LESSON 6 OF 11

Failure Patterns and Causal Evidence

YOU WILL BE ABLE TO

Distinguish primary failure from secondary damage.
Preserve and compare evidence before assigning a cause.
Explain competing causes and identify unresolved findings.

TOOLS AT THE BAY

Inspection light — Illuminate damage, lubricant deposits, surrounding joints, and neighboring components before cleaning.
Inspection mirror — Inspect concealed surfaces around seals, brake hardware, welds, and attachments.
Camera — Record damage distribution and component relationships before disassembly or adjustment.
Clean sample containers and labels — Preserve relevant lubricant and contamination samples with their collection locations clearly identified.
Tire pressure gauge — Record current inflation as evidence without assuming it establishes the tire's inflation history.
Dial indicator — Investigate component movement when looseness is a plausible contributor; obtain applicable service criteria before judging acceptability.

A damaged part shows what happened to it, but does not necessarily prove why it happened. Organize the investigation around the reported symptom, the primary failure, secondary damage, and any contributing conditions; describe an unresolved cause as unresolved rather than selecting a convenient warranty category.

Before cleaning or adjusting the assembly, inspect the distribution of damage and compare related locations, such as opposite wheel ends or adjacent frame joints. Preserve relevant contamination and lubricant samples in clean, identified containers when analysis is warranted, because cleaning can remove evidence needed to distinguish competing causes.

Bearing discoloration and scoring indicate distress but do not alone distinguish inadequate lubrication, incorrect adjustment, contamination, or excessive heat from another source. Examine lubricant condition, seals, races, rollers, fits, and neighboring brake components before assigning a cause.

Uneven tire wear can result from inflation history, alignment, looseness, load distribution, or suspension condition; tread appearance alone does not establish a defective axle. Unequal brake wear likewise requires checking adjustment, actuation, contamination, hardware movement, and drum or rotor condition rather than automatically condemning the friction material.

For a structural crack, document its location and inspect surrounding welds, attachments, deformation, corrosion, and evidence of prior repair. A crack beside a weld is not by itself proof of a defective weld, and secondary damage can obscure the original initiation point.

How It Works

Damage is evidence, not a verdict

The reported symptom, primary failure, secondary damage, and contributing conditions are distinct parts of the investigation. A seized bearing or cracked joint shows a damaged condition, but the initiating cause must be supported by evidence rather than a convenient warranty category. Record observations separately from interpretations and leave the cause unresolved when competing explanations remain.

Preserve patterns before disturbing them

Before cleaning or adjusting, document where damage is concentrated and compare affected and unaffected locations under equivalent conditions. Lubricant condition, contamination, and the relationship between neighboring damaged parts can help distinguish competing causes, so preserve relevant samples in clean, identified containers when analysis is warranted. Cleaning or adjustment can erase those relationships and make a later explanation harder to defend.

Trace mechanisms through the complete system

Lubricant-film loss, contamination, incorrect bearing adjustment, and heat from nearby brakes can produce overlapping signs of bearing distress. Tire and brake wear likewise reflect interacting operating conditions, while structural damage can spread away from its initiation point. A defensible conclusion explains how the proposed mechanism fits the whole pattern and which findings weaken alternative explanations.

Step-by-Step Procedure

0/6 done

1.List plausible causes before selecting a failure category.
2.Inspect the complete affected system before cleaning away evidence.
3.Compare affected and unaffected locations under equivalent conditions.
4.Preserve samples when their condition could distinguish causes.
5.Separate observed facts from interpretations.
6.State which evidence supports or weakens each remaining explanation.

Diagnostic Chart

Symptom	Check	Spec / Expected	Likely cause & fix
Bearing discoloration and scoring	Before cleaning, document the damage pattern and inspect lubricant condition, seals, races, rollers, fits, and neighboring brake components. Compare the opposite wheel end under equivalent conditions and preserve relevant samples when analysis is warranted.	Discoloration and scoring establish distress but do not alone distinguish inadequate lubrication, incorrect adjustment, contamination, or heat from another source.	Keep the competing explanations open until the complete wheel-end evidence supports or weakens each explanation. Basis: General shop practice (not from a downloaded manual)
Uneven tire wear	Investigate inflation history, alignment, looseness, load distribution, and suspension condition; compare related tire positions under equivalent conditions.	Tread appearance alone does not establish a defective axle.	Inflation history, alignment, looseness, loading, or suspension condition may contribute independently or together. Basis: General shop practice (not from a downloaded manual)
Unequal brake wear	Inspect adjustment, actuation, contamination, hardware movement, and drum or rotor condition before condemning the friction material.	Unequal wear requires investigation of the brake system rather than automatic rejection of the friction material.	Adjustment, actuation, contamination, restricted hardware movement, or drum or rotor condition may explain the wear pattern. Basis: General shop practice (not from a downloaded manual)
Structural crack beside a weld	Document the crack location and inspect surrounding welds, attachments, deformation, corrosion, and prior repairs. Compare adjacent joints and distinguish visible damage from any proposed initiation point.	A crack beside a weld is not by itself proof of a defective weld; secondary damage can obscure the original initiation point.	Weld condition is a competing explanation, not an established cause; evaluate surrounding conditions and leave initiation unresolved if evidence is insufficient. Basis: General shop practice (not from a downloaded manual)

At the Bay: Diagnose the Case

CASE 1 / 4

Bearing discoloration and scoring

Test
Compare
Correct

Select the next check

Basis: General shop practice (not from a downloaded manual). A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.

Common Bay Mistakes

✕ Cleaning away contamination before documenting or sampling it.
✕ Treating a damage pattern as proof of its cause.
✕ Comparing locations without checking whether conditions are equivalent.
✕ Choosing a warranty category to hide an unresolved cause.

Quick Check

Practice only — not scored.

A bearing is discolored and scored. Which response best supports a defensible diagnosis?

A crack is found beside a frame weld. Which conclusion is justified?

The bay reports: Bearing discoloration and scoring Which check addresses this condition?

Plan the Service Sequence

Failure Patterns and Causal Evidence

Planning practice only. Follow the applicable manufacturer's instructions, employer safety program and qualified supervision on real equipment.

Plan the first action. Compare the service steps with the bay mistakes.

Basis: General shop practice (not from a downloaded manual)

Source: General shop practice (not from a downloaded manual)

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