9.3 Bearing Heating Limits, Fits & Removal Tools

Precision bearing performance depends on exact shaft journal and housing bore fit tolerances. Standardized fit systems ensure correct radial interference without causing inner ring cracking or housing distortion. When bearings reach the end of their operational lifecycle, millwrights must utilize appropriate dismounting tools and conduct systematic root cause failure analysis.

ISO Shaft and Housing Fit Tolerance Systems

The ISO Tolerance System (ISO 286 / ISO 1101) uses alphanumeric codes to specify fundamental deviations and tolerance grades for shaft journals and housing bores.

• Letter Designations: Lowercase letters denote shaft fit tolerances (e.g., g, h, j, k, m, p); uppercase letters denote housing bore tolerances (e.g., F, G, H, J, K, P). Letters A-H indicate clearance fits, J-N indicate transition fits, and P-Z indicate interference press fits.
• Number Designations: Numbers represent the International Tolerance (IT) Grade (e.g., IT5, IT6, IT7). Smaller numbers represent tighter manufacturing tolerances.

+---------------------------------------------------------------------------------------------------+
|                                 COMMON INDUSTRIAL ISO BEARING FITS                                |
+-----------------------+-----------------------+-----------------------+---------------------------+
| ISO Fit Designation   | Fit Classification    | Physical Fit Condition| Typical Industrial Usage  |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Shaft Fit: j6         | Transition Fit        | Light interference to | Non-critical equipment,   |
|                       |                       | light clearance       | smooth light loads        |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Shaft Fit: k5         | Light Interference    | Light press fit       | Normal industrial shafts, |
|                       | (Press Fit)           | (0.0002" to 0.0006")  | electric motor rotors     |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Shaft Fit: m5         | Medium Interference   | Heavy press fit       | Heavy shock loads, gearbox|
|                       | (Heavy Press Fit)     | (0.0005" to 0.0012")  | input shafts, crushers    |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Housing Fit: H7       | Clearance Fit         | Free sliding clearance| Standard non-locating     |
|                       | (Slip Fit)            | (Allows axial drift)  | floating bearing housings |
+-----------------------+-----------------------+-----------------------+---------------------------+
| Housing Fit: P7       | Heavy Interference    | Tight press fit in    | Vibrating screens, rotating|
|                       | (Press Fit)           | housing bore          | outer rings, wheel hubs   |
+-----------------------+-----------------------+-----------------------+---------------------------+

Non-Locating vs. Locating Bearing Fits

In a standard two-bearing shaft assembly, one bearing is designated as the locating bearing (fixed axially to prevent shaft floating), while the other is the non-locating bearing (free to float axially inside housing bore fit H7). This axial float accommodates thermal expansion of the shaft during operation without preloading the bearings.

Precision Metrology & Journal Measurement

Before installing new bearings, millwrights must measure shaft journals and housing bores to verify geometric accuracy.

+---------------------------------------------------------------------------------------------------+
|                             SHAFT & BORE METROLOGY INSTRUMENTS                                    |
+-----------------------+-----------------------+---------------------------------------------------+
| Measurement Device    | Primary Application   | Required Measurement Protocol                     |
+-----------------------+-----------------------+---------------------------------------------------+
| Outside Micrometer    | Shaft Journal Outer   | Measure at 3 axial planes (A, B, C) and 2         |
| (0.0001" / 0.002mm)   | Diameter (OD)         | perpendicular angles (0° & 90°) to detect taper   |
|                       |                       | and shaft out-of-roundness (ovality).             |
+-----------------------+-----------------------+---------------------------------------------------+
| Dial Bore Gauge /     | Housing Inner Bore    | Calibrate against setting ring or micrometer block.|
| Telescoping Gauge     | Diameter (ID)         | Sweep bore at top, middle, and bottom to detect   |
|                       |                       | bell-mouthing or hourglass distortion.            |
+-----------------------+-----------------------+---------------------------------------------------+
| 3-Point Internal Micro| Precision Bore ID     | Provides direct 3-contact self-centering readouts |
| (Tri-Micrometer)      |                       | accurate to 0.00005" for machine tool spindles.   |
+-----------------------+-----------------------+---------------------------------------------------+

Out-of-Roundness (Ovality) and Taper Limits

Shaft journals and housing bores must be checked for dimensional geometry. If a housing bore exhibits ovality (out-of-roundness) exceeding 0.0005" (0.013 mm), the outer bearing ring will distort when clamped, leading to pinching of rolling elements, localized high stresses, and rapid fatigue spalling.

Bearing Dismounting Tools & Techniques

Removing tight press-fit bearings requires specialized tools to prevent shaft damage.

