| Author |
 Topic  |
|
PeterL
United Kingdom
1731 Posts |
 Posted - 07/06/2012 : 10:31:37
|
Colin B's comments on the MOT thread, which I do not wish to hijack, have caught my eye.
"…no one has commented on my point about crack-testing steering knuckles and steering drop arms…. please tell me how the current MOT Testers can be certain that the steering components are not full of cracks (as they most probably are after 80 years of bouncing about on British roads.)… John Thornley was very concerned about these items... The T Racers also insist on crack-testing these bits every year as part of their race regulations."
How often do people crack test theirs? I did mine before I fitted them and have not touched them since. I don't race, I do tour. Do I need to worry?
Cheers
P |
|
|
spitfire
United Kingdom
371 Posts |
Posted - 07/06/2012 : 11:17:45
|
Does not anyone produce, or have considered producing this front axle beam.
Something that really is a safety fulcrum point of the MMM car?
I suppose we'll get the string of "Spitfire,don't you know blah, blah.." replies.
How much, in this computer exact (ha ha) world, would it take to refabricate these components.
Even if, through this site/club it involved a set amount of orders being taken?
Isn't that the strength of a club, supplier interface?
Richard Hardy, you're pretty good at rising to the challenge!
I know through the MGOC, engineering CAD workshops that make racing components. Now if something can withstand todays racing G forces... They've also just made a complete vintage Rolls Royce gearbox. |
 |
|
|
Martin Warner
United Kingdom
85 Posts |
Posted - 07/06/2012 : 17:11:52
|
| I had all the front axle components on the KN crack tested a couple of years ago as I have been punishing them for nearly forty years now. All passed with a clean bill of health. I was so suspicious of this that I had them re-tested by another company using a different process. Again a clean bill of health. So maybe we worry too much. Usual disclaimers if something of yours breaks! |
|
|
|
spitfire
United Kingdom
371 Posts |
Posted - 07/06/2012 : 17:15:25
|
Hi Martin. Me, being a goon ... What are the two different testing processes? |
Edited by - spitfire on 07/06/2012 17:16:08 |
|
|
|
bloodysalmon
Spain
1482 Posts |
Posted - 07/06/2012 : 17:20:18
|
I would be very interested in who we take, for instance, drop arms too to get crack tested and at what cost this may be! Midlands preferably ...
Chris Blood (D0407, D0430 & F0753) |
|
|
|
John Reid
United Kingdom
705 Posts |
Posted - 07/06/2012 : 18:42:30
|
Spitfire, this was the late Paul Duncombe's reply in 2006 to a similar question about the tools and techniques for crack detection:
The principle non destructive testing methods are Ultrasonics, Radiography, Visual Optical, Eddy current, Liquid dye penetrant, and Magnetic particle. The selected method depends on a number of factors such as availability of equipment, access and required sensitivity. Ultrasonics is very sensitive but relatively expensive kit is required. Radiography is quite sensitive and the kit is very expensive. Visual Optical is obviously cheap (most of us have two Mk1 eyeballs) but very insensitive (by the time you see the crack in the component it's probably about to fracture!) Eddy current tests are not too expensive to set up but geometry can influence results; they can be very sensitive. Liquid Dye penetrant is inexpensive to carry out but is relatively low in sensitivity, and the same applies to Magnetic particle.
For the testing which you require on steel steering components with relatively uneven surfaces the techniques used can be radiography, Magnetic particle, or Liquid dye penetrant. Radiography is the most expensive of these three, Dye penetrant the cheapest. I have seen grinding cracks on a ground steel surface (sensitive to thermal cracking) which were just not visible by dye penetrant examination, however steering components are nothing if not tough (rather than brittle), they are designed to bend rather than break, they are malleable. But they can be suceptible to fatigue. Fracture will occur in these materials only after a substantial fatigue crack has grown to a size where the remaining material cannot support the dynamic loads imposed upon it. The cracks will be formed by repeated imposed loads exceeding the yield strength of the material. The imposed loads will concentrate around stress raisers like notches. It is therefore evident that non destructive testing of these components is looking for cracks that will tend to be "open" rather than very fine due to shrinkage of the surface due to, say, rapid cooling as in the ground surface cracking in the sample above. The fatigue fracture surface in steels will often contain rust due to penetration of moisture. Rust is porous. A liquid dye penetrant is likely therefore to take up residence in a crack which is exactly what we want it to do. Some will argue that any cost is justified when testing steering components, and few would deny it, but providing that the procedures for dye penetrant testing are strictly adhered to it is, for the amateur, a good method for crack detection. I used it myself on my steering components. I would say that if there is any doubt at all about a result then magnetic particle or radiography testing should be carried out as a double check.
The chemicals for liquid dye penetrant testing are readily available (eg RS Components, or LAS Aerospace).
John R |
|
|
|
spitfire
United Kingdom
371 Posts |
Posted - 07/06/2012 : 22:24:10
|
Thanks John. Comprehensive stuff indeed.
