Tips For Torquing Fasteners

May 18, 2000
In the manufacturing and assembly world, tightening, controlling, and measuring torque on fasteners is imperative for production efficiency.

Troy Mountz
Customer Service Technician
Mountz Inc.
San Jose, Calif.

Using the proper amount of torque to attach fasteners is crucial to the fastener's performance.

Inadequately torqued fasteners can vibrate or work loose. Conversely, if tension is too high, fasteners can snap or strip their threads. Faced with these problems, manufacturers are realizing that precise torque control spells the difference between safe, reliable, and economical products and complete disaster.

Precise control of torque is also key to good assembly. It can help ensure products perform as expected. In many cases, companies spend a great deal of time and money for scrap or re-pair of parts damaged during assembly by improper torquing. Worse yet, if these products make it to market, manufacturers face customer dissatisfaction if the product falls apart due to loose screws or stripped threads.

The following nine tips for precision torque control will help engineers and OEMs streamline the design and production processes:

1. Determine torque requirements. When determining correct torque specifications, engineers must consider the maximum load placed on the fastener, the strength of the materials being joined, and whether the joint is hard or soft. Hard joints connect materials directly and fasteners used in them rotate a few degrees to develop full clamping forces after they encounter the material. Soft joints contain a gasket or involve compressible materials and require additional tightening after fasteners make contact to achieve full clamping force.

One recognized method of determining torque requirements is to destructively test the actual fastener and material being joined. Evaluations are usually conducted with 10 parts, 10 fasteners, and a calibrated torque-control tool with a transducer. First, the fastener is tightened to the point of failure. This is then repeated several times to verify the consistency of the failure point. Another series of tests uses torque to 75% of the failure point. Depending on how parts will be used, the tightening can be reduced by any degree necessary. For example, if parts will experience heavy vibrations, 85% of the total force may be necessary for good torque control.

2. Pick the right tool. A wide variety of tools are available to control and measure torque applied to fasteners. They run the gamut from electric screwdrivers to large industrial wrenches and include analyzers, sensors, and multipliers. These tools use calibrated torque-setting mechanisms that may be factory set or user definable. After reaching specified settings, the tool gives a visual, audible, or tactile signal.

Choosing a tool for a given application is based on the anticipated production output, type of materials being joined, amount of torque required, and specified fasteners. Lighter materials such as wood or plastic may require only lightweight tools. Similarly, heavy materials such as steel may require stronger or larger tools. Tools should also have connection ports for an RS-232 PC cable if torque data is to be gathered electronically.

3. Use torque analyzers. Torque analyzers can quickly and reliably calibrate torque tools to their proper settings. They can also be used for quick tests on the line or in the lab to determine whether torque tools are holding a given setting. Quality-control inspectors can use them to calibrate torque sensors and verify torque on fasteners. A quality torque analyzer should have enough memory to record several hundred readings, and it should store calibration data for multiple torque sensors.

4. Cooperation is necessary. Orchestrating a successful torque program requires extensive teamwork in all production-related departments to assure consistent adherence to torque specifications. Production planners, supervisors, engineers, quality-control technicians, and assemblers must work together to efficiently control the process. To avoid problems, consult everyone when changes are made to the use or type of fasteners.

5. Train the workforce. Professional torque-tool suppliers often conduct personnel training sessions and workshops. They should cover basic torque theory, types of tools available, how to operate specific tools, preventive maintenance, safety concerns, and job-related ergonomics.

6. Keep everyone safe. Safety programs and high-quality tools help prevent worker fatigue and injuries. They can also decrease the incidence of expensive lawsuits and product recalls. And reducing worker fatigue will help keep production consistent and cut the number of repetitive motion injuries

To avoid accidents, tools and the work area should be inspected regularly. Worn components should be replaced and unsafe conditions on the assembly line should be rectified as soon as possible. Torque-control tools which improve ergonomics and reduce the effort required for consistent tightening are available

7. Establish a calibration program. Calibration fine-tunes the torque-control process in production environments. Calibration should be checked periodically to determine whether torque tools are operating at their proper settings. Many tools lack a locking device, and their torque settings may be easily changed by users. When this happens, tools fall out of adjustment.

Regularly scheduled calibration programs let quality-control personnel correct settings, whether they're because of normal tool slippage over time or because of adjustments to the tool. Begin by setting calibration intervals initially based on the severity of the application and tool manufacturers' recommendations. If applied torque values are out of range, cut the calibration interval in half and retest the tools.

8. Conduct preventive maintenance. Torque tools must be checked periodically for wear and defective parts if they're to maintain consistent accuracy. Properly structured preventive maintenance programs optimize tool performance and reduce unexpected down-time, thereby saving time and money.

Monitoring the number of cycles per day and total hours a tool is used is the most accurate way to establish proper maintenance intervals. Tools should be serviced after 100,000 cycles, or if an inspection reveals old or dry grease, parts with signs of excessive wear, or loose screws or bolts.

9. Take advantage of supplier knowhow. Tool suppliers should offer recommendations and answers to manufacturers' torque-control challenges. They can answer questions such as: Will their tools let assemblers build faster and with less wasted motion? Will the tools give quality-control inspectors more time to check parts thoroughly? Will those tools reduce errors in the assembly process? And, will they save money and time for the company overall?

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