girdraulic blades-straightening


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Hi all, controversial subject this one, but as i have discovered that one of my blades is about 3/16-1/4" out of true, i am looking at the possibility of rectification i.e. straightening. :eek:
I have contacted a specialist firm in the Midlands, whose core business is heat treatment of alloys, and heating/straightening/re-heat treating of alloy components.
I am assured that this can be done, but the process is VERY temperature critical according to the material spec and each grade of alloy has its own temp at which it can be fettled. The information we have suggests that girdraulic blades are made from "L40", (although I have read also"RR56") and forged. Speaking to the chap at this company, he tells me L40 is a cast material, and if the blades are forged then unlikely that is the material used? Can anyone shed any light on the material originally used?
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John Cone

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It will interesting to see what comes out of this enquiry, as i have hanging up in my garage four fork blades that are bent at the top at thier thinnest point. I am very reluctant to throw them away.

Ken Tidswell

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The blades need heating in a salt bath to normalise them . I presume at several hundred Degrees Celcius, but well below the melting point of this particular alloy
that is to say reverse the process of cooling and hardening. They can then be straightened in a suitable jig. Mac Reid ( no longer active? ) used to offer this as a service, The forgings then reharden over time.

John Appleton

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The blades need heating in a salt bath to normalise them.

We had fork blade material analysed and came up with a heat treatment which worked well. A salt bath is not used. We had our local heat treat works anneal two legs, which I collected immediately on removal from the furnace. We then straightened them, all within six
hours as age hardening starts very quickly. the legs were then returned to the heat treatment works for artificial aging (tempering ).
In the annealed state the blades were very easy to manipulate and we had no problems with alignment using three spindles through the two spindle holes and the axle hole.
If John Cone or genuinely interested parties would like to contact me by PM I will be pleased to go deeper into details
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Pete Appleton

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Facts and figures

Looks as though the old man beat me to this as I have to work for a living and he keeps reminding me that he should be retired by now. But here is my contribution:-

I recently did a bit of research on this matter as the fork blades that I used in building my Rapide had suffered a fatal encounter with the side of a car, back in the sixties.

To start with we tried an odd and severely bent blade that was lying around the shed. When put under the press, with no pre-treatment, it put up a lot of resistance and then broke.

Consultation with my friendly, local heat treater produced some suprising information :-

Annealing of aluminium alloy is totally the reverse to steel. You take the aluminium up to a temperature, dependant on the chemical composition of the alloy, and then quench it.

Salt bath solution is just one type of heat treatment furnace. Mine was done in an air blast furnace with no ill effects.

Once annealed the alloy will then begin to re-harden with time. This once again depends on temperature and composition. The higher the temperature the faster the hardening will take place. I was advised to carry out the hardening within a day of the annealing and to keep the blades cool, i.e. not in front of a heater, until we had finished straightening.

Re-hardening is known as precipitation hardening or artificial ageing. This involves heating the material to a specific temperature for a time and then allowing it to cool slowly.

I sent a piece of the first, broken, blade to 'Bodycote materials testing Teeside' for destructive analysis. The results were as listed at the end of this. This seems to equate to a 2000 series aircraft alloy when checked on

Once annealed my blades straightened frighteningly easily under a fly press and the spindle holes were checked against a known, straight set. Some form of crack testing should be performed after re-hardening, especially if the bend was severe. We used penetrant dye (because we had some) but you could use ultrasonic or x-ray.

When given the chemical composition of the alloy the heat treatment works were able to look up the required treatment specifications.

All of this worked for me and has been ok for 15,000 miles - Try it yourself at your peril.

AL - Base
Cr <0.01%
Cu 2.25%
Fe 1.02%
Mg 0.67%
Mn 0.10%
Ni 1.21%
P <0.005%
Pb 0.02%
Si 1.03%
Sn <0.01%
Ti 0.10%
Zn 0.06%

Vickers hardness test 145 VPN


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I was concerned a few years ago about the heat from having my blades powder-coated. That Hayling Island fellow told me basically what you have related, that it would have been just fine, but by then I had painted the blades. Maybe next time.