3.4 Hoists, Overhead Cranes & Standard Hand Signals
Key Takeaways
- Pneumatic (air) hoists are inherently spark-resistant and ideal for explosive or high-duty environments, whereas electric hoists feature dual-speed control and friction/motor brakes.
- Cheater bars or pipe extensions must NEVER be used on lever hoists (come-alongs) because they bypass operator feel and shear internal safety pins or break lever handles.
- Crane working load limits must be clearly marked on bridge girders and hoist blocks; shock loading from sudden stops must be strictly avoided.
- Upper limit switches are emergency safety devices designed solely to prevent two-blocking; using them as routine operational stops is strictly prohibited.
- Standard International/CSA crane hand signals must be directed by a single designated signalperson (though ANY worker may signal Emergency Stop), supported by mandatory daily pre-use inspections of hooks, lines, brakes, and controls.
Industrial Hoists: Types, Drives, and Operational Safety
Industrial hoists are machinery units designed for vertical lifting and lowering of unguided loads. Millwrights install, maintain, and operate three main power configurations of hoists:
Manual Spur-Gear Chain Hoists (Chain Blocks)
Manual chain blocks use a hand chain to drive a reduction gear train (spur gears), lifting a load chain. They incorporate an automatic mechanical load brake (such as a Weston-style friction brake). The Weston brake uses load torque to force friction discs together, holding the load suspended automatically whenever hand chain pulling stops.
Electric Chain and Wire Rope Hoists
Electric hoists utilize electric motors equipped with electromagnetic motor brakes and secondary mechanical load brakes. Modern units feature two-speed or Variable Frequency Drive (VFD) controls for smooth acceleration and inching capability. Thermal overload sensors protect the motor from burning out during heavy duty cycles.
Pneumatic (Air) Hoists
Air hoists are powered by rotary vane air motors driven by compressed air. They offer distinct industrial advantages:
- Spark-Resistant & Explosion-Proof: Ideally suited for volatile chemical plants, refineries, spray booths, and grain elevators where electrical sparks present an explosion hazard.
- 100% Continuous Duty Cycle: Compressed air cools the motor internally, allowing continuous operation without thermal overheating.
- Precise Stepless Speed Control: Air throttling allows extremely fine millimeter-level load positioning.
Manual Lever Hoists (Come-Alongs)
Lever hoists utilize a ratchet-and-pawl mechanism operated by a short manual lever handle. They are used for short-pull positioning, machinery alignment, and rigging tensioning.
CRITICAL SAFETY RULE: Never use a pipe extension ("cheater bar") on a lever hoist handle. Manufacturers calibrate lever handle length so an average operator applying maximum hand force cannot exceed the hoist's safe rated load. Adding a cheater bar increases leverage, which will shear internal safety pins, deform ratchet pawls, bend the handle, or overload the load chain, causing catastrophic load dropping.
Overhead Cranes and Capacity Ratings
Overhead cranes move loads vertically and horizontally along building bays or localized work cells.
Industrial Crane Configurations
- Overhead Bridge Cranes: Consist of parallel runways integrated into building structures with a traveling bridge girder. Can be top-running (bridge end trucks travel on rails mounted on top of runway beams; highest load capacity) or underhung (end trucks ride on the bottom flange of runway beams).
- Gantry Cranes: Feature bridge girders supported by rigid vertical legs traveling on floor tracks (full gantry) or with one end supported on a building runway (semi-gantry). Used in outdoor storage yards and fabrication shops.
- Jib Cranes: Feature a horizontal boom (jib) mounted to a building column or floor-mounted pillar. The hoist trolley travels along the boom, providing 180° to 360° rotation for localized machine loading.
Capacity Markings and Dynamic Hazards
The Rated Load / Working Load Limit (WLL) must be clearly stenciled on both sides of the crane bridge girder and on the lower load block, legible from the shop floor. Operating a crane beyond its marked capacity is illegal under OHS regulations. Operators must avoid sudden acceleration or rapid plugging (reversing motor direction abruptly), which introduces dynamic shock loads.
Upper Limit Switches and Two-Blocking Safety
All power-driven hoists are equipped with an upper limit switch (either a paddle/weight type or a screw-geared rotary limit switch).
CRANE TROLLEY FRAME
[==============]
|
| <--- Upper Limit Switch Paddle
|
[ LOAD BLOCK ] <--- Moving upward toward drum
|
V (Striking frame = "TWO-BLOCKING" catastrophic failure)
Two-Blocking Defined
Two-blocking occurs when the lower load block travels too high and physically strikes the hoist drum, trolley frame, or upper sheave assembly. Continued hoisting against a stuck block will snap the wire rope or tear the hoist from its trolley, dropping the load instantly.
Absolute Limit Switch Rule
The upper limit switch automatically cuts electrical or pneumatic power to the hoisting motor before two-blocking can occur.
MANDATORY SAFETY RULE: The upper limit switch is an emergency safety stopping device ONLY. It must NEVER be used as a routine operational stop to end a hoist lift cycle. Relying on the limit switch during routine lifting causes contact arc welding, lever fatigue, and premature failure, leaving the crane unprotected against two-blocking.
Standard Crane Hand Signals and Pre-Use Inspections
When a crane operator's view of the load is obstructed, or during precision rigging operations, standard International / CSA hand signals must be used.
Key Standard Hand Signals
- Hoist (Raise): Arm vertical, forefinger pointing up; move hand in small horizontal circles.
- Lower: Arm extended downward, forefinger pointing down; move hand in small horizontal circles.
- Bridge Travel: Arm extended forward, hand open and slightly raised; make pushing motion in direction of travel.
- Trolley Travel: Palm up, fingers closed, thumb pointing in direction of trolley motion.
- Stop: Arm extended horizontally, palm down; move arm back and forth horizontally.
- Emergency Stop: Both arms extended horizontally, palms down; move both arms back and forth rapidly horizontally.
- Dog Everything (Pause All Operations): Hands clasped in front of body.
Communication Protocol
- Designated Signalperson: Only ONE designated, qualified signalperson shall give directional signals to the crane operator at any time. The operator must take signals only from that designated person.
- EMERGENCY STOP EXCEPTION: ANY worker on the job site who observes an immediate safety hazard is authorized to give the Emergency Stop signal, and the crane operator MUST obey it immediately.
Daily Pre-Use Crane Checklist
Before operating an overhead crane at the start of each shift, the operator or millwright must conduct a pre-use inspection:
- Check hook for throat expansion, twist, cracks, and safety latch operation.
- Inspect load chain or wire rope for broken wires, kinks, or wear.
- Test upper limit switch under no-load condition at slow speed.
- Test pendant control switches (E-stop button, direction buttons).
- Test hoist brake holding capability with a light load.
- Check for oil leaks, unusual gear noise, or loose structural bolts.
What is the primary safety rule regarding the upper limit switch on an electric overhead bridge crane hoist?
During a multi-worker overhead crane operation, who is permitted to give an Emergency Stop hand signal to the crane operator?
Why is the use of a pipe extension ("cheater bar") on the handle of a manual lever hoist (come-along) strictly prohibited in industrial maintenance?