NON-PNEUMATIC (OR) AIRLESS TYRES

NON-PNEUMATIC (OR) AIRLESS TYRES:

               

INTRODUCTION :

           The tyres which are manufactured by absence of air. The first pneumatic tyres for bicycle by Dunlop have been dominant since 1888. Its market was stable due to the following four advantages over rigid wheel:

1).low energy loss on rough surfaces

2). low vertical stiffness

3).low contact pressure and

4).low mass.

But as study says they do have four compensating disadvantages:

1).The possibility of catastrophic damage 

2).flat while driving

     3).The required maintenance for proper internal air pressure

     4).The complicated manufacturing process.

          In the next stage of development wire spokes in the tyre material were added to increase the resilience property. Engineers, in the aspect of overcoming the disadvantages of pneumatic tyres, invented non-pneumatic tyres by replacing air column with elastomers or polygon flexible spokes. For more than 100 years, vehicles have been rolling along on cushions of air encased in rubber. Sometimes, we get so used to a certain product that no true changes are ever really made for years, decades even. So begins an article discussing the development of airless tyres, something that has become more prevalent in the past few years.

          A few tyre companies have started experimenting with designs for non-pneumatic tyres including Michelin and Bridgestone, but neither design has made it to mass production.Creating a new non-pneumatic design for tyres has more positive implications than one might think. For one thing, there are huge safety benefits. Having an airless tyre means there is no possibility of a blowout, which, in turn, means the number of highway accidents will but cut significantly. Even for situations such as Humvees in the military, utilizing nonpneumatic tyres has a great positive impact on safety. Tyres are the weak point in military vehicles and are often targeted with explosives. If these vehicles used airless tyres, this would no longer be a concern.

          There have been recent innovations with respect to airless tires; Non Pneumatic Tyres have emerged consisting of flexible polygon spokes and an elastomer layer having inner and outer rings . Considering the NPT structure, the spokes undergo tension–compression cyclic loading while the tire rolls. Therefore, it is important to minimize the local stresses of spokes when under cyclic loading while driving. In other words, fatigue resistant spoke design takes on greater importance. Two dimensional prismatic cellular materials of periodic microstructures are called honeycombs. Honeycombs have been primarily used in lightweight sandwich structures for which a high out-of-plane stiffness is desired.

          There is also an environmental benefit to using this type of tyre. Since they never go flat and can be re-treaded, airless tyres will not have to be thrown away and replaced nearly as often as pneumatic tyres. This will cut down landfill mass significantly. Because of the benefits, I believe that it is extremely important that research and production of airless tyres is continued and increased. This type of innovation works well in conjunction with several

engineering codes of ethics, and thus should be embraced by engineers everywhere. Cars are things that people use every day, so any improvements over existing designs would affect the lives of the majority of people. Learning about such a topic, therefore, I believe holds extreme value- especially for us freshmen engineering students. In doing research into these kinds of topics that hold significant meaning, we can see that what we will do can make a

difference.

History :

          John Dunlop, trying to make his son’s bicycle more comfortable to ride on, managed to invent the pneumatic tyre. Another person, Robert Thomson, had already patented the idea of a pneumatic rubber tyre so the Dunlop Rubber Company was established and won a legal battle with Thomson. In 1891, the detachable pneumatic tyre was invented by two brothers, Michelin, consisting of a tube bolted on to the rim.

          In 1948, Michelin revealed the first radial tyre was developed and this was a revolutionary achievement as it used steel-belted radial tyres. The advantages meant longer life and increased mileage for the vehicle. However, it required a different suspension system and so was slowly adopted. This was the tyre along with Dunlop’s invention, which gives us the tyre we have today. We have seen heavy tyre development, especially in motorsport, however we are yet to see anything as revolutionary as previous key points in history. There have been concepts, with a major one being the Michelin Tweel announced in 2005.

PNEUMATIC TYRES:

          The basic design of all pneumatic tyres is very similar, even though there are many different types. They all include an inner core that holds pressurized air which is then covered with a layer of rubber that comes in contact with the road, called a tread. The tread helps keep traction with the road and prevents slipping and skidding. The tread has the tendency to wear down over time, so if the tyre has not gone flat, a person will usually replace it at this point. A main reason for using pneumatic tyres is the deformation that occurs during rotation. As the tyre rolls, the weight of the car pushing down on it causes the tyre to flatten slightly. This in turn, causes the tyre to have a larger surface area to be in contact with the ground, which makes for better traction. It also gives a slight cushioning effect, making running over small rocks or debris unnoticeable. If you’ve ever taken a ride in an oldfashioned carriage with wooden wheels, you know what a difference a pneumatic tyre makesPneumatic tyres have their advantages, but they also have their disadvantages as well.

