Metal Spraying Crankshaft query ?

[Brian]

I think this is one of those areas where we have to keep our minds open to improvements in technology, sometimes something that wasnt any good 20, or even 10 years ago is now quite viable.

I've never had anything repaired by metal spraying myself but I did work in a industry that regularly had shafts repaired by this method and to my memory they never had a failure, but that was with large industrial low rpm machines.

I was around when nikasil was first brought out as a bore liner, at first it was a failure. The nikasil used to peel off the cylinder walls sometimes with very little use. Now it is very common and lasts for many thousands of miles.

So what I am saying is keep an open mind, just because "a mate" had something done 20 years ago and it was a disaster today it may well be a very good way of repairing something. Any technology that keeps our bikes on the road has to be good.

[metalflake11]

Wise words Brian.

[bsa-bill]

Fair comment Brian, trouble is when you get on a bit 20 years ago seems like yesterday 

[M20Mike]

Hi, I'm new to the site but have a fair bit of engineering experience and a few BSA's.
I've had no experience of metal spraying but having read the thread on this topic I have a couple of comments to make.
Crankshafts should not be hardened during any remedial work.  A crankshaft will move (bend) under the action of loading reversals so I would steer clear of nitriding or any other form of hardening.  An alternative to metal spraying is to have the damaged pin welded up and reground back to its original diameter.
The procedure is approved by Classification societies for all sorts of marine applications.  In small engines, Norman Hyde offered a similar procedure for converting the BSA and Triumph triples to 1000cc.  He had the crank pins welded up then machined eccentrically to increase the crank throw.  These engines are still performing well and as a performance engine, are likely to sustain greater loadings than an A10 engine.
I have had a couple of A10 cranks built up in this way and reground back to standard and all still working well.
Welding up a crank needs to be done carefully and is not something that can be achieved by anyone.  Heat treatment is very important both for grain structure and prevention of distortion.  Obviously, this process is not cheap, but the results speak for themselves.
I would recommend speaking with Norman Hyde for more advice on the subject.  He advertises in most of the decent mags.
I would also recommend removing the sludge trap and cleaning that during any crank work.

[Brian]

Hello M20Mike and welcome to our little world of A models BSA's.

As you say having a journal welded and reground is a common way of repairing a crank and as far as I know very successful. Its probably the most common way of "stroking" a car engine.

I'm curious about your comments on nitriding however as this is a subject I have been trying to find out more about as I am in the process of making a big end pin for a 1910 Triumph I'm restoring and I intend to get the pin gas nitrided, this is just a pin however, not a full crank.

Most crankshafts do flex to a certain degree and I can see the possibility of fractures if the crank is too rigid. Gas nitriding is a surface hardener and the depth depends on the process so the crank should still have the ability to "flex" to a certain degree.

Out of all the bikes built Moto Guzzi probably have the best reputaton for having cranks that will do huge mileages, their cranks are nitrided from new. And incidently run plain bearings, not a ball or roller to be seen. Obviously there is a lot of difference between a Guzzi crank and any British vertical twin but for them nitriding has been very successful.

I'm always keen to learn about these type of things.

[MG]

Hi Mike and welcome!

Can't argue about a high-quality weld repair, if done properly this deffo is a worhtwile (and common) method of repairing a worn shaft.

But I strongly have to disagree with your remark on nitriding causing problems with dynamically loaded parts. Quite the opposite is true, nitiriding introduces residual compressive stress in the thin outer (hardened) layer and the bonding layer underneath, leading to an increase in endurance limit and fatigue strength.
From personal experience in professional life as well as hobbyist projects, I can only highly recommend (and have done so several times in older posts!) having a reground crank (camshaft, followers etc) nitrided. Not only will it improve fatigue behaviour of the component, but also add a very thin but hard surface layer, thus improving wear resistance and tribological properties (improved oil adhesion to microscopic pores on the surface).
It is common practice to nitride cranks, camshafts, followers, gears, valves and many many other components in automotive industry nowadays, repro camshafts very often are machined from QT (quenched and tempered) steels and the finished component is nitrided without any further previous heat treatment (quenching/surface hardening), and will give many miles of trouble-free operation.
The process temperature of nitriding is low enough not to disturb the hardened layers of either martensitic (surface heat treated) or case hardened type, and there is virtually no increase in volume or warpage involved (except on very long and thin parts such as multi-cylinder camshafts which have to be straightened afterwards, albeit only a few thou runout usually). So nitriding can be used on the readily machined component easily. But steer clear of any combined process including carburizing, such as nitro-carburizing, Tuftriding, Teniferizing and whatever the trade name is. It is too easy to disturb the delicate equlibrium of a previously case hardened part by introducing further carbon to the metal matrix!

