High Joint Performance Fastening Solutions For Sheet Metal.
High Joint Performance Fastening Solutions for Sheet Metal Applications and Thin Walled Boxed and Closed Sections.
In the drive to reduce weight many designers are turning to lighter materials such as aluminium, magnesium and composites together with relatively new manufacturing techniques such as hydro-forming. These new materials and techniques often require mechanical fastening solutions, as they cannot be welded. This Case Study considers the various fastening systems available to provide load bearing threads in this new generation of materials and closed section components, including the latest option of high strength threaded inserts.
The Requirements for Component Weight.
The need to reduce component weight whilst increasing mechanical performance is prevalent in many industries. The automotive sector is no exception. In recent years automotive manufacturers have been faced with the need to produce lighter weight vehicles. Consumers demand increased performance, power, improved economy, better safety and more comfort. Legislation requires lower emissions and imposes harsher crash testing criteria and pedestrian impact countermeasures. Furthermore, the highly competitive nature of the industry demands significantly reduced production costs and lead-times to market.
Therefore, weight saving has never been more important. Cutting the weight of a vehicle's structure, suspension or door, bonnet and boot closures, enables a manufacturer to deliver improved fuel economy and emission results, or better performance for a given engine, or improved comfort or safety features for a predetermined weight. The industry then is turning towards lighter and more rigid structural parts.
One solution has been to make greater use of lightweight materials such as aluminium, magnesium, plastics and composites, especially in extruded, tubular, cast, moulded and thin panel forms. Components made from composite materials can range from body panels, radiators and ignition components to structural and semi-structural components such as bumper beams, front-end panels and leaf springs.
High Performance Fastening Solutions .
The prospect of lighter more durable components together with the increased design freedom, relatively low investment costs and parts consolidation opportunities that advanced composite technology offers, is expected to see its popularity grow. OEMs such as BMW and VW are looking at the amount of body panel applications that utilise composites; especially in the case of BMW's future sports car range with the company making a significant investment in research and development. However, in the past few years one of the key technologies the automotive industry has turned to is the hydro forming of steel and aluminium.
The process has been used since the 1980s, initially to form simple under body components and latterly more complex members. It can enable several components to be consolidated into a single hydro-formed shape with an improved uniformity of wall thickness, whilst often using less material and taking less time than manufacturing traditional stamped and welded parts.
Currently in North America it's estimated more than a million engine cradles a year are produced by hydro-forming and in Europe the technology is used in sub-frames by BMW and VW as well as by Ford for its Mondeo and by General Motors for the Vectra. It is estimated that by 2004, 50 per cent of all vehicles produced in North America will have chassis with hydro-form content. Many industry experts believe the next major application for hydro-forming will be a vehicle's space frame.
These changes, whether the switch to lighter materials or the greater use of sheet metal, extruded/tubular and hydro-formed components mirrors some of the parallel developments that are taking place in fastening systems technology. Furthermore, they have raised questions about the benefits of some of the more tried and trusted traditional types of fasteners.
To fully understand the process of fastening sheet metal and thin walled materials, you first need to consider the characteristics of a well-designed joint. This Study concentrates on the use of removable threaded fasteners. However, there are non-removable fastening methods such as riveting, welding, clinching, etc. For most removable fasteners to perform optimally, several factors need to be taken into account. These include:
It is important that these factors be considered early in the design of an application, rather than as an afterthought, to ensure optimal joint performance. Besides these elements, sheet metal and thin wall components present additional challenges to joint design. First is thread engagement.
Fastening directly into sheet metal or thin walled materials is often impractical due to a lack of material in which to obtain sufficient thread engagement. In many instances, the screw will strip immediately after seating. Another scenario is that fastening such minimal material thickness relies too heavily on the run-out portion of the screw where a thread is not properly formed, hence a secure joint can't be guaranteed. A further problem can be the deformation of the assembly because of the force required to gain clamp during the installation process.
There are many thousands of fastener variations designed to provide permanent load-bearing threads in thin sheet metal that are otherwise too thin to be tapped for component attachment or fabrication. However, there are only four main types that offer good pullout and torque loading.
Weld nuts have long since been established as the main type of fastener for installing load bearing threads into thin sheet metal. These fasteners have enabled the development of thin-metal designs, especially in the automotive and commercial appliance industries, that would otherwise have been impossible to achieve.
