Getting Maximum Performance, Life And Safety From Otr Tyres
Tyres are critical components of off-the-road (OTR) vehicles and serve several functions that affect the productivity of your equipment and subsequently affect your bottom line. They support the load of the vehicle by retaining the inflation pressure, provide traction and braking, control the direction of travel, absorb road shocks to smooth the ride (on most OTR equipment, the tyres are the only suspension for the equipment), and overcome road hazards.
The importance of tyre shape
Tyre designers use sophisticated tools such as finite element analysis and laser engraving to design and manufacture today’s OTR tyres. A very critical design factor of all these tyres, whether radial or bias construction, is the tyre shape. Designers very carefully design the mold shape to produce the resultant inflated tyre shape for maximum performance in the intended application. Tyre shape has a large influence on many tyre performance factors such as:
- Footprint shape – a critical factor in tyre wear, traction, and handling
- Vehicle stability – critical in equipment such as telescopic material handlers and wheel loaders
- Ride comfort
Loaded tyre shape – both bulge location and size must be planned for the desired load capacity, pneumatic cushioning and resistance to sidewall cutting and bruising from rocks.
Proper tyre shape is critical to ensure maximum tyre life and that a tyre is able to perform at its best for its intended function. Optimal tyre shape is maintained by proper tyre inflation pressure.
Correct inflation for the applied load maintains the designed tyre shape. The tyre can deflect as intended under load and achieve the best possible footprint shape and size for maximum traction, floatation and wear life. Correct inflation will result in the intended sidewall shape under load, which will maximize ride comfort and best resist sidewall cuts, bruises, and punctures from worksite rocks and other hazards. Correct inflation will generate minimal heat and minimize stresses on the tyre casing, leading to maximum tyre life and increasing the potential for a retread of the casing.
Inflating a tyre to even 110 percent of the needed inflation pressure for the applied load results in a distorted shape. The centre of the footprint will carry more of the load while the shoulders will be relatively unloaded. This will cause heat buildup in the tread centre and premature wear on the shoulders. Overinflation makes a tyre much more susceptible to cuts and impact breaks. Overinflation also magnifies the stresses on the tyre structure resulting in a reduction in original life and the potential for a retread. Overinflation will also reduce traction and flotation and compromise handling.
Underinflation = overloading
The most common occurrence is low inflation pressure, which is simply having less inflation pressure in a tyre than required to properly carry the applied load for the application. Underinflation can also be called overloading because if the inflation is less than required by the load, then the load is more than that supported by the inflation pressure.
A tyre that is underinflated is being applied at less than its optimal shape. Underinflation leads to overloading the shoulders and relative unloading of the centre. This will cause excessive heat build up in tyre shoulders as the rubber is usually thickest there. The tyre centre will scrub, causing excessive centre wear. As well as negatively effecting wear, the poorly shaped and unevenly established footprint of an underinflated tyre will lead to poor traction, flotation and handling. The excess stress on the overloaded shoulders may lead to belt edge separation or casing fatigue. Underinflation will shorten tyre life, compromise handling and can be a safety hazard.
Zipper tyres are dangerous, even lethal
A very important safety concern with underinflated tyres, especially steel/steel radials, is the possibility of creating a zipper tyre.
Tyre cords work with tyre beads and the correct inflation pressure to form a very strong yet flexible structure. When tyre cords are repeatedly over flexed and reloaded through many cycles of running or sitting for long periods while underinflated, the cords become weakened or may even break. This is a dangerous phenomenon that is not visible from the outside, especially in an OTR tyre due to the thick sidewalls. If a tyre is run underinflated, allowing repeated over deflection to weaken or break cords, or parked with little or no pressure in it, the result can be a zipper tyre.
Our first instinct when we encounter an underinflated tyre is to immediately add the needed additional inflation pressure. In this case, that could be a very dangerous, even fatal mistake. Any tyre that is found to be severely underinflated (SAE defines a tyre that is 20 percent or more underinflated as severely underinflated) must be dismounted and carefully examined by an expert for signs of cord degradation. The tyre should only be re-inflated after careful examination determines it is fit for continued service. As always, put the tyre and wheel in a safety cage or, if the assembly is too large, use a clip on chuck with a long enough standoff to allow the individual, along with a pressure monitoring gauge and control valve, to stand away from the sidewall outside the tyre “Blast Zone” which can extend 100 metres or more from the tyre.
