AdamR
|
| posted on 20/7/04 at 11:03 PM |
|
|
destressing...
That's 'destressing', not 'distressing'... at least I hope so.
Was chatting to my uncle at the weekend about my forthcoming Locost project... showed him The Book etc, in which he took quite and interest.
He made two convincing points about building the chassis (he has been an engineer all his life, and so he knows far more about this stuff than I
do):
1) I should have it destressed, which apparently involves heating it up in an oven to relieve the stresses caused by welding it all up. Makes
sense.
2) I should have it galvanised, otherwise over time it will rot from the inside out.
Having searched past posts, I have found that galvanising is not generally recommended as it makes the chassis weaker and heavier. But what do you
knowledgable people think about destressing? Is it necessary? Has anyone done it?
Cheers,
Adam 
[Edited on 20/7/04 by AdamR]
|
|
|
|
|
James
|
| posted on 20/7/04 at 11:19 PM |
|
|
As you say- galvanising not a good idea.
When it comes to destressing- I don't know of anyone having had it done and there's a lot of fantastic Locosts around that work perfectly
without it!
I think the aim is to construct it such that it doesn't need it- ie you weld at alternate points along the chassis and minimise the heating.
The main problems people have are the chassis banana'ing- this is the thing to avoid.
Hope that helps,
James
|
|
|
stephen_gusterson
|
| posted on 20/7/04 at 11:21 PM |
|
|
how long do you think it takes to rot 1.6mm steel from the inside when a std road car is approx half that thickness?
atb
steve
ps
how much are you going to drive an open car in the rain anyways.....
[Edited on 20/7/04 by stephen_gusterson]
|
|
|
thekafer
|
| posted on 20/7/04 at 11:25 PM |
|
|
This is usually done on aircraft fuselage frames that are made of chromoly (like 4130) I've never heard of doing it on a mild steel frame..
I'd rather have a bottle in front of me than a frontal labotomy...
|
|
|
Viper
|
| posted on 21/7/04 at 06:08 AM |
|
|
De-stressing? a bottle of Vodka does the trick for me, just apply liberaly to your insides ( oraly). Works every time
|
|
|
Cita
|
| posted on 21/7/04 at 06:38 AM |
|
|
Stephen it is not that long ago that FIAT car's rusted away in front of your eyes.Proper treatment has been a sales trick in the 80's to
give 3 or more years garantuee on a new car against rusting...from the inside out!
The problem with inside out oxidation is that it's very difficult to determine how weak the structure has become over a certain period of
time.On the other hand it indeed takes a very long time,depending on the conditions like a salt water enviroment,to completely destroy a 1.2 mm box
section.
Heating and controled cooling is used in the aircraft industry to bring the value of the material back to an acceptable level but weight is of such
importance in their case that they often work on the limit regarding material thickness,hence the need to recondition the steel after welding.
I would not wory about the locost frame rusting away and a few good coats of good quality paint will make the car last for years.
|
|
|
JoelP
|
| posted on 21/7/04 at 07:24 AM |
|
|
im sure ive heard it said that destressing a locost chassis would leave it hopelessly twisted. am i wrong?!
|
|
|
JAG
|
| posted on 21/7/04 at 08:10 AM |
|
|
No you're not wrong. If there is enough residual stress in the welds then destressing it can cause it to bend.
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
|
|
|
Terrapin_racing
|
| posted on 21/7/04 at 09:21 AM |
|
|
Your uncles advise is based on sound engineering - we do heat relief work a lot on the refinery.
However, please advise him that there should be no issue with this on a chassis like the locost. Your main concern should be to limit your welding in
one area (I usually weld / braze diametrically opposite points on a chassis at a time) to avoid distortion.
Galvanising - again not a problem technically if you want to really make the chassis rot proof ad infinitum. Some points to bear in mind.
It will add some weight to the chassis - a good galvanising firm should be able to calc this for you.
Watch who you use - heat distortion on a chassis like this during dipping is not such an issue (been there) - the gorillas that operate it are. Once
had a chassis dipped locally only for a stacker truck driver to run over a corner section!
