Extension springs

Gutekunst Z-091X lists 24.00 mm free length measured inside loop to inside loop, 3.881 N initial tension and 1.693 N/mm rate. At an illustrative 32.00 mm on the same datum, extension is 8.00 mm and nominal force is 17.425 N. Check hooks, alignment and load range for cyclic service. Z-091X online product table.

Identity and geometry

The attachments transmit axial tension to a coil whose wire works in torsion. Conventional extension springs lack a solid stop. A drawbar spring instead loads a compression spring. Newcomb Spring, What Are Extension Springs?, attributed to SMI.

Mean coil diameter is outside diameter minus wire diameter for round wire. Spring index divides it by wire diameter. Body length excludes the hooks. Lee Spring measures stock free length between inside loop faces. Century Spring, design information, Lee Spring, Extension Springs.

Active portions store and release elastic energy. In an extension spring, the body turns that open are active, and flexible attachment portions can also contribute to deflection. Active count is not automatically the number of turns visible in the body. Newcomb also notes that body diameter typically decreases during extension, which matters for mating clearances. Newcomb Spring, Active Coils in a Helical Spring.

Initial tension and force–extension behavior

Initial tension Fi is the internally established force holding close-wound coils together. It must be overcome before appreciable body-coil separation. It differs from the installed load, which depends on mounting length. A free, unattached spring does not exert an external pulling load merely because it has initial tension. In the separated-coil, approximately linear range:

δ = L − L0
F = Fi + k δ
k = (F2 − F1) / (L2 − L1)

Here lengths and extension are mm, rate k is N/mm and forces are N. L0 and L use the same attachment datum. The force equation is the manufacturer's nominal load model, not a description of the full transition from touching coils to separated coils. Century Spring, Total Force relationship; Newcomb extension-spring sheet, p. 1.

Newcomb's two-point rate measurement starts with definite coil separation. Extrapolating that straight portion gives Fi = F1 − k(L1 − L0). Its simplified Fi = 2F1 − F2 applies when the second extension from the free datum is twice the first, not for arbitrary test lengths. Initial tension also implies internal torsional stress: the sheet gives nominal τi = 8DFi/(πd³). With N and mm this is N/mm² (MPa); it is not a hook-stress calculation or a complete corrected maximum-stress assessment. Newcomb sheet, load graph and initial-tension formulas, p. 1.

Engineering synthesis: in the same linear range, an additional mounting stroke x raises force by kx; it does not add initial tension a second time. A rate without initial tension and a length datum is insufficient to predict the absolute pulling force.

Hooks, loops and local failure

Loops usually have a small gap; hooks have a larger opening. End forms include twist loops, cross-center loops, extended hooks, swivel attachments and threaded inserts. Some are formed from the final body coils; special forms can be made from straight tangent wire. These constructions change the load path and cannot be treated as interchangeable just because the bodies have the same diameter and rate. Newcomb Spring, Extension Spring Ends.

The body is mainly torsionally loaded, whereas hook transitions carry bending as well as torsion. Century identifies the transition between the last body coil and the hook as a region of elevated stress; a hook can fail while the body remains below its own limit. Loop forming can stretch, mark or distort wire, which also influences the relevant end-stress limits. Century Spring, stress discussion; Newcomb Spring, Static Extension Spring Stress, attributed to SMI.

Engineering synthesis: permanent hook opening, fracture at a local bend and loss of engagement are different potential failure modes. Their significance depends on hook radius and opening, anchor diameter and bearing contact, attachment orientation, lateral motion and operating force. A body-stress calculation does not establish hook strength, pin strength or retention. No universal hook capacity or safe extension follows from the end-style name.

Bounded example: Gutekunst Z-091X

Gutekunst's Z-091X sheet, printed 10.2026, page 1 of 1, specifies drawn EN 10270-1 wire, right-hand coiling and 1/1 German loops. Surface treatment is blank; no coating is established. Decimal commas are rendered as points. Manufacturer Z-091X PDF.

The sheet leaves mean diameter, active and total coil counts, maximum-length field, cycle count, frequency and application temperature blank. It specifies stock loops without openings (m = 0); a cut opening is an offered modification, not the depicted stock geometry. Z-091X PDF, p. 1.

The separate Z-091X online product table explicitly defines L0 from inner loop edge to inner loop edge and gives maximum length Ln = 46.64 mm on that datum. It lists initial tension as 3.881 N, one more decimal place than the PDF. Neither source assigns a cycle life to the example.

Derived nominal arithmetic: using the online table's Fi = 3.881 N, k = 1.693 N/mm and L0 = 24.00 mm, an illustrative inside-loop working length of 32.00 mm gives δ = 8.00 mm and F = 3.881 + 1.693 × 8.00 = 17.425 N. At the listed maximum length, δ = 46.64 − 24.00 = 22.64 mm; the same model gives F = 42.21052 N, close to the tabulated 42.22 N but not identical with rounded inputs. These calculations are not measured forces or fatigue ratings. The Fn tolerance belongs to that endpoint; no tolerance at 32 mm or independent numeric rate/initial-tension tolerance is established.

For an ideal round-wire body, nominal mean diameter would be 9.00 − 1.00 = 8.00 mm. This derived geometry does not fill the manufacturer's blank field or establish coil count, hook radius or stress margin.

Installation, static load and cyclic service

Maximum extension is a source-specific operating boundary. It is distinct from body length, added travel and inside-loop installed length. An extension spring does not gain a protective mechanical stop simply because a catalog maximum is stated. Assembly motion can impose extension during fitting as well as service; force and end geometry remain relevant in both states. This is engineering synthesis of the load model and the absence of a solid stop. Newcomb Spring overview.

Repeated loading introduces changing local stresses and potential fatigue; a maximum force alone does not specify a cycle life. Sudden unloading can produce lateral vibration, additional stresses and abrasion against neighboring parts if clearance is inadequate. Newcomb Spring, Extension Spring Clearance, attributed to SMI.

Sustained extension has a different concern: relaxation can reduce retained load with time. Newcomb identifies stress, time and temperature as variables that increase relaxation, and temperature also changes modulus and strength. Thus static holding force, a short-duration force test and endurance under repeated cycling answer different questions. Newcomb Spring, Spring Stress Relaxation, attributed to SMI.

Engineering synthesis: application qualification remains specific to installed and peak lengths, complete force range, hook and anchor contact, installation excursion, temperature and dwell, loading frequency, environment, required life and consequences of release. This article supplies no universal material choice, extension percentage, allowable body stress, hook safety factor or fatigue life.

Evidence scope

Sources were retrieved 2026-10-31 PDT. Both relied-on PDFs were opened, validated as one-page documents, rendered and visually read in full. The directly downloaded Gutekunst sheet is printed 10.2026; a search-service extract of the same URL was marked 02.2026. Those are separate accessed variants, not an established revision history. The Newcomb sheet has no visible edition or printed date. The cited HTML references have no identified substantive revision dates; website copyright and HTTP modification dates are not treated as technical editions.

A search-service retry of the Z-091X product page timed out; its direct HTML capture succeeded. No full EN, DIN or ISO standard was read, so the manufacturer's standard references are not interpreted as independent proof of compliance. No customer drawing, lot measurement, mounting geometry, machine motion profile or fatigue qualification was supplied. The bounded catalog example does not establish customer operating limits or approval.