I was wondering if there might be some "ancient knowledge" where you could tap and detect a dead pitch a la wheel spokes. Quite interesting how properties of, say, metal can alter dramatically but not in a visual sense.
Might cars in hard use retain a more maleable composure than those laid up?
Force, energy becoming a low heat but a uniform flex through the arm??? |
|
|
|
Ian Bowers
United Kingdom
948 Posts |
Posted - 08/06/2012 : 08:34:25
|
Metallurgically the only definitive non-destructive tests are ones which show cracks visually, by dye or radiography.
The cracks are from fatigue of the metal. Iron based materials have a lower threshold below which cracks do not develop or propogate. Above that threshold the forces will, over time, build up sufficient damage to create and propogate the crack. The forces involved vary according to the profile (in all three dimensions), and can be enhanced by local damage such as impact, scoring or filing/sawing marks. It is also possible to enhance the probability by such processes as chrome plating and to reduce it by polishing.
Due to the complex shaping of car parts, the varying metallurgical materials and production methods (cast performs very differently to forged materials) and the uncertain stress history, there is no certain way of predicting when cracks may occur or how and when the will propogate to failure.
Regular testing by non-destructive means is therefore advisable,
Ian Bowers
OD 6791
J3 3772 |
|
|
|
Colin Butchers
United Kingdom
1487 Posts |
Posted - 10/06/2012 : 10:22:20
|
I am delighted to read Martin Warner's comments, that he has had his K Type axle and stub axles crack tested twice wih no adverse result, so possibly my "doom and gloom" remarks were a tad over stated. From my own experience, I can confirm that the axle beam on my PA failed the crack testing and was deemed unsuitable for repair, and I recall that Dick Morbey also found a load of cracks in his PA beam, which I think he managed to polish out.
As to your enquiry about new axles being made, Spitfire, certainly there have been a number of K3 and K or N Special axle beams made in fairly recent years, but these were designed to provide racing strength with litte regard to originality in appearance. My guess is that to produce a small batch of original type drop forged beams for Js or Ps would be totally uneconomic even provided that a suitable manufacturer could be found. Stub axles are of course readily available.
I suspect that it is a question of balancing risk against practicality and whilst I know of several stub axles failing, I am not aware of an axle beam actually breaking. Perhaps someone out there knows better.
Colin B. |
|
|
|
spitfire
United Kingdom
371 Posts |
Posted - 10/06/2012 : 12:26:50
|
We are lucky to have the likes of Sports and Vintage, Vintage MG Parts and Barry Walker, amongst others spoiling us with new patterned parts. I notice a firm in the West country is producing stronger differential housings for MMM cars, with more to come.
I have been told that the axle beam and its eye is "the weakest part of the car."
These particular models, with racing and off road trialing, have been, it would be fair to say, HAMMERED!
If one is going to break. Wouldn't one of these witnessed events testify to the event?
To flesh out Colin B's question..
Does anyone know of anything actually BREAKING? |
|
|
|
John Haine
United Kingdom
208 Posts |
Posted - 10/06/2012 : 16:51:17
|
| My M-Type drop arm broke in half while I was doing a 3-point turn to get into the queue for the Test Hill immediately after passing scrutineering at the VSCC Driving Tests. It worried me that I had earlier driven down the A3 at 50 mph, but was assured this never happens on the road, only while manoevering. John |
|
|
|
Bob Stringfield
United Kingdom
854 Posts |
Posted - 10/06/2012 : 17:09:46
|
I had always understood that the main problem to look for with the MG axle beam was uneven 'upwards' distortion caused by cars being jacked up under the centre of the axle.
This may have an effect on king-pin angles.
Bob. |
|
|
|
tholden
United Kingdom
1640 Posts |
Posted - 11/06/2012 : 00:34:11
|
I have not heard of a J/P front axle breaking but have heard of stub axles breaking and know of at least two MW drop arms that have broken in the last couple of years one at slow speed and the other when manoevering in a garage.
TH |
|
|
|
bahnisch
Australia
674 Posts |
Posted - 11/06/2012 : 08:44:30
|
| I broke my PA steering arm (Pitman arm?) whilst manouvering into a parking spot over 50 years ago. It was an early car with a Marles-Weller steering box. |
|
|
|
Peter Frost
United Kingdom
57 Posts |
Posted - 11/06/2012 : 10:26:10
|
I can recomend Automotive Services in Northamton Tel:01604 722064.
I took the front Axle, 2 Steering knuckles,2 Steering Arms and the Drop Arm. I had polished some areas that I thought doubtful and pleased to say all passed. The process used was Magniflux and they Micro Peened the Drop Arm.
The chap I saw also went through other areas that would benefit from more polishing and explained "risers" and the changes in metal compostion used pre-war and now and why according to how they are used and age they fail.
Highly Recomended.
|
|
|
|
spitfire
United Kingdom
371 Posts |
Posted - 11/06/2012 : 14:59:19
|
| Thanks Peter. I work in Northampton, will give them a call. |
|
|
|
Topic |
|
Forum Locked
Topic Locked
