Face Knurling...

As oily Rag has mentioned about the dangers of stainless in high stress applications such as motorcycle wheel nuts can anyone give me some data.....

I dont doubt what people tell me but I like to see the data and context, also it seems to me to be a sweeping generalisation to lump all stainless steels as dangerous for such uses as motorbike wheel nuts.

Oily Rag was talking about the dangers of stainless axles, not nuts.

And the data can be deceptive. Stainless bolts can have same or even higher tensile strength than carbon and alloy steel fasteners. But it is stainless's tendency to work harden and fracture under repeated severe load cycles that causes the problems. So things that get a repeated hammering like axles or caliper mounting bolts can fail. Nuts, not so much. And stainless fasteners used for things like holding an oil tank or seat in place are fine.

Edited By Hopper on 12/04/2021 12:07:07

Thanks for clearing that up, one more related question: if you mix stainless with ordinary steel will the ordinary steel corrode more just like a battery?

I too dislike sweeping generalisations! Hard to avoid because few Model Engineers have a professional background in materials science, or the time and interest needed to tackle the subject in any depth. Thus we bundle vague ideas about entire families of alloys together, and often as not it's good enough to understand the broad differences between 'steel', 'aluminium', and 'brass', even though these all have wildly varying properties, good and bad! We're happy as long as metal machines reasonably well and doesn't break. Ignorance is bliss!

Fortunately, engineering for strength and reliability isn't often required in Model Engineering. As not much of my work is safety critical it doesn't matter my approach to metals is naive. I like Aluminium alloys because they are cheap, Brass because it it machines well, and mild-steel because it's strong and hard enough for what I need. I use what I have rather than buy in the most appropriate material for the job. My workshop choices are pretty brainless!

Slapdash methods aren't smart when the engineering has to be safe. Brakes, lifting gear, motorbike axles, wheel fixings, engine mounts, and anything where people get hurt if it fails need proper attention. To keep life simple there is a lot of general guidance around, such as building codes and type approvals, that tell constructors what to avoid, without going into detail. Behind the scenes, appropriately qualified engineers do the design, including selecting the suitable materials. Constructors can then apply their skills without needing to understand a mass of complicated design detail. It's iffy for constructors to modify or re-apply a design on the assumption they understand it! Might be OK, might not. Design decisions based purely on experience could be guesswork rather than understanding.

So as soon as someone might get hurt if a home-made part fails, it's necessary to look things up. Assume nothing. Read specifications, do the sums, and apply risk analysis. It's a thinking job. Bear in mind that the part might have to pass an amateur test such as magnetism; testing for stainless steel with a magnet is crude, and it might allow a mechanically unsuitable stainless through, whilst denying a well-chosen stainless because it happens to break the rules. Not daft, because the chap with the magnet is following a general guidance with a good chance of detecting a dud material with a simple test. Point is designs have to satisfy all the requirements, not just survive a short test drive.

If I wanted to make a wheel nut out of stainless, I'd start with the specification of the original and ensure the substitute was at least as strong. Where it gets complicated is the need to account for subtleties like work-hardening, fatigue cracking, and temperature sensitivity. This is why thousands of different steels are available, and several hundred different varieties of stainless, each designed to excel at a particular job. I'd also worry about why the original was drop forged with rolled threads - both methods are stronger than machining from stock.

WW2 Liberty Ships are an example of getting it wrong. A number, all virtually brand-new, fell apart at sea and in harbour.

Two reasons were identified. One was cutting cargo hatches with stress-raising sharp corners, the other was building the ships with a mild-steed steel alloy that becomes brittle below freezing point. The steel was fine for most stormy Atlantic crossings, but not for ships routed far North in some of the coldest winters ever recorded...

Using the correct materials and processes is about reducing risk. Replacing a motorbike wheel nut with a third-rate substitute doesn't guarantee failure: in practice, there's a good chance will last a long time - forever on a display bike. Doesn't mean the repair is acceptable. Foolish to put the same nut on a performance bike ridden aggressively over rough roads for hours on end.

