Torque is a measurement of rotational force. It refers to how tightly fasteners, such as bolts, nuts, and screws, should be tightened during assembly.
Following proper torque procedures is extremely important for creating safe, reliable, and long-lasting machine and product assemblies.
When hardware is not torqued correctly, it can lead to a number of problems. Overtorquing stretches and damages bolts and screws, compromising their strength.
Undertorquing leads to loose fasteners that won’t hold parts together properly, causing components to shift and rattle apart over time and through machine operation.
Using the right amount of torque ensures an assembly has the correct clamping force for its fasteners to function as designed.
This allows machines and products to operate smoothly without critical components coming loose or failing prematurely.
Proper torque procedures require using calibrated torque instruments, following specified tightening sequences, and adhering to torque specifications provided by manufacturers and engineers.
This article will explore the key aspects of torque technology and provide essential guidance for achieving correct and consistent bolt and screw tightening.
Measuring Torque
Torque is a measurement of rotational force. It indicates how much force is being applied when tightening or loosening a fastener.
The most common tool used to measure and apply a specific torque is a torque wrench.
Torque wrenches come in various styles but all operate on the same basic principle.
There is an internal spring-loaded mechanism that is calibrated to the desired torque setting.
As the fastener is tightened, it puts tension on the spring.
When the preset torque value is reached, the wrench will make a clicking noise and/or the head will move slightly to indicate the target torque has been achieved.
This allows the operator to tighten the fastener to the exact needed torque spec.
Most torque wrenches require calibration to ensure accuracy.
A torque wrench test bench is used to confirm the wrench is applying the correct amount of torque at all points along its range.
Torque wrenches should be recalibrated periodically according to manufacturer recommendations, usually annually.
Properly calibrated torque wrenches are essential for achieving proper clamping force on fasteners in many applications.
Tightening Sequence
Following the proper tightening sequence is critical when torquing bolts.
The sequence ensures the part is evenly clamped and reduces the chances of warping or distorting the part.
Most engine manufacturers specify a tightening sequence that starts from the center and spirals outward.
This allows the clamping force to be evenly distributed across the entire mating surface.
Some key tips on bolt tightening sequence:
- Always follow the sequence provided by the manufacturer. Don’t take shortcuts or make up your own sequence.
- If no sequence is provided, start from the center and work your way outward in a spiral or criss-cross pattern.
- For cylinder heads, start from the middle and move outward and alternate between sides as you progress.
- With multiple bolts in a four or six-sided hole pattern, progress diagonally across the pattern.
- Do not tighten one side fully before moving to the other. Go back and forth between sides to evenly distribute clamping force.
- Complete the recommended number of passes in the sequence. Usually 3-4 passes brings the bolts to the final torque spec.
Skipping tightening sequence can overstress one area of the part and lead to cracked mounting surfaces or warped parts.
Always take the extra time to follow the proper sequence for even clamping and optimal performance.
Bolt Lubrication
Lubricating bolts is an important step that should not be overlooked when tightening bolts to their proper torque specification.
There are several key reasons why lubricating bolts is necessary:
- Reduces friction and tension in the bolt. Without lubricant, the tension in the bolt would be higher than intended for the target torque value. This puts unnecessary stress on the bolt.
- Prevents galling or seizing. The friction of an unlubricated bolt turning against the threads of the hole can lead to galling, which is a form of surface damage. This can cause the bolt to get stuck.
- Achieves more consistent clamp load. Lubrication results in less scatter in tension among a series of bolts tightened to the same torque spec.
- Allows for more accurate torque. Lubricant reduces friction to enable a more precise measurement of tension based on torque.
- Extends bolt fatigue life. By reducing friction, lubrication minimizes wear and helps prevent crack initiation and propagation.
The most common types of bolt lubricants are:
- Oil-based lubricants – These leave a film of oil on the threads to reduce friction. Common options are motor oil, gear oil, or penetrating oils.
