layshaft failure

[peejay]

hi guys



this is an original layshaft from a 1959 series 1 Li 125

it was restored a couple of years ago and engine rebuilt with new seals and bearings.

luckily for the rider he was on a straight bit of road doing 40+ when it came to a rather rapid stop!

hub looks ok but probably may be ovalled so will be replaced.

layshaft failure is quite rare still i think, it's a risk we take i suppose using such old parts

[Adam_Winstone]

Hi Peejay,

Yes, very rare but we're hearing of more and more cases of it, which may be a result of better communication or because of old components failing over time. My failure on the way to Euro Austria was an later GP/SX spec Innocenti original. I note that all those that are failing do fail in the same way; base of cut thread.

In the same way that I thought I was happy playing the numbers game with tubes tyre blowouts, the numbers caught up with me for both blowout and sheared layshaft, and just as I since changed to tubeless, I have also recently fitted my first RLC / Casa Octopus rear hub and thicker threaded section layshaft. The design will need to prove itself over the decades and many thousands of miles but I needed to do something to address it as the gremlin of doubt keeps me wondering how I'll fare the next time one fails!

Time will tell. What I want to ensure is that my bike is as safe as it can be, without having to hang up my riding boots.

Adam

[hullygully]

Was the nut over torqued or was it the machinists end of shift? or was it just a material failure after 60yrs? the weight of a series 1 c/w latest road conditions...….
we could go on for ages here bud
we ALL need to check our scoots properly now adayz as we've got better tools & info @ our disposal
we don't bounce so well now

[dickie]

Was the nut over torqued or was it the machinists end of shift? or was it just a material failure after 60yrs? the weight of a series 1 c/w latest road conditions...….
we could go on for ages here bud
we ALL need to check our scoots properly now adayz as we've got better tools & info @ our disposal
we don't bounce so well now

I did a bit of work on this for an open university small module a couple of years ago.

It's basically down to three factors:

How many miles it's done.
How heavy the rider is
How bumpy the roads are that it's been ridden on are.

It has fatigued in all directions as it is continually flexed miniscule amounts as it rotates. Very few metals fail just because of age. Overtorquing may have an impact, but I found it hard to justify unless you put massive extra torque on the nut.

So old ones are more likely to fail as they've typically done more miles, but it is definitely not the only factor.

[Adam_Winstone]

^... LIKE

[lam1962brettagg]

Hi
having worked at a uni tech dept a very informed metal tech told me that as metal ages it becomes harder and more brittle. possibly leading to what we see here.
i changed to a mb race tour layshaft last year.

[hullygully]

its good to talk yoofs

[Warkton Tornado No.1]

Layshaft failure has been talked about on this Forum before, but, unfortunately, much of the talk has been conjecture as to why failures occur, with very little sound evidence, though efforts like those made by dickie are to be applauded.

However, in this particular instance, I would suggest that corrosion should be taken into consideration. In fact, I have never seen a layshaft as corroded as the one depicted in use!

The thing is, no matter how much ‘we’ talk about the potential of layshafts to fail, what is it that ‘we’ are prepared to do about it?

As Adam says, one solution might be to buy into a more expensive ‘upgrade’ but I remain unconvinced that that is the only option, or that it is guaranteed to be better than OEM layshafts bearing the correct logo thus carrying some indication of quality, even if it can’t be ‘aged’. Almost definitely, Innocenti, Serveta & SIL will have manufactured layshafts to a standard, from the forged billets right through the manufacturing process, even if those records are not to hand. I am openly sceptical about some components that have come to market, claiming to be an improvement on the generic, original design. Rear hub bearings are one example where safety has been compromised by their manufacture IMHO, but there are other 'upgraded' components such as drive shaft (shoudered) bolts that are too long & bottom out as the consequence, made by somebody that almost claims to have invented the wheel, whenever they express their opinion on the products made by their competitors!

Unfortunately, it will likely cost money to investigate if there is a way of ‘lifing’ existing &/or new layshafts made to the generic design.

I believe that there will be a means by which layshafts could be tested non- destructively, though I would imagine economies of scale would require a minimum batch size.

I have no idea of whether this idea would gain the support of the membership, but for the likely cost involved, I suggest we give the idea some serious consideration. Our lives are @ stake……

[dickie]

Hi
having worked at a uni tech dept a very informed metal tech told me that as metal ages it becomes harder and more brittle. possibly leading to what we see here.
i changed to a mb race tour layshaft last year.

Some metals do, but not steel, unless you bend it. And I'm only talking tiny amounts here if it's often enough.

Aluminium does become brittle over time without any mechanical input, but that's all I know about it, so I won't guess.

I'd be surprised if other metals didn't behave like aluminium but I don't know one way or another for certain.

[dickie]

However, in this particular instance, I would suggest that corrosion should be taken into consideration. In fact, I have never seen a layshaft as corroded as the one depicted in use!

Yes Mark, you have a point about corrosion, which would lead to crack propagation being easily exacerbated by the same mechanism as fatigue. In fact either would cause the other!

Just for clarity, the report I did was only worth 5 cats points of a degree and a degree is 360, so not exactly exhaustive! Useful nonetheless.

[Warkton Tornado No.1]

However, in this particular instance, I would suggest that corrosion should be taken into consideration. In fact, I have never seen a layshaft as corroded as the one depicted in use!

Yes Mark, you have a point about corrosion, which would lead to crack propagation being easily exacerbated by the same mechanism as fatigue. In fact either would cause the other!

Just for clarity, the report I did was only worth 5 cats points of a degree and a degree is 360, so not exactly exhaustive! Useful nonetheless.

It’s good that our passion for Lambrettas can sometimes cross over & be used as a tool in some manner in education.

(I tried my best to discourage my son from small wheels, but he was having none of it & his dissertation for his Maths & Computer Science degree was specifically the formulation (& formation) of expansion chambers to suit a Group IV Lambretta. He might still have been racing had he received the support he was promised, despite his obvious ability to match & beat Mugello powered riders on his underpowered Rapido. He’s a far better, ‘bigger’ person than I am & not bitter, but when you prove that you can ‘walk the walk’ not just ‘talk the talk’ it makes you realise that all the talk of encouraging youngsters to the sport is just the thing you can smell on Cattle Market day…. )