Q&A: Tightening Torque for Bearing Lock Nuts

Question:

Dear MISUMI Engineering Team:
We have several shafts on one of our machines that need to be removed periodically for maintenance. These shafts are supported with tapered bearings that are locked with a nut. How much torque should we put on the locking nut during reassembly? When I worked on larger truck axels in the past with similar bearings, we would just tighten the nut until it felt right – is there anything wrong with this approach?

Thanks, Mike R.

Answer:

Hi Mike,
These are great questions! With bearing lock nuts, it is common to have a tapered bearing on the end of a shaft or axel that supports the load (think of the wheel on your car). The rollers on the bearing are angled to match a sleeve, which is mounted inside the housing. The rollers need to stay in contact with the sleeve to provide support and also to keep the shaft centered. The bearing locking nut is threaded, either on the outside or inside, and screws into the hub and smashes the bearing on the sleeve. The nut is usually secured in position either by a key or a setscrew.

How much torque should be applied to the locking nut when you are tightening against the bearing?

The simplest answer to this question is to check with the manufacturer, which will typically specify the amount of torque needed either as a torque value or as an angle of rotation. Once you have this value, simply use a torque wrench and apply the correct amount. If an angle is given, hand-tighten the nut and then use a wrench to continue tightening through the angle specified. Sometimes it is necessary to back the nut off slightly to get the locking mechanism lined up (usually when using a key), and you may need a spanner wrench specifically designed to fit your nut.

We always recommend checking and following manufacturer specifications when tightening lock nuts, rather than following your intuition. While you might be able to guess the right amount of torque, over-torquing the lock nut will put stress on the bearing and can cause it to fail prematurely. Under-torquing the lock nut will allow play in the axel and bearing, which would also cause premature failure or possibly catastrophic breakage. If manufacturer-recommended numbers aren’t available, you can measure the size of your existing bearing and lock nut to find a comparable example, but this should only be used as a last resort.

Bearing lock nuts are an important part of many shaft bearing systems, and you should always be careful when tightening lock nuts, as both under- and over-tightening can cause serious problems. We hope your questions are all answered, but feel free to comment below or contact us at engineering@misumiusa.com with any additional questions!