Hi Friends,
Can anyone enlighten me as to the origin of the claim that the standard crank has a critical speed at 2500 Rpm. I have heard it mentioned on previous occasions but dont remember having read it in any authoritative text. Bill

.

Bill,

I do not know quite what you mean by 'critical speed';
when they break or when they vibrate.

My own 'noise and vibration' speed in top is around 27/28 mph. This equates to about 1800 rpm. I try to avoid this purely because it offends! In the gears these revs are passed through very quickly as a rule, so not a problem.

Two and a half thousand revs on the other hand ( around thirty-five in top) would be a complete pain.

Any further thoughts anyone?

Mike

Hi Mike,

Many thanks for your reply. In a previous message about axle ratios, a "Harmonic Barrier (2500rpm)" was mentioned and I was interested as my car does 14.75 mph per 1000rpm in top, (19*350 and 5.25:1)and this would equate to a speed of 36.87mph,a speed I might drop down to, on the odd occasion!
Although I have heard critical speed mentioned,I have never experienced abnormal vibration at any speed, apart from a buzzing in the seat of my pants at all speeds. I was wondering therefore, if the whole idea of a "critical speed" or "harmonic barrier" was a verbal inexactitude. Bill

Hi Bill,

Firstly in discussion with other local members we can't think where the 2500 figure came from.

However, the two bearing crank does whip and I think it is fairly well agreed that the centre of the crank distorts from it's central axis when revolving. Added to this is the reasoning that if you spin the crankshaft fast enough it becomes 'self supporting' and more rigid.

Putting these two together the whip must geat greater then decrease as the revs increase, maybe someone has determined that at 2500 rpm the whip is at it's greatest. Certainly we here believe that you won't break a crank at high revs, only when you drop the revs back down in certain conditions, or are reving up, are you likely to break a crank.

I know this doesn't answer your question but I thought I would make the point in hope that it triggers someones memory.

I always thought it was a "harmonic problem", like the suspension bridges collapsing, natural frequencys and resonance? I'm probably wrong, but maybe Rob Beck may add to this post as he has designed the floppy flywheel which would have an effect on this issue?

Steve

Cheers Steve!

No idea about the 2500rpm figure.

The issue as i see it is not the crankshaft going into its first harmonic - this probably occurs at low speed. The problem which causes the crank to fatigue is when the gyroscopic effect of the flywheel starts to overcome the whip of the crankshaft, the resulting vibration causes the fatigue. The lower the mass of the flywheel the higher the speed at which this occurs however at higher speeds it is more violent. The stiffer the crank the higher the speed at which it occurs. I dare say a 1 1/8 crank and standard flywheel go through the vibration at quite low speed, possibly 2500rpm. I tend to think the 1 5/16 cranks do it around 3500-4000rpm.

I am building an engine with a 1 1/2 crank and floppy flywheel (see previous posts and drawings on the association website). The taper end of the crank is a bit naff so i will regrind it and make a custom flywheel hub to fit. Will then make an ali flywheel outer, save cutting up a flywheel and to get the weight down. (vintage engine so the starter ring is on the clutch cover anyway).

Regards

Rob

Hi Bill

40 odd years ago, when the 750 racers were destroying A7 engines wholesale (but don't let's go there!) it was "common knowledge" that cranks "always" broke around this speed. When I saw your post, I thought it would be dead easy to go back to some ancient magazines etc. and find this in black and white. I failed. The best I can do is advice to "try and stay out of the engine harmonic period (about 2300 rpm)". How precise is that?! This comes from no less an authority than Martin Eyre (Austin Seven Companion p.249), so it may have some basis in fact. On the other hand...

John


(N.B. Martin's advice relates to racing. "Trying to avoid the engine harmonic period" is unlikely to be successful as a defence in speeding cases.)

