The Role of Lubrication
With the conventional method of tightening or loosening a nut using a spanner, lubrication plays a critical role since there are lots of surfaces that move against each other. The stud and the lower side of the nut move against the joint surface while the threads of the nut move against the threads of the stud. Overcoming the frictional force accounts for approximately 90% of your energy input applied to generate the overall load in the joint.
Typically, the behavior and reliability of any bolted joint are primarily determined by the magnitude and stability of the clamping force. In a wide range of threaded applications, it is estimated that at least 90 percent of the force needed to tighten anunlubricatedbolt may be utilized in overcoming the immense thread friction, which leaves only 10% of the actual work usefully translated into tightening the bolt. This means that with proper lubrication of your bolts, you can do up to ten times the work you usually complete on yourunlubricatedbolts.
Sometimes, the inaccuracies involved in determining the friction torque components may result in either underestimation or overestimation of the actual clamping torque in the joint. Underestimation of the actual force may lead to material failure as a result of overstressing while overestimation of the actual clamping may lead to failure as a result of fastener loosening. Therefore, it is critical for every mechanic out there to understand the effect of lubrication on tensioning and how the overall torque-tension relation. This article elaborates how lubrication affects both hydraulic and multi-joint tensioning.
Hydraulic Tensioning
Although we are talking about lubrication and how it can make your work easier, this might not be the case when it comes to hydraulic tensioning. Typically, when a standard hydraulic tensioning tool applies a clamping load to a joint, lubrication has no significant effect since the clamping load is usually applied directly to the stud and the joint.
Tension force can be described as a linear force that is applied in an axial direction, which means that there is no rotation required to generate the load. This process allows your nut to be turned down against the joint face under no or minimal friction. Since there is not any significant friction that comes into play, there is no need to lower the coefficient of friction through lubrication. The lack of friction in the application also permits much more accurate results.
How Lubrication Affects Multi-Joint Tensioning
When operating multi-jackbolttensioners, the use of a lubricant on the main bolt thread usuallydoesn’t affect thepreload. Experts recommend the use of a light film of lubricant with strong anti-seize properties to facilitate easiertensionerremoval. A more significant effect of using a lubricant on multi-jack bolttensionerscomes from the use of the lubricant on individual jackboltthreads, washer face and the jackboltbottoms of thetensioner.
It is also good to mention that proper use of lubrication is critical to safeguarding precise and repeatable load control in super bolt installations. If you are conversant with the operation of super bolts, you already know that they mostly use a graphite-based lubricant with a low friction coefficient and a pretty steady performance to achieve an overall positive impact on thepreload. The multi-jack bolttensionersare usually delivered with lubricant pre-applied to all the installed jackbolts.
An additional lubricant is also included for application to all the jackboltbottoms. For any subsequent installation, you will need to reapply the lubricant to achieve the intended performance.
Conclusion
In a typical bolted connection, it is essential to torque the bolts and nuts properly to achieve the desired bolting force that is critical to keeping all the various parts together. The bolt make-up torque is calculated based on the material, bolt size, and more importantly, the overall friction force between the nut face and the threads.
This friction force is significantly affected by the lubrication and the lubrication method that you use on the bolt. Lubrication is crucial especially if you want a smooth tightening process with most of your force being directed towards doing the rightful work that it was intended to do. Working withunlubricatedjoints can be stressing since it drains a lot of your energy yet only a small portion of it goes into completing the real work at hand.
Want to learn more about optimizing your bolting processes? Talk to the Aztec Bolting experts.
