Torque Control: Loose Bolts An Industry Problem

January 12, 2024

Loose Bolts: Introduction

Imagine if one of the one hundred and fifty-plus car engine bolts is under-torqued; it loosens over time and eventually destroys the engine.

What if the bolts are under-torqued in an airplane assembly and become loose mid-flight?

What if those bolts were to the exit door, which blew off mid-flight, and the cabin lost pressure at 10 km in the air?

Loose Bolts
Figure 1 Loose Bolts What happens when a threaded fastener is tightened?

This article examines the importance of controlling the tightening of a fastener so that things do not come loose.

We will examine the application of torque, discuss what loose bolts could do and some causes, and examine the pros and cons of several torque wrench types.

Loose Bolts: How Torque is Determined

Figure 2 Loose Bolts: Connection Joints
Figure 2 Loose Bolts: Connection Joints

The connection (joint) is made up of different pieces which are attached and held together.

The device (fastener) that holds the pieces together must be designed to be stronger than the total of all loads incurred (including all operational and environmental factors).

The tightening of the fastener creates a "clamping force."

The fastener is dimensioned and preloaded so that all forces acting to separate the connection are overcome.

Loose Bolts: Getting the Clamping Force Right
Figure 3 Loose Bolts: Getting the Clamping Force Right

To reach a desired torque value, enough force must be applied:

To overcome the friction induced by turning the fastener against its components (nut, threads, spring washer, etc.) and the attached and clamped materials.

Load the fastener and materials connected to a value inside their elastic limits.

The object of a threaded fastener is to clamp parts together with a tension greater than the external forces tending to separate them.

When appropriately torqued, the bolt remains under constant stress and is immune from fatigue.

 

Loose Bolts: The Importance of Torque Control

 

Figure 4 Loose Bolts: Vibration Can Loosen the Bolt

Figure 4 Loose Bolts: Vibration Can Loosen the Bolt

 

If the torque is not applied correctly and the tension on the bolt torque is too low, varying loads will act upon the bolt and fail.

Proper torque control during the tightening of bolts ensures that they are securely fastened.

Loose bolts can loosen over time from constant vibrations.

This can compromise the assembly's structural integrity and lead to potential failures.

Bolts are often used in structures and machinery where their integrity is critical.

If bolts are not tightened to the specified torque, the structural components may not be securely connected, leading to instability and a higher risk of failure.

What are the Industry Specific Causes of Loose Bolts

Various studies and reports within automotive, aerospace, and construction industries have consistently identified improper torque as a common cause of fastener-related issues.

These issues can range from minor performance problems to catastrophic failures.

Maintenance Incidents:

In maintenance and reliability reports, incidents related to fasteners are often listed among the top causes of equipment failures.

Failure to tighten fasteners to the correct torque specifications may result in unexpected downtime, increased maintenance costs, and potential safety hazards.

Aerospace Industry:

In the aerospace industry, where precision and safety are paramount, accidents have been attributed to improperly torqued fasteners.

These incidents underscore the critical importance of accurate torque control in this sector.

Automotive Industry:

In the automotive sector, loose or inadequately torqued fasteners can lead to vehicle malfunctions, accidents, and recalls.

Wheel lug nuts, for example, are critical components where improper torque can have severe consequences.

We wrote an article specifically on this as it happened to one of us.  That article can be found here.

Structural Failures:

Improperly torqued fasteners in construction and civil engineering applications can contribute to structural failures.

This can compromise the safety and stability of buildings, bridges, and other infrastructure.

Oil and Gas Industry:

In the oil and gas industry, where equipment operates in harsh conditions, improperly torqued fasteners can lead to leaks, equipment damage, and unplanned shutdowns, affecting overall operational efficiency.

Loose Bolts Industry Stats from UK CAA MORs in the 1990's

These statistics are older, though relevant, as many of these issues happen today.

They arise from a lack of training and employee disengagement.

Issues such as:

  • incorrect installation of components
  • fitting of wrong parts
  • electrical wiring discrepancies (including cross-connections)
  • loose objects (tools, etc) left in aircraft
  • inadequate lubrication
  • cowling, access panels, and fairings not secured
  • landing gear ground lock pins not removed before departure

Which is broken down further in the James Reason Report:

  • Fastenings undone/ incomplete (22%)
  • Items left locked/ pins not removed (13%)
  • Caps loose or missing (11%)
  • Items left loose or disconnected (10%)
  • Items missing (10%)
  • Tools/spare fastenings not removed (10%)
  • Lack of lubrication (7%)
  • Panels left off (3%)

Loose Bolts: Back Torque with Some Wrenches

What is back torque?

Back torque is when the energy of a click-type wrench comes back in the opposite direction and loosens a bolt.

The back-torque or kickback issue is with a specific "Bi-Directional Ratcheting" type of torque wrenches.

