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Leaf Springs for Motor Vehicle Suspension – Made to Customary U.S. Units (SAE J510‑2016)
SAE J510‑2016 provides concise specifications for leaf springs manufactured in customary U.S. units. It defines bar sizes, tolerances, surface‑decarburization limits, and spring‑type definitions, promoting a clear understanding between spring maker and spring user. The standard references SAE HS‑J788 (Manual on Design and Application of Leaf Springs) for fundamental design concepts and is stabilized (no longer subject to periodic review). 🛠️
Scope and Key References
This SAE standard is limited to essential specifications that support practical requirements in finished leaf springs. It supersedes J510 NOV1992 and is intended for use with customary U.S. units; for metric units see SAE J1123. The basic design concepts are fully addressed in HS‑J788. Other relevant SAE publications include J419 (Methods of Measuring Decarburization) and J1123 (Metric Units).
Bar Sizes and Cross‑Section Tolerances
Round‑edge flat spring steel is the adopted SAE standard. Bars have two flat surfaces and two rounded (convex) edges. The rounding radius of the edges is 65 % to 85 % of the bar thickness. Below are the standard widths and thicknesses, followed by the cross‑section tolerance table.
Standard Bar Widths (inches)
1.75, 2.00, 2.25, 2.50, 3.00, 3.50, 4.00, 5.00, 6.00
Standard Nominal Thicknesses (inches)
0.194, 0.204, 0.214, 0.225, 0.237, 0.249, 0.262, 0.276, 0.291, 0.307, 0.323, 0.341, 0.360, 0.380, 0.401, 0.423, 0.447, 0.473, 0.499, 0.527, 0.558, 0.590, 0.625, 0.662, 0.702, 0.744, 0.788, 0.836, 0.887, 0.941, 0.999, 1.061, 1.127, 1.197, 1.273, 1.354, 1.440
| Nominal Width Over (in) |
To and Including (in) |
Tolerance in Width −0.00 (in) |
Thickness Range (in) | Thickness Tolerance (± in) (1) |
Tolerance in Flatness (− in) (2) |
Max Difference in Thickness (in) (3) |
|---|---|---|---|---|---|---|
| 0.00 | 2.50 | +0.030 | 0.375 or under | 0.005 | 0.005 | 0.002 |
| 0.00 | 2.50 | +0.030 | Over 0.375 to 0.875, incl | 0.006 | 0.006 | 0.002 |
| 2.50 | 4.00 | +0.045 | 0.375 or under | 0.006 | 0.006 | 0.003 |
| 2.50 | 4.00 | +0.045 | Over 0.375 to 0.875, incl | 0.008 | 0.008 | 0.004 |
| 2.50 | 4.00 | +0.045 | Over 0.875 to 1.500, incl | 0.012 | 0.012 | 0.006 |
| 4.00 | 5.00 | +0.065 | 0.375 or under | 0.007 | 0.007 | 0.004 |
| 4.00 | 5.00 | +0.065 | Over 0.375 to 0.875, incl | 0.010 | 0.010 | 0.005 |
| 4.00 | 5.00 | +0.065 | Over 0.875 to 1.500, incl | 0.016 | 0.016 | 0.008 |
| 5.00 | 6.00 | +0.090 | Over 0.375 to 0.875, incl | 0.012 | 0.012 | 0.006 |
| 5.00 | 6.00 | +0.090 | Over 0.875 to 1.500, incl | 0.020 | 0.020 | 0.010 |
| (1) Thickness measurements shall be taken at the edge of the bar where the flat surfaces intersect the rounded edge. (2) This tolerance represents the maximum amount by which the thickness at the center of the bar may be less than the thickness at the edges. Thickness at the center may never exceed the thickness at the edges. (3) Maximum difference in thickness between the two edges of each bar. |
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🔍 Design Insight – Camber Control: During heat treatment, quench dies or fixtures are used to maintain the required camber within tolerances. Properly specifying and controlling camber ensures the spring meets load‑carrying and durability targets.
Surface Decarburization: Types and Acceptance
Surface decarburization can reduce fatigue durability, so it must be minimized. As‑received hot‑rolled steel bars typically have at least Type 3 decarburization (more than 50 % of the base carbon content remains at the surface). If decarb is Type 2 (50 % or less of base carbon remaining), the depth normally does not exceed 0.010 in for thicknesses 0.194–0.499 in, nor 0.020 in for thicknesses >0.499–1.440 in. In sections over 1.000 in, Type 1 (virtually carbon‑free ferrite) may occur.
⚠️ Common Mistake – Edge Decarb. The decarburization depth on the edges of the bar is generally greater than on the flat surfaces. Always account for this when evaluating as‑received steel and after forging/heat treating.
After forging and non‑atmospheric‑controlled heat treating, spring leaves will have greater decarb. Scale removal during processing reduces thickness, but the final decarb depth is usually greater than in the as‑received bars. The acceptable type and depth of decarb must be agreed upon between the steel producer and the spring manufacturer.
Definitions, Datum Line, and Design Insights
Leaf Spring Types
Leaf springs are classified as multi‑leaf (constant width, stepped leaves of constant thickness, or assemblies of tapered leaves) or single‑leaf. They can be full‑elliptic, semi‑elliptic, or quarter‑elliptic in shape.
Datum Line
The datum line (X‑X in the standard’s figures) is the reference for load and rate checking. For springs with eyes, it passes through the centers of the eyes; for other springs it passes through the points where load is applied near the ends. When checking load and rate, the spring ends must be free to move in the direction of the datum line.
Seat Angle Base Line & Loaded Dimensions
The seat angle base line is drawn along the tension surface of the main leaf through the terminal points of the active spring length at each eye. For springs without eyes it coincides with the datum line. Detailed definitions for loaded length, loaded fixed end length, and straight length are provided in the standard with specified tolerances.
🛠️ Engineering Design Insight – Cold‑Straightened Bars. It is recommended that leaf‑spring bars cold‑straightened by the mill be identified so the spring manufacturer can use them selectively. Bars are required to be free of kinks or twists and must meet straightness limits (gaps ≤ 0.05 in per foot of length for bars >3 ft).
Frequently Asked Questions
What standard bar widths and thicknesses are available?
Standard widths (in inches): 1.75, 2.00, 2.25, 2.50, 3.00, 3.50, 4.00, 5.00, 6.00. Nominal thicknesses range from 0.194 in to 1.440 in, with 37 specific values listed in the standard.
How are cross‑section tolerances measured?
Thickness must be measured at the intersection of the flat surfaces and the rounded edge. Flatness tolerance is the maximum allowable difference between edge thickness and center thickness (center may never be thicker than edges). The max difference in thickness between the two edges is also tightly controlled.
What are the decarburization types and acceptable limits?
The standard references SAE J419 for measuring decarburization. Type 3 (partial loss, >50 % carbon retained) is typical in as‑received bars. Type 2 (≤50 % carbon retained) is acceptable only to limited depths (0.010 in or 0.020 in depending on bar thickness). Type 1 (carbon‑free ferrite) may appear on thicker bars. Exact acceptance criteria must be agreed between steel producer and spring maker.
Why is the datum line important?
The datum line serves as the reference for load‑rate checking and for defining loaded dimensions. It ensures that spring ends can move freely in the direction of the datum line during testing, providing repeatable and comparable results.
