Almond's Flexible Metallic Tube is a flexible pipe built from a single metal strip wound helically and interlocked with itself so each turn hooks into the next. It solves the problem of carrying steam, gas, or fluids through a path that bends and vibrates without using rubber or leather. The interlocking profile lets the tube flex while the mating faces stay sealed under internal pressure. You will still find this construction today in strip-wound metal hose used on furnace doors, exhaust runs, and cable conduit.
Almond's Flexible Metallic Tube Interactive Calculator
Vary bore, helical strip pitch, and hook engagement to see the safe minimum bend radius and how the interlocked strip behaves.
Equation Used
The calculator applies the article bend-limit equation for a strip-wound interlocked tube. D_i is the internal bore, p is the axial pitch of the helical strip, and delta is the hook engagement depth. A larger pitch or smaller engagement increases the minimum safe bend radius.
- Static bend limit before hook disengagement.
- Bore, pitch, and engagement use the same length unit.
- Pressure, fatigue, packing wear, and end-crimp effects are not included.
Inside the Almond's Flexible Metallic Tube
The mechanism is a single ribbon of metal — usually brass, bronze, or steel — rolled with a precise S-shaped or hook-shaped cross-section, then spiral wound around a mandrel so each new turn locks into the previous one. The hook on one edge catches under the lip of the edge before it. When you pull the finished tube straight, the hooks bottom out against each other and the tube goes rigid in tension. When you bend it, the hooks slide over a small gap on the outside of the bend and close tighter on the inside. That sliding action is what gives the tube its flex without breaking the seal.
The seal between turns is mechanical, not bonded. A packing of asbestos cord, graphite yarn, or in modern strip-wound hose a PTFE or graphite ribbon, sits inside the hook before winding. When the strip clamps down, the packing gets squeezed into every gap. If your packing is undersized — say 0.2 mm short on cross-section — the tube leaks under the first pressure cycle. Oversize the packing and the tube will not bend without the strip popping out of its hook. The gauge of the strip matters too. Run a 0.3 mm strip when the spec calls for 0.5 mm and the hooks open up after a few hundred flex cycles, and you would be amazed how fast the tube goes from gas-tight to pinhole-leaking.
Failure modes are predictable. Hook unrolling at the ends — that is what happens when the field crew cuts the tube without re-crimping the last two turns. Crevice corrosion under the packing — that is what kills brass tubes in salty air. Work-hardening cracks at the strip edges — that is what kills any flexible metal hose run past its rated bend radius cycle after cycle.
Key Components
- Helical Strip: A continuous ribbon of metal, typically 0.3 to 0.8 mm thick, rolled with an S or hook profile. The strip width sets the pitch of the finished tube — wider strip means coarser pitch and tighter minimum bend radius. Brass and bronze were the originals; modern strip-wound hose uses 304 or 321 stainless.
- Interlocking Hook Profile: The cross-section of the strip is rolled so one edge folds into a J and the opposite edge folds into a matching catch. When wound, the J of turn N+1 hooks under the catch of turn N. The clearance between hook and catch is typically 0.05 to 0.15 mm — tight enough to hold packing, loose enough to slide during flex.
- Inter-Turn Packing: A soft sealing cord laid into the hook before the strip closes. Original Almond tubes used asbestos or cotton cord soaked in tallow. Modern equivalents use graphite yarn or PTFE tape rated for the service fluid. Packing cross-section must match the hook void within 5% or the tube either leaks or jams.
- End Fittings: Brazed or swaged collars that grip the last two or three turns of strip and present a threaded or flanged interface. The fitting must clamp tight enough that the strip cannot unroll under axial load, because once one turn lets go the whole tube unravels like a clock spring.
Industries That Rely on the Almond's Flexible Metallic Tube
The construction is over 130 years old but it earns its keep in any application where you need flexibility plus heat or abrasion resistance that a rubber hose cannot deliver. You see it anywhere a flexible link has to survive what would melt or chafe through an elastomer. The same interlocking-strip principle shows up under names like strip-wound metal hose, squarelock conduit, and flexible metal tubing.
- Industrial Heating: Flexible exhaust connections on Cleaver-Brooks and Fulton boiler vent stacks where rigid pipe cannot accommodate thermal expansion.