Mechanical Jaw Pullers & Split-Plate Pullers

• Mechanical 2-Arm and 3-Arm Jaw Pullers: Jaws must grip the shoulder of the inner ring directly. Pulling against the outer ring forces pulling tension through the rolling elements, damaging raceways and risking outer ring fracture. For flush-mounted bearings where jaw access behind the inner ring is restricted, a split-plate (knife-edge) puller is installed behind the inner ring face, providing a flat pulling surface.
• Hydraulic Pullers: High-tonnage hydraulic rams generate smooth pulling forces up to 50 tons, eliminating hand effort and heavy hammering during large industrial bearing removal.

+---------------------------------------------------------------------------------------------------+
|                             HIGH-PRESSURE OIL INJECTION DISMOUNTING                               |
+---------------------------------------------------------------------------------------------------+
| Operating Principle: High-pressure oil (100 to 300 MPa / 15,000 to 43,000 PSI) is injected       |
| through pre-drilled oil ducts in the shaft into distribution grooves under the bearing bore.      |
|                                                                                                   |
| 1. High fluid pressure expands the inner ring hydraulically, floating it on a micro-thin oil film.|
| 2. Friction drops to near zero, allowing the bearing to slide off the journal smoothly.           |
| 3. MANDATORY SAFETY WARNING: On tapered shaft seats, an axial stop nut MUST be threaded loosely  |
|    onto the shaft end! When oil pressure breaks the taper lock, the bearing releases with violent |
|    explosive force. The stop nut safely catches the dismounted bearing.                           |
+---------------------------------------------------------------------------------------------------+

Bearing Failure Analysis (ISO 15243 Standard)

When a bearing fails, millwrights perform Root Cause Failure Analysis (RCFA) based on ISO 15243 categories.

+---------------------------------------------------------------------------------------------------+
|                                 ISO 15243 BEARING FAILURE ANALYSIS                                |
+-----------------------+-----------------------------------+---------------------------------------+
| Failure Mechanism     | Visual Symptoms & Characteristics | Underlying Root Causes                |
+-----------------------+-----------------------------------+---------------------------------------+
| Sub-surface Fatigue   | Flaking, spalling, deep craters   | Normal end-of-life fatigue, heavy     |
| Spalling              | with sharp edges across raceways  | dynamic overload, extreme shaft flex  |
+-----------------------+-----------------------------------+---------------------------------------+
| True Brinelling       | Regular plastic indentations in   | Excessive static impact, hammer blows |
|                       | raceways matching roller spacing  | during installation, extreme shock    |
+-----------------------+-----------------------------------+---------------------------------------+
| False Brinelling      | Bright, polished elliptical       | Stationary micro-vibration & fretting |
|                       | depressions with red rust powder  | during transport or standby           |
+-----------------------+-----------------------------------+---------------------------------------+
| Electrical Fluting /  | Washboard pattern of transverse   | Stray electrical currents (VFDs, arc  |
| Arcing Damage         | ridges and micro-melted craters   | welding ground attached across shaft) |
+-----------------------+-----------------------------------+---------------------------------------+
| Fretting Corrosion    | Reddish-brown iron oxide dust     | Loose fit micro-motion between shaft  |
|                       | ("bleeding rust") on bore/OD      | journal and inner ring bore           |
+-----------------------+-----------------------------------+---------------------------------------+
| Thermal Discoloration | Steel turns yellow, brown, blue,  | Oil starvation, excessive grease churn,|
| & Overheating         | or purple; melted cages / seizure | tight internal clearance binding      |
+-----------------------+-----------------------------------+---------------------------------------+

Failure Mode Deep-Dive

1. True Brinelling vs. False Brinelling:
   • True Brinelling: Caused by static overload or severe shock impacts exceeding the elastic limit of bearing steel. Produces smooth plastic indentations that retain surface grinding marks.
   • False Brinelling: Occurs while equipment is stationary. Subtle micro-vibrations cause rolling elements to micro-frett against raceways, wearing away protective oxide films. Debris oxidizes into a reddish-brown powder (cocoa powder), forming polished depressions with ground marks worn away.
2. Electrical Fluting (VFD Current Discharge): When Variable Frequency Drives (VFDs) generate common-mode shaft voltages, current discharges across the bearing oil film. The localized electrical arcs melt micro-craters (5 to 20 µm diameter) in raceways. Under rotation, this damage progresses into a distinctive washboard pattern of transverse ridges, accompanied by blackened, carbonized grease. Mitigated by installing shaft grounding rings or insulated ceramic hybrid bearings.