View Full Version : Did you know our cranks are forged steel!
There has been much discussion on the US MKIV tech list about forged steel cranks so I thought I'd check with the guy who balanced my crank, & hey presto they are! Also the Yanks reckon the rods are too !
This all make me feel much happier, & explains why these cars can take such phenominal amount of power.
You can all sleep soundly in your beds tonight;)
Terry, did you have anything else done to your crank and rods other than balancing ? i.e. shot-peened or any other treatments.
Crank has been bladed, & both the crank & rods have been tuftrided & shot peened.
Mark Ayling
25-07-02, 14:52
Originally posted by Terry Saunders
Crank has been bladed, & both the crank & rods have been tuftrided & shot peened.
mm-hmm, uh-huh - yep, I know exactly what you're talking about :conf:
Bladed?
That's a new one on me.
Further lightening to the crankshaft can be achieved by
blading the crankshaft webs.
Originally posted by Terry Saunders
Further lightening to the crankshaft can be achieved by
blading the crankshaft webs.
I thought our crankshaft's were already bladed ?
Hmmmmmm....hmmmmmmmmmm....
So its a material removal process of some kind.
I've seen crank webs with streamlined edges to reduce windage losses and oil aeration inside the crankcase but I'm kinda guessing that's not what you are talking about.
Can you describe the blading process in more detail?
Without wanting to speak out of turn here: Spot peening increases the fatigue life of a material by inducing compressive stress into the surface. The idea being that in order to initiate a crack, any tensile load first has to overcome this compression. A similar principle to pre-stressed concrete in fact.
It can also remove machining marks which may otherwise act as tiny stress raisers in billet parts.
Peening can be used for one-offs or low volume stuff but the "mass production" alternative for crankshafts is fillet rolling, where the radii between the main bearings, crankpins and crank webs are compressed locally on a dedicated rolling tool, producng the same effect in the material.
Tuftriding: I'm guessing that's the same as or similar to nitriding? Its used for for increasing the surface hardness of the steel.
Originally posted by Darren Blake
Hmmmmmm....hmmmmmmmmmm....
So its a material removal process of some kind.
I've seen crank webs with streamlined edges to reduce windage losses and oil aeration inside the crankcase but I'm kinda guessing that's not what you are talking about.
Can you describe the blading process in more detail?
Without wanting to speak out of turn here: Spot peening increases the fatigue life of a material by inducing compressive stress into the surface. The idea being that in order to initiate a crack, any tensile load first has to overcome this compression. A similar principle to pre-stressed concrete in fact.
It can also remove machining marks which may otherwise act as tiny stress raisers in billet parts.
Peening can be used for one-offs or low volume stuff but the "mass production" alternative for crankshafts is fillet rolling, where the radii between the main bearings, crankpins and crank webs are compressed locally on a dedicated rolling tool, producng the same effect in the material.
Tuftriding: I'm guessing that's the same as or similar to nitriding? Its used for for increasing the surface hardness of the steel.
You have the anwers so why ask lol...
Blading does exactly what you describe & obviously removes weight from the crank as well. The other two processes are used to strengthen the parts.
Nick Harris
25-07-02, 18:49
How did they come to the conclusion that it is not a cast unit and is a forged unit!
They look identical to the eye.
Originally posted by Nick Harris
How did they come to the conclusion that it is not a cast unit and is a forged unit!
They look identical to the eye.
I assume that it reveals it's difference once machined.
Nick Harris
25-07-02, 19:47
In the ''old days you could immerse the crank in oil and accurately measure the displacement then weigh the crank and gain a reading for the mass, however this no longer works as the use of hybrid materials is rife and this is no longer reliable.
Machining would use the same cutters and it would be difficult to really be sure.
If any car has aforged crank then every company out there would crow about it, does the sales lit say it is forged 'cos if it don't then it ain't.
there is sales literature in the States to back this up Nick
Nick Harris
25-07-02, 20:08
Is that on MkIV.com?
Not sure, it was mentioned on the US MKIV technical list which is where I picked it up from. Apparently it's common practice for Toyota, Honda & Nissan to install forged cranks that they think it's normal. There is also a US magasine singing te praises of these manufactures for this practice.
The crankshaft section on MKIV.com is pretty scant compared to the other sections. The technical description of the engine mentions the material of just about every other component, but not the crank!
The NCF section doesn mention that the pins and journals are induction hardened, and I'm not sure if you can do this with cast iron so it may be a clue.
It probably would be possible to tell if it was a forging or casting by eye, but only by looking really closely.
I think forged rods are more common than forged cranks but its horses for courses. A high output engine is obviously more likely to have a forged crank, and the I6 is a naturally "bendy" design so it makes sense. Casting will always be the first choice on cost grounds, if it can be made strong enough though. You won't find many engines south of 90bhp per litre with forged crankshafts unless its an unusually skinny design.
The pictures on MKIV.com look like its a pretty stout crank, enough to look like it might work as a casting, but there's not enough pictorial evidence to say one way or the other. If someone in the States really has done the research then there's a good bet that it is steel.
PurpleHaZe
25-07-02, 22:28
Here in the US, there is a supra with 900RWHP with stock internals. :)
Nick Harris
25-07-02, 22:36
American horses
Originally posted by PurpleHaZe
Here in the US, there is a supra with 900RWHP with stock internals. :)
For how long? The dyno run?
