Tire Testing Systems and Tire Manufacture

TIRE TESTING SYSTEMS

As in all fields of technology a method of measuring performance is essential to progress. The major areas of a tire testing system are:

(a) Laboratory testing of cords compounds and composites
(b) Laboratory testing of tires
(c) Proving grounds testing of tires
(d) Commercial evaluation (taxi fleets truck fleets
(e) Quality control testing of production. There must be a constant feedback to tire research and     development at all stages. Each of these stages becomes a subsystem that must be         related to the product goals.

 Laboratory Tests

As described previously the components of a tire are strained in a cyclic manner in service. Tire cords for example are subjected cyclically to axial strain torsion forces and lateral compression and must be designed to operate for many millions of cycles without long term erosion of physical properties. In addition the tire components are not perfectly elastic and some of the energy is absorbed and converted into heat. This characteristic is termed hysteresis. Thus the materials also must be able to with stand the effects of heat.

Many tests have been developed to approximate these effects in the laboratory. With the advent of more sophisticated laboratory test equipment the need for a faster more accurate method of data processing was recognized. Thus it has become necessary to link the laboratory to a computer. An example of this is an Instron tester with a computerized data acquisition system. It also should be pointed out that certain information such as toughness or work-to-break cannot be obtained practically without a computer. In addition many material tests are conducted in environmental chambers under a variety of conditions.

The general approach is one of analysis of structure and materials.

The tire industry is continuously developing laboratory tire tests to fail tires purposely (sure-fail tests) and thus determine performance limits. Most laboratory tire tests are developed to produce one type of failure are accelerated tests generally lasting only several days and are semi-quantitative with respect to service conditions. Laboratory tire tests have been developed to evaluate separation resistance bead durability fatigue resistance flat-spotting, heat generation bruise resistance standing wave characteristics growth rolling resistance weathering resistance etc. The machine consists of a tire-mounting wheel on a carriage that can be loaded against a driven steel drum. Testing speeds can vary from 15 to more than 100 mph. Most tests are conducted from 50 to 70 mph. Some machines permit tire steer to be included.

Tire tests generally are performed on the outer surface of a steel drum 1/300 of a mile in circumference. Some tests however are run on the inner circumference.

Thermocouples are often used to record tire temperatures during tire testing.

Tire test machines now are being electronically programmed to act as simulators especially in testing aircraft tires. A complete cycle of taxi takeoff and landing can be continuously repeated. Speeds can go above 300 mph. In addition testing is monitored more efficiently by means of closed-circuit TV.

Most laboratory tire test machines are based on the principle of a steel flywheel. A unique tester designed by the Cornell Aeronautical Laboratory utilizes a flat bed (continuous steel belt) for the test surface.

The government tire standards published by the U.S. Department of Transportation National Highway Safety Bureau are based on laboratory tire testing. The government standards specify the following tests: plunger strength wheel endurance and wheel high speed performance. In addition the standards spell out requirements on tire labeling inflated tire dimensions and bead unseating resistance.

Nondestructive Testing

Most classical testing techniques are seeking performance limits and consequently result in destruction of the specimen. Considerable effort has been expended to develop nondestructive tests for application alone or in conjunction with standard tests.

Nondestructive tests have the potential to predict performance without destroying the product study the mechanism of product performance and enhance product reliability.

The tire industry has been evaluating many types of nondestructive testing techniques: infrared     ultrasonic microwave holography X-ray and force variation uniformity.

Infrared: This technique utilizes an infrared scanning system to obtain patterns from a tire during operation either on a test wheel or vehicle. The system continuously records the temperature gradients on the tire surface and identifies “hot spots” and incipient failures. Thermal analysis of a tire by infrared techniques also is a useful tool to determine the effects of tire construction and service conditions on rates of heat generation and modes of heat dissipation. The output from the infrared procedure can be readouts from strip recorders video tube displays or thermo-grams (photographs).

Ultrasonic: This technique is an immersion method and is based on the principle that ultrasonic signals travel for short distances through water. A submerged tire that is not dimensionally uniform will cause a change in reflectance of the signals. This method is useful for measuring variations in tire thickness.

Microwave: This technique uses radar-type frequency (small wavelengths) to penetrate the tire and detect changes in physical properties.

Holography: Holography is an extremely sensitive method of nondestructive examination of a product’s structure. It is accomplished by imaging without lenses. In this technique a laser beam is split into reference and object beams. The object beam is reflected from the tire into the hologram plane where it intersects the reference beam to produce a wave interference pattern.

