How SR Locks at 90° to the Shaft
SR 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. 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.
Two Set Screws Per Nut
M4 to M12. 2.2 to 60 Nm max
Four Hook Slots
More places for the wrench to seat
Ground Contact Surface and Thread
Maximum axial run-out 0.002 mm
No Thrust Washer or Keyway
Fewer parts, simpler assembly
Where SR Locknuts Are Used
Large Threads Above M130 x 2, Where the M12 Screws at 60 Nm Carry More Clamping Load Than MR's M10
Bearing and Gear Retention in Compact Gearboxes and Reducers
Installations Where the Housing Leaves the Wrench an Awkward Approach, and a Fourth Slot Decides Whether the Nut Can Be Turned at All
Impeller and Rotor Retention on Pumps, Fans and Blowers
Shafts Already at Their Limit for Section, Where a Keyway Would Take Out Strength the Design Needs
Find Your Size(M6 to M200)
Filter SR locknuts by shaft diameter, height, or pitch.
Products in this Radial Locking Series
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.
Tell Us What You Need. We'll Find the Locknut.
Give us the locknut code you use now and we'll find the NIKKI equivalent.
Describe the job, thread size, space, load, and we'll point you to the right NIKKI series.
Questions About the SR Locknut
The SR is the two-screw ground locknut in NIKKI's Radial Locking Series, in metric threads from M6 x 0.5 to M200 x 3 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 SR 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 SRZ is the same nut at 0.005 mm and a lower price.
SR carries two set screws and four hook slots at every size. MR carries three set screws at 120°, with three hook slots below M105 and six above. Both are ground to 0.002 mm in SCM440 at HRC 28-32.
The screw sizes cross over along the range: through the middle SR takes the lighter screw, M6 at 8 Nm on SR55 to SR65, where MR fits M8 at 19 Nm. From SR135 SR fits M12 at 60 Nm, while MR is still on M8 at that size and does not reach M10 until MR140. The envelopes are not interchangeable either, and MR continues to M260 x 4 while SR stops at M200 x 3.
Each SR size lists a catalog maximum per screw in the dimension table, from 2.2 Nm on the M4 screws of SR6 up to 60 Nm on the M12 screws fitted from SR135. Every size carries two screws. Tighten the pair gradually and alternately rather than one screw fully first.
Add L to the order code for a left-hand thread (SR30x1.5P-L) or U for an inch thread (SR30-U18). Standard right-hand metric versions are ordered by the plain name, for example SR30x1.5P.
SRZ is the precision-turned version of SR, in the same envelope, with the same two screws and four slots, at 0.005 mm maximum axial run-out instead of 0.002 mm and at lower cost. Specify the SRZ locknut where the drawing tolerance allows 0.005 mm, and SR where it does not.
No. SR 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.