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The stock unit is a mechanical bolt belt tightener with indirect temperature compensation.
There is no significant outward spring pressure to absorb belt length fluctuation. Hence, no rebound to dampen.
Ken,
The Audi unit strictly speaking is not a "hydraulic tensioner" but indeed they are called such! - In reality the Audi unit is a spring tensioned hydraulic damper and strangely enough so is the Porsche 928 tensioner. The cam followers are hydraulically tensioned not that it matters much.
Audi introduced their tensioner in 1994 Porsche deployed their tensioner in the first 928's delivered back in 1977 or whenever and they were granted a patent in 1980 - if it was a straight forward mechanical tensioner why would they be granted a patent?
The Belleville spring is very much what its name suggests it is a spring but it is oh so much more than that if you did but know it. It will compress to something like 95% of its conical height without deforming but as you noted stick it in a vice and flatten it and it will be well and truly buggered. Remember you stated the other day that when at temperature the pack is flat and cannot compress and quite understandably that reflects your thinking on the process but as I demonstrated the cone height is approx 0.8mm at zero C, at 100C the cone height reeduces to 0.6mm, this will likely compress down to 0.4mm when tensioned [my estimate] and that leaves about 0.35mm for further compression so not difficult to visualise that it can and will oscillate around an envelope of plus and minus 0.2mm and up to 0.35mm were it needed but I doubt they would push the envelope that far..
Now you will probably ask [as would most on this list] what is the relevance of the above? It would seem from your dialogue that you expect the damper to take out the amplitude of the flutter but as I understand that is just not the case. Think about it- we have an engine travelling at 6k rpms or 100 revs per second. The pulses are created by the cams creating resistance as the cam lifts and then motoring the camshaft as the valve springs help drive the camshaft on its decline. The 928 is an eight cylinder engine so for every revolution of the camshaft there are 8 cam events- 4 induction and 4 exhaust giving a total of 800 pulses a second. Flutter is a resonance effect - to eliminate flutter the traditional approach was to use a mechanical tensioner. When the 928 was introduced as I recall it was the longest ever cambelt put into production. Did American Vee 8's have a cambelt- err no they had pushrods. Did Ferrari and Lamboghini have cambelts?- as far as I know they had camchains. The more cylinders that are pulled the more tensile load on the cambelt and ever more superimposed load had to be applied to stop it fluttering. How do you reduce the load? - you fit a damper.
The entire idea of a damper is to stop resonance taking place in the first place by bleeding off the out of balance energy that the belt itself cannot absorb. The notion that the damper has to absorb pulsations at those frequencies I find somewhat amazing to say the least but who knows maybe I have that one wrong..
The thing that most folks on the planet do not know is that Belleville washers make very good dampers thus by a very clever piece of engineering Porsche managed to combine a tensioning device that could self compensate for temperature, tension the belt and dampen it. The oil of course helps transmit the energy away from the pack. Then of course they modified the design and in 1983 were granted another patent for their revised design that incoiporated a check valveand possibly some additional porting so that the pack could presumably pump the oil around the body of the tensioner to help bleed off energy. Whether or not this was connected with introduction of the HTD sprocket design I know not- my guess is that they felt it was a bit marginal and needed beffing up a bit but whatever the reason the upgraded version is still available and as far as I know the original version is not but the revised version can be used with the original sprocket design.
As to whether flutter is a problem with the stock item all I can say is that it would be nice to see the evidence and how such was concluded. I remember Eric telling us that Turbo Todd uses the racing belt to drive stiffer springs and hotter cams in his 1000 bhp brute - I suspect TT probably uses the stock tensioner but maybe Eric can chime in on that one. I know GB uses the stock tensioner on his torque monsters- so how exactly can it be concluded that the stock tensioners do not work?
Maybe they do not work as well as your PKT units [I have no clue on that one] but that does not qualify the stock item as a disaster. That the community has an option at one third the cost of the stock setup is something the community should be very thankful for as the stock items wear and lose their effectiveness with age. Maybe there is a good reason to use the racing belt after all if the stock tensioner can handle it? For sure it will resist the pulses that induce flutter.
Hopefully the above may induce some interesting technical thoughts but please- no PKT bashing there has been way too much of that!
Last edited by FredR; 07-23-2023 at 01:48 PM.
Reason: 1000 BHP brute
The Audi unit strictly speaking is not a "hydraulic tensioner" but indeed they are called such! - In reality the Audi unit is a spring tensioned hydraulic damper and strangely enough so is the Porsche 928 tensioner. The cam followers are hydraulically tensioned not that it matters much.
