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.

ePTFE Applications in Cable Manufacturing

Despite extensive research into a new product, we are often introduced to applications that we had perhaps not considered and which open a whole new avenue of possibilities for the item in question.

Given the sheer versatility of ePTFE as a material for sealing, filtration, vibration dampening and corrosion protection, it came as little surprise to us to learn that its electrical properties open up applications into the cabling industry.

ePTFE vs PTFE

ePTFE or Expanded PTFE is a variation of pure or solid PTFE. The material is processed in a way that infuses air into the solid PTFE to give it a spongy, malleable texture that makes it a preferred material for sealing applications. The same texture – being comprised of 70% air, also lends itself to vastly improving electrical conductivity and dielectric strength.

We already know the properties of pure PTFE in electrical applications make it an insulator of unparalleled effectiveness. The invention of ePTFE resulted in a material that was up to ten times lighter and nearly halved the dielectric constant from 2.1 to 1.3.

So while many high-performance cables use solid PTFE (by way of paste extruding PTFE tube on to a conductive core), wrapping the core in ePTFE offers added possibilities in cabling.

ePTFE Tapes in Cabling

ePTFE insulator tape can be made with tightly controlled thicknesses of as little as 0.05mm, with a uniform density, and dielectric constant. Wrapping individual conductors in ePTFE can cut interference, noise, cross-talk, and signal attenuation. In some applications, ePTFE tape helps limit phase shift to 4.3° and signal attenuation to 0.05 dB at 110 GHz.

High-dielectric ePTFE insulation can be up to 50% thinner than other materials.

At higher voltages, corona discharge also becomes a concern. We have modified PTFE for better performance in wires carrying 5 kV and higher voltages. Corona-resistant (CR) PTFE eliminates the microscopic voids between conductor and insulation that can be corona- discharge initiation sites, especially in high-altitude, military, and space applications.

Shielding is the furthest from the cable’s neutral axis, so it sees the greatest flexure stress. Cutting shield-to-conductor and shield-to-jacket friction deters heat generation and keeps stress off the shield.

Placing ePTFE binders on either side of the shield (with coefficients of friction as low as 0.02) lets each conductor slide past its neighbours and the outer shield with ease, making the cable as a whole more flexible in rotation and torque, and eliminating internal abrasion. Designers who know a cable will not lose strength over time through abrasion can tighten the design envelope and still extend cable life.

Any cable jacket must protect the shields and conductors from the environment and lend extra tensile and flexural strength. Like conductor insulators, jacket layers should be thin, resist tears, withstand fluid attack, and have high tensile strength.

Many applications use durable polyurethane (PU) jackets. For environments that require low particulation, polyvinylchloride (PVC) may be a better choice.

Jackets can also be made of ePTFE for additional insulation and resistance to chemical attack. If the cable assembly slides through other machine parts, abrasion-resistant ePTFE is a good choice for extending cable life.

The advancements in ePTFE manufacture allow for uniformly thick tapes in running lengths of over 1000 meters. This opens up a world of possibilities for cable manufacture that is only now being harnessed around the world.

Lubring (Turcite-B®) – Special Notes on Bonding and Finishing

As a globally reputed manufacturer of Lubring Slideways (Turcite-B®), we are frequently asked to provide technical assistance with regards to the bonding and finishing of the material.

Lubring is a slideway bearing material that is used primarily in machine tool building and reconditioning. As such, the machine builder is usually equipped with enough knowhow on the bonding and subsequent planing of the material, such that it forms the most effective bearing surface. However, we often supply to dealers or first time builders, who need more assistance on the bonding process.

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Preparation:

The metal surface to be mounted with Lubring Slideway 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. Slideway bearing material should be cleaned similarly.

Bonding:

For bonding of Lubring Slideway 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 slideway 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 0.1 – 0.3 kp/sq.cm 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 Lubring Slideway can be machined by conventional means. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of Lubring Slideway 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 Slideway 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 Lubring Slideway should be finished to 16 Ra for optimum performance. The surface finish must never be below 14 Ra or above 20 Ra as applicable to cast iron or steel. This surface finish should be obtained by grinding in the direction of travel. Do not lap or polish to obtain this surface finish.

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The above parameters provide an effective guideline not just for Lubring, but for bonding all PTFE-related items to metal.

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A Comparison of Dimensional Stability Among High-Temperature Polymers

In the world of engineering polymers, plastics capable of withstanding temperatures above 150°C come at a price. While Polyamides, POM (Delrin®), and PVDF (Kynar®) are all well suited to temperatures within this barrier, when we look beyond we find the options become rather expensive.

Polymers that can accommodate higher temperatures, such as PTFE, PEEK and Polyimides tend to be in the range of 3x-20x the price of lesser plastics. As a result, the cost implications of designing a system using high-temperature polymers are significant.

What do we mean by high-temperature polymer?

