Tuesday, February 28, 2017

Bearing Preload

Bearing preload is critical for machine tool spindles since it keeps the spindle rigid, especially under load. It's the amount of compressive force placed on the bearings at rest, and is usually expressed in pounds force or newtons force. For example, my mill spindle has approximately 100 pounds of preload.

High end machine tools use angular contact (AC) bearings which are ground such that, when clamped together back to back, automatically provide the required amount of preload. Unfortunately, the design of our mini lathes and mini mills makes setting the preload extremely difficult since setting it via the spindle nut is very imprecise, and you're also fitting the press fit of the bearing on the spindle.

Now that I've replaced my spindle bearings quite a few times, I've found several different ways to check the bearing preload. In practice I'll usually end up using all of them as double checks.

1. Calculate the torque required for the desired clamping force. I use the this http://www.engineersedge.com/calculators/torque_calc.htm online calculator. The problems with using torque is you need to account for the addition force necessary to move the bearing's inner race on the spindle and the amount of torque needed can change while tightening the nut.

2. Check the breakaway torque, meaning the amount of force required to make the spindle start to turn. This can be fairly easily calculated using a thin feeler gauge, a strong magnet, and an scale. You fix the end of the shim to the spindle using the magnet, measure the diameter of the spindle in inches where it's attached, hook the scale to the shim, wrap the shim around the spindle as far as it'll go, and then pull on the scale and see how much force in pounds is required to start turning the spindle. If you multiply that by the radius you get the breakaway torque in in/lbs. For example, I measure at the lock rings, which have a radius of 0.815". My breakaway force measured at 1.25lbs. Therefore, the breakaway torque was 1.02 in/lbs. For a mini lathe or mini mill using AC bearings, about 1 in/lbs is good, and I'd estimate a range of 0.6-1.5 in/lbs being acceptable. If you're using tapered roller bearings, then a breakaway of a little under 10 in/lbs is good (which also illustrates the much higher friction inherent in tapered roller bearings).

Since grease can effect the breakaway torque I'll turn the spindle in the opposite direction I'm going to measure and then turn it back just slightly and then measure. I've found this technique creates a "dead spot" in the grease and almost completely eliminates its friction. This is especially useful if I'm using sealed bearings.

Shim in place, wrapped around the spindle, and just starting to pull with the scale.

3. Bearing temperature run at top speed won't tell you if the preload is correct, but it can tell you if it's too high. Run the spindle at the top speed you'll be using and measure the temperature on the head directly next to the bearing (I've found it's only a couple degrees cooler there than directly on the bearing's outer race). If the temperature stabilizes below 60 degrees Celsius then it's ok.

Another method which I haven't tried yet, is using the power consumption of the motor to estimate the preload on the bearing. Since power consumption will go up with preload, you can increase the preload until you start to see the power consumption start to rise.

If you're setting preload on tapered roller bearings, then you want to make sure you snug down the spindle nut and then turn the spindle at least 10 times to insure the rollers properly seat in the racers. Otherwise, the preload will drop significantly, if not completely, as the rollers seat.Then you need to measure the axial end play of the spindle. To get the best measurement, shove the spindle in one direction while rotating it, and then shove it in the other direction while rotating it, and be sure to measure at the same spot on the chuck/spindle every time to account for tolerances. You want to keep adding preload until the end play just reaches 0. If you tighten it any more at that point friction and heat will rapidly rise.

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