Almond’s flexible metallic tube combines an open helix of round wire with an outer winding of triangular-section wire. The diagram enlarges the two sections to make their arrangement visible as the tube bends.
Almond's Flexible Metallic Tube Interactive Calculator
Explore the round-wire and triangular-wire construction. Compare bore, pitch and a joint allowance; the radius result is not a rated bending limit.
Equation Used
The calculator compares dimensions with R = (D_i + p)p/(2δ), using bore D_i, wire pitch p and an entered joint allowance δ in the same units. This dimensional expression is not a validated bend-limit equation for Almond’s two-wire tube and cannot establish a safe bend radius.
- Dimensional allowance comparison only; no physical bend-limit prediction.
- Bore, pitch and joint allowance use the same length unit.
- Pressure, fatigue, packing wear, and end-crimp effects are not included.
The drawing follows the historical two-wire tube: an open round-wire helix and a triangular outer winding. The allowance formula does not determine a verified minimum bend radius or pressure rating for this tube.
Two coils form the tube wall
An inner helical winding has rounded wire surfaces. A second winding has a triangular section that fits between adjacent turns of the inner coil. The contact between these shaped windings allows the tube to bend while its wall remains assembled.
Thomas R. Almond’s flexible-tube patent, US434748A, describes the outer triangular coil with concave faces matching the curved inner coil. That construction is different from a single strip rolled into interlocking hooks.
The enlarged section identifies the two windings. The main drawing exaggerates clearances and flexing to make their arrangement visible. It does not simulate contact pressure, sealing or deformation of the wire.
Dimensions to distinguish
- Bore: the open diameter inside the tube.
- Pitch: axial spacing of the winding turns.
- Wire sections: the contacting shapes of the inner and outer windings.
- End connections: the separate means of securing and connecting a particular tube assembly.
What the construction demonstrates
The tube illustrates how shaped helical windings can combine a continuous passage with flexibility. The useful question in this model is how bore and pitch change the visible construction, not which fluid or pressure it can carry.
A real flexible assembly must be selected for its contents, temperature, pressure, movement, end fittings and required life. Pressure hose, protective conduit and a flexible mechanical casing can look similar while having very different ratings. This page does not establish interchangeability between them.
Reading the dimensional comparison
The equation shown beside the calculator is R = (D_i + p)p/(2δ). With bore D_i, pitch p and allowance δ in millimetres, its result is in millimetres. The comparison diameter is 2R. The other outputs are p/D_i and δ/p expressed as percentages.
At fixed pitch and allowance, increasing bore increases R. At fixed bore and pitch, doubling allowance halves R. These are properties of the expression, not validated predictions of tube behavior.
The equation does not include wire profile, material stiffness, contact force, packing, pressure or repeated flexing. Its R_min notation must not be read as a manufacturer’s minimum bend radius. Use the rated limits for the specific assembly when designing an installation.
Worked dimensional example
For bore 25 mm, pitch 4 mm and joint allowance 1.2 mm, the comparison radius is (25 + 4) × 4 / (2 × 1.2) = 48.33 mm. The comparison diameter is 96.67 mm, pitch/bore is 16%, and allowance/pitch is 30%.
Keeping bore and pitch fixed while doubling the allowance to 2.4 mm halves the radius result to 24.17 mm. This demonstrates the equation’s inverse dependence on allowance.
Neither result specifies a safe bending mandrel or proves that the tube will remain sealed. No pressure, temperature or fatigue limit is calculated.
Different flexible-tube constructions
| Construction | Wall arrangement | Selection information needed |
|---|---|---|
| Almond two-wire tube | Shaped inner and outer helical windings | Complete assembly dimensions and documented service limits |
| Interlocked strip tube | A formed strip with engaging edges | Specific construction, lining or sealing details and intended duty |
| Corrugated metal hose | Corrugated tube wall, with reinforcement where specified | Pressure/temperature ratings, movement limits and compatible end fittings |
| Polymer or elastomer hose | Flexible wall with reinforcement appropriate to the product | Fluid compatibility, pressure, temperature and movement ratings |
These construction names do not supply a universal leak rate, service life or bend radius. Compare documented assemblies for the actual application.
Flexible-tube questions
Is the illustrated tube made from a hooked metal strip?
No. The illustration shows two wire windings: a rounded inner coil and a triangular-section outer winding. A strip-wound interlocked tube is a different construction.
Does the calculated radius tell me how tightly to bend a tube?
No. It is the result of a dimensional allowance expression, not a verified bending limit for this tube. Use the specified limits for the actual assembly.
Can I use these numbers to select a steam or gas hose?
No. The calculator has no pressure, temperature, sealing or material model. The complete hose and end fittings need documented suitability for the service.
Why are the wire clearances enlarged in the drawing?
Small contact features are difficult to see at the overall tube scale. The enlargement explains the arrangement; it does not specify manufacturing clearance or predict a leak path.
Does the animation predict flex life?
No. It shows a repeating bend for explanation. Material stress, contact wear and fatigue cycles are not calculated.
Can the page prescribe cutting, repacking or leak repair?
No. Those procedures depend on the exact tube, fittings and service. Isolate a leaking pressurized assembly and follow its supplier’s inspection and replacement guidance; do not diagnose it by bending it under pressure.
Routing around a separately actuated mechanism
If a moving assembly also carries a flexible tube, its routing and supports must accommodate the full motion within the tube’s documented limits. The tube is not a guide for an electric actuator or a structural restraint for the moving assembly.
For a separate actuator selection, compare FIRGELLI actuator families or use the replacement finder. Those tools do not establish the flexible tube’s service rating.
References & Further Reading
- Wikipedia contributors. Metal hose. Wikipedia
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