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JohnD

Triumph Crank Damper Pulley Survey

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Once again, I'm asking for help from the Faculty of Sideways!  And I'm asking as a genuine student!

I retired a while ago, and since then I've gone back to college, where I'm now studying for a degree in Motorsport Engineering.     I need a research project and chose to investigate the crank damper pulleys that Triumph fitted to all the six-cylinder engines it made.   The crank damper was needed to suppress 'torsional', twisting vibration in the long crankshafts, and has a rubber layer between the inner hub and outer ring.   They were suspect even in the day, when they were new.     Kas Kastner described in his Handbooks his technique to ensure that the outer hadn't shifted, which would make the timing marks on them completely false.      Worse, a faulty damper could lead to crankshaft failure!

Today, none of those dampers are  less than forty years old, and many show the rubber to have deteriorated.  How many still work as timing indicators, let alone to suppress crank vibration?   

My study has two parts:   In the workshop, I'm building an engine rig with sensors on the pulley and flywheel to detect vibration, and I'm asking Triumpheroes about their experiences with the dampers.   Please help me by completeing the short survey questionanire I've put up on Survey Monkey?

Go to https://www.surveymonkey.co.uk/r/GPTCDSG    There are only five questions and it will take you less than two minutes!

Thank you!

John

PS I'm posting this on several websites, to catch as many people as I can but no need to complete it more than once!

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Interesting.  But I think you needed to make more effort to separate out the 2 and 2.5 as the 2.5 is reputed to be much more liable to torsion vibration related failures.

For example - I've seen a busted crank (still have the short part) - from a 2.5, but I ticked the box for Vitesse/GT6 because that's where the majority of my experience is (and only allowed 1 selection.  Broken 2L cranks are very rare.  I've also seen 2 or 3 busted small crank 1300 cranks (no damper at all) and at least two of those were at least partly due to poor quality regrinds with poor fillett radii.

You should maybe make contact with James Shackford and Andy Thompson.  James made a batch of "Shackler" (based on the "Rattler") mechanical dampers and Andy has been "testing" one in his racing 2.5PI.  I've a feeling Andy may also have a broken crank - though it had had a long hard life before it met the Shackler!

I may have a few scruffy looking relics of dampers knocking about if you want some to test (would very interesting to see how consistent they are) and I think you can buy new again now - though goodness knows whether they are worth having and they seem to list the same part for 2L and 2.5, which is not very promising.

 

Nick

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John,

You deserve a degree just for tacking the question......waterfall plots etc.  Am looking forward to seeing the finished project on here!

I think the first  harmonic resonance on 6 pot cranks is at 5000rpm.....................well above my comfort zone.

Peter

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6 minutes ago, PeterC said:

I think the first  harmonic resonance on 6 pot cranks is at 5000rpm.....................well above my comfort zone.

 

Blink and you miss it on the way up :-)

Interesting comment though, in racing terms 5K is a blip, but 5K at a sustained high speed cruise?  Makes you really want to know when crank failures occur, it may not be very high rpm's but maybe lower sustained for a longer period at a critical rpm point.

NB. I have zero knowledge of such a thing, it just made me wonder.

Alan

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This is something I will be tackling, but Holtzer Analysis hat identifies the resonance zones is not for the faint hearted!

John

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I wonder if lightening the f'wheel adds inertia stresses to the rubber insert ,eg when blipping the throttle, that the damper was not designed to embrace?

Peter

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On ‎25‎/‎01‎/‎2018 at 11:46 PM, Nick Jones said:

The resonance that dismantles 2.5s is reputed to be about 6,500.  2L will be higher.

Nick

That's interesting.  My rev counter goes red at 5500 so why would Triumph have bothered with an harmonic damper to combat resonance 1000 revs above this?

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I think that the factor that "dismantles" 2.5 is more the mean piston speed than torsional vibration, although the two are allied.   Unless special parts are used, production engines are at risk if the MPS goes above 20m/s.    This "rule-of-thumb" figure is a surrogate for the actual peak speed, and the acceleration on the pistons through a stroke, but calculating that isn't trivial!    Estimates say that a standard TR6 piston and gudgeon pin weighs 346grams on the bench and 1G of 'acceleration', but at 6K would 'weigh' 881.7 Kilograms - nearly a ton!

