Technology: Keeping the pressure up

The 94 percent solution
According to Michelin, a tire carries only 6 percent of the vehicle’s load. That means the air in the tire supports 94 percent of the vehicle’s weight. Relying on the strength of an invisible support system calls for a reliable way to measure that support.

Companies who make tire pressure monitoring systems for the automotive industry are bringing more robust versions of their TPMs to the off-road construction market. Building on the sensor technology used in the automotive and long-haul trucking industy, the TPM systems for heavy equipment use small sensors contained in tough packaging to measure tire pressure and temperatures under rough conditions.

The two most common TPM configurations are direct and indirect. Direct TPMs measure the pressure in a vehicle’s tires, while indirect TPMs estimate differences in pressure by comparing the rotational speed of the wheels.

Passive TPM systems do not have their own power source and rely on the operator to physically look at the monitor to see if the low pressure signal has been activated. Typically, the screw-on valve stem cap is signal green when the tire pressure is adequate, yellow if the tire is 4 pounds underinflated, and red when the tire loses 10 psi or more.

Active systems carry a battery power source within the pressure sensor and actively draw the operator’s attention to the display monitor via a visual or audible signal.

Direct systems use tire pressure sensors (TPS) that are patched on the inside wall of the tire, attached to the tire wheel or snapped on the tire’s valve stem. The sensor reads the pressure and temperature of the tire, evaluates the information and then triggers a gauge or warning light on the dashboard or handheld device. TMP systems use SAW (surface acoustic wave) technology sensors that are powered by lithium batteries and can both measure and transmit. The SAW-based sensor continuously reads the tire’s pressure, temperature and torque by detecting miniscule strain changes. Each SAW sensor has its own integrated radio frequency identification tag that signals the receiving unit when a specific tire is losing pressure.

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Indirect systems use sensors that are already part of the vehicle’s electronic control system. Indirect TPMs work on the theory that an under-inflated tire is slightly smaller than a correctly inflated tire and has to rotate more times to cover the same distance as a good tire. By comparing the rotation speeds, the TPM module can sense which tire is smaller and underinflated. Other indirect systems detect underinflation by analyzing the vibration of individual wheels or load shift effects during acceleration and cornering.

Both types of systems are programmed to compensate for differences in tire temperature so that readings accurately report the air pressure while in operation. Roman Goltz at SmarTire Systems, a manufacturer of active tire monitoring systems for commercial and off-highway vehicles, says when a tire begins to lose pressure, its rolling resistance increases and the tire’s operating temperature rises. Simply looking at a working tire will not reliably measure its pressure because when the air inside the tire heats up, it expands and makes the tire look like it is fully inflated when in fact it can be up to 30 percent underinflated.

Currently, most TMPs for off-road construction tires are aftermarket direct systems. Sensors attach to the vehicle in several ways.

The TMPs simplest to install are owner-installed valve-stem sensors. The sensor is sealed in a screw-on hard plastic cap that replaces the dust cap on the tire valve. The owner programs the on-board receiver with each sensor cap’s radio frequency serial number, designates the tire assignment of the cap and the target air pressure for that tire. The sensor takes constant readings and sends the data on a predesignated schedule to the in-cab display. If it detects an immediate problem, the system will override the schedule and alert the operator to which tire is at risk.

Other systems’ sensors are encased in a molded plastic case and strapped onto the inside wall of the wheel. Each sensor has an individual identity RF signal and constantly reads the tire’s pressure and temperature, sending the data regularly to the cab’s monitor. This system can also be used by gate readers and handheld receiving units. Wheel rim sensors can be installed before the owner takes possession of a new machine or added later during routine tire maintenance. Each sensor needs to be tuned to the receiver and adjusted for the density of the vehicle – all best done by a trained technician. Alan Travelbea, project manager at Smartire, says the sensors can even work in tires that use liquid for ballast.

A third system is the patch. A dime-size sensor is permanently attached to the inside wall of the tire and cannot be removed to be used on another tire. The sensor has its own address that effectively becomes the tire’s identification. A lifelong history of that particular tire’s performance can be recorded for future maintenance and record keeping.

On the job
The effectiveness and reliability of TPMs on off-road heavy equipment tires has yet to be determined. The U.S. Department Of Transportation National Highway Safety Transporation Administration’s Tire Pressure Monitoring and Maintenance Systems Performance Report, tested TMP systems for passenger cars and long haul trucks. Some of the report’s findings may apply to off-road tires. The report says TPMs were accurate to within two to three psi and compensated for temperature variations and valve-stem systems display of an immediate alert if the tire fails or the sensor is lost.

Wheel-mounted systems were able to read tire pressures and temperatures, and reliably relay alerts from temperature increases greater than 20 degrees but were less reliable when temperatures increased only a few degrees, the report indicates. Sensors mounted on the rim of the wheel were susceptible to heat generated by the brakes and readings were less accurate.

The tested tire-mounted systems required a handheld or gate reader, instead of transmitting to an in-cab display. The report says drive-by readings were most accurate at speeds of less than five miles per hour.

Chris Bell, director of training for the Tire Industry Association, says TPM systems are not standardized and in some cases dealers are hesitant to install the system due to liability issues after the vehicle leaves the dealership. If the vehicle has an accident directly related to a non-functioning TPM sensor, the tire dealer may be held accountable for the event. Bell says, “Some dealers are even refusing to install the systems. They don’t like them.”

