What is torque load retention?
It is the measurement of how well a bolted joint holds the force applied to each bolt. When you start to tighten a bolt what happens first is that you overcome macro effects – bringing the bolt head into contact with the flange or bearing surface, drawing the gap closed and aligning the bolt to the flange face. Once macro effects are overcome, the micro effects come into play. These are primarily friction driven. When applying torque to fasteners, approximately 90% of it goes into overcoming thread friction – only about 10% goes into tensioning a bolt in a typical unlubricated system.
The torque load retention process begins by creating a seal by tensioning a fastener into the elastic range of its stress/strain curve. While it may seem counterintuitive, bolt stretch is desired in a sealing system as it can help maintain the seal. In fact, a flange, gasket, and fastener can be modeled as a spring system. This is important because of torque load retention – this is how you maintain a seal. Bolt stretch is very important in torque load retention because it will act as the spring to keep the flange interface sealed under dynamic loading, vibration and joint thickness reduction from creep relaxation of the gasket. This behavior occurs when the bolt stress is in the elastic range on its stress-strain curve. It is critical that the bolt remain in the elastic region to maintain the spring-like properties.
Literature research shows examples where the applied torque load on the bolts to achieve elastic bolt conditions are between 50-80% of the specified bolt yield stress. We recommend that you research the proper torque load for your application and your specific effects of loading the fastener to its elastic range. Each case will differ, making it necessary to calculate and test your assembly to ascertain its overall effectiveness for the duration of your application.
Other considerations for fastener type:
- Fastener material composition
- Galvanic corrosion
- Physical constraints
- System internal pressures
- Chemical requirements
Understanding the forces at the joint will help to prevent fatigue failure of the fastener.
The stiffness of the spring depends on the system inputs such as materials, temperature ranges, thicknesses, number of bolts, etc.… Note that stiffer bolts are not always better. In fact, for a system with the same external load applied, the fatigue stresses will be higher for a bolt with more stiffness relative to the flange. Forces at the joint are important to understand to prevent fatigue failure of the fastener. Continuing the subject of fatigue, where the theoretical loading occurs on the flange relative to where the joint is can influence the forces the bolt carries.
Phases of Fatigue Failure
- Initiation-Caused by repetitive stresses
- Propagation-Caused by a change in the direction of the stress point
- Final Rupture-Caused by the final stressor that breaks the part
Torque loss can occur in any bolted joint. For example, bolt relaxation, gasket creep, vibration in the system, thermal expansion, and elastic interaction contribute to torque loss. Proper gasket installation helps to reduce torque loss. Bringing the flanges together slowly and parallel, a minimum of four tightening passes and in the correct tightening sequence help to reduce maintenance later. Additionally, the thickness of the gasket is also important. If the material is thicker there is a higher likelihood that the gasket will creep and that can lead to torque loss. Even with each of these “rules” followed, loosening of the bolts will occur over time and maintenance will likely need to be done.
Other factors affecting torque loss include:
- Rate of heat up
- New vs. used bolts or studs
- Use of hardened steel washers
- Lubrication of bolts, nuts and nut facings
Additional factors when selecting fasteners:
When thinking about what fasteners to choose in an application there are additional factors that need to be considered. The gasket material’s min and max flange pressures-to-seal are important factors to understand how to prevent under/over loading of the gasket. The min flange pressure will be found equidistant between bolts; the max flange pressure will be found under the bolt. The length of bolt should be as long as possible to provide as much strain allowance to account for creep relaxation of the gasket and contraction of the flanges due to temperature. Strain is defined as “delta L” over “L” so for a given strain value, a longer bolt (L) will require a greater “delta L.”
Bolt diameter will affect flange pressure and can possibly lead to surpassing the max flange pressure of the gasket. Large bolt diameters torqued to, for example, 65% of yield stress will also impart higher loads on the flanges and can cause excessive bowing or flange hole deformation. If the gasket area is not adjusted to match the flange pressure, it can lead to gasket rupture and possibly leaking.
Temperature ranges need to be considered along with the coefficient of thermal expansion of all materials in the system. Especially in material mismatches. As temperature goes up, expanding material will increase flange pressure. As it goes down, it will decrease – possibly to the point of zero flange pressure. Additionally, creep of the fastener should not be ignored above 200C. Standard carbon steel fasteners at 300C can exhibit 25% stress loss. Ninety percent of industrial fasteners are made from carbon steels.
Torque control method is important because measurement accuracy can place too little or too much load depending on the proper flange pressure window. Even a perfectly designed system can have sealing issues if not torqued properly and with the proper equipment.
- Maintain seal by tensioning the fastener
- Stiffer bolts are not always better
- Bring the flanges together slowly and parallel
- Gasket thickness and min/max flange pressure are important
- Bolt diameter affects flange pressure
- Torque control method is important because it measures accuracy