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.
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.
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.
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.
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.
If you know of other retained fastenings that are appropriate for fixed guards, please email the details to firstname.lastname@example.org 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.