Threaded inserts are useful in lightweight alloys and plastics both for improved joint performance in new components and for repairs to damaged threads. Jon Severn reports on the numerous options available.
Threaded fasteners are essential components in most engineered products and have been ever since screw threads began to be standardised in the early nineteenth century. Often taken for granted, threaded fasteners are once again a focus of attention for design engineers who are seeking to create lighter and cheaper products. Whereas fixing a screw in steel is straightforward, with the screw material, size, threadform and tightening torque simple to specify, fastening into softer metals or plastics requires more careful consideration, particularly if the threaded joint might be disassembled and reassembled during the lifetime of the product.
Assuming that a threaded fastener is still the optimum type of fastener for the joint, threaded inserts can be a cost-effective way to provide the required joint characteristics. Available in many different styles, threaded inserts can be suitable for installation as standard in new components and also for thread repairs either in the field or to salvage a new component that might otherwise have to be scrapped due to a faulty thread.
Looking first at threaded inserts for use in metals, these are available as either solid inserts or wire thread inserts. The former are machined from solid steel or stainless steel, though other metals can also be used. Indeed, it should be remembered that if no suitable threaded inserts can be found in the various manufacturers' standard ranges, many of the manufacturers also offer a service to produce inserts that are tailored to the customer's requirements; geometry, material and surface finish can all be specified.
Typical of the solid type of insert is the product launched recently by Memfast. Offered in sizes from M4 to M18 (internal), this insert is machined from mild steel with course metric threads on the internal and external diameters. A RoHS-compliant trivalent chromium finish provides protection against corrosion, or the inserts can instead be made from stainless steel. Memfast says the inserts offer excellent durability and resistance to shock and vibration in softer alloys and engineering plastics, both for new components and repairs to damaged threads. Unlike other inserts that require specialist tooling, this type can be installed using a standard machine screw with a head diameter greater than the insert's outside diameter.
A further development of this theme is the Böllhoff Kobsert. As with the Memfast product, the Kobsert is machined from steel or stainless steel with internal and external threads and can be used for new components or thread repairs. Designed for installation in metals with low shear strength - such as aluminium and alloys of aluminium and magnesium - the Kobsert features a knurled safety flange that is pressed into the surface of the host component to give improved resistance to torque and vibration (Fig.1). In addition to standard through-hole variants, Böllhoff also offers through-hole and blind inserts with a sealing gasket preinstalled beneath the safety flange to give a gastight seal.
While the Kobsert uses a collapsible collar to enable the knurled safety flange to bite into the surface of the host material, the Time-Sert - from the Time Fastener Company - features a plain flange to prevent over-insertion and a locking feature at the bottom of the insert. When the flange contacts the counterbore in the host material, the threaded insertion tool screws further into the insert and, because the last three internal threads on the insert are not fully formed, the insertion tool forces this part of the insert outwards to lock it into the pre-cut thread in the host material. Another point to note about the Time-Sert is the way the internal and external threads are cut so as to correspond with each other, which enables the insert wall thickness and radial space in the host material to be smaller than might otherwise be the case.
Alcoa Fastening Systems (AFS) offers a similar concept in its Slimsert inserts. However, these thin-walled inserts are installed to the required depth in a tapped hole that has been prepared with a stepped drill. Once in place, the insert's upper end is flared into the counterbore to hold the insert in place.
An alternative technique used to provide threaded inserts with torque resistance is self-broaching keys that are forced down the side of the insert after it has been installed to the required depth, thereby locking the insert to the host material. Examples of these include Alcoa Fastening Systems' Keensert and Kelox, which is similar to the Keensert but with the self-broaching keys joined by an integral ring for driving the keys. Yet another type is ring-locked inserts that rely on a separate serrated locking ring whose internal serrations engage with corresponding external serrations on the insert; the external serrations on the locking ring are driven into the host material to lock both components in place.
Wire thread or helical inserts, often known by trade names such as Heli-Coil or Armacoil, are typically manufactured from coils of cold-rolled stainless steel, hence they have a surface hardness that is superior to that of most host materials. While they can be used in steel or stainless steel, wire thread inserts are more likely to be found in the following metals and their alloys: aluminium, magnesium, zinc, titanium and copper. In addition, they can be installed in plastics and other materials.
As well as giving a more robust female thread than one formed directly in the host material, these inserts give a more balanced distribution of static and dynamic loads for the full length of the thread engagement, plus the elasticity in the insert helps to compensate for any lead and angle errors. Because of their inherent advantages, wire thread inserts are often specified as standard on equipment in the aerospace, defence and general engineering industries.
If additional vibration resistance is required, companies such as Armstrong Precision Components can supply wire thread inserts with an integral 'grip coil' that imparts a prevailing torque on the screw. Further options for wire thread inserts include higher-specification materials (titanium, Nitronic60, Nimonic90, InconelX750, super duplex stainless steel and phosphor bronze, for example), as well as surface treatments ranging from cadmium plating and dry film lubricants through to silver plating and proprietary treatments that prevent galvanic corrosion between the insert and the host metal.
Solid and wire thread type inserts can be used equally well for creating strong threads in relatively soft materials and for repairing threads that are damaged either during manufacture or maintenance (Fig.2). In the case of thread repairs, however, sometimes the thread is too badly damaged or positioned too far off-centre to be repaired using conventional inserts. For these situations, products such as the Armacoil Twincoil or the Heli-Coil Twinsert can be used to salvage a component that might otherwise have to be scrapped or undergo a costly repair operation. These products consist of two wire thread inserts; the first is installed in an oversize tapped hole, then the second is installed inside the first.
