Lock Nut

What Is a Lock Nut

A lock nut is a threaded nut whose job is not merely to clamp but to stay clamped. In power transmission the term has a precise meaning: a bearing lock nut is a thin, low-profile round nut, slotted across its face, that is screwed onto a threaded shaft seat or onto the thread of a bearing adapter sleeve to locate a rolling bearing and apply a controlled axial force. Once tightened, it must not back off under vibration, thermal cycling or shaft reversal, so it is always combined with a separate anti-rotation device. This distinguishes it from an ordinary nut, which simply transmits clamp load and relies on thread friction alone to stay put.

The bearing lock nut is one of the small accessories that makes a self-aligning bearing arrangement work. On a parallel shaft seat the nut presses the bearing inner ring against a shoulder; on a tapered adapter sleeve the nut drives the sleeve up the taper, which expands the sleeve and grips the shaft while simultaneously setting the bearing internal clearance. In both cases the precision of the final axial position, and the squareness of the nut face to the thread axis, directly affect bearing running accuracy, preload and service life. A poorly seated lock nut shows up later as vibration, heat and premature bearing failure, which is why the nut and its washer are specified on bearing arrangement drawings rather than left to the fitter.

The second meaning of lock nut, common in general mechanical assembly, is the prevailing-torque fastener nut: a hex nut that locks a bolted joint against loosening through running friction built into the nut itself. The two best known forms are the nylon-insert (nyloc) nut to DIN 985, where a polyamide collar grips the bolt thread, and the all-metal nut to DIN 980, where a deformed top crown bites the thread metal-to-metal. These nuts resist loosening even when the joint loses clamp load, which an ordinary nut and plain washer cannot do. They appear on machinery, vehicles, structures and equipment by the millions, but they are fasteners, not bearing accessories, and they are dimensioned by the bolt they run on rather than by a bearing bore.

Historically, the slotted bearing lock nut grew up alongside the rolling bearing itself in the early twentieth century, as makers like SKF standardized adapter sleeve mountings that let a single bearing fit a plain shaft. The KM nut and MB tab washer pairing, codified later in DIN 981 and DIN 5406, became the de facto global standard for that job and remains so. The prevailing-torque fastener family is younger: the nylon-insert nut was patented in the United States in the 1940s and was standardized in Europe as DIN 985 and DIN 982, then harmonized internationally as ISO 7040, ISO 7041 and the self-locking ISO 10511 thin nut. All-metal designs such as the Stover and flexloc patterns followed for high-temperature and aerospace use.

For a buyer, the practical consequence is that the words "lock nut" on a request for quotation are ambiguous until the context is fixed. A bearing arrangement drawing calling out KM 8 and MB 8 wants a DIN 981 slotted nut and its DIN 5406 washer. A structural or vehicle drawing calling out M16 DIN 985-8 wants a nyloc hex nut in property class 8. The two are not interchangeable in price, geometry or function, and the rest of this guide keeps them clearly separated.

Lock Nut Types and Families

Lock nuts split into two top-level families with no overlap: bearing lock nuts, which locate and clamp rolling bearings, and prevailing-torque fastener nuts, which lock bolted joints. Within the bearing family the standard slotted KM type leads, joined by integral-locking and precision variants. Within the fastener family the nylon-insert and all-metal types lead. The table below sets out the principal series, their controlling standard and their job.

KM and KML slotted nuts are the workhorse of the bearing family. They are thin round nuts with four equally spaced axial slots in the face, gripped by a hook or pin spanner, and they are always paired with an MB lock washer from DIN 5406. KM denotes right-hand thread, KML denotes left-hand thread for shaft ends that would otherwise self-loosen under rotation direction. The series runs in catalog steps, roughly from KM 0 at M10x0.75 up to KM 40 at M200x3, so a single family covers small electric-motor shafts through to heavy industrial gearbox seats.

