This article reviews some of the retained fastenings that are available for use on fixed machine guards, as required by the new Machinery Directive 2006/42/EC and the guarding standard EN 953:1997+A1:2009.
One of the changes in the new Machinery Directive is that the Essential Health and Safety Requirements (1.4.2.1, fixed guards), states: "fixing systems must remain attached to the guards or machinery when the guards are removed." This new requirement has also been added to the amended EN 953.
Various types of retained fastening - such as captive screws and quarter-turn fasteners - are available, but machine builders need to specify these with care if they are to find the optimum combination of purchase cost, installation cost and ease of use. Also bear in mind the requirement that "fixed guards must be fixed by systems that can be opened or removed only with tools." What follows is a summary of some of the products and suppliers operating in this field.
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 Dzus 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 SBC Spinning Clinch Bolt (pictured right), which is both low cost and quick to install. 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. Recent additions to the range are the CAPS Coloured Access Panel Crews that feature a durable moulded plastic cap in a choice of colour. Note that many of the threaded fasteners from PEM are available with an ant-cross-threading feature.
Specialty Fasteners & Components 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.
Reliance Precision Mechatronics LLP sells captive screws in metric sizes M2 to M6, some 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 are also available from Moss Plastic Parts, which offers a choice of natural nylon (to suit thread sizes M2 to M8) and HDPE (to suit M3 to M16), and Skiffy, which offers LDPE anti-loss washers in red, blue or grey with internal diameters of 2.7-10.5mm (small retaining tabs protrude within the diameter).
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 Item 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 in which the washer or O ring can be accommodated.
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.