This is repeated with a low level of stress applied to the tire. The interference pattern is used to produce a three-dimensional picture or hologram. Only changes in the object itself introduce variations in phase which in turn produce the interference pattern. Thus it is an optical measure of tire surface displacement.

Consequently application of this method reveals irregularities in internal construction identifies incipient failures and detects damage in the tire from testing or use. Holography has been combined with acoustical emission (ultrasonic) in a new technique called holosonics.

As can be seen nondestructive testing can act as a testing method or can be used as an examination procedure after or during conventional testing.

Proving Grounds Tests

The best method of determining the behavior and integrity of a tire is to examine its performance when subjected to millions of road test miles. All types of tires (passenger truck earthmover farm etc.) are proof-tested on industry proving grounds in the heat of the southwest USA under accelerated conditions. Here tires are torture-tested to failure. These industry proving grounds generally have available the following test tracks and road courses:

(a) high-speed tracks (circle or oval)
(b) interstate and turnpike simulations (straight and curved)
(c) gravel and unimproved roads
(d) cobblestone roads
(e) cutting chipping and tearing courses
(f) Baja road hazard courses (bruise and rupture)
(g) skid pads (wet and dry traction)
(h) serpentine or slalom courses (esthetics and handling)
(i) tethered tracks (farm tire durability)
(0) glass roads (not generally available)

Here tires are tested under conditions more severe than encountered in normal driving.  The proving grounds site contains a banked five-mile high speed circle. Tires can be tested flat out at sustained speeds up to 140 mph without vehicle side force due to the parabolic curve of the roadway. The high-speed circle can be used to measure a tire’s resistance to high temperatures and centrifugal forces.

An eight-mile highway at the proving grounds permits cars trucks and buses to operate night and day on an interstate course with long straights and through sweeping turns. Tread-wear durability acceleration on freeway ramps passing and line changing can be evaluated. Effect of starting and stopping torque also can be studied. Extensive gravel courses are utilized for measuring tire performance on unimproved roads. Many tests combine highway and gravel conditions.

Huge cobblestones embedded in concrete provide a tortuous road surface designed to tear up tires.

Earthmover tires are tested for TMPH on a two-mile road of coarse aggregate. In addition a cut-rail road is used to chip cut and tear earthmover tires. This course is at a 10 percent slope and consists of railroad rails cut off and placed on end in concrete.

Tires are brutalized on a Baja road hazard course by bruising cutting or rupturing over stones plungers and chuck holes. Effect of enveloping and absorptive ability also can be studied.

Stopping ability of tires on all types of road surfaces wet and dry is determined on skid pads with spray equipment.

In addition traction road-holding curve control and hydroplaning are evaluated. Driving in the rain can be simulated. Instrumented cars are utilized to evaluate esthetics and other force and moment characteristics such as ride harshness driving comfort response nibbling noise cornering forces handling stability passing maneuvers and general vibration analysis. Serpentine or slalom courses often are used for these determinations.

A glass road facility permits direct observation of the tire foot print through 3-1/2-inch-thick optically clear glass set into the road bed. Under the glass is a data laboratory built below the road.

As a tire passes over the window section of the roadway a high speed camera photographs the action of the tire tread which permits traction hydroplaning and tread-wear behavior analysis.

Telemetry can be used to monitor continuously the internal forces of a tire being tested on a vehicle. Data from the car is transmitted to the mobile ground station and is analyzed by the computer in the vehicle. This is a powerful new tool for dynamic tire behavior analysis.

Proving grounds testing are utilized to evaluate general durability bruise resistance high-speed performance tread-wear under varying service conditions traction skid resistance cornering ability gravel durability hydroplaning esthetics cutting and tearing resistance fuel economy etc. Data from the proving grounds is often fed directly into a computer for analysis.

 Commercial Evaluations

Commercial value analysis is performed on fleet vehicles. In addition test data from all stages of tire evaluation is fed into computers for in-depth and regression analysis. As can be seen testing has progressed a long way from the days of the “tire kicker.”

The final stage of tire research and development is production; consumer satisfaction is the true test of a tire.

