Shot peening standing rigging hard fixtures

[smeaks]

Stop cracking by Shot peening standing rigging hard fixtures

Having read a recent post regarding cracks propgating in bottle screws it shows how mistreatment and repeated cycles of stress can significantly compromise the integrity of the rigging components. How often is a screwdriver used to torque a bottle screw fitting whilst tightening instead of using the correct spanner...? most time you get away with it but each time it is done the machined and cast sections are being subjected to higher than designed stresses. add to this the effect of cyclic loading from general use, every gust, change in direction and tack then the effects of metal fatigue will short life the components particulaly in the corrosive marine atmosphere.

To maximise the cycle life of these components the use of shot peening can induce compressive surface stresses that counter the destructive nature of the cyclic or abusive tensile stresses and can give huge extensions of working life to the components. the process is proved and inclusive in many areospace and automotive applications and would be just as effective in countering the effects found in steel rigging.

Prior to fitting the entire set of bottle screws could be processed relativly cheaply providing peace of mind and effective extensions in working life.

 

[wotayottie]

I'm sure you are right - in theory. But since I see very few cases of failure of what you call hard fittings, I can only assume that the stress levels are actually a failrly low % of max.

 

[Skylark]

We regularly use shot peening in my business, that is the manufacture of gears. I'm not a metallurgist and I stand to be corrected but I would have thought the cyclic tensile stresses impossed by the rigging on the bottlescrews act in a different plane compared to the benefit of shot peening the surface of the fixing, hence any benefit would be very small.
You suggest that sailing loads are higher than design stresses. Surely the yacht designed would ensure a reasonable factor of safety? This would be borne out by there being a low incidence of in-service failure. An interesting thought, none the less. What is the basis of your post? Do you have experience of using shot peening with this application?

 

[NormanS]

Having read a recent post regarding cracks propgating in bottle screws it shows how mistreatment and repeated cycles of stress can significantly compromise the integrity of the rigging components. How often is a screwdriver used to torque a bottle screw fitting whilst tightening instead of using the correct spanner...? most time you get away with it but each time it is done the machined and cast sections are being subjected to higher than designed stresses. add to this the effect of cyclic loading from general use, every gust, change in direction and tack then the effects of metal fatigue will short life the components particulaly in the corrosive marine atmosphere.

To maximise the cycle life of these components the use of shot peening can induce compressive surface stresses that counter the destructive nature of the cyclic or abusive tensile stresses and can give huge extensions of working life to the components. the process is proved and inclusive in many areospace and automotive applications and would be just as effective in countering the effects found in steel rigging.

Prior to fitting the entire set of bottle screws could be processed relativly cheaply providing peace of mind and effective extensions in working life.

What does the "correct spanner" look like?

 

[alahol2]

Here's one type... http://www.ebay.co.uk/itm/120895681398 but not really sure why it's any better than a screwdriver.

 

[VicS]

What does the "correct spanner" look like?

Probably not one of these

But it might be here somewhere

 

[NormanS]

Here's one type... http://www.ebay.co.uk/itm/120895681398 but not really sure why it's any better than a screwdriver.

Well I've never seen one of those, and I don't suppose I ever will! Maybe any suitable flat piece of metal would do, although
when you think of the relatively small load that you apply with a screwdriver, it's hard to see what harm you could do.
I used to have closed "Bottle screws". The only tool you could use was something very similar to a screwdriver.

 

[AngusMcDoon]

It's bad to use a screwdriver in this type of rigging screw...

It dents the edge which is a stress raiser.

 

[smeaks]

We regularly use shot peening in my business, that is the manufacture of gears. I'm not a metallurgist and I stand to be corrected but I would have thought the cyclic tensile stresses impossed by the rigging on the bottlescrews act in a different plane compared to the benefit of shot peening the surface of the fixing, hence any benefit would be very small.
You suggest that sailing loads are higher than design stresses. Surely the yacht designed would ensure a reasonable factor of safety? This would be borne out by there being a low incidence of in-service failure. An interesting thought, none the less. What is the basis of your post? Do you have experience of using shot peening with this application?

It is the fatigue failure that the peening offsets. The compressive stresses envelope the component thereby preventing the propagation of micro cracking from the cyclic stress loading. The design of the component will be sized and designed to accommodate the loads and should do hat job. This will protect from the loading calculated for the component, this design is based upon the expected load and the UTS of the material I.e as long as the load does not produce a force that will exceed this ultimate tensile strength. This loading will induce a general localised degradation of structure that eventually induces micro cracks that reduce the core integrity of he structure until the remaining material can not even take a single cycle under the same load.... At which point the component will fail abruptly. The shot peening whilst inducing the compressive stress in the surface will significantly increase the cycle count at which this occurs.