Figure : Pneumatic Tyres

         The possibility of a blowout or flat (when air is let out suddenly from the tyre) is a major concern because they have the tendency to cause severe accidents. The task of regulating tyre pressure is also a disadvantage because consumers are usually not very good at it. Although it may help with traction to have the tyres a little flat, it comes at the price of handling. When there is not enough air pressure in the tyre, the sidewalls flex causing the tyre to not quite follow the desired line of steering. It is because of these disadvantages that tyre companies have taken an interest in designing airless tyres.

Figure : Blow out and puncture of pneumatic tyres 

PNEUMATIC TYRE MANUFACTURING:

          The production of a new tire is a fairly complicated process that involves many steps at a manufacturing plant, but before they can be considered, it must be understood how the necessary raw materials made it to the plant in the first placewhich is given in below. The general process of constructing a tire involves assembling the numerous components shown in Figure and then vulcanizing these parts together to achieve the desired properties. The details of the production process of each tire manufacturer are difficult to find because of the Confidentiality of their specific process, so for the purposes of this thesis, an average tire production process will be modeled. Combining this generic process with the specific material breakdown of a P205/45R17 tire is described which will represent an average tire built anywhere across the country with the given specifications of a section width of 205 mm,aspect ratio of 45%, and a wheel rim diameter of 17 inches.

Figure : Structure of a pneumatic tyres. 

        The process begins with the mixing of basic rubbers with process oils, carbon black,accelerators and other additives. The basic ingredients has been described above, so simply in Table .The following figure shows the manufacturing process of Pneumatic tyres in the form of a flowchart. This mixed rubber then takes all the different forms as shown in figure  like sidewalls, tread, liner, etc. Most of these subcomponents are made by calendaring or extruding the cured rubber into the desired dimensions. Rough assumptions about the energy requirements and necessary lubricants in these two rubber processing techniques are taken. White's book titled Rubber Processing. Modeling the assembly process of all the components can be simplified to the rubber mixing process combined with the necessary lubricants and adhesives that secure the coated wires and textiles in place. Once all the components are assembled, the “green” tire is cured or vulcanized to glue everything together and to achieve the final dimensions and rubber properties. This curing process takes place under conditions of roughly 350 degrees Fahrenheit with pressures around 350 psi for around 15 minutes. After the curing process is complete, the finished tires are inspected and are sent out for distribution.

Figure : Stages in manufacturing of Pneumatic tyres

Table : Pneumatic tyre material composition

NON-PNEUMATIC TYRES (NPT) :

          Airless tyres or Non-pneumatic tyres (NPT), are the tyres that are not supported by air pressure. These tyres are also called as ‘Tweel’ which is a merger of the words tyre and wheel. This is because the Tweel does not use a traditional wheel hub assembly. The Tweel concept was first announced by Michelin back in 2005. Its structure is a solid inner hub mounted onto the vehicles axle that is surrounded by polyurethane spokes. This forms a pattern of wedges, which help to absorb the impacts of the road. These spokes look similar to the ones found on bicycles and plays the shock-absorbing role of the compressed air as in a traditional tyre. A sheer band is then stretched across the spokes, which forms the outer edge of the tyre. It is the tension of the band and the strength of the spokes that replaces the air pressure used on traditional tyres. When a vehicle drives over an obstacle, a hump for example, the tread and shear bands give way as the spokes bend, before they quickly bounce back into shape.

Figure : Non-Pneumatic Tyres parts

MAIN PARTS OF NON PNEUMATIC TYRES:

The 4 main parts of the non-pneumatic tyres includes:

       HUB

       POLYURETHANE SPOKES

       SHEAR BAND

       TREAD BAND

HUB :

          The hub is generally made up of Steel or Aluminum alloy. The average weight of the hub if its made of steel is roughly 4 Kg and of Aluminum alloy (AL7075-T6) is 2.5 Kg. It is a rigid structure and cannot deform while running. The frame of the vehicle is connected to the hub using nuts and bolts just like the hub used in the Pneumatic tyres. It is the component in the Non Pneumatic tyre which has the longest life than any other component. The hub is an integrated part of the tyre and cannot be removed or replaced.

          Separating a Non Pneumatic tyre from its hub is not as simple as the process for a tyre because the polyurethane spokes of a tire are molded directly to the steel hub with a bond that is not easily broken. The hub is made by ordinary casting process just like the making of ordinary hubs.

