Solar Tracker Bearings – Finding the Right Polymer Solution

Many believe that solar energy is the ideal path to ensuring a green, sustainable future. However, few realise that commercially, their financial viability is constantly threatening those very pioneers who boldly invest their funds into making this vision a reality.

We were recently asked to review the requirements of a 55 Mega-watt solar power plant in South India. Driving into the plant, one had o admit that the absence of smoke and noise alone made the idea of solar power appealing in contrast to its traditional coal-based variant. Things within the plant were not quite as peaceful. Despite the rated power, the plant was only operating at about 40 MW. This was because the unpredictability of sunlight made the whole operation highly subjective – with engineers having to take a call on whether or not to start the plant based on the extent of cloud cover on the horizon. Starting the plant is a 45-minute process which also consumes a lot of power. Hence, turning the plant on was both an investment in time and money. Unless the resulting power generated justifies this expense, there was no point in turning the system on. Add to this that unlike a coal powered plant – which can run non-stop – solar plants need to shut down at the end of the day. So each day, the financial decision to turn the plant on is visited anew.

Our own inputs were called upon for two reasons. First, the client was looking for a solution to prevent the shattering of the glass reflecting panels. Panels were arrayed along a stretch, but an irregular spacing between these panels meant that in some cases, the mirrors were too close to one another. In the event of strong winds, there was a high chance that they would collide and shatter. To remedy this, we were asked to look into the prospect of making a polymer fixture that would be sturdy enough to hold the frames of the mirror panels in place. When we inquired why a simple metal rod could not be used, we were informed that the power consumed in having the panels track the sun’s movement was significant – so a metal fixture would only increase the weight further, adding to this expense.

Another area of requirement was in the solar tracker bearing. All solar panels have a tracking mechanism to follow the sun’s movement. The mechanism that moves the panels needs to have an effective bearing element that can minimize the static coefficient of friction and endure for an extended period of time. Most specifically, solar tracker bearingsneed to encompass the following:

1. Self lubrication – since the volumes of these bearings tend to be large, it may not always be possible to lubricate the mechanism regularly. In such a case, self-lubricating bearings are ideal.
2. Lightweight – as mentioned above, solar plants rely on pockets of sunlight in which they can effectively generate energy. Hence, the aim to maximize the output when the plant is functional is critical to the financial viability of the system. One way to do this is to use materials that place as little load on the mechanisms as possible
3. UV and weather resistant – It goes without saying that any component used in a solar tracker system will be exposed to generous amounts of sunlight. In addition to this, the effects of rain and general weathering would also need to be accommodated. Plants are usually set up with a time horizon of at least 25-30 years. It is reasonable to assume that a good bearing solution will not need replacement during this time frame.
4. Wear resistance – again, the constant movement of the system entails a wear resistance bearing that will not yield during the life of the plant.
5. Cost effective – finally, the volume of bearings required in a single plant can be up to 2000 pieces per Mega-watt. This means that cost is also a criterion that must be looked into when designing the same.

The right polymer solution needs to encompass all the above properties. There currently exist a number of bearing types manufactured by companies in Europe and the USA. Most of these, we are told, rely on a combination of PA6 (Nylon 6) with either glass or molybdenum-di-sulphide. The filler materials provide the wear properties while Nylon 6 is reasonably light-weight, has good self-lubricity and is reasonably priced. In order to make the polymer UV resistant, the material is pigmented black.

In India, the interest in polymer solar tracker bearings has recently spiked. While many OEMs are keen to replicate the materials used abroad, care must be taken to ensure that the properties are all met factoring our local conditions.

Firstly, the harsher weather and heat in India calls for a more robust polymer. In addition to this, the ad hoc nature of the system means that the polymer is subjected to sudden loads, which it must be able to absorb.

While we have been recommending PA6 to a number of clients, we understand that for India, UHMWPE may be a better option. The material exhibits superior wear properties and has a coefficient of friction comparable to that of PTFE. Compared with PA6, UHMWPE is 20% lighter and is also capable of taking high loads. Finally, a price comparison would also show that UHMWPE is a more cost effective solution.

It remains to be seen whether the adoption of UHMWPE in solar applications sees any growth. The industry is alive with activity an innovation, so perhaps it is only a matter of time.