Comparing the 2L and 2.5l versions. 

The piston velocity of the two engines

2L, 76mm stroke

RPM

Mean speed in mm/sec

Mean speed in meters/sec

6000

15200

15.2

7000

17700

17.7

7500

19000

19.0

 2.5L, 95mm stroke

6000

19000

19.0

6500

20840

20.84

  John

Edited by JohnD

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I'm more bothered with balance than balancers.

I've chucked a set of 2.5 rods on a scale and found a surprisingly large variation.

No sign of any grinding other than flash removal so 'as is' into the engine at the factory

BL Quality control had gone out the door after the TR5 years.

15g, do the math!

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@ John,

About 1 ton of force at peak cylinder pressure pushing the piston down on the power stroke.

But little pressure  or force to deal with in the other three phases.

However peak combustion force will be at round 3500 rpm at peak torque, and will drop with VE as rpm rise.

So I'd go with cranks suffering at 6500 rpm from inertia loading than torsional loading from combustion pressure.

Which indicates the damper wont have much effect I think.

Peter

 

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Have to ask, then why did Triumph fit them?

They didn't to any four cylinder engines, and Sir John Black was notoriously keen to save fractions of a penny on production.   Unless the engineering design dept. said they were essential, surely he would have vetoed them?      Would those Standard-Triumph engineers have insisted on an unnecessary item on a production engine that would have been expected to spend its life at less than 6K?

John

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3 hours ago, Scooter said:

I'm more bothered with balance than balancers.

I've chucked a set of 2.5 rods on a scale and found a surprisingly large variation.

No sign of any grinding other than flash removal so 'as is' into the engine at the factory

BL Quality control had gone out the door after the TR5 years.

15g, do the math!

In my experience, if they are a true set, all coming from the same factory-assembled engine, the weight match will be pretty good.  Problems may arise once the engine has been through a rebuild or two in your "average" reconditioning shop and parts mixed and matched.

This might be interesting

http://www.eng-tips.com/viewthread.cfm?qid=427266

and this

http://www.epi-eng.com/piston_engine_technology/crankshaft_torsional_absorbers.htm

 

Nick

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9 hours ago, Scooter said:

 

I've chucked a set of 2.5 rods on a scale and found a surprisingly large variation.

 

Put a micrometer/vernier across the width of the BE journal and see what you get also

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This is for PeterC - I've posted this survey on the US sites, and one responder told me that they could not use their damper pulley,as their TR6 was supercharged.  The torque to drive tge 'charger was too much for the damper pulley, so they have resorted to a solid pulley, with no ill effects, they claim.       Could this be beacue a 'charged engine is niot taken high enough for torsion vibration to be a problem?

http://www.triumphexp.com/phorum/read.php?2,1509485,1509485

JOhn

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Thanks John, a blower could take up to 10 to 20hp depending upon boost, so, yes, I agree the rubber insert would soon fail. The Moss kit used a solid pulley. My DIY pulley uses the crank nose, replacing the fan boss. I read somewhere that the blower belt drive does a good job of taking out the torsional vibrations. But no numbers given as far as I can recall-

Peter

Quote

 

.

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Of course! Rubber, just attached top the crank in a different way, using its viscosity to remove vibration energy!

Doh!  But I'm not going to even try to model that.

Thnaks, Peter.

John

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Hi John

Just something else to throw in the mix re 6 crank dampers, not sure when Triumph started fitting them, but I know the 1600 vitesse and MK1 vitesse and GT6 2ltr did have them. They also had a massive metal fan wizzing around bolted to the front of the crank. That's going to make one hell of a difference to the crank, were they ever balanced?, do crank failures occur on cars fitted with crank fans or those without? also the TR6 have one hell of a big boss fan extension on the front of the crank.

