NFE 25-511 Washer Type Selection: Z/M/L Model Recommendations

1. Definitions and Geometric Characteristics of the Three Models

NFE 25-511 provides three radial width series for single-sided toothed conical spring washers with the same nominal diameter (matching bolt size):

For the same bolt size, the inner diameter $D_i$ is essentially identical, while the outer diameter increases sequentially, thus the width increases. Thickness and cone height also increase with the model, creating a gradient in flattening load, stiffness, and elastic compensation capacity across the three models.

2. Selection Criteria

The choice between Z, M, or L models is primarily based on the following constraints:

1. Preload $F_M$
   The washer must provide sufficient elastic force under the working compression and must not be flattened under the applied load. Therefore, the washer's flattening load $F_{flat}$ should be greater than the maximum working preload $F_{Mmax}$, with a safety margin.

2. Space Constraints

3. Installation space limits the washer outer diameter $D_e$ (e.g., counterbore diameter, wrench clearance).

4. If space is extremely limited, only the Z model is feasible; if ample, the L model can be chosen to increase bearing area and reduce surface pressure.

5. Surface Pressure Limits
   The softer the material of the connected parts, the larger the bearing area required to reduce surface pressure. The L model has the largest contact area, making it suitable for soft materials like aluminum alloys and plastics.

6. Elastic Compensation Requirements
   The L model has the highest stiffness, providing higher restoring force to compensate for the same embedding amount; the Z model has lower stiffness but allows a larger compression stroke (deflection). The choice depends on the expected relaxation displacement $\Delta l$ and the required residual clamping force.

3. Rapid Selection Process Based on Preload and Outer Diameter Constraints

1. Determine the bolt nominal diameter $d$, and initially check standard washer dimensions (inner diameter matches the bolt).
2. Obtain the maximum assembly preload $F_{Mmax}$ from VDI 2230 R6.
3. Calculate the required minimum washer flattening load:
   
   $$F_{flat,req} = (1.2 \sim 1.5) \cdot F_{Mmax}$$

A safety factor of 1.2~1.5 prevents the washer from flattening under working load or over-torquing.
4. Check space constraints: The maximum allowable outer diameter $D_{e,max}$.
5. Select a model from the standard series that satisfies the following conditions:
- $D_e \le D_{e,max}$ (space satisfied)
- $F_{flat} \ge F_{flat,req}$ (strength satisfied)
- If multiple models satisfy, prioritize the smaller outer diameter (to save space) or the thicker model (for higher load capacity).
6. Check surface pressure $p = F_{Mmax}/A_{nom} \le p_G$ (VDI 2230 R10); if not satisfied, increase the outer diameter (upgrade model or add a flat washer).
7. Check washer stress (OM point, uM point, including notch factor) to ensure no yielding or fatigue under working compression.

4. Reference Load Capacities for Each Model (50CrV4, Elastic Working Range, Compression ≤ 0.75h)

The following provides approximate flattening loads and recommended working load ranges (in N) for Z/M/L washers under common bolt sizes, for quick selection reference.

Note: The recommended working load is taken as 0.6～0.8 times the flattening load to ensure the washer operates in the approximately linear elastic range.

Usage: Based on the calculated $F_{Mmax}$, select a model whose recommended working load range covers this value; if the preload exceeds the L model's upper limit, increase the bolt size or use multiple washers in a stack (parallel arrangement) to share the load.

5. Mandatory Selection Under Space Constraints

When the installation space (e.g., counterbore inner diameter $D_{hole}$) strictly limits the washer outer diameter, the feasible model can be determined using the maximum outer diameter values in the table below:

If the allowable outer diameter is smaller than the Z model's outer diameter, the NFE 25-511 washer cannot be used directly with that bolt size. Consider: - Reducing the bolt size - Deepening the counterbore and recessing the washer into the hole (if structurally permissible) - Switching to a non-toothed elastic washer or other compensation solution

6. Elastic Compensation Comparison and Selection

When the joint needs to compensate for a certain amount of embedding or thermal expansion/contraction displacement $\Delta l$, the remaining compression margin $\Delta s = h - s_{work}$ of the washer under working load must be greater than $\Delta l$. The three models have different cone heights $h$ and thus different compression strokes (Z model smallest, L model largest). When a larger elastic compensation displacement is required, prioritize the L model or use multiple washers in series.

7. Complete Selection Example

Conditions: M10 bolt, grade 8.8, $F_{Mmax}=18\,000$ N, connected parts are aluminum alloy (low allowable surface pressure), installation space allows outer diameter ≤ 28 mm.

Steps: 1. From the table above, the outer diameters for M10 models are: Z=16, M=20, L=25, all ≤28 mm, so all satisfy the space constraint. 2. Load capacity: The Z model's recommended working load upper limit is 5 200 N, far below 18 000 N, so it is excluded; the M model's upper limit is 8 000 N, still insufficient; the L model's upper limit is 11 200 N, still below 18 000 N! This indicates the table data may not be for high-strength bolts. In fact, the L model's flattening load of 14 000 N is already less than $F_{Mmax}=18 000$ N, making it unusable. Adjustments are needed: either increase the bolt size or use multiple washers in a stack (two L models stacked, total flattening load doubles). If space permits, washers can be stacked in series: two washers under the same compression provide the same total force as one, but the maximum allowable load doubles (since each washer bears half the compression). However, this requires more axial space (increased total height). Another option is to upgrade to an M12 bolt or use a special high-load elastic washer. 3. Surface pressure: If using the L model with $D_e=25, D_i=10.5$, the area is approximately 377 mm², pressure $18 000/377≈48$ MPa. The allowable surface pressure for aluminum alloys is generally 150～250 MPa, so this is satisfied. 4. Conclusion: In this example, a single standard NFE 25-511 washer cannot independently bear 18 000 N. Two L models in a stack must be used, or another high-load elastic washer should be selected. This highlights the importance of selection—load capacity must be verified.

(If the preload were lower, e.g., an M10 bolt only using 8 000 N, the M model would suffice, and the L model would be even safer.)

Summary:
The choice between Z/M/L is essentially a balance between space constraints, preload level, and surface pressure. By comparing the outer diameter and load capacity from the standard size table, feasible models can be quickly identified, and final confirmation is made through washer stress and surface pressure checks. For high preloads or extremely tight spaces, combined use or customization may be necessary.