Should you try them?
Perhaps. Keeping tires at 100-percent inflation allows them to run at 100 percent effectiveness. Tire-related costs are the single largest maintenance expense for most contractors and while tire costs are not escapable they are containable. Some things to consider are:

  • Tim Good, Goodyear’s manager of global accounts, says improper tire inflation increases a contractor’s annual new and retread tire costs by about 10 to 13 percent.
  • According to Michelin, if a tire is under-inflated by 20 percent, the life of the tire decreases by the same percentage. Keeping tires at the target pressure increases the number hours they last and the equipment runs.
  • The American Trucking Association says a difference of 10 pounds in air pressure between over-the-road tires can drag the lower pressured tires 13 feet per mile. Tires running 10 percent under-inflated lose up to 16 percent of their tread life. Poor inflation rubs away the life of the tire.
  • The U.S. DOT reports that a 10-percent under inflated tire reduces fuel efficiency by 3 percent for each low tire (two tires low = 6 percent). Tires underinflated by 30 percent lost up to 10-percent fuel economy.

One thing to keep in mind: TPM systems are affordable. Valve-stem TPM systems from PressurePro start around $50 per sensor. Costs for other systems vary but they all are in a price range that make them a reasonable investment for the value they return. Keeping tires at their target psi maximizes fuel economy and a longer tread life. Proper tire inflation helps keep the vehicle under the operator’s control and prevent accidents. The data recorded by a TMP system can be uploaded to a tire management program to help predict future tire costs based on current performance. And a TMP system lets the operator see where the tires are heading before they run into trouble. Immediate notification of abnormal tire pressure lets the operator correct a situation before it becomes a problem, and avoid expensive repair downtime and damage.

Is it a law yet?
While there has been some discussion about extending the TPM laws to long-haul and dump trucks, no formal ruling has been handed down. A call to the DOT in Washington did not result in any definitive information.

Joe Packett, director of the Tire and Rim Association, says since the U.S. DOT and NHTSA do not have regulatory authority over off-road construction vehicles, he feels any rules making TPM systems mandatory on construction vehicle tires are far off, well after the two agencies tackle truck tires.

The notion of voluntary installation of TPMs on contractor’s machines is still trying to find traction. It’s not that the TPMs concept is a hard sell. It just seems to be one of those technologies that works well enough but hasn’t quite embedded itself in the construction industry. Smaller contractors still use the old familiar tire pressure testing methods, like kicking the tire in the right place and listening for the right sound. Some aren’t sure what a TPM sensor is.

Like other technologies that were acknowledged as good ideas but didn’t have the sexiness to create market demand, (like catalytic converters) TPM systems on heavy equipment will need time in the marketplace to show their value. More impressive feature/benefit relationships between maintaining proper tire pressure and lower tire maintenance costs need to occur before these monitors find their way into a contractor’s fleet.

TPMS Manufacturers
PressurePro’s TPM system transmits tire pressure by RF signal every 15 seconds to an in-cab display. When a tire is 12.5 percent below its target pressure an initial audio and visual alert is displayed on the operator’s monitor. A second more aggressive alert sounds when the pressure reads 25 percent or lower. Vanessa Zaroor, PressurePro’s marketing manager, says in case of a tire failure the sensors will step in and immediately alert the operator within seven seconds. website

SmarTire’s active monitoring system straps a sensor on each wheel rim and measures the internal tire pressure and temperature every 12 seconds, sending a reading to the wireless receiver about every five minutes. If the sensor detects a change in pressure of 3 psi or more, it overrides the regular schedule and immediately sends an alert to the receiver. The SmarTire system can also send data to a handheld reader for use in daily maintenance tire checks and replaces the less accurate method of thumping the tire with a hammer. website

Michelin’s eTireII is its second generation tire-mounted TMP system. Michelin’s first eTire tire monitoring system was larger and glued to the inside of the tire. At speeds greater than 50 miles per hour, the sensor module could malfunction or stop working entirely. The new eTireII sensor module, made by Honeywell, is considerably smaller. The dime-sized module is glued onto the sidewall and can’t be removed for use on another tire. Pressure and temperature data are sent wirelessly to a gate reader or a handheld unit that reads the tires while the equipment is in motion. The reader stores the information and can be uploaded to Michelin’s web-based tracking program. website.

TREAD lightly
In November 2000 the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act was enacted. The TREAD Act’s goal is safer driving, safer tires and better fuel economy. To reach that goal, after much wrangling and several court cases, the National Highway Safety Transportation Administration now requires all new on-road motor vehicles less than 10,000 pounds (except motorcycles) to be equipped with a tire pressure monitoring system (TPMs) as of September 2007. The Act applies to most passenger cars and light trucks.

TREAD requires the system’s trigger must engage when the tire is 25 percent under-inflated. Once the trigger’s threshold is reached, a warning light on the vehicle’s dashboard and audible alerts switches on and can not be turned off until the driver takes action to correct the tire pressure. The rule requires the TPM system be designed so it can not be disabled by the vehicle’s owner. Owners who disable the TPMs face a fine.