I am a rock climber. Climbing relies on skill, strength, knowledge, luck, and sound gear. Falling is a part of the sport, and with the right gear, the sport is safe. So far, I’ve enjoy no equipment failures.
I do not know, nor want to know, the MTBF (or MTTF) of any of my climbing gear. I’m not even sure this information would be available. And, all the gear I use has a finite chance of failing every time the equipment is in use. Part of my confidence is that the probability of failure is really low.
Equipment manufacturers tend to stress the robustness of the equipment, the strength of the design to shock load, and so on. The equipment is very strong relative to the load a falling climber may cause. There is very little overlap between the stress and strength distributions – meaning the equipment has a large safety margin or safety factor.
The equipment does wear out with use and time. The literature and climbing community stresses the use of equipment that is in good condition. As harnesses and ropes age, the strength reduces, and we tend to retire this equipment while it still has potentially years of useful life remaining. Again, we use a safety margin (if you are paying attention).
The expectation is the equipment will not fail during a reasonable duration of use and stress. This isn’t the only industry that enjoys this goal. And they do not use MTBF.
In the climbing industry, a failure tends to lead to death or very serious injury. This hazard reinforces the desire to have no equipment failures. Again, this isn’t the only industry where this is true. Yet, in the medical equipment, aerospace, and transportation industries, we commonly see the use of MTBF. That is unfortunate.
Do you have recommendations on what other metric to quantify robustness? Would you advocate radiation hardened parts to increase robustness rather than do a bunch of resiliency based approaches? I think economics partly answers why MTBF is used in the industries you have mentioned.
Hi Balaji,
I recommend using reliability (the probability of success over a specific duration).
If your product will encounter radiation beyond the expected background radiation of every day then using radiation hardened parts would be justified, but not to increase robustness alone.
I don’t quite follow the comment that economics is by MTBF is used… I don’t follow the logic there. Please elaborate.
Cheers,
Fred
One way economics can come into play is that it costs money and time to generate realistically accurate probabilities, especially for environments that are very difficult or impossible to replicate in a lab. Or if you have a product that sees significantly different stresses with different users that may result in several different failure modes.
Very true Brian. We always face a tradeoff of fully understanding the use/environment conditions an related failure mechanisms and the cost of doing so. Keep in mind that just because we cannot replicate conditions in a lab doesn’t mean it doesn’t exist in the field. cheers, Fred