Gearbox Nasties...

like Leakey water pumps mike. lol

Hi Mike!
No sweat.
The final snap was failure by torsion-overload which was after the cracks had made the effective shaft diameter smaller in the middle -- I can see that in the pic... They are Fatigue cracks, from the root of each spline, that take a long time to get that far -- probably 7 to 8 years -- so no sweat, just crack detect each shaft before you put it in --
all GOOD Bantam racers should crack detect their shafts before inserting... not foretting their viagra, of course.

There´s a new liquid form of the latter -- it has to be taken quickly because it stiffens up rapidly.

PS. I´ll probably get banned from this site now...
So, Cheers!
JayBee (for John Boy not yet extinct!).

26/21 or 24/23 can make no significant difference -- except perhaps a year more or less...

In that 300 hours you mention Mike -- how often do you reckon you changed -- up and down -- to the intermediate gear -- i.e. 2nd...?
Within a hundred either way will do...!!

I ask, because on accelerated fatigue testing the time before failure can
(obviously) be shortened by increasing the load or by increasing the number of times the load is reversed for a given level of load.

Before I shut up and Foff -- have you ever checked the concentric running of tops of the gear teeth -- or that of the collar on the end of the layshaft gear and the other gears??.

Your bikes -- I hadn´t realized there were two at first -- look really wonderful. You ought to go over to the IoM with them....

Hi John Blimy Viagra where do you put it in the tank. or box

sorry John don't agree with prognosis, no doubt a crack formed, but still can not see fatigue on a shaft so short, bending, I have my moments to, anyway fatigue "no" dont think so,

Still say either a fault or possibly even a machining defect or over hardedning or a inclusion, just can not buy fatigue' on such a short and in a low load area, got to be down to one of the isues already identified, and load unload test the teeth again would go first, these are far more prone to fatigue as the bending moments are far hire especially on the cored. these are so much smaller, so these would go first if it was fatigue, therefore must be down to start from some kind of inperfection, of the type already discussed
"But I could be wrong" ?

anyway John Ford ! Engineers sorry I dont rate any of the engineers Iv'e had the pleasure of working with, and Ive met a few, present companys on here excluded of corse, - "all pomp and bull".


Regds Derek

Beg to differ Derek!

In 60 years of automotive engineering I have seen thousands of fractured components, many of which I durability-test-rig-fractured myself. Some with very small alternating loads typically failing at thousands to millions cycles. Fatigue cracking only became really studied as important after World War when many failures could not be explained.

All rotating shafts experience some form of bending-stress even if they do NOT bend! Even turbine shafts!
The bending stress concentrates on an undercut with no radius or just a sharp change of section. May or may not initiate a crack.
Fatigue cracks can form from very small loadings over long periodfs of time...
Brittle fatigue cracks CAN be attributed to poor heat treatment -- Thro´ hardening, case hardening and normalising ...

The Bantam layshaft is a very stiff shaft.
Too stiff -- perhaps -- for the material elasticity to compensate for the undercut that does exist in the drawing I have seen on here (front page titled, Gearbox drawings -- unfortunately says nothing about heat treatment for layshaft) exactly where the spline root-cracks -- (crackS) -- propagated.

This shaft broke near the middle of the shaft where two gears are in mesh and there appears to be FATIGUE (long term) CRACKS starting at the root of each of the 5 splines -- which coincide with the undercut at that section...

Mike gave a "Distinction" in his reference to the broken layshaft being D1-D3 -- therefore it could have been manufactured in 1947--1950 time with different manufacturing methods and heat treatment of those shafts made later -- D5-D7 and on thro´ D10 ...

SO IT IS A VERY OLD SHAFT __ ALTHOUGH WE DON`T KNOW ITS SEVICE HOURS THEY COULD BE CONSIDERABLE HAD IT have BEEN IN A POST OFFICE BANTAM EARLIER.

I shall stick to this theory unless it be disproved:-
The small bending stress experienced by the shaft started the crack at the root of each spline, which over many years and maybe many hundreds or thousands of hours had the 5 cracks extend towards the centre of the shaft.

I state that because the cracks appear to be angled (to the plane of the shaft) from spline root which is TYPICAL of bending-fatigue...

Further, if the shaft is viewed with one spline at the bottom, any little bending stress will be acting in tension at the sharp corners of the undercut.
When the spline is at the top this undercut is under compression -- which happen 5 times per rev... since this root corner is about 1/4" wide and the undercut corner a virtual crack in itself the stress would be enormous. It would be an alternating compression/tensile stress acting at the root-corner, thousands of times ...

Which fits the description of a Fatigue Failure as described by a renowned Automobile engineer who said he was surprised at how small an alternating load was needed to produce a fatigue failure.

Reference "Ford engineers" -- I shall not comment except to say I went to Dunton when Simms was sold out to Joseph Lucas/CAV and left in boredom after 4 years to take up a job in Montreal where I had a real engineering challenge.