While the phrase seems fairly self-explanatory, high-temperature polymers need to be further evaluated to understand exactly how they behave. Usually, an OEM or product designer will look for the continuous service temperature to assure themselves that a part made using the polymer can withstand the conditions it will be subjected to.

Polymer	Common Brand Name	Glass Transition Temperature (°C)	Continuous Service Temperature (°C)	Melting Point (°C)
PTFE	Teflon®	-20	260	375
PEEK	Victrex®/Ketaspire®	150	250	340
Polyimide (PI)	Kapton®	400	450	NA
Polyetherimide (PEI)	Ultem®	220	185	250
Polysulfone (PSU)	Udel®	190	170	350
Polyphenylsulfone (PPSU)	Radel®	220	180	370

However, for the component manufacturer, the service temperature is less relevant than the melting point and the glass transition temperature of the material. This is because these are the temperatures that directly impact the production of the component – both in molding as well as machining. We will be focusing here on glass transition temperatures and trying to understand how this metric needs to be used in component design and manufacture.

What is glass transition?

Put simply, a material moves from crystalline to amorphous states beyond its glass transition temperature. All polymers, when in a crystalline state, have internal stresses that keep it dimensionally stable. These stresses are a culmination of the inherent molecular arrangement of the molded shape and further stresses lent to it during the machining stage. Heating the part above the glass transition point causes the molecules to realign, thereby relieving the stresses and causing dimensional changes to the part. As stress due to machining can be significant, most polymers are subjected to an annealing cycle prior to machining, to ensure that the stress build up does not cause the part to crack during the process. Polymers such as PEEK will crack under so little as a simple turning operation of not annealed beforehand.

The stresses are very relevant for machined components, as it ensures that machined parts subjected to temperatures within their glass transition point will not deviate dimensionally. However, it is equally true that in the event of higher temperatures, the deviation may result in part failure. This is typically the case for highly machined components.

Consideration for Dimensional Stability

Our experience with dimensional stability rests around the use of PTFE and PEEK. Both polymers behave very differently both during machining and after. We shall look at them one by one.

PTFE

Among high temperature polymers, PTFE is unique in that it has a glass transition temperature under 0°C. The implication of this is that PTFE is generally amorphous even at room temperature and therefore does not suffer the internal stresses that other polymers do. As a result, PTFE typically does not require annealing, although it is still done as a means to improve the hardness of the material. No internal stresses mean that the material undergoes minimal duress during machining and any cracking of the machined part is avoided.

The flip side of this property is that PTFE has a very weak dimensional stability when subject to applications where a high range in temperatures may be present. While PTFE can easily withstand high temperatures, close tolerances would need to be abandoned when subjecting it to these conditions as the material itself experiences an up to 3% deviation in linear dimensions between 0 and 100°C.

So although PTFE is capable of surviving the harshest of environments, a PTFE part machined with close tolerances is usually employed only in areas where the temperature, while high, must remain range bound within +/-15°C.

PEEK

In contrast to PTFE, achieving close dimensional tolerances in PEEK and difficult due to the constant build up of stress during machining. In our own experience, PEEK parts may sometimes need to be annealed multiple times to ensure that after each stage of machining, the internal stresses are adequately relieved so that the part does not crack/deform after the next stage.

Unlike PTFE, which constantly gives off heat as it is applied to it, PEEK needs external help in cooling it down. As a result, the use of a coolant is common in PEEK machining and helps reduce the extent of stress induced in the part.

Finally, while close tolerances of up to +/-0.01mm have been achieved on PEEK parts, there is no guarantee these tolerances will be retained should the part be subjected to a temperature above its glass transition point during application. In such an event, stresses induced during the final operation of machining will relieve themselves and cause the molecules within the PEEK material to realign slightly, causing dimensional deviations in the part.

So given the above hazards, why are PTFE and PEEK still so widely used? One reason is that there exist very few applications where strict dimensional stability in temperatures above 200°C are a co-requisite. Hence, we have applications of high temperature where the dimensional tolerances tend to be very lax and we have applications with tight machining tolerances, where the part may experience a maximum temperature of only 150-160°C.

Expanded PTFE (ePTFE) Joint Sealant – A miracle product with varied applications

Starting 2015, Poly Fluoro Ltd. will be among the few companies globally, with the capability to manufacture expanded PTFE (ePTFE).

While the uses of ePTFE are numerous, it falls upon a select few manufacturers to produce this material. As we have covered in an earlier article, the production of ePTFE is as diverse as its applications. Some of the variants that exist include:

1. Mono-axially stretched ePTFE tape
2. Bi-axially stretched ePTFE tape and sheet
3. ePTFE membranes
4. ePTFE tubes and rods
5. ePTFE gland packing

Each of these products comes with unique production methods and specific nuances. However, the most commonly used variant at this point is the mono-axially stretched ePTFE tape.

