Effective fastener design cuts assembly costs

In today's global economy with an ever increasing emphasis being placed on lean manufacturing and product rationalisation, it is important to realise that fastener engineering must play a key role.

In today's global economy with an ever increasing emphasis being placed on lean manufacturing and product rationalisation, it is important to realise that fastener engineering must play a key role.

No longer can the fastener design and cost be viewed outside the manufacturing process.

The design of the fastener and its impact on production costs and the product development process must be integrated and no longer based on the individual piece part price of the individual fastener.

Fastener development and design must now be based and thought of as a contributing process technology.

It must impact process and labour costs as well as addressing global trends such as carbon emissions, End of Life Vehicle (ELV) directives and the myriad of other demands placed on industry.

Fastener design businesses must invest in the latest tools of technology to design and deliver fasteners that rise to the challenge.

It is essential to stimulate greater investment by companies in product integrity by showing the commercial benefit of applying appropriate technology and by challenging traditional methods which no longer provide an adequate business case.

Sheet metal fasteners and their design have evolved from the 19th century based on machine V type form configured screws to be used with a free running nut or the many variants of internally threaded inserts.

Fastener designers have year by year virtually ignored the requirements of OEM for thinner material gauge and less cost.

This trend of reducing gauge thickness is also causing quality problems for sheet metal forming screws.

Purchasing fasteners based on these technologies from low cost economies such as China is cul-de-sac thinking that cannot deliver the requirements of modern manufacturing production techniques.

Future fastener designs must move away from the stretch the bolt mentality and in particular in the sheet metal industry must view the press and bending machines as an integral and important ally in future technology solutions.

Any design for the future must throw away the rule book, it is not cost effective.

We can no longer afford to punch holes of any shape into purchased material and then add labour, inventory and process cost to insert internal thread technology.

The way forward is to look at past design breakthroughs that fundamentally change the way we think about how products should be produced.

Fastener design has previously stated that we must utilise the existing processes and not add to them, if this can be achieved it should even reduce processes and cost.

The trend of using thinner gauge material has led to automotive and industrial manufacturers that use sheet metal to replace conventional fastener methodology with product design and geometry to achieve strength and stability over expensive and thick gauge materials.

This approach has the added advantage of reducing weight.

The HTFS product was originally designed to obey trends, work with existing processes and deliver the usability that the end customer requires.

In brief the V type form of thread was replaced by a square type buttress form that matched material thickness and a system to generate a geometrically complementary sheet metal helix was designed as an insert to fit into all existing press tool configurations.

This approach has led to the development of the customer creating the "nut" using existing processes and eliminating the imported female insert and associated insertion cost.

The HTFS product also creates a joint that is reusable, does not stretch the bolt and has the advantage of not requiring antivibration technology such as washers.

The HTFS joint design of controlling the attributes of the sheet material allows the joint to flex with the component requiring less assembly torque to achieve joint integrity and prevent panel slippage or assembly failure.

The design of this fastener owes more to the Playstation generation than it does to Brunel.

All original designs where created as CAD models with joint integrity prediction assisted by the deployment of finite element analysis.

The HTFS product has been deployed in such diverse products as Caterpillar generators, 3Com computer boxes, domestic cookers through to elements for Land Rover and Jaguar.

All these products demanded some or all of the attributes that the system has to offer.

The conceptual idea of creating a geometric helix thread form with a complementary buttress screw was conceived as a potential answer to a fundamental problem seen in everyday production.

So how do we hold material together, reduce costs and increase product efficiency all at the same time?.

Essentially, the design enhances the material and creates a re-usable durable fastener joint with many attributes still being found that deliver the cost savings required by manufacturers and assemblers of the sheet metal components.

This is especially true in the current economic climate with raw material prices soaring due to the growth in world markets.

Nut inserts will only ever be as strong as the material that they are being inserted into (totally relying on whatever means the design incorporates to stop the nut rotating or breaking away for the parent component), whereas the HTFS design is part of the component and with its controlled geometry, mimics the corrugation effect, which enhances the strength of the formed material.

The HTFS concept is also designed to achieve maximum engagement of the formed sheet material and thread profile, and is designed to accommodate a large range of material gauges for a given thread diameter.

The key element of the system is the relationship between material thickness and the diameter of the fastener where the pitch of the thread is directly related to material thickness.

Due to the nature of the HTFS design, numerous manufacturing processes can be removed due to the "nut" element of the fastened joint being built into the sheet material.

Time and money can be saved with pressed panels being available and ready for assembly directly from the production press with no need for secondary welding or mechanical processes.

The system is also unaffected by any painting processes that may be used during production which can be carried out before or after assembly.

