PTFE BRIDGE BEARINGS

Product write-up and technical details

Bridge bearings

Bridges typically consist of two components: the superstructure and the substructure. The superstructure is subject to various dimensional deformations due to the nature of loads placed upon it. These deformations could include:

• Thermal expansion/contraction
• Elastic deformation under live load
• Seismic forces
• Creep and shrinkage of concrete
• Settlement of supports
• Longitudinal forces - tractive/ breaking
• Wind loads

The nature of these forces makes it necessary to have a device in between the substructure (base) and the superstructure which allows for the required movement, while also giving stability and having the capacity to bear the loads placed on the bridge. The device most popularly used, is a bridge bearing which assumes the functionality of a bridge by allowing translation and rotation to occur while supporting the vertical loads.

Thus, a bridge bearing is an element of the superstructure which provides a vital interface between the superstructure and substructure.

PTFE in Bridge Bearings

The use of PTFE in such bearings has been steadily increasing, although its application does not extend to all variants of bridge bearings.

PTFE has an exceptionally low coefficient of friction and high self-lubricating characteristics, resistance to attack by almost any chemical, and an ability to operate under a wide temperature range.

Furthermore, while unmodified PTFE can be used to a PV value of only 1,000, PTFE filled with glass fibre, graphite, or other inert materials, can be used at PV values up to 10,000 or more. In general, higher PV values can be used with PTFE bearings at low speeds where its coefficient of friction may be as low as 0.05 to 0.1.

The low coefficient of friction exhibited by PTFE is unique for two primary reasons:

1. PTFE against stainless steel exhibits an even lower coefficient of friction that PTFE against PTFE. In fact, the coefficientsopf PTFE against steel have been found to be the lowest between any two solid materials

2. The coefficient reduces with increased pressure – allowing for coefficients as low as 0.03 (See table 1)

Average pressure on confined PTFE (MPa)	Max. value of 'µ' for lubricated PTFE	'µ' for un-lubricated PTFE
5	0.08	0.16
10	0.06	0.12
20	0.04	0.08
30	0.03	0.06

Table 1: Coefficients of friction of PTFE

To summarise, the following properties have driven the increased application of filled grade PTFE:

§ PTFE against steel has one of the lowest coefficients of friction

§ The load bearing capacity of the PTFE sheet is in the range of 130-140Kg/cm²

§ The PV values are found to be in excess of 10,000

§ Service temperatures of -250 to +250°C are possible.

PTFE is most commonly used in two types of bridge bearings:

Sliding bearings: A system of two plates, one sliding over the other makes one of the simplest types of bearings. These bearings permit translation in longitudinal and transverse directions, unless specifically restrained in any of these directions. No rotation is permitted unless specially provided in the form of articulation and only vertical loads are resisted / transmitted by these bearings.

Generally, plain sliding bearings are provided where span is less than 30m, because the movement capacity of these bearings is usually small.

The bearing is composed of two thick sheets of steel (preferably high-density carbon steel). Between the sheets are one layer of PTFE (with suitable fillers) and one layer of polished stainless steel. The stainless steel is welded to one of the bearing plates while the PTFE is bonded to the other plate. To provide for better bonding, a recess is created on the bearing plate into which the PTFE is fitted.

Their regular maintenance is very important, to keep a tab on friction otherwise the value of horizontal force transmitted to sub-structure will increase tremendously. Therefore, the frequency of lubrication has been prescribed as once in three years.

PTFE-POT Bearings: These consist of a circular non-reinforced rubber-pad (elastomer) fully enclosed in a steel pot. The rubber is prevented from bulging by the pot walls and it acts similar to a fluid under high pressure.

PTFE PLATE

While the bearings were initially created without PTFE, the necessity of horizontal movement in addition to load bearing capacity made it necessary to incorporate PTFE on the piston. The rotation, therefore, is provided by the elastomer due to differential compression and translation by steel and PTFE.

POT bearings offers a much higher degree of movement than standard sliding bearings, although it is tougher to manufacture due to the extended recess needed for the POT as well as the sealing elements needed to contain the elastomer within the POT. These seals must be metallic. The PTFE plate must be recessed into the piston and requires 'dimples' into which additional lubricants are placed during time of installation.

Typical working conditions for standard POT-PTFE bearings include:

• Provisions apply for temperature ranges of -10°C to +50°C
• POT bearing of diameter up to 1500 mm are within scope of these specifications
• Rotation up to 0.025 radians only considered
• PTFE can withstand bearing pressures in excess of 40MPa – depending on the filler used (See table 2)

Fig 3: PTFE-POT bearing

Type of PTFE	Average bearing pressure (MPa)	Max edge pressure (MPa)
Filled PTFE or unfilled recessed PTFE	24.5	35
Unfilled PTFE (not recessed)	14	35
PTFE with bronze	42	70
PTFE perforated metal composite	35	35

Table 2: Allowable pressures for PTFE

PTFE Bridge Bearings from Poly Fluoro Ltd.

Poly Fluoro Ltd. is one of India's fastest growing PTFE companies with the capability to mould, extrude, skive and machine PTFE sheets, tubes and components. We are an ISO 9001:2000 certified company and our ambition is to become India's largest and most innovative PTFE manufacturer. This is leading us into many new areas of PTFE application – bridge bearings key among them.

Poly Fluoro Ltd. has combined its expertise in PTFE skived sheets with know-how on PTFE bonding techniques to produce high quality and durable bridge bearings. We have opted to focus on PTFE bridge bearings – sliding bearings and PTFE-POT bearings – as these allow us to hone in and capitalise on our expertise in PTFE processing.

Our bearings usually employ PTFE sheets with a 25% glass filling as this gives us better creep properties than other fillers. Fillers of bronze, carbon and MoS₂ are also available. Depending on customer specifications, we can colour the PTFE sheets with a variety of pigments including gold, turquoise and green.

Due to our in-house milling, grinding and bonding capabilities, we are able to provide fully assembled bridge bearings as per our customer specifications. However, as our core competence lies in PTFE sheets, we are also able to provide standalone sheets for use in bridge bearings – should our customer wish to machine and assemble the bearing on-site. This versatility allows us to cater to both the end-user as well as other bearing manufacturers.

We are equipped with facilities for testing the tensile properties, deformation properties and shore hardness of our sheets before we employ them in bridge bearings.

Fig 4: PTFE sliding bearings

POLY FLUORO LTD.

260A. Bommasandra Industrial Area, Hosur Road, Bangalore – 560 099

(: 91-80-27832627. Fax: 91-80-27832549. Mail: sales@polyfluoroltd.com; www.polyfluoroltd.com

Note: This Information is based on our experience to date and believed to be reliable. It is intended only as a guide for use at your discretion. We cannot guarantee favourable results and assume no liability for use of this product. A local test programme is always recommended.