Windscreen support pillars

[John Murch]

I recently had a failure of a windscreen pillar, I was fortunate that the total failure (I assume it must have been partial earlier) happened at very low speed on a 'speed hump'.
Had it happened ten minutes earlier when I had been proceeding at considerable speed, I fear unpleasant consequences to my head would have occured.

The pillar would appear to be a brass casting, I have always thought the pillars looked very slim for the forces placed on them, any thoughts on replacements would be appreciated.
John

[Paul Spencer]

Oh dear! It sounds like there was no consequential damage to you and looks like none to the car, which is lucky. My windscreen is a James Pearce one from the 1970s. I don't know whether they still do them. If I were going for a new one, I would either go for a more traditional opening design or (more likely) one that sloped back a little to avoid being dazzled by following cars at night.

[Chris Card]

If, as Paul suggests, you have a new frame made that rakes back slightly, this will affect your hood.

Chris

[John Murch]

In the short term its being repaired, I'm trying to decide the best course of action, to replace with beefier cast pillars or pillars machined from solid.

The screen is hinged at the top, i have only once opened the screen, having got caught in a snow blizzard, opening it such that I looked through an inch slot, no snow came through and I felt i had better visibility than the modern cars with good wipers.

I'm not sure that I would go for a folld flat design, I cannot imagine when I would ever fold it down!

John

[Robert Craven]

John,
A screen that opens up doesn't legally need wipers or washer. I don't know what your old screen was like and whether you relied on this exemption (I do to the extent that I have just one wiper and no washer), but if you change to a windscreen which doesn't open you'll no longer be exempt from this requirement. I don't think it's anything to do with the age of the car.

PS, on a cold foggy day I've had to rely on opening the windscreen to see because mist condenses on both the outside and inside.

[Iain Warner]

John
Have you had quotes from the companies that advertise new hoodframes and windscreens for getting a set of replacement pillars made?
I think I would always be worried when driving about the integrity of the repair and would have to give the other pillar a very close inspection. Did it fracture in way of one of the bolt holes?
Iain

[John Murch]

Any repair is only a very temporary measure. I am in contact with the obvious suppliers. The fracture was about an inch above the upper (of two) fixing holes.
I would love to have an opinion on the merits of a casting versus a machined pillar.

[Iain Warner]

The only 'volume' car maker that that still uses this form of windscreen support that I can think of is Morgan.
Traditionally the frame supports were always cast but looking at my wifes 1976 4/4 the two supports bolted on the scuttle are cast with the machined bar side supports set into them and held in the channel by a couple of bolts.
You could always ask the Morgan factory if the will give the contact for their supplier.
I would have thought that the properties of the particular alloy used in either case is the main criteria for which I guess you need expert advice.
Iain

[Stephen Blakey]

[John Murch]

Stephen, many thanks for the interesting information. I am envious of your skills in being able to even attempt to forge one.
My thoughts in beefing up were in the region of 15% in the part against the body up to where the screen attaches. I'm now reading up on stress corrosion cracking

[Christopher Carnley]

The parts being up to 90 years old have served very well and I believe that from looking at the failure the pillar was made by hot stamping.

It is a victim of straightforward metal fatigue where the flexible lever arm has encountered the rigidly fixed part and the cyclical bending and twisting have proved rather too much and the alloy has crystallized.

Any metallurgist should be aware that stress corrosion cracking only occurs where metals are subject to internal compression forces, and in the presence of a mildly corrosive medium, which you don,t have here.

There is a very good UO film detailing this, when examining the cracking in the wrought aluminium alloy central fuselage member that holds the wings on. You would never suspect that such a part could be under water. The actual forging of the part from a big cylinder is a joy to watch, and the subsequent revised heat treatment to control the skin tension.

Having some experience of stainless forging (!) I would plump for carving them out of 303 stainless bar.

I watched the BBC4 film of the building of the Severn Bridge again last night, and found it very moving, it dwarfs our pathetic endeavours, many times over.

[Stephen Blakey]

Chris,

I wonder if you might wish to re-write that. You appear to be suggesting that a crack can be started by a compressive force?

Best wishes,

Stephen

[Christopher Carnley]

Stephen,

The research conducted by the MOD on the aircraft component revealed that the cracking of the main frame was due to service stresses on it amplified by the presence of condensation as the corrosive medium, and the fact that the incorrect heat treatment had by skin tension induced compression forces within the structure, ie stress corrosion.
Such a condition does not exist without being bathed in a corrosive medium, eg dilute acid.

[Barrie Warrener]

[Stephen Blakey]

Hi Barrie,

Mentioning molecules in the context of metals, which are ionic, is a risky business as you can find yourself on the receiving end of pedantry overload.

I'm no expert on concrete but I'd always understood that the failure of concrete compressive test samples was due to the lateral expansion causing all sorts of tensile and shear stresses. I could ask my pal who runs a concrete testing lab but, frankly this is getting so far removed from John's windscreen pillars that I'm getting to the point of wanting to agree to differ and move on.

I'm seeing the metallurgist again tomorrow so I'll ask about whether you can start a crack in a metal by applying compressive stress. It's nearly forty years since I studied stress analysis and a lot of it went over my head then. In those days it was taught by people who were obsessed by calculus and, in retrospect, liked to show off.

Stephen

[Christopher Carnley]

"natrual" Barrie? Since when did haired lime plaster really stick anyway,and the little side windows on the Comet 1 were not the problem source?

Crushing of concrete test samples usually results in diagonal shearing, as the aggregate dictates the direction of failure.

Any competent text book on Metallurgy states that for "stress corrosion cracking" to occur the component has to be in a corrosive medium, the cracking is then due to the interior of the component being in compression by the skin of it being in tension due to bad heat treatment.

Any engineer should know fresh metal castings have a core in compression, and the skin is removed to weather the casting, sometimes for months before the part it final machined.

I cannot understand why you find this so difficult to follow.

[John Murch]

My very nicely done repair failed about 6 weeks ago, fortunately without damage to me or the car!
I have had two new pillars made, they were done from a solid bar, machined up to approximate shape, it was then a week's work to finish them off.
I am delighted with the result. Collected the car from deepest, darkest Cornwall this afternoon. I placed the order a month ago and agreed a completion date of today. How often does that work out!
Full praise to Richard Prout for his excellent quality work.
http://www.rpcustommetalwork.co.uk