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Earlier this year, I asked people to contribute to this survey, and I can now publish the results.

Rather than take up everyone's bandwidth, I've done so on Sideways, at: http://sideways-technologies.co.uk/forums/index.php?/topic/7830-survey-on-crank-damper-pullies/

 

Turns out that particular forum has some odd restrictions, so you may not be able to reply there, so I started another thread at ST, for people to do so: http://sideways-technologies.co.uk/forums/index.php?/topic/7831-survey-on-crank-damper-pullies/

 

Or, of course, here!

 

Looking forward to plenty of criticism/comment!

John

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Nice work John!

Sorry, no criticism from me, but good read and a better understanding of the dampers purpose.

Others who are more knowledgeable in this area will comment for sure!

Best regards,

Waldi

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Yes, yes, Steve! Patience

The test rig is built - my electronic trials with being unable to get Megajolt programmed may mean that I have long battle in front getting the sensors to work!

John

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There was so much interest and cooperation with the survey. Has no one else ay comments?

 

Try posting it all here!

 

Introduction

A reciprocating engine is subject to different types of vibration. The crankpin is inherently unbalanced, so web extensions in the opposite direction to the journal can achieve static but not dynamic balance, as the shaft spins and journals in different planes cause a twisting moment about the shaft.

The crank will also be subject to primary and secondary vibration due to piston movement. The piston accelerates at each end of its stroke, reacting against the crank, and that motion is faster at the top than at the bottom of the stroke, so that secondary vibration occurs twice as fast as primary.

The straight-six engine is the equivalent of two three-cylinder engines with all those forces opposing each other, so that it is uniquely smooth running, but the longer crankshaft allows torsional vibration to become important. As each piston descends in the power stroke, against the load or just the inertia of the rest of the drivetrain, it bends the journal on the main bearings of the crankshaft, which rebound and oscillate in the same way as a struck tuning fork, at the ‘natural’ frequency of the part.

A vibrating tuning fork will cause another of the same size to resonate, at the same frequency as the first. If the input frequency into the journal is the same as its natural frequency, then it will resonate, amplifying the vibration. Resonance can shatter a wine glass at its resonant frequency, and there may be several “critical speeds” at which a crankshaft will resonate with the frequency of firing strokes.

Torsional vibration dampers absorb the energy of vibration and dissipate it as heat. There are many different designs, which depend on:

Viscous friction

Solid Friction

Fluid viscosity

Tuned elastomer mass dampers

The last is most commonly used for production car engines, as it can be made cheaply, is relatively light in weight and requires no maintenance.

Survey of crank damper pullies A survey was devised and placed on the website Survey Monkey. It was publicised via websites and message boards in the UK and America, and on Facebook pages, all devoted to the Triumph marque. Users were invited to visit the survey and complete it. The purpose of the survey was to find a denominator for the incidence of failed crank damper pulleys in Triumph engines. To promote compliance, it only contained five questions, four factual and a last question on their opinion if the broken crankshaft was due to the faulty damper.

The Survey ran from the beginning of January to the end of February 2018. A total of 207 owners responded to the survey.

Results

Q1. Duration of ownership

image.png.0c3cf1d3240c235f9776d421267414

Q.2. Models owned

image.png.a6df27c180a43281b11a239bfa8e48

Q.3.Faulty crank dampers seen image.png.bf0da6ae6c73f3b5971454e2502d24

Q.4. Known fractured crankshafts

image.png.725190f0a0093db142570fe32c5f80

Q.5 Opinions on cause of crank failure

image.png.bc4524a43586be1772863fd82d258e

Q.6. Free Text

This unstructured text box allowed responders to write what they liked. 109 responders offered opinions, difficult to categorise, but showing several had owned more than one Triumph for many years, rebuilding up to seven engines, all without any sign that the dampers were faulty. A rebuild involves finding TDC, to time the camshaft and ignition, so a faulty damper would be found if the builder compared that with actual piston position.

