PTFE Sliding Bearings: Calculating Coefficient of Friction

PTFE is a preferred material in sliding bearings for three very specific reasons:

1. Load bearing capacity
2. Weather ability (due to its overall chemical inertness)
3. Low coefficient of friction

The first two factors are well accepted and easily tested. The vertical load on a bearing is simply tested by placing the bearing under a hydraulic press of suitable capacity, applying 1.25 times the rated load of the bearing and holding this load for a period of 1 hour to observe any adverse impact on the bearing material. In our own experience, it is very rare that pure PTFE would fail in this instance, since:

1. Pure PTFE genuinely does have a very high load capacity and even in the event of over-loading, would tend to deform rather than break down
2. The design load for most bearings incorporates a safety factor of up to 60% – implying that while PTFE may be able to withstand a load of up to 40Mpa, it is designed with a load of only 16Mpa and is thus well within its own capacity to take the load applied

Weather ability is difficult to test, as this is a long-term guarantee that the material can stay in outdoor conditions without experiencing any degradation in properties. However, most clients are happy to take this assurance at face value – as long as they can satisfy themselves that the material being used is in-fact pure PTFE. Its should be mentioned here that in the event that reprocessed or recycled PTFE is used in sliding bearings (a gross violation of quality norms, but one that may occur all the same, especially if the bearing manufacturer is buying their PTFE from a third-party and is therefore not directly in control of the quality), there will definitely be a failure of the bearing after installation.

As mentioned in earlier articles, we have witnessed many deviations from expected performance when presented with reprocessed PTFE. Amongst these is the tendency of the material to become brittle and even crumble after being kept outside for a prolonged period (usually over a few months). No doubt, a similar effect would be experience by a material used in a bearing that is installed on a bridge or flyover – with the result that the bearing may fail after only a year of service.

Coefficient of friction

The problem with the coefficient of friction is that most people are not fully familiar with what it implies. So let us start with defining it and then look at the misassumptions surrounding it.

The coefficient of friction between two planes is defined as the ratio of the force needed to move one plane over the other divided by the force pushing the two planes together.

So in the case of a block resting on a table, coefficient of friction between the block and table would simply be the force needed to slide the block across the table, dived by the weight of the block itself.

Since the coefficient is a ratio of two forces, it does not have any units. A common mistake clients make is to ask us to define the unit we have considered for the coefficient of friction.

In the case of the PTFE sliding bearing, the coefficient of friction being considered is that between PTFE and polished stainless steel. Here again, a mistake is often made asking what the coefficient of friction of PTFE is. There is no such thing as a stand-alone value for coefficient of friction for any material. The coefficient between PTFE and polished stainless steel will no doubt be much lower that between PTFE and concrete. In other words, it would take more force to move a slab of PTFE across a concrete surface than it would to move the same slab across a polished stainless steel surface. Thus, when we talk about a coefficient of 0.04 between PTFE and stainless steel (the commonly accepted value for bearing manufacturers), we are saying that for a 1Kg PTFE block to slide across a polished stainless steel surface, it would require only 40 Grams of horizontal force. (Note: we are aware here that Kgs and Grams are units of mass and not force, but seeing as these are ratios of force, the values in Kgs/Grams against the values in Newtons would yield the same results).

Measuring coefficient of friction

We have already described that the value for the coefficient is derived by dividing the horizontal force over the vertical force. However, in practice, this is less straightforward. We would like to look at some of the methods that are used around the world to check these values as pertaining specifically to PTFE bearings, before describing what we feel is the most straightforward and easily implemented method.

1. Two-press method
   It is not possible to test the bearings simply by applying a vertical load and seeing at what horizontal load the bearing slides. This is because the bearing plate on which the horizontal load is applied also has another surface, which would be in contact with the vertical press and therefore be subject to friction from the press itself (which is likely to be very high).
   Therefore, the accepted method is to place 2 bearings, back-to-back (shown below) and exert load on the centre.
   This process is technically sound, but practically not always feasible. For starters, the bearing shape itself may not lend itself to being places back to back. It may have welded guides attached to it or be of an unusual shape. In addition to this, the process in expensive – requiring two hydraulic presses.
2. Load indicator method (laboratory)
   In this method, the stainless steel plate is placed horizontally with the PTFE plate on top. The PTFE plate is connected to a steel wire which is in-turn connected to a load indicator. The load indicator has a motor, which causes it to move upwards very slowly. As the indicator moves up and the wire gets tight, the load reading starts to show the horizontal load being applied. Once the PTFE plate starts to move, the reading is recorded and divided by the weight of the PTFE plate to give the coefficient of friction.
   This method is possibly the most accurate, as load indicators can offer values in grams. However, it again suffers from the issue that if the bearing plates are not totally flat or are too big for the equipment, they cannot be accurately tested. Furthermore, it is debatable whether such accuracy is needed in the realm of sliding bearings
3. Poly Fluoro method (inclined plane)
   In an attempt to find a quick, repeatable, logical and universally applicable method to check the coefficient of friction, our method follows the rather simple process of gauging the angle of incline.
   For starters, we do not believe that getting an accurate value of the coefficient of friction would add any value to the product. If we can confirm that the plates slide at a coefficient of friction set to 0.04, then it does not matter whether the coefficient is in-fact 0.03 or 0.036, as the product has met its required specification.
   
