The DIN 981 bearing lock nut is a slotted round nut used to locate and secure rolling bearings, adapter sleeves, and other machine components on threaded shafts. The product shown matches the four-slot KM-style form, which is commonly paired with an MB lock washer when positive mechanical locking is required. DIN 981:2009-06 is the current German standard for rolling-bearing locknuts. ISO 2982-2:2013 is the current international reference for locknut and locking-device dimensions and was reconfirmed in 2024. Typical buyers include bearing distributors, machine builders, gearbox manufacturers, maintenance teams, and industrial OEMs.
Product Overview and Key Features
KM-style bearing locknuts combine a thin circular body, metric fine internal thread, and equally spaced outer slots for tightening with a hook or locknut wrench. The clamping face and thread relationship is important because the nut establishes axial position at the bearing or sleeve.
- Four-slot round body: provides wrench engagement without the radial space required by a conventional hex nut.
- Metric fine thread: supports controlled axial positioning on bearing shafts and adapter sleeves.
- MB washer compatibility: smaller four-slot KM nuts are commonly secured with a matching lock washer and shaft keyway.
- Bearing-accessory geometry: standardized outside diameter, axial width, slot dimensions, and face runout support interchangeability.
- Wide machinery use: suitable for bearing mounting, shaft location, and related rotating-equipment assemblies when specified by the design.
DIN 981 Bearing Lock Nut Specifications
Specification Overview
| Item | Specification / Common Configuration |
|---|---|
| Current German standard | DIN 981:2009-06 |
| Product form covered here | KM-series four-slot rolling-bearing locknut |
| International reference | ISO 2982-2:2013 for locknuts and locking devices |
| Associated locking device | DIN 5406 MB lock washer for common four-slot KM sizes |
| Thread form | Metric fine thread for common KM sizes |
| Common material options | Steel; A2 and A4 stainless steel as market options |
| Common finish options | Plain/oiled, blackened or phosphated, zinc plated; stainless normally natural |
| Installation tool | Hook spanner or suitable locknut socket |
Selected KM Series Dimensions and Thread Details
The following table lists selected DIN 981 KM dimensions. d1 is the thread, d2 the outside diameter, A the axial nut width, b the slot width, and t the nominal slot depth. Some catalogs retain the older symbol h for axial width. Dimensions are millimetres.
| Designation | Thread d1 | d2 (mm) | A (mm) | b (mm) | t (mm) | Lock Washer |
|---|---|---|---|---|---|---|
| KM4 | M20×1 | 32 | 6 | 4 | 2 | MB4 |
| KM5 | M25×1.5 | 38 | 7 | 5 | 2 | MB5 |
| KM6 | M30×1.5 | 45 | 7 | 5 | 2 | MB6 |
| KM7 | M35×1.5 | 52 | 8 | 5 | 2 | MB7 |
| KM8 | M40×1.5 | 58 | 9 | 6 | 2.5 | MB8 |
| KM9 | M45×1.5 | 65 | 10 | 6 | 2.5 | MB9 |
| KM10 | M50×1.5 | 70 | 11 | 6 | 2.5 | MB10 |
| KM11 | M55×2 | 75 | 11 | 7 | 3 | MB11 |
| KM12 | M60×2 | 80 | 11 | 7 | 3 | MB12 |
DIN 981 also includes smaller and substantially larger locknuts, including other series and larger thread forms. Do not extrapolate the KM table into a continuous size range. When an MB washer is used, the shaft requires the corresponding groove for the washer’s inner tab.
Common Materials and Surface Finishes
Steel Locknuts
Machined steel is the standard market choice for bearing locknuts. Established industrial catalogs specify steel versions with minimum tensile strength around 350 MPa. Plain or lightly oiled parts are common where the assembly is internally protected. Blackened, phosphated, and zinc-plated configurations are also widely available.
Stainless Steel Options
A2/304-type and A4/316-type stainless locknuts are established market options for corrosion-sensitive machinery. Stainless material should be specified separately because the DIN 981 designation alone does not prove an exact stainless grade. Natural or passivated surfaces are common, while zinc plating is normally unnecessary on stainless steel.
Typical Applications and Selection Considerations
- Axial location of rolling bearings on cylindrical or tapered shafts.
- Bearing installation on adapter sleeves used in gearboxes, conveyors, fans, pumps, and motors.
- Machine-tool spindles, transmission assemblies, and rotating industrial equipment.
- General shaft assemblies requiring a compact slotted round nut and mechanical locking device.
For a related general-purpose round locking nut, compare Flybear’s GB812 round nut. The products are not automatically interchangeable because their standards, dimensions, and locking arrangements differ. Flybear’s industrial machinery fastening solutions provide additional application context. Select the locknut from the shaft thread, bearing or sleeve designation, locking device, required axial position, corrosion environment, and installation method.
Ordering and Quality Documentation
An RFQ should state DIN 981, KM designation or exact thread, material, finish, quantity, matching DIN 5406 lock washer if required, shaft or adapter-sleeve designation, and any controlled runout requirement. Buyers may request material verification, dimensional inspection, thread gauging, slot measurements, face-runout inspection, and coating checks. Flybear’s quality control and inspection page describes available inspection support; exact acceptance criteria should be agreed for the order.
Frequently Asked Questions
Is DIN 981 still a current standard?
Yes. DIN 981:2009-06 is currently listed by DIN as the active German standard for rolling-bearing locknuts. It replaced DIN 981:1993 and aligned several symbols and runout requirements with the ISO 2982-2 system.
What is the relationship between DIN 981 and ISO 2982-2?
Both address rolling-bearing locknuts, and DIN 981 was revised to align important symbols and tolerances with ISO 2982-2. They should still be stated separately on procurement documents rather than described as automatically identical in every requirement.
What does the KM number mean?
KM is the common designation for the narrow-series metric bearing locknut. Each KM number corresponds to a specific thread and main dimensions; for example, KM4 uses M20×1 and KM10 uses M50×1.5.
Does a KM bearing locknut need a lock washer?
When positive mechanical locking is required, common four-slot KM nuts are used with matching MB lock washers. The washer’s inner tab engages a shaft groove, while an outer tab is bent into one of the nut slots after adjustment.
What should I include in a quotation request?
Provide the DIN 981 or KM designation, exact thread, material, finish, quantity, matching washer requirement, shaft or sleeve reference, and inspection needs. Include a drawing when face runout, special thread direction, nonstandard slot geometry, or other controlled dimensions differ from the standard form.
Request a Quote for DIN 981 Bearing Lock Nuts
For a DIN 981 bearing lock nut quotation, send Flybear Industrial the KM designation, thread, material, finish, quantity, matching MB washer requirement, and shaft or adapter-sleeve details. Include drawings for special dimensions or inspection requirements. Submit the project through the Flybear fastener RFQ page for technical review and quotation.
