Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Shear Load Considerations for PTFE Sliding Bearings

PTFE sliding bearings have become a mainstay for any application that requires a combination of loads and movement. The key feature of the bearing is that it can accommodate high vertical loads, while offering a low coefficient of friction. As such, the standard model of PTFE against polished Stainless Steel allows for coefficients of friction as low as 0.03 (near rolling friction) while being able to sustain vertical loads of up to 400Bar.

The applications of this sliding arrangement include pipe support systems, boiler plants, bridges, and walkways. The plain PTFE sliding bearing does not lend itself to loads exceeding 150 Tonnes, hence the application does not extend to areas such as flyovers and railway bridges, where the loads are in excess of this value.

Design

Designing a plain PTFE sliding bearing involves an understanding of the various loads involved and ensuring that the PTFE does not succumb to these loads, even under their most extreme applications. For the most part, while PTFE is rated to a compressive load of up to 400Bar, we apply a 50-70% safety factor in design, keeping the design load within 120-200Bar.

Other loads such as lateral loads, uplift loads, and even rotation, can be factored into the bearing using metal bolts that can accommodate these loads. Care, however, must be taken in ensuring that PTFE is used in any area where there would be sliding. So, if metal bolts are used to prevent movement in one of the lateral directions, a PTFE element is employed around the metal bolt, such that the bolt itself does not rub directly against the sliding plate thereby increasing the friction.

One of the main design arguments that result with PTFE bearing is around the thickness of the PTFE pad employed. The thickness of the pad has long been specified as being between 3mm and 5mm as per the BS 5400 standard – which remains one of the only standards that pertains specifically to the design of PTFE sliding bearings. The standard recommends that the PTFE sheet should be 5mm in the care of a recessed bearing (a bearing where the PTFE pad is bonded within a recess on the metal plate) and 3mm in the case of an un-recessed bearing. The case for making the recess is quite sound. Bonding the PTFE within a pocket milled onto the surface of the metal plate ensures that the PTFE does not come loose of the metal over time. Barring cases where the shear load on the PTFE is likely to be low, a recessed option is generally preferred by designers.

Nonetheless, a lot of debate does arise as to why the values of 3mm and 5mm are found acceptable.

Shear Loads

Recently, we were asked to supply 8mm thick polypropylene sheets for a bearing project. Among the technical requirements was a shear load confirmation. Against a given horizontal load, we needed to confirm that the sheets would not deform, causing failure.

Given the Shear Modulus (G) of the material, the above formula can be used to calculate the value of F needed to displace the sheet. Since this was an infrastructure project, we assumed Δx to be 0.1mm (a very small displacement) and given the other properties, the force was calculated.

In our particular example, a force of 750Tonnes would be needed to displace the polypropylene sheet by 0.1mm. Consideting the actual loads expected were within 10 Tonnes, this safely allowed us to state that the bearing sheet would not be affected by the shear load.

The more interesting aspect of this, however, is the function that l (the thickness of the sheet), plays in the claculation. For the same shear strain, a higher value of l would imply a higher value of Δx. In other words, increasing the thickness increases the risk of deformity.

Thus, the sheet thickness – whether it is polypropylne or PTFE – needs to be minimised in order to minimise the effects of shear. However, too thin a sheet would risk damage from tearing and/or wear out. Furthermore, grit that finds its way between the bearing plates would be more likely to cause damage. PTFE normally swallows the grit, allowing it to stay embedded justt below the sliding surface, so that it does not compromise the sliding movement. If the sheet was under 3mm thick, larger pieces of grit would cause problems.

Because PTFE has such a low coefficient of friction, shear loads are not normally considered an issue. What ever the thickness of the PTFE, it would require a tremendous load to deform the sheet due to shearing. Nonetheless, the shear load is worth keeping in mind, especially when a design engineer argues that 3mm or 5mm is too thin and that a higher thickness of sheet is needed to accommodate the load.

