Turnbuckle, Jack Screw, Adjustable Linkage Rod or Push-Pull Rod: What Is the Difference?

What principle do they share?

Opposing threads. A turnbuckle body is threaded internally at both ends, one right-hand and one left-hand. Turning the body one way draws both end fittings inward at the same time, and turning it the other way pushes them out. Each turn therefore changes the length by two thread leads instead of one, and the connected parts do not have to rotate, so a link can be adjusted while it stays connected at both ends. Rigging screws use the same principle.

What does each name mean?

A push-pull rod describes the duty. It can be built as any of the first three.

Is a jack screw the same as a screw jack?

No. A screw jack is a lifting or positioning machine: a driven screw that converts rotary motion into linear motion or force and can push, pull or tilt a load. Its size, drive torque and self-locking are the maker's business. The "jack screw" in a linkage is only a threaded adjuster. Do not use it to lift or hold a load.

Is a rigging turnbuckle the same as one for a machine linkage?

The mechanism is the same, the duty is not. Rigging turnbuckles are made to tension cables and rods in a straight line, with end fittings such as eyes, jaws and hooks, and with ratings and inspection rules in standards such as ASME B30.26. Final selection of a rigging turnbuckle follows the manufacturer's documents. A linkage in a machine may also have to push, may swing through an angle, and may see vibration. See Q1-02 for sizing and Q1-03 for the push case.

How do you choose?

• The link only pulls, and you want a simple adjustable tie. A turnbuckle or a pair of rod ends with a jack screw.
• The link pushes as well. A push-pull rod, checked for buckling (Q1-03), with pinned ends.
• You need a fixed centre-to-centre length that you can trim in place. An adjustable linkage rod, with jam nuts (Q1-05).
• Space is tight. A solid-bar or hex-bar adjuster is compact. For a longer link a tube is stiffer for the same weight (K07-Q1-10).
• You need to adjust without disconnecting. Use a right-hand and a left-hand thread. The syzmachine articles on LH and RH threads explain the setup.

What do you check when choosing a jack screw adjuster?

Five things, in this order.

• Threads. The two male threads must match the female threads of the rod ends in size, pitch and hand (Q1-07).
• Travel. Pick the body length for the adjustment range you need and keep thread engagement at both ends of the travel (Q1-04).
• Load. The male thread, the thread engagement and, if the link pushes, buckling set the limit (Q1-02, Q1-03).
• Material. Choose for the environment first and then for strength (Q1-08).
• Locking. Fit a jam nut on each end, a right-hand nut on the right-hand end and a left-hand nut on the left-hand end (Q1-05). Relieve the load and back off the nuts before you turn the body, or the threads can gall (Q1-09).

Does a fine adjustment thread lock itself?

Not reliably. A fine thread has a small lead angle, so friction in the flanks can hold a static load. Vibration or shock can break that friction momentarily, and a screw-jack maker (ZIMM) warns that even static self-locking can be lost this way. A linkage therefore relies on jam nuts to hold its length, not on the thread.

What changes on a suspension or steering link?

On a vehicle the same parts turn up in safety-critical jobs: the adjuster in a tie rod, the track bar (Panhard rod) that centres an axle, the radius or trailing rods of a four-link axle. Depending on the layout, braking, drive and cornering put these links in both pull and push, so the push case in Q1-03 applies, not only the tension case. The adjuster exists to set geometry (toe, axle position, link angle). Those settings are made once and must then stay put, so locking (Q1-05) matters more than ease of adjustment. Choose the form with the questions above, and take loads and thread size from the vehicle maker's data. The linkage rods and tie rod pages are the product pages, and the linkage rod buyer's guide covers selection. Bushed vehicle links and other automotive parts are covered on the sister site, syzmachine.com.

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