5.5 Quick-Release Fasteners, Circlips & Keys
Key Takeaways
- Turnlock fasteners consist of a stud assembly, a grommet and a receptacle, and are used only on non-structural removable panels such as cowlings, fairings and access doors.
- A Dzus stud locks by a spiral cam slot engaging a spring wire, while Camloc and Airloc studs lock by a cross-pin engaging a cam receptacle.
- Turnlock studs are selected by grip length matched to the total panel thickness; the wrong grip either fails to engage or never pulls the panel tight.
- A circlip retains axially and provides no clamping force; over-expanding it past its elastic limit ruins it invisibly, and it must seat fully in the groove around its entire circumference.
- A key transmits torque through its sides with clearance at the top, is the intended weak link in the drive, and its keyway is a stress raiser where shaft cracks commonly start.
Closing Out Sub-Module 6.5.3
Sub-module 6.5.3 Locking devices is level 2 for A, B1, B3 and B2, and its content bullet reads:
"Tab and spring washers, locking plates, split pins, pal-nuts, wire locking, quick-release fasteners, keys, circlips, cotter pins."
Section 5.3 covered tab and spring washers, locking plates, split pins, pal-nuts, wire locking and cotter pins along with torque procedure. This section covers the last three families — quick-release (turnlock) fasteners, circlips and retaining rings, and keys — which lock or retain by geometry rather than by friction or by a deformed wire.
Quick-Release (Turnlock) Fasteners
A turnlock fastener secures an access panel, cowling or fairing with a fraction of a turn, so that a panel with twenty fasteners can be opened in seconds. They are used only on non-structural removable panels — cowlings, inspection doors, fairings, fillets and radome doors.
Every turnlock fastener has three parts:
| Part | Function |
|---|---|
| Stud assembly | Carried in the removable panel; contains the operating head and the locking element |
| Grommet | A hollow rivet-like eyelet that mounts the stud in the panel and takes the wear |
| Receptacle | Riveted to the fixed structure; contains the spring or cam that the stud engages |
The three classic types
Dzus. The stud has a spiral cam slot cut into its shank. The receptacle is a stiff spring wire. Pushing the stud in and turning it a quarter turn draws the spring wire up the cam slot, which pulls the panel down and holds it under spring tension. The stud is retained in the panel by the grommet, so it cannot be lost. Dzus parts are identified by a head-style letter, a size number and a grip length: in a designation such as F6½-80, F is the flush (countersunk) head style, 6½ is the stud size code (0.375 in major shank diameter) and 80 is the grip length in hundredths of an inch (0.800 in). Fitting a stud with the wrong grip is the classic error — too short and it will not engage, too long and it never pulls the panel tight.
Camloc. The stud carries a cross-pin through its shank. The receptacle contains a cam and a spring. A quarter turn rotates the cross-pin along the cam ramp, drawing the panel down; the spring then holds the cross-pin in the detent. Camloc studs are supplied in graded lengths to suit the total panel thickness, and the stud, grommet and receptacle are matched parts.
Airloc. Similar in principle to the Camloc — a cross-pin stud engaging a cam receptacle — but with a heavier stud and a receptacle designed for higher retention loads. The stud is again matched to the panel thickness.
Maintenance points
- Grip length is measured, not guessed: the total thickness of the panel and any doubler determines the stud length. Stud gauges are used to select the correct part.
- Turnlock fasteners rely on a spring. A receptacle spring that has taken a set no longer holds the stud, and the panel will drum, chafe and eventually depart in flight. Cowling fastener security is a routine walk-around item.
- Worn grommets let the stud float and the panel rattle; the grommet is replaced with the stud.
- Some quick-release fasteners are additionally safetied with lockwire or a lanyard where an in-flight release would be hazardous.
- Quick-release (ball-lock) pins are a related family: a pin with spring-loaded balls released by a plunger, used for seat, ballast, cargo-net and control-lock attachments. Their locking balls must be checked for full extension before the pin is relied upon.
Circlips and Retaining Rings
A circlip (retaining ring, snap ring, Seeger or Truarc ring) is a spring-steel ring that seats in a machined groove and forms an artificial shoulder to retain a component axially. It clamps nothing — it simply stops a part from moving along a shaft or out of a bore.
