Cam Belt-Drives

[saltfever]

If one was to believe advertising you would think that a belt drive was a magic excelsior to dampen all the bad harmonics coupled to the cam. Not being an mechanical engineer, it sounded like a plausible solution. However, as a technocrat suspicious of unsubstantiated advertising, I am always looking for good data proving the supposition. I am still neutral about belt drive claims. Testing and engineering analysis to prove belt-drive claims is non-trivial and expensive. In Experimental Aviation a belt-drive-prop-reduction-unit has always claimed, among other things, to be beneficial due to its dampening characteristics.  So, in absence of good data, it is refreshing to see an interesting article that appears to debunk the status quo.
http://www.eaa.org/experimenter/articles/2009-10_beltdrive.asp

Knowledge is horsepower. Too much is a good thing

[krusty]

I'm not an engine builder, nor do I wish to be one. That said, I think you are painting the automotive camshaft belt drive with a fairly wide brush when you state   " If one was to believe advertising you would think that a belt drive was a magic excelsior to dampen all the bad harmonics coupled to the cam."   Which belt drive manufacturers are claiming this?  I thought that the belt drive was developed (by Jesel, for one) to solve the problem of failed roller chains, primarily in endurance racing. Some dampening benefits were probably achieved, but I don't recall this as being the primary thrust of the advertising. I hope some engine builders will chime in on this - dynoroom?    Wayne Jesel?     I for one am Dodge happy that our record-holding 303" Chevy (d/grmr) has a belt drive.  I also do not believe that a prop drive (yikes! propster?!) and a cam drive are an apples to apples comparison.  YMMV      vic 

[desotoman]

Here is some other food for thought.

Tom G.

https://www.gates.com/file_save_common.cfm?thispath=Gates%2Fdocuments_module&file=MTZ.pdf

[saltfever]

Thanks, Tom for that great link. I am going back to read it a few more times. I note that the acoustic test (chain vs. belt) was done on an electrically driven motor and they say if it was done with internal combustion the amplitude would be less. Also, in paragraph 3.2 they describe the test setup but provide no quantitative results. The results are mentioned in the acoustic test done with electric drive. Your link is a good start. Also the reference might prove to be interesting. Thank you.

[saltfever]

Krusty: I wrote this before Tom posted the link above. I agree with you that current adds seem to minimize the claim.

My post was not about disparaging anyone. In fact, Wayne is a wonderful supporter of LSR, a highly esteemed competitor, and an astute businessman. He is a great guy and has even helped me with some tech questions while waiting in line at the Red Flame. I assumed that this thread might reach him.

As mentioned in the link, sometimes a supposition can be mentioned so many times in literature over the years that it becomes an assumed fact. The harmonic dampening phenomena has been mentioned so many times in magazine articles and adds it isn’t even disputed. Perhaps, since you are not an engine builder, the phenomena is not a focus for you or even an important issue to discuss. There are some wonderful claims about durability, changeability, precision, that appear to be reasonable. I thought the link might be interesting for those constantly filtering the noise around us and seeking a balanced opinion. Reference, throughout the years are too many to list. Magazines seem to just rubber stamp the issue. But for your viewing pleasure here are two current links from the manufacturers.

From Jesel:
http://jesel.com/index.php?categoryid=9
Patented High Torq Drive™ reinforced belt operates dry and spins with less friction than timing chains or gear drives and also absorbs harmonics.

From Comp Cams:
http://www.compperformancegroupstores.com/store/merchant.mvc?Screen=PROD&Store_Code=CC&Product_Code=6100B&Category_Code=TMNGBLT
Absorb crankshaft harmonics to guard against valve train instability

I agree with you that aircraft and cars are two completely different environments. And your point is important in that issues from one environment can’t be assume in a different environment. But the point was simpler than that. We are talking about coupling two inertias together with a belt. Of course, mass and stiffness will vary all over the place. And the extent that a belt couples or decouples unwanted harmonics is interesting. There are a lot of claims but no quantitative data.

[Speed Limit 1000]

Maybe Jon Amo from Gates will give some input

[John Noonan]

Maybe Jon Amo from Gates will give some input

Amen...

[SPARKY]

all I know is the more I beat on stuff, the more I tear up

 and in LSR it sure is easy to tear up stuff in a way that it can be a long time before you come up with the right question to the right person who then MAY share with you what he heard about someone else's problem and how they solved it!!!!!!!! 

I believe More STUFF than I use to ever imagined can, (and probabaly does) set up harmonics, bend, break--distort due to the slow rpm acceleration rate of our Eng when we get in the top gear and our Eng pulling their guts out

[John Burk]

The drive torque graph at the link Tom posted is true for idle . At high rpm the springs on the closing valves are too busy overcoming inertia to return the energy it took to compress them . The lobe on the up slope has to compress the spring plus overcome valve train inertia . The cam drive torque increases with rpm . Anybody want to help me with what the sine wave would look like at valve float rpm .
John

[Kato Engineering]

I use a lot of them on the  drag race stuff

it really depends on the weight of the parts and the forces involved, common V-8 is popular,
 but on a odd engine or a V-6, I would not!!! too many odd forces and imbalanced loads with no controll of "wrap up" and "unloading" with those engines.  That is probably why they ran gear drives back in the day of the Buick stage III engines.

you also have to be concerned withdirt / rocks getting inside

you also have to be concerned with the fact that at bonneville the rear tires unload and then grip, unload and then grip,
this could cause weird things to happen with the cam pulses to damage the belt.

[maguromic]

. Anybody want to help me with what the sine wave would look like at valve float rpm .
John

John, I can help you with that.  I need to have the cam profile lift table data in one degree steps, the weight of the parts, the spring data, and the stiffness of the pushrod if it has one.  PM me the info and I will put a chart together for you.  Tony

[saltfever]

Tony, you can do that?    Is that done in the FEA part of your CAD program?

[Glen]

google cam drive belts, there is some interesting stuff on there as well.

[saltfever]

edit  . . .  At high rpm the springs on the closing valves are too busy overcoming inertia to return the energy it took to compress them . The lobe on the up slope has to compress the spring plus overcome valve train inertia . The cam drive torque increases with rpm . John

Interesting, John. To simplify your point in terms I understand: In a two valve V-8 you have 8 valves opening and 8 valves closing. At idle or static the torsion input to the cam is about equal or non-cyclic. However, as RPM increases, the inertia loads impart a cyclic torsional input.

Can we say the stiffness of the coupling driving the cam will influence the amplitude and frequency of the torsion input if the coupling absorbs some of the energy? Some of these belts have Aramid, steel, or other stiff members in the composition. If the belt stiffness does not yield under the conditions, is it fair to say there is no difference between belt or steel?

[desotoman]

Here is a blower belt on a TAD dragster, run with a single and duel idler pulleys.

Tom G.

http://www.youtube.com/watch?v=TD9EdtaBdXw

The belt is as tight as we can make it. Before we start the car it feels like a rubber band! This video demostrates how the minute start/stop pulses of the engine combined with the 'flywheel' effect of the supercharger create slack and tension, which eventually break the belt.