Inspection Metrics for Complex Polymer Profiles

When manufacturing machined components, the key challenge is to ensure the consistency and repeatability of the dimensions and tolerances. With metals, this challenge is not so acute. There is a wealth of industry experience around the machining, punching and shaping of metals and therefore enough data to support how the material will behave when subjected to external forces. Metals also adhere to much closer tolerances. Precision metal part manufacturers will frequently cite tolerances of as low as one micron (1µm), which are unheard of in the polymer space. In addition to this, polymers are not as well traversed as metals and manufacturers who develop specific methods to ensure close tolerances will usually keep these as proprietary. Furthermore, each polymer behaves in its own manner and as such throws up its own limitations to what can be allowed by way of dimensional accuracy. A few such examples are given below:

  1. PEEK – tends to absorb stress during machining and can deform post machining (sometimes after days of being kept on the shelf), if not treated properly
  2. PTFE – does not absorb heat from the tool during machining, but can experience thermal expansion and its softness, can make it difficult to achieve close tolerances
  3. Nylons – can be affected by the heat from the tool, causing melting at the part’s surface. At the same time, coolant is not an option as the material tends to absorb moisture and swell post-machining. Methods need to be developed to balance the machining RPM with the feed rate to ensure tolerance and part finish are both achieved

The above examples are only a few to illustrate how each polymer behaves. There are literally thousands of polymers grades and each would come with its own set of properties to influence how it must be handled.

Non-contact inspection

One feature of most polymers is their softness when compared with metals. This is especially true when considering a thin cross section, where the part will deform even when held by a human hand.

For such thin sections, traditional inspection tools – such as callipers, micrometres and probes – do not work. The mere act of holding the part and applying the pressure of the tool will throw out the dimension, rendering the process useless. To remedy this, non-contact methods, such as profile inspection and vision inspection were developed.

Vision inspection is not only useful for non-contact inspection, but can also be used to inspect radii, chamfers, angles and other unique profiles that gauges and tools cannot be easily developed for. In addition to inspection, the process also allows for the dimensions to be captured on the part and a visual to be created to be send to the client or end-user as proof of the dimensional accuracy.

Inspection of complex profiles

More recently, we encountered a set of parts which can only be inspected by sectioning the final component and using the vision inspection system to capture the dimensions. Obviously, once the part is sectioned (cut through), the part itself is destroyed.

Given the size of the parts, this kind of destructive testing can be cost prohibitive. Furthermore, as the parts were made from an especially expensive PTFE-based compound, the challenge to inspect and approve the FAI parts before beginning manufacture was proving to be an expensive one. To be absolutely sure that the dimensions did not deviate during bulk manufacture, it would be optimal to perform this destructive test at the beginning, the middle and the end of the machining cycle. As such, a batch of say 50 such parts would require the destruction of 3 parts to ensure the dimensions were within tolerance. This represents a wastage of 6% - which is high and very costly.

To remedy this, we made three suggestions:

  1. We machine the parts needed for destructive testing from a polymer that is inexpensive. When making both PTFE and PEEK components, we usually machine a few parts in POM (Delrin) first. POM is dimensionally very stable and mirrors the behaviour of PTFE and PEEK to a high extent. Once we are happy with the dimension, we shift over to the more expensive polymer
     
  2. The second option was to machine the FAI part in pure virgin PTFE, which would most closely match the behaviour under machining of the PTFE compounded material. This would work out more expensive than option 1, but still cheaper than destroying the compounded parts
     
  3. The final option was to ask the client to specify the percentage of parts they needed tested and work this back into the commercials. In such a scenario, we would need to define a minimum batch quantity – say 40 parts – and come to an agreement that 1 out of the 40 would be checked, leading to a 2.5% increase in the costing

Eventually, because of the sensitivity of the part and the stringent need for quality, it was decided that option 3 would be used.

