Permissible Shaft Radial Runout Calculator
Calculate maximum allowable shaft runout relative to bearings
Calculation Parameters
Based on API 686 and machinery alignment standards
Calculation Results
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Runout Severity Assessment:
How the Calculator Works
Shaft Runout Definition
Shaft runout is the total indicated reading (TIR) of radial displacement measured while rotating the shaft slowly (typically by hand). It indicates shaft straightness and mounting accuracy.
API 686 Standard
API 686 provides guidelines for permissible shaft runout:
whichever is greater, where D is the shaft diameter in mm.
Speed-Based Limits
For high-speed machinery, more stringent limits apply:
- < 1800 RPM: Standard limits
- 1800-3600 RPM: 75% of standard
- 3600-7200 RPM: 50% of standard
- > 7200 RPM: 25% of standard
Location Factors
Runout limits vary by measurement location:
- At coupling: Most critical – use base limit
- Near bearing: 1.5× base limit
- At midspan: 2× base limit
- At overhang: 0.5× base limit (more stringent)
Causes of Excessive Runout
- Bent shaft from improper handling or storage
- Uneven bearing fit or misaligned bearing seats
- Rotor imbalance causing permanent deformation
- Thermal distortion from uneven heating
- Manufacturing tolerances and machining errors
Measurement Best Practices
- Use dial indicators with resolution of 0.001 mm (0.00005″)
- Support shaft in V-blocks or on centers
- Rotate slowly by hand, not under power
- Take readings at multiple axial locations
- Check for indicator sag on horizontal shafts
- Clean shaft surface before measurement
Correction Methods
- Minor runout: Precision balancing may help
- Moderate runout: Shaft straightening (cold or hot)
- Severe runout: Machining or shaft replacement
- Bearing-related: Check and correct bearing fits
Usage Examples & Value Selection Guide
Example 1: Pump Shaft at Coupling
Scenario: Centrifugal pump shaft check before installation
- Shaft Diameter: 60 mm
- Speed: 2950 RPM
- Machine Type: General machinery
- Bearing Type: Ball bearings
- Location: At coupling hub
- Surface Finish: Good
- Result: Max TIR = 38 μm (0.038 mm)
- Typical reading: 15-25 μm acceptable
Example 2: Precision Spindle
Scenario: Machine tool spindle for CNC machining
- Shaft Diameter: 80 mm
- Speed: 8000 RPM
- Machine Type: Precision equipment
- Bearing Type: Ball bearings
- Location: Near bearing
- Surface Finish: Excellent
- Result: Max TIR = 10 μm (0.010 mm)
- Critical: Must be < 5 μm for precision work
Example 3: Large Compressor Rotor
Scenario: Centrifugal compressor shaft inspection
- Shaft Diameter: 200 mm
- Speed: 5400 RPM
- Machine Type: Process machinery
- Bearing Type: Sleeve bearings
- Location: At midspan
- Surface Finish: Good
- Result: Max TIR = 50 μm (0.050 mm)
- Note: Midspan allows 2× coupling limit
How to Choose Values
Machine Type Selection
- Precision Equipment:
- Machine tool spindles
- Grinding machines
- Optical equipment
- Measurement instruments
- General Machinery:
- Electric motors
- Standard pumps
- Fans and blowers
- Most industrial equipment
- Process Machinery:
- Compressors
- Turbines
- High-speed pumps
- Critical process equipment
- Heavy Machinery:
- Rolling mills
- Crushers
- Large gearboxes
- Mining equipment
Measurement Location Impact
- At Coupling Hub:
- Most critical location
- Directly affects alignment
- Use strictest limits
- Check both faces and rim
- Near Bearing:
- Affects bearing life
- 1.5× coupling limit OK
- Check within 1 diameter of bearing
- At Midspan:
- Natural sag point
- 2× coupling limit acceptable
- Check for bent shaft
- At Overhang:
- Amplifies vibration
- Use 0.5× normal limit
- Critical for impellers
Surface Finish Effects
- Excellent (Ra < 0.8 μm):
- Ground or polished
- No measurement error
- Use for precision work
- Good (Ra 0.8-1.6 μm):
- Fine turned
- Standard for most shafts
- Minimal measurement error
- Average (Ra 1.6-3.2 μm):
- Normal turning
- May affect readings
- Clean before measuring
- Rough (Ra > 3.2 μm):
- Rough turned or milled
- Significant measurement error
- Consider machining
Speed Considerations
- < 1800 RPM: Standard runout limits apply
- 1800-3600 RPM: Reduce limits by 25%
- 3600-7200 RPM: Reduce limits by 50%
- > 7200 RPM: Reduce limits by 75%
- Rule: Higher speed = tighter tolerance
Practical Measurement Tips
- Setup: Use stable V-blocks or centers
- Indicator: 0.001 mm resolution minimum
- Rotation: Turn slowly by hand, mark high spot
- Multiple readings: Check several axial positions
- Temperature: Measure at room temperature
- Documentation: Record readings on shaft sketch
📘 Shaft Runout Calculator (API 686)
Determines permissible radial runout (TIR) per API 686 standard. Critical for high-speed equipment and precision mechanisms.
Rule: TIR ≤ max(50 μm; D/2000) where D is shaft diameter in mm.
💼 Applications
- New Shaft Acceptance: Ø80 mm shaft. Measured: 35 μm. Limit: max(50; 40) = 50 μm. Passes ✓
- Post-Repair Control: Shaft straightened after bending. Runout was 180 μm, after straightening: 45 μm. Ready for installation.
- Vibration Diagnosis: 1× vibration growing. Checked runout: 95 μm. Limit for Ø100: 50 μm. Cause: Bent shaft or worn bearings.
Measurement Methods:
TIR (Total Indicator Reading): Full dial indicator reading. Peak-to-peak runout (2× amplitude).
Measurement: Mount shaft in V-blocks or lathe. Place dial indicator on bearing journal. Rotate slowly, note min and max readings.