   The diagrams  show that when the planes are inclined, the coefficient of friction takes the value of the tangent of the angle of inclination. This allows us to easily check the coefficient of friction, as we simply set the ratio of Y to X to equal 0.04 and check if the PTFE plate slides down the plane, when placed on the SS plate. Again – note here that only in the event that the PTFE plate does not slide, can we conclude that the coefficient is greater than 0.04 (and hence outside tolerance). Whether the plate slides slowly or very fast, is of no consequence – as either way it confirms that the coefficient is at least 0.04.
   
   In the event that the client specifies a higher or lower coefficient, the same method can be employed, by simply changing the value of Y. So we first assess what the value of Y should be by multiplying the length of the bearing plate by 0.04 and then use a calibrated slip gauge to prop up the bearing on one side (preferably on a flat bed) so that the angle is attained.
   The method employed here is useful to check PTFE bearings as it can be applied to any design and multiple bearings can be checked from one lot if needed, without the hassle of making separate fixtures and modifications. Furthermore, it can be checked even before the bearing is assembled so as to confirm that the PTFE material being employed meets the parameters.

Static vs Dynamic coefficients

It must be mentioned that both static and dynamic coefficients of friction are relevant metrics and that the inclined plane method only measures the static coefficient. However, this is of no concern for PTFE bearings as PTFE is known to exhibit nearly identical values for both static and dynamic coefficients (a property unique to PTFE and not universally applicable).

We believe that the method described above is a more effective way to check for a very important parameter that bearings are based on.

Cantilever Load Considerations for PTFE Sliding Bearings

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As a manufacturer of sliding bearings in India, one of the challenges we face is that there does not exist an official book of guidelines specifically for this type of bearing. The closest we have is the IRC:83 – which is the code book for POT-PTFE bearings, and which details the specifications of the materials to be used and also the testing parameters for POT bearings. As such the IRC:83 does provide some guidelines – but does not address the finer design requirements for sliding bearings. Consequently, during design and testing of the bearings, customers rely on the Quality Assurance Plan (QAP) we provide them, with the option to question, refute and even modify the requirements as they see fit.

Although the BS:5400 and AASHTO standards do make specific recommendations for PTFE sliding bearings, customers are not always willing to accept their parameters as they may sometimes vary from those specified in the IRC – in places where the codes overlap. This can result in technical stand-offs, between the customer and manufacturer, as each tries to convince the other regarding a certain process or parameter, without any official rule book, to back up their view point.

Among the more interesting technical debates we have had recently has been regarding the thickness of the top plate (upper sole plate) in bearings with high movement requirements.

Given a specified vertical load, a PTFE sliding bearing will be required to have a PTFE pad with side determined by the compressive strength of PTFE (usually taken at 100-200 Kgs per sq. cm). This side in turn determines the side of the lower sole plate. Give these two dimensions, the movement of the bearing (specified usually by the client/ contractor) will give us the side of the Stainless Steel sliding element, which in turn will give us the side of the upper sole plate.

Our designing of PTFE sliding bearings began with taking standard designs for specified loads and movements (such as C&P) and recommending them to clients. As we became better versed with PTFE sliding bearings, we started designing from scratch – using the parameters of movement, load and rotation to design customized PTFE bearings for specific project requirements. As many of the original standard drawings recommended sole plates of 12-15mm thickness for bearings up to loads of 50-65 Tonnes, we too continued with this recommendation. After all, the constraint of a bearing in taking a vertical load depends purely on the PTFE – as when compared with steel, PTFE has a much lower compressive strength (100-200 Kg per sq. cm versus steel’s 14000-15000).

It came as a surprise to us therefore when a client expressed concern over the top plate thickness – citing that the bending moment caused by the vertical load would be in excess of what the top plate could accommodate, given the overhang of the top plate over the bottom. This led us to revisit a lot of the standard designs – only to find that for similar load-movement parameters, other bearing designs did not recommend a thicker sole plate that our customer was insisting on.

