1. Explanation of terms&Accuracy requirements (reference)

1, term explanation:

Radial runout: Radial runout refers to the runout in the diameter direction of the equipment shaft.

Axial runout: Axial runout is the runout in the direction of the shaft centerline.

2, accuracy requirements:

Ordinary machine tool spindle: radial runout ≤ 5–10 μm; axial play ≤ 3–8 μm.

Precision/CNC spindle: radial runout ≤ 1–3 μm; axial play ≤ 1–2 μm.

High-speed motor/bearing: radial runout ≤ 1–5 μm; axial runout ≤ 1–3 μm.

2. Differences in causes

(1) The spindle bearing is poorly preloaded, causing axial movement or radial runout.

(2) The bearing is seriously damaged, causing uneven rotation speed.

(3) The bearing allocation is unreasonable.

3. Common testing tools

Contact type: dial indicator/dial indicator (general purpose, low cost), lever meter (small space), LVDT displacement sensor (high precision).

Non-contact: eddy current sensor, capacitive sensor, laser interferometer (μm level, high-speed dynamic).

Tooling: standard mandrel/test rod, center base/V-block, magnetic meter base, axial loading tooling.

4. Radial runout detection (general steps)

1. Static/geometric accuracy detection (machine tool spindle/axis)

Clamping: Install the standard mandrel into the spindle taper hole/between the two centers to ensure there is no gap and coaxiality.

Installing the meter: The magnetic meter base is fixed on the machine bed/fixed parts, and the meter head is vertically pressed on the cylindrical surface of the mandrel (preloaded 0.1–0.3mm).

Measurement: Rotate the spindle slowly and uniformly for one revolution, record the maximum and minimum readings of the dial indicator, and the difference is the radial runout of the section.

Multiple cross-sections: Measure 1–3 cross-sections at the proximal, middle, and far ends of the mandrel, and take the maximum value as the final radial runout value.

2. Dynamic detection (high speed/under load)

The eddy current/capacitive sensor is used to collect data under no load or rated speed, and the comprehensive radial runout is obtained through software fitting.

5. Axial movement detection (general steps)

1. Static/geometric accuracy detection

Clamping: same radial runout, standard mandrel installed.

Install the meter: The meter head is pressed against the end face of the mandrel/shoulder end face (vertically) along the axis direction.

Loading and measuring:

Apply the specified axial force manually/with tooling (to eliminate backlash).

Rotate for one revolution and record the difference between the maximum and minimum readings, which is the amount of axial movement.

You can also push gently in both directions in the axial direction to measure the total movement (clearance + end face runout).

2. Dynamic detection

The axial displacement sensor collects real-time data and analyzes periodic axial fluctuations (same frequency as the rotational speed).

6. According to the listed fault points, you can refer to the following steps to solve the fault.

Step 1: After letting the CNC mold machining center run dry for a period of time, touch all the bearings on the spindle with your hands and find that the temperature rise is not obvious and the noise during the processing is small. This eliminates the fault caused by serious damage to the bearing itself.

Step 2: Since the preload force of the bearings is adjusted by the manufacturer before leaving the factory, directly replace a pair of new bearings. After installation, it is found that the processed workpiece still has serious drag marks, then the fault can be ruled out as being caused by poor preload of the bearings.

Step 3: After replacing the rear bearing with a smaller precision, re-run the machine tool and try processing the workpiece. If it is found that the drag tool marks have been significantly improved, it can be concluded that the fault is caused by unreasonable assembly of the bearing. Through calculation, the fault is eliminated after replacing the rear bearing with suitable accuracy.

The spindle bearing is an important part of the spindle components. The quality of the bearing assembly directly affects the radial runout and axial movement accuracy of the rotating shaft. Therefore, in order to ensure the radial runout and axial movement of the spindle, in addition to requiring a certain accuracy of the spindle and bearings, correct assembly methods must also be adopted.

(Source: Hunan University