Audi introduced their tensioner in 1994 Porsche deployed their tensioner in the first 928's delivered back in 1977 or whenever and they were granted a patent in 1980 - if it was a straight forward mechanical tensioner why would they be granted a patent?
The Belleville spring is very much what its name suggests it is a spring but it is oh so much more than that if you did but know it. It will compress to something like 95% of its conical height without deforming but as you noted stick it in a vice and flatten it and it will be well and truly buggered. Remember you stated the other day that when at temperature the pack is flat and cannot compress and quite understandably that reflects your thinking on the process but as I demonstrated the cone height is approx 0.8mm at zero C, at 100C the cone height reeduces to 0.6mm, this will likely compress down to 0.4mm when tensioned [my estimate] and that leaves about 0.35mm for further compression so not difficult to visualise that it can and will oscillate around an envelope of plus and minus 0.2mm and up to 0.35mm were it needed but I doubt they would push the envelope that far..
Now you will probably ask [as would most on ths list] what is the relevance of the above? It would seem from your dialogue that you expect the damper to take out the amplitude of the flutter but as I understand that is just not the case. Think about it- we have an engine travelling at 6k rpms or 100 revs per second. The pulses are created by the cams creating resistance as the cam lifts and then motoring the camshaft as the valve springs help drive the camshaft on its decline. The 928 is an eight cylinder engine so for every revolution of the camshaft there are 8 cam events- 4 induction and 4 exhaust giving a total of 800 pulses a second. Flutter is a resonance effect - to eliminate flutter the traditional approach was to use a mechanical tensioner. When the 928 was introduced as I recall it was the longest ever cambelt put into production. Did American Vee 8's have a cambelt- err no they had pushrods. Did Ferrari and Lamboghini have cambelts?- as far as I know they had camchains. The more cylinders that are pulled the more tensile load on the cambelt and ever more superimposed load had to be applied to stop it fluttering. How do you reduce the load? - you fit a damper.
The entire idea of a damper is to stop resonance taking place in the first place by bleeding off the out of balance energy that the belt itself cannot absorb. The notion that the damper has to absorb pulsations at those frequencies I find somewhat amazing to say the least but who knows maybe I have that one wrong..
The thing that most folks on the planet do not know is that Belleville washers make very good dampers thus by a very clever piece of engineering Porsche managed to combine a tensioning device that could self compensate for temperature, tension the belt and dampen it. The oil of course helps transmit the energy away from the pack. Then of course they modified the design and in 1983 were granted another patent for their revised design that incoiporated a check valveand possibly some additional porting so that the pack could presumably pump the oil around the body of the tensioner to help bleed off energy. Whether or not this was connected with introduction of the HTD sprocket design I know not- my guess is that they felt it was a bit marginal and needed beffing up a bit but whatever the reason the upgraded version is still available and as far as I know the original version is not but the revised version can be used with the original sprocket design.
As to whether flutter is a problem with the stock item all I can say is that it would be nice to see the evidence and how such was concluded. I remember Eric telling us that Turbo Todd uses the racing belt to drive stiffer springs and hotter cams in his 100 bhp brute - I suspect TT probably uses the stock tensioner but maybe Eric can chime in on that one. I know GB uses the stock tensioner on his torque monsters- so how exactly can it be concluded that the stock tensioners do not work?
Maybe they do not work as well as your PKT units [I have no clue on that one] but that does not qualify the stock item as a disaster. That the community has an option at one third the cost of the stock setup is something the community should be very thankful for as the stock items wear and lose their effectiveness with age. Maybe there is a good reason to use the racing belt after all if the stock tensioner can handle it? For sure it will resist the pulses that induce flutter.
Hopefully the above may induce some interesting technical thoughts but please- no PKT bashing there has been way too much of that!
Well, Ken may not say it but I will. That is so much BS about whether the devise is mechanical or hydraulic, that I cannot believe it, from Fred.
I do agree with FredR, with the below clarifications.
I do enjoy your comments Fred. Realistically, I think Porsche came up with another Patent opportunity, for a spring loaded tensioner. As you identified, the pulses from the cam train are 8 times the rotational speed. (per revolution, 4 intake valves and 4 exhaust valves) I wanted to add that the cam pulses are linear with rotational speed, and there are additional impulses from the combustion cycle, at 4 times per revolution, on the crank gear, which is engine load dependent, so the amplitude varies with engine load All these varying loads are accommodated by a spring tensioner and belt E.. Ultimately the belt does the heavy lifting to absorb all the primary impulse inputs through the elasticity of the belt.