TIRE MANUFACTURE

The basic operations include mixing the elastomers carbon blacks and chemicals to form rubber compounds; processing the various fabrics and coating them with rubber in a calendaring operation; tubing the rubber treads and sidewalls; assembling the various components at the tire-building machine; curing the tire under heat and pressure; and finally finishing and screening the product. Extensive quality control is practiced beginning with the incoming shipments of raw materials and continuing through to the finished tire.

Fabric

Carcass: Tire cord fabrics (rayon nylon polyester) for the car-cases are manufactured similarly. Beams of yarn are shipped to the fabric mill. The first operation is yarn twisting. In this operation the yarn is twisted on itself to the desired number of turns. After the initial twisting operation two or more yarns are twisted into a cord. For example if two 1260 denier nylon yarns are twisted together a 1260/2 nylon cord is formed. Similarly three 1300 denier polyester yarns will form a 1300/3 polyester cord.

The direction of twist in the cord is opposite to that in the yarn. Generally the twist in the yarn and the twist in the cord have an equal number of turns. More sophisticated twist relationships are possible. For example sometimes maintaining the ply twist 25 percent higher than the cable twist is more desirable. High cord twist permits the cord to be more like a coil spring and results in higher fatigue resistance but lower strength. Conversely low cord twist permits the cord to act more like a rod and gives lower fatigue resistance but higher strength. Thus the twist must be optimized and requires accurate and uniform cord geometry. Twisting is essential to impart fatigue resistance and durability to the product.

A cross section of a typical cord comprised of two yarns. Each yarn contains many individual filaments.

From final twisting the spools of cords are loaded into a creel. Creeling is the method for supplying many parallel ends uniformly spaced to a subsequent operation. The cords in the creel are fed into looms for weaving into fabric. The cords become the warp since tire cord basically is a unidirectional fabric. Small fill threads are added to space the cords in the fabric. These are required to facilitate handling in the factory. A roll of tire cord fabric will contain hundreds of individual cords.

Processing is the critical stage in preparing a cord fabric for a tire. During fabric processing an adhesive is applied and the woven fabric is treated under electronically controlled conditions of time temperature and tension. The processing of fabric is essential to apply an adhesive stabilize the fabric for factory operations and product dimensions optimize physical properties of the reinforcing materials to meet tire requirements and equalize differences due to source of fiber (i.e. different yarn producers).

When cotton was the primary fabric used in tires the fabric was calendared with little or no processing. When rayon entered the picture tire engineers quickly established that a method had to be developed to provide adhesion and reduce growth. With nylon the problem was even more complex. Nylon has a “plastic memory” and seeks to recover from any type of treatment.

Therefore it was imperative that systems be designed to process tire cord fabric to constant modulus maximum strength minimum growth and optimum dimensional stability. Each fiber and each fabric construction requires specific conditions tailored to the end use.

A simple flow-sheet of a unit is shown in . A roll of untreated tire cord fabric passes through an adhesive and into a series of ovens where the fabric is dried and heat treated under tension. The fabric then passes in a continuous process into a second adhesive and a second series of tempering ovens. Finally it passes to the roll windup.

This unit is more than seven stories high. A unit of this type can process 100000 miles of tire cord per day and is the largest piece of equipment in the tire manufacture process.

This unit also is the most expensive. A fully equipped fabric processing unit is a multimillion dollar piece of precision equipment. The entire process is controlled by a computer.

Adhesive Systems: Tire cord adhesives are a specialized science.

The adhesive makes up only one-half of one percent of the total weight but adhesives can have a very large effect on tire performance.

The task of joining tire cord and rubber compound in a dynamic structure is no simple task due to the wide differences in modulus. Tire cords have high strength with relatively low elongation. Rubber on the other hand exhibits high elongation with relatively low strength. The adhesive system must bridge this gap.

In addition each cord has a different reactivity. Rayon has many reactive hydroxyl groups. Nylon is less reactive but contains highly polar amide linkages. Polyester is quite inert. An adhesive system must be designed for each fiber.

In addition to supplying excellent adhesion adhesive systems must also conform to a rather rigid set of auxiliary requirements including a rapid rate of adhesive formation high fatigue resistance compatibility with many types of rubber compounds no adverse effect on cord properties heat resistance aging resistance in the factory and in tire service and mechanical stability. Most tire cord adhesives throughout the world are based on resorcinol formaldehyde resins and latex (RFL) in an aqueous medium.

An important innovation in tire cord adhesives is the use of two-phase systems. Originally introduced for polyester two phase systems (double dip) have gained wide acceptance.

