Over & Under-Torquing: Bolt & Fastener Risks

Ensuring Bolt Safety & Strength

Facing issues with bolt integrity? This post demystifies the dos and don’ts, ensuring your projects stand firm and safe. Follow these engineering-driven insights to confidently specify, measure, and control torque for optimizing equipment structural integrity, performance, and safety.

Torquing refers to the process of tightening bolts, screws, nuts, and other threaded fasteners to a specific target torque value using tools such as wrenches, drivers, or automated torquing systems. Proper torquing technique is absolutely critical for ensuring the safe, reliable, and optimal performance of the countless mechanical systems and machines that rely on threaded fasteners for structural integrity and functionality.

However, improper torquing through either over-torquing beyond the recommended specification or under-torquing by not reaching the minimum required torque value can lead to a number of failures with potentially catastrophic consequences. Understanding proper torque fundamentals and being diligent to apply appropriate torque is essential for engineers, technicians, and anyone working with threaded fasteners.

Understanding Torque

Torque is defined as the rotational force that is applied to a threaded fastener such as a bolt or nut to induce tightening or loosening. Torque has units of measure such as pound-feet, pound-inches, Newton-meters, or others. The required torque specifications for a given application depend on several factors:

The required torque specifications for a given application depend on several factors:

• Fastener Size – Diameter and thread dimensions affect the stresses and needed torque.
• Fastener Grade – Higher strength bolts and studs need more torque to induce clamping.
• Fastener Coatings – Lubricants or thread locking compounds can dramatically reduce needed installation torque.
• Fastener Material – Softer materials like aluminum need less torque versus hardened steel.
• Connected Materials – Dense materials like cast iron require higher torque versus softer plastics.
• Gasket or Seals – Compressing gaskets increases friction, raising needed torque.

Adhering to proper torque values ensures critical outcomes in an assembly:

• Uniform Load Distribution – Evenly spreads clamping forces to avoid stress concentrations. This prevents fatigue failures.
• Vibration Resistance – Maintains tight clamping to prevent loosening from cyclic loads.
• Sealing – Applies appropriate gasket compression force to maintain leak-proof joints.
• Alignment – Avoids distortion of clamped components that can bind moving parts.
• Safety – Ensures properly secured structural, electrical, or fluid connections.

Using calibrated torque wrenches for precision application of torque is vital, as is following torque specifications from manufacturers, engineers, or established standards for the particular application.

Torque calculations take into account several factors:

Torque (T) = Bolt clamp force (F) x Radius (R)

• F depends on bolt size, grade, and materials
• R is measured to the center of the bolt shank

Torque (T) = Coefficient of Friction (μ) x Clamp Force (F) x Bolt Diameter (D)

• μ depends on lubrication and surface finishes
• F is the target tensile bolt loading
• D is the nominal bolt diameter

Torque (T) = K x Clamp Force (F) x Bolt Diameter (D)

• K combines μ and other efficiency factors
• F and D must be consistent units (e.g. lbs, inches)

Did you Know

Proper lubrication technique is critical to achieve uniform clamping force in an assembly. Accounting for embedding through retorquing maintains long-term loading as materials relax.

Lubrication significantly affects torque:

• Dry threads have higher friction, requiring more torque
• Lubricants reduce friction, allowing lower torque
• Over-torquing dry bolts risks damage before reaching desired clamp force

Embedding occurs as clamped materials compress:

• Torque must increase to maintain target clamping force
• Retorquing is often needed to account for embedding settling
• Insufficient torque risks loosening over time as embedding relaxes clamp load

Consequences of Over-Torquing

Over-torquing refers to tightening a threaded fastener beyond the recommended maximum torque value. This introduces several possible modes of failure:

• Material Fatigue and Failure
• Excessive torque induces stresses beyond material limits, causing fractures in bolts or damage to internal threads that they are screwed into. Repeated cyclic stresses also accelerate crack formation, reducing component lifespan through fatigue failures.
• Stripped Threads
• The extreme forces of over-torquing can shear, strip, or deform the threads on the fastener and/or in the holes into which they are threaded. This often necessitates replacement of the damaged components.
• Compromised Seals
• Over-tightening gaskets, O-rings, or other sealed connections crushes, embeds, or warps the sealing surfaces. This creates leak paths destroying the tight seal integrity critical to many systems, especially hydraulics and pneumatics.
• Deformation of Components
• High torque forces clamped components like plates, brackets, or housings to bend, distort, or warp. This impacts alignment of mating parts and overall functionality of assemblies.

The effects of over-torquing failures can range from loss of product containment, imprecise equipment operation, to full structural collapse in critical connections.

