PTFE Slide Bearing Plates

In a wide range of applications, PTFE slide bearings are superior to conventional expansion plates, rollers and rocker arm type supports. They support petrochemical plant, 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 principle 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. Contact Us for a quote or more information.

General information

Slide bearings are used in a wide range of applications, petrochemical plant, 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 where used with pipework, pipe stresses and equipment loadings resulting from high frictional loadings are reduced. Carpenter and Paterson (Asia) Limited 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 CPAs range of pipe bases and clamp bases. In addition
to the standard designs, we offer a design service for slide plates to cover any situation where sliding needs to occur.

MATERIALS

  • 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 high
    temperature up to 500°C.

FRICTION

For PTFE on stainless steel, coefficient of friction reduces with increasing bearing pressure. The graph in Figure 1 gives an indication of how the applied loading affects the coefficient of friction. For graphite on stainless steel, coefficient of friction will be approximately 0.15.

ADVANTAGES OF CPA SLIDE BEARINGS

  •  The simplicity of the bearing design and its ease of
    fabrication and installation make the unit cost
    efficient.
  • The costs of a 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

Carpenter & Paterson (Asia) Limited offers a specialist service, based on many years’ experience of the design and application of slide bearings. Low friction slide plates with a coefficient of friction less than 0.1 are available.

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. 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 an 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.

NON-STANDARD OPTIONS

  • 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. Carpenter & Paterson (Asia) Limited recommend the use of Monolux 500.

BONDING OF PTFE

Chemical bonding is the recommended method for locating the bearing material on its support, because the shear 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.

INSTALLATION

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/ACCOUSTIC DAMPING PADS

Slide bearing units can be built with a variety of elastomer composite interlayers or backings to suit customers design parameters when acoustic or vibration damping is necessary.

DESIGN STEPS

The following steps will provide an indication of 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 indefinite life, since chemicals and
    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 minimise 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 results in cost efficiency.
  • Costs of a construction can be reduced thus designing for expansion rather than strain.
  • 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.