Air is to the tire what oil is to the engine. Without either, the actual life will be shortened. But what is the correct air pressure for OTR tires? That is a question that is always asked and can be difficult to address. A tire cannot perform without air, as this is the most important factor for any tire. Air carries the load, not the tire.
There are a lot of parameters to look at when deciding the correct air pressure to suggest. For example, if the tire is being mounted on a loader, you will need to know if it will be mounted on the front or rear position and the density of the material being loaded.
Then, you need to understand what the loader is doing: is it working the pit area, or doing load and carry, or both? A tire operating at the correct air pressure will help to maximize the performance of the tire and machine. The operator will find that the ride, handling and stability of the machine will increase, which translates into greater productivity.
How often should the air pressure be checked in a tire? Before each shift would be great, but many operators do a walk around, kick the tires, check the fluids and go to work. Tires will lose pressure naturally and need to be pressure checked and adjusted periodically.
Cold inflation checks are the best and most accurate. A cold tire is generally defined as one that has been sitting idle and not used for a minimum of 24 hours. From this point the pressure can be monitored more accurately. Hot inflation checks, in most cases, are the norm since many operations are 24/7. Therefore, knowing the cold inflation pressure of the tire will help track the rise in pressure in a working tire. This will then help to set the correct standards for warm inflation checks.
As a tire works, its internal temperature will increase, as will its air pressure. A hot tire must never be deflated. If the air pressure increases by 20% or more from the cold inflation reading, it is a signal that the tire is overheating and should be stopped. The tire, then, should cool down or moved to a different haul or operation that is less demanding. If there is any question regarding the continued operation of the tire, consult with a tire manufacturer representative.
A tire operating at the incorrect air pressure, either under or over-inflated, might not show any immediate issues but may fail later, even after its air pressure was corrected. Operating a tire with insufficient air pressure causes the tire’s operating temperature to rise. This can lead to irreversible damage of the tire’s internal components and will shorten its life expectancy. Overinflation, on the other hand, is not good either. It can cause rapid and irregular tire wear. In addition, the tire will be subjected to a greater change of sustaining cuts in the tread and sidewall due to improper flexing of the tire.
Because of the specialized nature of off-the-road vehicle usage, loads in excess of those in the appropriate load tables stated by the tire manufacturer are often encountered. Excess loads can be a result of the actual vehicle weight exceeding the design weight. This can be due to different attachments being added, varying density of the materials being moved, load transfer and other factors. Under these conditions, the actual tire load may exceed the load rating noted in the manufacturer’s load table.
If this is the case and excess loads are encountered, the cold inflation must be increased to compensate for the higher loads. As noted in the manufacturer’s data books, for each 1% increase in load for a radial tire, the inflation must be increased by 2%. The maximum excess load allowed for a radial tire is 7% with a maximum psi increase of 14%. For a bias tire, the excess load allowed is 15% with a maximum psi increase of 30%. The maximum pressure allowed for either a radial or bias tire is 120 psi cold. The customer must be advised that if they are running continually with excess loads, the tire won’t perform as well. Questions on proper tire inflation should be discussed with both the tire and rim manufacturer to obtain their approval.
Tires are marked with either a ply rating, load and speed index or star rating. This helps us understand the maximum load that the tire is designed to carry at a specific load and speed. In the past, a 1-star tire was for a loader and a 2-star was designated for a haul truck. Today, many manufacturers only produce a 2-star rated tire that is designed to work in either an earthmover or loader (E/L) application. This means that the tire will work on both a loader and haulage piece of equipment.
Knowing the application of the loader and the density of the material hauled will determine the psi at which the tire needs to operate to maximize its performance. For example, the chart above shows the allowable load based on the speed of the loader. Knowing this will help to match the best pressure for the tire based on the actual usage of the loader. This will help you figure out the cold inflation pressure of the tire.
When looking at the load tables found in manufacturers’ data books, you will find that for a certain star- or ply-rated tire, there is a specific psi shown. For example, a 2-star radial tire for a loader application (5 mph) is 94 PSI. If the same is installed on an articulated dump, the psi is 74 pounds.
In many cases, 95 pounds of air in the front tires of a loader is too high. Knowing what the density of the material is, the size of the bucket and what the machine is doing with the material will direct you to the proper air pressure required for the front tires. As for the tires on the rear of the loader, 25 pounds or less is a good designation. If you set a 29.5R25 on the front wheel positions at 85 psi, the rears should be set at 60 psi. This should provide the operator with the best ride and handling for the loader.
When I am asked what the air pressure should be for 26.5R25s on the fronts, I like to start with 75 psi and 65 psi for the 23.5R25 tires. Again, the rears should be about 25 pounds less. But, as I stated above, make sure you understand the actual weight of the material being loaded into the bucket and how the loader is operated. Is it used for load and carry or just loading material into a crusher or trucks?
As for articulated dumps, I like to set the pressure at 65 to 70 psi for all wheel positions. Again, based on the actual loads, speed and distance, the pressure may need to be adjusted accordingly.
What I would highly suggest though, is to find out what the equipment manufacturer recommends as to the best psi for all wheel positions on the specific machine. Also, you should confirm with the tire manufacturer about the psi they would recommend for their tire mount on the specific machine. This way, if there is any question about a tire’s performance, you can document the information provided.
What is the correct air pressure for a specific tire for a certain application? My answer is, I do not know – not until you have taken time to study the use of the machine and the loads carried. A successful tire program is only a pound or two away. TR
OTR industry veteran Tim Good has spent most of his career in the off-the-road industry, working with Maxam Tire and Goodyear Tire & Rubber Co., among others. His expertise includes major mining, aggregate, construction and port applications.