Weld nuts are installed using proven welding technology that attaches the weld nut to a pre-pierced sheet of metal. Despite their low cost, the initial capital investment in fixtures, clamps and robot welding equipment can be very high together with running costs such as the energy required and consumables such as electrodes. Therefore, as component specifications change the many disadvantages weld nuts suffer from are causing manufacturers to consider other types of fastening system. These disadvantages include:
Press-in Type Nuts Clinch Nuts.
Slightly more expensive than weld nuts, but still relatively low cost, self-clinching nuts again utilise a very tried and tested technology. They are usually installed in sheet metal by pressing them into place in correctly sized drilled or punched holes using a parallel-acting press (self piecing clinch nuts can be used in some applications).
Special hole preparation, such as chamfering or deburring is not required. The squeezing and pressing process causes displaced material to cold-flow into a specially designed annular recess in the shank of the fastener, locking it into place. A serrated, shaped or ribbed clinching ring prevents the fastener from rotating in the metal when tightening torque is applied to the mating screw. As a result, self-clinching nuts become a permanent and integral part of the panel, chassis, bracket or other component in which it is installed.
During installation the most important criterion is that the fastener be pressed into place between parallel surfaces. High pressure is required to make the sheet material flow into the recesses of the shank. Therefore, a squeezing force needs to be applied to totally embed the clinching ring. Over pressing can distort the threads and buckle the sheet metal.
For high volume assembly, an automated press can be used - some are specifically designed to feed self-clinching nuts automatically into pre-punched or pre-drilled holes and seat them correctly with a parallel pressing force. Feeding rates can be up to six times quicker than manual insertions and the pressing action is adjustable to compensate for variations in material thickness and hardness. Some self-piercing designs eliminate the need to align holes and thereby reduce tooling costs.
When correctly installed, clinch nuts will produce little or no distortion of the sheet metal or damage to the finished surface. In most instances fasteners should be installed after plating or finishing. However, some finishing processes will not affect the fastener. The major benefits of clinch nuts are:
It is for these reasons that a growing number of weld nut applications are being replaced by clinch nuts. However, both weld nuts and clinch nuts require both sides of a work piece to be accessible during installation and final component assembly. In addition, clinch nuts cannot be used in composites or plastics. Hence, for so-called ‘blind' applications created by the growing use of tubing, extrusions and hydro-formed components, one-sided fastener solutions such as threaded inserts and J-Nuts are now a designer's main options.
The J Nut is a threaded fastener with a side profile generally in the shape of a ‘J'. The fastener is designed to be easily pressed into a position over the edge of a panel and retained by a spring steel retainer or inserted into rectangular holes into centre panel locations. They offer floating alignment to hasten assembly, achieved by a retaining leg that easily snaps into the mounting hole allowing the fastener to shift or align without disengaging.
J Nuts will not slip or rotate when bolts are driven into them. They can also be applied after painting or plating operations, eliminating the need for masking or re-tapping operations. Available in numerous versions, J Nuts tend to be popular in lighter weight applications, as the use of high torque to the nut can break its spring ‘legs'. J Nuts require no special tooling and are easy to install. However, automation can be difficult as they need a large access hole, which if in the middle of a component can take up value space and also weaken it. The fasteners can also be expensive and prone to corrosion as well as hydrogen embrittlement.
Background to Threaded Inserts.
Textron Fastening Systems pioneered the development of blind threaded inserts in the 1950s for automotive and aerospace applications. Known in the market as ‘Nutsert®, they are used extensively in the assembly of white goods, furniture, all types of motor vehicles, electrical components, HVAC equipment, etc.
They offer a quick, reliable and low cost system of inserting high quality, permanent, and load bearing threads. The inserts can be installed rapidly from one side of the work-piece making them ideal for the assembly of boxed or enclosed sections.
Each installed threaded insert provides a securely anchored female thread in the parent material that may be sheet or tubular steel, aluminium extrusion, cast magnesium, moulded plastic or a wide range of composites. The body of the insert is allowed to bulb in a controlled, ductile fashion to create a large ‘footprint' on the blindside of the parent material that resists pullout loads. Minimal deformation of the parent material is caused.
