|
Authored by: |
Engineers at General Dynamics Land Systems, Sterling Heights, Mich., faced a challenge when designing the Expeditionary Fighting Vehicle for the U. S. Marine Corps. The amphibious combat craft needed to be made mostly of a lightweight aluminum alloy so that it would meet its mission objectives. But aluminum is relatively soft, and fasteners used to attach components to the aluminum frame needed to satisfy several requirements. They had to be lightweight, create watertight seals, lock in place solidly, resist extreme vibrations and shock, and withstand extended exposure to saltwater. The Marines also wanted be able to remove and replace the fasteners in the field.
In search of an insert
The design team wanted to use threaded inserts, a common practice when putting bolts in soft metals like aluminum and magnesium. Inserts, usually made of a tough metal like stainless steel, are installed into pretapped holes and permanently locked in place. Their interior threads give a strong, wear-resistant interface to install bolts into.
Although engineers have a few basic types of threaded inserts to choose from, including key-locking, ring-locking, and helical-spring, none met the U. S. Marine Corps demands. For example, if a conventional insert needs to be removed for any reason, it must be drilled out. This risks damaging the hole in the parent material. Plus, drilling can be clumsy and difficult if the insert is not easily accessible.
Another limitation is that inserts typically cannot create air and watertight seals without using a thread-locking compound. And even when thread-locking compounds are used, many inserts don’t offer ‘blind’ internal threads, so there is still a leak path.
And finally, there’s a limited range of standard threaded inserts on the market, and custom lengths or materials are typically accompanied by high prices and long lead times. This lack of flexibility often means engineers must alter their designs to accommodate available insert sizes or material options.
Faced with requirements it could not meet with commercially available threaded inserts, General Dynamics began internal design and development of an insert that would do the job without adding cost. Led by a talented and now-retired tool engineer named Fred Wheeler, the team came up with the Fredsert after 12 months of development, testing, and redesign. Ultimately, more than 6,000 Fredserts were installed on each Expeditionary Fighting Vehicle, and Fredserts have been used on several other General Dynamics vehicles and weapons, including the Mk 46 naval-gun turret, the Army’s Future Combat System and Joint Light Tactical Vehicles, the Navy’s Littoral Combat Ships built by Austal, Mobile, Ala., as well as more than 12,000 upgraded Humvees made by AM
General, South Bend, Ind.
Fredserts’ advantages
A Fredsert combines a tapered thread profile, 100% thread engagement, cutting flutes, and a flanged head to create a friction fit with material compression to reliably lock it in place. External threads are slightly oversized compared to other inserts and tapered in certain areas so that they create a friction-fit when installed into the parent material (such as aluminum). And all of the Fredsert’s threads are engaged with the parent material, much like the way taps interface with threaded holes. Other inserts use standard bolt threads that end up with significantly less contact with the parent material in terms of surface area.
The flanged head also helps in that it gets torqued down onto the parent material and contributes to the locking action. Fredserts are designed to exceed the tensile strength of bolts installed in them. In tests, bolts break before the Fredsert ever budges from the parent material.
For example, in tests at General Dynamics Land Systems Test Lab, inserts with M12 × 1.75 internal threads were put into 0.5-in.-thick plates of 6061 aluminum. In torque-out tests, Grade 8 bolts broke at about 160 lb-ft. In pull-out tests, the bolts broke at about 21,000 lb. In neither tests were the Fredserts damaged.
The combination of a tapered thread profile, 100% thread engagement, and a flanged head also deliver air and watertight seals on the insert’s internal threads. In addition, “blind” Fredserts deliver air and watertight seals on its internal threads because they don’t break through the bottom of the insert.
Although Fredserts can handle high torques and pull-out forces, they are not permanently locked in place. Instead, its patented geometry lets it “break away” when technicians apply approximately 80% of the recommended installation torque. If the insert is reinstalled into the same hole, the breakaway torque drops slightly to 70% of the original installation torque. This is because the Fredsert’s external threads remove small amounts of parent material when initially installed. Subsequent reinstallations will not cut any material. But the friction fit and material compression will still lock the insert in place. Fredserts have a generous safety factor, so even at the 70% level, the insert has more than twice the breakaway torque of the bolt, even if thread-locking compound is applied.
This lets soldiers or technicians quickly remove and replace bolts and inserts in the field, given they have a torque wrench and Fredsert drive tool, which is designed to mate with a Fredsert. By comparison, other inserts can only be removed by drilling them out and, in some cases, retapping to the next largest size.