- Wax-based lubricants – These include wax-based sticks that are rubbed directly onto the threads. The wax reduces friction between the threads.
- Anti-seize compounds – These lubricants also act as a protectant by isolating the threads from exposure to rust and corrosion. Common anti-seize lubes contain nickel or copper.
- Graphite – This dry lubricant can be used alone or added to oil or grease to enhance lubricating properties. The graphite leaves a solid film that minimizes rubbing.
Proper bolt lubrication is crucial prior to tightening to ensure the target torque results in the correct tensile stress for clamping while avoiding damage to the threads.
Using the right amount and type of lubricant allows for consistently accurate and reliable torque procedures.
Torque Specifications
Torque specifications provide the proper tightening force for fasteners during assembly.
Following the manufacturer’s torque specs helps ensure parts are tightened correctly for optimal performance and safety.
Torque specs are commonly found in:
- Owner’s manuals
- Service manuals
- Parts catalogs
- On the component itself
- Online spec sheets and databases
For many standard fasteners, typical torque values are:
| Fastener | Typical Torque Range |
|---|---|
| Grade 2 or 5 bolt | 8-12 ft-lbs |
| Grade 8 bolt | 12-16 ft-lbs |
| Aluminum bolt | 8-12 ft-lbs |
| Steel bolt | 15-30 ft-lbs |
| Wheel lug nut | 70-120 ft-lbs |
| Head bolts | 55-75 ft-lbs |
However, always defer to the manufacturer’s torque specifications for a particular vehicle or component when available.
Do not rely solely on standard torque tables.
Over or under-tightening fasteners can lead to part damage or failure.
Following the recommended torque procedure helps optimize clamping force for the application.
Overtorquing: The Dangers of Overtightening
Overtorquing is when a fastener is tightened beyond the correct torque specification.
This can happen when technicians believe that tighter is always better, but overtorquing can cause severe damage.
When a fastener is overtorqued, it stretches beyond its elastic range which leads to permanent deformation.
This can result in a loss of clamping force over time as the material relaxes.
Overtorquing fasteners like bolts leads to tensile failure, while overtorquing cap screws causes them to shear.
Excessive torque distorts flanges and joints, warping and cracking them over time.
It crushes gaskets, reducing their sealing effectiveness.
Overtorquing also leads to damage of product components, causing cracks or fractures in the housing material.
The economic impact of overtorquing can be substantial as well.
When fasteners fail prematurely due to being overtorqued, it leads to unplanned system downtime and costly repairs.
The damaged components then need to be replaced ahead of schedule.
Proper torque control requires diligence, care, and precise tooling.
Technicians should use a calibrated torque wrench and follow manufacturer torque specifications exactly.
Overtorquing may seem harmless in the short-term but can lead to system failures, accidents, and substantial costs if left unchecked.
Implementing a comprehensive torque auditing system helps ensure torque accuracy.
Undertorquing
Undertightening fasteners is extremely dangerous and can lead to a variety of issues.
When a fastener is not tightened to the proper torque specification, it can vibrate loose over time.
This can cause components to shift and move out of alignment, damage fastener threads, or even result in complete fastener failure.
Some of the potential dangers and issues caused by undertorquing include:
- Loose components resulting in rattling, squeaking, and other noises
- Leaks at gaskets and seals
- Reduced clamping force leading to slippage or movement between components
- Fatigue failure as forces are concentrated on a small number of threads
- Shearing or snapping of fasteners due to forces being applied when undertorqued
- Unsecured rotating components resulting in imbalance or catastrophic failure at high speeds
- Accelerated wear of components, fasteners, and joints
- Inability of fastener to handle expected loads and forces
- Electrical arcing due to loose connectors
- Compromised safety from failing safety components
Proper torque is essential for load distribution, resistance to external forces, vibration damping, and securing joints.
Undertorqued fasteners cannot achieve this, severely compromising the integrity of an assembly.
