Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Cantilever Load Considerations for PTFE Sliding Bearings

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.

PTFE Wear Plates: Misconceptions and Applications for Heavy Equipments

Although PTFE is used extensively for its wear resistant properties in a range of different products, its application as a wear resistant plate remains restricted and not widely known in certain areas where it would be ideal.

In most cases, the preferred composition for PTFE wear resistant material is PTFE with bronze (along with some friction reducing additives). As discussed in our earlier article (see: PTFE Compounds and their effects), this composition improves the PV value and wear rate for the material and although the coefficient of friction does increase, over-all it performs superbly as a replacement to metal bearing parts that require frequent lubrication.

Currently, some of the main applications for PTFE as a wear resistant material for which we supply include:

  1. Slideway bearings: Commonly referred to by the brand name “Turcite” (Poly Fluoro brand name: Lubring), PTFE slideway bearings are used widely in the machine tool industry, where they serve to either replace or reinforce standard phosphor-bronze LM guideways. The material was earlier used primarily to recondition old machines in which the guideways had worn out. However, increasingly it is incorporated in new machines as well – owing to the higher life and lower maintenance required in comparison to metal guideways
  2. Wear stripsPTFE wear strips are used either in running lengths or punched into flat components, which are used in sub-assemblies, like shock absorber struts and pistons. Usually the tolerance on thickness for such wear strips is very low – implying the requirement of a high precision skiving machine. In most case, where we supply these items, a tolerance of +/-0.02mm is maintained on thickness.
  3. Piston rings: Here, thin bands of PTFE wear material are machined and fitted on the piston shaft to absorb the wear resulting from a constant back-forth movement. As this process is wasteful (and therefore expensive) due to the machining involved, sometimes customers prefer to buy wear strips and bond them around the shaft. However, bonding PTFE is usually only recommended when there is minimal shear force being applied on the item – so this method is usually unsuccessful.
  4. Bushings: PTFE can either be machined into a solid bush, or be used as a layer on a metallic bushing (commonly called DU bushings). Again – the idea here is to create a self-lubricating bush, which can be installed within a sub-assembly and allowed to run without the constant need for lubricants.
  5. Wear plates: Used in more heavy duty applications, wear plates are usually employed in thicknesses exceeding 10mm and often require milling on the surface to create oil-grooves and holes for bolting. In most cases, their function is similar to that of a slideway bearing, however we have noticed that many OEMs remain apprehensive to employ PTFE wear plates into their equipments. In an attempt to clarify certain points regarding PTFE wear plates, we are going to be looking at 2 aspects of their usage:

 

1) The common pitfalls clients experience when using these bearings and misinformation regarding the same

2) Our own experience in the Die Casting Industry, where the success of these plates has led us to aggressively recommend it to OEMs

Issues hindering the adoption of PTFE wear plates

  1. Installation: We find that most people adopting PTFE wear plates do so because they have some prior experience with installing slideway bearings. Consequently, they assume the installation methods would also be the same. However, as slideway bearings are much thinner (going up to no more than 5-6mm) and because following installation, they remain subjected to very little shear loads, they can be bonded to the metal bed and this bond is likely to survive over a long period of time.In the case of wear plates, bonding is not an option as it is likely that there is some shear load which will get applied which, when coupled with the thickness of the plate would weaken the bonding and cause the plate to come loose in the medium to long term.

    The correct method of installation is bolting – although valid apprehensions exist with regards to this. For one: the plate needs to be milled with a stepped hole to allow the bolt to rest within the piece. Care needs to be taken to ensure that the bolt head does not rest above the surface of the PTFE plate. As an added measure, PTFE discs can be bonded to the head of the bolt to ensure that in the event that any extra pressure squeezes the PTFE plate, the contact between the bolt and the moving plate is not damaging. Furthermore, tightening the bolt too much can cause the PTFE plate to get crushed (a common reason cited by OEMs for not using a soft material like PTFE). Hence the correct method would be to use a metal bush to ensure the bolt is not tightened beyond a point (see below).

    The purpose of the bolt is to ensure the PTFE wear plate does not slide away during operation. As long as this is ensured, the plate will perform properly.

  2. Load bearingA common misconception relating to the load bearing capacity of PTFE leads many machine tool builders to write-off PTFE as a wear pad material. The assumption is that phosphor-bronze, being a metallic material, is the only option strong enough to take the load of heavy moving parts.

    In truth – PTFE has a compressive strength of at least 135-140Kg per square cm. This implies that a 100mm x 100mm plate would be able to withstand 13.5-14 Tonnes of vertical load. In most heavy-duty equipments, maximum loads of 5-6 Tonnes are present, meaning that the load bearing is not an issue at all. Furthermore, the coefficient of friction of PTFE against another surface only reduces with the application of pressure – implying that apart from taking the load, the effectiveness of the wear plate in ensuring a smooth functioning of parts is greatly enhances.

  3. MachiningClients who are looking to convert to PTFE wear pads frequently express two concerns pertaining to machining.

    The first is on tolerance: as the thickness on phosphor-bronze wear pads can be grond to within a few microns. In the case of PTFE – a maximum tolerance of 50 micros is possible – which we have found is acceptable in most industries.

    The other concern is around specific grooves and the exact positioning of holes. As PTFE can be milled (we use a CNC vertical milling centre) – any groove pattern and hole dimensions can be machined on to the surface of the wear plate.

  4. EnvironmentFinally – we have heard concerns over the conditions in which the equipment is used and whether PTFE will be able to withstand the same in the long term.

    Firstly – PTFE has the ability to withstand temperatures of up to 250 Degrees Celsius. In most industries we know, the actual heat generation never causes the surrounding temperature to go about 80 Degrees, so clearly there is no issue in using PTFE.

    The other concern is on the build up of dirt and whether grit and other hard particles will damage the surface of the PTFE plate. While the recommended option here would be to make a seal around the PTFE to ensure that dirt does not get accumulated between the PTFE and the other moving plate, it should also be noted that in case a particle does get lodged between the plates, PTFE has the unique ability to absorb the same so that it does not hinder the movement of the assembly.

 

Case Study: PTFE wear plates in the Die-casting industry

A client who was consulting on technical metrics with various companies engaged in aluminum die-casting approached us, a while back. The problem they were facing was that the wear plates that had been installed on as bearings between the platens was wearing out every 2-3 months, with the result that there was significant down time on the machines every time these plates needed to be replaced.

The plates being used were a fiber enforced resin plates and it was easy to see that a few months of usage had significantly worn out the plates leading to deformation and even cracks.

We offered them PTFE wear plates and these were installed on a few machines as a trial. The machines were run normally for a period of 3 months and the PTFE plates were analyzed with the following results:

  1. Wear out was minimal: In fact, the PTFE wear plates were much the same dimension as when they were installed. The customer felt that the load of 2.5 Tonnes being applied on the plate would compress the plate and lead to a deformation on thickness – but this was not the case.
  2. Lubricity was greatly enhanced overall: The plates had become completely smooth due to the constant sliding across its surface and this smoothness translated into the more efficient operation of the equipment. The customer also reported that while earlier there was some amount of “jerkiness” in the motion of the platens was no longer an issue.
  3. Improved cycle time: Apart from the fact that the down-time of the machine was no longer an issue as the plates were not worn out, the overall cycle time of the machine during production was also improved. This was mainly because there was no longer a need to continuously monitor the level of lubrication on the wear plate.

 

Following the successful trial of the PTFE wear plates, the material was adopted in all machines of the client and we are now working with a number of clients in the die-casting industry to replace their resin plates with PTFE plates.

The various forms of Sliding Bearings

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.