ESSENTIAL TERMINOLOGY WHEN SPECIFIC OVERHEAD CRANES
1. Crane Capacity - The rated capacity of crane is the maximum working load that can be lifted. A crane capacity is expressed in US tons or (metric) tonnes and is required by code to be marked on each side of the crane and hoist; and if the crane has more than one hoisting unit, each hoist shall have its rated load marked on it. Note that the load block, hook and ropes are not included in the rated load. If the crane is intended to be used along with a magnet, ‘C’ hook, grab, ladle or any other appliance, the dead weight of such appliance should be specified so that the safe working load of the crane could be determined appropriately.
Note: The regulations set forth by OSHA 1910.179 N paragraph (k) states that the crane shall not be loaded beyond its rated load except for test purposes. The load test shall be conducted at or near 125% of the rated load.
Caution: Never pick more than the rated working load limit (WLL) since so many factors go into the design of a crane and one has to look at the safety factor of all the components such as the wire rope, the motors, bearings, sheaves, drums, wheels, rails, hoisting speeds, and beam and steel sizes. In many cranes this will be printed as the safe working load (SWL).
2. Long travel - ‘Long travel’ is the direction of travel of the bridge along the rails.
3. Cross travel - ‘Cross travel’ is the movement of the ‘crab’ from one side of the bridge to the other.
4. Hook Height - The hook height is the distance from the datum to the highest position of the hook. This dimension is critical in most applications as it determines the height of the runway from the floor and is dependent on the clear inside height of the building. Effective lift is the distance between the beam and the floor, minus the height of the hoist.
5. Runway Height - The distance between the grade level and the top of the rail.
6. Side Clearances - Side clearances are measured from the center of the supporting rail to the face of the supporting column and are required for operation, safety and wheel maintenance.
7. Vertical Clearances - The crane must be able to travel within the building while avoiding obstructions in the building such as lights, equipment and structural framing. The vertical clearance is based on the size of the crane bridge, location of the hoist and trolley, rail and safety
allowance. The lower the crane headroom is, the higher will be the vertical clearance and the lower could be the building height.
8. Clear Span - The span of the crane is the horizontal center distance between the rails of the runway on which the crane is to travel. Typically the distance is approximately 500 mm less than the width of the building. How much span a crane requires depends on the crane coverage width dictated by the application. Crane coverage is the horizontal crane coverage and is defined as the crane span less the left side hook approach and right side hook approach.
9. Runway Length - The longitudinal run of the runway rail parallel to the length of the building.
10. Hook approaches - The maximum hook approach is the distance from the wall to the nearest possible position of the hook. The smaller the distance, the better will be the floor area utilization.
11. End Approach – This term describes the minimum horizontal distance parallel to the runway, between the outermost extremities of the crane and the centerline of the hook.
12. Bridge, Trolley and Lift Speeds - The rate at which the bridge or trolley travels or at which the hoist lifts is usually specified in feet per minute or FPM. The crane operating speeds are selected to allow safe operation while using the pendant. Dual operating speeds, normally a fast and slow speed with a ratio of 4:1, are commonly used, but for optimum control a variable speed control system is preferred.
13. Electrical Requirements - Ideally 480 volt, 3 phase, 60 hertz for US requirements. The runway power is usually by a conductor bar, and a hoisting trolley by festoon cable. The control circuit voltage at pendant push-buttons shall not exceed 150 volts for AC and 300 volts for DC.