Choose from the middle of the range

(Of course this isn’t really about motors.)

Designing machinery, it’s a rule of thumb to choose components from the middle of each supplier’s range. Say MoCo offers industrial motors in 0.5hp, 1hp, 2hp, 8hp, 10, 12, 16, 20, 30, and 50hp. You want to stay away from the far ends of that range. Probably their motors from 8hp to 20hp are pretty good, but those at the far ends are pushing the limits of their design. Here’s how that happens.

“Make it just like the smaller model, only bigger.”

There’s an original design, say for a 12hp electric motor, that works well. The company sells a lot of them, and they make other models from 8hp to 20hp on the same design, and those are good products too. Then a customer wants a 30hp motor for some application. MoCo doesn’t make one of those, but this doesn’t stop the salesman, who promises one right away. So the engineering department has to design a 30hp motor right away. They take the 20hp design and embiggen it.

The original design relies from parts in the middle of the range of MoCo’s own suppliers. To get 30 horse power, MoCo’s engineer should use different product lines, maybe from different suppliers. There’s never enough time, people, or money for that (if there was, MoCo would lay people of until there wasn’t.) Just doing the paperwork to get a new supplier into the system would take more time than the engineer has. Instead, he has to choose parts from the far ends of existing suppliers’ products. Remarkably this works, and they put 30hp into their product line. What happens when they need a 50hp? The engineers enlarge the 30hp. This process only stops when a product fails dramatically.

The same thing can happen at the low end, as engineers try to cram stuff that doesn’t fit into a package that won’t hold it. Now sometimes you do get lucky, and the 2 is a cut-down 8. It’s over-designed, solid as a rock, won’t wear out, and never needs attention. It seems like this has become less and less common, but maybe that’s just me getting old and bitter.

So whatever you’re choosing, choose from the middle of the range.


2 Replies to “Choose from the middle of the range”

  1. In the words of my generation:

    This is why so many people who get into gaming in a big way hate X or Y computer companies– Dell is an example I’m familiar with, so I’ll use them.
    When I got into the Navy, I saved up a nice chunk of money, was enjoying some MMOs and invested in a zing-bang all-the-dials-set-to-eleven laptop.
    It died less than a week after I got it. They sent an on-site repair lady the next day, fixed it…same thing happens. I got to know that lady REALLY well…. the dang machine just kept dying from overheating. Obvious to me now, looking back nearly a decade, but worrying back then. (I always had it properly setup and all, the system just wasn’t designed to deal with all the heat that was pumped out.)
    Eventually, she “accidentally” put in a new part that did slightly less, which opened up a bit of room inside the machine for airflow, which stopped the heat-deaths. The design was sound, the parts were good, but it was on the far edge of what is possible. This happens a LOT with any big computer company– you buy from the bleeding edge, you’re likely to get cut.

    Trying to explain this to most “computer experts” won’t get you anywhere, though. *sigh* They might know about building rigs, they probably know more than I do about improving the rigs when you already have them, but design is a foreign country. Sort of like the kids who will answer “where does milk come from?” with “from the store” and just can’t understand it comes from somewhere before that….

    I’m so going to have to steal this explanation, by the way– it might get through a lot better!

    1. Yes, that laptop sounds like an example of marginal design. You’re welcome to the explanation; I’d like more people to understand this.

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