Nitrogen Summary study of private fleet greg stephens 2014

Lean Transit Email: [email protected]

Greg W Stephens of 4 4/28/2014

Reducing Tire Expense and Failure

Summary

A private fleet with roughly 1000 trailers and three distribution centers initiated a project

to develop a scheduled maintenance program for trailers, improve productivity, reduce

cost of the maintenance function, and specifically reduce tire expense and tire failures. A

similar project for tractor maintenance was subsequently approved.

Although the project was broad, one outcome (not an objective) was to replace

compressed air with nitrogen for tire inflation. This discussion paper is limited to the

project findings and activities related to tire life.

Background

Since the trailer fleet was not equipped with hub-odometers it was unknown what

mileage tires were generating prior to replacement. It was estimated to be about 70,000

miles for a radial tire.

Tire Tread Life

The factors that impact tread wear and tire failure are well understood. The relative

impact depends to what extent each factor is present on the vehicle. The following are

the key factors that cause tread wear all of which are related to friction.

1. Improper tire inflation – every study documents this as the number one cause of

excess tire wear1.

2. Wheel alignment

3. Improperly installed or bent rims and worn or improperly installed bearings.

4. Wheel alignment on tractor and axle alignment on trailers.

5. Wheel balance

6. Mismatched tread patterns.

7. Tires of materially different tread depth.

8. Load factors. Trailers moving empty have excess tire wear as do overloaded

trailers

9. Speeds in excess of 60 MPH

10. Use of un-dried compressed air (i.e. containing water vapor).

11. Vertical forces due to suspension system characteristics.

Tire failures

There has been extensive research into the causes of tire failures. Still tire failure is less

well understood than tire wear as wear can more easily be measured through controlled

experiments. DOT studies indicate that tire failures are primarily caused by:

1. Road hazards and truck operation: 39%.

2. Heat buildup, caused primarily by under inflation and secondarily by improper

maintenance of brake system: 30%

3. Manufacturing defects: 16%

4. Undermined or multiple causes including using tires older than 5-6 years2: 15%

1 A random check of 64 tires at the docks of a distribution center on 10/14/10 indicated that over 40% of

tires were under inflated by 15% or more.



Compressed Air

Many studies have shown that tire life and safety is negatively impacted by the use of

compressed air for inflation. The reason is that compressed air contains oxygen which

oxidizes the rubber in tires and causes the casing and tread to deteriorate. The oxidation

reduces casing strength and changes how tire tread interacts with the road thus

accelerating tread wear.

The concept is similar to using Modified Nitrogen Packaging in the food industry.

Nitrogen preserves the fresh taste of the food while the air in oxygen would allow the

product to go ‘stale’. Nitrogen is also used in a variety of industrial processes where air

would cause oxidation of the either the product or some component of the manufacturing

process.

A similar process occurs in tires inflated with compressed air. Tires (like some foods)

have both single and double chemical bonds. Double bonds are subject to oxidation. In

order to reduce oxidation tire manufacturers add anti-oxidant coatings to the inside of the

casing. However, compressed air ‘uses’ up those anti-oxidants over time. Using up the

anti-oxidants accelerates as fleets ‘top off’ the air in tires to maintain pressure.

The oxidation of tires occurs from the inside-out. As the oxygen ‘attacks’ the rubber air

permeates the tire from the inside creating microscopic holes in the rubber. Over time

holes penetrate the surface of the tire and actually change the physical properties of the

casing and tread. The casing loses strength while the tread softens thus increasing

friction between the tread and the road surface. Friction is the only source of tread wear.

If the compressed air that is being used is not dried, water in the air collects inside the tire

causing the wheel to oxidize and can damage or cause failure of the value stem.

Oxidation of the rim causes small leaks around the tire bead. Freezing of water in tires

can cause valve failure.

Nitrogen

Nitrogen is a totally inert and dry gas. It does not chemically react with tires. Thus while

a commercial tire will lose from 2 to 7 lbs of pressure per month, a nitrogen filled tire

will lose about ½ lb of pressure per month.

Nitrogen inflated tires maintain pressure better and run cooler thus reducing heat buildup

which causes tire failures. Nitrogen inflated tires are safer. Nitrogen is used in military

applications and applications which require the highest tire reliability. Nitrogen is

required by Federal law to be used in airline tires. Since deterioration of the tire casing is

much lower using Nitrogen; airline tires are re-treaded as many as 12 times. There is an

80% probability the commercial flight you are on will be using re-treads.

