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Eight Steps to Precision Torque Control Of Fasteners

Wed, 12/11/2002 - 11:32am
Gordon Wall, Engineer Manager, Mountz, Inc.

Torque is defined as the force causing rotation or torsion in machinery. In the manufacturing and assembly world, tightening, controlling, or measuring torque on fasteners is imperative for production efficiency. An inadequately torqued fastener can vibrate or work loose. Conversely, if tension is too high, the fastener can snap or strip its threads. Most manufacturers realize that precise torque control can spell the difference between a safe, reliable, and economical product and complete disaster.

The following 10 torque-control tips can help streamline any production process:

1. Determine torque requirements

When determining correct torque specifications, the engineer must consider the maximum load placed on the fastener, the strength of the material joined, and whether the joint is hard or soft. A hard joint connects materials directly. In this case, the fastener rotates very few degrees to develop full clamping force after it encounters the material. Since a soft joint contains a gasket or involves compressible materials, it requires additional tightening after the fastener makes contact, to achieve full clamping force. One recognized method is to perform a destructive test with a calibrated torque-control tool on the actual material and fastener to be joined. An evaluation is 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, then repeated several times to verify the consistency of the failure point. Then another series of tests is begun where the joint is torqued to 75% of the failure point. Depending on how the parts will be used, the tightening can be reduced by any degree necessary. If parts on a machine are subject to heavy vibration, a high percentage of the total force might be necessary for good torque control.

2. Choose the right torque tool

A wide variety of tools are available to control and measure torque applied to fasteners, from electric screwdrivers to large industrial wrenches, analyzers, sensors and multipliers. These tools utilize calibrated torque setting mechanisms that may be factory pre-set or user-definable. When the specified setting is reached, the tool gives a visual, audible or tactile signal.

The selection of tools for a given application is determined by the anticipated production output, the type of materials being joined, the amount of torque required and the specified fasteners. Lighter materials such as wood or plastic may require only lightweight tools. Heavy materials such as steel may require stronger or larger tools. Tools should have connection ports for an RS-232 PC cable if torque data must be gathered electronically.

3. Use torque analyzers

Use of a torque analyzer is a fast and reliable method of calibrating torque tools to their proper settings. Analyzers can also be used for quick tests on the line or in the lab to determine whether torque tools are holding a given setting. They also allow quality-control inspectors 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. Stress teamwork

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 control the process. To avoid problems, consult everyone whenever changes relating to the use or type of fasteners are instituted.

5. Train employees

Professional torque-tool suppliers often offer personnel training sessions and workshops. Topics to cover include basic torque theory, types of available tools, how to operate specific tools, preventive maintenance, safety concerns, and job-related ergonomics.

6. Stress safety

Worker fatigue and potential injuries can be avoided with safety programs and high-quality tools. In critical applications where safety is an issue, the proper use of tools can decrease the incidence of expensive lawsuits and product recalls. 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 before injuries occur.

Reducing worker fatigue must also be considered as a means to achieve production-line consistency and reduce repetitive-motion injuries. Torque-control tools are available that improve ergonomics and reduce the effort required for consistent tightening.

7. Establish a calibration program

Calibration of the torque-control process should be checked periodically to determine that torque tools are operating at their proper settings. Many tools don't have a locking device, and torque settings may be easily changed by users.

A regularly scheduled calibration program enables quality-control personnel to correct divergence from proper settings, whether due to normal tool slippage over time or adjustments to the tool. Begin by setting a calibration interval initially based on the severity of the application and the tool manufacturer's recommendations. If the applied torque values are out of range, cut the calibration interval in half and re-test the tools.

8. Use preventive maintenance procedures

To maintain accuracy, torque tools must be checked periodically for wear or defective parts. A properly structured preventive-maintenance program optimizes tool performance and reduces unexpected downtime. Monitoring the number of cycles per day and total hours that a tool is used is the most accurate way to establish proper maintenance intervals. It is recommended that tools be serviced after 100,000 cycles or if an inspection reveals old or dry grease, parts that show signs of excessive wear, or loose screws or bolts.

Precise control of torque is a key to quality assembly and can ensure that products perform as expected. In many cases, companies spend a great deal of time and money for disposal or repair of damaged parts during assembly, as the result of improper torquing. Also, look for torque-control tools that enable quicker, more efficient assembly; give quality-control inspectors more time to check parts; and will reduce assembly errors. Use of tools that provide all of these qualities will result in time and money savings.

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