Ball Screw Locknut

Tianjin OuNaiDa Transmissions Technology Co., Ltd. is one of the leading manufacturers and suppliers of ball screw locknut in China. If you're going to buy high quality ball screw locknut, welcome to get more information from our factory. Also, custom service is available.

There is a failure mode that looks exactly like a broken screw but is actually a two-dollar part fitted wrong. The shaft snaps at the journal, just behind the bearing seat. The buyer assumes material defect and asks for warranty. Nine times out of ten the locknut face was not square to the thread axis, and the shaft end had been bending a fraction of a millimetre on every single revolution until it gave up.

Worth knowing before you tighten anything.

What the locknut is actually holding

At the fixed end of a ball screw, the angular contact bearings need axial clamping. The shaft journal is turned down, threaded, and a locknut pulls the bearing inner races against a shoulder. That clamp sets the bearing preload, and the bearing preload is what gives the axis its axial rigidity.

Get it right and the bearing stack behaves as one solid unit. Get it wrong in either direction and you have problems: too loose and the whole axis has axial play no amount of nut preload can fix, too tight and the bearings run hot and die early.

RN series dimensions

Source: OND precision transmission components catalogue, Locknut RN Series, P45.

Note the thread pitches. M10×1.0 where a standard coarse M10 would be 1.5, M15×1.0 against a coarse pitch of 2.0. Fine pitch is not a stocking accident - it gives finer control of clamp force per degree of rotation, and it resists loosening under vibration better than coarse. If a distributor offers you a coarse-thread nut as an equivalent, it is not one.

The -N variants carry the same dimensions with a different surface treatment. Specify plainly if your application is washdown or humid.

Matching the nut to the shaft end

The locknut thread has to match the journal thread that was cut on the screw. For our BK-series fixed-end machining, the journal threads run as follows:

Source: OND catalogue, Fixed Side BK Machining Size, P46.

Read across: a 20 mm screw running in a BK15 support gets an M15×1 journal, which takes an RN15. A 20 mm screw in a BK17 support gets M17×1 instead - different nut. The support unit choice determines the journal thread, and the journal thread determines the locknut. Same screw diameter, two different answers.

This is the single most common ordering error on shaft-end hardware. People order by screw diameter.

The 0.01 mm number

Here is the figure that explains the snapped journals.

The bearing seating face on the shaft and the thread axis of the locknut must be perpendicular to each other, and the opposite faces of the locknut must be parallel to each other. If they are not, tightening the nut bends the shaft end, and repeated bending breaks it. HIWIN specifies the limit directly: the deflection measured at the end of the screw shaft, before versus after the locknut is tightened, must not exceed 0.01 mm (HIWIN Ballscrews Technical Information, Appendix A2-3).

That is a straightforward check with a dial indicator and thirty seconds of your time. Indicate the shaft end, tighten the nut to working torque, watch the needle. Over a hundredth, something is not square - pull it apart and find out what before you run the machine.

Also from the same source: sharp corners on the shaft end should be avoided, because they concentrate stress. A proper journal has a radiused or chamfered transition at the shoulder, finished to Ra0.4. If a supplier hands you a shaft with a sharp internal corner at the bearing shoulder, that is a fatigue crack waiting to start.

Fitting it without causing the problem you are trying to avoid

Clean the thread and the seating faces. Any chip trapped under the nut face tilts it, and a tilted nut is the whole failure mode.

Tighten progressively, and check the shaft-end indicator reading as you go rather than only at the end.

Then lock the set screws. Two hexagon socket set screws with brass set pieces bear on the journal thread and stop the nut backing off. The brass pieces exist so the set screws do not bite into and damage the thread - if they are missing from a cheap kit, do not substitute plain steel screws directly onto the thread.

Do not use a hammer and punch on the nut slots. It works, it is fast, and it deforms the nut face enough to reintroduce the squareness error.

Ordering

Tell us the screw model and which support unit you are using at the fixed end - those two facts determine the journal thread and therefore the nut. If you are machining shaft ends yourself, ask for the BK machining drawing for your size and follow the journal diameter and effective thread length exactly. We supply locknuts loose or pre-fitted with the support unit as a matched set, which removes the matching problem entirely.

Related reading

• How To Select Ball Screw Support Units
• Ball Screw Support Unit BK12 FK15 EK10
• Ball Screw SFU1605 SFU2005 C7 Flange Nut
• Maintenance And Care Of Bearing Seats

Tianjin OuNaiDa machines ball screw shaft ends to drawing and supplies matched locknuts and support units. Send your screw model and fixed-end arrangement and we will confirm the journal thread and nut size before anything is cut.

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