As a former designer of elevated material handling equipment (boom and bridge cranes, telescopic forklifts, aerial manlifts, et cetea) I’ll expand on Craneop2’s excellent post. While it would be feasible to put electronic measurement devices on cranes that link to a control system (and I actually worked on the “envelope management system” for one large telescopic manlift which prevented exceeding the load envelope), it would also add a lot of cost to the design, and would also transfer liability from the operator (who is licensed and provided load charts for proper operation) to the manufacturer, who now by default assumes liability should the EMS fail to prevent the vehicle from tipping. As you can guess, this isn’t a feature that manufacturers are particularly enthused about implementing, and the margin on cranes and other material handling equipment isn’t large, nor do they sell enough volume to invest millions into design and test of such a system unless there is a market ready to pay for it.
The other thing to consider is that cranes and handling equipment are often modified in the field, or are used in ways that the original designers did not intend or analyze for, especially unlicensed cranes and forktrucks that don’t require periodic inspection. If you had a limit switch on a boom and it was preventing the operator from performing an operation that he believed the crane to be capable of, he’d probably just disable the thing. Of course, you can make a control system failsafe, such that if it doesn’t get feedback from the limit switch it goes into a safe mode that prevents operation, but the reality is that there isn’t much demand for this. A good experienced operator is actually a very sensitive controller, and provided he isn’t drunk, high, under extreme pressure to load a barge as quickly as possible, or operating equipment that has not been properly maintained and inspected, is probably more reliable than the type of control system that would be designed by a crane manufacturer, just as a good driver can operate a car more precisely with the traction control off.
True story: while working at one of the major telescopic material handling companies, we got a call from marketing asking what safety factors were used in designing the boom. Factors of safety, for those who don’t know, are factors that you apply to the loads (or take away from allowables) to account for the fact that your analysis may not capture the absolute worst loading case for structural failure or tipping, and are also often intended to give a margin against damage tolerance; basically, they are your safety cushion against a failure that pushes you just past your design envelope. For structural safety on these machines, it was typically something like FS[SUB]Yield[/SUB]=1.25 for non-critical parts and FS[SUB]Yield[/SUB]=1.5 for critical structures, with a FS[SUB]Static Tipping[/SUB]=1.1 and a 1.5 dynamic load factor. You get the idea; there is some margin built in to protect against manufacturing defects, minor design analysis errors, in-use damage, and your basic average idiocy.
Well, it turned out that marketing wanted to up the rating on our machines by removing all of the design factors of safety! That’s right; our 10,000 lb machine would now be rated for 15,000, but if you lifted 15,001, it would collapse. When we explained in plain terms why that was not a practical solution (uncertainty in analysis, the precision of measuring the load being lifted, et cetera), the marketing group argued that they would just put a more visible sticker in the operator’s cage warning against exceeding the load; maybe yellow with orange text. :smack:
Stranger