general industrial terms this product is known as PTFE. (Polytetrafluoroethylene)
FLUOROGOLD® is the industry designation for the type of PTFE used in our slide bearings. It is a special formulation PTFE, reinforced with a glass fiber aggregate. This reinforcement is what allows for its exceptional compressive strength. Fluorogold® is a registered trademark of Saint Gobain and is used with permission through exclusive license.
Note: Teflon® is a registered trademark of the DuPont Corporation. The generic name for that product is Polytetrafluoroethylene, or more simply, PTFE.
Fluorogold® is the trade name for the type of PTFE used in our slide bearings. It is a special formulation PTFE, reinforced with a glass fiber aggregate. This reinforcement is what allows for its exceptional compressive strength. Fluorogold® is a registered trademark of Saint Gobain and is used with permission through exclusive license.
Teflon® Slide Bearings can be found in many configurations, but the basic principle is true for all of them. Teflon® bonded to some kind of steel backing plate. Their function is to allow for expansion, contraction or movement in any structure, bridge, pipe support, etc.
To keep it simple, here is the most common and most basic construction.
10 gauge steel is 1/8" thick (+/- 1/64"). The Teflon® material used on our slide bearings is known as Fluorogold®. It is 3/32" thick, so an upper and lower assembly combine to be 7/16". As shown in the drawing above, the upper element, as the movement element, is usually longer and wider than the lower element which is stationary. This serves two purposes;
- As movement occurs, the full surface of the lower slide bearing always has an even load on its entire surface.
- By always covering the lower element particles of debris, grit, etc. cannot settle onto the Teflon® surface. This prevents premature failure due to abrasion occurring during movement.
In the drawing above the PTFE extends to the edges of the 10 gauge steel plate. This will provide maximum surface area, however the heat generated while welding can cause the teflon to de-laminate. The bond is quite strong and welding with attention paid to not overheating the plate can avoid this problem. One additional step to assure de-lamination does not occur is to recess the PTFE back from the edge. In the drawing below, note the PTFE is recessed 1/4" from all edges. This is especially important if the steel plate is to be fully welded all the way around its perimeter.
The configuration can change but the fundamental design is fairly consistent. Often we see drawings which require the Slide Bearing to be thicker. For instance 3/32" thick Fluorogold® Teflon bonded to 1/2" thick steel plate. This can be made to order, but a work around would be to use the stock Slide Bearing and weld to another 3/8" thick filler plate, thereby creating the same thickness. This controls the cost and can be done quickly. Any such alterations should be approved by the Engineer or Architect responsible for the project.
An alternative design, not seen as often employs a polished stainless steel upper member. In this case the Teflon® surface of the lower member mates with the polished stainless steel surface of the upper member. The advantage of this system is its ability to carry a heavier load. The standard Fluorogold® Teflon Slide Bearings we keep in stock have a load limit of 2,000 psi. So a bearing assembly with a lower element surface area of 8" x 6" would have a load limit of 96,000 lbs. The same assembly with a polished stainless steel upper member would have a load limit of 192,000 lbs.
Critical to the understanding of the strength and durabililty of these Slide Bearing Assemblies is the difference between Virgin PTFE and the fiberglass aggregate Fluorogold® Teflon used in both our stock and made to order bearings. Virgin Teflon® cannot support the weight a slide bearing is placed under without compressing and distorting. As mentioned above, the specially formulated Fluorogold® PTFE used in our slide bearings has a strong glass aggregate that provides far greater compressive strength and yet retain the low friction capability of virgin PTFE. As well the glass aggregate PTFE remains chemically inert, allowing it to remain in service indefinitely.
PTFE can also be bonded to other substrates such as commercial grade neoprene. This applies to situations where cushioning is required, such as machinery mounting and loading dock ramps. Note that while PTFE bonded to commercial grade neoprene has a co-efficient of friction similar to PTFE bonded to steel, it does have a lower compressive strength.