In your gears the mesh of the gears roll each engaging tooth into tension as it engages into the train forcing the reverse radii into compression. Therefore the similar cyclic loading is apparent in each tooth root every time it engages in power transmission. The peening protects the tooth from the surface regardless of the plane of loaded stress apparent across the root.

 

[Salty John]

I've never seen cracks in bottle screws, I suppose they do occur. The hairline cracks which signal trouble are usually at the swaged terminal.
Bottle screws are often re-used when rigging is replaced such is their durability.

 

[Skylark]

It is the fatigue failure that the peening offsets. The compressive stresses envelope the component thereby preventing the propagation of micro cracking from the cyclic stress loading. The design of the component will be sized and designed to accommodate the loads and should do hat job. This will protect from the loading calculated for the component, this design is based upon the expected load and the UTS of the material I.e as long as the load does not produce a force that will exceed this ultimate tensile strength. This loading will induce a general localised degradation of structure that eventually induces micro cracks that reduce the core integrity of he structure until the remaining material can not even take a single cycle under the same load.... At which point the component will fail abruptly. The shot peening whilst inducing the compressive stress in the surface will significantly increase the cycle count at which this occurs.

I agree that a material will be selected based upon load, UTS of the material plus a suitable factor of safety. Fatigue is generally related to the cyclic application of load. Failure is usually categoriesed as high cycle or low cycle. Crack initiation normally starts from a grain boundary. No one seems to be suggesting that there is a high incidence of rigging failing. It would be normal practice to examine any failed parts to determine the root cause of failure. It could be related, for example, to inclusions or other morphographic defects or cyclic fatigue, as you seem to be suggesting. The first example is a material processing issue, the second may be a design issue. Easiest "fix" to the design issue would be to either increase section or move to a higher grade material. Using a process such as shot peening is commonly used in industry but I can't get my head around it making a significant difference to a problem that doesn't seem to exist.

Most of my industrial experiences of fatigue have been with aluminiums and steel alloys. Unless it was proved to be, say, vibration induced over load, fatigue cycle count tends to run into the millions.

Most providers of shot peening do it to MIL-specifications. This also tends to suggest that there is good science behind the application.

Please tell us more because I'm intrigued?

 

[wotayottie]

Using a process such as shot peening is commonly used in industry but I can't get my head around it making a significant difference to a problem that doesn't seem to exist.

Couldn't agree more. When I worked at a leaf spring manufacturing plant, we used to shot peen single leaf springs for Ford amongst others because this raised the fatigue life several times. It was in the multi millions of cycles anyway. But a leaf spring face cycles back and forwards from compression into extension all the time. In contrast a rigging screw is permanently in extension and its typically in the 15 to 50% of its maximum working load. If its an austenitic stainless fitting then the shot peening would work harden the surface but I'm not enough of a metallurgist to know what this would do to its fatigue life. What I do know is that it would be way easier to get round any problem simply by fitting a heavier fitting in the first place.

 

[Little Sam]

Hi.

I just happened on this thread and it also happens to be right up my line of work.

We as a company have recently made Titanium hard deck fixings for one of the most exclusive yacht builders in the world, and i can categorically inform you ALL those parts where indeed shot peened.

Thanks.

 

[smeaks]

Hi.

I just happened on this thread and it also happens to be right up my line of work.

We as a company have recently made Titanium hard deck fixings for one of the most exclusive yacht builders in the world, and i can categorically inform you ALL those parts where indeed shot peened.

Thanks.

That's interesting and titanium is very responsive to shotpeening, were they done for fatigue life improvement or visual effect. Many architects specify peening because of the pleasant surface Finnish.

 

[Little Sam]

All the parts where shot peened for fatigue relief primarily the nice finish comes as an added bonus.

Firstly the parts where heat treated at the roughed out phase, then they where finish machined polished and shot peened. Heat treatment and shot peening together greatly reduces the stresses contained in metals, it allows the metal to be more elastic.

 

[Skylark]

All the parts where shot peened for fatigue relief primarily the nice finish comes as an added bonus.

Firstly the parts where heat treated at the roughed out phase, then they where finish machined polished and shot peened. Heat treatment and shot peening together greatly reduces the stresses contained in metals, it allows the metal to be more elastic.

Hang on a mo......

