Expansion segments of a coiler or uncoiler mandrel: the wedge-driven shells that grip the coil bore, and how they are guided and held
Expanding mandrels of strip coilers and uncoilers · segments on wedge or cam faces moved by a draw rod · closing bars between segments · guides, anti-friction faces and holding against centrifugal force · spring-held versus pressure-held expansion
A coil cannot be wound onto, or paid off, a mandrel of fixed diameter: the mandrel has to grow to grip the bore and shrink to let the coil go. The parts that do this are the expansion segments, shells arranged around the mandrel shaft that slide outward on wedge or cam faces when a draw rod moves, and back in when it returns. Their guides, wear faces and the way they are held in the expanded position decide whether the mandrel keeps its shape under strip tension and speed.
Expansion segments of expanding mandrels on strip coilers, recoilers and uncoilers (decoilers): the segments, wedge or cam faces, draw rods, closing bars, lateral guides, wear plates and the means that hold the segments expanded. Complete motorised decoilers are covered on the Motorized single-mandrel decoiler page and hot-strip downcoilers on the Downcoiler page. Thermal expansion joints, expansion valves and loose bearings are outside this page.
CNFX editorial research · Sources checked October 19, 2026 · 3 sources
Segments on wedge or cam faces, moved by a draw rod through the shaft
SMS Siemag's patent US7975956 sums up the principle: to coil, the expansion segments are pushed radially outward until their outer surfaces form a circle; to uncoil, the mandrel must become smaller than the coil's inside diameter. A mandrel of this kind moves between two diameters, and only the fully expanded one is completely circular. Earlier designs, it notes, moved the segments on wedge surfaces driven by push or pull rods, or swung shell portions about an outer pivot with cylinders. [1]
Svenska Metallverken's US3507455 describes the same mechanism on a winding head: a ring of segments carries pairs of converging cam surfaces that bear on matching cam surfaces of the rotating core, so pushing the segments axially drives them radially outward; a rod through the centre of the core does the pushing. [3]
In Demag's US4131240 for hot-strip mandrel reels the wedge faces sit directly on a square-section mandrel shaft with fins, with opposing wedge faces inside each segment, and a radially expanding star unit on the tie rod engages pockets inside the segments; the expanding cylinder can pre-expand, re-expand, fully expand and provide an emergency expansion stroke. [2]
SMS Siemag adds that between the segments closing bars can be fitted; in the expanded position segments and closing bars form a closed circle, and in the contracted position gaps open between the segments because the closing bars sit further in. [1]
For a replacement segment set, give the mandrel drawing, number of segments and closing bars, expanded and contracted diameters, wedge angle and draw-rod stroke, segment length, and the coil weight, width and strip tension the mandrel carries.
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What the segments carry on a hot-strip mandrel
Demag lists the duty its mandrel reel was designed for: strip up to about 500 °C, 20 to 25 mm thick and up to roughly 2,300 mm wide, bundles of about 45 t, and threading speeds of 14 to 15 m/s for thinner strip. It also requires the segments to be held against centrifugal force at high speed and the mandrel to stay true to form so the coiling rollers can be set to an exact gap. [2] SMS Siemag cites a coiler design, EP 0 504 296 B1, for strip tension forces of 10 to 1,000 kN. [1]
Editorial calculation: at 14 m/s strip speed, a mandrel with an assumed 762 mm (30 in) expanded diameter turns at 14 ÷ (π × 0.762) ≈ 5.85 rev/s, about 350 rpm, at the start of coiling; the diameter is an example, not a figure from the patent. [2]
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Guides, wear faces and holding segments in the expanded position
Demag guides each segment on wear-resistant plates fixed to the fin flanks, with a spring in a spline at the wedge angle that holds the segment in any expanded position against centrifugal force; removable clamps and an annular crown piece hold the segment ends and let segments be slipped off for exchange or repair, and all guide faces carry anti-friction material such as special bronze. [2]
SMS Siemag warns that wedge designs must stay within a narrow range of friction behaviour, otherwise segments jam or shift under load. Its answer guides each segment by a web in an axial groove of the mandrel and clamps it with a hose that expands under pressure; it notes that the holding force doubles with each additional pair of frictional contact surfaces. [1]
Svenska Metallverken points to the risk of mandrels held expanded by hydraulic pressure: they need a constant fluid connection through wearing rotary seals, and a pressure drop can let the segments collapse during winding. Its design holds the segments expanded with a spring and uses a hydraulic piston only to contract them. [3]
Ask the supplier which holding method the mandrel uses, what keeps the segments expanded if hydraulic pressure is lost, and the wear-face material and replacement interval.
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Segment data to send with an enquiry
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 19 October 2026
Buyer questions
How do mandrel expansion segments expand?
A draw or push rod moves the segments along wedge or cam faces on the mandrel shaft, which drives them radially outward. [1][3]
Why fit closing bars between segments?
In SMS Siemag's design they close the gaps so segments and bars form a full circle when expanded. [1]
What holds the segments if hydraulic pressure is lost?
Svenska Metallverken's design keeps them expanded with a spring and uses hydraulics only to contract. [3]
Sources and what still needs a direct check
Sources checked 2026-10-19. Three patents from mandrel builders: Demag (hot-strip mandrel reel), SMS Siemag (coiler mandrel with closing bars and hose clamping) and Svenska Metallverken (spring-held expansible mandrel).