That broken layshaft of Mike´s could be even older...
--
When did you acquire it Mike? If you don´t know its history you realize, of course,
that it could have been an original DKW. That, since the Bantam, its design and manufacturing tools were taken as compensation from Germany after WW2 for starting it ...and two 125DEEKS stood in Royal Enfield´s experimental dept car-park until taken by BSA as a "favour" to RE Directors in 1946 or so to introduce BSA, D1 Bantam in 1947...

Since there´s a bit of a lull before Mike gets the pics out -- (I must see the other broken end of the layshaft soon, at least before I popp my clogs ...) -- I shall explain a bit about the "Notch Effect" which I referred to earlier on and I may have confused some readers, with, on here.
...
Looking thro´some of my ancient notes I came across the "Wöhler Curve" I cribbed from the Germans which shows the effects of surface finish on fatigue cracking tests.
The test rig applied compression and tensile stresses that were less than the elastic-limit-stress of each material:- steels in this case from 300N/mm² -- 1400N/mm²
--- 20tons/in² -- 90tons/in²...
...
Just imagine the hours of tesing that lot took -- even with several test rigs!!
...
One test piece -- #3 below -- had a Notch that was effectively a 60° annular gash in the test piece and 0.1mm deep.
YES! --- 4 THOUsands of an inch DEEP!
...
Wöhler stated that good quality, good surface-finished steel would last thro´ 2,000,000 cycles to 10,000,000 stress cycles with 20 to 90ton/in² steels --
But -- note below***...
...
The Wöhler Notch Curve refers to 7 test pieces with a ratio, `ß´, of rough/polished
surface quality which were:-
1. ß = Superfinished, surface finish -- 0 to 1micro-meter; 100% of ß "/"....
2. ß = Ground -- 2.5 to 6micro-meter. 95%at 20t/in² to 90% at 90t/in².
3. ß = Rough Machined NOTCHED test piece. 83% " "/" to 40% " "/"
4. ß = Rolled surface -- not machined. 80% " "/" to 30% " "/"
5. ß = Surface corroded by tap-water. 67% " "/" to 18% " "/"
6. ß = " " " sea-water. 50% " "/" to 12% " "/" ....

What all that bullshine means is a rusty layshaft is more likely to suffer fatigue cracking -- with subsequent failure -- than a highly polished one...

*** the Wöhler Curve shows that with increase of tensile stress -- from `20Ton´ to `90Ton´ steel the failure by fatigue stress is more likely in the stronger steels
and that surface quality is extremely important.

Although this suggests that a score mark -- or scratch -- could initiate a fatigue failure I thought this might put some minds at rest over Mike´s layshaft breaking and show the importance of all stressed parts -- particulary rotating shafts -- having a good surface finish.

I´ll end this bit by saying -- if you have an ancient gearbox, get the shafts out²²² and crack-detect them, polish bits that are under repeated bending or tensile stress and after that -- Pray!

²²² --not straight away -- when you split the crankcases for some other reason or just happen to have half a dozen OLD layshafts as spares -- get them crack-detected.
Even after cracks have started the shafts last quite a long while before total disaster!!

All together now --- Amen!

Hey Mike!?

I looked at your very first statement on this Topic where you said,
"Robin exploded his gearbox on Saturday morning -- it´s the heavy flywheels..."

I know you were only joking but that comment gave me the feeling you felt the gearbox bothers you both suffered had something to do with `Shock loading´ and that led to the thought of harsh and vicious gear changing...

Do you experience some sort of "Road Rage" when practising & racing that the gearlever suffers severe abuse...??

Only joking there -- of course.

What was Robin´s explosion caused by? I´m interested in another shaft failure...?

Cheers!

PS... did you mean Robbie & not Robin?

No more photos Mike?

Not worried any more -- about layshafts breaking ...?

-- I am now! because my suggestion of the break being a long-time-coming Fatigue Cracking failure -- taking many hours -- MIGHT --I say, MIGHT -- be wrong BASED on the only picture I have, which is the one you showed at the beginning here.

I say that because right in the centre of the broken face is a black spot and just below that, is another black dash in a flash of white down to the bottom spline root which might be poor photography or something Derek suggested.

AND!? -- Could be something else altogether, from what I suggested...

Be sure to take a spare layshaft to Cadwell and do have a good weekend´s racing.

By the way, do not be too concerned over crankshaft weight*** -- I am feeling fairly confident that the resonant exhaust pipe (and in consequence engine torque) has more, MUCH MORE, effect on the sudden-push effect than the heavy flywheels have in a Bantam.

*** however light flywheels get the engine revs up quicker to the effective resonance speed.

I know there´s nothing psyches you out at race meetings, I remember Lydden 2005, -- I just added that bit in case. Try to keep looking frontwards -- at Lydden you had your head on back-to-front several times -- very worrying!
All the best, show them how its done!
CheerS!
JayBee.

Looking at the race tuning article -- for the 1st time, this morning -- I see Bantam crankcases wherein sit ball-bearings for the layshaft.

...hmmmmm!?

...none broken at Cadwell?

...subject closed -- I take it?!