FAQ

'Rolling' or 'dishing' of a spiral ring occurs when the thrust load causes the ring to twist, leading to premature ejection. To prevent this, the groove must have a sharp corner (maximum radius of $10\%$ of material thickness $t$) and a sufficiently deep wall. The 'Groove Deformation' load $P_g$ is often the limiting factor rather than the 'Ring Shear' load $P_r$. $P_g$ is calculated using $P_g = \frac{D \pi G S_y}{K}$, where $G$ is groove depth, $D$ is shaft/bore diameter, $S_y$ is the yield strength of the groove material, and $K$ is a safety factor (usually 2.0). If the housing is a soft material like aluminum, a wider groove and thicker ring are mandatory.

316 Stainless Steel (UNS S31600) is often chosen for its molybdenum content which provides superior resistance to pitting and crevice corrosion. However, in marine environments, 316 can still be susceptible to Stress Corrosion Cracking (SCC) if the combination of tensile stress, chloride concentration, and temperature (usually $>140^{\circ}F$) is met. For spiral rings, the coiling process and the 'clinging' tension in the groove create a baseline stress. If the environment is highly aggressive, a nickel alloy like Inconel 625 or X-750 may be necessary, as their higher nickel content makes them virtually immune to chloride-induced SCC.

A spiral retaining ring will begin to expand and lift out of its groove if the centrifugal force exceeds the ring's 'clinging' force. The maximum allowable RPM $N$ is calculated as $N = \sqrt{\frac{4.8 \times 10^{11} E t G^2}{w (D_o^3 R_m)}}$, where $E$ is the Modulus of Elasticity, $t$ is the thickness, $G$ is the groove depth, $w$ is the radial wall, $D_o$ is the outer diameter, and $R_m$ is the mean radius. If the application speed exceeds this value, a self-locking feature (a tab and slot) is required to mechanically prevent the ring from expanding radially. This is critical in high-speed automotive transmissions and aerospace turbine shafts.

Relaxation is the gradual loss of load when a spring is held at a constant deflection over time, exacerbated by high temperatures. The rate of relaxation follows an Arrhenius relationship, where the loss in load $\Delta P$ increases exponentially with temperature $T$. For a carbon steel spring at $250^{\circ}F$, relaxation might be $5-10\%$ over 1000 hours. If the application requires a precise preload (e.g., in a mechanical seal), the spring must be 'heat set' by the manufacturer. This involves compressing the spring at a temperature higher than the operating temperature, which pre-relaxes the material and stabilizes the load for service.

Shim ends, also known as 'flat ends,' are produced by flattening the last half-wave on each end of a multi-turn spring. This creates a $360^{\circ}$ contact surface rather than point contact at the peaks. This is critical for applications where the spring must provide uniform pressure on a seal or a bearing race. While shim ends increase the solid height $H_s$ by $2t$, they significantly reduce the 'tipping' moment and ensure that the load is transmitted axially. The spring rate calculation must be adjusted because the shim ends act as inactive turns, reducing the effective number of active waves.

In MRI and sensitive aerospace electronics, materials with low magnetic permeability $\mu < 1.01$ are required to prevent field distortion. Standard carbon steels and 400-series stainless steels are ferromagnetic and unsuitable. 302/304 stainless steels are paramagnetic in the annealed state but can become slightly magnetic when cold-worked into springs. For strictly non-magnetic applications, Phosphor Bronze (ASTM B159) or Monel K-500 (UNS N05500) are used. Phosphor Bronze offers excellent conductivity but lower strength, while Monel K-500 provides high strength and excellent corrosion resistance while remaining non-magnetic even after heavy cold reduction.

The maximum load $P_{max}$ is reached when the peak stress $\sigma$ equals the yield strength $S_y$ of the material, adjusted for a safety factor. Rearranging the stress formula: $P_{max} = \frac{4 b t^2 N^2 S_y}{3 \pi D_m}$. For most spring steels, the design stress at work height is limited to $80\%$ of the minimum tensile strength to ensure long-term stability. If the calculated $P$ required by the system exceeds $P_{max}$, the designer must either increase the material thickness $t$, increase the radial width $b$, or select a material with a higher $S_y$, such as 17-7PH over 302 stainless.

Wave shifting occurs when the peaks of a multi-turn wave spring do not remain vertically aligned during compression. This shift causes the spring to behave as a hybrid between a series and parallel assembly, leading to an unpredictable spring rate and localized overstressing. It is often caused by lack of radial constraint or high-frequency vibration. To prevent this, 'Crest-to-Crest' springs can be manufactured with 'alignment dimples' or 'shim ends'. In extreme cases, switching to a 'nested' design where the wire is wound continuously on top of itself can eliminate shifting, though this changes the load-deflection profile significantly.

Tall Crest-to-Crest wave springs (where $L_{free}/D_m > 1.5$) are susceptible to buckling or 'snaking' when compressed. The radial wall thickness $b$ contributes to the lateral stiffness. To prevent buckling, the spring should be guided either by a shaft or in a bore. The clearance should be minimal but sufficient to allow for the diameter expansion during compression. If no guide is possible, the engineer must increase the radial width $b$ to improve the moment of inertia $I = \frac{b t^3}{12}$ relative to the axial direction, effectively increasing the critical buckling load $P_{cr}$.

Coiling wave springs from cold-rolled flat wire introduces significant residual tensile and compressive stresses throughout the cross-section. Without stress relieving, these internal stresses can cause 'creep' or dimensional instability over time. For SAE 1070-1090 carbon steel, stress relieving is typically performed at $600^{\circ}F$ to $700^{\circ}F$. This process stabilizes the microstructure, reduces the risk of stress corrosion, and ensures that the elastic properties are consistent. It also minimizes 'spring back' variations, allowing for tighter control over the free height $L_{free}$ and wave diameter.