Preventing undertorque failure requires carefully following OEM torque specifications, using properly calibrated torque tools, following the recommended tightening sequence, and re-checking torque values after initial tightening.
Undertorquing may seem harmless, but can quickly lead to catastrophic and dangerous failures.
Torque Angle
Torque angle refers to the degrees of rotation after the fastener has been initially tightened to a specified torque value.
It provides a more consistent measurement of tightness than relying on torque alone.
As a fastener is tightened, the tension builds up in the bolt and the components being fastened.
This causes the fastener to stretch like a spring. The torque angle represents how much the fastener yielded or stretched during final tightening.
Measuring torque angle requires tightening the fastener to a specified torque target, then further tightening it a specified number of degrees or fraction of a turn.
This ensures the fastener stretches the proper amount to create the desired clamping force.
Torque angle is especially useful for critical joints or gasketed joints.
It compensates for factors like surface finish irregularities, variability in lubrication, and differences in grip length.
Using a torque angle procedure prevents under or overtightening the fastener.
The proper torque angle spec depends on the fastener type, size, thread pitch, and materials involved.
Many torque angle values range between 30 to 180 degrees. Specifications are typically provided by the manufacturer based on standards and testing.
Properly measuring torque angle requires the use of a torque angle meter or torque wrench equipped with an angle gauge.
The mechanic tightens to the torque target, then further rotates the fastener until the angle gauge indicates the specified torque angle.
Overall, torque angle provides a more reliable installation procedure than torque alone. It ensures bolts are stretched to the optimal level during tightening.
Understanding proper torque angle procedures leads to improved joint integrity.
Torque Audit
A torque audit is a critical process that ensures proper tightening procedures have been followed, validating that fasteners meet the correct torque requirements per manufacturer specifications.
Auditing torque is an essential quality control measure that prevents product defects or failures that could occur from incorrect torque levels.
The importance of a torque audit cannot be understated. Incorrect torque is one of the leading causes of fastening failures.
Overtorquing or undertorquing fasteners can lead to a variety of problems such as broken bolts, damaged threads, cracked components, loose connections, or leaks.
By verifying that proper torque procedures were implemented, manufacturers can avoid unsafe products, recalls, or warranty issues.
The torque testing process typically involves selecting a random sample of fasteners from a production batch.
The torque is then measured using a calibrated torque wrench or testing system. The results are compared to the target torque value to identify any outliers.
If certain fasteners show a torque variance outside the acceptable range, the findings can be used to adjust the tightening process.
Auditors should test an adequate sample size to ensure statistical confidence in the results.
The sampling may focus on critical applications, safety components, or areas prone to torque errors.
Comprehensive audits will examine the full torque process, including tool calibration, operator training, and torque sequence.
By identifying where deviations occur, corrective actions can be taken to avoid torque mistakes going forward.
In summary, regular torque auditing provides quality assurance and prevents creating products with serious torque defects.
Verifying proper tightening procedures through testing and analysis is vital for safety and performance.
A robust torque audit program demonstrates a commitment to manufacturing excellence.
Conclusion
Proper torque procedures are absolutely critical in ensuring that bolted joints are properly fastened.
By adhering to torque specifications, using the correct tightening sequence, properly lubricating threads, and avoiding under or overtorquing, mechanics can avoid a host of problems including joint failure, bolt breakage, stripped threads, gaskets leaks, and more.
While torque specifications are readily available from OEMs and repair manuals, simply having the specs is useless if proper techniques are not followed.
Torque audits should be performed regularly to confirm work is being done to specification.
Though it may seem tedious at first, making torque procedures a habit will ensure joints stay together and minimize expensive repairs down the road.
Proper tightening truly pays off when it prevents dangerous and catastrophic joint failures.
By reviewing torque fundamentals, understanding the consequences, and sticking to OEM procedures, technicians can make safety and quality their top priorities.
