What types of retained fastenings can I use for fixed guards?
Posted to News on 30th Jan 2024, 15:00

What types of retained fastenings can I use for fixed guards?

You may have heard or read something about particular types of fastenings being required for securing machine guards in position. Derek Coulson, machinery safety consultant at Safe Machine, looks at the requirement for fixed guards to be attached with retained fastenings, and unravel what exactly that means, and lists some of the suppliers that offer various different types of retained fastening.

What types of retained fastenings can I use for fixed guards?

A retained fastening, sometimes called a captive screw, is a fastening that remains attached to the guard or machinery when the guard is removed.

A fixed guard is defined as a guard affixed in such a manner (for example, by screws, nuts, and welding) that it can only be opened or removed by the use of tools or by destruction of the means by which the guard is affixed' (BS EN ISO 14120:2015, Safety of machinery Guards General requirements for the design and construction of fixed and movable guards).

This definition is all but identical to that found in BS EN ISO 12100:2010, Safety of machinery General principles for design Risk assessment and risk reduction.

If fastenings are not retained when a guard is removed, there is a chance that they will be lost, leading to the guard being replaced without its full complement of fixings. Someone could then force the guard open and reach into the danger zone. As fixed guards are not equipped with guard switches, the machine can continue to operate in this dangerous state.

The requirement for fixed guards to be secured with retained fastenings is laid down in several places. BS EN ISO 14120:2015, sub-clause 5.19 (Retained fastenings) states When it is foreseen, for example maintenance, that the fixed guards will be removed, the fastenings shall remain attached to the guard or to the machinery'. Note that this sub-clause also outlines some circumstances under which retained fastenings are not required but extreme caution should be applied if the designer is considering not using retained fastenings.

It is important to note that BS EN ISO 14120:2015 is harmonised to the EU Machinery Directive and is a designated standard for the Supply of Machinery (Safety) Regulations in Great Britain. In both cases, complying with the standard is the simplest way to meet the relevant essential health and safety requirements (EHSRs).

If a machine is being CE marked for placing on the market in the EU, EEA or Northern Ireland, the Machinery Directive 2006/42/EC requires certain EHSRs to be met. Of these, (Fixed guards) states fixing systems must remain attached to the guards or machinery when the guards are removed.'

On 20 January 2027, the EU Machinery Directive will be replaced by Machinery Regulation (EU) 2023/1230. The corresponding EHSR in the new Machinery Regulation, (Fixed guards) has been slightly reworded but is essentially the same: Their fixing systems shall remain attached to the guards or to the machinery or related product when the guards are removed'.

For machinery being placed on the market in Great Britain (ie England, Scotland and Wales), the relevant requirements for UKCA marking are the same as for CE marking. At the time of writing, it is unclear whether or not the equivalent GB legislation, the Supply of Machinery (Safety) Regulations, will be revised in line with the new EU Machinery Regulation but, either way, the need for retained fastenings on fixed guards will remain.

Suppliers and types of retained fastening

Southco manufactures an extensive range of captive (retained) and quick-access fasteners, many of which can be used on machine guards. Models include the series 47 captive screws that come in a variety of head styles and threads, and with coloured heads for the corresponding series 4C Prism series.

In addition, the company offers the series 52 miniature captive screws, F5 flush captive screws and 51/53/58 captive screws. Where rapid access is required from inside the guarded area, Southco offers 9/12/17 fast-lead captive screws and the full Dzus range of quarter-turn fasteners.

PEM, a PennEngineering brand, supplies the SCB Spinning Clinch Bolt, which is both low cost and quick to install. It is also offered as a self-retracting version incorporating a lightweight compression spring. Similarly, the SFN Spinning Flare Nut is a one-piece, flanged hex nut that is installed by pressing into a pre-punched embossed mounting hole. The nut is permanently captive and still spins freely in the sheet.

In addition, PEM has a broad range of PF series self-clinching panel fasteners with various head styles and threads, with some also benefiting from a floating design that compensates for panel misalignment. Many of the threaded fasteners from PEM are available with a MAThread feature to prevent cross-threading.

Clarendon Specialty Fasteners (CSF) supplies captive screws that are all variations of the reduced shank' concept. One style features a retraction spring and uses a countersunk retainer nut to secure the fastener sub-assembly housing to the guard, though the housing stands proud of the of the guard surface and this may be unsuitable for some machines.

Another style retains the screw by means of an aluminium grommet that snaps into a hole in the guard, but note that the machine will need a small counterbore to clear the protruding tail of the grommet. The third style secures the fastener sub-assembly to the guard by swaging; as with the first style, this leaves a fastener housing that stands proud of the guard, so may not suit all applications.

Finally, CSF also supplies plain reduced shank screws for inserting through guards that have a hole tapped with the same thread. Alternatively, the reduced shank screws can be supplied with threaded washers for use in guards having clearance holes. All of these products are manufactured from 316 (A4) grade stainless steel in a variety of thread sizes (M3 to M8 and Imperial threads), lengths, head styles and drive types. For fixed guards that are secured from inside the guarded area, CSF also supplies a range of light-, medium- and heavy-duty quarter-turn fasteners.

Reliance Precision sells stainless steel 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 reduced shank diameter. Options include Imperial sizes and alternative materials.

WDS Components supplies similar 316 (A4) grade stainless steel reduced-shank captive screws in metric sizes M3 to M12, all in a choice of lengths. These are for use with guards that have a threaded hole. Alternatively, for guards having clearance holes, WDS offers 316 (A4) grade stainless steel threaded washers for retaining the screw on the guard. If required, the captive screws and washers can be supplied as kitted sets.

