This article reviews some of the retained fastenings for fixed guards (captive screws for machine guards) as required by the Machinery Directive 2006/42/EC and the machine guarding standard EN ISO 14120 (BS EN ISO 14120:2015 in the UK).
European Machinery Directive 2006/42/EC lists Essential Health and Safety Requirements that must be met and 184.108.40.206, fixed guards, states: "fixing systems must remain attached to the guards or machinery when the guards are removed." This requirement is mirrored in ISO 14120, sub-clause 5.19: "When it is foreseen, e.g. maintenance, that the fixed guard will be removed, the fastenings shall remain attached to the guard or to the machinery."
Various types of retained fastening - such as captive screws and quarter-turn fasteners - are available as standard components but machine builders need to specify these with care if they are to achieve the optimum combination of purchase cost, installation cost and ease of use. Also bear in mind the requirement (ISO 14120, 3.2 and ISO 12100:2010, 3.27.1) that fixed guards must be attached in such a manner that they can only be opened or removed by the use of tools; this implies that straight-slotted screws are unsuitable. Another point to watch out for is the issue of how to replace a fastener or receptacle that sustains damage in use.
Note that ISO 14120 restricts how quick-release fasteners such as quarter-turn screws can be used. Sub-clause 5.3.9, Removal of fixed guards, states that these shall not be used to secure fixed guards from outside the guarded area. However, their use is not prohibited if they are only accessible from inside the guarded area.
What follows is a summary of some of the products and suppliers operating in the field of retained fastenings for fixed guards on machinery.
Suppliers of retained fastenings for fixed guards
Southco manufactures an extensive range of captive (retained) and quick-access fasteners, many of which can be used on machine guards. Models include the 47/4C captive screws in a variety of head styles, threads, and with coloured heads for the Prism series (pictured right). In addition, the company offers the 52 series miniature captive screws and the 51/53/58 series of captive screws. Where fast access is required, Southco offers the 9/12/17 series fast-lead captive screws and the full Dzus range of quarter-turn fasteners.
PEM UK, a subsidiary of PennEngineering, supplies the SCB Spinning Clinch Bolt (pictured right), which is both low cost and quick to install. It is also offered as a self-retracting version incorporating a lightweight compression spring. In addition, PEM UK has a broad range of self-clinching panel fasteners with various head styles and threads, with some also benefiting from a floating design that compensates for panel misalignment. Note that many of the threaded fasteners from PEM are available with a MAThread anti-cross-threading feature.
Clarendon Specialty Fasteners (previously known as Specialty Fasteners & Components, SFC) supplies numerous products that are suitable for use on guards. These include push-turn fasteners, quarter-turn fasteners, captive bolts (pictured right) and captive screws that snap into position with thumb pressure and are retained by an aluminium grommet. Some products are equipped with a spring to provide a self-retraction function.
Reliance Precision sells captive screws in metric sizes M2 to M6, most of which are offered in a choice of lengths. These screws are of the type that have a long neck of a diameter smaller than the thread so that they can be unscrewed without causing the guard to be jacked off the part to which it is attached. Options include Imperial sizes and alternative materials.
Machine Building Systems offers the Item Safety Fastening Set Multiblock 8 that features a flanged, button-head M6 screw (bright zinc-plated steel) with a necked shank; this is secured on the panel by means of a stainless steel retaining spring. Although the fastener is designed for use with Item Multiblocks or Item Multi Brackets on Item aluminium profile frameworks, it can also be used with conventional tapped holes, rivet nuts or captive nuts.
A second option from Machine Building Systems is a simple, low-cost anti-loss washer. Available in sizes to suit M4, M5 and M6 screws, the plastic (natural PA) washer simply pushes onto the screw after it has been inserted through a clearance hole in the panel before the panel is attached to the machine frame. As with the Item Safety Fastening Set described above, the anti-loss washer can be used with Item Multiblocks or Item Multi Brackets on Item aluminium profile frameworks, as well as tapped holes, rivet nuts or captive nuts.
Similar anti-loss plastic washers (retaining washers) are also available from Essentra Components (previously known as Moss Plastic Parts), which offers a choice of natural nylon, nylon 6/6, HDPE and PE. The full range covers metric thread diameters from M2.5 to M8, as well as imperial thread sizes. This design of retaining washer features small tabs that protrude within the inner diameter. In most cases, free samples are available on request.
Jet Press offers Bolt Retainers that are austempered carbon steel washers with small flexible teeth. The Bolt Retainers can be pushed onto screws in a similar manner to the anti-loss washers described above, and they are available for M4, M5, M6, M8, M10 and M12 screw threads. Note that these are equally suitable for use with panels containing clearance holes or slots.
In a similar vein, machine builders sometimes use fibre washers or O rings that are an interference fit on the screw. However, depending on the thickness of the retainer and the design of the corresponding female threaded part, if the panel is to fit flush then it is necessary to introduce a counterbore to accommodate the washer or O ring.
Rudolf Rafflenbeul has developed the Savetix captive screw technology specifically to meet the new requirements in the Machinery Directive. Savetix fasteners comprise a metric screw with a slender shank, and a mating retainer. After the screw has been passed through a plain washer and the panel, a shallow conical spring steel retainer is pushed over the thread to ensure the screw does not become detached from the panel (or the screw is screwed through the retainer when the panel is assembled to the framework for the first time). Once the screw is tightened, the conical retainer is forced flat. A clever option for panels that do not need to be removed frequently is for the washers to be supplied with a self-adhesive ring so that the washers can be pre-applied to the framework, making installation much easier when multiple screws are required for attaching a guard. Screws are available in M4, M5, M6, M8, M10 and M12 sizes, in a variety of head styles, material specifications and finishes for the screw and retainer.
Wixroyd's catalogue includes precision captive screws manufactured from 303 grade stainless steel (other ferrous and non-ferrous materials are available on request). Head types are straight-slotted (not normally suitable for use on guarding), socket head, pan head Phillips and countersunk Phillips, with threads ranging from M2.5 to M12, depending on the head style. When ordering, customers can also specify threaded captive washers and a variety of locking washers.
For machine guards that require non-standard fasteners because of, for example, particular component geometries, Accu offers a custom design service for captive screws that enables designers to specify the head style, metric or imperial thread size and length, undercut diameter and length, whether or not a captive washer is required, the material, finish and general tolerances. Note that Accu has an extensive choice of head styles, including options for using Torx and security drivers in applications where machine builders are concerned about unauthorised removal of guards by people with access to common tools such as screwdrivers and Allen keys (hex drivers). Accu can manufacture custom captive screws in small batches or large quantities.
Alcoa Fastening Systems, under its Camloc brand, manufactures numerous captive quarter-turn and quick-release fasteners, many of which are suitable for use as retained fastenings for fixed guards. Most of the various fastener series are offered with a choice of head styles, though it should be noted that some do not require the use of a tool and are, therefore, unsuited to guarding applications.
If you know of other retained fastenings that are appropriate for fixed guards, please email the details to so the information can be added to this page.
There is also an informative article on MachineBuilding.net that explains how to specify fasteners for fixed guards on machinery and Procter Machine Guarding has published a free White Paper, Fixings for fixed guards, that considers the definitions of a 'fixed guard' and which fixed guards on a machine might require the use of captive fasteners (or retained fasteners) and which might not.