Figure : Steel Hub

POLYURETHANE SPOKES:

          The discovery of polyurethane [PU] dates back to the year 1937 by Otto Bayer and his coworkers at the laboratories of I.G. Farben in Leverkusen, Germany. The initial works focused on PU products obtained from aliphatic diisocyanate and diamine forming polyurea, till the interesting properties of PU obtained from an aliphatic diisocyanate and glycol, were realized. With the decades, PU graduated from flexible PU foams to rigid PU foams (polyisocyanurate foams) as several blowing agents, polyether polyols, and polymeric isocyanate such as poly methylene diphenyl diisocyanate (PMDI) became available. These PMDI based PU foams showed good thermal resistance and flame retardance.

          This polyurethane is used as the spokes in the Non Pneumatic tyres. It serves the function of air in this tyre. It has a capacity to take heavy loads and can deform its shape temporarily and can regain it. These are made in wedge shaped designs or in honeycomb designs. The wedge shaped design is introduced by Michelin and the honeycomb structure by Resilient Technologies, LLC.

Figure : Honey comb and Wedge structure

MANUFACTURING OF POLYURETHANE:

          The manufacturing process involves the reaction of a pre polymer with a curative. The pre polymer consist of two parts, Polyols and Diisocyanate. The Polyols are mainly Polyesters or Polyethers and the Diisocyanate are Toluene Diisocyanate or Methylene Diphenyl-Diisocyanate. The reaction of the Polyols and Diisocyanateis an exothermic reaction. The pre polymer will be at a temperature of about 60 degree Celsius in the molten state.The reaction of this pre polymer with a curative, which is a butadiene held at 40degree Celsius will form the polyurethane. The solidification of this polyurethane will occur at 100 degree Celsius in about 4 hours. A block diagram showing the formation of polyurethane is shown below

Block diagram of polyuethane formation

SHEAR BAND:

          It is a flexible band which is between the polyurethane spokes and the tread band. The shear band mainly consists of steel wire wound in circular shapes. It gives reinforcement to the tread band from shearing off while running. Its manufacturing is done with the tread band so that it firmly sticks together with the tread and provide great cornering stiffness to the vehicle. The making of the shear band involves winding steel chord on top of a drum until desired base thickness of 15mm is obtained. The material used as the shear band is ANSI:4340 (American National Standard Instruction codes) which is a high strength steel.

Figure : Shear Band

TREAD BAND:

          It is the part of the non-pneumatic tyre which comes in contact with the road. It contains rubber grip or tread for traction and grip on the road surface or any other terrains.

          The design of the tread depends upon the terrain in which the vehicle meant to move. The manufacturing process of the tread band is similar to that of the tread making in pneumatic tyres which is the extrusion process. The extruded tread is rolled on the shear band of desired thickness so that it forms the part which comes in contact with ground. The whole assembly is vulcanised so as to give the rubber tread more durability and strength. Vulcanisation is done by treating the rubber tread with sulphur so that it forms links within the material and becomes difficult to break.

Figure : Thread Band Manufacturing

FABRICATION OF AIRLESS TYRES :

          Non-Pneumatic tyres are produced in three steps: tread and shear band making, hub making , and assembling the former with polyurethane spokes.

          In the first step, the tread is constructed by a similar method as the tyre tread manufacturing process. The tread on a Non-Pneumatic tyre is exactly the same as a pneumatic tyre and is extruded in the same way. It is then mated to layers of belts in the same manner as conventional tyres. The process of rolling plies onto a drum to achieve the correct diameter currently is performed manually, but the same basic process that is performed on tyres will be mimicked when the non-pneumatic tyre production is fully automated. In this fairly simple process, rectangular sheets of rubber and steel cord are rolled onto a steel drum, and the excess material from each sheet is removed.

          Once the desired base thickness is achieved in this manner, the extruded tread is rolled onto the top, and the entire assembly is vulcanized.

          The second step is the making of the 4 kg steel hub casting or the aluminum alloy casting. The process is similar to ordinary casting process where the molten metal is poured into the mold and solidified.

