Joint relaxation occurs to some extent in all fastened joints. It is caused by the surface of part(s) embedding or by “soft parts” such as gaskets, plastics or spongy material, which collapses under the clamping force created in a torque fastening condition. For correctly designed joints, relaxation is small and can be virtually ignored. However, relaxation is a particular problem on joints where gaskets, or parts such as spring washers and plastics, are utilized. On these fastening joints it can take a long time before the joint settles, which results in a reduction of the clamping force until the joint condition stabilizes.
The physical phenomenon for the collapse of material is often referred to as “creep.” All materials and solids are subject creep to some extent. The creep effect is most obvious and dramatic right after the clamping force has been applied. In many tightening applications the majority of the creep, the reduction of clamp load (and sometimes static torque), appears within the first 10-50 milliseconds. There are four common techniques that can be used to reduce the effect of “creep”.
Fundamentals of Tightening Fasteners and Relaxation
1. Torque the fastener down, then un-tighten the joint and retighten it.
2. Redesign of the fastening joint (for example, replace soft gaskets with a sealing compound).
3. Torque the fastener, wait briefly and then apply torque again (can be repeated in several steps).
4. Use a power assembly tool with a low RPM setting to apply the final torque.
With the introduction of sophisticated electric screwdrivers and other assembly tools, it has become more and more common for these torque tools to be chosen to help resolve any “joint relaxation” from incurring during fastening process. Depending on the joint design, technique option 3 is quite often selected.
When an electric screwdriver runs down a fastener and the tool clutches off once the preset torque is achieved there can be some joint relaxation that can occur. Mountz offers a solution with some of its electric screwdrivers that feature a selectable “double hit mode” or a programmable “multi-hit mode” option for soft joint applications. The “Plus” models for the Mountz NF-Series and BF-Series brushless electric screwdrivers feature the “double hit mode” option. While the Mountz YF-Series offers “multi-hit mode” option.
The “double hit mode” allows the electric screwdriver to perform a second hit to stabilize the torque for any joint relaxation that may occur. For hard joint applications there is not really a need to use the double hit mode. The clutch of the electric driver operates twice at the set torque under the double hit mode. This mode will increase the repeatability accuracy at the target torque by double checking. The YF-Series power tool features a “multi-hit mode” option. The clutch of the YF electric driver works multiple times at the set torque under this mode setting. The YF tool allows an user to program the number hits to be performed on the joint application (selectable setting of 1 to 10 times).
Auditing of Fasteners to Prevent Failures
The goal of auditing is to improve process results. To do this it is necessary to gather information for evaluation and analysis. Fastening errors can never be eliminated if proper technique and tools are not used. Therefore, auditing is a necessary interrogatory to gain information about how a job was performed even when the process is being controlled perfectly. There are three common torque auditing test methods that have been established to provide an accurate reference to the applied torque.
When setting up a workstation, don’t overlook an ergonomic solution. Assembly operators often spend long periods standing and/or performing repetitive motions and lifting over the course of their shift. Ergonomic solutions can considerably reduce the physical impact of these daily industrial tasks. An ergonomic mindset leads to a safer work place. Learn more about reducing injuries and workers’ compensation costs.
Also, consider improving your assembly process and prevent costly screw fastening errors with a screw counter. Eliminate the potential problem of missing screws from occurring. Designed to detect cross-threading, omissions, unfinished rundowns and cycle incompletes, a screw counter ensures the assembly process is done correctly the first time. The intent of the screw counting process is to ensure that all screws are accounted for during the assembly process.
Using a quality electric screwdriver and the proper fastening technique makes a safer world through accuracy and precision. Controlling torque is essential for companies to ensure their product’s quality, safety and reliability isn’t compromised. The failure of a three-cent fastener that isn’t properly tightened can lead to catastrophic or latent failures. Fasteners that are insufficiently fastened can vibrate loose and excessive torque can strip threaded fasteners.
Contact us with any questions you may have. We can help you make the right choice, every time.