Consequences of Under-Torquing

Under-torquing – due to insufficient torque also introduces significant risks:

Loose Connections – Inadequate torque leads to threaded fasteners loosening over time, especially under vibrational loads. This causes dangerous detachments of components, structural connections, or entire assemblies.

Uneven Load Distribution – Fasteners tightened below their target torque specification have uneven loading distributions with localized stress concentrations. This leads to premature wear, galling, or fatigue.

Compromised System Integrity – The combined effects of loosened structural connections, detached components, and leaks from poorly sealed joints leads to unacceptable degradation in performance, equipment malfunctions, and serious safety hazards.

• Detached Fasteners – Inadequate clamping allows fasteners to loosen under shock, vibration, or thermal cycling.
• Fluid Leaks – Improper gasket compression promotes leakage in pipe joints, pump housings, gearboxes, etc.
• Control Failures – Electrical connectors can short or open circuits. Hydraulic fittings can leak pressure.
• Accelerated Wear – Fretting and galling occur at interfaces between under-clamped components.
• Equipment Damage – Loosened fasteners can migrate and jam moving parts. Unsecured covers or guards can interfere.
• Hazardous Releases – Loose process piping or vessels result in leaks of hot, toxic or flammable chemicals.

The Importance of Proper Torquing Techniques

Achieving correct torque specs requires proper tools, training, and techniques:

• Torque Wrenches – Allow controlled, measured tightening. Should be regularly calibrated and suitable for application.
• Manufacturer Specs – Critical to precisely follow recommended torque values. Consider material grades, coatings, seals, etc.
• Fastener Preparation – Proper cleaning and application of lubricant improves consistency. Allow proper engagement into blind holes.
• Bolt Patterns – Systematic tightening in a star pattern evenly loads joints. Perform additional passes to account for joint settling.
• Training – Develop skilled, experienced technicians through hand-on practice and testing. Audit procedures periodically.

Tightening Patterns

Tightening bolts in the proper sequence is critical:

• Star Patterns – Tighten bolts in a crisscross star pattern to evenly load the joint. Apply torque in increments of 30%, 60%, 100%.
• Spiral Patterns – Sequence tightening in a spiral pattern from the center out. This also evenly loads the joint.
• Diagonal Patterns – Sequence bolts diagonally across a rectangular joint to provide uniform clamping.
• Circular Patterns – Tighten bolts in a circle pattern to load the joint radially in all directions.

Why Patterns Matter

• Prevent warping – Applying torque to one side causes uneven stress.
• Achieve uniform clamping – Maximizes strength by engaging all bolts.
• Account for settling – Joints settle after initial torquing, requiring re-torque.
• Allow controlled assembly – Progressively loads joint without overloading.

Pro Tips

• Mark bolts with paint pen to track tightening sequence.
• Follow multiple passes of 30%, 60%, 100% of torque spec.
• Use torque angle gauges to monitor degree of rotation.
• Check for gaps opening up indicating uneven loading.

Following proper torquing patterns is just as important as using the correct torque spec for optimal joint loading.

Application-Specific Torquing

The required torque levels vary significantly across applications based on desired outcomes:

• Automotive Wheel Lugs – High torque (100+ ft-lbs) ensures wheels stay attached even under hard braking.
• Aircraft Control Surfaces – Low torque levels (in.-lbs.) prevent airload induced bending of connected parts.
• Oilfield Pipe Joints – Thousands of ft-lbs. provide leak-proof seals up to 10,000 psi pressures.
• Load Bearing Structural Bolts – High bolt tension clamping requires hundreds of ft-lbs. of torque.
• Electronic Device Assembly – Precise torque control (only in.-oz.) prevents damage to fragile components.

Safety Begins with Proper Bolt Torque Technique

In summary, proper torque application through calibrated torque tools, manufacturer specifications, training, and robust fastening practices is absolutely vital for maintaining structural integrity, optimal equipment reliability and performance, and operational safety. Thorough inspection and vigilant preventative maintenance helps detect and correct under or over-torquing issues before they escalate to equipment failures with potentially dangerous and costly consequences. Understanding proper torque principles allows engineers and technicians to confidently design, analyze, and maintain equipment relying on threaded fasteners.

Notes

• Use torque equations to calculate target values
• TORQUE does not equal TENSION or CLAMP FORCE
• Follow all specifications for your application
• Use calibrated torque wrenches
• Washers or gaskets can compensate for embedding
• Account for lubrication effects
• Validate specifications through testing
• Retorque to account for embedding
• Train technicians on proper technique