Hey there, engineering enthusiasts and seal aficionados! If you’ve ever delved into the world of mechanical seals, you know that garter springs play a crucial role in keeping things tight—literally. These unassuming coiled wonders provide the radial force needed for radial lip seals to function effectively, preventing leaks in everything from automotive engines to industrial machinery.
Today, I’m diving into a timeless technical bulletin from the Rubber Manufacturers Association (RMA), originally published in 1984 and revised in 1992. Titled Garter Springs for Radial Lip Seals (OS-5), this document lays out the nuts and bolts—er, coils and wires—of designing, manufacturing, and inspecting these springs. I’ll rewrite it in a more digestible, blog-style format, breaking down the key concepts without the formal jargon overload. Whether you’re a designer, manufacturer, or just curious, let’s uncoil this together!
What Are Garter Springs and Why Do They Matter?
At their core (pun intended), garter springs are close-wound extension springs formed into a loop. They’re inserted into radial lip seals to exert consistent compressive force, ensuring the seal lip maintains contact with the rotating shaft. Without them, seals could loosen up, leading to oil leaks, contamination, or outright failure.
This bulletin establishes standard terminology, design guidelines, tolerances, and inspection methods for these springs. It’s a collaborative effort from industry heavyweights like CR Industries, Federal-Mogul, and Freudenberg-NOK, among others. Though it’s from the ’90s, the principles still hold up in modern applications—proving good engineering is timeless.
Key Terminology: Speaking the Language of Springs
Before we get into the specs, let’s define some essential terms. These aren’t just buzzwords; they directly impact how springs perform in real-world seals.
- Initial Tension: The force needed to start separating the adjacent coils. Think of it as the spring’s “resting grip.”
- Spring Rate: Independent of initial tension, this is the force required to stretch the spring by a unit distance (e.g., per inch or mm).
- Stress Relieving: A heat treatment process for uncoiled springs to eliminate stresses from manufacturing. It’s not always necessary for low-temp applications (below 212°F or 100°C), but for hotter environments, it’s a must. Common temps: 400°F (204°C) for carbon steel, 500°F (260°C) for stainless. Pro tip: This reduces max initial tension, so consult your manufacturer to avoid production headaches.
- Compressive Force: The radial “squeeze” the spring applies in its working position, measured in force per unit of inner circumference (like ounces per inch).
- Spring Load: Total tension at a specific length—combining spring rate and initial tension. Handy for quality checks.
- Free Length: The unassembled length (excluding the tapered “nib” end). Calculate it as π times (assembled inner diameter + wire diameter).
- Test Length: Where you stretch the spring to measure load—ideally the design stretch or 110% of free length.
- Passivate: A chemical treatment to make metal surfaces less reactive, boosting corrosion resistance (especially for stainless steel).
Materials: Choosing the Right Wire for the Job
The bulletin specifies tried-and-true materials to ensure durability and performance.
Carbon Steel Spring Wire
Go-to options include grades like ASAE 1050-1095 (or AISI-C1050 to C1095), ASTM A227, or Music Wire (ASTM 228). These are reliable for general use.
Stainless Steel Spring Wire
For corrosion-prone environments, use SAE 30302-30304 (AISI Type 302-304).
- Coatings: Wires come with lubricants like lime/soap, lime/oil, or copper to ease coiling and reduce die wear. Leave the choice to your supplier unless copper’s needed for ID purposes.
- Passivation: Dip in 15-25% nitric acid at 140-160°F (60-70°C) for 5 minutes, then rinse. It removes coatings and enhances resistance but adds cost—skip if not essential.
Other Materials
For niche needs (e.g., extreme heat or corrosion), consider brass or Inconel. These are specials, so chat with your spring supplier for tailored advice.
Tolerances: Keeping It Precise Without Overkill
Tolerances ensure consistency, but the bulletin advises flexibility to make manufacturing feasible. Over-specifying can drive up costs!
Wire Diameter
Treat this as a reference to prioritize tension specs. If needed, aim for ±0.001 inch (0.03 mm). Tighter tolerances (down to ±0.0003 inch/0.008 mm for small wires) are possible but only when critical. Maintain a coil-to-wire ratio of at least 5.3:1.
| Wire Diameter (mm/inch) | Tolerance (± mm/inch) |
| 0.15-0.25 mm / 0.006-0.010 inch | 0.008 mm / 0.0003 inch |
| 0.28-0.38 mm / 0.011-0.015 inch | 0.010 mm / 0.0004 inch |
| 0.41-0.48 mm / 0.016-0.019 inch | 0.013 mm / 0.0005 inch |
| 0.50-0.69 mm / 0.020-0.027 inch | 0.015 mm / 0.0006 inch |
| 0.71-0.86 mm / 0.028-0.034 inch | 0.018 mm / 0.0007 inch |
Coil Diameter
±0.005 inch (0.13 mm), with variation within one spring ≤0.003 inch (0.08 mm). Slight joint bulge is okay if within tolerance.
Assembled Inner Diameter
Based on wire diameter for accuracy—tighter wires mean tighter control.
| Wire Diameter (mm/inch) | Tolerance (± mm/inch) |
| 0.15-0.28 mm / 0.006-0.011 inch | 0.20 mm / 0.008 inch |
| 0.30-0.48 mm / 0.012-0.019 inch | 0.30 mm / 0.012 inch |
| 0.50-0.76 mm / 0.020-0.030 inch | 0.40 mm / 0.015 inch |
| 0.80-1.40 mm / 0.031-0.055 inch | 0.50 mm / 0.020 inch |
Spring Load
±0.50 ounces (0.14 Newtons) or ±20%, whichever is larger. Specified at a test length, not elongation, to account for length variations.
Inspection Methods: Ensuring Quality Every Time
Quality control is key—here’s how to verify your springs meet the mark.
Assembled Inner Diameter
Use a taper gauge (as shown in the original Figure 1). Avoid calipers or verniers—the spring’s flexibility skews results.
Spring Load
Requires specialized gear: Extend to test length with ±0.001 inch (0.02 mm) accuracy and measure force within ±0.05 ounces (0.014 Newtons). Suggested extensions: 0.200/0.400 inch (5.08/10.16 mm) or 5%/10% of free length.
Spring Joint (“Nib”)
- Strength: Test with a gauge at 1.35x nominal I.D. If it passes without disassembling, it’s good (destructive test—discard after).
- Allowable Gap: Max two wire diameters (see original Figure 2).
Spring Wind-Up
If the spring forms a “figure eight,” drop it 12 inches (300 mm) onto a hard surface. Acceptable ones snap back to circular.
Stress Relief
Heat an unassembled spring to spec temp for 30+ minutes, cool, and recheck load—it must match original specs.
Wrapping It Up: A Nod to the Contributors
This bulletin wouldn’t exist without input from companies like Chicago-Allis, JM Clipper, John Crane, CR Industries, Dana, Federal-Mogul, Freudenberg-NOK, Garlock, Orcomatic, Albert Trostel Packings, and Yale South-Haven. It’s part of a broader RMA series on oil seals—check out related pubs like OS-1 (Shaft Finishing) or OS-4 (Application Guide) if you’re hungry for more.
While tech has evolved since 1992, these fundamentals remain rock-solid. If you’re designing seals today, consider modern updates, but this is a great foundation. Got questions or experiences with garter springs? Drop a comment below—I’d love to hear!
Source: Rubber Manufacturers Association Technical Bulletin OS-5 (1992). All figures and diagrams referenced are from the original document.