A KM locknut on its own is a poor fastener. It is thin, it has no flange, and its fine thread will happily back off under the vibration of a running shaft. What makes it a locking system is the flat steel washer underneath it - the MB - and the two only work as a pair. Fit the nut without the washer and you have not locked anything; you have just spun a nut onto a shaft.
How the Pair Works
The MB lockwasher is a plain annular disc with two features: an inner tang that folds down into a keyway machined along the shaft (or the slot in an adapter sleeve), and a ring of outer tabs around its rim.
- The inner tang keys the washer to the shaft, so the washer cannot rotate.
- The KM nut is threaded on and tightened against the bearing inner ring.
- One of the outer tabs is bent up into one of the slots machined in the face of the KM nut.
The nut is now mechanically tied to the shaft through the washer. It cannot rotate without either shearing the tab or shearing the tang. This is a positive locking method - it does not depend on friction, preload or thread adhesive, which is exactly why it is used on rotating machinery where a loose bearing would be catastrophic.
The one thing to remember: the nut carries the clamping load, but the washer carries the locking. A KM nut fitted without its MB washer - or with the tab left unbent - is not a locknut at all. It is the single most common cause of bearings walking along a shaft.
Reading the Designation
KM numbering looks arbitrary but it is a simple series index, not a dimension. KM 10 is not an M10 thread - it is M50 × 1.5. That catches people out constantly when ordering, so read the number as a series position and look the thread up.
The rules are mercifully simple:
- The washer number always matches the nut number. KM 10 takes MB 10. There is no cross-sizing.
- The thread is always fine pitch - 0.75 mm, 1 mm, 1.5 mm, 2 mm or 3 mm depending on size. A fine pitch gives finer preload adjustment and resists loosening better than a coarse thread.
- Above KM 42 the series changes to HM, a heavier nut with a deeper section for larger shafts.
Nuts in this family are also catalogued under DIN 981 (the nut) and DIN 5406 (the tab washer), so a drawing may call out either the KM/MB designation or the DIN number for the same part.
KM Size Chart
Thread, outside diameter, width and the matching lockwasher for every size in the standard KM range:
| Locknut | Thread | O/D D | Width b₁ | Slot width | Lockwasher |
|---|---|---|---|---|---|
| KM0 | M10×0.75 | 18 | 4 | 3 | MB0 |
| KM1 | M12×1 | 22 | 4 | 3 | MB1 |
| KM2 | M15×1 | 25 | 5 | 4 | MB2 |
| KM3 | M17×1 | 28 | 5 | 4 | MB3 |
| KM4 | M20×1 | 32 | 6 | 4 | MB4 |
| KM5 | M25×1.5 | 38 | 7 | 5 | MB5 |
| KM6 | M30×1.5 | 45 | 7 | 5 | MB6 |
| KM7 | M35×1.5 | 52 | 8 | 6 | MB7 |
| KM8 | M40×1.5 | 58 | 9 | 6 | MB8 |
| KM9 | M45×1.5 | 65 | 10 | 6 | MB9 |
| KM10 | M50×1.5 | 70 | 11 | 6 | MB10 |
| KM11 | M55×2 | 75 | 11 | 7 | MB11 |
| KM12 | M60×2 | 80 | 11 | 7 | MB12 |
| KM13 | M65×2 | 85 | 12 | 7 | MB13 |
| KM14 | M70×2 | 92 | 12 | 8 | MB14 |
| KM15 | M75×2 | 98 | 13 | 8 | MB15 |
| KM16 | M80×2 | 105 | 15 | 8 | MB16 |
| KM17 | M85×2 | 110 | 16 | 8 | MB17 |
| KM18 | M90×2 | 120 | 16 | 10 | MB18 |
| KM19 | M95×2 | 125 | 17 | 10 | MB19 |
| KM20 | M100×2 | 130 | 18 | 10 | MB20 |
| KM21 | M105×2 | 140 | 18 | 12 | MB21 |
| KM22 | M110×2 | 145 | 19 | 12 | MB22 |
| KM23 | M115×2 | 150 | 19 | 12 | MB23 |
| KM24 | M120×2 | 155 | 20 | 12 | MB24 |
| KM25 | M125×2 | 160 | 21 | 12 | MB25 |
| KM26 | M130×2 | 165 | 21 | 12 | MB26 |
| KM27 | M135×2 | 175 | 22 | 14 | MB27 |
| KM28 | M140×2 | 180 | 22 | 14 | MB28 |
| KM29 | M145×2 | 190 | 24 | 14 | MB29 |
| KM30 | M150×2 | 195 | 24 | 14 | MB30 |
| KM31 | M155×3 | 200 | 25 | 16 | MB31 |
| KM32 | M160×3 | 210 | 25 | 16 | MB32 |
| KM33 | M165×3 | 210 | 26 | 16 | MB33 |
| KM34 | M170×3 | 220 | 26 | 16 | MB34 |
| KM36 | M180×3 | 230 | 27 | 18 | MB36 |
| KM38 | M190×3 | 240 | 28 | 18 | MB38 |
| KM40 | M200×3 | 250 | 29 | 18 | MB40 |
All dimensions in mm. The lockwasher number always matches the nut number - KM 12 takes MB 12, with no exceptions. Sizes above KM 42 move to the heavy HM series; ask us for those dimensions.
