KM and KML Lock Nuts

Overview of KM and KML Lock Nuts

KM and KML are two of the most commonly used and classic metric shaft lock nut series from SKF. They form the foundation of industrial bearing locking solutions.

Shared Core Characteristics:

1. Design Purpose: Specifically designed for the axial positioning and locking of rotating components such as bearings, gears, and belt pulleys onto a shaft. They are used in conjunction with a lock washer to form a reliable anti-loosening system.

2. Locking Principle: They belong to the category that requires a keyway. The nut itself does not have a built-in locking mechanism. Its anti-loosening function is achieved through an independent lock washer. The inner tabs of this washer engage with the keyway in the shaft, while the outer tabs engage with the slots on the nut's face, preventing the nut from rotating relative to the shaft.

3. Installation Method: Both nuts feature four equally spaced slots around their circumference. These slots accommodate a hook spanner or impact spanner for tightening and removal, making them particularly suitable for confined spaces or applications requiring high preload.

4. Thread Type: Both utilize metric threads.

5. Alternative Names: Often referred to as shaft nuts or withdrawal nuts.

6. Reusability: The nut body can be reused provided the threads and load-bearing faces are not damaged. However, the lock washer is considered a consumable part and should be replaced with a new one every time the corresponding nut is installed to ensure locking reliability.

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Core Difference Between KM and KML Lock Nuts

While functionally identical, KM and KML differ in one key design aspect, which directly influences their application scenarios.

Core Difference: Cross-Sectional Height

In simple terms: The KML is the "low-profile" or "thin" version of the KM nut. They function identically, but the KML is thinner for space-constrained applications.

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Technical Details & Selection Information

• Size Range:

  • Both cover a wide metric thread range from M10x0.75 to M200x3 (corresponding to size codes 0 to 40).

• Compatible Lock Washers:

  • Both can be used with the standard MB series lock washer.

  • For applications requiring higher locking force, both can be upgraded to use the thicker, stronger MB A series lock washer.

• Application Schematic:

  • The combination of KM/KML nut + MB/MB A washer + shaft keyway forms the complete anti-loosening system.

Summary & Selection Guide

• Functional Identity: Both KM and KML are metric shaft lock nuts that achieve locking via a lock washer and keyway.

• Key Difference: The KML is a low-profile design variant of the KM, primarily used to save axial space.

• How to Select:

  • Step 1 - Determine Space: Check the axial space at the installation point. If space is ample, the standard KM series is preferred. If space is tight, the KML series must be used.

  • Step 2 - Determine Specification: Select the corresponding nut and washer models based on the shaft's thread size.

  • Step 3 - Determine Load: For常规 loads, the MB washer is sufficient. For heavy loads, high vibration, or high-reliability requirements, the stronger MB A washer is recommended.

  • Always Remember: Always use a new lock washer during installation and ensure its inner tabs fully engage the shaft keyway.

Therefore, when selecting, the engineer should first decide between the standard (KM) or low-profile (KML) type based on the axial space constraints of the mechanical design. Then, the specific model and compatible washer are chosen according to the thread size and load conditions.

The primary dimensions of the SKF KM series lock nuts are compatible and consistent with the DIN 981 standard, but they are not 100% identical. SKF has introduced optimizations and enhancements upon this base standard.

We can understand this relationship from the following aspects:

1. Core Conclusion

• Interchangeability: In the vast majority of cases, for the same thread specification, KM series nuts are interchangeable with nuts conforming to the DIN 981 standard. They can be installed on the same shaft, used with the same lock washers (e.g., the MB series), and fitted with the same spanners.

• Based on, and Superior to, the Standard: The SKF KM series is designed and manufactured based on the DIN 981 standard. However, as a premier bearing and component manufacturer, SKF typically adheres to stricter internal control standards and optimizes aspects such as material selection, heat treatment, and surface finishing to deliver higher reliability and performance.

2. Primary Identical Dimensions (Key to Interchangeability)

The following critical dimensions are essentially identical to the DIN 981 standard, forming the basis for interchangeability:

• Thread Specification: Metric threads (M).

• Core Installation Dimensions:

  • Thread Major Diameter (d)

  • Width Across Flats (S): This dimension, used for hook or socket spanners, must match.

  • Nut Outside Diameter (D): Relevant for installation space.

  • Cross-Sectional Height (m): The nut's thickness, a key parameter previously discussed.

• Number and Position of Spanner Slots: The standard four equally spaced slots.

3. Potential Minor Differences (Typically Non-Critical to Function)

These differences usually lie in non-critical dimensions or design details, aimed at enhancing performance:

• Spanner Slot Width and Root Radii: SKF may optimize the slot geometry to improve stress distribution and reduce the risk of cracking under high-impact tightening torque.

• Face Chamfers or Surface Treatment: The flatness of the load-bearing face, surface hardening, or anti-corrosion coatings (e.g., Dacromet) may meet specifications exceeding the base DIN standard.

• Material and Heat Treatment: While DIN 981 specifies a performance class (e.g., Strength Class 8), SKF employs specific high-grade steels and precisely controlled heat treatment processes to ensure consistent, high-level mechanical properties (such as hardness and toughness) in its products.

4. Practical Recommendations

1. For General Replacement and Design: You can confidently consider the KM series as a high-quality commercial product conforming to DIN 981. Specifying the purchase of KM series for drawings calling for a "lock nut per DIN 981" is perfectly acceptable.

2. For Critical or Extreme Service Applications: If the application involves heavy loads, high vibration, or high-reliability requirements, it is strongly advised to use SKF's product catalog and complete technical drawings for final verification and selection, rather than relying solely on the DIN standard number. The KM series may offer performance guarantees beyond the standard.

3. How to Verify: The most direct method is to obtain the DIN 981 standard drawing and the SKF KM series product drawing for the same thread specification (e.g., M30x1.5) and compare all dimensional parameters side-by-side. You will find that the key dimensions (d, S, D, m) are identical, while some secondary dimensions (e.g., slot width n, radius r) may have slight variations.

Summary:
The KM series is functionally equivalent, interchangeable, and matches the core installation dimensions of DIN 981. You can understand SKF's KM series as a high-performance product that "conforms to and exceeds" the DIN 981 standard. For the vast majority of industrial applications, they can be directly substituted.

Alternative options for KML:

If you are looking for a low-profile lock nut that conforms to a DIN standard and is fully interchangeable with the KML series in terms of both dimensions and locking system, there is, in fact, no independent DIN standard number for such a component. The KML is a specialized low-profile variant developed by SKF based on its KM series (which approximates DIN 981), and it retains SKF's unique washer-based locking system.