Learning Objectives
After completing this lesson you will be able to:
Field Scenario
You are replacing a worn drive pulley on a high-load sorting conveyor. You slide the new pulley onto the motor shaft — it goes on very loosely, rocking slightly side-to-side. You think, "The key and set screw will lock it tight anyway," so you tap the key in, cinch down the set screw, and boot up the line.
Twelve hours later, the pulley spins completely free, destroying the shaft. When you pull the hub off, the square key has sheared in half and the shaft keyway is completely ripped open and wallowed out.
What happened? The new pulley bore was machined slightly oversized, turning a strict Transition Fit into a loose Clearance Fit. Because the hub could wiggle on the shaft, every startup shock hammer-loaded the key sideways instead of transferring torque across the metal surface. The key didn't fail from torque — it failed because you accepted an out-of-tolerance fit.
Understanding Tolerances
It is impossible to machine a batch of steel components to an absolute, single dimension. Every component is machined to survive within an engineered Tolerance Zone — a controlled envelope bounded by maximum and minimum allowable sizes.
Visual Aid 4.7-01 — Keyway Interface Cross-Section
The Three Primary Fit Classes
Shaft maximum < Bore minimum. There is always a physical air gap — the parts can rotate or slide freely. Used for: sleeve bearings, sliding carriages, pump shaft-in-bronze-bushing assemblies.
Tolerance zones of shaft and bore overlap. May be a light slip or light push depending on where each part falls within its tolerance range. Used for: coupling hubs, belt pulleys, precision locating registers that require routine disassembly.
Shaft minimum > Bore maximum. Zero clearance — parts must be forced together by hydraulic press or thermal scaling. Once assembled, the metal layers compress to create an immense friction lock. Used for: motor rotor cores, bearing inner rings, gear hubs.
Visual Aid 4.7-02 — Tolerance Zone Overlap Diagram
Power Transmission Fit Reference
| Assembly Element | Ideal Fit Class | Key Style | Field Rule |
|---|---|---|---|
| High-Speed Motor Coupling Hub | Transition | Parallel Square Key | Key must slide into keyseat with light thumb pressure only — never file flanks tapered |
| Heavy Industrial Gearbox Output Gear | Interference | Parallel Rectangular Key | Use anti-galling lubricant during hydraulic press assembly to prevent metal tearing |
| Pump Cam / Crankshaft Element | Interference (thermal) | Woodruff Key (half-moon) | Rock Woodruff key perfectly level in the curved pocket before mounting the hub |
| Bronze Sleeve Bearing / Bushing | Clearance | None (fluid film support) | Verify oil film gap meets designed running clearance spec — too tight will seize |
Common Failure Modes
When a hub fits loosely on a shaft, it bangs laterally against the sides of the key every time the motor starts, stops, or reverses. This hammer-loading mushrooms the keyseat walls outward, progressively widening the slot until it is completely destroyed. The key is not the primary cause — the oversized bore is. Always verify fit class before accepting a replacement component.
A catastrophic failure where a mechanical jam or massive torque spike shears the key cleanly along the shaft-to-hub split seam. The key is designed intentionally as a mechanical fuse — it must shear before the far more expensive motor shaft or gearbox teeth are destroyed. A sheared key is diagnostic evidence of an overload event, not just an installation failure.
Attempting to force an oversized key into a keyseat by filing the flanks at an angle creates line-contact point loading instead of full flank surface contact. The tapered key rolls over and strips out under operational loads. Never file key flanks to force a fit. Source the correct key stock dimensions or re-machine the keyseat.
Field Application — Measuring & Qualifying a Shaft-Bore Interface
Interactive Exercise 4.7-01 — Tolerance Fit Calibrator
Enter your micrometer readings below (in inches, to four decimal places). The calculator will compute the allowance, identify the fit class, and flag whether the assembly is within engineered limits for a keyed hub application.
Knowledge Check
You inspect a heavy conveyor drive setup and discover the square parallel key has sheared in half along the shaft-to-hub split line. The shaft's keyseat walls are severely rounded over, wallowed out, and mushroomed wide. What is the fundamental mechanical root cause of this structural destruction?
What to Document
Lesson Summary
| Concept | Key Point |
|---|---|
| Tolerance Zone | Tz = Dmax − Dmin — every machined part lives within a controlled size envelope |
| Clearance Fit | Shaft max < Bore min — air gap always exists; free rotation/sliding |
| Transition Fit | Zones overlap — either light slip or light push; used for removable keyed hubs |
| Interference Fit | Shaft min > Bore max — press or thermal force required; permanent friction lock |
| Allowance Formula | Allowance = Shaft − Bore: positive = interference, negative = clearance |
| Key Drive Surface | Side flanks only — top ceiling face requires clearance gap, never contact |
| Keyseat Wallowing | Caused by loose clearance fit — hub hammers key flanks under load cycles |
| Key Shear | Key is a mechanical fuse — shear indicates overload event, not just poor install |
| Escalation Limit | Shaft worn >0.003" undersize — stop and escalate for machining/metalization |