Several commented that the outer edge of the rubber in the damper showed signs of deterioration, being swollen and cracked (see title page). Some had replaced the damper, either by having it rebuilt, with another, better looking damper or with a non-Triumph damper pulley. One had shaved off the swollen cracked edge, found the rubber beneath in good condition and used it again, with no apparent ill effect.

Three commented that they had seen broken crankshafts, one twice, but they were in racing Triumphs. Another had seen one while the car was still under the manufacturer’s warranty. The last had seen one in an earlier Triumph, a TR3A, but that was an earlier, four-cylinder engine with no damper.

Others commented on seeing faulty dampers on other cars, including the Volvo B21 red-block engines, Ford F150 V8 and Triumph Stag.

Conclusion

The owner who had seen a crank broken under guarantee thought that this might have been due to a faulty damper, but this seems unlikely so early in the car’s life. It is more likely to have been faulty manufacture of the crank, not deterioration through age.

The survey showed that nearly two thirds of responders had owned one for more than twenty years, so their experience may be expected to be typical. However, they owned a range of those vehicles that is not in line with the known production figures:

Model

% Produced of Total

% Owned in last 20 years

Saloon 2L/2.5

66

14

TR5/6

19

51

GT6/Vitesse

15

35

The attraction of a “British sports car” as a preservation project in the TRs and less so in the GT6 and Vitesse models contrasts with how few of the saloons have survived. This, however is not relevant, as all these cars had very similar viscous rubber mass dampers, only varying slightly as the 2.5liter engines had wider pulleys

Of those who responded, there were 41 (20%), known crank damper failures, and 32 known crank failures (16%), but only 12% of responders considered the damper failure as the cause of the crank fracture. They were not asked to amplify what other cause might have led to such a major malfunction, but despite the obvious association there are many other potential causes, and a list would have been only confusing.

The opinion of experts, while not contradicting the experience of owners, contradicts each other and is not helpful in this matter. The statistics must decide, and that a fifth of the owners of surviving cars had known of a failed damper is a large proportion. The concern of owners on this problem is justified.

However, the worried owner has no method of resolving his concern. Inspection of almost any used Triumph damper will show apparent deterioration of the rubber, but four out of five will still function adequately. A method of testing dampers is needed, to reassure owners, or to demonstrate that the expense of a new damper is essential.

This project justifies further research to discover such a method.

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  • 6 months later...

Research Report

Last year, you may have contributed to my survey about Triumph six-cylinder crank dampers.  Thank you!  And thank you in particular to those who have sent me pulleys that they suspect are faulty, so that I can test them.   Now, I'm ready to start testing!      I've equipped my engine rig with sensors, and  am able to see torsion vibration on the screen. See pic.

And I've actually looked at the 'bad' pulleys I've been sent, and got a surprise!    They are different from all the pullies I've had before!    The latter have all been either 2L or 2.5L from saloons, and have solid hubs, that includes the grooved pulley.   The damping ring and rubber sits on a boss on the hub.

The 'bad' pulleys have a spoked hub, with a separate grooved pulley and a larger damping ring.   The rubber in between goes right through the pulley.  I hope that description is sufficient, but here is a pic of my 'collection' that shows the difference from the front.  "Bad" pullies on top.

So I know, can anyone confiorm that the pattern of my "bad" pulleys is from TR6/TR250 engines?  I'm guessing, as I can't find an online source that shows different Triumph damper pullies.

Thanks!

John

Frequency analysis view.JPG

P1040107.JPG

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Very interesting. Did you run any solid pulleys? I have one on my supercharger setup? 

Cheers

Tim

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Hi John,

Interesting study.

I have the spoked type on my early CP engine but cannot confirm if this is “original”, but besides painting it required no work. I have checked position of TDC mark with a dial gauge, it was spot on.