The ability to remove processes also helps in improving the efficiency and output of the labour force.

It gives companies the opportunity to evaluate and distribute the labour in a more effective and efficient way to increase output without increasing costs.

Process removal also saves energy and the environment and allows companies to reduce expenditure on capital equipment.

The geometry of the system does give the fastener system enhanced mechanical performance characteristics which are integral within the design when compared to conventional sheet metal fasteners.

Firstly the geometry enhances the strength of the sheet material due to the corrugation effect.

It also has the ability to withstand extreme vibration conditions.

This is achieved by controlling the small amount of deflection within the helix form, which tries to reset itself to its original position when tightened down.

The flexibility within the helix has a spring effect which gives it its vibration resistant qualities and enables the fastener to be utilised in automotive applications where vibration resistance is a key attribute.

One of the major advantages that the system brings is inventory reduction.

The system allows for the removal of inserts, spring washers etc which means that fastener inventory will be reduced by a minimum of 50% as the nut element of the system is built into the sheet material.

The HTFS product has also assisted many customers where they have been able to rationalise on the panel gauges that they are using.

In one example, the customer was previously using 1.2mm stainless steel and a "Pem" style insert to assemble all its sheet metal work.

Due to the nature of the application it was difficult to understand why the material being used was so thick.

After further investigation and discussions with the customer it was found that the company had to maintain that thickness due to the nature of the insert that it was using.

As a result of converting to HTFS the customer has reduced the panel thickness to 1.0mm which again is an added cost saving.

The system is free running on insertion right up until lock down so the insertion torque required is significantly reduced which makes the assembly process easier for the operator and ultimately quicker.

The system is also totally re-usable and can be used over and over again without loss of function which is particularly useful for inspection panels.

The system also reduces the possibility of repetitive strain injury due to the shock impact as high insertion forces are required with some fastener designs.

Repetitive strain injury occurs from repeated movements over and over again which is something that assembly operators within the manufacturing industry are prone to.

This is something which employers have started to take seriously in recent years and it is particularly prevalent in today's modern era of personal injury claims and compensation.

The HTFS product is being used by a number of customers in the information technology industry particularly on their racking systems.

In one example the HTFS is now being used to fasten various components onto the main chassis as well as the main enclosures.

The customer was previously using pierce and plunge fastening method which was giving them a high percentage strip rate during their assembly process.

This meant that a large amount of their sheet metalwork either needed to be reworked or in some cases scrapped altogether.

By converting over to High Torque the company's strip rate has been reduced to zero and the possibility of swarf getting into the circuitry eliminated.

It is also important to understand how the fastener functions to give these benefits.

With a conventional fastener the bolt is put under tension, which can require extreme torque loading, which will work loose over time.

Much work has previously been done on this and how joints are prone to self-loosen.

The theory can be understood if we consider a simple block/incline analogy.

A thread can be thought of as an inclined plane wrapped around a cylinder.

What stops a block sliding down the incline is friction.

A small proportion of the weight of the block will act down the incline, and a frictional force, also created by the weight of the block will remain stationary.

If an external force now acts on the block, friction will resist this force as well.

However if this external force continues to be increased, a point is reached when the frictional resistance is overcome and the block will slide down the plane.

A similar process is involved with the threads of bolts.

Here it is the tension in the bolt rather than gravity that generates the frictional force to hold the parts in place.

The HTFS product follows the same principal except that the helical form is also put under tensile loading, thus reducing the stress in the bolt.

The locking engagement in the fastener arises from a relatively small deflection in the helical form, requiring less torque to achieve sufficient securing tension.

With the small amount of deflection needed to achieve lock up, the helical form tries to reset itself to its original formed position where this flexibility acts as a natural spring.

The flexibility of the HTFS product means that it can be used in a range of applications and it therefore does not restrict itself to a particular type of fastener application.

Different product applications have their own advantages for adopting the system.

On the one hand some applications adopt the system purely for cost reduction purposes without loss of performance where inserts are being used, and on the other hand the system may be brought in to improve quality or design issues that may be encountered with thread forming fasteners.

The flexibility of the system allows it to be used in very thin gauge material (0.5/0.6mm) right the way through to thicker gauge (2.5/3.0mm).

The HTFS product is yet to be exploited within many manufacturing industries where they have not yet considered the technology.

Ultimately the system lends itself to any industry that fastens sheet metal.

A continuing programme of development exists for third party examination of HTFS products into new materials and applications.

It is only by pushing forward with an ongoing process of questioning the way we do things that innovation will continue to develop and find new and improved ways to increase efficiency and meet the market demands.

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