Mike Frye

Mark, I don't think he's saying your concerns aren't valid, I think he's saying that he's done the research required to satisfy himself that his engine will be safe to run with this setup.

Only time will tell if it works or not. I doubt he plans to 'take it easy' on the thing just to prove a point, and the proof will be, as they say in the pudding. (or in the tasting, or something).

Anyway, I think the title of the thread says it all: 'One man's tensioner solution'. If it turns out to be a true solution, others may follow.

I, for one, think it's brilliantly concieved and if he's going to be his own test case, I say go for it!!!

leperboy

I don't have any comment about the Volvo tensioner, but...

I think Mark makes a good point that I see confusion about regarding the tensioner. Most newer tensioners are spring-loaded, but the 928s isn't. I think people instinctively think the 928 tensioner works as a spring. But really it works more like a contstant pressure on the belt. It is a shaft acting on the roller, not a spring. In effect, the length of the rod is supposed to vary with engine temp to match the expansion of the engine when warm.

Any spring-loaded system is not constant. It will react differently. Regardless of spring rate - which can be assumed to be linear in the tiny range here - it is a spring and does not apply constant pressure. Its pressure will vary depending on belt movement.

An ideal solution might be a liquid-filled shock where the liquid expands and contracts in direct proportion to engine expansion and contraction.

Matt

blown 87

Mark, the Belleville washers are springs, just a different type.
They are used in many different applications, here is one for a recoil reduction stock on a competitive shotgun (Perazzi MX2000 with a PFS stock). These give a couple of inches of travel.

When you say the stock type is not spring loaded is just not true.


Pics






here is a link to some spring washers.

http://www.mubea.com/tellerfedern/?k...JeXRptgbZELaVS

mark kibort

but why would you design a tensioner on a 928 that had compliance, due to the fact that failures do happen, and they could be due to the little compliance the belt has inherently? i know people that have lost engines due to belt failure. most have been neglect, some still a mystery. what we do know is that a loose belt can cause failure. remember my last Speed GT race, in qual, i got one lap and at 6000rpm, i had a real bad missfire. belt slipped 3 teeth. tension was fine. so, what is the issue here? oil, belt worn, etc etc??? what i do think, is that it could be a fluke thing. i changed the belt, tensioned it up slightly higher than spec, or the high range of the spec. and havent seen a problem since. water pumps could introduce play as well. Lots of factors and this design seems to introduce one more variable, that i was asking about. do we know the forces of the belt at different accel rates? are those rates similar to the forces of the system that this design came from (ie volvo)?
if the belt can act on the spring (which it can, because thats the design), will it creat slack in the system? or does a spring keep the tension better and counteract the compliance of the belt itself.
these are the questions of the design i have. It could be a revolutionary design. but then it could also be a time bomb. im just asking the questions and making the comments because it looks scary at first glance.

mark kibort

At matt says for our 928 design, they are not being used as a spring, and to my comment, for all relative comparisons, it is not a spring design. The spring shown has a spring rate that is much different than a set of washers.

i think the only analogy i can think of is the chassis in our car. It is not a spring like our suspension but it does act like a spring in extreme cases.

mk

blown 87

Mark, so what you are saying is they have a spring in the design, but they are not using it as a spring. Is that what you are saying?

If they did not want it to be a spring they would have put all of the washers in the same orientation, not in stacks turned in different directions.

If you do not think that a stack of Belleville washers is a spring, then no facts will convince you.

leperboy

Going back to basic physics: A spring can be assumed to have a force of X lbs/ft times the distance it is compressed. I assume a linear spring rate.

When the belt is tensioned properly, the spring would be compressed X feet (or mm) giving you the proper tension.

The problem might be that as the engine expands, the spring crompresses more, applying even more pressure and possibly putting you out of proper tension. If the belt expands over time, the spring lengthens and the pressure it exerts is less, exacerbating the looseness.

The end result could be a system that is really hard to keep properly tensioned. But, I don't know the spring rate and the travel distances, so I don't know that. It's just theoretical. The Belleville washers ensure that pressure stays the same during engine expansion and contraction. They may have spring qualities, but that is not their application in the tensioner. They are used for the expansion and contraction.

Mark commented on resonation in the belt and spring, but I think that should be minimized by the idler roller. It may not be a signficant concern. But there is still the fact that sudden looseness or vibration in the belt may be exacerbated by a spring, but not a constant pressure.

Having said that, there must be a better way to skin this cat and I applaud anyone who takes a knife to it. I'm too chicken to try myself, so I hope I can benefit from what others do.

Matt

blown 87

Well said.

the flyin' scotsman

Hammer..........there are many of us believe the Porsche design can be updated/ bettered and most of us are not racing our cars. Perhaps thats why Porkens design/invention is sold out.

After all, if we all remained status quo then the world would have remained flat and there would be no Guinness in North America to celebrate St. Patricks day.

cheers and good luck

leperboy

They are placed in opposing stacks so when they heat up or cool they either:
- flatten and pull closer together (compensating for engine expansion) or
- dimple and push away from each other (compensating for contraction int he engine).

They are convex metal, so they have spring qualities, but I think that is negligible and not their purpose.

Matt

PorKen

These bi-metal Belleville washers are really only springs only when cold.

AFAIK, besides the increased danger of skipping a tooth, the WSM always cautions to turn the engine CW to protect the tensioner washers from being squashed.

blown 87

Ken, to state that they are springs only when they are cold, come on.

I can not think of a engine with a t-belt that the manufactures do not warn about turning backwards.

Maybe the metals they use in the washers in the tensioners are not up to being compressed, that I do not know.

But in the case of the stock pictured above, I will promisse you they will work when very hot, to hot to touch and I have flatned them out well over 100,000 times on this stock alone.

Mike Frye

I think the WSM is referring to the cam chain tensioners, not the belt tensioner.

mark kibort

No, as matt says, having all the washers aligned, then it couldnt compensate for temp expansion and contraction.

All you need to do is see what the spring rate of the washers is. Ive tried to compress them and they are solid for all practical purposes. Give me that spring and I'll tell you how much force is needed to compress it .75mm. I'm sure its an order of magnatude less than the washers.

now, if the engine was going the other direction , the spring tensioner may make a litle more sense to me. what side of the crank was this "volvo" designed used on?

it could work, i just get a little nervous thinking about a little movement that can be introduced, could cause a failure or slip. since the belt is running CW and the crank pressure is on the CW side of the belt teeth, as well as on the cam pulley. the slack is taken up by the tensioner roller . I guess if the spring tensioner can keep the slack constant, there should be no issues.

again, it makes me nervous thinking about a spring, where basically a rigid shaft would be.

Mk

blown 87

http://en.wikipedia.org/wiki/Belleville_washer