Integral-locking nuts (KMK, KMFE) remove the separate washer. A radial set screw, bearing on a soft brass or polyamide pad so it does not damage the shaft thread, locks the nut in any angular position. This speeds mounting and dismounting, is useful where the shaft has no keyway for a tab washer, and is favored on adapter and withdrawal sleeve mountings where the nut is also used to push the bearing off the taper during removal.

Precision nuts (KMT, KMTA) trade cost for accuracy. Three equally spaced grub screws act through hard-drawn brass pins onto the shaft thread, locking the nut and, crucially, letting the fitter square the clamping face to the thread axis within a tight runout tolerance. They are reusable for many cycles and are the standard choice for machine-tool spindles and super-precision bearing sets where the bearing must run true. SKF lists, for example, the KMT 8 in M40x1.5 with a 65 mm outer diameter, 22 mm width and 210 kN axial static load capacity, with a published locking-screw torque of 18 Nm.

In the fastener family, nylon-insert nuts (DIN 985 low profile, DIN 982 standard height, ISO 7040 and ISO 10511) suit ambient-temperature vibration duty, while all-metal nuts (DIN 980, often the Stover pattern) cover heat, fire and chemical exposure where nylon would fail. Both are selected by bolt size and property class rather than by a bearing bore, and both are covered in more depth in the chapters on mechanisms and materials.

Locking Mechanisms

Whatever the family, a lock nut earns its name through a defined anti-loosening mechanism. There are four mainstream mechanisms across the two families: the tab lock washer, the integral set screw, the multi-pin precision screw, and prevailing torque from friction. Each has a different reuse profile, cost and accuracy. The table below compares them on the criteria that matter at selection.

The tab lock washer (MB to DIN 5406) is the classic bearing solution. The washer has an inner tongue that sits in a keyway machined along the shaft thread, which prevents the washer from turning, and a ring of outer tabs. After the KM nut is tightened the fitter bends one outer tab down into the nearest of the nut's four face slots. The nut is then mechanically keyed to the washer and the washer to the shaft, a positive lock that cannot creep loose. The tabs are mild steel and fatigue if bent and straightened repeatedly, so good practice fits a fresh MB washer at every rebuild even though the KM nut itself is reused.

The integral set screw (KMK, KMFE) dispenses with the washer entirely. A radial grub screw is tightened against the shaft thread through a soft brass or polyamide pad, which spreads the load and protects the thread flanks from damage. The advantage is speed and the freedom to lock at any angular position, with no keyway required. It is well suited to adapter and withdrawal sleeve mountings, where the same nut is later used to jack the bearing back down the taper for removal.

The multi-pin precision screw (KMT, KMTA) takes the set-screw idea and makes it precise and balanced. Three grub screws, spaced at 120 degrees and acting through hard-drawn brass pins, let the fitter both lock the nut and dial in the squareness of its clamping face to compensate for small angular errors in adjacent components. Because the load is shared by three pins and the brass protects the thread, KMT nuts can be removed and refitted many times, which is why machine-tool spindle builders favor them. SKF specifies a defined grub-screw tightening torque for each size, so the preload is repeatable rather than felt by hand.

Prevailing torque is the fastener-family mechanism. A nylon-insert nut creates running friction because the polyamide collar is slightly undersized; the bolt thread cuts its own path through the nylon, and the elastic grip resists rotation even with zero clamp load. An all-metal nut achieves the same through a top section that is pinched or slotted at manufacture, so the threads deform elastically around the bolt. Prevailing torque is the extra torque required to turn the nut before it touches the joint face, and it is the measurable proof that the locking feature is present. Both insert and all-metal locks lose grip with repeated use, so these nuts are treated as limited-reuse parts and renewed on safety-critical joints.

Threads, Materials and Standards

Thread, material and the controlling standard together decide whether a given lock nut will fit and last. The bearing family is built on fine metric threads and low-carbon steel; the fastener family follows the general bolt-and-nut property-class system. Getting the thread designation wrong is the single most common ordering error, because a KM nut and a hex nut of the same nominal diameter use entirely different pitches.

