Controlling the turn of the screw.
Today, the implementation of tightening strategies that ensure joint integrity and product reliability is emerging as a new priority. This is apparent in a host of new fasteners, adhesives, and tightening systems. Most recently, developments of interest have come from Lawrence Technological University (Southfield, Mich.), Loctite Corp. (Troy, Mich.), E/M Corp. (West Lafayette, Ind.), GSE Inc. (Farmington Hills, Mich.), Magni Group Inc. (Birmingham, Mich.), Phillips Screw Co. (Burlington, Mass.), Raymond Engineering Inc. (Middletown, Conn.), Technologies Inc. (Jenkintown, Pa.).
"People have been slowly waking up to the complexity of the bolted joint and its importance," said John Bickford, a consultant for Raymond Engineering. While detailed studies and instruction on other fastening methods such as welding are relatively extensive, he said, the bolted joint, which is just as ubiquitous, hasn't received the same attention.
But as manufacturers realize the importance of optimum clamping, several firms are developing and producing products that are aimed at maintaining the maximum possible control over tightening. Most of these products have found their biggest market niche in critical applications on engines, axles, and transmissions in the automotive industry, where joint failure can lead to costly repair. Probably the most frequent application involves engine cylinder head and connecting rod bolts.
The Big Three automakers in the United States and German manufacturers such as Mercedes-Benz and BMW are using electronically controlled assembly systems produced by SPS Technologies to tighten engine and transmission bolts. In addition, a consortium of Chrysler Corp. (Highland Park, Mich.), Ford Motor Co. (Dearborn, Mich.), GSE, andt Raymond Enginering is sponsoring a project at Lawrence Tech that aims to integrate ultrasonics into assembly systems for eventual use in automotive production plants.
The prevailing obstacle to achieving predictable results during tightening lies in the countless variable parameters that effect the clamping force in a joint. These include the materials used, geometry of joint and fastener, surface finishes, clearance, type of thread, burrs, heat treatment, and types of lubricants and platings. "One of the things that we've known about for a long time and had trouble controlling is the coefficient of friction involved in the fastener," Bickford said. To combat these undesirable variations, engineers often use products that produce consistent lubrication in bolted assemblies and provide increased control over the tightening process. An added benefit of many of these products is that they also increase the reliability of the bolted assembly.
In addition to providing consistent lubrication, the anaerobic and preapplied adhesives produced by Loctite offer adhesion, corrosion resistance, and sealing in a bolted joint. Solid film lubricants produced by E/M Corp. also help resist corrosion, galling, and seizing and can be used in high temperatures where greases and oils cannot be applied; zinc-rich coatings produced by Magni Group offer an alternative to other zinc, cadmium, and phosphate oil coatings and provide designed-in frictional properties, corrosion protection, and freedom from hydrogen embrittlement.
For commercial fastening applications, Phillips Screw is offering a variation on the Phillips head bit. The device is a proven technology in the aerospace and aircraft industries, and company officials claim higher insert and removal torque with less risk of recess failure and damage to the surrounding surface than other cross-recessed and straight-slotted driving bits. According to the company, the bit is ideal for production fastening of screws because it helps overcome the tendency of the driving bit and driven screw recess to separate during tightening. Precision Bolting
Realizing that controlling the tightening process is essential to maintaining the integrity of a bolted joint, mechanical engineers over the years have studied methods of determining the most accurate clamping load so that a joint can withstand external forces. "Too small a tightening force can cause leakage or loosening in a joint, and ultimately the parts can come apart," said Sayad Nassar, director of the Fastener Research Laboratory at Lawrence Tech. "Too much tightening can deform the joint so that it becomes excessively distorted. It can also cause cracks or failure of the bolt itself." The research has led to the development of tightening equipment that produces an accurate clamp load on a bolt.
The familiar bolted joint is subject to forces from piston acceleration and deceleration, combustion pressure, and shear forces produced by the flywheel. In an effort to achieve the optimum clamping load on a joint, automotive technicians typically employ three tightening strategies: tightening to a predetermined torque, a measurement of the turning force applied to the fastener; tightening to a fixed torque plus the angle that the socket has turned through; or tightening to the bolt's yield point, the point at which it undergoes a permanent elongation.
"Achievement of a specific torque alone has never been a guarantee that the right clamp load has been reached in a joint," said Paul Wallace, president of the Assembled Systems Division of SPS Technologies. "For example, increases in the friction of the threads would allow torque to build up without the corresponding clamp load being developed." Indeed, the three tightening modes produce varying levels of clamping load accuracy. According to Wallace, the torque-angle method typically has a minimum of [+ or -] 25 percent variation, the angle method [+ or -] 15 percent, and the yield method [+ or -] 8 percent.
