How to Order Custom Springs: The 7 Specifications You Must Provide
**Direct Answer:** To order custom springs correctly, you must provide seven critical specifications: wire diameter, outer diameter, free length, number of active coils, material grade, end type, and required load at a defined deflection. Missing even one of these values leads to production delays, tooling rework, and springs that fail in your assembly. This guide details each requirement with industry-standard tolerances and pricing impact, based on our 20 years of manufacturing experience at BQUQ in Dongguan.
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Why Incomplete Spring Specifications Cause 90% of RFQ Failures
When we receive an inquiry for custom springs at BQUQ, roughly 90% of initial quotes are missing at least two of the seven critical dimensions. The most common omissions are wire diameter (often confused with inside diameter) and the exact number of active coils. This is not a minor paperwork issue. A spring with a wire diameter error of just 0.05 mm will shift your load capacity by 15-20%, which can cause premature fatigue or plastic deformation in your mechanism.
The cost of rework is significant. For a typical compression spring in stainless steel 302, our standard tolerance on wire diameter is ±0.025 mm for diameters under 3.0 mm. If you specify only the outside diameter and free length, we have to guess the wire size based on your load requirement. This "reverse engineering" adds 2-3 days to the quoting process and increases the risk of a field failure. Provide all seven numbers upfront, and we can quote within 12 hours with zero ambiguity.
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The 7 Mandatory Specifications for Any Custom Spring
### 1. Wire Diameter (d) – The Foundation of Your Spring Rate This is the thickness of the wire itself, not the hole it fits over. For compression springs, the wire diameter directly controls the spring rate (k) using the formula: k = (G × d⁴) / (8 × D³ × N), where G is the shear modulus of the material, D is the mean coil diameter, and N is the number of active coils. A 10% increase in wire diameter increases the spring rate by 46% (due to the fourth power). Our standard tolerance for wire diameter is: - Under 1.0 mm: ±0.010 mm - 1.0 mm to 5.0 mm: ±0.025 mm - Over 5.0 mm: ±0.050 mm
### 2. Outside Diameter (OD) or Inside Diameter (ID) – Specify One, Not Both Always give us the outside diameter (OD) if your spring sits in a bore, or the inside diameter (ID) if it slides over a rod. Do not give both unless you also state the wire diameter, because OD = ID + 2 × d. If you give us both OD and ID, we will verify the wire diameter matches; if it does not, we will reject the drawing. For precision springs, our standard concentricity tolerance is 0.5% of the OD, but we hold 0.25% for high-speed applications.
### 3. Free Length (L) – Uncompressed Length This is the overall length of the spring with no load applied. It includes the end coils if they are closed and ground. For a compression spring, free length tolerance is typically ±1.0% or ±0.5 mm, whichever is greater. If you need tighter control for a safety-critical valve spring, we can hold ±0.25% with secondary grinding operations, but this adds 5-8% to the unit price.
### 4. Number of Active Coils (N) vs. Total Coils (Nt) Active coils are the coils that actually compress and store energy. Total coils include the inactive end coils. For a closed and ground end spring, total coils = active coils + 2. This is the most frequently misquoted value. If you give us free length, wire diameter, and OD but no coil count, we cannot calculate the pitch. You must specify either active coils or the pitch (distance between wire centers). Our tolerance on coil count is ±1/4 coil for springs under 10 active coils, and ±1/2 coil for larger springs.
### 5. Material Grade – Not Just "Stainless Steel" Saying "stainless" is not a specification. We need the exact grade. For custom springs, the three most common materials we use at BQUQ are: - Music Wire (ASTM A228): Best for high tensile strength, max operating temperature 120°C. Cost benchmark: $0.02 per piece for a 10 mm OD spring. - Stainless Steel 302 (ASTM A313): Corrosion-resistant, max temperature 250°C. Cost benchmark: $0.035 per piece for the same geometry. - Chrome Silicon (ASTM A401): For high stress and fatigue, max temperature 230°C. Cost benchmark: $0.06 per piece. If you do not specify, we default to 302 stainless, but this may fail in high-temperature or corrosive environments.