Dave

Edited By SillyOldDuffer on 12/04/2021 14:39:30

To reply to a few of the comments made here:-

Jon - Anodised aluminium of HE30 specification is fine for nuts, so long as thread engagement is 1.5x diameter. Anodising is a surface treatment which imparts a hard skin on the alloy, so stress raisers are nullified. The thread does need to be suitable though, for maximum strength I always advocate BSW (see my other post on F1 engine failure with M11 threaded Main Bearing Stud after 300km but successfully replaced by 7/16th BSW)

Chas, mgn, and Hopper - Stainless spokes are OK if they are made by the correct process! which will inevitably be a drawn rod, upset forged head and a rolled thread. Here the process is minimising the chance of a stress raiser which would otherwise be a 'notch sensitive area'. I would hope that the stainless stock is ultrasonically inspected for possible 'voids' in the drawing process - this was the procedure for materials from Speciality Steels Division of British Steel in Sheffield. I could not guarantee anything from China being so tested though! For Indian supplied Stainless spokes, if they come from Tata Steel there is an excellent chance they have checked it correctly as the equipment from British Steel was shipped out to Pune. My 540 RE Continental GT in India managed to break rear spokes - but given the pot holes that was no surprise.

Colin - Yes, a sore point that Titanium is banned under ACU reg 14. My BSA failed the 'magnet test' when applied to the axles (a small iron insert in the hollow spindle got around the test!!). The bike also has Titanium spoke nipples along with nuts & bolts all over the place. It even has a Titanium cam wheel (silver plated to prevent galling). Fortunately they never checked the fork stanchions or the yoke head bolt! I was happy with using the bike like this as it was all BSA works parts and was originally from the World Championship Moto crosser. I did after the second season have all the parts X rayed to check for any potential risks.

SOD (Dave) - I think you have summed up the risks and concerns admirably well, and given an excellent graphic of when things go badly wrong (the Liberty ships).

Just as a final point - it is not about just the material but also a crucial part is the process involved. As Jason pointed out the thread rolling process is superior for thread strength AND for preventing unwanted stress raisers. The process starts with the alloying of the material in the furnace (single electric arc, open hearth, double vacuum remelt) the stock manufacturing (hot rolled, cold drawn, forged billet) and then the physical component manufacturing and subsequent heat treatment and allied processes.

As I started out in my initial reply - a little knowledge CAN be a dangerous thing - we owe it to ourselves to understand all these possible pitfalls and ask each other for advice where we are unsure. I have been asked on several occasions to make something which I have considered to be a 'critical component issue' and have disagreed and refused to make with the offered material because the material specified was either completely unknown or totally unsuitable. I never make critical items from Ti230 for instance, always preferring Ti316.

Martin

Two reasons were identified.

Rather more than those two reasons in that case, Dave.

The steel used was that used for building ships at the time - what changed with Liberty ships was that the original (British) design was for "traditional" rivited construction & the Americans redesigned them to be all welded to allow construction of subassemblies and speed construction. With the riveted construction, if a plate cracked the run of the crack was limited to the plate concerned. With the welded construction the cracks ran through welds and ajoining plates. Add in to that poor quality welding perfomed by poorly trained, unskilled, welders (it seems that many if not most cracks started in poor welds or weld preps - that would appear to be the case in the example you quoted), the lack of knowledge at the time of the effect of welding the type of steel used & design shortcomings that put welded joints in stress raising positions due to the prefabricated construction were as much to blame as the material used. One "workaround" used to alleviate the problem was to rivet (not weld) a substantial steel plate belt around the ships below deck height.

I find the whole mobilisation & mass production of war materiel in the US during the war fascinating & I doubt that such feats could be accomplished today. The whole shipbuilding program was an amazing feat - largely starting from scratch, rethinking building techniques & many hard lessons were learned that laid the foundations for what we largely take for granted today. Interestingly, the only remaining part of one of the larger Liberty ship builders, Kaiser, is the company healthcare scheme set up at the time.

Nigel B.

It certainly is interesting. Considering they built those Liberty ships in as little as five days from laying the keel to launch. Even with fitting out, they averaged about 40 days from start to finish. The aim was to churn them out faster than the U-boats could sink them I guess. One thing the Yanks can do is get things done. I saw a video on YouTube about Chrysler built a factory, including foundry, to build the engines for Liberator bombers, from a greenfield site in less than a year, including the parking lot for 17,000 workers. Thinking big is their other strong point.

And those old Liberty ships were still chuffing around the world as late as 1970, some with hulls that had been extended by 70 feet to increase capacity. So they must have got the cracking issue sorted out. My old man sailed on one with Blue Funnel in the 1950s (and several Victory ships/Sam boats). Imagine getting paid to run a working museum complete with triple expansion engine!