5.4 Systems of Limits, Fits, Clearances & Shaft Alignment

5.4 Systems of Limits, Fits, Clearances & Shaft Alignment

In aviation mechanical systems, operational safety and component longevity depend on maintaining microscopic dimensional relationships between mating components. Aircraft gearboxes, turbine rotor shafts, landing gear trunnions, and flight control bellcranks operate under extreme cyclic loads and severe thermal gradients. A licensed maintenance engineer must master the ISO system of limits and fits, understand the metallurgical mechanics of shrink and interference fits, accurately measure shaft alignment and radial runout, and interpret Aircraft Maintenance Manual (AMM) wear limits.

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The ISO System of Limits and Fits (ISO 286)

To ensure global interchangeability of precision mechanical components, the International Organization for Standardization (ISO) developed ISO 286, establishing a standardized code of letters and numbers to define limits, allowances, and tolerances.

                    BASIC ISO 286 FIT CALLOUT: 40 H7 / g6

             [ 40 ]             [ H7 ]                   [ g6 ]
               |                  |                        |
          BASIC SIZE          HOLE FIT                 SHAFT FIT
           (40 mm)       (Upper-case Letter:      (Lower-case Letter:
                          Hole Basis System,       Clearance Fit Shaft,
                           IT Grade 7)              IT Grade 6)

Essential Metrological Definitions

• Nominal Size: The commercial or general descriptive size of a feature (e.g., a "40 mm bearing journal").
• Basic Size: The exact theoretical dimension from which all limits of size are calculated via deviations. The basic size is identical for both the hole and the shaft.
• Upper Deviation (ES for holes, es for shafts): The algebraic difference between the maximum limit of size and the basic size (ES = Upper Limit - Basic Size).
• Lower Deviation (EI for holes, ei for shafts): The algebraic difference between the minimum limit of size and the basic size (EI = Lower Limit - Basic Size).
• Fundamental Deviation: The deviation (upper or lower) that lies closest to the zero line (basic size). It is designated by a letter code (A through ZC for internal features/holes; a through zc for external features/shafts).
• International Tolerance Grade (IT Grade): A standardized group of manufacturing tolerances designated by numbers (IT01, IT0, IT1 through IT18). Lower IT numbers denote ultra-precision machining (e.g., IT5-IT7 for aerospace journals), while higher numbers represent general fabrication (e.g., IT11-IT14 for stamping and casting).

                      HOLE BASIS SYSTEM (H-HOLE)

             +-------------------------------------------+ (Upper Limit: Basic + ES)
             |               HOLE TOLERANCE              |
    ZERO LINE============================================= (Lower Limit = Basic Size)
    (Basic Size)                                           EI = 0.000 mm

           Clearance Fit Shaft        Transition Shaft      Interference Shaft
              (e.g., g6)                 (e.g., k6)            (e.g., p6)

    ZERO LINE========================================================================
                 - es                   + es                    + es
             +-----------+          +-----------+          +-----------+
             |   SHAFT   |          |   SHAFT   |          |   SHAFT   |
             | TOLERANCE |          | TOLERANCE |          | TOLERANCE |
             +-----------+          +-----------+          +-----------+
                 - ei                   - ei                    + ei

Hole Basis System vs. Shaft Basis System

1. The Hole Basis System (Hole Standard):
   • Designated by the fundamental deviation letter H (upper-case).
   • In the H-hole system, the lower deviation is exactly zero (EI = 0).
   • The minimum permissible hole diameter equals the basic size.
   • Why Preferred in Aerospace: Standard manufacturing tooling (such as reamers, broaches, core drills, and plug gauges) produces holes to fixed standard sizes. It is far more economical to machine standard H-holes and produce the desired clearance, transition, or interference fit by precision grinding the external shaft diameter.
2. The Shaft Basis System (Shaft Standard):
   • Designated by the fundamental deviation letter h (lower-case).
   • In the h-shaft system, the upper deviation is exactly zero (es = 0).
   • The maximum permissible shaft diameter equals the basic size.
   • Application: Used where a single continuous length of ground steel shafting or torque tube passes through multiple structural brackets requiring different classes of fits along its length.

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Three Classes of Mechanical Fits

A fit is the relationship resulting from the dimensional difference between two mating parts before assembly.

