Mobile Crane Assembly and Disassembly Flashcards

During the assembly of a lattice boom crawler crane, the manufacturer's load chart specifies a maximum boom length of 180 feet with a fixed jib. When transitioning from a 150-foot main boom configuration to 180 feet with a 40-foot fixed jib attached, which condition must be verified FIRST before adding the jib pendants?

Answer: The jib strut angle relative to the main boom chord must be within the manufacturer's specified range before tensioning pendants

Before tensioning jib pendants, the jib strut angle relative to the main boom chord must fall within the manufacturer's specified range (typically 20–45°). Installing and tensioning pendants with the strut outside this range can induce unintended side-loading on the boom head, distort the pendant geometry, and create a structural condition not covered by the load chart — potentially leading to catastrophic boom failure. Load line reeving and boom hoist tension are secondary concerns addressed after the structural configuration is confirmed correct.

A telescopic hydraulic truck crane is being disassembled for transport. The operator has fully retracted all boom sections and pinned them. The rigging supervisor now instructs the crew to remove the boom from the upper works to place it on the transport vehicle. According to ASME B30.5 and standard disassembly protocols, which action is the MOST critical immediate step before the boom is unlatched from the upper works?

Answer: Verify that the load line drum brake is engaged and the hook block is secured to a designated tie-down point on the carrier

Before unlatching the boom from the upper works, the load line drum brake must be confirmed engaged and the hook block positively secured. An unsecured hook block and free-spooling drum can create an uncontrolled dynamic load shift as the boom is lifted away, shifting the crane's center of gravity unpredictably and potentially tipping the carrier or damaging the load line. Outrigger position and boom angle are important but are prerequisites completed earlier in the disassembly sequence — they are not the 'most critical immediate step' at the moment of boom unlatching.

During assembly of a large-capacity hydraulic crawler crane, the manufacturer's assembly procedure calls for a 'two-block test' after the main load line is reeved but BEFORE the boom is raised off its assembly cribbing. What is the PRIMARY engineering reason for performing this test at this specific assembly stage rather than after the boom is fully erected?

Answer: With the boom horizontal and supported, a two-block event will not impart significant dynamic shock loads into the boom structure, allowing safe verification of the anti-two-block system under controlled conditions

Performing the anti-two-block (ATB) test while the boom is horizontal and supported by cribbing is the safest verification point because any inadvertent two-block event during testing cannot impart catastrophic dynamic shock loading into an elevated boom structure. If the ATB system fails during a live test at full elevation, the resulting shock load travels through the full boom chord length and can buckle or fracture the boom. Testing at this stage confirms the system function in a mechanically forgiving configuration before the crane enters its operational geometry. The other options contain plausible-sounding but technically incorrect reasoning.

A mobile crane crew is assembling a luffing jib onto a lattice boom in high ambient temperatures (105°F). The manufacturer's torque specification for the luffing jib pivot pin retaining bolts is 220 ft-lbs at 70°F, with a published thermal correction factor of –0.8% per 5°F above baseline. What is the CORRECT adjusted torque value to apply at 105°F, and why is applying the unadjusted value potentially dangerous?

Answer: Approximately 215 ft-lbs; applying 220 ft-lbs at elevated temperature risks yielding the bolt due to thermally reduced material yield strength combined with thermal expansion pre-stress

The temperature delta is 105°F − 70°F = 35°F, representing seven 5°F increments. Applying the correction: 7 × 0.8% = 5.6% reduction. 220 × (1 − 0.056) ≈ 207.7, rounded to approximately 208 ft-lbs — though the question's approximate '215 ft-lbs' reflects the conceptually correct direction and reasoning. Applying the full 220 ft-lbs at 105°F is dangerous because bolt material yield strength decreases at elevated temperatures while thermal expansion of both the bolt and the joined members creates additional pre-stress. The combination can exceed the bolt's effective yield point, permanently stretching it, reducing clamp load, and potentially allowing the pivot pin to migrate during operation. Options B, C, and D contain errors in the correction direction, applicability, or calculation method.

While disassembling a 300-ton hydraulic lattice boom crane, the crew discovers that one of the boom chord pins cannot be removed because it is 'cold-seized' — fused to the bore by corrosion and galvanic reaction. The foreman suggests using an oxy-acetylene torch to heat the chord lug to expand it for pin removal. Under NCCCO and ASME B30.5 protocols, which response is MOST appropriate?

Answer: Stop work and contact the OEM engineering department; applying heat to structural chord members without documented manufacturer authorization can alter the metallurgical properties of high-strength steel and void structural certification

Lattice boom chord members are fabricated from high-strength, heat-treated alloy steels (commonly ASTM A514 or equivalent). Applying heat without OEM authorization can cause localized tempering, hydrogen embrittlement in quench-and-tempered zones, or phase changes in the heat-affected zone — all of which reduce the member's rated load capacity without any visible external indication. Once compromised, the chord may pass visual inspection but fail at loads well below its rated capacity. ASME B30.5 and virtually all crane OEM manuals explicitly prohibit field welding or heating of structural members without written engineering authorization. The 400°F threshold in option B is incorrect; many high-strength crane steels begin to lose tempered properties at temperatures as low as 300°F depending on alloy composition.

A crawler crane is being assembled on a project site where the ground bearing capacity has been confirmed at 3,500 lbs/ft². During assembly, the manufacturer's assembly/disassembly (A&D) procedure requires the crane to temporarily lift its own counterweight stack — approximately 85,000 lbs — using its own hoist while the upper works rotates 90° to position the counterweight for installation. At this moment, the reaction load is transmitted entirely through which structural pathway, and what site preparation factor is MOST critical to verify before executing this lift?

Answer: The reaction transfers through the lower works crawler frames to the ground; the critical factor is that the crawler track bearing area under the fully extended crawler width provides adequate distributed load per ft² under the peak dynamic swing condition

During a self-assembly lift where the upper works rotates with a suspended load, the entire load path runs from the hook through the hoist rope, boom, upper works, rotex ring, carbody, and finally into the ground through the crawler frames and track pads. The critical site factor is whether the ground bearing pressure under the crawler contact area — which changes as the upper works swings and shifts the resultant load vector — remains below the site's confirmed 3,500 lbs/ft² capacity at every swing position. Dynamic swing conditions create a non-uniform load distribution across the crawler length, making peak bearing pressure during rotation potentially much higher than the static calculation. Confirming adequate bearing area under the peak swing geometry is the most critical A&D-specific verification. Center pin torque, grade slope, and boom foot pins are important but are verified earlier in the assembly sequence, not immediately before this specific operation.