A couple of tactical comments:
1. My 924 Audi motor does not use any spring belt tensioner, strictly an idler pulley to set cold tension. The belt works within the Elasticity of the belt design.
2. The Bellville washer spring description of characteristics, see https://www.engineersedge.com/belleville_spring.htm
3. There is some positive temperature correlation, relaxing the pressure as temperature rises.
4. There is likely some Hysteresis damping within the Bellville stack, with the resultant heat dissipated through the oil. Based on the bellville springs sliding in micron movement, disk on disk.as they deform under load
5. Porsche limited the flutter by adding rollers, as possible (within mechanical space along belt travel), to limit the amplitude of the flutter. We all have experienced resonant frequency issues, such as why tires imbalance peaks around 60 MPH, because the suspension has fundamental frequency points, just like a guitar string. Recognizing the inherent leverage the flutter has along the total unrestrained length. Back to the guitar, if we shorten the unrestrained string length, we change the fundamental frequency, or increase belt tension. (belt tension has limits based on belt construction, I suspect the racing belt has much higher E, spring modulus of Elasticity) But the more important part is the belt has mass, so the flutter is swinging a mass, only limited by the unrestrained length of belt, hence the limit rollers. But, when you view Ken's videos there is no perceptible movement in the tensioner, flutter or not. It would be of value to determine if there is any movement of the tensioner stack. Based on what I see, the answer is NO, in micro time. For there to be hydraulic action, there must be motion.
(Note: I have just scratched the surface of this issue, way more detail to be discussed)
My telehandler has at airtube clamps the same mechanism.
Looks like washers who compensate the effect of heat
Airboost tube
The type of clamp fastening that tube is what is known as a constant tension clamp and the only thing it can respond to is changes in ambient temperature - why the designers felt they needed such on a piece of hose like that i have no idea- it has Belleville washers and the ones fitted to that clamp respond to ambient temperature changes whatever it is they are made from. Note how the washers are fitted in series to give more travel and less tension.
Good to see the lights coming on between your ears!
I first saw the cambelt tensioner around February 1999 when the local main agents checked the belt tension and adjusted it a tad. Not too surprisingly my first inclination was that it was a mechanical tensioning device and nothing more than that. Wind the clock on a few years and my immaculate 90 S4 was wrecked in a big crash caused by some dickwit on a mobile phone [not me] and that caused me to purchase my current 928 GTS with the TBF'd engine. We stripped my 90S4 for parts harvesting and pulled the original motor out of the 93 MY GTS for analysis. The GTS motor was completely trashed with cracks in the main bearing webbing and of course I had a spare tensioner to play with.
I pulled the tensioner apart and was amazed at what I saw - there was a concentric annulus, there was a check valve in the base and it seemed pretty clear to me that this had to be a spring tensioner with a hydraulic damping capability. I did some research into how the Belleville pack worked, did some simple tests to prove that the pack did in fact expand and contract with heat and sure enough it did. Belleville packs are normally configured either in parallel to increase the spring rate or in series to reduce the spring rate and increase the deflection capability. In my later type tensioner the Belleville pack is configured as 7 packs in series with each pack consisting of 5 springs in parallel. The only reason I could think of as to why 5 washers were stacked in packs like that was to facilitate damping. Thus as you queried "what motion are they damping?"- it seems to me that there is an expectancy that these things have to swing about in the breeze but that is just not the case- I reckon those packs can absorb significant amounts of energy with very little movement because of the nature of the beast- think of it as a vibration if you like the point being that if it stops flutter it is doing its job. Excessive flutter causes premature belt failure- I have never seen or heard of such "mysterious" failures in a 928.
To my way of thinking the only real world difference between the stock tensioner and the Audi unit is the latter is self adjusting - I have no reason to believe one is better than the other- hopefully they both function adequately. My expectation is that they both have to apply equal amounts of tension or at least something in the same ballpark and I suspect we are talking about numbers in the 10's of KgF range as in 20kg, 30kg - whatever.
To my way of thinking the only real world difference between the stock tensioner and the Audi unit is the latter is self adjusting - I have no reason to believe one is better than the other
I think that is less a tensioning difference, than given the obvious next question that Jim brought up with his disassembly photo...is HOW they dampen differently.
Which is..again...related to how the OE belt compared to the gates racing belt, react differently as one part of the entire system. (Less stretch = less compliance to absorb those cam/crank shocks).
07-21-2023, 03:54 PM