Loose Bolts: Can a Torque Wrench Loosen the Bolt? The Cog Assembly naturally creates a "kickback" torque component.
Figure 5 Loose Bolts: Can a Torque Wrench Loosen the Bolt? The Cog Assembly naturally creates a "kickback" torque component.

More specifically, back-torque is when the setpoint of the compression spring is overcome, the roller assembly climbs over the cog, and a load component is created in the direction of the torque being applied.

As the roller assembly comes off the backside of the cog, it naturally creates a kickback load component in the reverse direction of the torque being applied.

Some Pros of click-type torque wrenches:

  • Provide consistent results.
  • Reduce operator error reading and interpreting scale.
  • Get the right wrench from the kit and use it.
  • Independent verification is rarely needed.

    Figure 6 Loose Bolts: Figure Showing What Happens When an Operator Makes the Wrench Click Twice
    Figure 5 Loose Bolts: Can a Torque Wrench Loosen the Bolt? The Cog Assembly naturally creates a "kickback" torque component.

 

Some Cons of click-type torque wrenches:

  • There is a tendency to over-torque a fastener by 15-20 percent using these tools.
  • Humans cannot react quickly enough when the click mechanism is tripped.
Figure 7 Loose Bolts: Graph showing only 75 % of the proper torque applied.
Figure 7 Loose Bolts: The graph shows only 75 % of the torque applied.

 

 

 

 

 

 

 

 

 

 

 

Note: Not all click-type wrenches produce this kickback. 

The best recommendation is to ask the manufacturer if the wrenches are impact-free or have any kickback.

Loose Bolts: Conclusion

The significance of torque control in fastener applications cannot be overstated, as highlighted throughout this exploration.

The potential consequences of loose bolts extend beyond mere inconvenience, encompassing catastrophic failures, safety hazards, and structural compromises across various industries.

Proper torque application is not just about tightening bolts; it is a critical aspect of ensuring the integrity and reliability of assemblies.

As examined, the clamping force generated by correctly torqued fasteners is designed to withstand external forces and environmental factors, providing stability and immunity from fatigue.

Torque control is particularly important in automotive, aerospace, construction, and oil and gas industries, where precision, safety, and operational efficiency are paramount.

Instances of loose bolts have been identified as significant contributors to equipment failures, accidents, and unplanned shutdowns.

Furthermore, the exploration of torque wrench types, such as click-type wrenches and the potential back-torque issues, emphasizes the need for careful consideration in selecting the right tools.

While these tools offer consistency and reduce operator error, awareness of their limitations, such as the tendency to over-torque and the potential for kickback, is crucial.

Controlling torque is not merely a technical detail but a fundamental practice that safeguards against a cascade of repercussions.

As industries evolve and technology advances, the commitment to precise torque application remains a cornerstone in ensuring structures and machinery's longevity, safety, and reliability.

The journey from understanding the clamping force to exploring the intricacies of torque wrenches underscores torque control's critical role in the seamless functioning of our engineered world.

Applying To Engineer Is Human To Metrology

Engineering failure is measured in two ways: human death toll and materials lost. This is just one of many insights from Henry Petroski’s book To Engineer is Human: The Role of Failure in Successful Design.  As the president of a force calibration laboratory that has seen over 120 years of success in the industry, I found the content of this book to be both fascinating and indispensable. Petroski analyzes case studies and describes tragic yet avoidable failures in engineering; thus, he proves that with better practices, lives need not have been lost.

The book opens by mentioning a simple design error that, on July 17, 1981, caused a walkway collapse at the Hyatt Regency Hotel in Kansas City, Missouri. This collapse resulted in the death of 114 people and injured another 216.  The disaster, a product of a corporate culture of profound neglect, subsequently became a case study in engineering ethics.

Petroski discusses what happened in detail and how the cause of the eventual failure was determined.  Simply put, the constructed walkway created double the amount of force on a nut than what was originally intended in the design. How and why did this egregious error occur?  From the first chapter onward, I considered how these details relate to Metrology. More specifically, how does this example relate to Measurement Risk?

Did the manufacturing company know the proper requirements? Did they have the right equipment to measure force? Did they follow the correct documented processes? Further investigation would reveal the answers to these questions.

The above questions could be posed about other illuminating case studies that Petroski explores. Petroski's examples captivated my attention from beginning to end, from buttons wearing out on Texas Instruments Speak & Spell toy to the cause of the 1979 DC-10 crash in Chicago to how an oversized waterlily inspired the Crystal Palace. I could not help but draw parallels between the DC-10 crash and more recent airplane failures, and I found myself asking, “Why have we not learned from previous failures?”

Petroski addresses this issue by stating, "Failures appear to be inevitable in the wake of prolonged success, which encourages lower margins of safety. Engineers and the companies who employ them tend to get complacent when things are good; they worry less and may not take the right preventative actions." Petroski's claim about complacency might merely describe human nature or point to the old but well-known motto, "If it ain't broke, don't fix it."