Sorry for the long recitation, but like stated by others, you got to be careful with spreading hearsay and rumours as fact. I have no personal experience with metal spraying whatsoever, so can't and won't comment on that  

Best, Markus

[M20Mike]

Hello Brian, I am a marine engineer and obviously there is a difference in scale between marine engines and bike engines, but Classification society rules are quite precise on hardness of crankshafts.  Though nitriding is case hardening, it is a hardening of the grain structure and not simply an external layer over the metal.  As such, it is susceptible to fracturing under bending stress.  The fractures will continue to grow through the entire grain which will eventually result in a broken shaft.  Of course, that depends entirely on the grade of steel used and the size of the shaft as much as the loading.  It may be that case hardening is acceptable on automotive crankshafts.  I know camshafts are case hardened and they see much higher stress reversals that does the crankshaft.
On a related matter, I have seen a marine engine connecting rod split down the middle like a banana skin.  It turned out that during manufacture, the top end was bored out too big and was reclaimed by chrome plating then ground back to correct size.  A fracture initiated in the boundary layer between the chrome and the steel and it travelled down in way of the oil hole running the length of the connecting rod until it gave way!  Obviously there were two metals in that instance but it pays to do some research before jumping in to something you're not entirely sure about.
I hope this is of some help.

[MG]

Mike, surface/case hardening is absolutely common with automotive crankshafts, either by induction hardening of steels with sufficient carbon content, or by case hardening (carburizing, nitro-carburizing. nitriding etc) and other diffusion processes. All these are based on microstructural transformation and are not external layers on the base material natch as you say. Sorry if my last post was unprecise or mistakable in this respect.
I am not familiar with marine engineering standards, but I suppose the statistical and technological size effects make a big difference,, compared to car and motorcycle components. A larger component statistically has more defects in the stressed area than a small one, and not only their number, but also their size as well as the size of the grains in the material differ markedly. Also the positive effect of residual compressive stress introduced by surface hardening processes is much smaller due to the comparatively smaller penetration depth in a larger part.
I think it is safe to say that nowadays ANY automotive crankshaft is surface hardened by one or the other method, to the best of my knowledge the last unhardened crankshafts were those in pre-war vehicles, paired with soft white metal bearings (which is still common practice with marine engines I think?). With the increase of power output and the need for better (harder) bearing material the need for hardened journals arose to increase crank life.
So, to cut a long story short, what is no good for a huge marine Diesel might still be very useful for a car or bike part.

[BSA_54A10]

Mike I also find your comments a bit confusing.
The biggest positive with nitriding was a massive increase in the fracture toughness of plain carbon steels.
We must have smashed hundreds of Charpy & Izod bars and run dozens of rotation fatigue specimens to establish the relationship between Nitrogen content and increase in toughness as Nitriding was a fairly new process in the 70's and carbo-nitriding was yet to become commercial.

[Toby]

""The biggest positive with nitriding was a massive increase in the fracture toughness of plain carbon steels.
We must have smashed hundreds of Charpy & Izod bars""

Charpy and Izod are Notch Ductility tests surely ?.

[MG]

Charpy and Izod are Notch Ductility tests surely ?

Yep, they are notch impact tests. The Charpy test incorporates a three-point bending setup, Izod is is a cantilever beam impact test.

[BSA_54A10]

Charpy and Izod are Notch Ductility tests surely ?.

Actually they are notch sensitivity tests, not quite the same thing, from which you can derive ductility ( or lack there of )

And how would you measure toughness ?
By definition it is the stress required to propagate a crack within the ductile region of the material.
So we used the Izod multiple times to produce a crack and continued till complete failure.
The fracture surface was then photographed, enlarged and measured from which we could calculate an approximate fracture toughness.
Once we knew how to make a "standard" crack, we cracked the sample on the Izod then transferred it to the Charpy to continue with a 3 point load test.

While none of these complied with what is now a standard tensile or 3 point load tests it was good enough to be able to plot apparent fracture toughness against nitrogen content and that was the aim of the exercise
I was teaching at TAFE ( Trade School ) at the time and "Fracture Mechanics" was the realm of NASSA & Defence in those days.

[jaytee1960]

Ho Folks, another newbie here,.... however back to the original question, metal spraying or not? I also spoke to SRM and they advised against it as such and suggested speaking to SEP in Kegworth Nottingham, who do another form of welding... which will not peel off under load. Just wondering if anyone has experience with SEp ??
Thanks
JohnT