- Foundry & Glass: Cooling water hose to the gob feeder mechanism on Owens-Illinois IS glass-forming machines, where radiant heat would destroy a rubber line in hours.
- Electrical: Squarelock and interlocked-armour cable conduit (the Anaconda Sealtite family is a direct descendant) for routing wiring through machine tools where cutting fluid would attack PVC.
- Aerospace Ground Support: Hot-air ducting between APU ground carts and aircraft pneumatic ports — the strip-wound stainless hose handles 200 °C bleed air without softening.
- Locomotive & Rail: Flexible steam connections between locomotive and tender, and between coaches on steam-heated passenger trains — the original 1890s application that made Almond's design famous.
- Process Plant: Flexible jumpers on hot-oil heat-transfer systems running Therminol or Dowtherm at 300 °C, where the alternative is an expensive bellows assembly.
The Formula Behind the Almond's Flexible Metallic Tube
The number that decides whether a strip-wound tube survives is the minimum bend radius. Bend it tighter than the limit and the hooks pry open on the outside of the curve and the packing extrudes. The bend radius is set by the strip pitch and the hook engagement depth. At the loose end of the typical range — pitch around 6 mm on a 25 mm bore — the tube bends almost like a garden hose and you can route it through tight corners, but axial stiffness drops and it kinks if you pull too hard. At the tight end — pitch under 3 mm on the same bore — you get a stiff, near-rigid tube that holds pressure cleanly but only flexes through a gentle arc. The sweet spot for general industrial service sits around pitch = 0.15 to 0.20 × bore.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Rmin | Minimum static bend radius before hook disengagement | mm | in |
| Di | Internal bore diameter of the tube | mm | in |
| p | Pitch of the helical strip (axial distance per turn) | mm | in |
| δ | Hook engagement depth (how deep one turn sits inside the next) | mm | in |
Worked Example: Almond's Flexible Metallic Tube in a steam-heat jumper on a heritage locomotive
You are specifying a strip-wound brass flexible tube to carry 100 psi saturated steam between the locomotive footplate and the leading passenger coach on a heritage railway restoration. Bore is 25 mm, strip pitch is 4 mm, hook engagement is 1.2 mm. You need to know how tight you can route it around the buffer beam without breaking the seal.
Given
- Di = 25 mm
- p = 4 mm
- δ = 1.2 mm
Solution
Step 1 — at nominal pitch p = 4 mm with a 25 mm bore and 1.2 mm hook engagement, compute the minimum static bend radius:
Step 2 — at the loose end of the typical range, pitch p = 6 mm (a coarse-wound tube favoured for tight routing), hook engagement drops slightly to about 1.0 mm because the strip rolls thinner:
That seems backwards until you realise coarser pitch puts more strip-edge stress on each hook during a bend. The tube feels floppy in your hands but the hooks need a gentler arc to stay seated. You can pull it through a 100 mm bend by hand all day, but anything tighter and you will hear the strip clicking as hooks pop.
Step 3 — at the tight end, pitch p = 3 mm with a generous 1.5 mm hook engagement (a fine-wound steam-tight tube, like the original Almond locomotive jumpers):
This is the sweet spot for steam service. The tube is stiff to handle but it bends through a 30 mm radius cleanly and the packing stays put under 100 psi. Above this pressure you would step the bore up rather than tighten the pitch further.
Result
Nominal minimum bend radius is 48. 3 mm — call it 50 mm in the field. In practice that means you can wrap the tube once around a 100 mm diameter mandrel without leaks, which is plenty for the buffer-beam routing on a heritage coach. The range is wide: a coarse 6 mm pitch needs nearly 95 mm of bend radius even though the tube feels loose, while a fine 3 mm pitch tightens to 28 mm and is the configuration the original Almond locomotive hoses used. If your installed tube weeps steam at the bend before reaching rated pressure, suspect three things in this order — packing cord undersized by more than 5% so the hook void is not filled, end fitting swaged on only the last single turn instead of the required two-to-three turns letting the strip unroll axially, or a sharp pull during installation that exceeded Rmin momentarily and pried two adjacent hooks apart by 0.1 mm or more.