As a purely mechanical fastening, threaded inserts will not damage the surface coating of a component so they can therefore be installed in pre or post- finished applications as well as used to join dissimilar materials. With a typical assembly cycle of just three seconds, the installation can be fully automated for in-line processing to provide high-speed precision assembly with much lower set-up costs compared to welding equipment. Alternatively, hand-actuated tools are available for small batch and repair work and pneumatic tools for medium volume production. Standard blind inserts are cold formed from annealed low-carbon steel. The steel is first cold formed and then heat treated to increase ductility. Finally, the fastener is tapped and plated or coated for corrosion resistance. Other materials include stainless steel, brass or aluminium.
Ranges are available to suit differing parent material thickness and hole shapes. In certain applications holes are pre-drilled and a round splined insert can be used. Pre-punched, laser cut or moulded holes can be square or hexagonal to suit square or hexagonal shaped inserts that are designed to offer increased resistance to turning within the parent material.
A significant disadvantage of standard threaded inserts is that the female thread is weaker than the male thread of the mating screw or bolt. This is not a concern if maximum screw tightening torque guidelines for the insert are followed. However, when the application demands that the clamp load be maximised and the tightening torque is increased, (either by introducing thread lubrication or specifying a high tensile bolt or screw metric property class 9.8. 10.9 or 12.9), then there is a risk that the thread of the insert, being weaker than that of the screw, will strip before the maximum torque capability of the screw is reached.
High Strength Threaded Inserts.
There are a number of solutions to this problem including through-hardened and tempered inserts, nitride-hardened products or more recently, the High Strength Hexsert ® .
The high hardness of through hardened inserts results in good strip-out performances, but poor installation properties. This is due to the very high pulling forces required to place them, due to the fact that the bulbing part of the insert is far less ductile than a standard insert. Hence, they are not suitable for placing with either standard hand-held placing tools or automated systems.
Through hardened inserts also tend to be expensive because of the high cost of the material and a short tool life, due to the problems associated with placing them. High torque machines are required for the tapping of bigger sized inserts. The heat treatment processes are also more complex and thus more costly. Nitrided inserts are made from conventional, low cost carbon steel. Being harder than standard inserts, the thread offers good strip-out performance. However, the additional nitriding process is very slow and costly. There is also some reduction in ductility when placing.
Avdel ® High Strength Hexsert ®
To overcome these problems the solution has been to combine the relatively soft and ductile portion of a standard threaded insert to anchor onto the blindside of the parent material, with a much harder female thread. The result is the Avdel ® High Strength Hexsert ® – a high performance blind threaded insert with a double work-hardened thread portion designed to complement high tensile metric screws up to property class 12.9.
At the heart of the production of the High Strength Hexsert ® is a carefully controlled ‘ band annealing' operation, which selectively heats only the bulbing region of the insert, but retains the work hardening of the cold-heading in the nut portion. A subsequent roll-tapping process enhances the work hardening of the female thread material. A comparison of the hardness profiles of a sectioned standard Hexsert and a High Strength Hexsert ® clearly reveal the nett effect of this special processing – see below.
The bulbing region of the High Strength Hexsert ® is slightly harder than that of its standard counterpart to enhance torque to turn and pullout strength. But, the female thread region is a great deal harder and able to withstand the highest torque loading normally applied to metric higher strength screws. If excessive torque is applied, it is the screw shank that will fail not the insert. In many instances, it is then possible to remove the screw and refit a new one in the same insert with the correct tightening torque. Hence, costly and time-consuming reworking is avoided.
In many cases the high torque capability of the High Strength Hexsert ® also allows designers to save weight, space and cost without compromising joint performance by using a smaller screw and insert combination. The harder thread better resists frequent removal and refitting, especially where correct torque tightening is not applied.
Dimensionally, the High Strength Hexsert ® is similar to a standard Avdel ® Large Flange Steel Euro Hexsert ® , (the hexagon hole sizes required are identical) which enables it to achieve a very high torque to turn resistance and push through inline with its high strength thread performance. In addition, the large flange gives good hole reinforcement for thin sheet metal applications.
The High Strength Hexsert ® is slightly longer and features an increased female thread length in comparison with standard Hexserts. This difference also ensures that standard and high strength threaded inserts are never confused or mixed. The current range covers the three most common sizes, M6, M8 and M10 in low carbon steel. In most applications smaller sizes do not require the higher strength performance while above M10 diameter, the thread performance of the standard insert appears to satisfy most applications.