Fredserts are available as standard components in several sizes and materials. In addition to those made from stainless steel, there are also titanium versions which are 40% lighter than stainless-steel inserts of the same size and more corrosion resistant. Inconel Fredserts handle high-temperature application such as well drilling, engine installations, and nuclear-power plants. And phosphor-bronze Fredserts offer a high level of corrosion resistance in saltwater, are nonmagnetic, and make excellent electrical conductors.
While designing the initial amphibious attack vehicle, General Dynamics engineers determined that the vast majority of Fredserts would have no problems with corrosion. But about 10% of them are in areas where saltwater can become trapped, leading to galvanic corrosion of the aluminum. To prevent this, General Dynamics coated the head (flange) of those Fredserts with Alodine ec2, which was developed and patented by Henkel Corp., Cleveland, and independently proven effective by General Dynamics through salt-fog and durability testing. The electroceramic coating creates a barrier between the titanium and aluminum. Because Fredserts create their own seal upon installation, there is no need to coat external threads or any other part, only the head. The initial amphibious vehicle has been the only application where this coating was deemed necessary. Other vehicles have used uncoated titanium and stainless-steel Fredserts with no corrosion issues.
Although most commercial inserts are only available in female configurations, there are several styles of Fredserts that meet a variety of needs. Female, the most common style, creates a wear-resistant bolt interface. Male versions let designers place threaded studs on soft metals. Appurtenance Fredserts are similar to female versions, except that they have a specified flange thickness that lets them be used as standoffs for armor plates or electronic boxes. Appurtenance Fredserts let designers eliminate or limit the use of aluminum weld bosses. Hex Fredserts let bolts be installed from the opposite side of the metal plate as the inserts. This is useful in areas where threads are needed, but access and reach are limited. And fitting Fredserts create hydraulic or pneumatic connections through metal plates or housings
Simplifying installation
One of the most important features that differentiate Fredserts from other threaded inserts is that installation will soon be automated. General Dynamics is currently working to develop ways to install Fredserts with CNC machines and robots. The CNC method will use a toolholder similar to a tension/compression tap holder to take a Fredsert from a tray and install it into a pretapped hole. This approach will let companies combine machining and insert installation in one setup, a major step toward Lean Production. Essentially, the user would machine a part as they do today, but after the holes are tapped, the machine would pull the Fredsert installation tool from the tool magazine and install Fredserts while the part is still set up in the machine. Other threaded inserts on the market would be difficult or impossible to automate, due to the mechanical clamping aspect of their installation. Fredserts are simply installed with torque, so a CNC machine has all the necessary abilities to install them.
General Dynamics is also developing a robotic installation cell consisting of a robotic arm with an electric nut-runner that picks Fredserts from a tray and installs them into pretapped holes. Such a setup would let manufacturers install inserts into several surfaces of a component in a single setup. This would be a plus for applications where a CNC machine cannot reach all the installation locations due to part size or shape. Automated threaded-insert installation could dramatically change the way aluminum components and structures are assembled in aerospace, defense, automotive, and other industries.
Other future developments include a Fredsert designed for composites. Such an insert should overcome limitations in traditional “potted” inserts which use epoxy-type resins to permanently bond inserts to composite parent materials. A less-costly and more-effective insert for composite applications will save time and money for companies building lightweight aircraft, mobile shelters, and other composite structures.
While General Dynamics has used Fredserts in military platforms for nearly a decade, the company has only begun offering it to commercial markets in the past several months. The Fredsert design and configuration are controlled by General Dynamics using a 3D model tied to a 2D manufacturing drawing. This lets designers plug in key dimensions to the model to create custom insert designs, as well as a fully dimensioned manufacturing drawing, in seconds.
Fredserts are manufactured by Miller Precision Industries in Ohio, and marketed through sales representatives across the U. S. Standard metric sizes range from M4 × 0.7 to M36 × 4.0 internal threads. Standard inch sizes go from #8-32 to 1-8 internal threads. At production quantities, stainless-steel Fredserts cost $5 to $25 each, depending on size. Titanium versions typically cost 30 to 40% more, based on the cost of titanium.
The introduction of Fredserts gives engineers and designers new fastening and joining choices. The inserts have also simplified vehicle design, improved producibility, and reduced logistical costs.