Here I would like to touch on two subjects that came together during a single project. 1) Anticipating abuse of equipment and, 2) balanced designs. Abuse is self-explanatory. I use the term balanced not in terms of center of gravity but a design that has all components of similar strength or life expectancy. No single component is excessively weaker or stronger than the rest of the system.
A little background is required before the real world example.
A balanced design suggests there are no significantly weak or strong components in a system as compared to the rest of the system. For example, a well-maintained engine can operate for hundreds of thousands of miles before rebuild. If this were true except for one component, say a timing chain requiring replacement every 10,000 miles this would be an unbalanced design. An external timing belt requiring replacement every 60,000 miles may be acceptable since replacement is easier and it lasts 6 times longer than the first example. There must be some tradeoff for using a 60,000-mile belt vs. a timing chain and creating a weak link.
Weak links do exist and with good purpose, tires for example. My vehicle has traveled over 200,000 miles on the original engine and transmission but I have lost count of how many sets of tires and brakes I have replaced. Some weak links are desirable because they provide superior performance. Traction in a variety of road conditions, comfort of ride, and road noise are tradeoffs over rock-hard 200,000 miles tires. These are known and acceptable tradeoffs.
Returning to the engine as an example, a high quality high production engine may have all the components designed specifically for that one engine. A less expensive engine (any system) may use a large portion of existing parts originally designed for another applications or no particular application. Some may be much stronger than required but still be economical to use due to mass production of the component. Other components may be marginal requiring replacement one or more time before the entire engine is worn. For a one-of-a-kind device addressed below the attempt at a balanced design can really go out the window if ever considered.
A few years ago, a client received a unique order for a very large volume of a specific component. The component normally shipped within a complete assembly with only a limited number of this particular component per assembly. The labor of loading tens of thousands of these 300+ lb items on to railroad flat cars was a bottleneck never before experience by the client.
The client designed a beam that lifted 25 components at a time. Since this was a one-off design, there was no optimization performed to balance the design. What came with that approach was a potential safety hazard.
The staff purchased off-the-shelf cables with a lift rating of 1,000lb each and fabricate the lifting bar with 25 attach points. The cables were available at the local supply store. The danger here is that each set of cables has a rating tag indicating 1,000lb capacity. A worker not familiar with the design may assume the lifting bar could take 25,000lbs. The unbalance in this lifting device comes in the middle of the system. Each of the cable sets (2 legs x 25 sets) was good for 1,000lb. The overhead gantry crane was 20 or 40 tons. The weak link was the lifting beam, which was well below a 25,000lb capacity if a high factor of safety was maintained.
When equipment is being assessed or designed, the engineer must endeavor to find likely methods of abuse, misuse, and safety issues. My final report emphasized the unbalance and potential safety issue. Shortly after submitting my report, I spoke to the client’s safety engineer. He said one of the worker suggested using the lifting device for (25) 1,000 lb components. Exactly what my caution predicted could happen.
In this example, the abuse of an unbalanced design would not just result in an early repair due to a weak link, but could have resulted in deadly accident. The weight capacity marked on the cables and the overhead crane provided a false impression of system capability.
This story recalled another incident that demonstrates that a balanced design can develop a weak link depending on use of the system. I will save that for another article.