Bearing lock nut threads are fine metric, matched to the bearing shaft seat or adapter sleeve thread rather than to a coarse bolt thread. The KM series steps through a fixed table: small sizes such as KM 1 are around M12, mid sizes such as KM 8 use M40x1.5, and large sizes run to M200x3 at KM 40. The fine pitch gives a high mechanical advantage for accurate axial drive-up and a flat torque-versus-displacement curve, which is exactly what controlled bearing seating needs. Inch shaft nuts in the N and AN series instead use UNF threads and pair with W-series tab washers, a parallel system common in North American machinery.

Bearing lock nut materials are usually a low-carbon steel such as C20 (1.0402) for catalog KM nuts, with a plain or zinc-plated finish, because the nut carries axial load but is not a high-tensile fastener. Where corrosion or hygiene matters, stainless variants in A2 (1.4301) are offered, and precision KMT nuts add hard-drawn brass locking pins and steel grub screws. The mating MB washers are mild steel chosen for ductility, since the tabs must bend cleanly without cracking. The table below summarizes representative KM and KMT sizes verified against SKF and FAG catalog data.

Fastener lock nut materials follow the nut property-class system. A DIN 985 or DIN 982 nyloc nut in carbon steel is offered in class 8 and class 10, and in stainless as A2-70 or A4-70, the same grades as a plain hex nut, because the nylon insert adds locking but not strength. The nylon ring itself is polyamide 6 or 6/6, which sets the temperature ceiling: it keeps its elastic grip from roughly minus 40 to plus 120 degrees Celsius, softens above that, and is permanently damaged beyond about 130 to 140 degrees. The all-metal DIN 980 nut uses no polymer, so it carries the full property-class temperature range and is the correct choice for exhaust, manifold, kiln and fire-rated joints.

The governing standards are worth keeping straight, because drawings cite them directly. DIN 981 and ISO 2982-2 dimension the KM slotted bearing nut; DIN 5406 dimensions the MB tab washer that mates with it; ABMA 8.2 covers the equivalent inch AN and N bearing nuts. On the fastener side, DIN 985 and ISO 10511 cover the low nyloc nut, DIN 982 and ISO 7040 the tall nyloc nut, and DIN 980 the all-metal nut. A specifier who quotes the right standard pair removes nearly all ambiguity from a purchase order.

Key Specification Parameters

Reading a lock nut spec sheet is a short skill, but a few parameters carry all the selection weight. For a bearing lock nut the decisive figures are thread designation, axial load capacity, width and the locking method; for a fastener nut they are bolt size, property class, prevailing-torque rating and temperature limit. Each is explained below.

Thread designation is the first and most error-prone field. It must match the shaft seat or adapter sleeve thread exactly in nominal diameter, pitch and hand. A KM 8 is M40x1.5 right-hand; ordering a coarse M40 hex nut, or the left-hand KML 8 by mistake, gives a part that either will not start or will loosen in service. Always copy the thread from the bearing arrangement drawing or the sleeve marking rather than inferring it from the shaft outer diameter.

Axial load and clamping capacity tells you whether the nut can hold the bearing seat under operating thrust without yielding the thread. Precision nuts publish a number directly: the SKF KMT 8 carries 210 kN axial static load. Catalog KM nuts are rated through their thread strength and the recommended drive-up, and in practice the nut is rarely the weak link, but on adapter-sleeve mountings the drive-up force that sets bearing clearance must stay within the nut and sleeve ratings.

Width and outer diameter govern fit in the surrounding envelope. A KM 8 is 58 mm across and 9 mm wide, a slim profile that suits crowded bearing housings, while the precision KMT 8 is 65 mm across and 22 mm wide because it must house three grub screws and a stiff square face. Always check the nut width against the available thread length on the shaft and the clearance to the next component.

