• Florian Buder

Torque down those hold-down bolts!

Updated: Jun 20

The importance of applying proper torque values

Image 1: Skills competition in Canada - Alignment

Drive trains being misaligned is a common phenomenon in plants around the globe. Luckily the majority of industries invested in alignment tools and training to minimize the parallel and angular offsets in machine trains. It affects the lifetime of assets, reduces power consumption and optimizes production capabilities.


SYMPTOMS

It is not always easy to detect misalignment on machinery that is running. The radial forces that are transmitted from shaft to the shaft are challenging to measure externally. By using vibration analysis or infrared thermography, it is possible to identify primary symptoms of misalignment such as high vibration readings in radial and axial directions or abnormal temperature gradients in machine casings. Still, without such instrumentation, it is also possible to identify secondary machine problems, which can indicate inaccurate shaft alignment.


CAUSES OF MACHINE BREAKDOWN

  • Loose or broken foundation bolts

  • Loose shim packs or dowel pins

  • Excessive oil leakage at bearing seals

  • Loose or broken coupling bolts

  • Some flexible coupling designs run hot when misaligned

  • If the coupling has elastomeric elements, look for rubber powder inside the coupling shroud

  • Similar pieces of equipment are vibrating less or have a longer operating life

  • An unusually high rate of coupling failures or wear

  • An excessive amount of grease or oil inside coupling guards

  • Shafts are breaking or cracking at or close to the inboard bearings or coupling hubs


There are many aspects to machine alignment, such as the understanding of what it means when the center lines of shafts are not colinear, the proper shimming, what is a soft foot, what kind of coupling types exist, what are the tolerances and best practices. One other major factor in the appropriate machine installation and alignment is the torque that is applied to the hold-down bolts.


THE IMPACT OF TORQUING

Image 2: Vibration pattern - Courtesy of Mobius Institute

Hold-down bolts need to be torqued each time in the right sequence to avoid that the bolts are too tight or too loose. Otherwise, fasteners can break, or increased vibration occurs. The looseness is usually picked up by the vibration team. (1x and occasionally its harmonics as shown in image 2)


It is imperative that the clamping forces are as even as possible during the soft foot measurements but also during and after the actual alignment process.






WHAT IS THE RECOMMENDED TARGET TORQUE

The target torque depends on many factors. An OEM should have specified the values originally. If not, one needs to go by the grades of the bolts, thread type and nut factor. Each fastener grade has specific limits based on the strength of its material. The usual grades are 5 or 8.


How to identify fastener/bolt grades/thread size/nut factor?

Image 3: Fastener grade table

The grade can easily be determined via the grade markings on the bolts, as shown in image 3. Grade 5 has three lines, and Grade 8 has six lines on its head. The bolt grade must be identified correctly.


Since now the grade is determined, what is the recommended target torque value?


There is another factor that plays a role in identifying the proper target torque. The thread size needs to be considered, too.

Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger thread form relative to the bolt diameter, where fine threads have a smaller thread form relative to bolt diameter.


Lastly, the nut factor, K, sums up the combined effects of many variables affecting the difficulty of tightening the bolt, such as friction.

Some of these factors include:

  • The type and material of the bolt, washer and nut

  • The presence and nature of any plating, coating or lubrication

  • The pitch or angle of the bolt threads

  • Corrosion and wear