Types
| Type | Fits | Notes |
|---|---|---|
| External (shaft) ring | A groove in a shaft; retains bearings, gears and collars | Expanded to fit over the shaft, then contracts into the groove |
| Internal (bore) ring | A groove in a housing bore; retains bearings and seals | Compressed to enter the bore, then springs out into the groove |
| Lug (Truarc/Waldes) type | Both | Has two lugs with holes for dedicated circlip pliers |
| Plain type | Both | No lug holes; removed with a hooked pick |
| E-clip | Shaft, applied radially | Snaps sideways onto a shaft groove; used on light control linkages and clevis pins |
| Spiral-wound ring | Both | Two or more coils, no lugs, gives a continuous 360° bearing face |
| Bowed / beveled rings | Both | Deliberately dished to take up end float or to preload against a tapered groove face; must be installed the correct way round per the maintenance data |
Installation and inspection rules
- Use the correct circlip pliers, sized to the ring's lug holes. Expanding an external ring or compressing an internal ring with pliers that are too large over-strains the ring past its elastic limit; it will then never grip properly, even though it looks undamaged.
- Expand or compress only as much as needed to clear the shaft or bore. Permanently deformed rings are scrapped, not re-used.
- After fitting, verify the ring is fully seated in the groove around its entire circumference. A ring sitting partly on the shoulder will jump out at the first axial load.
- Check the groove as well as the ring: a worn, burred or corroded groove reduces the retention capacity even with a perfect ring.
- Direction matters for bowed, beveled and some stamped rings. Follow the AMM or the component maintenance manual — the load-carrying face is defined by the manufacturer.
- A circlip is a retaining device, not a load-bearing device: it takes axial thrust up to the rated value only. If a bearing is transmitting significant axial load, look for a nut, a spacer and a torque figure, not just a circlip.
- Removing an internal circlip from a housing containing a compressed spring is a stored-energy hazard — the ring can launch. Eye protection and a shield are standard practice.
Keys and Keyways
A key is a parallel or tapered bar sunk half into a shaft and half into the mating hub, transmitting torque between the two while positively preventing relative rotation. It is a locking device in the sense the syllabus intends: it stops one component turning on another.
| Key type | Form | Application |
|---|---|---|
| Parallel (square or rectangular) key | Straight bar in a milled shaft keyway | Gears, pulleys and couplings on accessory drive shafts |
| Gib-head key | Tapered key with a head for extraction | Where the key must be driven out during disassembly |
| Woodruff key | Semicircular disc sitting in a semicircular pocket | Self-aligning on tapered shafts; magneto and accessory drive couplings |
| Feather key | Long parallel key fixed to the shaft, allowing the hub to slide axially | Sliding couplings |
| Splines | Multiple integral keys machined into the shaft and hub | High-torque drives — engine accessory pads, gearbox shafts. Splines share the load over many teeth, so no single key can shear |
Key points for the exam:
- The key is deliberately the weak link in torque transmission: it will shear before an expensive shaft or gear is damaged. A sheared key is a symptom — find out why the drive overloaded before fitting a new one.
- A keyway is a stress raiser. Cracks in shafts very often start at the sharp corner of a keyway, which is why keyway corners are radiused and why NDI concentrates there.
- Keys must be a firm fit on the sides (the sides transmit the torque) with clearance at the top. A key that beds on its top face and rattles on its sides will pound out its keyway.
- Look for fretting debris around a key or spline — brown or black powder means relative micro-movement, which means the fit has gone and the joint is failing.
- Splined couplings are wet or dry: dry splines need periodic anti-fret lubrication in accordance with the AMM, and running a dry spline unlubricated produces exactly the fretting failure described in Section 4.2.
Exam Traps
- Turnlock fasteners are for non-structural panels only. Any answer that puts a Dzus or Camloc into a structural joint is wrong.
- Dzus works by a spiral cam slot and a spring wire; Camloc and Airloc work by a cross-pin engaging a cam. Do not swap the mechanisms.
- Turnlock stud selection is by grip length, matched to the panel thickness — not by "the next one out of the bin".
- A circlip retains, it does not clamp. Questions offering "provides axial clamping force" as the function of a retaining ring are testing this.
- Over-expanding a circlip past its elastic limit ruins it invisibly; damaged or previously over-stressed rings are single-use.
- The sides of a key carry the torque; there must be top clearance. A "tight on top" key is incorrectly fitted.
How does a Dzus quick-release fastener lock a cowling panel closed?
A technician uses oversize circlip pliers to fit an internal retaining ring, compressing it well beyond the diameter needed to clear the bore. What is the consequence?
How should a parallel key be fitted between a drive shaft and a gear hub?
Which statement about quick-release turnlock fasteners is correct?