2 “Comments on Tire Aging: Docket 2005-21276”, 2 JUNE 2008, study performed on behalf of NHTSA

by Strategic Research and Strategies, Inc., Rehobeth, MA.



Aircraft manufacturers such as Boeing use nitrogen membranes in their On-Board Inert

Gas Generation Systems (OBIGGS) to "top" fuel tanks with nitrogen - an inert gas that

does not support combustion.

Nitrogen Inflation Economic Benefits

There have been a number of studies quantifying the economic benefits of nitrogen

inflation3. A summary of several of those studies follows:

Wal-Mart is converting to Nitrogen for its private fleet moving between

distribution centers and stores. Their initial assessment is a 50% reduction in tire

wear.

Tread wear in the Transport Canada study showed an 85% improvement in tread

life. Nitrogen filled tires delivered roughly 31,000 miles per 1/32 inch of tread

wear while compressed air inflated tires delivered roughly 17,000 miles per 1/32

inch of tread.

Because Nitrogen inflated tires lose pressure over 5 times slower than

Compressed Air inflated tires, fuel consumption studies sponsored by Transport

Canada for long-haul trucking show a 4% improvement in fuel economy. Other

studies show a 2-6% improvement depending upon how the study was performed.

Nitrogen inflated tires fail ½ as often as tires filled with compressed air. A 50%

reduction in cost of risk from tire failures and emergency road services should be

realized.

Nitrogen inflated tires can be retreaded multiple times generating the same

mileage as the new tire at 50-70% the cost. Thus buying a tire at a significant

discount generates savings over and above the increase in tire life on the original

casing.

Nitrogen inflation is a consummate ‘green’ project. Increased tire life, multiple

retreading of casings, and reduced tire failures all reduce the quantity of tires put

in landfills and the cost of disposal.

3 Studies have been performed by Clemson University, Transport Canada, Ford Motor Company, Akron

Rubber Development Laboratory and the California EPA. Some of the most comprehensive studies were

performed by Lawrence Sperberg of the Probe Forensic and Testing Laboratory of El Paso, TX in 1985

and 1996.



Observations of Initial Impediments

There was initial resistance to switching from compressed air to Nitrogen. This

resistance came from the following factors:

Perception that Nitrogen is an ‘exotic’ gas. Air is approximately 80% Nitrogen

Perception that Nitrogen is unsafe. Unlike air Nitrogen will not support

combustion.

Nitrogen is expensive. An Ingersoll-Rand Nitrogen generator with a 120 gallon

storage tank costs about $14,000. There are other manufacturers of nitrogen

generators such as Parker Hannifin and pricing is competitive.

Nitrogen generators are expensive to maintain. Maintenance costs are low and

consist primarily of changing an inexpensive membrane that separates Nitrogen

from air.

Existing ‘relationships’ with tire vendors.

Infra-structure investments. Other than the cost of the Nitrogen generator and

storage tank there are no additional investments. The same lines and fittings as

used is compressed air generators work with Nitrogen generators.

Experiments

There were a number of experiments designed to quantify the benefits to this particular

private fleet. These were controlled experiments using trailers with new tires and hub-

odometers to track mileage. Laser tread depth measurement instruments from MTI

Instruments will measure tread depth at an accuracy of 8/100,000ths of an inch.

Shearography and ultrasound inspection was used on tire casings.

Since the project was much broader than just tire inflation the experiments overlapped for

this particular application.

Constraints

In order to generate the maximum potential benefits from nitrogen inflation the chassis

should be maintained according to an appropriate schedule dependent upon operational

environment. In any event, regardless of inflation method, misaligned axles, unbalanced

tires, mismatched tread, excessive suspension wear, worn kingpins, etc. will still cause

excessive tread wear.

Fleet owners that have some maintenance capability in-house will find nitrogen inflation

much easier to accomplish due to the fact that nitrogen is not yet widely used in the U.S.

Owner-operators would find the conversion particularly difficult.

Like any other ‘commercial’ endeavor the players often have a vested interest in the

outcome. Makers of Nitrogen generators want to sell generators. Makers of tires want to

sell more tires.

As an opinion, the most compelling evidence comes from the fact that D.O.T. requires

commercial airliners to use nitrogen and those tires are retreaded as many as 12 times.

Nitrogen Summary study of private fleet greg stephens 2014

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