What This Collection Delivers in Raw Numbers This is a ready-for-customization line of extension tension springs, zinc plated for corrosion resistance. You get wire diameters fro
What This Collection Delivers in Raw Numbers This is a ready-for-customization line of extension tension springs, zinc plated for corrosion resistance. You get wire diameters from 0.3 mm to 8.0 mm,
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This is a ready-for-customization line of extension tension springs, zinc plated for corrosion resistance. You get wire diameters from 0.3 mm to 8.0 mm, free length up to 300 mm, and a load tolerance of ±2% at specified deflection. Hardness lands between 42 and 48 HRC after full heat treatment. Lead time is 5 working days for prototypes, 12 for production runs. Every spring ships with a dimensional report showing measured OD, free length, and rate.
Wire Grades and Zinc Plating Thickness That Actually Matter
We start with music wire (ASTM A228) for diameters under 3.0 mm and oil-tempered chrome silicon (ASTM A401) for heavier loads. Zinc plating runs 8–12 µm per ASTM B633, SC1 type, giving 72 hours of neutral salt spray resistance before white rust appears. No chromate conversion here—we use trivalent passivation to keep your parts RoHS clean. If your environment demands more, we offer clear or yellow zinc as a drop-in option.
Load Rate Precision: How We Hit ±2% Every Time
Tension springs fail when the rate drifts. We control this by coiling on CNC machines with servo-driven pitch control, not mechanical cams. After coiling, each spring is stress-relieved at 260°C for 20 minutes to stabilize the wire. Then we test load at 80% of maximum deflection on a digital force gauge. If the rate deviates more than 2% from your drawing, it goes to scrap—no rework, no exceptions.
End Configurations: Loops, Hooks, and Custom Bends
Standard ends are full loops (German type) or extended hooks (English type). Need a cross-over center loop or a reduced OD loop? We do those too, with a positional tolerance of ±0.5 mm on the hook opening. For high-cycle applications, we can grind the last coil flat to reduce stress concentration—this increases fatigue life by up to 30% in our internal testing. Tell us the orientation on your assembly drawing; we mirror it exactly.
Surface Finish and Runout Control After Plating
Zinc plating can build up at the wire ends, altering the fit. We counter this by pre-plating the wire diameter to a minus tolerance, so after plating, the final OD stays within ±0.03 mm of your spec. Runout (straightness) on the free length is held to 0.5 mm per 100 mm of spring length. We measure this on a granite surface plate with a height gauge—not by eye. Surface roughness after plating is Ra 1.6 µm max, which prevents snagging on mating components.
Inspection Before Every Shipment: Not a Random Sample
Every single spring gets checked for free length, OD, and hook alignment. A 100% load test is optional but available on request—we use a 5 kN test stand with a 0.5% accuracy load cell. For random QC, we pull 5 pieces per batch and measure wire hardness on a Rockwell tester, plus verify zinc thickness with an X-ray fluorescence gauge. You get a PDF report with batch number, date, and all measured values. This is the same process we use for automotive clients who demand PPAP level 3.
Lead Time, MOQ, and How We Beat Your Deadline
Prototype springs: 5 days. Production: 12 days for up to 50,000 pieces. No MOQ—we ship 10 pieces or 100,000 at the same per-unit quality. Tooling is zero for standard wire diameters; for non-standard sizes, we make a new coiling mandrel in 2 days. Zinc plating is done in-house, so we control the entire cycle. If you need faster, we can air-freight from our Dongguan factory to Hong Kong International Airport in 4 hours after final inspection.
When to Choose a Custom Extension Spring Over a Catalog Part
Catalog springs have fixed rates and ends. Your application likely needs a specific free length, a hook that fits your bracket, or a load that matches a motor torque curve. We make custom springs from your drawing or from a simple sketch. If you have a sample, we reverse-engineer it in 24 hours—measuring wire diameter, coil count, and load at two deflections. We then send you a confirmation drawing before production. No guesswork.
Parameter
Value / Range
Wire Material
Music Wire (ASTM A228) / Chrome Silicon (ASTM A401)
Wire Diameter
0.3 mm – 8.0 mm
Free Length
5 mm – 300 mm
Hardness (After Heat Treat)
42 – 48 HRC
Load Tolerance
±2% at specified deflection
Surface Finish (Post-Plating)
Ra 1.6 µm max
Zinc Plating Thickness
8 – 12 µm (ASTM B633 SC1)
Runout (Straightness)
0.5 mm per 100 mm length
Lead Time (Prototype)
5 working days
Lead Time (Production)
12 working days
MOQ
No MOQ (10 pieces accepted)
What is the minimum order quantity for this collection?
No MOQ—we accept 10 pieces for prototyping and scale to 100,000+ without changing the per-unit price structure.
Can you match a specific hook geometry or wire diameter not listed?
Yes, we custom coil any wire diameter within 0.3–8.0 mm and fabricate hooks, loops, or torsion ends to your drawing with ±0.5 mm positional tolerance.
How do you ensure quality on a small batch order?
Every spring is measured for free length and OD, plus 100% load testing is available; random batches get hardness and plating thickness verification with a full report.
What is the fastest lead time for a production run of 5,000 pieces?
We can ship 5,000 pieces in 7 working days if wire stock is available—zinc plating adds no extra delay as it is done in-house.
This is not a stock catalog. This is a precision manufacturing service where every parameter—wire grade, plating thickness, end form, load rate—is tuned to your drawing. Made in China, but with inspection standards that match German or Japanese suppliers. We have held tolerances of ±0.02 mm on wire diameter for over 20 years, and our CNC coiling machines repeat within 0.01 mm on pitch. For more on selecting the right cutting tools for your spring hooks or mating parts, see our CNC cutting tool selection guide. If you are calculating fits for your spring ends, check the CNC machining tolerances guide to match your mating holes.
Get a Firm Quote Within 12 Hours—Send Your Drawing
Send your spring drawing or a sketch with wire diameter, free length, number of coils, and end type. We will return a firm quote with load curve data and a DFM note if we spot any manufacturability issues. Email sc@bquq.com or WhatsApp +86 13713157787. Reply time is under 12 hours, including weekends. No MOQ, no tooling fee for standard wires, and a 5-day prototype guarantee.