Hi Rob, Correct me if I am wrong. I think what you are saying is the greater the speed - the greater the gyroscopic effect of the flywheel which holds the end of the crankshaft as it tries to deflect, due to the centrifugal and inertia loads, which as a matter of interest, increase as the square of the speed. However, the harmonic theory suggests that there is a speed at which a serious vibration occurs and which fades as the speed increases.
I have been searching the Seven Companion and on page 249 in an article attributed to Martin Eyre, there is a statement, " Try to stay out of the engine harmonic period (about 2300 rpm)". Unfortunately there is no elaboration on the origin. Bill

I think I understand what Rob is saying, in that the fluctuations of the flywheel and the whipping of the crank can become "in phase" thus becoming harmful?


As a matter of interest my car has a harsh period around 55 ish and clears at 60mph (these are inaccurate speedo readings but are useful as an indicator to me).

I would presume that the 3 bearing crank would exhibit this effect at a totally different engine speed due to the addition of another supporting bearing?

Like other members of the Cornwall club I now let it rev quite freely but always try to change down a gear before the revs drop when going uphill to avoid labouring at low revs. This was worrying at first but Malcolm Watts soon egged me on from the passenger seat to let it rev, whilst I was itching to change up to soon!!!

I seem to remember that the 750 companion has something about this? Mine is down the garage covered in oily thumbprints as it should be, so I will try to look at it tomorow?

Steve

Right, at last I seem to be able to post on this site, I've had 'time-outs' trying to reply to this topic when it first started.

I mentioned 2,500 revs as being the engine speed at which my RP went from being very noisy to much quiter and that seemed to equate to approx 36mph in top with a 1:5.125 CWP - as stated in previous posting, it's not a 1:5.25 as I marked up the flexible coupling and rear wheels to count the revs.

Bill's use of the word CRITICAL is wrong, it was the speed at which the worst harmonics occur apparently, although from my car it seems everything between 400 and 2,500 is bad. I know it's probably the engine rear bearing rumbling, but what bliss over 36mph....

I'm not sure the cranks break at around 2,500 revs, I thought most went at low speeds (unless of course you are reving up and over 5,500 or something like that), or they broke on over-run, which is why when going down hill, I tend to let the engine idle for a while, much to the chagrin of some Cornwall Club members, especially as I have been known to overtake them in this mode!!!!

Sandy

PS - I hope at long last this arrives on the Forum

As I understand it, over revving does the damage, as I noted previously the loading on the crank is proportional to its speed squared ie the load at 5000rpm is 4 times the load at 2500rpm. Every period of over revving causes fatigue and as this builds up,the crank can fail at any time and at any speed. Minute cracks, apparently common in A7 cranks, accentuate the effect. Bill

Bill, i dont know that you can apply typical shaft harmonics theory to an austin crank for two reasons:
1) It rapidly goes so out of balance as the shaft deflects - and it will flex a long way - dad had to fit 2 head gaskets to his 750 racer in the 60s to stop 2 and 3 hitting the head

2) The forces from the conrods

I dont think the crankshaft comes out of the harmonic whip due to the changing frequecy, it does it because it is forced to by the flywheel acting as a gyroscope.

If you have not tried it already take the flywheel cover off the engine and start it up then watch what happens as you rev the engine. At certain revs the crank just disapears into a blur.

Cranks do seem to go in a couple of different ways, sometimes you can be flying along and just starting to think "blimey this is going well" BANG no warning sounds at all. Alternatively it can sound like someone the entire innerds of the engine are trying to get out before the crank lets go. Dad pulled a crank out a few years ago that was so noisy he decided not to drive the car onto the ramps first. The crack goes about 7/8ths the way through the crank and you could probably bend the end off by hand.

The actual mechanism of the vibration really doesnt matter, the only way to stop the crank breaking is to prevent it, hence the floppy flywheel!

Regards

Rob

Hi Rob, I understand your comment and agree that if there is a "harmonic", then after having been stressed by over revving or possibly, periods of use at the harmonic speed, the crank could fail at any speed but most likely the harmonic.
I had better do as you suggest and have a look at my engine at 2300revs. Regards, Bill

Right I think I have got it now!!!
I misunderstood Rob at first but now see were he is coming from!
I too have run the engine without a gearbox attached and you could see various vibrations, although my flywheel is not quite true, which of course is not good.
Can I now assume that basically it will break when it feels like it!!! (unless I fit a floppy flywheel!) .

Steve.