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Walt Fricke's Avatar
 
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Well, new pressure fed tensioners come without oil in them. I've just put them in, cranked the motor until there was oil coming out a loosened line (plus you can see the oil pressure gauge stirring), and fired it up. Never had an issue. The air in both the supply and working chambers will quickly be pushed out, one past the ball on top, and the other up the spiral windings and out the hole in the side of the piston top.
Old 09-27-2017, 04:43 PM
  Pelican Parts Technical Article Directory    Reply With Quote #81 (permalink)
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Thanks to everybody for this great thread, and the cutaway views. Where did you get those? They are not in the 3.2 workshop manuals that I have.

Here is what I have gathered after reading the thread, and working with my tensioners:

Failure Modes

Clogged restrictor orifices. The restrictor orifices are pretty small, so they could get clogged by debris from some sort of post-filter internal situation. This would prevent oil from getting into either the supply chamber or the piston chamber, or both even. The result would be a collapsed tensioner.

Broken main spring. If the main spring is broken, the piston can collapse when the engine is shut down. On startup, it will take a longer time to pump up, or it may never pump up.

Internal check valve stuck open. - The tensioner will not pump up. This will exhibit the symptoms of a collapsed tensioner.

External check valve stuck open. - The supply chamber will not pressurize. This will exhibit the symptoms of a collapsed tensioner.

Engine operating modes:

Brand new engine or brand new tensioners. - Engine oil pressure fills up the supply chamber. The external check valve vents out the air as it fills. The supply chamber inlet orifice is sized so that, once it is filled with oil, the oil pressure is not great enough to pop open the external check valve, so it closes. Oil flows into the piston chamber, past the internal check valve. Air is forced out the spiral leakdown passage. Once the piston chamber is full of oil, the pressure from the chain presses on the piston and closes the internal check valve. The tensioner is now ready for normal operation.

Normal operation. - As the engine warms up, the spacing between the cams increases. If the tensioner did not have some leakdown, the tension would become too tight. As the chain pushed harder on the plunger, oil is forced out the spiral groove. The spiral groove acts as an orifice. Once the force on the chain is in equilibrium with the rate of oil loss through the spiral groove, the normal operating condition is achieved. The internal check valve and the cylinder orifice work together with the spiral groove to set the operating tension just right.

Shutting down. - The oil supply pressure dwindles down to zero, but the supply chamber should still be full. The chain is still pressing down on the plunger, so oil will still leak out of the spiral groove. This lets the plunger collapse a small amount, until equilibrium is reestablished.

Subsequent starts. - The system stays filled with oil. If the engine has cooled down completely, the chain will be a little slack again, but the plunger spring will keep things tight until engine oil pressure refills the tiny amount of oil lost at shutdown.

Other notes. - I would say pressurizing the tensioner with air is not a good idea. The orifices are not sized for air, so you could get too much force on the internal check valve, and maybe blow it loose. I think priming it with oil, even just partially, is a good idea, but not mandatory. It shaves a few seconds off the time to achieve tension. The tensioner and its internal spring do not quickly adjust tension. It is a gradual thing done only to compensate for the expansion of the engine. The damping of dynamic motion of the chain is handled by the ramps.

Old 11-19-2017, 09:46 PM
  Pelican Parts Technical Article Directory    Reply With Quote #82 (permalink)
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Quote:
Originally Posted by Speedy Squirrel View Post
Thanks to everybody for this great thread, and the cutaway views. Where did you get those? They are not in the 3.2 workshop manuals that I have.

Here is what I have gathered after reading the thread, and working with my tensioners:

Failure Modes

Clogged restrictor orifices. The restrictor orifices are pretty small, so they could get clogged by debris from some sort of post-filter internal situation. This would prevent oil from getting into either the supply chamber or the piston chamber, or both even. The result would be a collapsed tensioner.

Broken main spring. If the main spring is broken, the piston can collapse when the engine is shut down. On startup, it will take a longer time to pump up, or it may never pump up.