Demystifying Rulon

We have earlier looked at Turcite B* and explained how it is a result of a very successful branding exercise that has stood the test of time. In truth, as we now know, Turcite* is a PTFE based composition and has been successfully substituted in many applications with equivalent PTFE formulations.

Another very successful branding venture has been that of Rulon*. Although we do not see the demand for Rulon* being as high as that of Turcite*, there has been a very conscious and well thought out strategy which has kept the compositions of this brand ambiguous, to the point that clients find it very tough to accept any alternatives.

In addition to this, the unique pigmenting of each Rulon* grade offers further ambiguity. Visually, a client is unable to reconcile with a substitute when the colors do not match. It should be mentioned here that in many cases, we have seen that pigments help alter the properties of PTFE in a very measurable and positive way. For example, the green-blue pigment of Turcite* has been proven to offer better PV values than the same composition in, say brown color. Therefore, while the pigmenting of Rulon* does help the branding considerably, we would assume the pigments themselves were not chosen randomly, but by testing different variants and choosing the one that had maximum impact on the properties required.

We have done some research to try and lay out the compositions of the most popular Rulon* grades, in the hope that it will make the choice a little easier for an OEM or manufacturer. In most cases, these appear to be regular PTFE grades that have been made unique using pigments. In some cases, such as Rulon J*, the grade is not regular, but can be easily blended as long as one knows the composition. The table below shows the various compositions and attributes of the most common grades of Rulon*.

Product Description	Filler Details	Max Load (Mpa)	Max. PV ((psi-fpm); Mpa-m/s)	Properties	Colour
Rulon LR	PTFE+15% Glass	6.9	10,000; 0.35	High creep and abrasion resistance	Maroon
Rulon AR	PTFE+25% Glass	6.9	10,000; 0.35	Wear resistant, improved hardness, lower thermal expansion, lower deformation under load	Maroon
Rulon 142	PTFE+Bronze (40-60%)	6.9	10,000; 0.35	High thermal conductivity; better creep resistance; linear bearing material	Turquoise
Rulon 641	PTFE+15% Mineral	6.9	10,000; 0.35	Used mainly in food processing, FDA approved	White
Rulon J	PTFE+15% Polyimide	5.2	7500; 0.26	Good friction against soft metals	Gold

We would like to point out a few things pertaining to the values of this table:

1. The Load values of Rulon* across grades seem to be considerably lower than those of comparable regular grades of PTFE. For example, PTFE+15% Glass has a tensile strength of >20 Mpa when tested in-house – which is almost 3 times what Rulon* offers. The reason for this lowering of load metrics is not quite known. Most likely the addition of pigments causes some sacrificing of load values
2. The PV values are comparable with regular grades of PTFE, however not so vastly different that it makes Rulon* superior in any obvious way. For example, Rulon LR* offers a PV of 10000, whereas PTFE+15% Glass offers only 7500. However, Rulon AR* also offers a PV of 10,000, whereas PTFE+25% Glass offers 12,000.

In a nutshell, we do not believe that the uniqueness of Rulon* pertains to any significant improvement in properties, but to a branding push given when PTFE was still an ambiguous material for many buyers. In recent times, many clients have adopted substitutes as they rightly feel the premium attached to Rulon* material is unjustified. Although rigorous testing is first done to prove that the substitute matches up with Rulon*, we have found that regular materials are more than equal to the task.

* Rulon is a brand name of Saint-Gobain Plastics; Turcite is a brand name of Trelleborg Sealing Solutions

The wonder that is Lubring (Turcite®)

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It is only rarely that a marketing push succeeds so thoroughly that the brand name becomes synonymous with the product itself. When Turcite B ® was first introduced, PTFE itself had been around for quite a while. Yet, so effective was the branding of this variant of PTFE that everything from the properties, to the composition to the colour merged under a single umbrella and became known simply as “Turcite ®”. The material itself has become a mainstay in the industrial goods market with machine tool builders, re-conditioners and bearing manufacturers demanding it for their applications.

As the market expanded, new manufacturers developed their own variants under different brand names (ours being Lubring), which all succeeded in their own way. However, to this day, clients will initially demand “Turcite ®” – following which there will be a brief discussion about the fact that our material is equivalent to Turcite ®, but that we brand it under a different name. Usually the client is happy as long as the properties match and that the colour of the material matches the turquoise-green shade developed specifically for Turcite ®.