Heat treatment is a generic term. Could be to normalise to remove any residual stresses from some form of prior work / processing. Could be one of the many forms of hardening, depending upon material. What do you mean by "allow the metal to be more elastic"? Normalising should improve ductility but likely as not, post machining the material will need to be hardened. This finishing process will restore strength via UTS.

Shot peening puts compressive stresses at the surface under controlled conditions on the premise that cracks do not propogate from metals under such conditions. This has nothing to do with elasticity / ductility.

Having said my bit, I need to add that I've never worked with Titanium so I'm not familiar with its properties.

My background is automotive and, like wotayottie, I'm used to measuring fatigue in millions of cylces and I'll stick to my earlier view that I can't get my head around it making a significant difference to a problem that doesn't seem to exist.

I'm not being provocative, I'm merely trying to learn something................but for now I'm a skeptic!

 

[Little Sam]

Ive asked our Metallurgist at Ferrari F1 today about this, as this subject is getting above my head and becoming a pure metallurgist subject.
I only told you what i know as a mechanical engineer, the Metallurgists decide how a component should be treated..shot peend et al.

its something i dont want to make public knowledge on a worldwide forum.

OK onto your next statement of "A problem that doesnt exist"

I find the above statement as a bit daft . Myself and many engineers like me, are forever trying to find ways of making things faster lighter and stronger......and cheaper ! Its evolution and the cutting edge of engineering, engineering know-how that gradually gets passed on. BUT NOT YET: We are forever trying to improve, make headway, even if its only by a percentile, its an improvement and the tiniest one counts.

Just ask the dead passengers of flights all over the world if you could , that have crashed because of Component fatigue. This is a very real problem that does exist, i grant you, you may have been referring to boats where a component failure is hardly likely to kill someone unless of course a mast or boom happens to drop on some unfortunate souls head. But, i do work in a job where component failure is very likely to kill in a blink of an eye.....

Yes we do heat our titanium up to normalize it to a certain temperature and we also choose the grain direction in how it should be machined and we also choose exactly how it should be shot peened. All the above I'm not for saying how and why that's because if everyone knew we'd all be making parts for F1 teams.....or you'd be trying to get my job !

So take it from me.
Heat treatment (a certain kind)
Machined in the correct grain direction.
Shot peened

Increases component life span.

End of story. Ciao.

 

[NormanS]

And how relevant is that for ordinary sailing as we know it?

 

[Little Sam]

And how relevant is that for ordinary sailing as we know it?

Because if you read the thread, we are not talking about "ordinary sailing".

We are talking about taking boat components to a new level. Well at least i am, and the OP, by the sounds of him too.

 

[Skylark]

Ive asked our Metallurgist at Ferrari F1 today about this, as this subject is getting above my head and becoming a pure metallurgist subject.
I only told you what i know as a mechanical engineer, the Metallurgists decide how a component should be treated..shot peend et al.

its something i dont want to make public knowledge on a worldwide forum.

OK onto your next statement of "A problem that doesnt exist"

I find the above statement as a bit daft . Myself and many engineers like me, are forever trying to find ways of making things faster lighter and stronger......and cheaper ! Its evolution and the cutting edge of engineering, engineering know-how that gradually gets passed on. BUT NOT YET: We are forever trying to improve, make headway, even if its only by a percentile, its an improvement and the tiniest one counts.

Just ask the dead passengers of flights all over the world if you could , that have crashed because of Component fatigue. This is a very real problem that does exist, i grant you, you may have been referring to boats where a component failure is hardly likely to kill someone unless of course a mast or boom happens to drop on some unfortunate souls head. But, i do work in a job where component failure is very likely to kill in a blink of an eye.....

Yes we do heat our titanium up to normalize it to a certain temperature and we also choose the grain direction in how it should be machined and we also choose exactly how it should be shot peened. All the above I'm not for saying how and why that's because if everyone knew we'd all be making parts for F1 teams.....or you'd be trying to get my job !

So take it from me.
Heat treatment (a certain kind)
Machined in the correct grain direction.
Shot peened

Increases component life span.

End of story. Ciao.

Senti, raggazzino,

You've gone too far drawing comparisons with "dead passengers". That's poor taste, IMHO.

Try re-reading the OP. Your comments about incremental improvements being woven within the culture of engineering is bang on the money. However, the OP talks about using a screw driver to tension the bottle screw and refers to cyclic loads from general use. Very much ordinary sailing. You're the guy bringing in super-duper yachts.

Nothing more to add. Anche per te, sperriamo.