Internal check valve stuck open. - The tensioner will not pump up. This will exhibit the symptoms of a collapsed tensioner.

External check valve stuck open. - The supply chamber will not pressurize. This will exhibit the symptoms of a collapsed tensioner.

I think what is happening in this case, is air would be allowed in the chamber after shutdown. The bleed valve, or the external check valve as you say, is a check valve, to let out excess oil while not allowing air in which might happen after shutdown. After shutdown the cooling motor will contract with the effect of the chain tension relaxing so the tensioner's piston could extend possibly drawing in air through the defective bleed valve.


Engine operating modes:

Brand new engine or brand new tensioners. - Engine oil pressure fills up the supply chamber. The external check valve vents out the air as it fills. The supply chamber inlet orifice is sized so that, once it is filled with oil, the oil pressure is not great enough to pop open the external check valve, so it closes. Oil flows into the piston chamber, past the internal check valve. Air is forced out the spiral leakdown passage. Once the piston chamber is full of oil, the pressure from the chain presses on the piston and closes the internal check valve. The tensioner is now ready for normal operation.

The bleed valve, or external check valve as you call it, will open at around 10psi. So air and oil will vent out. Interesting that oil is being constantly allowed to bleed out of the bleed valve during operation. (It doesn't appear to have any noticeable affect on oil pressure.)


Normal operation. - As the engine warms up, the spacing between the cams increases. If the tensioner did not have some leakdown, the tension would become too tight. As the chain pushed harder on the plunger, oil is forced out the spiral groove. The spiral groove acts as an orifice. Once the force on the chain is in equilibrium with the rate of oil loss through the spiral groove, the normal operating condition is achieved. The internal check valve and the cylinder orifice work together with the spiral groove to set the operating tension just right.

The oil supplied to the supply chamber is constantly being released out of the bleed valve. The supply chamber only maintains enough pressure to make sure that the working chamber is always supplied with oil, when it is needed. The tensioner's spring does most of the work with the oil acting mainly to dampen the plungers movement and to keep it from collapsing. Perhaps the 10psi or so in the supply chamber is just the correct amount to balance out the oil lost through the spiral groove.

Shutting down. - The oil supply pressure dwindles down to zero, but the supply chamber should still be full. The chain is still pressing down on the plunger, so oil will still leak out of the spiral groove. This lets the plunger collapse a small amount, until equilibrium is reestablished.

Subsequent starts. - The system stays filled with oil. If the engine has cooled down completely, the chain will be a little slack again, but the plunger spring will keep things tight until engine oil pressure refills the tiny amount of oil lost at shutdown.

Other notes. - I would say pressurizing the tensioner with air is not a good idea. The orifices are not sized for air, so you could get too much force on the internal check valve, and maybe blow it loose. I think priming it with oil, even just partially, is a good idea, but not mandatory. It shaves a few seconds off the time to achieve tension. The tensioner and its internal spring do not quickly adjust tension. It is a gradual thing done only to compensate for the expansion of the engine. The damping of dynamic motion of the chain is handled by the ramps.
The diagram came from an old posting here. Probably from a tech bulletin. I added the red annotations.

My notes are from my observations, and open for debate, since there seems to be no official documentation.
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Last edited by Trackrash; 11-20-2017 at 11:32 AM..
Old 11-20-2017, 10:31 AM
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For those of you with curious minds here is a tensioner disassembled. Note that the check valve can be removed and it is the same as the external bleed valve.

It seems from my experience that the precise operation of these check valves is critical to the proper function of the tensioner.

The older hydrolic tensioners can be repaired and rebuilt with great success. No so with the pressure fed ones, I guess.
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Old 11-20-2017, 11:14 AM
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Not to confuse, but it's easier to follow if the written description uses the same terminology as shown in the illustration and vice-versa.

For example: Is the "external check valve" in the description referring to the bleed valve or the not-labeled ball check at the oil supply port?

Sherwood
Old 11-20-2017, 11:14 AM
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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.
Old 12-10-2017, 12:28 PM
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