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Nitrogen Summary study of private fleet greg stephens 2014

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Nitrogen Summary study of private fleet greg stephens 2014

  1. 1. Lean Transit Email: info@leantransit.com Greg W Stephens Page 1 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.
  2. 2. Lean Transit Email: info@leantransit.com Greg W Stephens Page 2 of 4 4/28/2014 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.
  3. 3. Lean Transit Email: info@leantransit.com Greg W Stephens Page 3 of 4 4/28/2014 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.
  4. 4. Lean Transit Email: info@leantransit.com Greg W Stephens Page 4 of 4 4/28/2014 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.

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