### 6. End Type – Closed, Ground, or Open Your end type changes the effective length and the load-bearing surface. The three standard options are: - Closed and Ground: Ends are flattened and ground flat. This provides a square bearing surface, reduces buckling, and is required for precision compression springs. Adds 2-3 days to lead time. - Closed (Not Ground): Ends are closed but not ground. Suitable for light loads, lower cost. - Open (Plain): No closure. Rarely used for compression springs because it causes off-axis loading. For extension springs, specify loops (full loop, cross-over loop, or side loop). For torsion springs, specify the leg length and angle.
### 7. Load at Deflection – The Functional Requirement This is the force your spring must exert at a specific compressed length. For example: "10 N at 20 mm compressed length, from a free length of 30 mm." This gives us the spring rate directly. If you only give free length and OD, we cannot guarantee the force. Our load tolerance is ±10% for standard springs, but we can achieve ±5% with 100% load testing. Load testing adds a cost of $0.01-$0.03 per piece depending on volume.
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Data Table: Standard Tolerances and Cost Impact for Custom Springs
| Specification | Standard Tolerance | Precision Tolerance | Cost Increase (Precision) | Lead Time Impact | --- | --- | --- | --- | --- | Wire Diameter (d) | ±0.025 mm | ±0.010 mm | +10% | +1 day | Outside Diameter (OD) | ±0.10 mm | ±0.05 mm | +8% | +1 day | Free Length (L) | ±1.0% or ±0.5 mm | ±0.25% | +12% | +2 days | Active Coils (N) | ±1/4 coil | ±1/8 coil | +15% | +2 days | Load at Deflection | ±10% | ±5% | +20% (100% testing) | +3 days | Surface Finish (Ra) | 0.8 µm | 0.4 µm (barrel tumbling) | +5% | +1 day | Operating Temp (max) | 250°C (302 SS) | 300°C (Inconel X-750) | +150% material cost | +5 days |
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*Table 1: BQUQ standard manufacturing tolerances and cost multipliers for custom springs. Prices based on 10,000-piece order, 10 mm OD, 20 mm free length.*
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Additional Specifications That Prevent Field Failures
### Operating Temperature and Environment If your spring operates above 150°C, standard 302 stainless will lose its tensile strength by 20%. For engine valves or high-heat applications, specify Inconel X-750 or 17-7 PH stainless. Conversely, for cryogenic applications below -40°C, we recommend music wire, which actually increases in tensile strength at low temperatures. Always state the minimum and maximum ambient temperature.
### Surface Finish and Pre-Stress Shot peening is a post-treatment that induces compressive residual stress on the wire surface, increasing fatigue life by up to 50%. For springs cycling over 1 million times, we strongly recommend shot peening (adds $0.005-$0.01 per piece). For corrosion resistance, specify zinc plating (costs $0.01/piece) or electroless nickel (costs $0.03/piece). If you skip this, bare steel springs will rust within 48 hours in a humid environment.
### Direction of Helix (Left or Right Hand) This is often overlooked. For compression springs, the direction rarely matters unless you have two springs nested together—then they must be opposite hand to prevent tangling. For torsion springs, the direction is critical because it determines the winding direction. Specify "right-hand" or "left-hand" on your drawing; if omitted, we default to right-hand, which may cause assembly issues.
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Practical Recommendations for Faster and Cheaper Spring Orders
1. **Provide a 2D drawing with GD&T symbols, not just a 3D model.** Our engineers can extract dimensions from a STEP file, but a PDF with explicit tolerances eliminates back-and-forth emails. We have seen 3D models with hidden radii that caused a 0.2 mm mismatch in the end coil.
2. **Use the metric system consistently.** While we accept imperial, mixing units (e.g., wire diameter in mm, OD in inches) is the #1 cause of quote errors. Convert everything to mm or inches before sending.
3. **Order prototype quantities first.** For a new design, order 50-100 pieces with precision tolerances before committing to a 50,000-piece run. This costs $50-$150 for the prototype, but saves you $1,000+ in tooling corrections later. Our prototype lead time is 5-7 working days.
