4.5 Gauges, Tolerances and Fits, and Torque Measurement
Key Takeaways
- Tolerance is the total permissible variation in a dimension, equal to the difference between the upper and lower limits.
- A clearance fit always leaves space between shaft and housing, an interference fit always requires force or heat, and a transition fit may be either.
- Bearings are normally mounted with an interference fit on the rotating ring and a clearance fit on the stationary ring.
- Feeler gauges measure clearance and can be stacked, but the stack should use the fewest possible leaves to limit cumulative error.
- When an extension is added to a torque wrench along its axis, the wrench must be set below the target torque, because the effective lever arm has increased.
Gauges that transfer or compare rather than read
Not every precision instrument has a scale. Several of the most useful ones transfer a dimension to a micrometer or compare against a known standard.
| Gauge | What it does | Typical maintenance use |
|---|---|---|
| Feeler (thickness) gauge | Direct-reading leaves of known thickness | Coupling gap, valve clearance, soft foot, bearing internal clearance |
| Telescoping gauge | Expands to a bore, then is locked and measured with a micrometer | Housing bores, bushing bores |
| Small-hole (ball) gauge | Expands a split ball into a small bore | Small bores below telescoping gauge range |
| Taper gauge | Wedge marked with width | Gap width in a slot or crack |
| Thread pitch gauge | Leaves matching standard pitches | Identifying an unknown fastener or pipe thread |
| Radius gauge | Fillet and corner radii | Verifying a shaft fillet will clear a bearing corner |
| Plug and ring gauges | Go / no-go comparison | Production inspection of bores and shafts |
| Straightedge | Reference plane | Checking shaft and sheave alignment, flatness of a mounting surface |
Two disciplines govern the transfer gauges: do not force them, because forcing springs the gauge and reads oversize, and measure the gauge immediately with a micrometer using normal ratchet pressure.
When stacking feeler leaves, use the fewest leaves that make the required thickness. Every additional leaf adds its own thickness tolerance and a little surface film, so a four-leaf stack is less trustworthy than a single leaf of the same nominal size.
Tolerance, allowance, and fits
| Term | Definition |
|---|---|
| Nominal size | The designation used to identify the part, such as a 2-inch shaft |
| Basic size | The theoretical exact size from which limits are derived |
| Limits | The maximum and minimum permissible sizes |
| Tolerance | The total permissible variation: upper limit minus lower limit |
| Allowance | The intentional difference between mating parts at their tightest condition |
| Bilateral tolerance | Variation permitted in both directions, such as 2.000 ± 0.002 |
| Unilateral tolerance | Variation permitted in one direction only, such as 2.000 +0.000 / −0.003 |
A shaft specified as 2.0000 / 1.9985 inch has a tolerance of 0.0015 inch. If the mating bore is 1.9995 / 1.9990, the parts will always interfere at some combinations and clear at others — a transition fit.
The three fit classes:
- Clearance fit — the shaft is always smaller than the hole. Assembly is by hand. Used for sliding parts and for the stationary ring of a bearing.
- Transition fit — depending on where each part falls within its tolerance, the fit may be slightly loose or slightly tight. Used for locating parts that must be accurate but removable.
- Interference (press) fit — the shaft is always larger than the hole. Assembly requires force, cooling the shaft, or heating the housing. Used for the rotating ring of a bearing and for coupling hubs.
The governing rule for bearings, which returns in Chapter 7: the ring that rotates relative to the load gets the interference fit; the ring that stands still gets the clearance fit. On a normal pump or motor, that means an interference fit on the shaft and a clearance fit in the housing.
Torque measurement
Torque is force applied at a distance:
Tightening a fastener to a specified torque is a proxy for producing a specified clamping force. That proxy is imperfect: roughly 85 to 90 percent of applied torque is consumed by friction in the threads and under the head, so lubrication state matters enormously. A dry torque specification applied to a lubricated fastener over-tightens it, often past yield.
| Wrench type | Characteristics |
|---|---|
| Beam | Simple, inherently stable calibration, must be read while pulling, no overtorque protection |
| Click | Signals the setting with an audible click; store at the lowest setting |
| Dial | Continuous reading, good for measuring breakaway torque on an existing fastener |
| Digital / electronic | Readout with peak-hold and angle capability |
| Torque multiplier | Gear reduction for large fasteners; output equals input times the stated ratio |
Rules that produce assessment items:
- Never use a torque wrench as a breaker bar and never add a cheater pipe to the handle.
- Pull smoothly to the setting and stop. Re-clicking a fastener adds torque.
- Follow the specified pattern — star or crisscross on flanges and bearing housings — in graduated passes, typically 30 percent, 60 percent, and 100 percent of final torque.
- If a crowfoot or extension is added along the axis of the handle, the effective lever arm grows, so the wrench must be set lower than the target torque:
where L is the wrench length from the drive to the handle grip and E is the extension length. Mounting the crowfoot at 90 degrees to the handle does not change the effective lever arm, so no correction is needed.
Worked example. Target torque is 100 ft-lb. The wrench is 18 inches and a 3-inch crowfoot is added in line. Set the wrench to 100 times 18, divided by 21, which is 86 ft-lb.
A shaft has limits of 3.0000 and 2.9982 inches. What is its tolerance?
A bearing inner ring rotates with the shaft while the load direction stays fixed. What fit should the inner ring have on the shaft?
A specification calls for 120 ft-lb. The mechanic uses a 20-inch torque wrench with a 4-inch crowfoot mounted in line with the handle. What should the wrench be set to?