Almond's Flexible Metallic Tube vs Alternatives
You have three realistic options when you need a flexible pressure or gas line: an Almond-style strip-wound metal tube, a hydroformed metal bellows hose, or a reinforced rubber/PTFE hose. They sit at different points on cost, temperature ceiling, and flex life.
| Property | Almond Strip-Wound Tube | Hydroformed Metal Bellows Hose | Reinforced PTFE/Rubber Hose |
|---|---|---|---|
| Maximum continuous temperature | 500 °C (stainless), 250 °C (brass) | 650 °C+ (Inconel) | 200 °C (PTFE), 120 °C (rubber) |
| Working pressure (25 mm bore) | 100–250 psi | 300–600 psi | 150–3000 psi (depends on braid) |
| Minimum bend radius (multiple of bore) | ~2 × Di | ~6 × Di | ~4 × Di |
| Flex cycles to failure | 10,000–100,000 | 100,000–1,000,000 | 1,000,000+ (rubber), lower (PTFE) |
| Relative cost per metre | Low to medium | High | Low |
| Gas-tight without external jacket | No (relies on packing) | Yes (welded) | Yes (continuous bore) |
| Best fit application | Hot exhaust, conduit, steam jumpers | High-pressure hot-fluid lines, vacuum | Hydraulics, chemical transfer, low-temp |
Frequently Asked Questions About Almond's Flexible Metallic Tube
Bending opens the hook gap on the outside of the curve. If the packing cord is sized correctly, it stays compressed in the gap and seals. If the packing has compacted over time — common with old asbestos cord that has lost its tallow — the gap exceeds what the packing can fill and you get a weep on the outer radius only.
Quick check: pressurise straight, then slowly bend. The leak point will be on the outside of the bend. Repack with graphite yarn one size up from nominal and it usually solves it.
Strip-wound wins on cost and tight bend radius. Bellows wins on absolute leak tightness and pressure. For Therminol or Dowtherm at 300 °C the deciding factor is usually the consequence of a small leak — hot oil dripping onto a hot surface is a fire risk, so most plants spec a welded bellows hose despite the cost. If you are running hot air, exhaust gas, or low-pressure steam where a slow weep is tolerable, strip-wound is the right call.
Yes, but you have to immediately re-secure the last two or three turns or the strip unrolls. The factory method is to braze or swage a fitting onto the cut end within seconds of cutting. Field method: clamp the last three turns with a hose clamp before you cut, then fit a compression collar that grips at least two full turns of strip. If you cut and walk away, the tube will be a useless coil of ribbon by morning.
Two causes dominate. First, packing degradation — heat or chemical attack hardens the cord, and once it loses compliance the hooks cannot slide over each other during flex. Second, debris or scale building up in the hook gap. On steam service, carbonate scale wedges into the hooks and locks them. You can sometimes recover flex by working the tube back and forth while flushing with descaler, but if the strip itself has work-hardened the tube is done.
Aim for pitch ≈ 0.12 to 0.15 × bore for high-cycle flex applications. Coarser pitch reduces flex cycle life because each hook sees more travel per bend. Finer pitch increases stiffness and the cumulative friction wears the strip edges. For a 25 mm bore that means 3 to 3.8 mm pitch. Also specify a stainless strip — brass work-hardens and cracks in under 50,000 cycles in dress-pack duty.
The formula gives the static disengagement radius — the point at which hooks geometrically cannot stay seated. Real tubes often tolerate a tighter bend short-term because the packing stretches and holds the seal even when the hooks open slightly. The catch is fatigue: every bend below Rmin shortens hook life. If you measured a tube bending to 70% of predicted Rmin without leaking, it is not a free pass — it is a warning that you have used up bend cycles you cannot get back.
Almost always it is the end fitting installation, not the tube itself. The fitting must grip at least two full turns of strip with even radial pressure. A swage that catches only one and a half turns will let the strip slowly axially creep under pressure pulses, opening the first hook just enough to weep. Pull the fittings on the leaking unit and count how many full turns of strip are inside the collar — if it is less than two, that is your problem.
References & Further Reading
- Wikipedia contributors. Metal hose. Wikipedia
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