We therefore went back to theory to gauge exactly how much of a bending moment would be caused by our given load and assess whether there was a case for changing our top plate design.

The formula for calculating the bending moment is as follows:

M= (P x L²)/2

Where:

P = Pressure (Kg/cm2)
L = Length of overhang (cm)
M = Maximum Bending Moment (Kg)

Once we know M, we divide by the Maximum Allowable Bending Stress (S) of the material (1650Kg/cm for steel) – to derive the thickness of the steel plate required.

In the example in question, the value of M obtained was 3941 Kgs, which when divided by S gave a thickness of 3.78cm – or 38mm.

This was a shocking revelation, as our recommendation was for 15mm – clearly insufficient for the load in question. Nonetheless, we wanted to dig deeper to find out how most standard bearing designs only called for a 12-15mm thickness – when the theory clearly showed that this was inadequate.

In most of our discussions with consultants and industry experts, the value of 38mm was ratified and we were told that this was in fact the thickness needed. They were unable to explain, however, why the standard designs did not tally with the theoretical calculations. Our question was finally answered when we spoke with a contractor who studied the drawings and design details and confirmed that while a thickness of 38mm was indeed required, the thickness of the insert plate also needed to be taken in to account.

The insert plate is installed at site and is simply a 25-30mm thick steel plate grouted/ cast along with the concrete substructure and/or superstructure. The bearing is welded or bolted to this plate and the load on the bearing is transferred through the plate as well. Thus, a bending moment will act through the layers of both the upper sole plates and the insert plates – meaning that even with a sole plate thickness of 15mm, the total thickness through which the load acts is 40mm – which is more than sufficient in our example.

We went back to our client with this, but were informed that they were not using an insert plate in this particular project and that the sole plate thickness needed to be at least 40mm as per our calculations.

The exercise was an eye-opener, because we had never before been questioned on the bending moment for sliding bearings, nor had we come across any design calculations for this in the various codebooks. However it is a critical point in the design of a PTFE sliding bearing – as there is always an overhang of top plate over the bottom. As a rule, one needs to verify that an insert plate is being used and of what thickness it is. Adding this thickness to the sole plate thickness, the designer needs to verify whether the bending moment is being accommodated. If not, the thickness of the upper sole plate must be re-worked.

The various forms of Sliding Bearings

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The application of PTFE in load bearings is not new. Amongst its many other attributes, PTFE also has an excellent compressive strength, allowing it to absorb pressures of up to 200 Kgf/cm2 (2900 psi). This is approximately double the compressive strength of neoprene (the material used in most elastomeric bearings) meaning that PTFE bearing pads can be much smaller and manage the same load.

In addition to the load bearing capacity, PTFE also exhibits a low coefficient of friction (the lowest of any known solid) – which only goes lower with the addition of more pressure and is exceptionally low when PTFE slides against polished stainless steel (the lowest between any two known solids).

This combination of load bearing strength and low-friction makes PTFE the preferred material for sliding bearings – where both load bearing and sliding movement are required to create an effective bearing assembly.

The use of sliding bearings is fairly widespread. Some of the areas we have supplied to are:

1. Oil and gas pipelines
2. Waterways and water pipelines
3. Conveyor systems (both indoors and outdoors)
4. Boiler plants
5. Minor bridges
6. Power plants

The compact size and overall effectiveness of the bearing makes it an ideal choice in lower load applications (under 100 Tonnes). Furthermore, the simplicity of slide bearing design ensures that as long as the basic design specifications are adhered to, there exists a lot of latitude as to the exact dimensions and form of the bearing. This is useful for clients, who would prefer to design their structures independently and have the bearing modified to suit their overall design.

It must be pointed out that in India, there is no official rulebook for the design of sliding bearings. For the most part one refers to standards such as BS:5400 and AASHTO – taking care to cross check against the IRC:83 (the Indian code book for POT-PTFE bearings) to ensure that the material specifications match.

As a manufacturer of these bearings, this does add a lot of flavour to the task of design. Very rarely do two separate projects look for the same bearing design – there are always nuances and specific constraints against which the bearing must be altered to accommodate the client’s requirements. And although the constraints may be somewhat common – the method of accommodating them can vary significantly.

Movement

In many cases, the bearing requires a sliding movement in only one direction. This results in the requirement of guides. Our experience with guides is that as long as there is negligible horizontal load on the bearing (under 2 tonnes), any of the two following guiding elements can be used.

