Fasteners are an important element of any assembly. About 70 percent of all joints are connected with mechanical fasteners. However, choosing the correct fastener, from more than 500,000 types of metal and plastic fasteners, can be overwhelming. And the decisions don't stop there. Engineers also must decide how much torque to apply to the fastener; how to feed and install the fastener; and how to ensure that the assembly has been fastened correctly. The following case studies demonstrate how some manufacturers overcame challenges when they decided to go the fastener route when assembling their products.
Plastic Fasteners Offer Design Flexibility
According to the American Plastics Council, fasteners are not only critical to optimizing product functions, but also to the ultimate disposal and recycling of that product. Plastic fasteners give products useful design for the environment features. Some plastic fasteners are easily detached for quick disassembly. This allows products to be upgraded or repaired with ease, or recycled at the end of the product's life.
Screws, nuts and bolts, cable ties, panel fasteners and grommets can all be manufactured from plastic. Some fastener materials are nylon, acrylic, high-density polyethylene, polycarbonate and polypropylene.
These fasteners are used in a wide variety of industries. The automotive industry uses plastic wire harness retainers, panel and molding clips, and gas and brake line clips. Other industries that use plastic fasteners include toy and appliance manufacturers. Although not as strong as their metal counterparts, plastic fasteners have unique properties that make them suitable for use throughout industry.
The advantages of plastic fasteners are:
- low cost.
- light weight.
- high strength.
- heat resistance.
- chemical and corrosion resistance.
- electrical insulation.
The disadvantages of plastic fasteners are:
- poor resistance to ultraviolet radiation.
- stress relaxation.
- dynamic load failures.
- lower mechanical properties than metal.
The advantages of plastic fasteners were recently discovered by a manufacturer of X-ray based bone scanners. These scanners determine a patient's bone density.
The company faced a design issue. Most people who work in medical environments don't want to hear loud noise from mechanical equipment. Patients don't want to hear it either. The scanner used two stepper motors to move the X-ray head along leadscrews, longitudinally and latitudinally, in 2-millimeter increments. However, the rigid construction required for precision and accuracy allowed vibration from the motor and screws to transmit through the machine's frame and cause a high-pitched noise that would not be tolerated in a medical environment.
E-A-R Specialty Components (Indianapolis) recommended that the manufacturer isolate both the motors and the leadscrew brackets from the structural frame with highly damped Isodamp C-1000 Series grommets.
The manufacturer now uses four grommets to mount each stepper motor in place and six grommets in a stiffer formulation at the leadscrew attachment points. The grommets created soft connections for the motors and screws, isolated the frame from vibration, and prevented the vibration from becoming noise.
Power Tools Ease Assembly
Screwdriving is a repetitive task. Although assemblers strive to make the process productive and cost-effective, choosing the right tool for the right job can be challenging. Each application has its own challenges, and when different screw sizes with different torque requirements are added into the mix, this can lead to assembly problems. Instead of just a screwdriver, an application may require a number of tools. And automating a screwdriving application can expand into a full-blown automated system complete with robotic loading and unloading.
Brewster Plastics Inc. (Patterson, NJ) is a custom injection-molding company. The company supplies injection molded products to the medical, dental, consumer and industrial markets.
Recently, Brewster Plastics was awarded a project that required molding and assembling parts for a 10- by 14-inch doll case. However, before it could be awarded the project, the company had to reduce the project's overall cost.
A major hindrance to reducing the project's cost was installing 11 Phillips head screws that fasten the telescopic handle to the case. Brewster Plastics contacted Weber Screwdriving Systems Inc. (Mt. Kisco, NY) to provide the equipment. Weber's biggest challenge was cycle time. "We wanted to produce a part with a certain number of screws in a given number of seconds, and the company had timed what it would take an operator to prepare, load and unload that part. For Weber's screwdriving system to be efficient, it had to meet that cycle speed-if not exceed it," says Brett Wallace, vice president of operations at Brewster.
At first, Brewster Plastics had its own plans for an assembly machine, and it simply asked Weber to deliver screwdriving components for that equipment. The robotic equipment that had been planned for the project proved unsatisfactory, and Brewster Plastics turned to Weber for assistance.
The suitcase assembly operation is incorporated into a single workcell. And the equipment that Weber provided includes the Model SEB-0360 screwdriver mounted to an X-Y articulating system. This articulating system locates the screwdriver over each of the 11 screw positions. A blow feeder delivers the fasteners from two Poly 7 feeders that are built into the base of the workcell. Two feeders are required because the case uses screws of two different sizes. The shank length on one screw is slightly longer than on the other. The two feeders also allow the operator to differentiate between the two screw sizes. One size attaches the top handle to the handle tubes. The other size attaches the telescoping handle to the side of the case.
Because the two screw sizes have different torque ranges, Weber also supplied Brewster Plastics with the SureTork control system. It can drive screws to a variety of torque ranges, depending on the application.
