The quarter turn fastener has proved a mainstay in panel fastening over the years

Written by: Paul Gay | Published:

It all started around 80 years ago, when William Dzus, an American engineer of Ukrainian descent, filed a patent for a turnlock fastener intended to be used to secure skin panels on aircraft and other high-performance vehicles.

In the late 1930s, the World was preparing for another war and development of fabrication techniques, especially in aircraft manufacture, was gathering pace. The 1936 patent would become the foundation of what has become an important fastener type.

Based on the 1936 patent, the Dzus fastener is particularly suitable for securing plates, doors, and panels that require frequent removal for inspection and servicing. It a type of proprietary quarter-turn lock fastener that is often referred to as quick-action panel fasteners. The fastener is of an over-centre design, requiring positive sustained torque to unfasten and any minor disturbance to the fastener will tend to correct itself rather than lead to further loosening as it would in threaded fasteners. The aerospace industry adopted the Dzus whole heartedly, the fastener finding its way into many warplane designs including the famous Spitfire.

Over the years, the design has developed and other turnlock fasteners have become available on the market. The Dzus fastener is now joined by other designs and are also referred to by their maker's trade name, such as Camloc, and Airloc.

The current Camloc fastener, the 4002 series, consists of four principal parts: the receptacle, the grommet, the retaining ring, and the stud assembly. The receptacle is an aluminium alloy forging mounted in a stamped sheet metal base, compare this with the spring wire arrangement of the Dzus where the fastening pin has a slot that turns a quarter turn onto the spring to fasten.

Camloc's receptacle assembly can be riveted to the access door frame, which is attached to the structure of the aircraft. The grommet is a sheet metal ring held in the access panel with the retaining ring. Grommets are furnished in two types: the flush type and the protruding type. Besides serving as a grommet for the hole in the access panel, it also holds the stud assembly. The stud assembly consists of a stud, a cross pin, a spring, and a spring cup. The assembly is designed so it can be quickly inserted into the grommet by compressing the spring. Once installed in the grommet, the stud assembly cannot be removed unless the spring is again compressed.

There is also a high- strength version of the Camloc, which is a quick-release rotary fastener that may be used on flat or curved panels. This fastener may be distinguished from screws by the deep No 2 Phillips recess in the stud head and by the bushing in which the stud is installed.

A threaded insert in the receptacle provides an adjustable locking device. As the stud is inserted and turned counter clockwise one-half turn or more, it screws out the insert to permit the stud key to engage the insert cam when turned clockwise. Rotating the stud clockwise one-quarter turn engages the insert. Continued rotation screws the insert in and tightens the fastener.

The Airloc fastener is similar to the Camloc but consists of a receptacle, stud, and cross pin. The stud is attached to the access panel and is held in place by the cross pin. The receptacle is riveted to the access panel frame. Two types of receptacles are available, either fixed or floating type. The floating type makes for easier alignment of the stud in the receptacle. Several types of studs are also available. In each instance the stud and cross pin come as separate units so that the stud may be easily installed in the access panel.

The Airloc receptacle is fastened to the inner surface of the access panel frame by two rivets. The rivet heads must be flush with the outer surface of the panel frame. When you are replacing receptacles, drill out the two old rivets and attach the new receptacle by flush riveting.

Dzus fasteners are currently available in two types. One is the light-duty type, used on box covers, access hole covers and lightweight fairings. The second is the heavy-duty type, which is used on cowling and heavy fairings. The main difference between the two types is a grommet, which is only used on the heavy-duty fasteners. Otherwise, their construction features are about the same.

The light-duty Dzus includes a spring and a stud. The spring is made of cadmium-plated steel music wire and is usually riveted to an aircraft structural member. The stud comes in various designs and mounts in a dimpled hole in the cover assembly. When the panel is being positioned on an aircraft, the spring riveted to the structural member enters the hollow centre of the stud. Then, when the stud is turned about a quarter turn, the curved jaws of the stud slip over the spring and compress it. The resulting tension locks the stud in place and secures the panel.

In the 1930s, vehicles were not as powerful or as capable as they are today and the higher vibration levels experienced these days, especially on the race track, are capable of much more disturbance that the traditional Dzus design is capable of handling. Development work on a new design began and fastener engineers at Specialty Fasteners and Components (SFC) came up with a new design, especially suited to motor sport but equally valuable as a premium product for access panel fastening.

SFC had listened to its motorsport customers who were experiencing problems during the frenetic atmosphere of qualifying and pit stops. It became obvious that a more reliable, easy-to-use and consistent self-ejecting fastener was something that was sorely needed.

Working with engineering partner Camloc, SFC came up with what it considers an ideal solution. The design includes some interesting features including a cross-head stud that operate the lock. This removes the risk of accidental tool damage to panels during operation.

A locking spring ensures consistent, positive lock performance and light weight construction is in keeping with motorsport requirements. The design offers the benefits of high strength without any weight penalty.

The mounting plate is fully interchangeable with any panel prepared for EHF series fastener and helps distribute the load over a greater area. A hold out spring lifts the stud for easier panel removal. And for installations, where use of a screwdriver is difficult, the bail handled version can be specified. The AeroLoc quarter turn fastener is now in production and has proved it is able to overcome the short comings of the 80 year old design.

There is no doubt that panels traditionally fixed with quick access quarter turn fasteners do occasionally vibrate loose and come undone. Motorsport spectators will see the ultimate machinery racing around a track with many of its quick release fasteners taped up with ubiquitous pit lane tape that all teams stock.

The Aeroloc, which is designed in the UK and made in Germany, overcomes these problems and will surely enjoy a long and effective future.



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