You will need to use a isolating seam sealer between the chassis and any bare aluminium (zinc and aluminium + salt water = reaction!)
|
|
|
Aloupol
|
| posted on 21/7/04 at 11:22 AM |
|
|
Destressing is often used on mild steel frames, ie in jig manufacturing. The aim is in that case to make it stable over years with high acuracy
(0,1mm/m or something).
When destressed the frame is banana twisted, and then is milled on functional points.
Imho not at all the locost purpose.
Galva: is used on some steel car frames or bodies. For the Locost I don't know the best anti rust treatment, able to protect the inside of tubes
and of welds.
|
|
|
craig1410
|
| posted on 21/7/04 at 12:03 PM |
|
|
If you want to rust protect the chassis then drill numerous holes in the tubes and pump in waxoyl. Then fit grommets where the holes were drilled.
Alternatively just ensure that all of your tubes are airtight by closing off the ends and taking extra care with your welds. If you paint the chassis
afterwards with a good etch or zinc-rich) primer then any remaining small airholes will be filled by the paint. The other thing to bear in mind is to
get good quality steel tubing to begin with and keep this well protected from damp to avoid any rusting during construction.
As others have said, the car is a fair-weather vehicle and will probably get much more attention (spit and polish) than a normal road car. It also
probably won't get driven very much when salt is on the road - I don't know about you guys but my car will have the best part of 200BHP in
a 650Kg RWD car which would not be funny in the ice and snow!!
In short - don't worry about either de-stressing or galvanising, save your worrying for the SVA test!! 
Cheers,
Craig.
|
|
|
andkilde
|
| posted on 21/7/04 at 12:08 PM |
|
|
Hmmn
If hot dip is out, how about the zinc undercoat paint they use to protect bridges?
Just out of curiosity, sevens have been around for ages, as have space frame specials and racers -- is corrosion a serious issue over the long term?
Or is a good lick of paint and careful construction (Sikaflex between all the ali sheeting and chassis, closed rivets, well designed drainage, etc)
enough to keep things rot free?
Cheers, Ted
|
|
|
Terrapin_racing
|
| posted on 21/7/04 at 12:26 PM |
|
|
Yes, done it!
be brave
|
|
|
Terrapin_racing
|
| posted on 21/7/04 at 12:47 PM |
|
|
PS: Agree , you will get no guarantee and you need to discuss with their engineer the most appropriate way to dip (not slowly from one end) and you
need to read up on vent hole locations also. Can be done but there are risks if not done properly! be warned
When steel sections or fabrications are immersed in molten zinc, their temperature is raised to that of the molten zinc which is typically 455 455oC.
C. The rate at which the steel will reach this temperature across its entire surface will depend on;
- the thickness of the individual sections making up the item.
- the total mass of the item.
- the dimension of the item.
At galvanizing temperatures, there is no change to steel's metallurgical microstructure and the galvanizing process is not hot enough to have
any
heat treating effects on the mechanical properties of the steel after galvanizing. THE PRINCIPLES OF A AVOIDING OIDING DIST DISTOR OR ORTION TION
However, at galvanizing temperatures, the yield strength of steel is lowered by approximately 50%. If the adjacent
steel is not at the same temperature and any stresses exist, the weaker area will be subject to movement by the stronger area. There is a
responsibility on the designer, the fabricator and the galvaniser to co-operate in ensuring that distortion risks are minimised or eliminated.
Ensure that the structural design of the item is sufficient to support its own weight at 50% of the steel's specified yield strength. Consider
temporary bracing if potential to yield exists. Avoid using large areas of thin flat plate. Guillotine cut plate is preferred to oxy cut plate.
Design to use uniform thickness sections throughout the fabrication. Ensure welding and assembly techniques minimise stresses in components making up
the item. Ensure that venting and draining are adequate. This will allow the item to be immersed in and withdrawn
from the molten zinc as quickly as possible.
|
|
|
Terrapin_racing
|
| posted on 21/7/04 at 12:48 PM |
|
|
Lets not forget electro plate galvanising also - no heat involved!
|
|
|
AdamR
|
| posted on 21/7/04 at 12:50 PM |
|
|
Thanks for the replies all. I thought there must be some good reasons why I hadn't heard much about destressing on the forum.
