[DeTomaso] Here's a weird one
JEFFREY COBB
zumzum at cox.net
Sun Nov 6 08:48:22 EST 2011
Hey guys,
All I was saying is that when parasitic and torsional loads are removed then an engine can and will be more productive and live longer. A crankshaft spin path is one of rotation and orbiting due to oil clearances, it orbits while dealing with massive input and output loads. So a crank has the ability to orbit off center what ever clearance it has to its main bearing and in a pattern that is defined by how many cylinders. A star shape load that in a Pantera will be eight pointed. I believe that I have seen frontal main bearing top half surfaces worn more than the other bearing shells in my 30 plus years of owning a repair shop and I think this is because of the increased upward tension of timing belts, overtightened belts and timing chains set to tight. These all pull up ever so slightly the bearing free play of # 1 main cap and in effect to a very small degree cause the crank to rotate under bind and with the crank center front pointing upward.
Remember the old days when we had multiple belts, the alternator belt was the first one to wear out or break because it was the one with the highest load fluctuations even though it's width was the same or 1/8" thinner than the a/c belt. Remember the double alt belt setup of the Benz's of the 60s, 70s and 80s for longer life and they only used a 35, 60 or 80 amp alt. The p/s was a soft load input and the compressor had a slipping mag clutch with rubber sometimes so it's load was not a sharp input. The alternator has much more of a dramatic load demand and it tends to run 3.5 to 5 times the crankshaft speed. Most alternators max rpm is about 15,000 rpm. So a high inertial load would cause high peak loads in a opposite rotational load direction than the wavering crank rotational moments. I believe that this sprag allows the crank pulley to be freed up from negative energy inputs.
Here is part of an old VW diesel site thread. Hope my input was worth it?
Take care,
Jeff Cobb
If we study harmonic balancers, they are there to mitigate torsional load seen along the crank, flywheel and main trans shaft, whatever is causing this additional stress (remember the VW 1.6 TD, and the 1.6 NA applications did not have these problems, so we might assume there was less stress there) might help create some early failures in the crank shaft, or other elements tied to it. So here’s what I find interesting, VW group added an over run clutch to the alternator and it is integrated into the pulley of many of its other group cars! What’s fascinating is the use of this Sprag clutch has found it’s way into other German cars, including all Porsches!
Hi George... happy to clarify further if I can... some thoughts below:
>The one way, or sprag clutch alternator pulley is the very first I've
>ever heard deployed anywhere to solve a problem like this.
On the early days of their serpentine belt implementation on the AAZ engine (MK3 chassis 1993+) VW used a standard (solid) alternator pulley, meaning that the alternator is directly connected to the crankshaft drive pulley via a serpentine belt.
This engine (and its gasser cousins the ABA and VR6) unfortunately became a bit infamous for wearing out the crankshaft sprocket, key, and keyway. It's generally accepted that a large contributing factor to this problem was that the alternator has a small pulley and a lot of angular momentum; during deceleration it tends to want to overdrive the engine, putting large stresses on the crankshaft drive pulley (and the keyway) therein in a direction opposite to normal. Over time acceleration and deceleration results in the crankshaft sprocket rocking loose... on a diesel the result can be catastrophic. The small pulley on the alternator exacerbates the problem (over previous designs) by increasing the alternator RPM and giving it a mechanical advantage when overdriving the larger crankshaft pulley.
At some point just before they moved to the TDI engine VW switched to a 1-way ratcheting alternator pulley (which they continued with on the TDI engine). This pulley allows the alternator to automatically de-clutch from the drivebelt and freewheel on its own... preventing the alternator from applying reverse torque to the crankshaft pulley when the engine decelerates.
On Nov 6, 2011, at 5:05 AM, Roland Jaeckel wrote:
> There is no need to disconnect to save fuel. As long it takes no load, the
> power to drive it is close to zero.
>
> Roland
>
> --------------------------------------------------
> From: "Tomas Gunnarsson" <guson at home.se>
> Sent: Sunday, November 06, 2011 11:55 AM
> To: "Roland Jaeckel" <pantera874 at t-online.de>; <detomaso at realbig.com>
> Subject: RE: [DeTomaso] Here's a weird one
>
>> Thanks Roland. That explains a lot, among other things that it doesn't
>> disconnect the alternator drive to save fuel. That would require a
>> different
>> approach.
