MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Growlerizing Halfshafts - Shot peen vs Nitriding

I am looking at getting a couple of half shafts "Growlerized" ready for when I replace my knackered diff.

http://www.chicagolandmgclub.com/techtips/midget/ra_hs_break.html

I have an uncle that can sort the machining etc, but gets stuck at shot peening.

A bit of a google suggests shot peening is generally a bit more specialist than say nitriding, which seems very trendy these days.

I can get nitriding done at a few places locally, would it be a suitable surface treatment in this situation? I am not to hot on this aspect of materials science.

Thoughts appreciated.

Cheers,
Malcolm
Malcolm Le Chevalier

I'd say not. The idea of shot peening is to relieve surface stress, especially after machining. Nitriding, as far as I know is something different which actually alters the properties of the metal. I could be wrong though! I'm sure there will be someone more knowledgeable along soon.

Been a while since Growlerising has been discussed on here, I wonder what he's up to these days?
john payne

I had a pair of shafts heat treated at the ends but one of them broke fairly quickly. The idea was to relieve fatigue but it didn't work. See my new thread though about half shafts!
L B Rose

I would go with the shot blasting. Nitriding produces a very thin hard surface and is good for wear but I don't you'll benefit from it in the halfshafts. Shot blasting produces a compressive stress in the surface of the material and so makes it more difficult for cracks to form as cracks only occur in tensile stress regions.
David Billington

Should have said peening not blasting. Naturally there is a wiki page on the subject https://en.wikipedia.org/wiki/Shot_peening
David Billington

I recall that Growler was very specific re: shot peening just because of its stress relieving ability

I wonder how he is these days?
Bill sdgpM

erm why?

playing Devil's advocate here so don't take this the wrong way....

I've had standard late midget halfshafts in my K series car for 5 years. I've not broken one yet. It has 121bhp and 98lbft. It gets driven hard, and has done two track days.

Just saying....
Rob Armstrong

Bob,

Growler was right to recommend shot peening but maybe he didn't understand the full reasons behind it as it is not a stress relieving operation technically as it leaves behind a residual beneficial compressive stress in the surface. Stress cracks only start and propagate in tensile stress regions so by having a compressive surface stress in place the load applied to the part has to overcome that compressive stress before a tensile stress occurs, this results in a lower tensile stress for the same load so the part will be less prone to cracking.
David Billington

why?

because there are plenty of people that have broken half shafts. Les on the other thread has, my rear axle had one newer shaft in, suggesting one has broken in the past. The axle that my "new" shafts came from was the same situation.

so for minimal outlay I would like to try something that might reduce the chances of a break. especially as I am going to go to the cost and effort of fitting a new diff.

I agree, their ultimate power/torque capacity is above and beyond what my engine puts out. but its well known they have a patchy record when it comes to longevity.

cheers
Malcolm
Malcolm Le Chevalier

It's a pity Ashcroft Transmissions don't make half shafts and diffs for our cars. The landy lot swear by them. Might be an idea to contact them to see what treatment they would recommend.
J White

Rob
Your day will come .... there are three things in life that are certain for Midget owners: Death, taxes, and broken halfshafts! Sooner or later. It's an inhernetly weak design using fairly average material even in the later ones. That's why we like to get in first.
Paul Walbran

Get some of Kim's ones - they are about as strong as you can get - and not overly expensive - in fact, I suspect by the time you've bought a decent pair and paid for treatment - it would be cheaper (and certainly easier!)

www.magicmidget.co.uk
rachmacb

At £165 each? Yikes! I am not so sure, but then I have no idea how much heat treatment would cost... But at the moment I have £280 to play with on that price.

I assume the shafts don't have any special treatment as standard, just plain EN17? So doing anything to them should make them better, right?

Malcolm
Malcolm Le Chevalier

Peter May's are EN24 but they're £176 each. They stand up to racing pretty well.
David Smith

There is also Denis Welch

http://www.bighealey.co.uk/catalogues/sprite-midget/axle-4

Looks like they come pre Growlerised.
David Billington

Stumbled across this, same failure mode on a different type of car. Interesting details for those form an engineering background.

http://www.sciencedirect.com/science/article/pii/S2213290216300086

Malcolm
Malcolm Le Chevalier

Malcolm,

Useful information about the material and heat treatment for the shafts. EN24 is the British specification equivalent to 4340 so that is useful. The T in EN24T denotes the tensile strength of the material as supplied, see http://www.westyorkssteel.com/alloy-steel/engineering-steel/en24t/ so would need to be heat treated to get the properties mentioned for the shafts but is easily machined in the T condition.
David Billington

While the conclusion that the halfshaft should have been heat treated to achieve design spec is indeed important, what I also found intersting about that analysis is the lack of consideration of the stress concentration effect at the end of the splines where they are simply cut into the shaft as they were - even when the analysis showed that's where the stresses peaked and failure started.

Waisting the shaft behind the splines to leave them proud of the shaft greatly reduces that stress concentration. We were taught it in engineering 101 back in the 60's, and it wasn't new then. I'm suprised that a formulae SAE team of budding engineering minds made them this way.

BTW, we use 2767 (aka EN30B IIRC) for our shafts, they have proved much more durable than the 4340/EN24B ones we made initially, though the were latter vast improvement over the originals of course.

Paul Walbran

You know what... I did FS back in 2009 and I think I/we made a similar mistake, I can't find any pictures of it though. Shame.

I can't speak for other teams, but we had a team of eight students, half of which generated more work than they completed! In order to build a car from scratch in nine months, along with everything else going on in our final year, not much was really thought through and properly designed. Stuff just got chucked into CAD, checked it would go together and then made.

Holy crap that year was stressful! Ha ha.

Malcolm


Malcolm Le Chevalier

"we had a team of eight students, half of which generated more work than they completed!"
Same the whole world over I'd say Malcolm - son Andrew was in the Auckland Uni team about 10-12 years ago with exactly the same experience.
Paul Walbran

This thread was discussed between 18/05/2016 and 23/05/2016

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.