Prior to 1980, almost all production assembly shops used pneumatic equipment for bolt tightening. As a result, Wallace noted, control of torque was highly variable and there was little feedback on whether the equipment was performing as intended. However, increased control capabilities accompanied the development of strain-gage sensors that gave precise electrical signals proportional to torque and angle sensors that measured socket rotation. Also, electronic control systems were developed that drove fast-acting pneumatic shut-off valves to stop the tightening tool.
"This range of measurement and control capability led to control algorithms based on the torque produced when a bolt is tightened," Wallace explained. One of the most useful algorithms developed, and one that SPS Technologies employs in most of its electronic tightening equipment, is based on the concept of bolt yield control with torque and angle monitoring. Engineers found that by tightening fasteners to their predetermined yield point, the most accurate maximum clamp load could be applied to the joint. In addition, the yield point, which is influenced by heat treatment of the fastener, could be designed into the fastener and used in the product manufacturing process for the first time. Electric Efficiency
From 1983 to 1988, major changes in engine and transmission assembly saw electric motor-driven tools exceed the popularity of pneumatic drives. "The initial reasons for changing to electric equipment included cleaner systems that did not emit an oil mist and led to a reduction in noise levels," Wallace said. "Studies showed that the reduction in energy consumption was considerable. This was due not only to the efficiencies of electric motors but also to persistent losses in air systems due to leaks and exhaust of air during idling of tools."
SPS Technologies has developed a range of electronically controlled assembly equipment that employ electric, pneumatic, or hydraulic tools for single- or multiple-spindle or robot-guided applications. Currently, about 80 percent of SPS Technologies' products shipped to automotive manufacturers are of the electrically driven type.
A typical electric SPS Joint Control System employs a computercontrolled combination of a motor, gearbox, and torque and angle transducers in a straight or offset drive configuration. The transducers, which use a strain gage for torque measurement and an optical angle sensor to measure the bolt angle of rotation, are mounted into the system behind each socket. Based on its torque, torque angle, and adaptive-yield-control modes, the system can tighten multiple bolts simultaneously or sequentially.
The adaptive-yield-control mode, which can tighten a bolt to its yield point, is derived from the torque and torque angle measurements and accommodates the effect of frictional conditions and joint stiffness variations, Wallace said. Controlled by the electronics and guided by signals from its sensors, the system's motor inserts and pretightens the required number of bolts to the snug position, shuts off, and then starts up again to simultaneously tighten the designated bolts.
SPS Technologies bas also developed a computer-controlled torque wrench for use with the Joint Control System that it claims offers up to a 50 percent increase in clamp load over conventional torque tightening techniques. Sensors, a microcomputer, and a liquid crystal digital display are built into the wrenches. Dubbed the Sensor 1, the handheld wrench can be operated in the same fashion as traditional torque wrenches. Using the Sensor I's computerized tightening modes, the operator can measure the angle of rotation of the socket, get a changing digital readout of the torque induced in the joint as tightening proceeds, pinpoint "breakaway torque" or the point at which the bolt begins to loosen when turned in a counterclockwise direction, or identify and tighten to the yield point of the bolt.
So far, the wrench has been used on low-strength gearbox casings, high-pressure compressor rotor assemblies for aircraft engines, engine cylinder heads to reduce gasket leakage, computer disk drives to maintain assembly alignment, transmission ring gears to compensate for low clamp loads caused by misalignment of bolt holes, and engine connecting-rod bolts to eliminate fatigue failures. The Ferrari Formula One Racing Division (Turin, Italy) uses the wrench for critical joints on its high-performance race cars. Ultrasonic integration
Research into incorporating ultrasonic control into automated assembly systems is currently under way at Lawrence Tech in a project led by Sayad Nassar. The effort is being supported by Chrysler, Ford, Raymond Engineering, and GSE. Nassar's group is developing a computer-controlled assembly system that would facilitate the simultaneous tightening of multiple fasteners during critical applications on an automotive production assembly line.
The ultrasonic system would employ sound waves to determine when the optimum bolt length has been reached during assembly, adding a fourth dimension to the other three choices of tightening to the torque, torque turn, or yield point. "Our computer-controlled instruments and equipment measure the load in the joint, the gasket pressure, the angle of torque of the bolt, the tension of the bolt, and the torque components in the joint," Nassar said.
