I think you are stuck. If a bolt did come loose, then that means there is a bolt missing and that missing bolt needs to be replaced. Since no one has proposed replacing the missing bolt, it's no longer an engineering problem and is a human interaction problem.

Has anyone checked the exterior bolts for missing or replacement bolts? Often, when you lose a bolt and you don't find it, you might possibly replace it with a non-OEM replacement.

Is the bolt that damaged the pump available for inspection? You can possibly narrow down the possible original location of the bolt by the thread and length.

TTFN (ta ta for now)
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First I want to thank you guys for your responses.

We sell hyperlubricant oil/hydraulic fluid additives that bond at the molecular level to create a shield of lubricity, combined with a bypass filtration system that removes water and particles down to one micron, which, in hydraulics, drops peak operating temps by 20 degrees while increasing the pour point temp. The combination basically preserves hydraulic systems from needing overhauls or refilling for many years (no overheating, no varnish buildup, no internal particulate erosion). I made a deal with the owner of a large construction company in Texas, after which I heard lots of complaining from the maintenance staff about how they tried this a couple of years ago and it loosed a bolt from WITHIN the hydraulic system and ruined the pump. I immediately became suspicious.

All I'm going on is the equivalent of an old wives tale that is preventing willing cooperation from maintenance. Rather than "lower the boom" and create potential enemies throughout that camp, I prefer to prove scientifically that the event never happened as described. I've since examined many schematics of hydraulic systems and I think it's fairly easy to see that the bolt was external, and the chief mechanic at the time didn't dare admit it. So while I don't want to appear tyrannical to the entire maintenance staff, I don't mind making an enemy out of a liar.

Like 3DDave said, it's no longer an engineering problem -- it's a human interaction problem.

Again I thank you for your responses -- you're way above my pay grade.

If there was a bolt that damaged a pump it had to enter through the suction port. I'd look at any in-tank accessories installed with fasteners that go through the reservoir walls. I can not imagine oil velocity in the reservoir being high enough to carry a bolt or nut.
Was there a strainer on the pump intake reservoir port?
Hydraulic fluid, no matter its fabulous properties, do not cause fasteners to loosen.

Ted