Pressure plays a very big role. Think about it... the closer you get to the center, the less movement there is, so it's not necessarily wear from routine use. However, under excessive load, the trunion would suffer stress whether it moved or not, but especially under heavy load, which most systems with a high lift cam and stiffer springs will routinely see. I can say that the rollers that I have replaced for customers or on the Sprint car were under alot of load. Keep in mind, though, that the real reason aluminum rollers were put to use was for racing, not for longevity. A race engine, and it's parts, rarely have to go for more than 500 miles, and sometimes as few as 1/4 mile. Also, you get what you pay for. No offense intended, but Sig Erson, for example, is K-mart quality. The ford rollers are actually top of the line Crane rollers that Crane customized for Ford. GM uses top quality rollers as well, but still lists them as a part to replace at 100k miles as routine maintenance. In my opinion, based on what I've seen, I wouldn't really recommend aluminum rollers for a daily driver. The difference between aluminum rollers and steel rollers isn't noticable in a mild street engine. But, steel rollers wear faster than sled-fulcrumed rockers too.
Take care,
-Chris
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