Currently, the researchers are nearing the end of the first of three phases of development of the prototype. In the first phase, a single-spindle tool was developed that tightens one bolt at a time. Before the end of this year, Nassar's group hopes to complete the development of a multispindle tool that would simultaneously tighten up to 20 bolts. In the third and final phase, Nassar will undertake a feasibility study to determine whether the system can be implemented on an assembly line in an automotive plant.
In the single-spindle prototype, an ultrasonic system developed by Raymond Engineering relays ultrasonic signals from a transducer to a microprocessor. The transducer is mounted into a socket and sends sound waves via the head through the center of the bolt at a rate of 200 pulses per second during tightening. The wave reflects from the end of the bolt and sends a signal back to the microprocessor.
The computer measures the time that has elapsed between sending the wave and receiving it back. During tightening, as the bolt meets resistance from the joint, it begins to stretch. As the bolt stretches, the sound path through the bolt becomes longer and the time required for the sound waves to return to the microprocessor increases.
The relationship between the elongation of the bolt and the applied turning force is determined experimentally and the data are fed into the computer. When the bolt stretches to the required length, the microprocessor sends a signal to shut off the tightening tool that drives the socket. Each sound wave reflects the amount the bolt stretches, as tightening proceeds, to an accuracy of within .00001 inch.
However, the ultrasonic system alone cannot be relied upon to perform and monitor the tightening process. "What it can't do is find many of the practical problems encountered in automated production," said Bickford, who has helped develop the ultrasonic instruments used in the system. "For example, it doesn't recognize if the bolt is the wrong size, if it hasn't been hardened, if it has crossed threads, or if it is going in crooked and is jammed in the hole." Hence, the ultrasonic subsystem is hooked into a fourth channel on a torque-turn-tension microprocessor-based subsystem, developed by GSE, that tightens to torque, torque angle, or the yield point. There are several practical obstacles that have to be overcome, however, before the multispindle tool, which would perform the simultaneous tightening, is fully developed. Currently a couplant (a mixture of glycerin and water) must be added to the bolt head to facilitate the transmission of sound waves through the head. In addition, the raised markings on the bolt heads that indicate grade and tensile strength must be ground off. The researchers are working hard to develop methods of circumventing both of these procedures, which slow production and can affect the strength of the sound signals. Possible solutions lie in using indented markings on the bolts and an automated system to spray the couplant onto the bolt heads.
Perhaps the biggest challenge in developing the multi-spindle system, however, lies in multiplexing the ultrasonic signals to channel all of the individual transducers to a single microprocessor. Currently, each transducer requires its own ultrasonic processor. All told, the equipment needed for each spindle now costs $10,000 to $20,000, so multiplexing would be an important factor toward the goal of making the system economically feasible for use on the production assembly line. Fight Friction
In addition to controlling the tightening process, achieving consistent lubrication in a joint helps reduce friction variations. By controlling friction, engineers can increase the amount of torque transferred to clamping pressure during assembly. It is estimated that only 10 to 15 percent of the applied torque goes toward clamp load, with the remainder being used primarily to overcome bolt underhead and thread friction. Experts in the industry say that joint friction can differ by [+ or -] 25 percent, depending on such variables as thread tolerances, thread sizes and configurations, lubrication, and the existence of burrs and nicks.
In many critical threaded fastener applications, technicians apply chemical adhesives to fill the space between the male and female threads in a bolted assembly. Clearance between threads is cited as one of the main reasons why fasteners loosen when subjected to vibration and forces such as bending of the clamped assembly, differences in thermal expansion of the materials, and gasket relaxation. By adhering to the threaded surfaces and polymerizing, these thread-lockers reduce the potential for loosening and corrosion and provide lubrication and sealing in a bolted assembly.
Anaerobic adhesives made by Loctite are used extensively on power train bolt applications in the automotive industry such as on engines, axles, and transmissions. Other applications range from bolted assemblies in heavy construction equipment to military and aerospace systems. Chemical threadlockers are especially useful with electronic tightening systems that rely on accurate predictions of torque tension, said Janice Richards, account manager at Loctite's Preapplied Adhesives and Sealants Industrial Group.
Loctite's anaerobic adhesive is a methacrylate ester liquid resin packaged in a container in the presence of oxygen. Applied to a bolted assembly, the resin is deprived of oxygen and cures as it reacts with metal ions in the joint to form a solid polymethacrylate. The anaerobic adhesives come in different strengths for different bolt sizes and applications. One of the products, Drylock 201, is tailored to withstand temperatures as high as 400[deg.]F.