We want to take a closer look at this material, because despite it’s widespread usage, there are always questions from clients regarding the application and installation of this material. We will look at the following aspects:

1. What is Turcite ® (Lubring)
2. Where can it be used? What are the possibilities and limitations?
3. How must it be installed?

What is Turcite ® (Lubring)?

Quite simply – Turcite ® is PTFE impregnated with fillers and additives that serve to enhance the wear properties of the material. It is used, most often in a sheet form, in thicknesses ranging from 0.5mm (0.02”) to 4mm (0.16”), although in some applications, it is also used as a bush and in more rare applications it is used as a thick plate.

Being based on PTFE, the material cannot be extruded like a normal plastic sheet and instead needs to be “skived” – the process most commonly used to make thin PTFE sheets (See: PTFE – Myths Busted!). Also, the material will not easily adhere to other surfaces – another feature resulting from its PTFE base. Therefore a chemical etching is required on one surface of the material, so the sheet can be bonded to other articles.

In a broad sense, Turcite ® (Lubring) offers the following key advantages:

• Very low friction for reduced power loss
• No stick-slip for positional accuracy / control
• Good specific bearing loads
• Low wear for long life
• Excellent chemical resistance / fluid compatibility
• Unlimited shelf life
• High temperature resistance
• Absorbs vibration during machining

Applications of Turcite ® (Lubring)

Most commonly, Turcite ® (Lubring) has been used in the machine tool industry where it serves 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.

As mentioned above, Turcite ® is also used as a bush – which needs to be specially moulded and machined as per the customer’s requirements. These bushes are usually replacements for metal bushes – especially in areas where the lubrication of the metallic bush is as issue. Turcite ® – and in fact all PTFE grades – has self lubricating properties which means it can function deep within a sub-assembly taking enormous wear loads and does not need to be lubricated constantly to avoid damage.

However, the material is not an out-and-out replacement for metal. Being PTFE based, the material has a compressive strength limited to 150 Kgs per square cm (2,200 psi). This means that a single square foot of Turcite ® can accommodate a load of up to 150 Tonnes – which is more than sufficient for most applications. However, it is also a soft material (Shore D Hardness: 50-60) – meaning that point loads and excessive squeezing of the material can cause deformation.

Another limitation is with regards to insulation. Although the material has excellent temperature resistance (up to 260 Degrees Celsius/ 500 Fahrenheit), it does not have any electrical insulation.

In all, the industries for which we have supplied Turcite ® (Lubring) include:

• Automotive
• Machine tool
• Infrastructure
• Nuclear power
• Casting and forging
• Textiles
• Pumps and valves
• Pipe liners

Installation guidelines for Turcite ® (Lubring)

Preparation: The metal surface to be mounted with Lubring can be prepared by the normal machining methods such as, grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm. Once roughened the surfaces can be cleaned with Trichloroethylene, Perchloroethylene or Acetone.

Bonding: For bonding of Lubring the following resin adhesive can be used: Ciba Geigy's Araldite - Hardener - HV 953U; Araldite AW106. The Araldite should be applied both to metal and Slideway and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the surface brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.

Hardening: After mounting the Slideway a clamping pressure of between 30-35 Kg/cm2 is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening time for various temperatures is: 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.

Finishing: After curing of the adhesive, the PTFE can be machined by conventional means – if required. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of Lubring use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open ’stone. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment.

Oil Grooves: Lubring pads can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the Lubring Slideway. Oil grooves should be away from the edges by 6mm.

Metal Mating Surface: The metallic mating surface running against the PTFE pad should be preferably Stainless Steel (SS) 304 with a grade #8 mirror finish. Against this material, Lubring will have a coefficient of friction of between 0.1-0.12.

Conclusion

The recent surge in PTFE prices has obviously had a substantial impact on the price of Turcite ® (Lubring). However, owing to the ambiguity surrounding the material’s composition (many clients know it simply as “Turcite ®” and are unaware that it is PTFE based), there has been genuine confusion as to why the price has increased recently. While it take times and patience to convince clients that the price of Turcite ® is governed by the price of PTFE, it does serve as yet another reminder of how an effective branding campaign can truly give a product it’s own identity.

Turcite® is the registered trademark of Trelleborg Sealing Solutions

PTFE Wear Plates: Misconceptions and Applications for Heavy Equipments

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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 strips: PTFE 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 bearing
   
   A 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. Machining
   
   Clients 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. Environment
   
   Finally – 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.