Machinery Directive creates a new market for captive fasteners
An engineer’s choice of fastener for a particular application is normally driven by factors such as function, purchase cost and installation cost. In some cases there are also standards that must be adhered to, and aesthetics may need to be considered. From 29th December 2009 there will also be something else to take into account: if the fastener is used to retain a fixed guard on a machine, then the fastener must adhere to the requirements of the new Machinery Directive 2006/42/EC.
In common with its predecessor 98/37/EC this new Directive regulates the placing on the market, and the putting into service, of machinery in the European Union. The recast Directive is intended to extend the scope, improve clarity and eliminate some of the acknowledged flaws in the existing Directive. For manufacturers of some types of machinery the new Directive also introduces an optional route to conformity assessment – through quality assurance – and the Directive contains provisions for market surveillance (enforcement). EU Member States must implement the Directive through national legislation.
Apart from cartridge-operated fixing and other impact machinery, which were previously excluded from the Machinery Directive, there will be no transition period; the existing Directive and national regulations continue to apply until the new Directive becomes applicable and the new national regulations come into force. Preparations in advance of 29th December 2009 are essential. For example, EC type-examination certificates issued under the existing Directive will not be valid after that date.
An apparently minor change to the Directive that will nevertheless impact on almost all machine builders relates to fixings for fixed guards. Section 1.4.2.1 of the new Directive states: ‘fixing systems must remain attached to the guards or machinery when the guards are removed’. In other words, conventional screws will no longer be acceptable for retaining fixed guards.
Various types of captive screws, quarter-turn fasteners and other fasteners are available, but remember that guarding standards dictate that fixings for guards must require a tool for their removal. Captive screws and fasteners can differ significantly in their purchase cost and installation cost, as well as their ease of use. Machine builders therefore need to make the decision with care if they are not to regret it later.
Added value
Before looking in detail at some of the options available, all of which are likely to cost more than conventional screws, it is worth considering some of the advantages offered by captive fasteners.
It would be difficult to estimate the total annual cost to European machine builders of dropped fasteners. If a fastener has to be retrieved from the depths of a machine, the time required is likely to be substantially greater than the cost of the fastener. In fact it can be argued that it is more expensive to bend down and pick up an M3 washer from the floor than to leave it there and take another from a box on the bench. While this example is rather trivial, a fastener dropped inside a machine or control cabinet could potentially cause a breakdown. Using captive fasteners could therefore prove to be a cost-effective insurance policy against having to retrieve dropped fasteners or attend to subsequent breakdowns caused by stray fasteners.
With machinery for processing food, drink or pharmaceuticals, it is imperative that fasteners are not lost within the machine, as they could find their way into the end product – unless they are identified by inline metal detectors.
If captive screws are used on fixed guards, it is likely that removal and replacement of the guards will require less time – which therefore saves downtime and reduces costs. Moreover, as the revised Machinery Directive suggests, guards will not be replaced with some of the screws missing – which might otherwise enable a person to reach behind the guard and access the hazardous machinery.
Fastener options
One of the simplest captive screws available is the PEM Spinning Clinch Bolt (SCB) from Penn Engineering (Fig.1). Available with axial float or a full-thread jacking feature, the SCB is a one-piece threaded fastener that is installed simply by pressing it into a hole of the correct size using a simple punch and anvil. When the fastener is clinched into the hole, a raised feature under the head of the fastener causes the metal surrounding the hole to deform plastically in a controlled manner, thereby reducing the size of the hole so that the fastener cannot be removed, yet it can still turn freely.
These fasteners are available in M3 and M4 sizes (as well as similar Unified thread sizes) in lengths of 6, 10 and 12mm. The required hole size is 3mm or 4mm diameter, respectively, with a tolerance of +0.08mm.
An alternative type of captive fastener from Penn Engineering is the PF11, PF12, PF13 and PF14 family of self-clinching panel fasteners (Fig.2). These benefit from Mathread anti-cross-threading technology that helps to reduce assembly times and avoid the wasted time and cost associated with repairing damaged threads. For machine guarding applications, the PF fasteners are available with Phillips and six-lobe recess styles, with an optional knurled cap to aid manual assembly and disassembly. M3 to M6 metric thread sizes are offered, as well as similar Unified threads.
Similar products are also available from Southco, albeit without the anti-cross-threading feature. However, there is a wider choice of head styles in the Southco 47 range, including coloured caps in the Prism 4C family (Fig.3). As well as the aesthetics of using standard or custom-coloured caps, these Prism fasteners are said to eliminate problems associated with scratches on painted or powder-coated hardware. Another useful option offered by Southco is a ratchet head that enables the fastener to be assembled by hand, yet a tool is required for removal.
While these fasteners may prove too costly for simple machine guard applications where access is seldom required, Southco’s F5 flush captive screws are more economical. These can be installed in steel or aluminium panels as thin as 1.3mm and are flush when installed – with the screws able to float 0.5mm for easier alignment. M3 and M4 versions are available, all manufactured from stainless steel (Fig.4). Alternatively, Southco’s 9/12/17 series fast-lead captive screws feature a modified Acme thread to give quicker tightening and loosening than a standard thread, while still achieving a good clamping load. These screws are installed through a hole then retained by a stainless steel or nylon ring. However, because of the non-standard thread, the screws require a special receptacle to be installed on the mating component.
Quicker access
In some cases, such as when frequent access is required, captive quarter-turn fasteners may be preferred, either in push-to-close or turn-to-close format (Fig.5). Dzus quarter-turn fasteners – available through Southco and other distributors – come in a choice of sizes and head styles, including flush versions. As with the fast-lead captive screws, quarter-turn fasteners require the use of special receptacles on the mating component.
Specialty Fasteners & Components is both a manufacturer and distributor of fasteners, with in-house facilities for design, development and manufacture of specialist fasteners. Three of the company’s product ranges that will be of interest for machine guarding applications in particular are quarter-turn fasteners, push-turn fasteners and captive screws. As with the products already outlined above, the final choice depends on a number of factors, but Specialty Fasteners & Components has a broad selection from which to choose. Most come in a variety of head styles – including styles suitable for use on machine guarding applications – as well as a choice of materials, sizes and finishes.
One product worth highlighting from this company’s range is the snap-in captive screw. This fastener is installed using just using thumb pressure, then an aluminium grommet retains the screw in the panel. Self-ejecting versions are also available for sliding fit panels.
While the foregoing illustrates the wide range of captive fastener styles available, the fact that machine guarding applications require fasteners that can only be removed with a tool rules out many of the captive fasteners on the market (bearing in mind that a slotted head is not acceptable).
Furthermore, the number of suppliers of captive screws appears to be more limited than might be expected. For companies building small numbers of machines, the emphasis may be on the cost of installation, as set-up costs may be disproportionately high. On the other hand, manufacturers of higher volumes may find that the product purchase cost is the deciding factor. Either way, an apparently small change to the Machinery Directive is going to force almost every manufacturer of machinery to look in detail at the simple question of how to attach fixed guards.