Brewster Plastics has developed a system where all of the parts are molded at the same time and assembled immediately. This minimizes warehousing. "The workcell encompasses the molding machine that molds the case and cover, along with the Weber screwdriving system. Subassemblies enter through different workstations within that cell," says Wallace. A Star Automation Inc. (Menomonee Falls, WI) robot on the molding machine grabs the case and cover out of the mold. One part of the case is presented to one side of the workcell. The other part, in which the screws are driven, goes to a cooling fixture to allow the part to reach a predictable shrink dimension. Allowing the part to cool ensures that the screw holes line up correctly. Once the part is cooled, the robot picks it up and brings it over to the second side of the workcell. The molded case is then placed on a fixture that accurately locates the part in the Weber workcell. The operator activates a pair of palm buttons, and the 11 screws automatically screw together the suitcase handle assembly. After that, the case and lid are hinged together and passed to final inspection and packaging.
Correct Torque Makes a Difference
Controlling torque is crucial to ensuring product quality and reliability. Fasteners that are not tightened to the correct torque can vibrate loose, or the fastener threads can strip. Parts that loosen over time cause major joint failure. The parts can either fall apart, or fatigue and break.
When determining torque specifications, several factors should be considered:
- the maximum load that is placed on the fastener.
- the strength of the material being joined.
- joint hardness.
The fastener also makes a difference. The fastener material and thread form can affect the torque specification. Even the way the fastener was made can make a difference.
Torque control is a major concern at BMW's new engine manufacturing plant at Hams Hall, England. The plant has a capacity of over 400,000 units per year. One of the most automated plants in the world, it relies on more than 700 torque wrenches and a full range of calibration equipment supplied by Norbar Torque Tools Inc. (Willoughby, OH) to keep it running smoothly.
BMW has specified Norbar for all manual torque tools, testers and calibration equipment used at Hams Hall, from the smallest 5-newton-meter tools up to large 760-newton-meter wrenches. Most are "P" type production wrenches, which have no scale and have a sealed adjustment, making them perfect for the production environment.
Paul Ward is metrology manager at Hams Hall and has overall responsibility for calibration and measurement on site. He also meets with outside suppliers on these issues.
"After a careful assessment of possible suppliers, we chose Norbar because they could supply the complete package of tools and calibration equipment," says Ward. "They also offer good technical support."
Each of the 10 production teams has its own multifunction room where quality checks are carried out and tools stored. Every multifunction room is equipped with two Pro-Test professional torque tester instruments, which enable operators to check that their wrenches are accurately calibrated before use. Each Pro-Test comes with three easily interchangeable torque transducers, enabling wrenches of up to 1,500 newton-meters to be tested.
Each wrench has its own unique reference number to ensure traceability, and all operators are issued with their own set of torque tools to encourage a sense of ownership and responsibility for their care.
The engines made by Hams Hall are hardly touched by human hands, and Norbar torque wrenches play a vital role in keeping this BMW plant in perfect tune.
Feeding Fasteners Can Be Problematic
Automatic delivery of screws to the screwdriver can be a great time saver. This eliminates most of the parts handling that might actually take more of the operator's time than actual screwdriving.
There are several methods of feeding screws to the driver. Screw presenters feed screws to a single collection point. Using the screwdriver, the operator picks up one fastener at a time and installs it. Screw presenters are primarily used in low- and medium-volume manual applications, but they can also be used to feed screws to benchtop Cartesian robots.
Blow feeding is used in medium- and high-volume applications. Individual screws are blown through a tube to the nosepiece of the driver. Bulk screws are fed to the tube with a vibratory bowl or a rotating drum. Blow feeding can supply screws to handheld tools or fully automatic equipment. With this technique, fasteners can be supplied to the tool almost as a fast as an operator or machine can drive them. However, blow feeding can only accommodate a limited range of screw sizes.
The best way to feed and drive screws was recently an issue at Phillips Plastics Corp. (Eau Claire, WI), a custom injection molder of plastic and metal. The company provides contract manufacturing services to original equipment manufacturers in the automotive, appliance, communications, consumer electronics, industrial, medical and recreational markets.
Automatic screw feeding and driving had always been the bottleneck on the Phillips' assembly line. Although the company used handheld screwfeeders from Visumatic Industrial Products (Lexington, KY), the operators couldn't keep up with the desired 7-second assembly time, because of all the product varieties being assembled. This manual process caused many mishaps. Screws were sometimes left out of the assembly or dropped into the assembly.
Looking for a way to eliminate these problems, Phillips swapped the handheld screwdrivers for the Visumatic VIPer, a fully automatic screw feeding and driving system. The workhorse of the system is a four-axis Cobra 600 SCARA robot from Adept Technology Inc. (Livermore, CA). The robot is equipped with a DC electric screwdriver and automatic screw feeder. The tool can monitor both torque and angle. The robot can be integrated into an automated assembly system, or it can operate as a stand-alone system. Its work envelope is more than 7,000 cubic feet. The robot is available with a variety of control options, including vision guidance.
After switching to the VIPer, Phillips has noticed a reduction in cycle time at the screwdriving station. The cycle time has decreased from 11 seconds to 6.3 seconds. The unit also provides quality control of the assembly. It counts the number of screws per part that are driven correctly within the torque specifications. This has helped the company reduce the number of rejects.