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Re: Re: Dampers and Rough Roads.

Ronin:

Resonance? Sure! Dampers (whether external or inherent to the system like cast iron is inherently damped) in a system ensure the resonant response is not infinite. An undamped system, such an empty glass to an opera singer would resonate itself apart. Similar problems lead to damping of cranks, which operate through their resonant frequencies. Without a damper you'd better stay far away from the resonant frequency or else!

Neil Deshpande

***

Quote:
Originally posted by ronin
correct. so, since we're on the topic of oscillations and one-dimensional stresses and strains, how about we shift this sucker into high gear and expand into resonance...

wonder what kind of cool graphics I could come up with analogous to Galloping Gertie

Brit, I was serious about you using that pic as your av! and don't be surprised if your latte next week is a bit 'bitter'
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1987 944 NA 5-sp
Old 12-09-2003, 06:34 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #41 (permalink)
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Re: Re: Damper, Balance Shaft Function, Bounce Test, etc.

MrPants:

Wow, talk about OT discussion!

Any crank and slider mechanism is going to have some side-to-side motion of the conrod.

Visualize it ... the piston is going up and down and the crank is going round and round. The conrod converts the round-and-round to up and down. So, one end of the conrod goes up and down and the other end goes side to side (as well as up and down). Therefore, the conrod is wagging around a bit, right? Bah, those words sound horrible and untechnical! No matter, I think this is easy enough to visualize.

Anyway, this conrod motion is what happens at twice-crank for a 4-cyl motor. Since there is no other element in the engine to cancel it out you need to add a balance shaft ... or two.

Let me find a good site and post it. One of my weaknesses is I'm a bit too mathematical (part of my Indian too-theoretical education, I'm afraid) so I find it hard to provide good everyday analogies at times. In fact, the vibration from a conrod is like an infinite series with 2nd, 4th, 6th order, etc., in there. Ideally, you would have infinite balance shafts to reach the balance on a flat-four, inline-6, etc., but that is impractical. The series drops off in strength with order so if you take care of the 2nd order, the bulk of the vibration is eliminated.

What you say about inertia and such is true, of course! It is a comfort and refinement issue, AFAIK, but low vibes are good for all machines anyway.

Neil Deshpande

***

Quote:
Originally posted by MrPants
interesting. i tried to find some info about second order vibrations but i was unable to find much. why is the frewuency of the vibration twice that of the crankshaft? i didnt even realize this was a concern. is it a performance or a comfort concern? it seems that balance shafts, to funtion, would add inertia which isnt desirable, correct?
i did find some cool pictures though
some dampers for the engine below, more like motor mounts than balnace shafts though

and this giant engine produces 15710kW or 21058 horsepower!
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Old 12-09-2003, 06:41 PM
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Re: Re: Re: Damper, Balance Shaft Function, Bounce Test, etc.

Quote:
Originally posted by neildeshpande
Let me find a good site and post it. One of my weaknesses is I'm a bit too mathematical (part of my Indian too-theoretical education, I'm afraid) so I find it hard to provide good everyday analogies at times. In fact, the vibration from a conrod is like an infinite series with 2nd, 4th, 6th order, etc., in there. Ideally, you would have infinite balance shafts to reach the balance on a flat-four, inline-6, etc., but that is impractical. The series drops off in strength with order so if you take care of the 2nd order, the bulk of the vibration is eliminated.
ok i see the force of vibration is directed pi/2 rad away from the direction of amplitude. so it must increase with amplitude and frequency, right? is there a simple formula to express that relationship, or to calculate the force of the vibration? no need to dig around for it if you dont have it handy.
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Old 12-09-2003, 10:35 PM
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Re: Re: Re: Re: Damper, Balance Shaft Function, Bounce Test, etc.

MrPants:

I'll dig around a bit for the equations for the crank-and-slider mechanism today and post a link or something. I know there are several very nice ones on the Internet.

As for force in general, F=ma (Newton's First Law), and as per the equations your quoted on page 2 of this thread, a=-omega2*y and m is fixed.

So, force is in a direction opposite of displacement (amplitude is max disp and fixed while displacement, which is the same as distance from the mean position but with direction information as well, varies with time) and proportional to frequency squared times amplitude. That is general for all simple harmonic motion.

So, F=-m*omega*omega*y, where
m=mass
omega=angular frequency (for an engine=RPM/60)
y=displacement from mean position

Neil Deshpande

***

Quote:
Originally posted by MrPants
ok i see the force of vibration is directed pi/2 rad away from the direction of amplitude. so it must increase with amplitude and frequency, right? is there a simple formula to express that relationship, or to calculate the force of the vibration? no need to dig around for it if you dont have it handy.
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Old 12-10-2003, 02:34 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #44 (permalink)
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MrPants:

That would be Newton's Second Law. Sorry.

Neil Deshpande
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Old 12-10-2003, 03:27 AM
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