Blowout
Blowout occurs with parts used in dynamic applications, including wheels, tires and rollers. When polyurethane elastomers undergo cyclic loading and unloading, mechanical energy is transformed into heat energy and the elastomer will heat up. If this heat buildup is too great, then the polyurethane could melt and the part could experience blowout.
Potential Causes
Improper Choice of System
Some polyurethane elastomers have inherently better dynamic performance than others. High-performance polyether systems are generally preferred for optimum dynamic performance. Within the high-performance class of materials, however, some systems perform better than others.
Wrong Hardness Elastomer
With dynamic applications, the greater the cyclic deflection of the elastomer, the greater the heat buildup. If the deflection is higher than 5-10%, then the part has a greater chance for blowout. Therefore, specifying a harder elastomer is usually recommended to minimize deflection. However, there are typically other performance attributes that also need to be considered, such as ride comfort or traction, where use of a softer formulation may be desired.
Incorrect Stoichiometry
The dynamic performance of an elastomer is optimized in the 90-95% stoichiometry range. Performance will decline as a formulation moves significantly away from this stoichiometry.
Uneven Loading
Even when the correct formulation has been used, the loading on a wheel, tire or roller can be uneven, leading to localized meltdown and blowout in the region of maximum load. This happens frequently with rollers when the ends are tightened down to increase the nip pressure. It also happens with tires and wheels that have an improper crown.
Too Great of a Load/Speed
Dynamics are a function of load and speed. If either is too great, then the part may not be able to dissipate the heat fast enough to avoid a blowout. This can happen even with a properly designed part.
Solutions
Choose a System with Improved Dynamics
Generally, one will want to use a high-performance polyether-based elastomer. Many applications use TDI/Mboca/PTMEG elastomers with great success. Within that class of materials, some perform better than others. Products such as Airthane® prepolymers usually provide the highest performance materials in this class. In addition, alternative curatives have been developed which can offer dynamic improvements over conventional curatives. Consult your supplier for recommendations on the appropriate system.
Choose the Correct Hardness
Generally, an in-use deflection of 5–10% is acceptable, although a deflection of under 5% is desirable. The table below provides the formula to calculate the deflection. Choose a system that will provide this level of deflection or less, within the other parameters which may constrain your choice of hardness.
| Percent Deflection Formula |
- Where
- D = deflection in inches
L = loading in pounds
IR = inside radius of polyurethane in inches
OR = outside radius of polyurethane in inches
E = compression modulus, psi
W = actual width, inches |
| 750 |
52 |
— |
| 1000 |
62 |
— |
| 2200 |
78 |
— |
| 4400 |
90 |
40 |
| 9000 |
95 |
50 |
| 28000 |
— |
60 |
| 36000 |
— |
70 |
Optimize Stoichiometry
With most systems, a stoichiometry of 90-95% is ideal for optimizing dynamic performance, with 90-92% being preferred. Obviously, this is system-dependent.
Load Evenly
Proper installation of the part into service is critical. Loading across the face of the part should be as uniform as possible. Crowning of the wheel or roller may be required.
Redesign the Part/Application to Conduct Heat Away
Sometimes with the existing design, the combination of load and speed may be too great for the required polyurethane. In this case, it may be necessary to redesign the application. This can be accomplished by redesigning the wheel/roller to have a larger diameter, effectively reducing the speed. The part can also be made wider to distribute the load over a wider area. Also, additional wheels/tires can be added to spread the load. In some instances, the part could be redesigned to more readily dissipate heat away from the polyurethane.
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