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An automotive technician is rebuilding a heavy-duty suspension assembly subject to extreme cyclic shock and severe thermal variations. The technical specification requires replacing an unmarked standard low-carbon steel bolt with a fastener capable of handling a minimum tensile strength of 150,000 psi, paired with a positive mechanical locking device that physically prevents the nut from backing off even if clamping preload is lost. Which bolt head marking and locking fastener combination correctly meets these requirements?

  1. A
    A bolt head with 3 radial lines, paired with a nylon-insert locknut and a standard flat washer
  2. B
    A bolt head with 5 radial lines, paired with a split-ring lock washer and a standard finished hex nut
  3. A bolt head with 6 radial lines, paired with a castellated nut, a drilled bolt shank, and a steel cotter pinCevap
  4. D
    A bolt head stamped with property class 8.8, paired with an external-tooth star washer and a standard hex nut

Cevap

A bolt head with 6 radial lines, paired with a castellated nut, a drilled bolt shank, and a steel cotter pin
The combination featuring 6 radial lines on the bolt head paired with a castellated nut, a drilled shank, and a cotter pin satisfies all engineering constraints. The 6 radial lines represent an SAE Grade 8 medium carbon alloy steel fastener with a minimum tensile strength of 150,000 psi. The castellated nut with a steel cotter pin provides positive mechanical locking, meaning rotation is physically blocked regardless of thermal degradation or clamping load loss.

Adım Adım Çözüm

1
Determine the required SAE bolt grade for a 150,000 psi tensile strength requirement.
SAE Grade 8 provides a minimum tensile strength of 150,000 psi (compared to Grade 5 at 120,000 psi and Grade 2 at 74,000 psi).
High-stress structural and suspension applications require high-strength alloy steel fasteners.
2
Calculate the radial line marking on the hex bolt head.
Six radial lines on the bolt head denote an SAE Grade 8 bolt (formula: number of radial lines + 2 = SAE Grade).
SAE grade markings use radial lines extending outward from the center of the bolt head.
3
Select the locking mechanism providing positive mechanical retention.
A castellated (or slotted) nut secured with a cotter pin through a drilled shank physically locks the nut to the bolt.
Friction-based locking methods (nylon inserts, split washers, serrated tooth washers) can fail under severe vibration, dynamic shock, or extreme temperatures, whereas a pinned castle nut provides fail-safe positive retention.

Anahtar Kavram

Fastener tensile strength ratings (SAE bolt grades) and positive mechanical locking methods
Tahmini Süre:1m 30s
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Bolts, Nuts, and Washers

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