Belt: A word should be said about the high modulus belt cords fiberglass and wire. Fiberglass is received from the fiber producer with adhesive and generally goes directly to twisting and weaving without processing. Woven fabric is produced for subsequent operations. Wire is received from the manufacturer with a plating of brass. The brass functions as an adhesive. The spools of brass plated wire normally are placed into a creel directly in front of the calendar bypassing the normal fabric operations. Wire can be woven into fabric if desired.

Rubber

Incoming elastomers carbon blacks and chemicals are automatically weighed and transferred into an internal mixer for two or three minutes at high temperature and pressure. The ban-bury mixes the raw materials into a rubber compound. This compounded rubber then is sent to a series of mixing mills that knead the rubber compound. Finally the rubber is pelletized for use at the calendar in bead building and in tread extruding. Each application utilizes its own special rubber formula.

The calendar is a heavy-duty machine equipped with three or more rolls revolving in opposite directions. The calendar continuously coats processed cord fabric with a sheet of rubber compound. Each cord is insulated on all sides by rubber matrix.

The amount of compound deposited onto the fabric is determined by the distance between the rolls and is monitored automatically by beta gauges. The rubber coated fabric is wound into a liner. The calendared fabric next is cut on the bias to the desired width and angle. This cut fabric is now ready for the tire building operation.

Some of the rubber is used to coat the bead wires. Bead wire is received on large spools. Bundles of wire are passed through an extrusion die and given a coating of rubber. The rubber coated wires are wound into a hoop of specific diameter and thickness and sent to the tire building machine.

Most of the rubber compound is used to form the tread and sidewall by extruding from a tuber. In this operation compounds are forced through a die in a continuous stream. The continuous flow of rubber is bevel cut to a predetermined length and is weighed cooled and cemented. The treads then are sent to the tire building machine. In large tires the tread can be laminated by contour winding a strip of tread compound around the carcass.

Tire Building and Curing

All tire components are assembled at the tire building machine. The focal point of this machine is the cylindrical building drum.

The process begins with the application of a thin layer of special rubber compound to the drum called the inner-liner. Next the plies are placed on the drum one at a time. This step is followed by setting the beads in place. The plies are turned up around the beads. Belts if any are now applied. Finally the tread and sidewalls are added to complete the tire. The drum is collapsed and the green uncured tire is removed. In bias and bias/belted tires the green tire resembles a barrel with both ends open. In radial tires before the belts are applied the green tire generally is expanded from a cylindrical to a toroidal shape.

Then the belts and tread are added. The final green tire has a toroidal shape.

The green tires are loaded into automatic tire presses and cured (vulcanized) at high temperatures and pressures. In the curing process the rubber is cross-linked in a series of chemical reactions. The press contains the mold that determines tread design and tire dimensions. After curing the tire can be mounted on a rim and permitted to cool while inflated to relieve internal stresses. This step is called post-inflation.

Finished Tire

Finishing the tire after cure involves trimming buffing balancing and visual inspections. A new quality control dimension added to tire manufacture in recent years is the adoption of automated nondestructive testing to check tires. Two techniques have been widely adopted force variation analysis and X-ray inspection.

Force Variation: This nondestructive system measures tire uniformity based on force variation. In operation the cured tire is deflected against a steel drum and rotated.

Measurements of variations in force required to maintain constant deflection are electronically recorded. Radial force variation (force perpendicular to the road) and lateral force variation (force parallel to the axis of rotation) are recorded. The recordings show the non-uniformities that might be created by the tire on a smooth road. A regular low-amplitude waveform will be characteristic of a good tire; a large amplitude and highly irregular waveform is undesirable. The curve actually consists of a series of multiples called harmonics. All harmonics add together to form the original curve. The force variation machine can be programmed within limits to correct a tire automatically for uniformity by grinding.

X-Ray: Nondestructive testing by X-ray permits examination of the internal structure of a tire without having to destroy it. The X-rays literally take a penetrating look into the tire. The resulting picture is projected onto a TV screen. Electronic image intensification enhances its usefulness. Video tape recordings also can be produced for future reference. The system is applicable to green or cured tires. After inspections the tire is shipped to a warehouse for distribution.

Tire Manufacturers

There are 13 tire manufacturers in the United States:

• Goodyear
• Uniroyal
• General
• Armstrong
• Mansfield
• Dunlop
• McCreary
• Firestone
• Goodrich
• Gates
• Cooper
• Mohawk
• Denman