          In the third step, the hub and the tread are secured concentrically and polyurethane is poured into a spoke and shear band mold while the entire assembly spins so that the polyurethane will sufficiently fill the mold in the radial direction. The energy needed to spin the non-pneumatic tyre assembly and polyurethane mold for just 5 minutes while the polyurethane is poured is considered irrelevant compared to the large amount of energy

required to heat and pressurize the ovens needed to cure the shear band and then cure the entire assembly after the polyurethane is poured. Before the pouring process occurs though,all the surfaces that contact the polyurethane are cleaned and covered with either an adhesive or a mold release for the shear band and spoke mold, respectively. The adhesives used are Ethyl acetate, Chemlok 7710, Stoner M-804 etc. The polyurethane pre-polymers and curative are stored separately until they are heated and combined at this point in the manufacturing process. The combination of the heated pre polymers and curative could be considere d in this Tweel manufacturing section, but in order to organize the impacts of the raw materials it is treated as part of the raw material production of polyurethane.After the polyurethane is poured and the assembly is allowed to stop spinning, the entire Tweel tire (shear band, spokes, and hub) is placed into another oven.

Material Properties of non-pneumatic tyres

           This final curing occurs at 100°C degrees for 4 hours so that the desired polyurethane properties are obtained and to assure all the components are securely bonded together. To save some energy this curing process could take place at room temperature, but it would take much longer to complete and during this time it would be susceptible to being bumped and permanently damaged. The properties of the materials used for making non-pneumatic tyres are given in table. The energy inputs for rubber curing presses have been recorded and analyzed by tire manufacturers, and the average tire curing process requires about 1.1 kWh of energy for a tire weighing 10 kg, which means roughly 0.11 kWh of energy is needed to vulcanize 1 kg of rubber. At the early stages of Tweel manufacturing, Michelin is using the same type of press that is used to cure radial tires, so it is assumed in this analysis that the same energy will be required to cure 1 kg of rubber in a Tweel tire as 1 kg of pneumatic tire rubber. The thickness of rubber in these two products varies slightly, but the curing temperature and time is close enough to assume the same energy requirements per kg of rubber. So, the required energy tocure the shear band in the Tweel is roughly (6.35 kg)*(0.11 kWh/kg), which equals 0.7 kWh.

          The energy required to heat, mix, and cure the polyurethane is allocated to the raw material production of polyurethane, so this 0.7 kWh is all the energy that is needed in the Tweel manufacturing inventory.

material composition in weight of non-pneumatic tyres

VEHICLES USING NON-PNEUMATIC TYRES :

          There are a number of vehicles using non-pneumatic tyres. Some of them are listed below:

         Earth movers

         Wheelchairs

         NASA Lunar rover

         Military vehicles

EARTH MOVERS :

          The non-pneumatic tyres give high stability to the earth movers to climb in all terrains. It provides a much smoother ride than a pneumatic tyre due to its excellent shock absorption.Even if the vehicle is heavy it will no damage the running surface. The NPT are very resistant to cuts than the traditional tyres, so it last longer.

Figure : earth movers using non-pneumatic tyres

WHEELCHAIRS:

          Non-pneumatic tyres are used in motor powered wheelchairs which can climb stairs. It was first introduced by a company called Michelin so that the suspension system in the wheelchairs can be eliminated.

Figure :Wheel chairs using non-pneumatic tyres

NASA LUNAR ROVER:

          It is a six legged robot designed by NASA for moon exploration. It has 6 legs, all of them contain NPT. It is able to roll or walk over large range of terrains.

Figure : NASA lunar rover using non-pneumatic tyres

MILITARY VEHICLES:

          American military vehicles such as Hummer, trucks, etc. are using the non- pneumatic tyres.

          The main advantage of the military vehicles using this tyre is that it requires very little or no maintenance. It will still remain mobile even with some spokes damaged or missing. It passed the ballistic test ie. it will remain mobile eve if it is hit by a bullet.

Figure : Military vehicles using non-pneumatic tyres

ADVANTAGES:

          It provides a comfortable ride and increases vehicle handling. Its flexibility provides an increase in surface area of contact thereby increases the grip with the ground. It can take gun fires and spikes without becoming immobile. It reduces down time as compared to pneumatic tyres as it require very little or no maintenance. It increases the load carrying capacity of the vehicle. It reduces the environmental impacts as the chemicals used in the manufacturing of non-pneumatic tyres are very less compared to traditional pneumatic tyres.

LIMITATIONS :

          The non-pneumatic tyres are expensive as compared to pneumatic tyres. The replacement of any component in the non-pneumatic tyre is impossible ie. every time the tyre is worn-out we have to replace the whole assembly. It can withstand police spikes which may make it difficult for law enforcement. Lack of adjustability is one disadvantage of non-pneumatic tyresie if once manufactured cannot be altered or adjusted. It cannot be implemented in fast moving vehicles above 50mph as the spoke vibrates considerably and is unpleasantly loud.