This is a collection of torsion springs engineered specifically for brass buckle shafts—the pivot points in belt buckles, luggage clasps, and quick-release mechanisms. Each spring delivers a consisten
This is a collection of torsion springs engineered specifically for brass buckle shafts—the pivot points in belt buckles, luggage clasps, and quick-release mechanisms. Each spring delivers a consistent torque output with a tolerance of ±0.02 mm on wire diameter and ±0.05 mm on leg position, a surface finish of Ra 0.4, and a hardness of 48 HRC after heat treatment. You get a working life exceeding 50,000 cycles without measurable fatigue, with a standard lead time of 7 days for prototypes and 15 days for production runs.
The leg angle determines how much preload your buckle shaft receives at rest. We set this angle to ±1 degree, which translates to a torque repeatability of ±3% across a batch of 10,000 parts. Free position—the angle between legs when unloaded—is held to ±2 degrees, ensuring your assembly line doesn't need to re-calibrate each unit. For a standard 4 mm shaft diameter, we deliver a spring rate of 0.15 N·mm/degree, adjustable to your exact load curve.
We start with either 302 stainless steel (UNS S30200) or 65Mn spring steel (GB/T 1222). 302 gives you corrosion resistance for marine environments; 65Mn provides higher tensile strength at 1,600 MPa. Both materials go through an oil-quench and temper cycle at 320°C for 90 minutes, yielding a hardness of 46–50 HRC. This prevents the spring from taking a set when the buckle shaft is twisted to its maximum working angle—something we verify with a 10,000-cycle test on every sample batch.
We produce wire diameters from 0.3 mm to 4.0 mm, with a length-to-diameter ratio up to 12:1 without coil binding. Leg forms include straight torsion, double torsion, and hooked ends—the latter being common for buckle shafts where the spring wraps around a D-shaped pivot. For a 6 mm shaft, we recommend a wire diameter of 1.2 mm and 4.5 active coils, which gives you a maximum deflection of 90 degrees before stress reaches 80% of yield. We machine the legs to a square-cut finish, avoiding the burrs that cause shaft misalignment.
Runout on the coil axis is held to 0.03 mm total indicated reading (TIR). We achieve this by using a CNC coiler with a servo-driven pitch control, followed by a centerless grinding pass on the outer diameter. For buckle shafts that rotate continuously, this runout limit prevents wobble that would wear the brass housing. Our inspection records show a Cpk of 1.33 on runout across the last 40 production lots, meaning 99.7% of parts land within your tolerance band.
Every batch goes through a three-step check. First, a torque test at 10%, 50%, and 100% deflection using a digital torque gauge with 0.01 N·mm resolution. Second, a fatigue test on 5 randomly sampled parts—each cycled 10,000 times at 2 Hz to confirm no relaxation exceeds 5%. Third, a dimensional audit with a profile projector for leg angles and a micrometer for wire diameter. We attach a signed inspection report (IMDS format) to every carton, and you can request a video of the test run before we ship.
| Specification | Value |
|---|---|
| Material options | 302 stainless steel, 65Mn spring steel, 17-7PH |
| Wire diameter | 0.3 mm – 4.0 mm (±0.02 mm) |
| Spring rate | 0.05 – 5.0 N·mm/degree (±3%) |
| Hardness | 46 – 50 HRC (oil-quenched and tempered) |
| Coil outer diameter | 2.0 mm – 20.0 mm (±0.05 mm) |
| Leg angle tolerance | ±1 degree |
| Free position tolerance | ±2 degrees |
| Surface finish | Ra 0.4 (ground), Ra 0.8 (as-coiled) |
| Runout (TIR) | 0.03 mm max |
| Lead time | 7 days (prototype), 15 days (production) |
| MOQ | No MOQ (1 piece accepted for prototype) |
If you're machining the brass buckle shaft itself, the tool geometry directly affects how the spring seats. A sharper cutting edge reduces burr formation on the shaft groove, which in turn prevents the spring leg from catching. Refer to our CNC cutting tool selection guide for feed rates and corner radii that keep your groove walls square—this ensures the torsion spring's hook leg sits flush without additional deburring.
The spring's coil inner diameter must mate with your shaft's outer diameter with a clearance of 0.1–0.2 mm for smooth rotation. Tighter than that, and the spring binds; looser, and it rattles. Our CNC machining tolerances guide explains how to specify a shaft tolerance of h8 or g6, depending on whether you want a press fit or a slip fit. We coordinate with your shaft drawing to match the spring's ID to that tolerance class.
Yes, we have no MOQ—you can order a single prototype spring for testing, and production pricing starts at 100 pieces.
Yes, we modify the wire diameter, coil count, and leg angle to hit your exact torque at a given deflection, and we'll send a free torque curve chart with your sample.
We perform 100% dimensional inspection on critical angles and a 5-piece fatigue test per batch, plus we provide a signed IMDS report with every shipment.
Prototype springs ship in 7 days via express courier, and production orders are completed in 15 days—both counted from drawing approval.
Send your drawing with wire diameter, coil count, leg style, and target torque. We'll return a firm quote—including tooling cost (if any), unit price, and delivery date—within 12 business hours. Email sc@bquq.com or WhatsApp +86 13713157787. No MOQ, precision manufacturing made in China, and your first sample is free with a production order.
| Parameter | Capability |
|---|---|
| Materials | SUS302/304/316, SWC piano wire, SWP music wire, 65Mn, phosphor bronze, beryllium copper |
| Types | Compression, extension, torsion, wire forms, battery springs, touch springs |
| Wire Diameter | 0.05 - 12.0 mm |
| Tolerance | ±0.02mm standard, ±0.01mm on request |
| Surface | Zinc plating, nickel, black oxide, passivation, shot peening |
| Machines | CNC spring coiling machines, torsion machines, wire forming machines |
| Prototype | 72 hours, no MOQ on samples |
| Inspection | Load test per batch, fatigue test data on request |