Loctite's preapplied concept is an extension of its basic anaerobic adhesive technology. Instead of applying the liquid resin to threads during an assembly operation, an engineer can install a fastener that already has a dry-to-touch antiseize adhesive on its threads. The preapplied coating employs a polyvinyl alcohol binder to encase the liquid resin, and the materials cure like the packaged adhesive during assembly. Loctite has estimated that preapplied compounds, which currently account for 10 percent of total use in thread-locking assemblies, will increase to 50 percent within 10 years. Consistent Lubrication
In addition to adhesives, fastener coatings affect lubricity, and engineers have available a variety of products that offer consistent torque-tension properties. Cadmium and zinc are among the more conventional coatings used in industry today, but a number of manufacturers are producing products that are designed to meet specific criteria such as reducing torque-tension variability, resisting harsh contaminants and high temperatures, and preventing galling, seizing, and corrosion.
E/M Corp. is producing a variety of bonded solid film lubricants that can be applied to a bolt by spraying, dipping, or brushing before installation. The lubricants consist of a blend of a binder matrix (such as an acrylic, epoxy, phenolic, or silicone) and minute particles of solid lubricating and rheological materials (such as molybdenum disulfide, graphite, or fluorocarbons) in a liquid carrier. After application, the carrier evaporates and the resin is cured by air-drying or oven-curing, binding the solid lubricating particles to the part's surface.
The lubricating particles prevent surface-to-surface contact in the joint, reducing friction and wear between mating parts. "Through selectively combining lubricating pigments in a variety of options we have come up with a coating that reduces torque-tension variability by 70 to 80 percent over zinc-, cadmium-, or plain oil-coated bolts," said William R. Frederick, automotive marketing manager at E/M Corp. "That allows the engineer to stay within an existing design but to increase clamp load by coating with our lubricant." E/M's solid film lubricants can be used in temperatures ranging from - 395[deg.] to 2000[deg.]F, offering applications in areas where greases and oils are ineffective.
Corrosion-resistant coatings that also offer consistent lubrication are produced by Magni Group and applied to bolts employed in automotive manufacturing by GM, Ford, and Chrysler. Two of the products, Dorrlflake, a silver zinc-rich coating, and Dorrlmate 400, a black zinc-rich coating, are designed to provide above-normal levels of corrosion protection and freedom from hydrogen embrittlement.
During the Dorrlflake coating process, the bolt undergoes a zinc phosphate treatment before being coated with a zinc-rich epoxy resin and an aluminum-tint organic topcoat. Dorrlmate 400 consists of an organic black zinc-rich film under a black pigmented phenoxy topcoat that is applied over a zinc phosphate-treated steel. According to Robert Keagy, a spokesman for Magni Group, the products have designed-in frictional coefficients in the coatings, which reduce torque-tension variability. "Our coatings provide consistent bandwidth," he said, "for the input torque required to achieve a minimum predetermined tension level on each joint." Space Screwdriver
Seeing a need to achieve higher levels of torque in tightening slotted-head cap screws, Phillips Screw has developed and patented the ACR (anticam-out ribs) Phillips driving device. Often mistaken for a Phillips bit because of its shape, the driving device is used extensively in the aircraft and aerospace industries, but it was introduced only two years ago for use in commercial applications.
According to Phillips Screw, the product offers the best available method of resisting cam-out during tightening, which is what happens when the driving bit overcomes the load that keeps it mating with the screw head and lifts up. The company also makes special screws that have a cross-recessed head specifically designed to interlock with the anticam-out driver. The anticam-out technology incorporates horizontal ribs on the cruciform walls of the driver that grab onto the recessed walls of a Phillips screw head when torque is applied. When used with ACR Phillips screws, the driving bit's ribs interlock with opposing ribs designed into the recesses. The system permits higher insert and removal torque compared to other cross-recessed or straight-slotted methods, according to John Souza, vice president of Phillips' commercial division.
Introduced to the aerospace market in 1978, the ACR Phillips system has been used by astronauts during the repair of critical components on spacecraft. In commercial applications, Digital Equipment Corp. (Maynard, Mass.) is using the system for assembly of its computer hardware, and Ford is reportedly using the system during automotive assembly. According to Souza, the most promising applications for the ACR Phillips system are in the automotive and construction industries, where it would be useful for applications requiring off-angle driving, high-torque screws, stick-fit between the bit and recess, insertion of vibration-resistant screws, fastening of plastic or composite materials, and tightening of gasketed doors and panels.