    CLEARANCE FIT                 TRANSITION FIT               INTERFERENCE FIT
 (Positive Allowance)         (Overlapping Zones)            (Negative Allowance)

    +-------------+              +-------------+                +-------------+
    |    HOLE     |              |    HOLE     |                |    HOLE     |
    +-------------+              +------+------+                +------+------+
          |                             |                              |
    [ Clearance ]                 [ Play or ]                   [ Overlap / ]
          |                       [ Interf. ]                   [ Press-Fit ]
          v                             v                              v
    +-------------+              +------+------+                +------+------+
    |    SHAFT    |              |    SHAFT    |                |    SHAFT    |
    +-------------+              +-------------+                +-------------+

1. Clearance Fit (Positive Allowance)

• Condition: The minimum hole size is always larger than the maximum shaft size across the entire tolerance band. The components assemble freely by hand.
• Allowance: Minimum Clearance = Minimum Hole (MMC) - Maximum Shaft (MMC) > 0.
• Aerospace Applications: Flight control pulley bushings, bellcrank pivot pins, flap track rollers, hydraulic spool valves.
• Standard ISO Examples:
  • H7/g6: Precision running fit for sliding gears and hydraulic spools.
  • H8/f7: Normal running fit for bronze sleeve bearings lubricated by grease or oil.
  • H11/c11: Loose clearance fit for exposed linkages subject to dirt and thermal extremes.

2. Transition Fit

• Condition: The tolerance zones of the hole and shaft overlap. Depending on the actual manufactured dimensions of the mating parts, the assembly may yield either a slight clearance or a slight interference.
• Allowance: Spans from a small positive clearance to a small negative interference.
• Aerospace Applications: Precision locating dowel pins, inner raceways of ball bearings on shafts, removable gearbox splines.
• Standard ISO Examples:
  • H7/js6 or H7/k6: True transition fit assembled with light mallet tapping; ensures concentric location with zero radial play without requiring hydraulic press force.

3. Interference / Shrink Fit (Negative Allowance)

• Condition: The maximum hole size is always smaller than the minimum shaft size across the entire tolerance band. The shaft is physically larger than the hole prior to assembly.
• Allowance: Negative Allowance = Smallest Shaft (LMC) - Largest Hole (LMC) > 0 (as interference).
• Aerospace Applications: Landing gear trunnion bushings, turbine rotor sleeve collars, press-fit gear hubs.
• Standard ISO Examples:
  • H7/p6: Light press fit requiring arbor press assembly.
  • H7/s6: Heavy interference/shrink fit requiring thermal assembly techniques.

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Reference Table: ISO Fit Classifications

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Assembly Techniques for Interference Fits

Attempting to force an interference-fit bushing into an aircraft structural housing with raw mechanical force will gall, score, and seize the mating metal surfaces, ruining an expensive component. Technicians employ three primary assembly techniques:

1. Mechanical Pressing: Utilizes a precision hydraulic or manual arbor press equipped with an alignment pilot drift that applies uniform axial force square to the housing bore. High-pressure anti-seize assembly lubricant (such as molybdenum disulfide grease) is applied to prevent micro-galling.
2. Thermal Expansion (Heating): The female housing is heated in a temperature-controlled oven, induction heater, or hot oil bath (typically 120°C to 180°C / 250°F to 350°F). Thermal expansion temporarily expands the hole bore. Strict Warning: Direct heating with an open oxy-acetylene torch is strictly prohibited; localized flame heating degrades the metallurgical temper and heat-treatment properties of aerospace alloys.
3. Cryogenic Contraction (Chilling): The male component (bushing or pin) is submerged in dry ice (solid CO2, -78.5°C / -109°F) or liquid nitrogen (-196°C / -320°F). The severe thermal contraction shrinks the outside diameter of the steel bushing, allowing it to drop freely into the housing bore by hand. Once ambient room temperature is restored, the bushing expands to create an immovable interference joint.