The solid height $H_s$ of a Crest-to-Crest wave spring is the height at which the spring is fully compressed and the waves are flattened. It is calculated as $H_s = Z \times t$, where $Z$ is the number of turns and $t$ is the material thickness. However, in practice, a 'theoretical solid height' may be slightly higher due to the presence of the 'gap' or overlap in the turns. Design engineers must ensure the maximum work height $H_{min}$ is at least $20\%$ greater than $H_s$ to avoid 'bottoming out,' which causes an infinite spring rate and leads to catastrophic mechanical failure of the assembly.

Non-linearity in Crest-to-Crest springs typically occurs at the beginning of the stroke due to 'flatness' issues and at the end of the stroke as the spring approaches solid height. If the load-deflection curve shows an early 'lag,' it indicates that not all wave peaks are contacting the mating surfaces simultaneously. This is often resolved by tightening the parallelism tolerances on the spring ends. If the rate spikes prematurely, it suggests the waves are 'nesting' or shifting radially and contacting the bore/shaft. Using a shim or a 'level-end' design, where the first and last waves are flat, provides more uniform contact.

Nesting involves stacking multiple single-turn wave springs in parallel to increase the load capacity (load scales by the number of springs). The primary risk is friction between the layers, which introduces hysteresis in the load-deflection curve and generates heat during cyclic operation. Misalignment of the wave peaks between nested layers can also cause localized stress spikes and non-uniform loading. To address this, nested springs should ideally be manufactured as a single continuous 'interlaced' or multi-turn nested spring where the coils are produced together, ensuring perfect synchronization of the waves.

A286 (ASTM A638) is an austenitic precipitation-hardenable steel that maintains excellent ductility and impact strength at cryogenic temperatures, unlike ferritic or martensitic steels which become brittle. For cryogenic wave springs, the material is typically solution treated and age hardened to optimize the gamma-prime precipitates. The thermal expansion coefficient $\alpha$ must be accounted for in the assembly design, as the spring will contract more than the surrounding carbon steel housing at liquid nitrogen temperatures. The Modulus $E$ also increases slightly as temperature drops, leading to a higher spring rate in service.

The number of waves $N$ has a quartic relationship with the spring rate $k \propto N^4$. Increasing $N$ significantly increases the stiffness, allowing for higher load capacity within a very short axial space. However, as $N$ increases, the maximum allowable deflection $f_{max}$ decreases because the shorter arc length between peaks increases the bending stress for a given displacement. Engineers must balance $N$ to achieve the required $P$ at $H_w$ without exceeding the stress limit $\sigma < 0.8 S_y$. For high-deflection applications, a lower $N$ with a thicker material $t$ is often preferred.

Permanent set, or 'taking a set,' occurs when the spring is compressed to a height where the internal stresses exceed the material's elastic limit, leading to plastic deformation and loss of free height. To mitigate this, manufacturers often perform a 'preset' or 'remove set' operation during production. The spring is compressed to its solid height (or a height lower than the minimum work height). This induces beneficial residual stresses in the direction of the load, effectively increasing the apparent yield strength for subsequent cycles. This process ensures the spring remains stable at the design work height.

When a wave spring is compressed, its diameter expands slightly. If installed in a bore, the clearance between the spring OD and the bore ID must accommodate the expansion $\Delta D \approx 0.02 \times (f/N)$ per wave to prevent binding. If installed on a shaft, the ID must have sufficient clearance to avoid 'clinging' during deflection. Furthermore, the seating surfaces must be flat and hard; soft housing materials like aluminum may suffer from 'fretting' or 'brinelling' due to the concentrated point loads at the wave peaks, necessitating the use of a hardened shim washer.

Inconel X-750 (UNS N07750) is specified for wave springs when the application requires high strength at temperatures up to $1300^{\circ}F$ or resistance to chloride-induced stress corrosion cracking (SCC) found in subsea environments. It is often heat-treated to the NACE MR0175 standard for sour gas service. The material's high nickel content provides immunity to many reduction environments, while chromium provides resistance to oxidizing conditions. Its relaxation rate at $1000^{\circ}F$ is significantly lower than that of A286 or 17-7PH, ensuring long-term seal integrity in downhole tools.

While the Wahl factor is traditionally used for helical springs, a similar curvature correction is applied to wave springs to account for the non-linear stress distribution across the radial width $b$. The stress is higher at the inner radius $R_i$ than the outer radius $R_o$. The adjusted stress $\sigma_{adj} = C \cdot \sigma_{nom}$, where $C$ is a function of the ratio $D_m/b$. Ignoring this factor in compact designs where the radial width is large relative to the diameter can lead to unexpected yielding, as the peak fiber stress may exceed the yield strength $S_y$ by $15-20\%$.

Fatigue failure in wave springs usually initiates at the inner or outer diameter of the wave peak where tensile stress is maximized. Using the Goodman Criterion, the safety factor $S_f$ can be estimated by $\frac{\sigma_a}{S_e} + \frac{\sigma_m}{S_u} = \frac{1}{S_f}$, where $\sigma_a$ is the alternating stress and $\sigma_m$ is the mean stress. Common failure triggers include hydrogen embrittlement in carbon steel (SAE 1070) if not properly baked after plating, and surface pitting in corrosive environments which acts as a stress concentrator. To extend life, shot peening the surface can induce compressive residual stresses, effectively shifting the mean stress downward.