3.5 Rigging Screws & Turnbuckles

Rigging Screws & Turnbuckles

Rigging screws and turnbuckles are adjustable tensioning fittings used to set precise lengths in standing rigging, guardrail and lashing systems, sling legs, and guys. Although they perform no lifting motion themselves, they are lifting accessories when incorporated into a lifting or load-retaining system, and they are examined against the same marking, rating, and inspection disciplines as shackles and eyebolts.

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1. Construction: Rigging Screw vs. Turnbuckle

     RIGGING SCREW (TUBULAR BODY)                 TURNBUCKLE (OPEN BODY)

  (EYE)===[======= BODY =======]===(FORK)      (EYE)===[REIN]   [REIN]===(FORK)
        RH thread |  LH thread                            open frame
        ^ inspection hole drilled
        through body wall

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2. The Adjustment Principle: Right-Hand + Left-Hand Threads

Because one end of the body carries a right-hand thread and the other a left-hand thread, rotating the body in one direction draws both terminals inward simultaneously; rotating it the other way releases them. This gives precise, positive length adjustment without rotating the attached rope, stay, or sling—and without twisting the terminals.

• Locking: Once the correct tension is set, the terminal lock nuts (where fitted) must be tightened hard against the body to prevent the assembly vibrating loose.
• No shock loading: Rigging screws and turnbuckles are designed for steady tension. Impact or snatch loads can strip threads or bend the frame; they must not be used as dynamic shock-absorbing links.
• Adjustment range: Never screw the terminals out beyond the maximum safe take-up; adequate thread engagement must be maintained at all times (see the inspection hole rule below).

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3. The Drilled Inspection Hole (Thread Engagement Check)

A rigging screw's tubular body hides the threads, so the user cannot see whether each terminal is safely engaged. To solve this, the body is drilled with an inspection hole across each end:

• Screw each terminal in until its threaded end appears at (or beyond) the inspection hole.
• If the terminal end does not reach the hole, engagement is inadequate—the fitting could pull out under load.
• When adjusting out, never back a terminal so far that its end retreats behind the inspection hole.

This simple drilled witness hole is a frequently examined detail: its purpose is to facilitate checking that the threaded portion of the terminal fitting has adequately engaged with the body.

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4. Rating, Marking & Safe Use

• WLL marking: The assembly must be marked with its Working Load Limit, manufacturer's mark, and traceability code; the rating applies to straight axial tension only.
• Matching threads: Terminal fittings must match the body thread form and pitch exactly (metric coarse, UNC, or Whitworth); mixing thread forms strips the first threads and gives a false sense of security (the same fatal hazard described for eyebolts in Section 3.3).
• No side loading: Load must remain axial. Side pulls bend the frame and open the thread flanks.
• No welding or heating: As with all certified lifting accessories, welding or flame heating destroys heat treatment and is prohibited.

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5. Inspection & Rejection Criteria

As lifting accessories, rigging screws and turnbuckles require a pre-use visual check before every use and a thorough examination by a Competent Person at least every 6 months (LOLER Regulation 9(3)(a)(i), Section 1.3).

Practical note: When used for sling-leg length adjustment on asymmetric loads (Section 8.4), a turnbuckle forms part of the load path—its WLL, not merely its adjustment range, must be verified against the calculated leg tension.