>>
>> Tomas
>>
>> -----Original Message-----
>> From: Roland Jaeckel [mailto:pantera874 at t-online.de]
>> Sent: den 6 november 2011 11:36
>> To: Tomas Gunnarsson; detomaso at realbig.com
>> Subject: Re: [DeTomaso] Here's a weird one
>>
>>
>> Thomas,
>>
>> here is a good explanation:
>> http://www.litens.com/vibprod1.cfm
>>
>> Roland
>>
>> --------------------------------------------------
>> From: "Tomas Gunnarsson" <guson at home.se>
>> Sent: Sunday, November 06, 2011 10:55 AM
>> To: <detomaso at realbig.com>
>> Subject: Re: [DeTomaso] Here's a weird one
>>
>>> I can understand that an on/off clutch can do that but what I know as a
>>> sprag clutch can in this application either be in:
>>>
>>> A: the locked position where the pulley drives the alternator
>>> or
>>> B: the freewheeling position where the alternator turns faster than the
>>> pulley
>>>
>>> How do sprag clutches work according tou you?
>>>
>>> Tomas
>>>
>>> -----Original Message-----
>>> From: JEFFREY COBB [mailto:zumzum at cox.net]
>>> Sent: den 6 november 2011 01:17
>>> To: Tomas Gunnarsson
>>> Cc: JEFFREY COBB; detomaso at realbig.com
>>> Subject: Re: [DeTomaso] Here's a weird one
>>>
>>>
>>> The sprag is used in many German cars. The clutch removes most parasitic
>>> drag and vibrational periods of the alternator for fuel economy reasons
>>> and
>>> belt life. The alternator can now keep up a higher speed.
>>> ---------------------------------------------------------------------------------------------- On Nov 5, 2011, at 6:12 PM, Tomas Gunnarsson wrote:
>>>
>>>> When's a sprag on an alternator pulley supposed to be of use? To allow
>>>> the
>>>> alternator to keep spinning when you turn the engine off?
>>>>
>>>> Just asking...
>>>>
>>>> Tomas
>>>>
>>>> -----Original Message-----
>>>> From: detomaso-bounces at realbig.com
>>>> [mailto:detomaso-bounces at realbig.com]On Behalf Of Will Kooiman
>>>> Sent: den 5 november 2011 03:32
>>>> To: detomaso at realbig.com
>>>> Subject: [DeTomaso] Here's a weird one
>>>>
>>>>
>>>> My wife called me last week while she was on the way to work.
>>>>
>>>>
>>>>
>>>> She said she heard a crunch, the power steering went out, and all of the
>>>> lights on the dash came on. I'm assuming it was more than just the
>>>> check
>>>> engine light, but I didn't see it so I can only go by what she
>>>> described.
>>>>
>>>>
>>>>
>>>> The same thing happened to me in her car 2 years ago. The water pump
>>>> cracked badly around the bearing. It dumped water on the ground, plus
>>>> it
>>>> made the serpentine belt pop off. $800 later ('00 Boxster-S), and it
>>>> was
>>>> all better. I figured the same thing had happened this time.
>>>>
>>>>
>>>>
>>>> Not so. This time it was a failed alternator clutch. Yep - not a typo.
>>>>
>>>>
>>>>
>>>> Her car has a sprag clutch on the alternator pulley. A sprag is a one
>>>> way
>>>> ratcheting bearing. It free wheels in one direction only. The clutch
>>>> had
>>>> been locked up for who knows how long. The failure was the inner shaft
>>>> walking off of the sprag. It eventually walked enough to make the
>>>> serpentine belt pop off.
>>>>
>>>>
>>>>
>>>> It's a good thing it wasn't the water pump. I wasn't looking forward to
>>>> another $800 repair bill. This time it was only $440 - not counting the
>>> tow
>>>> truck.
>>>>
>>>>
>>>>
>>>> I'm going to try to talk her into a Mangusta.
>>>>
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