- Bracketed guides – these are normally two guide plates welded/ bolted to the side of the top or bottom plate

- Dowel guides – guide pins can be used either at the center of the plate or on the sides

In case the load is higher than 5 Tonnes, a centre dowel guide is always preferable. Some designs may also specify a guide that is monolithic with the top plate. While this is the definitely better from a load bearing stand point – it is often expensive, as the plate needs to be either cast or machined out of a much thicker plate.

In any case, as the horizontal load increases beyond 10-15 Tonnes, it becomes viable – both technically and commercially – to look at POT-PTFE bearings.

Rotation (lateral)

Rotation along the horizontal axis (perpendicular to the direction of the vertical load) is not a common requirement.

It is most easily achieved by employing a circular dowel pin at the centre of the bearing around which the top plate can rotate.

In case the load is high, you could also look at a hybrid POT bearing – where a PTFE disc is used in place of the elastomer and a polished stainless steel sheet is affixed on the piston to allow for rotational sliding movement.

Rotation (vertical)

Vertical rotation (around the direction of the vertical load) is most easily achieved by employing an elastomeric pad along with PTFE. In most design specifications, there is a stainless steel sheet required in between the PTFE and the elastomer.

In more heavy-duty applications, a fully reinforced elastomeric bearing may be employed. The bearing is affixed (either by bonding or during vulcanizing) to the base plate housing the PTFE.

However – as discussed earlier – the lower compressive strength of elastomeric bearing material (such as neoprene) would require the size of the PTFE bearing to be defined by the size of the elastomeric bearing required. In some cases, where space is a constraint, designers opt for spherical bearings to accommodate the vertical rotation.

The benefit of a spherical bearing is that it can be compact and that the radius can be changed to match the extent of the rotation required. In contrast, to accommodate higher rotation in an elastomeric bearing, the thickness of the bearing would need to be increased – making it more expensive and bulky.

On the other hand, the smoothness of the rotation provided by an elastomeric bearing (which is effectively using it’s elasticity to accommodate the rotation) is compromised in a spherical bearing. Although in most spherical bearings a PTFE-SS match is created to allow for smooth rotation – it will perform slightly less effectively than an elastomeric bearing. Ultimately, this is a trade-off that the designer will need to assess depending on the requirement of the project.

Arc bearing

Arc bearings are normally used in pipelines, as the bearing needs to take the curved shape of the pipe. The most common arc type bearings we have come across employ two sets of PTFE-Neoprene pads, which have been heated and bent to form the required radius needed to match the pipe. One set of PTFE-Neoprene is bonded with the pipe, such that the PTFE layer faces downwards. The second set is bonded to the concrete base, such that the PTFE surface faces upwards. When the pipe is lowered on to the concrete base, the PTFE layers mate, such that there is sliding along the length of the pipe. Also, due to the neoprene layers – there is rotation allowable.

This bearing can also be made using stainless steel to replace one of the PTFE layers. However, bending the stainless steel to match the radius of the pipe is more expensive than bending PTFE (which can be done using heat and a cheap metal die). Furthermore, it is likely that there would be slight variations on-site in the radii of the pipe and the concrete support. In this case, the stainless steel may develop kinks/ irregularities on the surface once the load is applied whereas PTFE, being much more pliable, will accommodate the same quite easily.

Two-way bearing

Our experience with this type of bearing has been mainly in the erection of conveyor systems. Often, along with the vertical load exerted on the bearing, there is some amount of horizontal load (along with restricted horizontal sliding movement in one direction) and some upward load. Usually, these loads are very small – within 2 Tonnes – so a complex or heavy-duty solution becomes wasteful

The concept of a low-cost, but effective bearing has let us to consider 2 alternate designs as shown below.

The simple design would employ side guides to form a bracket around the lower plate – allowing sliding movement in one direction and ensuring any uplift is contained. However, as the guides are welded, their strength is limited to within 2 Tonnes at most.

In case the uplift load is higher than 2-3 Tonnes, one would need to look at the second design – where a bolting arrangement allows the total load to go much higher. The second arrangement is altogether more elegant and compact – but comes at a much higher cost, owing to the extensive fabrication required and the extra thickness on the top plate needed to accommodate the guide-cum-anchor pin.

Rocker bearing

Although rocker bearings are usually stand-alone metal bearings, we have seen them used along with a PTFE sliding arrangement to give a rocking-cum-sliding arrangement.

The base plate housing the PTFE is usually the top plate of the rocker bearing.

Conclusion

We have described here only some of the bearing types and features that can be designed, based on the requirement of a specific project. Considering that projects take many forms and the constraints they may present could be very unpredictable, the above list could only be a fraction of the complete set of sliding bearings that can be envisaged. However, our experience in this field suggests that these are the primary features which are required of a given bearing and that ultimately, most bearings would be a combination of the above design forms.