What is the correct inflation pressure?
Since we have established that the proper inflation pressure is very important for optimal tyre performance and safety, we need to understand what correct inflation pressure is and how best to maintain it. For example, what is the correct inflation pressure for a 20.5R25 * OTR tyre? The only correct answer is: “It depends.”
First of all, we must know what the tyre is designed to do. That is indicated by the tread code. OTR tread codes are usually one or more of the following: G-Grader Service, E-Earthmover Service, L-Loader/Dozer Service.
Correct inflation pressure depends on the specific application
The chart on the bottom shows that the design inflation pressure is 35 percent higher for loader/dozer service than for grader service, but the corresponding load is a whopping 205 percent more. The load and inflation for earthmover service is in the middle. Obviously, it is very important to select the proper tyre and to inflate it to the needed inflation pressure.
From the chart, the 20.5R25 1* L tyre is designed for a maximum loader service load of 20,900 pounds at an inflation pressure of 73 psi. The Tyre and Rim Association allows up to 15 psi additional inflation pressure for front-mounted loader tyres due to the very high loads temporarily encountered on “breakout.” Most tyre manufacturers, including BKT, recommend adding 10 to 15 psi to the front tyres of a loader. Rear loader tyres have a lighter load than front tyres and should be inflated for the actual load in operation (with empty bucket) being careful not to go so low as to allow rim slippage, which can damage the tyre bead area. Such things as operating cycle speed and distance, as well as material density, should be considered when optimizing tyre inflation pressure. Your service dealer or tyre manufacturer can help you customize inflation pressure to your application.
What does inflate to 65 psi really mean?
You may at first think that is a silly question, however, the answer is not simple. First of all, stated inflation pressure targets, unless otherwise labelled, are for cold inflation pressure.
Cold inflation pressure is the pressure contained in a tyre/wheel assembly when the tyre, wheel and inflation medium are at ambient temperature. That is true even if ambient temperature is quite high. Inflation measured under any other condition is commonly called hot inflation and is definitely not to be confused with cold inflation.
So who cares? Sometimes it’s hot, sometimes it’s not. In 24-7 operations tyres are seldom, if ever, “cold.” The important point to remember is that gases (air or nitrogen) expand when heated. Therefore, inflation increases just from the hysteretic (the lag of an effect after its cause) heat generated by tyres as they operate. Since this operational increase in inflation pressure is variable and depends on many factors, ideally you should set, check and adjust inflation pressure when the tyres are cold.
A good plan is to check and adjust inflation pressure, if possible, before restarting equipment after your weekly shutdown.
It may take 24 hours or more of rest for large mining tyres to come to equilibrium with their surroundings; therefore, you may have to monitor and adjust “hot” inflation pressure.
Hot inflation will generally be approximately 7 to 15 percent higher than cold inflation but may, in some cases, exceed 20 percent depending on many operational factors. If a tyre is inflated while hot to a cold inflation target of 100 psi, then it might only be 80 psi cold inflation pressure and thus “severely underinflated.” Your service dealer and tyre manufacturer, along with your own operational historical data, can help you determine the needed hot inflation if you cannot measure cold inflation.
One more note on the effect of temperature on inflation is in order. When inflating tyres during cold weather, you must consider the temperature effect on inflation pressure. A tyre and wheel stored overnight in a warm tyre shop, inflated from an air or nitrogen source also inside the warm shop, then put into service in a colder environment, will be underinflated unless initial inflation pressure adjustments are made.
There are charts available from manufacturers which are helpful for such a situation. For example, a tyre with a target inflation of 100 psi that will be used for an application outside at 21 degrees F, should be inflated to 111 psi in a warm 65 degrees F shop. These charts are available from your service dealer or tyre manufacturer.
The correct inflation pressure in an OTR tyre will result in optimum tyre performance and longest possible tyre life. It will maximize your return on your tyre investment and reduce the risk of tyre related accidents. An educated and planned approach to inflation- pressure maintenance is a page to put into your operations manual. Whether you train and equip your personnel to manage inflation internally, or contract your service dealer to do so, inflation pressure maintenance is an essential management practice.
Inflation pressure and load capacity for a 20.5-25 20 PR bias tyre and a 20.5R25 1* tyre designed for use in three different applications: G-Grader Service, E-Earthmover Service, L-Loader/Dozer Service.
Source: Heavy Equipment Guide