Cam Belt-Drives

[saltfever]

Neat video, Tony. . . sharp people are always collecting data.  I see similar slack in chain drives on the non-drive side. It is the drive side of the coupling that transfers the harmonics. I wonder, when a belt is pulled "stiff" and tight, if it is as friendly as most assume.  

[maguromic]

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

Because I have to use a mesh model I don't use a FEA program to do this. The big problem I see in using a mesh model is it makes approximations in the cam lobe, and I don’t think that it would be that accurate doing that. It was a lot work but a friend of mine wrote a program in VB that uses the math model.  It still takes a lot of time with all the numbers but I think it’s more accurate.  Tony

Neat video, Tony. . . .  

 
Tom posted the cool video link.  Tony  

[jackson]

I never found FEA to be very useful on synchronous drives.  I think its the old garbage-in-garbage-out syndrome.  It wasn't for lack of trying though.  I learned more from load sensors and strobe lights than I have ever been able to figure out from anything else.

Just my two cents here, the modulus of modern synchronous belt is VERY high.  The only real advantage I see in harmonics is that a belt drive should produce a slight reduction in the chordal effect produced by a chain drive.  Keep in mind I am no valvetrain expert, but I would assume this influence is much smaller than any input from the rest of the valvetrain.

FYI - We run a gear drive for the cam and only run belts on the dry sump and fuel pump.

[saltfever]

Jackson, thanks for mentioning the chordal effect.
http://chain-guide.com/basics/2-2-1-chordal-action.html
Just one more input to consider. I note the effect is not eliminated on belt drive only that it may be minimized.

Tom: sorry about my confusion. That is is a great video!

[SPARKY]

GEEZE
 I am living the old folk saying "the older I get the dumber I find out I was"

At the rate you guys are going,  by the time I get this new car on the ground I will be qualified         Dr. Southern Farm Boy Enginering----keep it up  great stuff 

[John Burk]

Tom the principle of valve train input energy increasing with rpm is true of any engine with valve train and I don't have numbers for a specific engine . At idle if it goes + 1  -1  +1 , at 11,000 rpm (complete valve float) it would go +2  zero  +2  zero .

With a desmodromic valve train with open and closing lobes and no spring , at idle the energy sine wave for the combined lobes would be nearly flat line at idle and at 11,000 rpm it would go +1 +1 +1 +1 .

A batter works harder to hit a home run than a bunt .

John

[hitz]

Sparky............Thats it! Dr. Southern Farm Boy Engineering! I think I already might qualify Thanks

Harvey

[Rex Schimmer]

I think that we need to seperate power drive belt sizing from drive stiffness sizing. Sizing a belt drive to carry a torque load, basically belt tensile strenght times pulley radius along with a "service factor" (dependent if the load is high shock, vibration etc) and you can come up with the proper belt to drive the load that will provide satisfactory service. Yes belt speed is also very important. Sizing for stiffness requires that you know the inertias of the various parts of the drive system and then you set a frequency that you want to design the drive for and go to it. General Electric, back in the 60s, did lots of development on machine tool drives and they came up with a method that allowed you to take all of the inertias of a drive system and "reflect" it back to the drive motor, this inertia along with combining all of the drive components stiffnesses, which can be combined by adding the inverse of the components spring rates allows you to come up with a system natural frequency. So once you were done doing all of this mathmatic "dancing" and you came up with a natural frequency for the drive system.They developed basic rules of thumb as to how much higher this frequency should be in relationship to the drive motor response ability. If you had the drive frequency 3-4 time higher that the response time of the drive motor then the system would be stable. BYW Drive response of a IC engine is pretty slow.

So after all of this "jabber" how does that apply to cam drives, my cut is that a belt cam drive is probably so stiff combined with the inertias of the pulleys and the cam shaft that the belt probably doesn't even see the cam event frequencys as anything that would excite it at its natrual frequency and the belts inherent dampening charateristics are probably a plus. Jackson hit it on the head, strobe lites, load sensors, and testing is what will tell you what is happening.

Rex

[saltfever]

(major edit . . . ) Jackson hit it on the head, strobe lites, load sensors, and testing is what will tell you what is happening.

What I find interesting is that with all the Spintrons with sophisticated analytical equipment out there; we don't see any test data on belt drives. Come to think of it . . . we don't see much of anything that a Spintron can deliver. Has any company with a Spintron delivered an SAE paper?

[jacksoni]

Though I hesitate to use the word "nascar" on this site, those guys spend hundreds of thousands looking for any tiny advantage. They all use belt drives. Whether is HP or reliability or what, seems to work, regardless of the physics/engineering- which are of course interesting. The "why" seems to be more the subject here and with good info.