This motto has haunted our society in various ways long after Petroski first published To Engineer is Human. Consider the BP Texas City Refinery explosion, where the root cause was a combination of cost-cutting, production pressures, lack of preventative maintenance, procedural workarounds to compensate for deteriorating equipment, and collapsing cranes due to premature removal of the pins.

Unfortunately, this attitude, which leads to negligence, may be present in our Metrology community today.  Some companies might be using outdated equipment, or, like the BP Texas City Refinery, they may have established procedural workarounds to compensate for not having the right equipment.  In applications of force, some companies use equipment beyond its lifecycle. They could be avoiding upgrades to the proper equipment or using the wrong adapters, another common and unethical practice.

Such adapters could range from being unsafe to improperly machined, allowing the user to fixture the force-measuring device in the frame to apply forces and record output at those forces. Many do not realize that failing to use proper adapters to calibrate load cells, truck scales, aircraft scales, tension links, dynamometers, and other force-measuring devices can produce significant measurement errors that pose serious safety hazards. Another pitfall is that some purchase an adapter based on a copied design, assuming that it can work for a similar one because it worked for one application.

Petroski’s book enriched my appreciation for specific design elements and the importance of design safety factors while enhancing my awareness of what aspects of a design may fail. Petroski cites studies where a successful design was copied and failed because the engineer may not have understood concepts such as resonance. For instance, a successful bridge design in a certain area accustomed to low winds may fail under high winds. This information is even more relevant today since our climate produces more severe storms, higher winds, and the destruction of structures that may have been built to withstand only weather conditions that were once typical for their location.

To Engineer is Human discusses why understanding failure is essential to understanding and achieving success. Those who achieve success do so because they recognize how failure can occur and design accordingly. Because today's designs are pushed to the limit in terms of complexity, thorough knowledge of failure is imperative. A crewed mission to Mars would be an example of a highly complex project with a higher probability of failure. In a world of ever-increasing complexity, knowledge truly is power.

Essentially, Petroski highlights just how successful engineers can be when they choose to learn from failure. His book is one of my top ten all-time favorites because it remains as important today as it was over 35 years ago. I encourage everyone in the Metrology community to read it.

-Henry Zumbrun, Morehouse Instrument Company

Companies worldwide rely on Morehouse for accuracy and speed. The company turns around equipment in 7-10 business days so customers can return to work quickly and save money.

About Morehouse Instrument Company

Morehouse Instrument Company, a trusted and accredited provider of force and torque measurement services for over 100 years, offers measurement uncertainties 10-50 times lower than the competition.

Morehouse helps commercial labs, government labs, and other organizations lower their measurement risk by lowering equipment uncertainties for torque and force measurement. Contact Morehouse at info@mhforce.com or www.mhforce.com

More Information about Morehouse

We believe in changing how people think about force and torque calibration in everything we do.

This includes setting expectations on load cell reliability and challenging the "just calibrate it" mentality by educating our customers on what matters and what causes significant errors.

We focus on reducing these errors and making our products simple and user-friendly.

This means your instruments will pass calibration more often and produce more precise measurements, giving you the confidence to focus on your business.

Companies around the globe rely on Morehouse for accuracy and speed.

Our measurement uncertainties are 10-50 times lower than the competition.

We turn around your equipment in 7-10 business days so you can return to work quickly, saving you money.

When you choose Morehouse, you're not just paying for a calibration service or a load cell.

You're investing in peace of mind, knowing your equipment is calibrated accurately and on time.

Contact Morehouse at info@mhforce.com to learn more about our calibration services and load cell products.

Email us if you ever want to chat or have questions about a blog.

We love talking about this stuff.

Our YouTube channel has videos on various force and torque calibration topics here.

More Information from Morehouse

For information on our Torque Calibration Capability and Service, Click Here.

For more insights into force and torque calibration, metrology, and load cell reliability, explore our comprehensive blog at https://mhforce.com/blog/.

We believe in changing how people think about force and torque calibration in everything we do.

This includes setting expectations on load cell reliability and challenging the "just calibrate it" mentality by educating our customers on what matters and what causes significant errors.

We focus on reducing these errors and making our products simple and user-friendly.

This means your instruments will pass calibration more often and produce more precise measurements, giving you the confidence to focus on your business.

Companies around the globe rely on Morehouse for accuracy and speed.

Our measurement uncertainties are 10-50 times lower than the competition.

We turn around your equipment in 7-10 business days so you can return to work quickly, saving you money.

When you choose Morehouse, you're not just paying for a calibration service or a load cell.

You're investing in peace of mind, knowing your equipment is calibrated accurately and on time.

Contact Morehouse at info@mhforce.com to learn more about our calibration services and load cell products.

Email us if you ever want to chat or have questions about a blog.

We love talking about this stuff.

We have videos on various force and torque calibration topics on our YouTube channel here.

# Torque Control: Loose Bolts an Industry Problem

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