Talking torque
Ten easy tips to achieving precision torque control of fasteners

Torque: a) The moment of a force, a measure of its tendency to produce torsion and rotation about an axis.  b) Broadly, a turning or twisting force.

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.  But, if tension is too high, the fastener can snap or strip its threads.

Faced with these problems, manufacturers are realizing that precise torque control can spell the difference between a safe, reliable and economical product and complete disaster.

The following 10 tips for achieving precision torque control give your company vital information needed to streamline its production processes:

Tip 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, tighten the fastener to the point of failure, then repeat several times to verify the consistency of the failure point.  Then, begin another series of tests where the joint is torqued to 75 percent of the failure point.  Depending on how the parts will be used, tightening can be reduced by any degree necessary.  If you subject parts on a machine to heavy vibration, perhaps 85 percent of the total force is necessary for good torque control.

Tip 2: Pick the right torque tool
A wide variety of tools are available to control and measure the torque applied to fasteners, from electric screwdrivers to large industrial wrenches, analyzers, sensors, and multipliers.  These tools utilize calibrated torque setting mechanisms that are factory pre-set or user-definable.  When you reach a specified setting, the tool gives a visual, audible or tactile signal.

Determine tool selection for a given application by the anticipated production output, the type of materials being joined, the amount of torque required and the specified fasteners.   Lighter materials (i.e. wood or plastic) may require only lightweight tools; likewise, heavy materials (steel) may require stronger or larger tools.  Tools should also have connection ports for an RS-232 PC cable if you intend to electronically gather torque data.

Tip 3: Use torque analyzers
Effective use of a torque analyzer is a fast and reliable method of calibrating torque tools to their proper settings.  Also use analyzers for quick tests on the line or in the lab to determine whether torque tools hold 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.

Tip 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  whenever instituting changes relating to the use or type of fasteners.

Tip 5: Employee training
Professional torque tool suppliers often provide personnel training sessions and workshops.  Topics include basic torque theory, types of tools available, how to operate specific tools, preventive maintenance, safety concerns and job-related ergonomics.

Tip 6: Employee safety
Avoid worker fatigue and potential injuries with safety programs and high-quality tools.   In critical applications where safety is an issue, proper tool usage decreases the incidence of expensive lawsuits and product recalls.

To avoid accidents, regularly inspect tools and the work area.  Replace worn components and address unsafe conditions on the assembly line before injuries occur.   Also, reduce worker fatigue by achieving production line consistency and reducing repetitive motions.  Torque control tools are available which improve ergonomics and reduce the effort required for consistent tightening.

Tip 7: Establish a calibration program
Calibration is fine-tuning the torque control process in a production environment.   Check calibration periodically to determine whether torque tools operate at their proper settings.  Many tools don't have a locking device, and their torque settings may be easily changed by users.  When this happens, the tool falls out of adjustment.

A regularly scheduled calibration program enables quality control personnel to correct divergence from proper settings, whether it's from normal tool slippage over time or from 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.

Tip 8: Preventive maintenance
To maintain consistent accuracy, periodically check torque tools for wear or defective parts.  A properly structured preventive maintenance program optimizes tool performance and reduces unexpected downtime, thereby saving time and money.

Monitoring the number of cycles per day and total hours that the tool is in use is the most accurate way to establish proper maintenance intervals.  A recommendation is to service tools 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.

Tip 9: Torque control increases quality control
The 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, the result of improper torquing.  Worse yet, even if these products make it to market, manufacturers are faced with customer dissatisfaction if they fall apart due to loose screws or stripped threads.

Tip 10: Increase return on investment
Tool suppliers offer recommendations and answers to manufacturers' torque control challenges.

Will their tools enable assemblers to build quicker and with less wasted motion?   Will they give quality control inspectors more time to check parts thoroughly?   Will they reduce errors in the assembly process?  And, will they save money and time for the company overall?

Information compiled by Troy Mountz and Darren Figg of Mountz Inc., a provider of high-quality torque control tools.

This article appeared in the December 1999/January 2000 issue of MRO Today magazine.  Copyright, 2000.

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