Regards,

Waldi

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Richard,   as you can see from the above pic, all my damper pulleys are 'used'.  To have one that really is 'known good' would be invaluable by providing a control to the study.  I would be most grateful if you would loan it to me!    My time table is to complete the study before March, so I can return it to you then.   It won't be cheap to post, but I'll be glad to recompence you.

Ed,    Thats' the bunny!   Ignorance is all - I had no idea that TR6 damper pulleys were so different!  Which explains the need for a solid pulley on a supercharger, Tim D., as unlike the 2L/2.5 saloon (?) design, the stress of driving the 'charger goes through the rubber!

Below is a pic of the three TR6 pulleys, in a row up with the keyways lined up by eye.   The left one clearly has moved the outer ring!   The three ohers have a similar outlier.   So they will get tested in comparison with the other two, and Richard's  if he will be so generous.

JOhn

P1040110.JPG

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No problem, PM me with your address...………...I think the reason I never used it was that it may have been mis-aligned during rebuild, I emphasise think! #1 TDC aligns with the keyway in the hub with TDC on the outer pulley at around 16deg to the right of the keyway in order to align with the timing pointer. Can anyone confirm this?

See attached pic showing nice new rubber...…...

Damper.jpg

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I have seen a quite a few North American TR6 pulleys and they all have been 'spoked' and I have two TR250 engines which also have the 'spokes'.

Hope this helps.

Cheers

Graeme

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Ok John,

what I understand is the crankshaft is forced by torsion and vibrations?

And the damped pulley on the front end is there to reduce damages on the crankshaft and bearings?

Is this a correct summery?

 Ciao Marco 

 

.

 

 

Edited by Z320
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Hm,

the crankshaft runs not continious but with vibrations.

Isn't the damper there to keep the vibration away from the belt drive, water pump and generator?

What is the part of the belt and it's tension?

The pulley to me seems too light to do anything for the crankshaft when I think of the heavy fixed fly wheel on the other side of the crankshaft?

Ciao Marco  

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 Have you read my piece on Sideways, linked to in my first post here? 

No, the damper is there to protect the long crank of a straight six.    Most moderns will use one, but to cut down on "Noise, Vibration and Harshness" for a quieter, smoother ride.

J

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If I would own a 6 cylinder I would check the vibrations with damped and solid pulley, also with belt to water pump and generator AND without belt.

But as I do not own a 6 cylinder I'm out of this thread.

Ciao Marco

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Marco,

think of it as a vibrating rod clamped in a vice: if ticked, it starts vibrating.

now when you put your finger tip on the rod, the amplitude instantly drops.

that is what a damper does, it prevents repeative (harmonic) movements to further increase a  movement from a previous cycle to the point where the can result in damage.

In mechanical engineering, movement and stress are related (Hooke’s law), so if you limit the movements in a construction, you limit the stresses. The stresses in a crackshaft are varying (from low to high and back)or even worse, going from tensile to compression, which will lead to fatigue at much lower stress levels than when a continuous load (stress) is applied.

Regards,

Waldi

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Hi John, i’ve been looking forward to the results of your tests !

keep up the good work 

 

steve

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Thank you, Waldi, you made that very clear!

I'd add that any stiff object, a rod in a vice, a tuning fork or a crankshaft, has its own resonant frequency.    A tuning fork is simple, just one (for purposes of argument) frequency, whereas a crank may have many.   Think of a swing, push a child regularly and in time with the 'resonant' frequency of the swing, and the child goes higher and higher!      A crank will have 'critical' frequencies, rev speeds at which the impulses from the power strokes coincide with a resonant frequncy, and the vibration, like the swing, can build up until the child goes over the top damage occurs!

The damper changes the resonant freqency and absorbs the energy of vibration, converting it to heat, preventing that build up and protecting the crank.

JOhn

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True, but the damper has its own resonance since it is a mass-spring system in its own right. If that resonance coincides with one of the modes of the crankshaft it can reinforce rather than reduce the vibration. That's why it may not be a good idea to fit any old damper rather than one tuned for your particular engine.

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