Just some idle thoughts

Alan

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Thanks, Alan!     I'm grateful, as always to the Faculty of Sideways U. for thinking on my behalf!  Please continue the seminar, as it makes me think to reply to points you make. 

 I've read about TR4s needing a damper if they ran fanless, I presume with an electric one.  They had an odd four bladed fan that was like a St. Andrew's cross, diagonal, with unequal spacing between the blades, a long extension boss on the pulley to put it right behind the radiator, and no damper.

But the 6-fan was bolted to the central hub of the pulley, and the fan blades were light, should have been in good balance as the bolts had semicicrular slotted washers to balance it, or not, of course if your DPO hadn't bothered.  And an out of balance fan would deteriorate primary crankshaft balance, causing a couple about the crank nose, not make torsional vibration worse.    You will recall that  it's the repeated impulses from the power stoke as each journal that twist the crank causing the torsional vibration, and meanwhile, the outer ring of the pulley would still have been free to move on the rubber and absorb the vibrations. (see https://www.canleyclassics.com/triumph-gt6-mkiii-fan )

   So I can't think that a fan would contribute much to torsional vibration, and that for sixes the fan doesn't matter.  That it is said to do so for TR4's  is beyond me.

John

 

 

 

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image.png.e8cbe1c6b7b38d7e9cc8e31688cce0d7.png

Firing order 1342. - one bang every 180 deg equally spaced in time

The blades are 80 then 100 deg apart approx.

I wonder if that fan was doing a job damping?  Perhaps when cyl 2 then1 fired it helped damping to have the fan blades closer together ?

Was the fan position tied to the crank position with a keyway and asymmetric bolts in the hub? If so that could be a clue to there being more to the fan than blowing air.

Peter

 

Edited by PeterC

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This quote from the Revington website re the 4 pot engine:

Q: If I fit a plastic fan to the original fan extension or remove the fan and extension altogether, would I be risking crankshaft breakage or damage due to changing the dampening effect of the original fan/extension arrangement?
A: Irrespective of whether you use a wobbly aluminium fan or a wobbly plastic fan, the effect is the same. It helps (just) to cool the engine and that is all. In our opinion the amount of damping it offers is negligible. Dampers have been in use since the 20's where necessary. If Triumph (one of the largest sports car manufacturers in the world at the time) thought is was necessary they would have fitted one. We have been building TR2-4A engines with no fan and no extension for 25 years with NO broken crank related failures. As a result we are quite happy that an engine in good shape, from standard up to all bar the highest revving competition engines should not suffer ill effect by having the fan substituted for a later TR6 plastic type or having it and the extension removed altogether. If you do remove the fan altogether we recommend using a properly designed washer and bolt arrangement to hold the pulley to the crankshaft.

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Hi,

I've responded to John's survey a few days ago, claiming that I haven't seen any broken 6cyl. crank so far, but recalled only later that about 5 years ago I was at a traditional local engine part shop (I think they even used to rebuild engines a few decades back)...

The owner told me he used to have big Triumph sedans, as a new car. He did like them a lot, they were fast back then, but on on 2 cars he had a broken crank. He used the cars for business trips, which means possibly long traveling at high revs  into Germany in the 70ies (no speed limitation, no traffic jams...)

I don't know, whether  he was talking about 2l or 2,5l engines though.

However, above story supports two facts:

  1. Triumph 6 cylinder cranks may fail with a new damper, or a new damper is no guarantee against crank failure.
  2.  The 6.cyl is not made for running constantly at higher revs.

On the other hand, blueprinted 2,7l TR6s with 230hp red-line between 7500 and 8000 rpm with OE cranks. (as stated by long term Triumph-Competition racer) But in a race revs vary a lot...

So the culprit could be the word "constantly" .

A local Triumph 10CR hero, with a quick 1500 Herald estate, told me he tries always to vary the revs when driving on long highway journeys, in order to "statistically" keep the crank possibly out of a resonance situation.

I try to follow this advice, but nowadays, with all that traffic, it's anyway difficult not to do so...

Patrick

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