PTFE Slide Bearing Plates
PTFE slide bearings are superior to conventional expansion plates, rollers, and rocker arm type supports in a wide range of applications. They support the petrochemical plants, heavy machinery, pipelines, buildings, and bridge girders: they accommodate lateral and/or axial movement of any structure as a result of thermal, seismic, or differential forces.
Bearings for such applications must operate at high loads and low speeds, and it is under these conditions that the self-lubricating properties of PTFE are at maximum. This factor, together with its no stick-slip and anti-weathering characteristics, is the principal reason why PTFE has proved to be so successful as a slide-bearing material.
The slide bearing possesses an extremely low coefficient of friction, thus enabling the stainless steel plate to slide easily against the PTFE material. Slide bearing plates are often purchased with spring supports such as constants and variables.
Slide bearings are used in a wide range of applications, petrochemical plants, heavy machinery, pipelines, buildings, and bridge girders; they accommodate expansion, contraction, and other reciprocating motions of any structure that moves as a result of thermal, seismic, or differential forces. The use of low friction materials means that supporting structures can be designed more economically, and were used with pipework, pipe stresses, and equipment loadings resulting from high frictional loadings are reduced.
We produce standard slide plates using PTFE with stainless steel and also graphite with stainless steel. The standard designs are suitable for loadings up to 1000kN and are compatible with our range of pipe bases and clamp bases. Slide plates for attachment by welding and also using countersunk head bolts are provided. In addition to the standard designs, we offer a design service for slide plates to cover any situation where sliding needs to occur.
- Virgin PTFE sliding on stainless steel. This configuration gives the lowest possible coefficient of friction for unlubricated materials. PTFE is suitable for use at temperatures up to 200°C.
- 25% glass-filled PTFE on stainless steel. The use of glass-filled PTFE allows higher bearing pressure so that more compact designs can be used. The coefficient of friction is slightly higher than with virgin PTFE.
- Graphite sliding on stainless steel. Graphite is normally used where the sliding surface is at a high temperature. It can be used at up to 500°C.
- FRICTION For PTFE on stainless steel, the coefficient of friction reduces with increasing bearing pressure. For graphite on stainless steel, the coefficient of friction will be approximately 0.15.
ADVANTAGES OF OUR SLIDE BEARINGS
• The simplicity of the bearing design and its ease of fabrication and installation make the unit cost-efficient.
• The costs of construction can be reduced by designing for expansion rather than strain.
• Coefficient of friction over the bearing surface remains constant, even under worst-case conditions.
• The bearings are maintenance-free – both PTFE and graphite are inherently self-lubricating, while PTFE will absorb small dirt or grit particles. Only simple protection is required against the significant
ingress of dirt.
DESIGN / SELECTION
We offer a specialist service based on many years of experience in designing and applying slide bearings. Low friction slide plates with a coefficient of friction less than 0.1 are available. They are designed specifically for the loads and movements required. The standard slide plate assemblies SLB01 to SLB10 (pages 4-6) are designed
to operate efficiently at the loadings shown.
STANDARD BEARING ASSEMBLIES
• SLB01 to SLB04 (page 4) use 3mm thick PTFE bonded to an 8mm thick backing plate. Bonded PTFE bearings are the least expensive type of slide
bearing. They can be used up to a maximum temperature at the sliding face of 130°C.
• SLB05 to SLB08 (page 5) use 5mm thick PTFE bonded to a recessed backing plate. The recessed design allows use up to 200°C at the sliding face and increased bearing pressure. The thicker PTFE allows greater absorption of dirt/grit into the PTFE.
• SLB09 and SLB10 (page 6) use 5mm thick graphite contained in a recessed backing plate. These items can be used at temperatures up to 400°C at the sliding face and up to 500°C if alloy steel or stainless steel backing plate is specified.
• For all slide bearing assemblies, it is essential that the correct movement range is selected to ensure that the PTFE or graphite is in full contact with the stainless steel sliding plate. It is possible to choose a smaller movement range if the direction of movement is known and the upper plate is offset
relative to the lower plate when installed.
- Alternative PTFE grades.
- Alternative backing plate material e.g. stainless steel.
- Other shapes.
- Higher loading.
- Greater movements.
- Different backing plate thickness.
- Different bolting configurations.
SLIDING FACE TEMPERATURE AND THERMAL INSULATION
It is important to note that the maximum service temperature relates to the sliding face. As a rule of thumb, under normal conditions the temperature falls by 200°C for every 100mm from the heat source (in ambient air) – for example, a typical horizontal vessel operating at 500°C will have a bottom-of-saddle temperature of about 150°C. Where PTFE is used and the temperature of the bearing face is likely to exceed 200˚C by conduction through the bearing components, a thermal barrier can be interposed between the heat source and the sliding unit. We recommend the use of Monolux 500. Thermal barriers are not usually needed on conventional high-pressure pipework supports.
BONDING OF PTFE
Chemical bonding is the recommended method for locating the bearing material on its support because the sheer value of the epoxy adhesive is greater than that of the PTFE. All bonded PTFE elements are not adversely affected by exposure to ultraviolet light, providing the minimum thickness requirement of 1.5mm is met. Site bonding of PTFE is not recommended – strictly controlled conditions of cleanliness, pressure, and temperature are required to obtain a satisfactory bond between the PTFE and the substrate.
The bearing components can be located to the installation by bolting, tack-welding, full welding, or mortar embedment, and the appropriate type of bearing should be chosen according to the installation method.
The PTFE should be adequately protected against weld spatter, paint spray, metal swarf, etc., during installation.
VIBRATION/ACOUSTIC DAMPING PADS
Slide bearing units can be built with a variety of elastomer composite interlayers or backings to suit customer’s design parameters when acoustic or vibration damping is necessary. Elastomers may be used when
simple angular or rotational movements are required.
The following steps will indicate the slide
bearing requirements for a particular application:
1. Determine the load of the structure – this will indicate the total bearing area required at a suitable bearing pressure.
2. Decide the number and positions of the bearings according to the rigidity and function of the structure.
3. Take account of operating temperature limits, and specify any necessary thermal insulation.
4. Consider any unusual conditions affecting the bearings, such as the need for additional thermal insulation, damping pads, etc.
5. Decide the most appropriate method of mounting the bearings.
6. Select the types of bearings required and specify their dimensions.
TECHNICAL ADVANTAGES OF PTFE SLIDE BEARINGS
• PTFE has the lowest coefficient of friction of any known solid engineering material, including lubricated metal.
• There is no stick-slip action.
• They have an indefinite life since chemicals and the weather have no effect on PTFE – moisture absorption is less than 0.01% even under icing conditions or immersion and the material is chemically inert.
• No maintenance is required, PTFE will never cold weld to itself and therefore requires no lubrication.
• The bearings are easily installed, either pre-assembled or on-site.
• PTFE bearings are far less bulky than alternative assemblies.
• There is no possibility of fatigue failure.
• Electrical and thermal insulation minimize galvanic corrosion and heat loss.
• Vibrations are damped.
• Small particles which may become embedded do not cause binding of the surfaces.
• The slide bearing can accommodate some misalignment in construction without setting up stress corrosion along a leading-edge, as can occur in conventional bearings
Key advantages of PTFE slide bearing:
- Simplistic design and ease of fabrication and installation result in cost efficiency.
- Costs of construction can be reduced thus designing for expansion rather than strain.
- The coefficient of friction over the bearing surface remains constant, even under worst-case scenarios.
- The bearings are maintenance-free therefore PTFE is inherently self-lubricating, while dirt particles are absorbed into the material. Only simple protection is required against the significant ingress of dirt.