CRACKING DOWN ON COUNTERFEIT BOLTS
Reports earlier this year that Trans World Airlines and United Airlines were purging their passenger jet fleets of allegedly substandard bolts are symptomatic of what may be a much larger problem for the fastener industry.
Of an estimated 200 billion fasteners used each year in the United States, millions of them have been reported substandard or counterfeit, following an 18-month investigation begun in 1986, which was conducted by a team of several federal agencies, including the U.S. Customs Service, the Defense Criminal Investigative Service, and the Naval Investigative Service. These faulty fasteners have shown up in helicopters, trucks, buses, buildings, airline terminals, highway systems, nuclear power plants, and aerospace and military equipment. The products have come mainly from foreign manufacturers who collaborated with American importers to market substandard fasteners at reduced costs, according to federal documents.
In response to the wave of shady dealings, Congress has passed the Fastener Quality Act and ASME has initiated the Fastener Accreditation Program. Both aim to stem the flow of counterfeits in industry, ensuring that fasteners can be traced to their manufacturer and that they are made to specifications.
Indeed, as part of the Act, the Secretary of Commerce will approve fastener accreditation laboratories to monitor and test firms' manufacturing procedures and quality control of fasteners. As a result, shipments of fasteners will be accompanied by documents that show the products have been made to specifications and tested by an accredited laboratory.
The Fastener Advisory Committee has been established to offer advice on the implementation of the law to the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. NIST reports to the Secretary of Commerce.
The 16-member committee, made up of representatives from fastener manufacturers, standards organizations, importers, distributors, and users, has designated responsibilities to several task force groups, which are studying different aspects of the law. Research issues include procedures for fastener sampling, the minimal requirements for testing, methods of marking fasteners, procedures for dealing with noncomplying fasteners, and procedures for fasteners that are manufactured to proprietary specifications, Through NIST, the Secretary of Commerce is also charged with approving foreign fastener accreditation organizations. Implementation of the law will likely begin toward the end of 1992, according to NIST. Voluntary accreditation. In May of this year, ASME introduced the voluntary Fastener Accreditation Program. Under the program, ASME will issue a certificate to a manufacturer or distributor that establishes its own program in conformance with the ASME standard, "Quality Assurance Program: Requirements for Fastener Manufacturers and Distributors." The standard outlines the requirements for engineering drawings, tooling, production planning and lot control, procurement, raw material control and traceability, personnel training, and other elements of quality control:
To obtain accreditation, companies must submit written documentation of their quality-control practices and undergo an on-site evaluation by an ASME survey team. A certificate of accreditation generally covers a three-year period during which ASME monitors the approved company via on-site inspections. Bogus bolts. In one frequently recurring counterfeit example, over 30 million SAE Grade 8 fasteners were found in the inventories of the Defense Industrial Supply Center. The U.S. military has over 400 weapons systems that use these high-strength heat-treated bolts. In some cases, low carbon boron steel Grade 8.2 bolts were mismarked and sold as medium carbon alloy steel Grade 8 products. In addition, about 80 percent of the fasteners were coated with zinc instead of cadmium. More expensive than zinc, cadmium offers more resistance to corrosion and requires less torque during installation. Hence, torque-tension calculations that are created for cadmium-plated bolts do not apply to zinc platings, and will result in looser joints if misused, making the bolts more vulnerable to failure.
Other manufacturing flaws in the counterfeit scam revealed wavy threads that increase stresses in joints and false markings that lead to the belief that bolts were heat-treated when they were not. For Further information
For information on ASME's Fastener Accreditation Program contact Sandra Bridgers, Manager of Accreditations, ASME, 345 East 47th Street, New York, NY 10017-2392; (212) 705-7442.
For information on ASME's fastener standard FAP-1, 1990, Quality Assurance Program: Requirements for Fastener Manufacturers and Distributors," contact ASME Service Center, 22 Law Drive, Box 2900, Fairfield, NJ 10770-2900; (800) 843-2763.
The following publications are also of interest: * "The Threat from Substandard Fasteners: Is America Losing its Grip," Federal Subcommittee on Oversight and Investigations of the Committee on Energy and Commerce, U.S. Government Printing Office, Washington D.C. * "Fastener Application Advisory," Industrial Fasteners Institute, Cleveland, Ohio.
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|Date:||Sep 1, 1991|
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