As for the galvanisation/rust prevention, I'm tending to agree with the good lick of paint is sufficient theory.
Cheers
[Edited on 21/7/04 by AdamR]
|
|
|
Terrapin_racing
|
| posted on 21/7/04 at 12:58 PM |
|
|
Adam, you might want to try
http://www.enginewise.co.uk/zinc-galvanising.php
|
|
|
Alan B
|
| posted on 21/7/04 at 01:47 PM |
|
|
Destressing?
In 30 years of engineering I've never heard that word.....maybe just me I guess...
I assume we are all talikng about stress relief?
If so I agree with the concensus....it's not necessary.....also neither is galvanising IMO..
|
|
|
Terrapin_racing
|
| posted on 21/7/04 at 02:35 PM |
|
|
I haven't either Alan, but picked up the thread!
Mind you with two daughters I now have to deal with this generations meanings of
cool, mingin, gay, so yesterday! etc.
|
|
|
spunky
|
| posted on 21/7/04 at 05:00 PM |
|
|
Adam if you want the chassis to last for ever go for Waxoyle, as Craig said. It really is good stuff. Had a Rover SD1 that had been treated early in
its life, it was 17 years old and not a spot of rust, and we know how a SD1 could decompose overnight.
Finish all your welding before filling the tubes though
John
The reckless man may not live as long......
But the cautious man does not live at all.....
|
|
|
Mark Allanson
|
| posted on 21/7/04 at 05:38 PM |
|
|
quote:
Originally posted by Alan B
Destressing?
In 30 years of engineering I've never heard that word.....maybe just me I guess...
I assume we are all talikng about stress relief?
If so I agree with the concensus....it's not necessary.....also neither is galvanising IMO..
My thoughts exactly, I have only stress relieved anything welded when it was subsequently going to be machined, milled etc.
I did a couple of prototype gearbox cases (MOD bound) in 19mm plate, these were stress relieved before they were line bored for the shafts etc. The
finished articles were going to have cast casings
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
|
|
|
crbrlfrost
|
| posted on 21/7/04 at 08:38 PM |
|
|
I don't know much about galvanizing, so I won't touch on that. Stress relieving on the other hand probably shouldn't be an issue on
a mild steel space frame. Typically stress relieving is done for high precision parts to be machined, as stated earlier, or on high alloy steels and
aluminium. This usually consists of welding, fixturing, annealing and then re-heat treating to ensure homogenous strength of the part. In race cars
sake, that usually is limited to control arms, and other highly stressed components. A complete chassis would be very difficult and there are
arguments to the effect that its better to weld chromoly with a gas rig due to the greater heat dispersion zone is comparison to tig. The theory being
since the heat is spread over a greater area, there won't be such a rapid change in composition versus something like TIG. A good point being
that 4130 was originally developed back around WWII to be a high strength steel specifically for gas welding (TIG and MIG wasn't around yet).
Sorry for the length, Cheers!
|
|
|
NS Dev
|
| posted on 22/7/04 at 07:47 AM |
|
|
both stress relieving and galvanising will warp the chassis. Stress relieving will also scale it up badly unless you get it done in a controlled
environment furnace, which will cost a lot to do. A friend of mine built a 4 wheel car trailer and had it hot dip galvanised, this warped the deck
downwards at the back by about 1.5 inches, he was not best pleased!!
|
|
|
Hugh Jarce
|
| posted on 24/7/04 at 01:37 AM |
|
|
I once made a bike trailer with a bit of box tube for the axle and a bit of channel to hold the bike. I had it hot dipped and it looked like something
from a cartoon with the axle bent like a bananna.
|
|
|
mangogrooveworkshop
|
| posted on 25/7/04 at 11:47 AM |
|
|
Mad4x4 got his done in a high tec cold galv process used on a north sea oil rig. nothing to look at but very good reports.
|
|
|