4. **Specify a minimum and maximum load, not a single target.** For example, "8 N to 10 N at 15 mm deflection." This allows us to adjust the coil count within tolerance without rejecting the batch. Single-point load specs increase the rejection rate by 30%.
5. **Ask for a spring rate verification report.** For any order over 5,000 pieces, we provide a free load test report showing the actual force at three compression points. This is your engineering proof that the spring meets the functional spec, not just the dimensional spec.
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FAQ-Style Tips for First-Time Spring Buyers
**Q: What if I only know the OD and the load I need?** A: Send us the OD, load, and deflection. We will calculate the wire diameter and coil count for you. However, this is a "design consultation" and takes 24 hours, not 12. You will also need to accept a ±15% load tolerance because we are optimizing for your envelope, not your exact spring rate.
**Q: What is the minimum order quantity (MOQ) at BQUQ?** A: For custom springs, our MOQ is 500 pieces for standard materials (music wire, 302 SS). For exotic alloys like Inconel, the MOQ is 2,000 pieces due to material purchasing constraints. We do offer 50-piece prototypes at a 3x unit price premium.
**Q: How do I specify a spring that must not buckle?** A: Buckling occurs when the free length is more than 4 times the mean diameter. If L/D > 4, you must specify a guide rod or a spring with closed and ground ends. We will flag this on your drawing and recommend a slenderness ratio below 2.5 for unsupported compression springs.
**Q: Does the surface finish affect the spring's fatigue life?** A: Yes. A rough surface (Ra > 1.6 µm) creates stress concentration points. For high-cycle springs, specify a polished or tumbled finish (Ra 0.4 µm). This reduces surface micro-cracks and improves fatigue life by 40%. We perform barrel tumbling as a standard step for all precision springs.
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Conclusion and Next Step for Your Custom Spring Project
Ordering custom springs is not a simple "send us a drawing" process. The seven specifications—wire diameter, outside diameter, free length, active coils, material grade, end type, and load at deflection—are non-negotiable for a manufacturable, functional spring. By providing these with clear tolerances, you reduce your quote time from days to hours and eliminate the risk of a failed batch. At BQUQ, we have 20 years of experience in CNC machining and spring manufacturing, and we hold our tolerances to ±0.01 mm on wire diameter for medical and automotive clients. We are ready to review your specifications today.
**Send us your drawings or a list of your seven specifications, and we will provide a detailed quote with load test data within 12 hours.** Email your inquiry to sc@bquq.com or message us on WhatsApp at +86 13713157787. Visit our website at www.bquq.com to download our spring specification checklist. Your custom springs should not be a guess—let us engineer them with precision.
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Frequently Asked Questions
What are the seven critical specifications I need to provide when ordering custom springs?
You must provide wire diameter, outside diameter (or inside diameter), free length, number of active coils, material grade, end type, and required load at a defined deflection. Missing any of these can cause production delays, tooling rework, or spring failure in your assembly.
Why do incomplete spring specifications cause RFQ failures?
About 90% of initial quotes at BQUQ are missing at least two of the seven critical dimensions, most commonly wire diameter and active coil count. A wire diameter error of just 0.05 mm can shift load capacity by 15-20%, risking fatigue or deformation. Reverse engineering adds 2-3 days to quoting, while complete specs allow quoting within 12 hours.
What is the standard tolerance for wire diameter in custom springs?
Our standard tolerances are ±0.010 mm for wire under 1.0 mm, ±0.025 mm for 1.0-5.0 mm, and ±0.050 mm for over 5.0 mm. For stainless steel 302 compression springs, we hold ±0.025 mm on wire diameter under 3.0 mm. A 10% increase in wire diameter raises spring rate by 46% due to the fourth power relationship.
Should I specify outside diameter or inside diameter for my spring?
Specify only one: give outside diameter (OD) if the spring sits in a bore, or inside diameter (ID) if it slides over a rod. Do not provide both unless you also state wire diameter, since OD = ID + 2 × d. Our standard concentricity tolerance is 0.5% of OD, but we hold 0.25% for high-speed applications.
