The shortage has resulted in a sense of tire-care awareness among mine operators that is probably unmatched in the annals of modern mining, along with some fairly bizarre economics. At Caterpillar Global Mining’s annual dealer forum held earlier this year in Tucson, Arizona, Cat Senior Consultant Pete Holman presented some surprising numbers.

“Lack of proper road maintenance can dramatically affect the operating cost of a haulage fleet,” Holman explained. “For instance, one metal mine observed that nearly three quarters of all tire failures were caused by cuts and impacts. If only one 37.00R57 tire fails prematurely—say half the normal life—the cost is in excess of 250 hours of operating a motor grader, an important haul road management tool. Or, as one mine manager put it, ‘One cut tire is about the same price as a pickup truck. But I can get another pickup. I can’t always get tires.’”

To extend the pickup truck example even further, if tire damage that disables a large truck is caused by a rock in the haul road, face area or dump approach, the rock itself has elevated its own worth to the price of a pickup truck in terms of the cost of a new tire, which could run anywhere from $25,000 to $45,000 depending on size. And, if the truck must be taken out of service because the tire can’t be replaced, the overall cost to the mine’s production plan quickly spirals into the stratosphere.

All of the ingredients that enter into the tire-life mix can make for a complex recipe, as shown in the accompanying figure. However, there are certain factors that may appear obvious—but are sometimes overlooked or bypassed in the rush to meet tonnage figures. Others are less apparent but can be equally as worthwhile, or equally costly if ignored.

One of the most obvious factors of tire care is proper haul road management. Mine managers, for example, recognize that spillage from truck bodies is a prime cause of tire damage and are increasingly devoting more resources to minimize spillage—and once it occurs, to have it reported and removed.

What may be less well known, according to Holman, is that even with a clean surface, repeated use of the same path on a haul road lane can lead to increased tire wear. “If haul units consistently use the same path in the respective haul lanes, the concentrated load will eventually wear ruts or furrows. Running a tire on a rutted road can cut the sidewall and put stress on the carcass when entering and leaving the rut. Once in the rut, a tire will wear unevenly due to surface variations. Operators should be encouraged to use different area within the haul lane to avoid this,” according to Holman.

Another obvious fact is that increases in road grade can adversely affect tire life, cutting it by up to 30% as grades approach 15%. Holman cautions against grades higher than 1:10, but also notes that it’s not enough just to keep grades moderate—they should be consistent as well. “It is advisable to have one single consistent grade rather than allow multiple grade changes. Tires undergo less stress with a consistent grade due to fewer transmission shifts. However, mine operations sometimes are faced with challenges of short and steep grades for given distances. Proper mine planning can minimize such situations,” said Holman.

Load, vehicle speed, and proper inflation are all familiar tire-care issues, but what are the actual dynamics of failure in a tire that may be overloaded and underinflated? The culprit is “deflection,” explained Holman, with eventual damage depending on the degree and frequency of deflection, the catalyst being inflation pressure. “Responsible factors for deflection are load and speed. Harm is amplified if the inflation pressure is low. The result is higher wear, stress in tread and plies, weakened bonding, and heat generation. All of these spell lower tire life. Key benefits of proper inflation include providing the tire with maximum ground contact area, optimum flexibility and reduced heat levels in the tire. This results in less downtime.”

Proper suspension operation also enters into the mix, Holman noted. “Properly charged struts provide spring action and do not allow impact loads to come onto the tire directly. When cylinders bottom-out— we call them hard cylinders—and undulations in the road cause the truck to bump, excessive stresses come onto the truck frame as well as the tires, exceeding the cushioning capacity of the tires.”

Ten Areas for Awareness
At the most recent MineWest conference, sponsored by Mining Media International, Eduardo Santillanez, mining market segment manager for Michelin, offered some practical tips on tire care practices. His “Ten Areas of Attention That Will Increase Tire Life” include: