By Drew Troyer, CRE
CMRP, Editor
Have you and/or your team included threaded-fastener management in your proactive maintenance and plant-reliability goals? You should. Sooner than later. The reasons for doing so are clear.
In the most basic of terms, poor threaded-fastener connections create vibration in mechanical systems. Vibration, of course, reduces the life of components by applying unintended forces. Looseness is damaging in electrical connections. As much as half of all electrical problems in electric motors are fastener-related. So mechanical
and electrical fasteners definitely should be a priority for your reliability-management strategy.
Fastener looseness can be detected with vibration analysis by looking for an increase of vibration at 1X running speed and harmonics at multiples of running speed, frequently exhibiting half harmonics. On the time wave form, you’ll often see flatheads on the sinusoid.
Poor
electrical connections are usually revealed by thermographic analysis given the fact that a poor connection reduces conductivity and generates heat as a byproduct. However, ultrasonic analysis can reveal tracking and arc-spark emissions associated with the accumulation of static electricity.
Let’s discuss threaded mechanical fasteners here.
The
objective of effective fastening is to achieve a sufficient level of clamping force. Clamping force is achieved by properly tensioning threaded fasteners to leverage the elastic properties of metal.
When
tightened, fasteners elastically deform to create tension between the threads (much like a spring). Some metals can elastically tolerate more force than others. In general, the higher the SAE grade for a bolt, the higher the tension it can tolerate and, accordingly, provide greater clamping force potential.
It’s important to note that
over-tensioning a fastener beyond its elastic limit, i.e., the limit below which the threads will “spring” back into their normal place upon loosening, will permanently damage the fastener (a phenomenon called "plastic deformation").
Think of over-tensioning and pushing a fastener beyond its elastic limit this
way: If you pull on it, a spring returns to its normal state. That is unless you pull too hard. In that case, a spring will not return to its normal state.
(From an engineering perspective, the elastic limit of
materials is defined by Hooke’s Law. If you want to learn more, an Internet search on Hooke’s Law will produce plenty of useful information.)
BEST PRACTICES
Consider the following best practices when developing a fastener program at your plant:
* Select bolts carefully. Fasteners must be sized properly and fabricated from the appropriate material. SAE grade 5 (ISO 8.8) bolts have much higher elastic tension limits than SAE grade 2 (ISO 4.6). SAE grade 8 (ISO 10.9) bolts have even higher tension limits than SAE 5 (ISO 8.8). I suggest that you
eliminate fasteners below SAE 5 (ISO 8.8). Use SAE grade 8 (ISO 10.9), or even the unofficial SAE grade 9 (ISO 12.9) for high-vibration applications.
* Always lubricate fasteners, preferably using a purpose formulated fastener lubricant.
* Always use a properly sized and calibrated torque wrench. Employ hydraulic torque wrenches for very large fasteners.
* If washers are required, use only properly, sized, hardened, flat designs. Do not use spring-type lock washers for fasteners that are 3/8” (M8) or larger, since they would simply flatten out and reduce the effective clamping surface area.
* Fasten using a crossing pattern to ensure the clamp load is evenly distributed.
* Tighten to 1/3, then 2/3, then final torque required using the crossing pattern described above. Perform a final-pass torque check before concluding the job.
* Make sure electrical-connection points are free of debris or oil, which can compromise electrical conductivity.
* Perform periodic torque checks, particularly for high-vibration machines, such as shaker tables. For critical applications, employ an ultrasonic bolt-stretch meter, which measures tension more directly.
* Avoid excessive shimming and carefully inspect foundation grouting. No amount of precision fastening can overcome an inherently poor foundation.
* Incorporate fastener and fastening specifications in all work plans. Don’t make your craftspeople guess. (I'll provide more details on this particular issue in a future article on FLAB-related matters. FLAB stands for fasteners, lubrication, alignment, balancing.)
* Regularly monitor for mechanical looseness using vibration-analysis. Use infrared thermography to identify poor electrical connections.
Finally, be sure to train all craftspeople on fastener physics, mechanics, and best practices.