Fitting It Correctly
- Check the keyway first. The shaft (or adapter sleeve) must have a slot for the washer's inner tang. No slot, no lock - a KM nut on a plain threaded shaft has nothing to react against.
- Fit the washer before the nut, tang down into the shaft keyway, sitting flat against the bearing inner ring.
- Run the nut down against the washer using a proper C-spanner or a socket-type KM wrench that engages the slots squarely.
- Tighten to seat the bearing, not to a headline torque figure. On an adapter sleeve the nut is driving the sleeve up the taper, so the target is the bearing's specified radial clearance reduction - measure it with feelers on a spherical roller bearing rather than guessing.
- Bend one tab up into a nut slot. If no tab lines up, nip the nut a fraction tighter until one does - never slacken back to find a slot, as that gives up your bearing setting.
- Bend the tab fully home against the flat of the slot so there is no free play. A half-bent tab fatigues and snaps.
Four Common Mistakes
- Reusing the washer. A tab that has been bent up and flattened again is work-hardened and will crack. MB washers are consumables - replace them every time the joint is opened.
- Driving the nut with a punch. Hammering a drift against the slots deforms them, so the next tab will not seat and the next spanner will not engage. Use the correct C-spanner.
- Backing off to find a slot. Slackening to align a tab loses the bearing clearance you just set. Always go tighter, never looser.
- Bending more than one tab. One tab is the design intent. Bending several does not add security - it just makes the washer harder to remove and more likely to break tabs off inside the housing.
When You Need the HM Series
Beyond KM 42 the standard nut section becomes too slender for the loads involved, and the range continues as HM - a heavier, thicker nut for large shafts, typically on big spherical roller bearings in crushers, mills and marine gearboxes. We stock HM 44 through HM 76 alongside the standard KM range, with MB washers to match.
If your shaft has a trapezoidal rather than metric thread, or the nut is an unusual thickness, send us a photograph with the thread pitch and the outside diameter and we will identify it.
Need KM Nuts or MB Washers?
KM 0 to KM 42 and the heavy HM 44 - HM 76 series, with matching MB washers from stock in Mumbai.
View Fastener RangeFrequently Asked Questions
Is a KM 10 locknut an M10 thread?
No. The KM number is a series index, not a thread size. KM 10 is M50 × 1.5. The smallest nut, KM 0, is M10 × 0.75 - so the two do not line up anywhere in the range. Always look the thread up in a chart rather than assuming it from the number.
Which MB washer goes with which KM nut?
The numbers always match: KM 0 takes MB 0, KM 25 takes MB 25, and so on through the range. There is no cross-sizing and no substitution - a washer one size out will either not clear the thread or will not engage the nut slots.
Can I use a KM locknut without the MB washer?
No. The nut is thin and fine-threaded and will loosen under vibration on its own. The MB washer is what provides the locking: its inner tang keys into the shaft keyway and one outer tab bends up into a slot in the nut, tying the nut mechanically to the shaft.
Can an MB lockwasher be reused?
It should not be. Bending a tab up and flattening it again work-hardens the steel at the fold, and a tab that cracks off inside a bearing housing is expensive. Treat MB washers as consumables and fit a new one every time the joint is opened.
What torque should I use on a KM locknut?
On an adapter sleeve there is no meaningful torque figure, because the nut is driving the sleeve up a taper and the correct setting is defined by the bearing’s radial clearance reduction - measured with feeler gauges - not by torque. On a plain threaded shaft, tighten firmly to seat the bearing against its shoulder, then bend a tab.
What is the difference between KM and HM locknuts?
HM is the heavy series that continues the range above KM 42. The nut has a thicker section to suit the larger shafts and higher loads found on big spherical roller bearings. We stock KM 0 - KM 42 and HM 44 - HM 76, with matching MB washers.