Crawler Crane Anatomy
2 hours
Learning Objectives
- •Identify and name the major structural and mechanical components of a lattice-boom crawler crane
- •Explain the function of the friction clutch and band brake winch system and how it differs from a hydraulic hoist
- •Describe the purpose of the boom butt, intermediate sections, and boom top in a lattice-boom system
- •Explain the role of the counterweight, carbody, and crawlers in crane stability
Topics Covered
- •Carbody (lower works): crawler frames, track chains, drive sprockets, idlers, track shoes — distributes machine weight over a large footprint
- •Upperstructure (upper works): revolving frame, cab, engine, drums, counterweight — rotates on the center pin above the carbody
- •Center pin (king pin): the structural pin connecting upper and lower works and transmitting loads between them
- •Boom butt (heel section): the base boom section pinned directly to the boom foot pivot on the revolving frame
- •Intermediate boom sections (inserts): standard and half-length sections pinned together to build desired boom length
- •Boom top (head section): the uppermost section with sheaves for load line and boom hoist line
- •Boom hoist line: the wire rope system that raises and lowers the boom; typically reeved through a mast or A-frame
- •A-frame (gantry/mast): the structural A-shaped frame on the upperstructure from which the boom hoist pendants hang
- •Pendants (boom hoist cables): the static structural wire ropes connecting the boom top to the A-frame
- •Friction winch drum: mechanical drum with a friction clutch to engage the drum and a band brake to control load lowering
- •Friction clutch: engages or disengages the drum from the engine drive; controlled by the operator via clutch lever
- •Band brake: wraps around the drum to slow or stop rotation; controlled independently of the clutch
- •Main hoist and auxiliary hoist (whip line): two separate winch drums allowing independent load handling
- •Counterweight: heavy steel blocks at the rear of the upperstructure that counterbalance the boom and load
- •Load block and hook: the multi-sheave bottom block and hook assembly suspended from the load line
- •Boom angle indicator: shows the boom angle relative to horizontal; used to determine load radius
Resources
Self-Check Questions
Question 1: What is the difference between the friction clutch and the band brake on a crawler crane winch drum?
- A. The clutch slows the drum; the brake engages it to the engine
- B. The clutch engages the drum to the engine drive; the brake slows or stops drum rotation(correct)
- C. They are two names for the same device
- D. The clutch controls the boom hoist; the brake controls the load line
Show Explanation
Explanation:
On a friction winch system, the clutch engages the drum to the engine power train (power to lift), while the band brake wraps around the drum to slow or stop it during lowering. Both must be in working order before any lift — worn linings on either cause loss of load control.
Question 2: What is the purpose of the A-frame (gantry) on a lattice-boom crawler crane?
- A. It is the structural frame on which the operator cab is mounted
- B. It is the base section of the lattice boom that connects to the revolving frame
- C. It is the structure from which boom hoist pendants hang to support the boom(correct)
- D. It houses the main hoist drums and engine
Show Explanation
Explanation:
The A-frame (also called the gantry or mast) is the steel structure mounted to the revolving frame. The boom hoist pendants run from the boom top, through sheaves at the A-frame top, and down to the boom hoist drum. This system controls boom angle.
Question 3: What is the function of the boom pendants (pendant cables) on a lattice-boom crane?
- A. They are spare wire rope sections stored on the A-frame
- B. They are static structural wire ropes connecting the boom top to the A-frame that carry the boom's static load(correct)
- C. They are the running ropes that raise and lower the boom
- D. They connect the counterweight to the upperstructure
Show Explanation
Explanation:
Pendant cables are NOT running ropes — they are fixed structural members that support the boom top. The boom hoist running rope is separate and reeves through sheaves on the A-frame to change boom angle. Inspect pendants for broken wires, kinking, and corrosion at every shift.
Question 4: What is the center pin (king pin) on a crawler crane?
- A. The large pin that connects the boom butt to the revolving frame
- B. The structural pin connecting the upperstructure to the carbody, allowing rotation(correct)
- C. The pin at the top of each lattice boom section
- D. A spare pin stored on the machine for field boom assembly
Show Explanation
Explanation:
The center pin connects the revolving upperstructure to the fixed carbody. All vertical loads, horizontal loads, and overturning moments from lifting pass through the center pin. It must be inspected for wear, cracks, and proper locking during periodic inspections.
Question 5: Why does a crawler crane provide better stability than a truck crane of the same rated capacity?
- A. Crawler cranes always have more counterweight
- B. The crawler track footprint is much wider and longer than truck outriggers, distributing load over a larger area(correct)
- C. Crawler cranes use hydraulic cylinders that prevent overturning
- D. Truck cranes are not permitted to lift as high as crawler cranes under OSHA rules
Show Explanation
Explanation:
The crawler tracks distribute the machine's weight over a large area, providing excellent stability and allowing full-rated lifts over a wide arc without outriggers. This is why crawler cranes can often lift higher rated loads than similarly sized truck cranes and can work on soft ground with appropriate ground preparation.
Question 6: During a pre-shift inspection, you find that the band brake lining on the main hoist drum is worn through in one section. What is the correct action?
- A. Reduce the load to 75% of capacity and complete the shift
- B. Use the auxiliary hoist for all lifts and defer repair to end of day
- C. Remove the crane from service until the brake lining is replaced(correct)
- D. Test the brake under load and proceed if it holds
Show Explanation
Explanation:
A worn band brake is a removal-from-service condition. The band brake is the primary means of controlling load descent on a friction winch crane — a failed brake means an uncontrolled free fall. The machine must be taken out of service before any lifting begins.