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As already stated by others, the main engineering drive force is cost. That would be total cost of producing the item, not just the cost of the sub-assemblies that make it up. The most crucial cost element is the time required to manufacture the final item from the parts that make it up. Most of the time, an item is engineered to go together as easily and as quickly as possible when assembled in a specific order on a manufacturing line. Time, in seconds, is worth tens of dollars in this environment. That is why, for example, you have to remove several working items to finally get to the item that has failed. For example, you can't replace a failing water pump by removing just the pump itself. No, you have to remove other things. In some equipment, you have to remove LOTS of other things. It mainly depends on where in the production line process that water pump was installed. If it's installed as part of the engine sub-assembly (meaning the whole engine arrives in a crate from the engine build line or plant ready to bolt into a tractor), you're proabably in for a whole day of fun removing other things first. Another thing that causes service frustrations, is the fact that many machines are designed as semi-universal self-propelled powerplants. Once the unit is earmaked for a particular purpose, other engineers design add-on items for the base unit. This means that they had no say in the design of the base machine, and any add-on items will likely get in the way of servicing the base unit. For example, a tractor gets built as a basic utility machine. Then, a different group adds on a front-end loader and a backhoe. Sometimes, just getting on such a machine is a challenge, never mind working on it. I'm always mindful of the JD 420 (you knew that a tractor had to fit in here somewhere) crawler. It started as a wheel tractor, and service was easy. Then, tracks were added, making things more difficult. Next, add a bulldozer to it, and things get more cramped. Finally, if you add a loader instead of the dozer, forget even getting to the dipstick to check the oil. Now, your task is to change out a faulty clutch! Have fun!! Have 'loads' of fun! Always keep in mind that the final cost of the unit had to be kept below a certain limit, or no one would buy it. Many manufacturers learned this the hard way. It doesn't matter what era one is in, or even what it is one builds. The first rule is to not price yourself out of your market. Go back to the 420 example, and the economics of altering an existing design become apparent. You can't go off and build a crawler-loader from scratch until you've established yourself into the market as a player. Which is exactly that JD did with the M/40/420/430/440 line up. Made themselves a player in a different market (utility/industrial) than their other machines were in. Worked well, it did. Some items are designed for servicability right from the start. They can be found with price tags higher (sometimes MUCH higher) than any of the competition. At the time of sale, servicability doesn't come across as being worth the extra cost. Only after the buyer has a failure (maybe years later) will they think that maybe, just maybe, that other one that was easier to service might have been worth the extra cost. Usually, that's when they're looking at the labor charges on the bill. That doesn't help the manufacturer of the easy-service item any. They're out of business because no one bought their more costly item. As a final example, I give you Motorola Televisions. They had a wonderful idea in the early 1970's. Build their sets to be easily serviced. The electronics were housed in a sliding rack that pulled out of the front of the set. All major circuits were built onto their own seperate boards and plugged into a main board in the rack. To repair it, all you needed to do was slide open the drawer and swap the faulty board for a good one. Fast, easy, and the repairman didn't even have to move the set away from the wall. Motorola was driven out of the television business three years after this idea hit the market. Why? First, the set cost 25% more than the next guy's set did. Second, the competition ran ads suggesting that Motorola needed this gimmick because their set designs were suspect to begin with. Ouch! You make something easy to service and the competition zaps you for it! Anyway, this engineer that started out as a service tech will get off his soapbox now. Hope I opened a few eyeballs!
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