How to Measure Shaft and Select Bore and Key
Accurate shaft measurement techniques and coupling bore selection guidance.
Accurate shaft measurement is essential for proper gear coupling fit. This guide covers measurement techniques and bore/key selection to ensure reliable coupling installation.
Why Accuracy Matters
Incorrect measurements lead to:
- Poor coupling fit (too loose or tight)
- Fretting damage
- Key failure
- Hub cracking
- Vibration problems
Shaft Measurement Techniques
Required Tools
- Micrometer (accuracy 0.01mm)
- Vernier caliper (for reference)
- Depth micrometer (for keyway)
- Dial indicator (for runout)
- Steel rule
Step 1: Clean the Shaft
- Remove all rust, paint, burrs
- Clean with solvent if oily
- Inspect for damage
- Mark measurement locations
Step 2: Measure Diameter
At multiple points:
- At coupling mounting position
- At inboard and outboard of mount zone
- In two planes 90° apart
Record readings:
| Location | Plane 1 | Plane 2 |
|---|---|---|
| Inboard | ||
| Center | ||
| Outboard |
Calculate:
- Average diameter
- Out-of-round (max - min)
- Taper (end to end difference)
Step 3: Measure Keyway
Dimensions needed:
- Width at bottom and top
- Depth from shaft surface
- Length
- Corner radii
Standard Check:
| Dimension | Tolerance |
|---|---|
| Width | +0.025 to +0.050mm |
| Depth | As per standard |
| Corners | Clean, no burrs |
Step 4: Check Runout
Mount dial indicator and rotate shaft:
- Record TIR (Total Indicator Reading)
- Acceptable: <0.05mm for precision
- Marginal: 0.05-0.10mm
- Problem: >0.10mm
Selecting Bore Size
Standard Fits
| Fit Type | Application | Tolerance |
|---|---|---|
| Clearance (H7/g6) | Easy assembly, light duty | Gap exists |
| Transition (H7/k6) | Moderate duty, removable | Light interference |
| Interference (H7/p6) | Heavy duty, permanent | Press/heat fit |
Recommended for Gear Couplings
| Application | Recommended Fit |
|---|---|
| General industrial | H7/k6 (transition) |
| Heavy duty | H7/p6 (interference) |
| Frequent removal | H7/h6 (close running) |
Interference Calculation
For interference fit, calculate expansion:
Heating temperature:
ΔT = (Interference + 0.05) / (Bore × 11.7 × 10⁻⁶)
Where:
- Interference in mm
- Bore in mm
- Result in °C
Selecting Key Size
Standard Key Selection
Key size based on shaft diameter:
| Shaft Dia (mm) | Key Width × Height |
|---|---|
| 22-30 | 8 × 7 |
| 30-38 | 10 × 8 |
| 38-44 | 12 × 8 |
| 44-50 | 14 × 9 |
| 50-58 | 16 × 10 |
| 58-65 | 18 × 11 |
| 65-75 | 20 × 12 |
| 75-85 | 22 × 14 |
Key Fit Requirements
In shaft keyway:
- Side fit: Close running or light interference
- Bottom fit: Clearance
In hub keyway:
- Side fit: Transition or interference
- Top fit: Clearance
Key Length
Minimum length:
L_min = T / (σ × b × d)
Where:
- T = Torque
- σ = Allowable stress
- b = Key width
- d = Shaft diameter
Rule of thumb: Key length ≈ 1.0 to 1.5 × shaft diameter
Common Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Single measurement | Miss taper or out-of-round | Multiple locations |
| Dirty shaft | Incorrect reading | Clean thoroughly |
| Wrong key size | Failure, damage | Use standard tables |
| Loose key | Shear, fretting | Proper fit |
| Tight bottom fit | Key lifts hub | Bottom clearance |
Documentation
Record all measurements:
- Date and location
- Equipment identification
- All readings with positions
- Calculated values
- Selected bore and key sizes
- Any noted issues
Contact SMI for bore and key specifications with your coupling order.
Further Reading
What to send us to get a bore quoted
Measuring is the hard part; ordering should not be. Send these six items and we will machine the bore to your shaft rather than shipping a standard bore for you to have opened up locally:
- Measured shaft diameter — both shafts, at the point the hub will sit.
- Keyway width and depth, and whether it is a single or double keyway.
- Whether the shaft is worn, and by how much if you know.
- The fit you want — interference, transition, or clearance with a grub screw.
- Whether the coupling will be heated on during fitting.
- Any taper, and its ratio and end dimensions, with a sketch.
A photograph of the shaft end with a rule or vernier in shot is worth more than a paragraph of description. If your specification requires the bore and keyway to conform to a particular standard, say which one — our technical library sets out what AGMA 9002 actually specifies for coupling bores and fits, which is the reference most commonly cited on this point.
We machine to what is on your machine. That is the whole advantage of ordering from a manufacturer rather than a catalogue.
Frequently asked questions
Should I give the nominal shaft size or the measured one?
The measured one, always. Shafts get reworked and worn, and keyways widen under a loose key. A shaft nominally 50 mm may measure 49.6 mm after years of service. We machine to what is actually on your machine, so a measured diameter prevents a coupling that will not grip.
How do I measure a keyway correctly?
Measure the width across the slot and the depth from the top of the shaft to the bottom of the keyway, and state whether it is a single or double keyway. Parallel keys to IS 2048 are standard on most Indian equipment — if yours is a different standard or a non-standard key, say so with the enquiry.
What bore fit should I ask for?
It depends on how the coupling is fitted and what it drives. Interference fits suit high-torque and reversing duties and are usually heated on; transition fits with a key suit general drives; clearance fits with a grub screw suit light duty and frequent removal. Tell us the duty and we will recommend the fit.
Super Mech Industries — Odhav, Ahmedabad, Gujarat 382415, India. Phone: +91 63510 70577. Email: smigvs@gmail.com. ISO 9001:2015 certified.