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 reduced shank diameter; this is secured on the guard by means of a sprung stainless steel retaining washer. 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.

An alternative product 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 a standard screw after it has been inserted through a clearance hole in the guard before the guard is attached to the machine.

Essentra Components supplies retaining washers in a choice of PC, PA, HDPE, LDPE and PVC. The full range covers metric thread diameters from M2.5 to M12, as well as Imperial thread sizes. This design of retaining washer features small tabs that protrude within the inner diameter.

The company also sells threaded captive washers for use in conjunction with captive screws having a reduced shank diameter. These are available in grade 303 (A2) or 316 (A4) grade stainless steel to suit screw threads from M2.5 to M20. Free samples of most sizes of plastic retaining washer are available on request.

Wixroyd, which is part of Essentra Components, supplies reduced shank captive screws that can be used on guards having tapped holes of the same thread size. These screws are available with a variety of head styles, drive types, thread sizes (M2 to M6) and lengths. They are manufactured from 303 (A2) grade stainless steel as standard. If required, Wixroyd can manufacture specials to a customer's design and specification, including alternative materials and finishes.

Another product that can be used in conjunction with this style of captive screw is threaded retaining washers, which enable the screws to be used in guards that have clearance holes rather than threaded holes.

Jet Press offers a range of slimline Push Nut Bolt Retainers that are austempered carbon steel washers with small flexible teeth to push over standard screws with thread sizes from M4 to M12. These are equally suitable for use with panels containing clearance holes or slots. Jet Press can provide free samples upon request.

Rudolf Rafflenbeul developed its Savetix captive screw technology specifically to meet the requirements for fixed guards to be secured with retained fastenings. Savetix fasteners comprise a metric screw (cap head or hex head) with a reduced shank, and a special shallow conical retaining washer. After the screw has been passed through the guard, the conical retaining washer is pushed over the thread (it is a clearance fit for easy assembly).

When the guard is fitted and the screw tightened for the first time, the conical retainer is forced flat and the internal diameter is reduced so it becomes a close fit on the screw's reduced shank and no longer a clearance fit over the thread. This means that if the screw is undone, it is retained on the guard.

A clever option for panels that do not need to be removed frequently is for the conical washers to be supplied with a self-adhesive ring so that the washers can be pre-applied to the machine to make installation quicker and easier when a guard is secured with multiple screws. Screws are available in M4, M5, M6, M8, M10 and M12 sizes. Special sizes can be manufactured to order, and the company also offers alternative finishes such as black anodising. Free samples are available on request.

Accu has a similar product but with a significant difference. The Accu Captive Washer is conical but has a threaded hole, so it is screwed onto the reduced-shank screw rather than being a clearance fit. When the guard is secured to the panel for the first time, the conical washer is flattened. Because the washer's internal diameter has been reduced and the thread deformed, the washer can no longer be unscrewed from the captive screw.

Accu Captive Washers are offered in M3, M4, M5 and M6 sizes, all in A2 (304) grade stainless steel, and with a choice of natural or matte black finish. As a simpler and lower-cost alternative, Accu also supplies conventional retaining washers for use with reduced-shank screws. These come in a broad choice of sizes in austempered carbon steel (with either a bright zinc plated or phosphate and oil finish) or plastic (HDPE, nylon or PA).

If none of its standard products are suitable Accu can manufacture small or large batches of bespoke captive fasteners, including variants with tamper-resistant drives.

As well as the suppliers listed above, many others sell reduced shank captive screws, as well as plain and threaded retaining washers. Some will also manufacture custom products.

Which retained fastenings are best?

When specifying retained fastenings fixed guards, beware that not all types and variants on the market are suitable. Fixed guards must be attached in such a way that they can only be opened or removed using tools (BS EN ISO 14120:2015 sub-clause 3.2, BS EN ISO 12100:2010 sub-clause 3.27.1, EU Machinery Directive 2006/42/EC Annex III sub-clause, and Machinery Regulation (EU) 2023/1230 Annex III sub-clause

A fastening with a straight slot for a screwdriver would not be suitable, as this can be undone using an improvised tool such as a steel ruler or coin. Knurled knobs and wing nuts are also unsuitable.

Another point to watch out for is that quick-release fasteners, including self-clinching fasteners and quarter-turn screws, can only be used if they are accessible from inside the guarded area, not outside (BS EN ISO 14120:2015 sub-clause 5.3.9).

Many of the fixings described above feature a reduced shank. Care is required when incorporating these within machine designs, especially those with rigid guards, to ensure the screw thread can be undone fully from the machine while the guard is still in position. This might require, for example, a deep tapped hole in the machine frame, counterbored to a depth greater than the length of the screw thread.

For captive screws that are retained by a washer of any type, either the machine frame or guard must have a shallow counterbore to accept the retaining washer, otherwise the guard will not sit flush against the machine.

Machine builders sometimes convert standard screws to captive types by using fibre washers, rubber washers or O-rings that are a light interference fit on the screw thread. However, if the guard is to fit flush, then either the guard or machine must be counterbored to accommodate the washer or O-ring. Moreover, if the guard is rigid and does not come away from the machine as the screw is undone, the screw can easily be unscrewed from the retaining washer or O-ring, which could then be lost.

Finally, some types of retained fastener will need specialist tooling for installation, some will be more tolerant of misalignment, costs can vary significantly from one type to another both in terms of purchase cost and installation time, and some will be more difficult to replace if the fastener is damaged.

Each fixed guard will have its own requirements. It might be preferable to use a single type of retained fastening throughout a machine but, in some cases, it could be more cost-effective to specify different types if the requirements are not the same throughout.

Safe Machine Ltd

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