[Walt Fricke]

Squirll's synthesis is interesting, but I think Gordon's annotations are closer to what is happening.

What I rather dogmatically keep trying to say here is that one should think of the chain tensioner as a combination coilover and shock absorber in the suspension. The spring does the tensioning, and the shock absorber does the dampening of the otherwise inevitable oscillations.

An ancillary feature is to compensate for dimensional changes, either due to heat in the normal operating cycle, or wear over time, thus keeping the dampening pressures constant.

The main difference here is that a shock absorber is a closed system - oil which passes the internal valves on compression flows back under extension. Here some gets squeezed out under compression (the spiral orifice is the equivalent of the valve in a shock), and on extension gets refilled from the supply chamber. The upward movement of the piston is sufficient to draw oil from the supply chamber to put the working chamber back to where it was before it was compressed.

The original tensioners got the resupply oil into the supply chamber by gravity from all the oil mist that inhabits the chain boxes. If you don't think there is much there, try running with an open hole in a chain box cover (I did when I replaced a collapsed pressure fed with a solid tensioner at an event, but had no good way to plug the hole in the cover). It was not practical, and as soon as the plastic plug I had tried popped out, I got black flagged.

So the purpose of the pressure feeding is a more reliable resupply. Note the quantitative information found in this discussion: the bleed valve opening pressure is 10 psi or so. I forget - did someone calculate what 10 psi times the area of the piston is in terms of force? And what the spring force is at, say, the piston's maximum and "normal" extensions? It is a fairly stiff spring. I'd go measure a disassembled one, but my garage is at more than its usual state of disorganization due to installing insulation and I would have trouble finding the one I have.

*My guesses on failures:
Blockage of spiral groove = excessive tension, chain wear, and chain gear wear. Maybe more noise, but not the "chain rattling against an oil drum" sound of a "collapsed" tensioner.
*Failure of check valve = excessive movement of chain, just like a failed shock on the suspension. Chain rattling noise at certain engine speeds where the harmonics whip the chain about.
*Excess wear of the piston or cylinder = same as check valve failure.
*Failure of the bleed valve - same as check valve failure - dampening action failed. This is trickier, though. I had this failure - the valve blew out. There is really only one restrictor - the one right at the inlet, so the oil fed in went right out, leaving roughly zero pressure in the supply chamber. This led to the classic sound of tensioner failure. But why? The supply chamber must have stayed full, like an overflowing bathtub. Plenty of oil for the piston to draw past the check valve, just like the older gravity feds. Is the spring in the Carrera tensioners a bit weaker than in the older two versions (I think the internals of the Turbo are the same as their immediate predecessor)? Does the Carrera rely on that extra 10 psi to provide supplemental pressure against the chain? So this is where my theoretical model breaks down a bit.

Obviously blockage of the inlet restrictor is going to upset the apple cart. So is a broken spring, though I wonder if anyone has ever seen that happen. Springs are steel and last a long time, since they are basically unaffected by movement below their elastic limit. I suppose over time, if overstressed, they may lose length (distance between coils). But they never lose rate (same with torsion bars - they can take a set, but rate doesn't change).

But my mantra still is: it's a shock absorber and coilover.

Walt
p.s. I have to keep referring to the diagram to keep the terminology straight, as that is a useful point. But, by the way, there is no "not-labeled oil check at the oil supply port." The small circle in the fine cutaway diagram is not a ball check valve - it represents where the oil from the external oil line gets in. The gray area to its right is the plug for the drilling which comes in at a right angle to the intake threaded hole.