 

ePTFE tape – also referred to as ePTFE joint sealant or PTFE Gasket Tape, is widely used for creating a sealing joint between pipes and other mating metal parts. Specifically, as a chemically inert material, this tape finds application in chemical plants, biotech plants and oil and gas pipelines. In fact, any pipe-lining application, where two pipes are connected using flanges, would benefit from using ePTFE tape, as the pliable nature of the material ensures that no gaps are left unfilled.

The tape has a soft texture and is easily compressed. Typically, the compression set of the tape is one of the parameters that define the material. The tape needs to be soft enough to compress under minimal load and ensure that it decompresses just enough to guarantee a complete sealing. At the same time, the tape needs to have adequate tensile and compressive strengths to allow for heavy loads to work upon it, without fatigue. In addition to its excellent chemical resistance, ePTFE tape also has a high temperature resistance, which allows it to be employed in applications involving high temperature fluid transfer.

ePTFE tape comes in 2 variants: adhesive and non-adhesive. The application of the adhesive is not highly complicated, but for on-site convenience, it is preferred as else the installation of the tape is difficult.

The application of the tape is very simple. As the visual below shows, the tape is laid along the flange and allowed to overlap at the ends. The soft texture of the tape means that at the point of overlap, the tape does not bulge once compressed, by simply compressed further to make a uniform seal.

ePTFE joint sealant tape is among the most sought after materials for all sealing applications.

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PTFE Tubing: Process Parameters And Their Impact

PTFE Tube extrusion is among the most difficult processes within the polymer space. All polymers have their peculiarities and these certainly play a part in both their processing and machining. But PTFE tube comes with a set of so many different process parameters, that finding a combination that works consistently is something that not every tube manufacturer is able to discover. We have undertaken so many trials on tubes, each time assuming that we have looked into all the aspect. However, even after years of manufacturing, a new parameter may present itself that had hitherto gone unnoticed.

We would like to take a look at some of these parameters and their effect on the end-product:

1. Handling
   Handling resin is among the most easily overlooked aspects of PTFE processing. While many resin manufacturers specifically lay out guidelines for limiting the shear on the resin before processing, these become even more important where tubing is concerned. Due to the structure of PTFE tubing, the fibrils that form during extrusion are paramount to the strength of the final tubing. Excessive shearing of the resin before extrusion can cause a poor formation of fibrils and seriously hinder the achievement of good final properties
2. Blending
   The parameters within blending include the type of extrusion aid used (the surface tension of the aid needs to be less than that of PTFE, while also not having a volatility and/or flash point that can cause fires during sintering), the amount of extrusion aid used, the RPM of the blending process and the post blending storage of the fine power mixture. Since our unit is in India, we need to follow a slightly different process to that in colder countries. For starters, we need to artificially cool the resin to allow of a more easy mixture of the PTFE with the extrusion aid. Such nuances are only learnt through extensive trial and error. But unless the blending is done in the correct manner, the final extrudate will be either too soft or too dry. Furthermore, unless the blend is uniform, the preform billet will have uneven densities, causing issues during extrusion.
3. Preforming
   Preforming is done purely as a means to create a shape that can be fitted into the extruder. Preforming has two functions: first, it gives shape and second, it removes any air pockets from within the material. The process needs to be done keeping in mind that too little pressure will not allow for an adequate venting of the air within the material. Air pockets result in bursts during the extrusion, which damage the tubing and render it unusable. Too much pressure and the extrusion aid may get squeezed out of the preform, causing the extrudate to be too dry and increasing the extrusion pressure required to form the tube.
4. Extrusion
   While extrusion is understandably the most important step, by the time the preform billet is loaded into the extruder, the preceding processes have already defined a lot of the tube’s final characteristics. Nonetheless, extrusion offers the tube it’s final shape and this process needs to maintain both adequate pressure on the billet while ensuring the concentricity of the final tube. If the pressure is too high or too low, the tube will experience either too much shear, or too little pressure to form a proper end-product respectively. Concentricity is dependent not only on the tooling within the extruder (which needs to be precise and offer the correct extrusion angles depending on the size of the tube being drawn), but also on the uniformity of the billet’s density (discussed above). Finally, the extruder itself needs to be capable of offering a uniform load, so as to ensure the billet is under constant and non-erratic pressure throughout the extrusion run.
5. Sintering
   When heating the tube, the temperature needs to account for both a drying section as well as a sintering section. The drying section needs to be warm enough to evaporate all traces of vapour from the tube. At the same time, if it is too warm, there is a risk of the vapours igniting.
   Sintering needs to account for the fact that if the tube is heated too quickly, there is a chance of over-sintering. Also, although PTFE does not melt, it may under its own weight, elongate during sintering, causing dimensional deviations. Therefore the temperature has to be sent to ensure that the PTFE reaches its ‘gel state’ just before it leaves the sintering chamber, so it can cool down at room temperature.

Aside from the above-mentioned parameters, PTFE tube also undergoes pigmentation, addition of anti-static fillers and extrusion of specific profiles. Each of these needs to re-look at all of the above processes and understand how they need to be modified to allow for a proper end-result.