Joint Integrity Technology for Threaded Fasteners

By: Brian Reuter

A critical element in the manufacturing of many products is the fastening of parts or subassemblies using threaded fasteners. A total systems approach is required to carefully plan every step so that the reliability that exists on paper can be implemented at assembly. The manufacturer's objective is to maximize the long term joint integrity while minimizing the product cost.

The standard technique for controlling the clamp force in a threaded joint is torque. However, it is known that torque is a poor indicator of the clamping load. Recently, the automotive industry has begun using other methods, such as torque/turn and yield tightening, in an attempt to improve the reliability of their products' critical fastening applications. Since the ideal method is some form of direct load control, various organizations have been working on developments in this area.

The key to successful fastening is to obtain sufficient clamping force so that each joint survives the field service environment. In order to do this, the clamp force at assembly must be great enough to compensate for the relaxation and other load effects which modify the initial clamp force. The importance of the bolt preload (or clamp force in the joint) can not be overemphasized. It is the major factor influencing the reliability of the bolted joint. It is usually the root cause of joint failure, resulting in joint separation, leakage, loosening or fatigue.

Without proper clamping forces, distortions of bearing bores, cylinder heads, and gasketed joints may occur. Fixing these problems is less costly if they are caught before the assembled product leaves the factory. This is why the automobile industry expends a lot of effort to make sure defective joints are identified and corrected at each assembly station.

An ultrasonic control technology originally developed by Ultrafast has made direct control of bolt load practical and cost effective for production processes. This technology produces a piezo-electric thin film on the fastener to create an ultrasonic load transducer. The film is permanently applied by a sputtering vacuum process. This is a proven process used to deposit coatings for many commercial applications. Bolts for high volume applications can be coated at a cost of just pennies per bolt. Thus each fastener functions as a transducer in the assembly.

There are a number of patents involving tools, methods, and devices utilizing torque and angle of rotation of the fastener during tightening. Inventors are still finding ways to utilize the angle of rotation to solve fastening problems or improve control processes. A broad spectrum of companies are active in this area. Automotive companies, equipment companies and many others have contributed.

There has been a great amount of activity in the ultrasonic measurement and control technology field. Most of this activity centered around the use of piezo-electric films being applied to the fastener for the purpose of controlling the tightening of fasteners.

The main focus of the tool companies seems to be improving the torque control shutoff mechanism for various types of tools. The tightening process patents which they issued involved ways to compensate for gasket relaxation, tool overshoot (tool inertia), and joint stiffness (torque rate) variations to improve torque or angle control accuracy. Additional efforts are being spent on impulse tool refinements.

In the near future, direct load control will be utilized and will result in the following:

-100% quality assurance of clamp load at assembly
-Reduced labor costs due to reduced reliance on tool testing and calibration
-Use of less expensive tools
-Optimum joint designs and process reliability
-Reduced requirements for torque-tension testing of fasteners
-Improved gasketed joint assembly methods

Overall, there are several forces shaping the future of the clamp load control markets:

Better bolt load accuracy will be achieved in production assembly processes. Torque accuracy below the ? 5% level will be of lesser importance in assembly areas. Tool operator safety and injury prevention will continue as a major influence on tools
Productivity and cost reduction pressures will continue to increase due to world competition.

It is Guideline's view that torque control developments will no longer be sought after. This area has reached the point where improvements in clamp load control are not possible because the main controlling factor is friction. The law of diminishing returns has taken over in the torque control area. Both yield control and torque/angle applications have reached a plateau. Direct load control via bolt ultrasonic measurements will be emerging as the next trend.

It is Guideline's view that the ultrasonic control technology, such as that developed by Ultrafast, will make major changes in the assembly tool business and the manufacturing processes. Because these changes involve many different facets, viz., fastener suppliers, tool suppliers and manufacturing processes, it will take a somewhat longer lead time to implement applications, initially. The physical requirements are not difficult to accomplish, but because many areas are affected, the change-over will require coordination of many groups. Change is not easy but the benefits of this technology will outweigh any difficulties.

Our forecast indicates that the assembly business will be revolutionized as ultrasonic control technology starts to displace the torque control methods in major automotive companies around the world. This will change the competitive focus of the tool companies, just as the electric driven tools have changed the business over previous years.

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