The client was not comfortable with option 1. This is understandable, since as we have described above, no two polymers behave exactly alike. While we may be comfortable using POM as an approximation for PTFE and PEEK for internal dimensional verification, the same cannot always be considered accurate for bulk manufacture.

In order to better map the behaviour of POM with PTFE and PEEK, we will be undertaking a correlation study to be done over 3 months across 200-300 parts. Once the data is collected, it would put us in a better position to recommend option 1 to clients in the event that cost restrictions limit the level of destructive testing allowed.

The effects of Carbon Nanotubes on PTFE Properties

It has long been known that the properties of PTFE, while diverse and highly sought after, can be further enhanced with the addition of fillers. Given the high-melting point of PTFE (>350°C), most such fillers need to be stable at high temperatures in order to maintain their integrity during the sintering (curing) process that takes PTFE from a raw state into its final, workable form.

Over the years, fillers such as glass, bronze, stainless steel and carbon have become commonly used. Each of these fillers – used anywhere between 5% and 60% by volume - adds something to the property of the PTFE, while also taking away. For example, the addition of bronze greatly enhances the wear resistance of the material but increases the coefficient of friction and also renders the PTFE unsuitable for any electrical applications.

Over the past few months, we have been working extensively with carbon filled PTFE, in an attempt to improve the anti-static properties of the material. Being an excellent electrical insulator, PTFE tends to allow static charges to build up on its surface, rather than allow any vent for the charge to dissipate. While this property is useful in applications where high voltage loads need to be contained, it poses some problems in areas where a spark – caused when the static charge reaches some critical mass - can cause other materials around the PTFE to ignite. In order to dissipate the static charge through the material, carbon fillers are usually added. The resulting material retains much of the dielectric properties of PTFE but gives just enough conductivity within the material to allow static charges to dissipate. Such anti-static materials have become popular, calling for both machined components and PTFE tubes to be made using the compound.

The quantum and variants of carbon used in such compounds can offer differing values for the surface resistivity of the material. The standard grade of PTFE+Carbon employs a 25% content of the filler by volume. By most accounts, this is rather high. In addition to being a little too conductive, the compound also tends to “shed” some quantum of carbon in the form of microparticles. This can cause big problems in precision equipment, where clean-room levels of non-contamination are called for both within and around the apparatus. As a result, compounds have been developed using Vulcan.

Vulcan is a form of specialty carbon black which can be used in percentages as low as 2% to induce conductivity in PTFE. The resulting material is a lot softer and smoother than PTFE+25% Carbon – which, apart from being less ‘clean’, is also tougher to the machine – and is ideal for electrical applications calling for anti-static properties in sanitized environments.

Carbon nanotubes

We recently used Vulcan to manufacture anti-static tapes but were informed that the tests were unsuccessful. The client’s application called for a surface resistivity of 1 x 104 Ohms, whereas our material was only able to offer 1 x 103 ohms. The failure led us down a path of discovery to see what other materials could be added to further improve the properties as required.

Many new studies have pointed to the addition of carbon nanotubes to achieve the surface resistivity required.

Carbon nanotubes have been around for a while and its property as a high-strength to weight material has made it the stuff of the future. In addition to its impressive load bearing properties, the material also acts as a semiconductor. Although the applications into PTFE are relatively new, studies have prescribed its addition as a substitute for carbon black. According to these studies, the addition of as little as 1% of carbon nanotubes into PTFE can offer a surface resistivity of 1 x 104 Ohms, as required by the application.

In addition to the electrical properties, fillers of 7-10% have been shown to improve the wear properties of the material significantly. Another study looks at a composition of PTFE+20% Ekonol+1% CNT, claiming that at this composition, the tensile properties and elongation reach an optimum.

The mixture of carbon nanotubes to PTFE as a dispersion allow for enhanced corrosion resistance and deliver a superhydrophobic layer.

In the wider scheme of things, both PTFE and carbon nanotubes are relatively new materials. We are still discovering properties of each and there is no doubt we will experiment extensively with different compositions to see what effects arise.