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Date:         Fri, 2 Apr 2010 12:11:47 -0700
Reply-To:     Alistair Bell <albell@SHAW.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Alistair Bell <albell@SHAW.CA>
Subject:      Re: more VC obsession
Comments: To: mark drillock <mdrillock@cox.net>
In-Reply-To:  <4BB63AA2.5050506@cox.net>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed

Hi,

oh I agree about the fill quantity affecting agressiveness of coupling.

but don't you wonder how overall pressure increase affects the plate to plate contact in hump?

I mean, each side of the plate is exposed to the same pressure increase, why would the plates move closer unless there is a differential pressure? i believe, as illustrated by the diagrams i just posted, that it is localised pressure differences that initiate hump.

Back to fill quantity. The air left in the housing will not be one "bubble" at the top, but it will dispersed throughout as tiny wee bubbles. This, i believe, is the key. The amount of air left in the vc will determine the amount of tiny wee bubbles throughout the vc. Then you can see how localised pressure differences between plates can be moderated by the air bubbles right in that region. (geez, not comfortable with the term bubbles, its more like a air in silicone emulsion, and emulsion is not the right term either).

The more i read, the more I believe that the hump condition is *the* torque transfer mechanism when the vc being called upon.

One more thing,I like this discussion but sometimes it is hard to convey ideas and concepts through email.

Alistair

On 2-Apr-10, at 11:42 AM, mark drillock wrote:

If you look at Table 2 in Dr. Mohan's SAE paper titled "Comprehensive Theory of Viscous Coupling Operation" you can see that the initial fill ratio of the special fluid inside plays a huge role in the onset of the hump state with just a 1% or 2% change in fill having a large effect. This implies to me that overall pressure in the unit is a predominant factor in VC performance. In fact around the world replacement VC offerings have included options for fill capacity to tailor to specific needs.

IMHO, in the distant past there have been some false assertions on the Syncro list that led to misunderstanding of the VC behavior. Notably this involved an assertion that the torque level involved in the hump phase was higher that the engine was capable of and thus that hump phase was not important to VC operation. This error arose from a failure to account for the torque multiplying effects of the transmission in the lower gears.

Mark


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