All of the inserts discussed so far require the pilot hole to be tapped before the insert is installed. However, there are inserts on the market - such as the Tap-Lok types from Groov-Pin - that are self-tapping. If suitable for the application, these can eliminate a thread-cutting operation, thereby saving time and costs. For softer metal alloys and plastics, the Tap-Lok Slotted series features a slot cut across the bottom of the insert, with the leading edges of the slots acting as thread-cutting features. For similar or slightly harder materials, the Tap-Lok Hole series has radial holes that create a series of thread-cutting features around the bottom of the insert. In these two cases, the slots and holes, respectively, also help to lock the insert in place and resist vibration-induced loosening.
Alloys are increasingly being used to save weight in products, with threaded inserts playing an important role as described above, but designers are also specifying plastics in a bid to cut further weight and cost from their designs. Although thermoplastic components can be designed to have adequate structural strength and stiffness, fixings have the potential to be a weak point. Fortunately, properly specified and installed threaded inserts can deliver the required functionality at an acceptable cost (Fig.3). They can also overcome a problem that does not exist with threaded joints in alloy components, namely creep.
It is not unknown, however, for threaded insets to be incorrectly specified or installed, resulting in unreliable joints, time-consuming assembly processes and high reject rates. Given the current state of the economy in Europe, manufacturers experiencing any of these three would be wise to look very carefully at the joint design and assembly process with a view to eliminating the problems.
For example, numerous different designs of inserts are available to suit installation during moulding and post-mould operations such as self-tapping, press fitting in cold components (or components still warm from the moulding process), or installation with the aid of heat or ultrasonics. In a few cases - such as the Fitsco Multifit or the Tappex Multisert - the insert is designed to be suitable for installation by means of press fitting, heat or ultrasonics. This gives the advantage that all three methods can be trialled to identify the optimum for the particular application.
It is usual for a successful application to rely as much on the installation method and equipment as it does on the insert and joint design. Considering the assembly requirements as a whole and revising the process accordingly can result in an insert installation process that delivers greater throughput, higher-integrity joints and reduced scrap, as well as eliminating the need for secondary operations. In a recent project, Spirol Industries supplied a customer with a model PHplaten style multi-tip heat insertion system for use with SpirolINS 29 brass inserts. This replaced an ultrasonic machine that installed the customer's own design of insert into plastic housings. Not only did the new process eliminate rejects and the need for secondary operations to remove excess material, but throughput increased six-fold due to the new machine being capable of processing up to six parts simultaneously (two inserts per part) in contrast to the ultrasonic equipment that required the operator to install one insert at a time. This production improvement alone has enabled the machine to pay for itself in less than one year.
In the above example, the operator was required to load the components and inserts manually, but insert installation can be integrated as part of a complete assembly cell if production volumes are sufficiently high. However, if press-fitted or self-tapping inserts are to be used, it can be better to install these immediately after the plastic part has been moulded, as the plastic flows more easily and is less liable to cracking. Moreover, if the moulder undertakes the insert installation operation, it can simplify logistics and assembly operations for the equipment manufacturer.
Selecting the correct insert for use in plastics cannot be undertaken in isolation; rather the entire joint design and assembly process should be considered as a whole. Application requirements vary; for example, torque resistance or pull-out strength might be more important, or ease of installation might be the main factor. Expansion inserts are easily installed, with the threaded screw forcing the knurling into the sides of the moulded hole to provide grip, but these are only suitable for light-duty applications. Designers should also take care in the design of the mating component to avoid the insert being pulled out if, for example, the clearance hole is larger than the diameter of the insert. On the other hand, if the fastener is too long or the pilot hole insufficiently deep, the screw can jack the insert out of the hole as the joint is tightened.
Placing the insert in a mould tool and allowing the molten plastic to flow around it might seem to be the ideal way to secure the inert, but the need to place the inserts in the mould tool increases the moulding cycle time and raises the possibility of the mould tool being damaged in the event of an insert being displaced. In most cases, therefore, inserts are installed after the moulding operation, with insert security being achieved through the optimum combination of joint design, insert and installation technique.
Most threaded inserts for plastics are machined from brass, but materials such as stainless steel can also be specified, as can wire thread inserts. Wire thread inserts certainly offer advantages over holes tapped directly into the host material, but there is a drawback in that the drilling, tapping and insertion processes take longer in a production environment, whereas this procedure is more likely to be acceptable for thread repairs. In an attempt to reduce the installation time and cost, Alcoa Fastening Systems has developed the Smartsert that is coiled from a wire with a cross-section that is more slender on the side facing the host material than is the case with the usual diamond cross-section wire. This means there is no need to tap a thread, and the insert itself does not need to cut its own thread; instead the host material is forced to flow around the insert as it is installed. According to Alcoa, this not only reduces assembly time and costs, but it also helps to avoid cracking and other damage to the host material. When originally launched, the stainless steel Smartserts were described as being suitable for use in aluminium alloys and plastics, but these products are now sold as thread-forming wire thread inserts for plastics.
Spirol Industries has published a helpful design guide for inserts used in plastics, and there is plenty of other information available from suppliers, as well as software-based insert selection utilities and applications advice. While threaded insert technology is now mature, there are still developments in this area and a thorough investigation of each joint in terms of detail design, insert selection and installation process can yield dividends through improved productivity and joint integrity.