The table below gives the dimensional data of the High Strength Hexsert ® range.
Dimensional data of the High Strength Hexsert ®
Although increased the hardness of the product has a positive effect on pullout, torque-to-turn and push-out values, the important performance criterion is the increase in maximum torque, which is directly related to clamp up and therefore joint performance. For both M6 and M8 sizes the screw shank of a 12.9 class screw failed prior to the insert when over tightened. Furthermore, the same insert can then accept a new screw. There is no need to replace the insert. No insert failure appeared even at the maximum applied torque specified for the M10 High Strength Hexsert ® insert.
The table below compares typical performance data between three sizes of High Strength Hexsert ® and their standard Large Flange equivalents. The intention is to illustrate the significant differences in performance. Torque/tension traces produced by Textron Fastening Systems are available on request.
Performance Data Comparisons.
HSH – High Strength Hexsert ®
LFH – Large Flange Hexsert ®
1. Recommended maximum torque as applied to a joint with a static top plate. Thread of insert will not be damaged. However, this torque value may exceed the strength of the screw or the bolt in question. Always refer to recommended tightening torque limits for the screw or bolt also.
2. All threaded inserts tested in mild steel test plates.
The High Strength Hexsert ® can be placed with the standard Avdel ® 742 power tool as well as multi-head and fully automated systems. The lightweight, easy-to-use 742 hydro-pneumatic tool utilises ‘spin-pull' technology, which generates the high pull forces required to place large diameter and thick wall inserts. This method of operation reduces wear of the drive screw resulting in lower maintenance and longer tool life. The tool can be hand-held or suspended to suit production requirements and requires the pulling stroke to be manually set.
Multi-headed machines can be designed to install several inserts into a component at one time, while the fully automated Avdel ® Autosert Assembly System can be robot mounted and integrated into unmanned production cells for high volume installations.
High Strength Hexsert ® Initial Development - Automotive Application.
Companies such as General Motors, Daimler Chrysler and Volkswagen have only recently applied hydro-formed aluminium or steel structural components in volume. The technology allows the car manufacturer the opportunity to develop lighter, more fuel-efficient vehicles with a stiffer and stronger structure.
In one instance, VW designed a hydro-formed steel crossbeam as a structural component of a new utility van. The kinked tubular shape of the beam meant there was no access to the blind side to apply conventional fasteners such as weld nuts or clinch nuts. The only other type of nut that could feasibly be used was a spring steel J Nut. However, this was an expensive option that could not be practically incorporated into a fully automated assembly process.
Besides automation capabilities, because of the high cost of manufacturing the hydro-formed component, VW was looking for a fastening system that could deliver a high joint performance and was robust against damage during assembly so as to avoid expensive rework or even the scrapping of components. Initially, the automotive manufacturer considered using a standard threaded insert with an extended nut thread for increased resistance to the possibility of thread stripping caused by over-tightening a Metric Property Class 10.9 screw. This proved unsatisfactory. To solve the problem Textron Fastening Systems began development work on the High Strength Hexsert ® .
The resulting high torque capability of the High Strength Hexsert ® ensured that the shank of the installed Metric Property Class 10.9 screw would break before any thread damage occurred. Therefore, should excess torque be applied during assembly or indeed during subsequent repair or service work, the broken screw could simply be removed from the insert and replaced without any risk of damage to the beam.
The High Strength Hexsert ® was designed to use identical hole sizes to the standard Avdel ® Euro Hexsert ® range as well as retain its relatively soft bulbing properties, which was critical for the use of automated placing equipment.
The alternatives to the High Strength Hexsert ® that could deliver a similar performance in terms of strength, were either through-hardened inserts, which need very high pulling forces to be placed or nitride-hardened products that are costly to produce and not as strong.
In a series of verification tests undertaken by Textron Fastening Systems in the UK and Germany as well as VW's own laboratories, the High Strength Hexsert ® demonstrated excellent mechanical performance.
Time consuming rework of parts due to over-tightening problems was significantly reduced. Four Avdel ® Autosert ® 2000 autofeed placing heads mounted on robotic arms install 29 M6 Hexserts and M8 High Strength Hexsert ® s per beam in a total cycle time of just 95 seconds.
Submitted by Textron Fastening Systems.