Locking method and tightening data close out the bearing side. A washer-locked KM nut needs the matching MB washer and a shaft keyway; a KMK needs nothing extra; a KMT needs its published grub-screw torque, such as 18 Nm on the KMT 8, applied with a torque wrench so the preload is repeatable. For prevailing-torque fastener nuts the parameters shift to the bolted-joint world:

• Property class: class 8 or class 10 in carbon steel, A2-70 or A4-70 in stainless, matched to the bolt grade so the nut is not the weak member.
• Prevailing torque: the running torque before seating, the proof that the lock feature is present; standards set minimum first-use and maximum reuse values.
• Tightening torque: the total wrench torque is the bolt clamp-load torque plus the prevailing torque, not the bolt torque alone.
• Temperature limit: about 120 degrees Celsius for nylon-insert nuts, full property-class range for all-metal DIN 980.
• Reuse policy: limited for both insert and all-metal locks; renew on safety-critical and vibration-critical joints.

Corrosion and finish apply to both families. Plain steel KM and nyloc nuts are fine in dry indoor service, but outdoor, washdown, marine and food applications call for zinc, A2 or A4 stainless, with the food and pharmaceutical sectors often requiring stainless throughout for cleanability. Mixing a stainless bolt with a plated steel nut, or vice versa, invites galvanic corrosion and galling, so the nut finish should be specified to match the joint, not left to whatever is in the bin.

Selection Decision Factors

To turn the previous chapters into an actual order, follow the sequence below. Most selection mistakes are not exotic; they come from skipping the first step, which is deciding which family you are even in. Treat this as a fixed RFQ template and the wrong-part rate falls close to zero.

1. Confirm the family: bearing lock nut or prevailing-torque fastener nut. A bearing arrangement, adapter sleeve or spindle points to KM, KMK or KMT; a bolted structural, vehicle or machine joint points to DIN 985 or DIN 980. This single choice eliminates most wrong parts.
2. Copy the thread exactly: nominal diameter, pitch and hand, taken from the drawing or sleeve marking. For bearing nuts this is the fine metric KM thread (for example M40x1.5); for fastener nuts it is the bolt thread. Do not infer pitch from the shaft diameter.
3. Choose the locking mechanism: tab washer (KM plus MB, needs a shaft keyway), integral set screw (KMK, no keyway), or precision multi-pin (KMT, for running accuracy). For fasteners, nylon insert for ambient duty, all-metal for heat and fire.
4. Set the load and preload basis: for bearings, the axial drive-up or clearance-reduction target, and the nut axial capacity (for example 210 kN on a KMT 8). For fasteners, the property class matched to the bolt and the prevailing-torque rating.
5. Check the envelope: nut width against available thread length, outer diameter against housing clearance, and spanner access for the hook, pin or grub-screw tool. A nut that fits the thread but not the wrench is useless.
6. Specify material and finish: plain steel, zinc, or A2 / A4 stainless, driven by corrosion, hygiene and galvanic compatibility with the mating parts. Match nut and bolt materials in the fastener family.
7. Define temperature and environment: below the nylon ceiling near 120 degrees Celsius for nyloc, all-metal above it; account for vibration, washdown and chemical exposure that may rule out a polymer lock.
8. Plan reuse and spares: renew MB washers and prevailing-torque nuts at each service; stock KMT nuts as reusable assets; confirm the matching washer or pin part numbers are on the bill of materials so the lock is complete, not just the nut.

One last dimension that is easy to overlook is serviceability and tooling. A lock nut is only as good as the tool that sets it and the spare that renews it. Hook and pin spanners must be sized to the nut series, KMT nuts need a torque wrench and the right hex key, and MB washers must be re-stocked because they are consumed at every rebuild. SKF, Schaeffler FAG, NSK, NTN and Timken hold broad metric bearing-nut stock through MISUMI and bearing distributors, while Whittet-Higgins serves the inch and high-strength market in North America. Confirming tool, washer and spare availability at the quoting stage prevents a five-cent washer from stranding a bearing change years later.