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Metrological Methods for Checking Fits & Clearances

Verifying dimensional clearances during overhaul requires precision metrological tools matched to component geometry:

                 PLASTIGAUGE CRUSH MEASUREMENT PRINCIPLE

   1. Place Plastigauge strip       2. Torque bearing cap to     3. Remove cap & measure
      across clean journal             AMM spec (NO ROTATION!)      flattened strip width

        +-----------------+              +-----------------+          +-----------------+
        |   Bearing Cap   |              |   Bearing Cap   |          |  Calibrated Card|
        +--------v--------+              +--------v--------+          |  [--] 0.05 mm   |
         ....[Thread]....                 ====[Crushed]====           |  [---] 0.038 mm |
        +-----------------+              +-----------------+          |  [----] 0.025 mm|
        |  Shaft Journal  |              |  Shaft Journal  |          +-----------------+

1. Plastigauge for Plain Bearing Clearances

• Principle: A calibrated, precision-extruded plastic thread of known cross-section that deforms plastically when crushed between mating surfaces.
• Application: Measuring radial oil clearances in engine crankshaft journal bearings, connecting rod big-ends, and accessory drive sleeve bearings.
• Measurement Protocol:
  1. Thoroughly degrease the journal and bearing shell surfaces.
  2. Cut a strip of Plastigauge matching the bearing width and lay it axially across the crown of the journal.
  3. Install the bearing cap and torque all retaining bolts to the exact specification specified in the AMM.
  4. CRITICAL MANDATORY RULE: DO NOT ROTATE THE SHAFT. Any rotation will smear the plastic filament across the journal, destroying the test specimen and yielding false data.
  5. Remove the bearing cap. The plastic filament will be flattened into a wider strip adhered to either the journal or bearing shell.
  6. Match the width of the flattened strip against the calibrated graduation scale printed on the Plastigauge envelope to read radial clearance directly in thousandths of an inch or millimetres.

2. Feeler Gauges (Thickness Gauges)

• Spring-tempered steel blades ranging from 0.03 mm to 1.00 mm (0.0015 to 0.040 in).
• Technique: Insert the gauge squarely into the gap (e.g., piston ring end gap, flight control hinge gap). The correct blade thickness provides a light, sliding friction drag (a distinctive "feel"). A blade that falls through without resistance is too thin; a blade that must be forced or buckled is too thick.

3. Telescoping Gauges & Bore Micrometers

• Used to measure internal cylinder bores to determine ovality (out-of-round) and taper.
• Measurements are taken at the top, middle, and bottom of the cylinder bore in two perpendicular planes (X and Y axes). The difference between X and Y indicates ovality; the difference between top and bottom indicates taper.

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Shaft Alignment, Runout & Geometric Tolerances

Rotating drive shafts (such as auxiliary power unit drive shafts, starter-generator shafts, and flap torque tubes) must remain strictly aligned and free of structural bow or twist.

                       DIAL TEST INDICATOR (DTI) RUNOUT SETUP

                                  +-------+
                                  | (DTI) | Dial Indicator (0.01 mm)
                                  +---┬---+
                                      | Plunger Contact Tip
                                      v
                 ===================( O )===================
                 [V-Block]         SHAFT         [V-Block]
                 +-------+                       +-------+
                 |  | |  |                       |  | |  |
                 |   V   |                       |   V   |
                 +-------+                       +-------+
             =================================================
                        GRANITE SURFACE TABLE

Measuring Radial Runout with a Dial Test Indicator (DTI)

1. Setup: Clean the shaft and support its bearing journals on a matched pair of precision hardened steel V-blocks placed upon a calibrated granite surface plate.
2. Indicator Placement: Mount a Dial Test Indicator (DTI) on a rigid magnetic base. Position the indicator stylus perpendicular to the shaft surface at the mid-span location (or where maximum bow is anticipated).
3. Pre-loading: Depress the DTI stylus against the shaft to pre-load the gauge mechanism by approximately half a revolution (0.5 mm). Lock the bezel with the pointer aligned to zero.
4. Rotation: Slowly rotate the shaft by hand through 360 degrees in a smooth, continuous motion while observing pointer movement.
5. Total Indicator Reading (TIR) / Full Indicator Movement (FIM): Record the maximum positive needle deflection and the maximum negative needle deflection. The Total Indicator Reading is the absolute algebraic sum of the total sweep:

TIR = (+Max Deflection) - (-Min Deflection)

Example: (+0.04 mm) - (-0.02 mm) = 0.06 mm TIR

Calculating True Shaft Eccentricity (Shaft Bow)

Because the dial indicator rides on the outer surface of an eccentric rotating cylinder, the Total Indicator Reading reflects twice the true centerline bow:

True Shaft Eccentricity (Bow) = TIR / 2

True Bow = 0.06 mm / 2 = 0.03 mm

Measuring Axial Float (End Play)

To measure shaft end play (axial clearance):

1. Mount the DTI with its contact plunger aligned parallel to the shaft axis, touching a finished shoulder or shaft end.
2. Pry or push the shaft gently against its internal thrust bearing stop in one axial direction. Zero the dial.
3. Push the shaft in the opposite axial direction to its opposite mechanical stop.
4. Read the total needle displacement on the dial face, which represents total axial end play.

