How MRN Locks at 90° to the Shaft
MRN locks through a brass pad rather than through the set screw itself. Each of the two screws enters through the outside diameter, meets the shaft at 90°, and drives its pad onto the thread flank, spreading the load along the flank instead of denting the crest. On a shaft of six or eight millimeters that distinction decides whether the thread survives the first strip-down.
The full explanation is on the Radial Locking Series page.
Open-End Wrench on Two Flats
No hook wrench anywhere in the range
Smallest Locknut NIKKI Makes
M5 x 0.5 at 12.5 mm outside diameter
Precision-Turned Contact Surface and Thread
Maximum axial run-out 0.005 mm
Two Set Screws Per Nut
M3 to M6. 0.9 to 8 Nm max
Where MRN Locknuts Are Used
Small Shafts from M5 x 0.5, Below the Range of Every Other Type in the Series
Light Transmission Work: Small Pulleys, Sprockets, and Timing-Belt Drives
Assemblies Built and Serviced with Ordinary Hand Tools, Where a Hook Wrench Is Not on the Bench
Instruments and Small Mechanisms, Where a 12.5 mm Outside Diameter Is What the Housing Allows
Cost-Driven Builds Where a Drawing Tolerance of 0.005 mm Is Enough
Find Your Size(M5 to M40)
Filter MRN locknuts by shaft thread, pitch, or height.
Products in this Radial Locking Series
All seven lock the same way, with set screws at 90° driving a pad onto the thread flank, most in a body shorter than the 30° flank equivalent. What differs is the machining grade, the wrench the nut takes, the number of set screws carrying the clamping load, and in two cases the material and the duty the type is built for.
Grinding
MR
Grinding
Hook wrench · three set screws
M6×0.5 to M260×4 · h 8–30 mm
MR is the general-purpose radial locknut, ground to 0.002 mm, with three set screws at 120° and the widest range in the series. Use it where the drawing calls for the ground grade, and wherever the thread runs above M200 × 3.
SR
Grinding
Hook wrench · two set screws
M6×0.5 to M200×3 · h 8–32 mm
SR is the two-screw ground type, with four hook slots at every size where MR has three hook slots below M105 and six above, and heavier set screws at the top of the range, M12 at 60 Nm from SR135 where MR stays at M10 and 36 Nm.
MRRA
Grinding
Hook wrench · two set screws
M15×1 to M150×2 · h 20–42 mm
MRRA is the type for high radial torque, taller and heavier in the body than the rest of the series. Sizes from M20 to M50 are offered in two outside diameters, so the same shaft thread can carry a much larger contact face. Ground only.
Precision turning
MRZ
Turning
Hook wrench · three set screws
M6×0.5 to M200×3 · h 8–30 mm
MRZ is the precision-turned version of MR at 0.005 mm, matching it size for size up to M200 × 3 and adding fine-pitch rows at M55, M60, M65, M70 and M75 that MR does not carry.
SRZ
Turning
Hook wrench · two set screws
M6×0.5 to M200×3 · h 8–32 mm
SRZ is the precision-turned version of SR, with the same envelope, the same four hook slots and the same set screws at 0.005 mm run-out.
MRN
Turning
Open-end wrench · two flats
M5×0.5 to M40×1.5 · h 5–25 mm
MRN is drawn for light-duty transmission work and takes an ordinary open-end wrench on two flats. It reaches M5 × 0.5 at a 12.5 mm outside diameter, the smallest locknut NIKKI makes.
MRS
Turning
Hook wrench · three set screws
M6×0.5 to M50×1.5 · h 8–14 mm
MRS is fully SUS304 stainless, locking pin and set screws included, for food, pharmaceutical, chemical and semiconductor equipment. Dimensionally it matches MRZ across its range.
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Questions About the MRN Locknut
The MRN is the light-duty transmission locknut in NIKKI's Radial Locking Series, in metric threads from M5 x 0.5 to M40 x 1.5 with 0.005 mm maximum axial run-out. The questions below are the ones buyers ask before a first order.
Contact UsMRN carries two parallel flats for an ordinary open-end wrench, given as dimension L in the table, from 11 mm on MRN05 up to 50 mm on MRN40. Every size in the range has them, so no hook wrench is needed at any point; MRN is the only type in the radial series drawn that way.
MRN05 covers M5 x 0.5 in a body 12.5 mm across and 5 mm high, which is the smallest locknut NIKKI makes. The next size, MRN06, takes M6 x 0.75 at 13.5 mm. Below M6 x 0.5 no other type in the radial series offers anything at all.
The maximum axial run-out of an MRN locknut is 0.005 mm. NIKKI finishes both the thread and the contact surface by precision turning, which is what sets the run-out at this level and keeps the cost down. There is no ground version of MRN.
Each MRN size lists a catalog maximum per screw in the dimension table, from 0.9 Nm on the M3 screws of MRN05 up to 8 Nm on the M6 screws fitted from MRN25. These are the lightest screws in the series, so a torque wrench is worth using rather than judging by feel. Tighten the pair gradually and alternately.
Only at the small end. Through M8 x 1 and below MRN is the shorter nut, and from M10 x 1 to M15 x 1 the two stand level at 8 mm. Above that the wrench flats need body, and MRN grows: 25 mm high at M40 x 1.5 against 14 mm for the MR locknut. Specify MRN for the access and the small sizes, not for minimum height.
No. MRN locks between the set screws, the brass pads, and the thread flank, all inside the nut, so neither a thrust washer nor a shaft keyway is required. Nothing else goes on the shaft, and the nut is reusable if handled properly.