Legitimate Overload Situations
Earlier I wrote about “Rated Capacity” and “Capacity” in determining what an overhead crane can legitimately lift. There are in fact two occasions in which it is legitimate to overload a crane. The first case is the initial load testing of the crane after installation which is a mandated overload test prior to releasing the crane for operation. The crane is to be tested at the ASME prescribed 125% of the Rated Capacity. This overload is usually performed by the crane builder and/or installer. The other legitimate crane overload is called a “Planned Engineered Lift,” which may never happen in the life of the crane.
A Planned Engineered Lift
A “Planned Engineered Lift” is a lift in excess of the “Rated Capacity” of the crane. Its name is especially appropriate in that the above Rated Capacity lift is to be a “planned” event and under the auspice of “engineer(s).” In other words, this is not an “ad hoc”, spur of the moment event. Further, the entire lift “system” is to be analyzed.
Finding the Weakest Link
This is a good point to remember the old saying about “you’re only as strong as the weakest link.” In a “Planned Engineered Lift” the chain includes;
1. The rigging attached to the load
2. the lower block assembly
3. the wire rope
4. the hoist mechanicals
5. the hoist trolley
6. the bridge beam
7. the end trucks
8. the bridge beam to end truck connection
9. the runway rail
10. the runway support beam
11. the runway columns
12. the column footings
Overloading a Crane by the Book
Specification ASME B30.16-2007, Section 16-3.2 explains how these special overload lifts are to be conducted. Cranes are allowed to lift weights in excess of their rated capacity in two circumstances.
A. Initial Installation Load Test- mandatory overload test
B. Planned Engineered Lifts- overload condition that may never be incurred
In this post, we’ll delve into the latter, Planned Engineered Lifts.
PLANNED & ENGINEERED
As the name would imply, this is not a “willy nilly” over capacity lift, but rather a planned event that requires detailed written instructions with appropriate engineering calculations. Here’s the criteria;
1. Only cranes/hoists over 5 tons in capacity are eligible for this process
2. Cannot exceed 125% of Rated Capacity, except as provided in line item 4
3. Shall be limited to two such events in any continuous 12 month period
4. The hoist manufacturer must be consulted if load is to exceed 125%
Each Planned Engineered Lift event must include the following;
1. a review of the history of the hoist, including past mods and maintenance
2. an engineering analysis of the crane including, mechanical, electrical, pneumatic, and hydraulic components
3. a review of the hoist supporting structure
4. a full hoist/crane inspection prior to the lift
5. the lift shall be undertaken under the supervision of a single designated person
6. the initial lift shall be made such that it just clears the ground and then is stopped to check that the hoist brake can handle the overload
7. upon the brake successfully holding the load, the Planned Engineered Lift can proceed
8. upon completion of the Planned Engineered Lift, a complete inspection of the crane shall be performed
9. the inspection records and a writeup of the Planned Engineered Lift shall be included in the equipment file
The Intent of the “Planned Engineered Lift”
This procedure provided for two things;
1. A safe process for special circumstance above capacity lifts
2. The total elimination of the old concept of making “ad hoc” over capacity lifts, because the safety factor has been built into the crane.
An Extra Precaution (CYA)
Although not required, I would suggest that a representative of the hoist company or crane builder be on-site for the lift. Most modern cranes are equipped with a complex electronic overload protection system. If this system is bypassed for the lift (which will have to be done), you could void your warranty. Further, you’ll want to make sure the overload protection system is back in operation after the Planned Engineered Lift is completed. Most importantly, by having a factory rep on site and performing the overload bypass, none of your maintenance crew need become acquainted with the process of defeating the over load system.
The above article is an overview of the topic, and not intended as a replacement of the ASME specs. For a detailed writeup of Planned Engineered Lifts, see ASME B30.16-2007 Overhead hoists, page 20, Section 16-3.2 Handling the Load.