How MR Locks at 90° to the Shaft
MR locks through a brass pad rather than through the set screw itself. Each of the three 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. A screw sitting square to the axis needs only its own diameter of height in the body, which is where the shorter nut comes from.
The full explanation is on the Radial Locking Series page.
Six Millimeters Shorter Than MF
14 mm against 20 mm at M50 x 1.5
No Thrust Washer or Keyway
Fewer parts, simpler assembly
Ground Contact Surface and Thread
Maximum axial run-out 0.002 mm
3 or 6 Set Screws Per Nut
M4 to M10. 2.2 to 36 Nm max
Where MR Locknuts Are Used
Bearing and Gear Retention in Compact Gearboxes and Reducers
Large Threads Above M200 x 3, Which No Other Type in the Series Covers
Robot Joints, Servo Drives, and Assemblies Contesting Every Millimeter of Axial Length
Impeller and Rotor Retention on Pumps, Fans and Blowers
Shafts Already at Their Limit for Section, Where a Keyway Would Take Out Strength
Find Your Size(M6 to M260)
Filter MR 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 MR Locknut
The MR is the general-purpose ground locknut in NIKKI's Radial Locking Series, in metric threads from M6 x 0.5 to M260 x 4 with 0.002 mm maximum axial run-out. The questions below are the ones buyers ask before a first order.
Contact UsThe maximum axial run-out of an MR locknut is 0.002 mm. NIKKI grinds both the thread and the contact surface by a high-precision process, which is what holds the run-out at this level. The precision-turned MRZ is the same locking principle at 0.005 mm and a lower price.
Both lock the same way in SCM440 at HRC 28-32. MR is ground to 0.002 mm maximum axial run-out, MRZ is precision turned to 0.005 mm and costs less. Dimensionally the two match from M40 x 1.5 upward, with M165 x 3 the one exception, but below M40 the MR body runs a few millimeters larger in outside diameter, so the grades are not always interchangeable in a tight bore at the small end of the range.
Each MR size lists a catalog maximum per screw in the dimension table, from 2.2 Nm on the M4 screws of MR6 up to 36 Nm on the M10 screws fitted from MR140. MR carries three set screws at every size, with three hook slots up to MR100 x 2 and six from MR105 x 2 upward. Tighten gradually and alternately rather than one screw fully first.
Add L to the order code for a left-hand thread (MR25x1.5P-L) or U for an inch thread (MR25-U20). Standard right-hand metric versions are ordered by the plain name, for example MR25x1.5P.
Choose the SR locknut where the clamping load is better carried by two heavier screws than three lighter ones, since SR takes M12 at 60 Nm from SR135 where MR stays at M10 and 36 Nm. Choose the MRRA locknut for high radial torque, where a taller body and a larger outside diameter on the same shaft thread are worth the extra envelope. MR remains the type to specify above M200 x 3, as neither SR nor MRRA goes that far.
No. MR 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.