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Aircraft Maintenance Manual (AMM) Limits of Wear

Technical overhaul manuals (OHM) and maintenance manuals (AMM) provide a standardized Table of Limits that categorizes dimensions into three distinct legal thresholds:

+-----------------------------------------------------------------------------+
|                        AMM TABLE OF LIMITS SPECTRUM                         |
+-----------------------------------------------------------------------------+
| [ Manufacturing New Limits ]       -> Dimensions as manufactured at factory |
|                                       (e.g., 25.000 mm to 25.015 mm)        |
+-----------------------------------------------------------------------------+
| [ Permissible In-Service Limits ]  -> Allowable clearance during operation  |
|                                       (e.g., up to 25.045 mm)               |
+-----------------------------------------------------------------------------+
| [ Reject / Condemning Limits ]     -> MANDATORY PART RETIREMENT OR OVERHAUL |
|                                       (e.g., > 25.050 mm: SCRAP PART)       |
+-----------------------------------------------------------------------------+

1. Manufacturing Limits (Fit When New): The tight dimensional tolerance band required of newly manufactured components leaving the production line.
2. Permissible In-Service Limits: The allowable expanded clearance permitted during scheduled maintenance checks (such as A-checks or C-checks). Components within these limits are safe to remain in service until the next maintenance interval.
3. Reject / Condemning Limits: The absolute maximum clearance or wear threshold permitted. If a measured dimension exceeds this limit, the component is legally unairworthy and must be immediately replaced, bushed, or machined to an approved oversize dimension per structural repair procedures.

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Realistic Maintenance Scenario: Overhaul Inspection of a Starter-Generator Drive Shaft

During a scheduled C-check on a regional turboprop, an engineer inspects the starter-generator drive quill shaft:

1. Shaft Runout Check: The shaft is placed on V-blocks on a granite table. A DTI positioned at mid-span shows a minimum reading of -0.015 mm and a maximum reading of +0.025 mm. The Total Indicator Reading (TIR) is 0.025 - (-0.015) = 0.040 mm. True shaft bow is 0.040 / 2 = 0.020 mm. The AMM specifies maximum permissible runout as 0.050 mm TIR. The shaft passes the runout check.
2. Bearing Fit Check: The drive spline bearing housing bore is measured with a bore micrometer as 47.012 mm. The new drawing calls for 47 H7 (+0.025 / -0.000 mm). The measured hole falls within the H7 tolerance.
3. Plastigauge Bushing Clearance: The technician installs the quill sleeve bushing, torques the retention cap to 25 N·m without rotating the shaft, and removes the cap. The flattened Plastigauge ribbon measures 0.076 mm. The AMM Table of Limits gives:
   • New: 0.020 to 0.045 mm
   • In-Service Limit: 0.080 mm
   • Condemning Limit: 0.090 mm The clearance (0.076 mm) is within the permissible in-service limit, allowing the assembly to be returned to service with an entry in the aircraft maintenance log.

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Common Exam Traps & Pitfalls

Exam Trap 1: Conflating Total Indicator Reading (TIR) with true shaft eccentricity. A dial indicator measures the total peak-to-peak surface excursion. The true eccentricity (shaft bow) is always half of the TIR (Bow = TIR / 2).

Exam Trap 2: Rotating the shaft during a Plastigauge measurement. Rotating the shaft wipes the plastic thread across the journal, ruining the reading. The shaft must remain completely stationary while the bearing cap is torqued and loosened.

Exam Trap 3: Confusing the Hole Basis System with the Shaft Basis System. In the Hole Basis System (H-hole), the lower deviation is zero (EI = 0), meaning the smallest permissible hole equals the basic size. In the Shaft Basis System (h-shaft), the upper deviation is zero (es = 0).