PurpleHaZe
26-07-02, 01:32
the guys on Supraforums.com, his name is Jesus. If you look at his sig, he has 904 RWHP on stock internals.
I've seen all this, but remember we measure HP in DIN, you measure it SAE, a 20% power difference, meaning hes really running, 720 REAL horses, and as I said, for how long?
PurpleHaZe
26-07-02, 01:37
I have no idea for how long, his car is a weekend car I believe. So, you guys measure HP differently huh ? So, if the M3 in Germany is measured at 333HP its actually more then the 333HP the USA M3 has ?
Yep, different measuring standard, which is why our BPU figures are lower vs yours. :)
Nick this is a quote from someone on the MKIV Tech list:
"Mr. Okuma at GReddy once told me that Toyota and Nissan, in particular,
use forged cranks so much, its considered no big deal, more of a
standard than an exception. I also recall reading something to similar
effect when Sport Compact Car magazine was building an SR20DET motor for
one of its project cars; that is, making the same comment about Toyota
and Nissan and including Honda in the mix when it came to forged cranks."
HTH
Nick Harris
26-07-02, 10:34
I ask because in straight sixes, a cast unit will be superior to a forged unit, machined all round and prepared correctly it is less likely to break than a forged unit, fours and V8's are better with forged.
Originally posted by Nick Harris
I ask because in straight sixes, a cast unit will be superior to a forged unit, machined all round and prepared correctly it is less likely to break than a forged unit, fours and V8's are better with forged.
I'd be interested in seeing the justification for this. Firstly, design for design a forged crank will always be stronger than a cast one. The choice of material or process is more to do with the requirements from the crank and the crank design.
IMHO you can't make a sweeping statement that I4s are better forged, I6s are better cast, V8s are better forged. It depends entirely on the geometry of the crank and the requirements from the engine.
Casting is a lot better than forging for production. The tooling is cheaper for any crank layout, cycle times are faster, and the process is more accurate so you can minimise the amount of material that needs to be removed by machining. Machining a crank all over is something that is avoided like the plague on production cranks because it piles on cost. So with a cast crank you end up with the cheapest part, which is why in my experience it is always the first choice.
Forged cranks are stronger and will turn up where the crank has an extra-long throw and hence very little bearing overlap, or has fewer main bearings than is strictly desirable, or where the engine layout makes for an inherently weak crank (like a four-bearing 60degree V6 with even firing or an I6). If the crank is not "flat" then the forging tooling will be complex and multi-staged instead of a roughpress / finepress / cliping process. Anyone who has been inside a drop-forging shop will know that the process is brutal to say the least. The tolerances are much larger so machining all over is more common, and the cycle times and part cost are proportionally higher. In return for all this extra ball-ache the crank is, however, stronger than the same design as a cast part.
Typical applications of forged cranks may be where an existing engine is being enhanced, or the stroke is being increased without the crankpin or main bearings being enlarged, or as mentioned above when the crank layout is inherently weak in relation to the engine output.
Nick Harris
26-07-02, 16:31
>I'd be interested in seeing the justification for this. Firstly, design for design a forged crank will always be stronger than a cast one. The choice of material or process is more to do with the requirements from the crank and the crank design.<
Strength is only part of the equation, forged cranks lack ductility, they are very harsh on the bearings and caps, there rigidity it not compatible with a long block, all 6s flex longitudinally to a far greater extent than the 4/V8, our 6s are extended 2.5s the added piece at the top of our blocks is the only potential weak point, so it is vital that the flexure is not made worse by having a crank that opposes this flexure, in doing that the flex will transfer up the block to the weakest point.
>IMHO you can't make a sweeping statement ............It depends entirely on the geometry of the crank and the requirements from the engine<
I'll think about that.
Originally posted by Nick Harris
Strength is only part of the equation, forged cranks lack ductility, they are very harsh on the bearings and caps, there rigidity it not compatible with a long block, all 6s flex longitudinally to a far greater extent than the 4/V8, our 6s are extended 2.5s the added piece at the top of our blocks is the only potential weak point, so it is vital that the flexure is not made worse by having a crank that opposes this flexure, in doing that the flex will transfer up the block to the weakest point.
[/B]
I think we may have to agree to disagree on this. :cool:
Ductility is a measure of a material's ability to be elongated or drawn without fracture. I suspect you may mean "resilience" which is the ability to absorb impact. In a crank this would be the gas load on the power stroke. The inertia load of the piston at TDC on the exhaust stroke, which is the largest tensile load, requires good tensile properties.
Definitions aside, whatever the case a stiff crank is a good thing. You don't want it bending around and going out of control under load. You want it to be as stiff as possible.
Considering a single bay, the gas load is going to get transferred to the main bearings no matter what happens. The crank just has to be resilient enough to do it again and again and again. Because the gas and inertia loads act in opposite directions, a crank is subject to fatigue. Fatigue life is a function of the ulitmate tensile strength (UTS) and yeild point of the material. Again, these are tensile properties (althouh their compressive couterparts do exists). This brings us rather nicely back to shot peening, who's purpose it is to extend fatigue life in the loaded parts of the crank.
Considering two adjacent bays, there will be loads acting in opposite directions at the same time to create a bending force, but again, bending strength is a function of UTS and crank geometry. Cranks tend to crack across a straight diagonal line joining the main bearing and crankpin fillets (again, the areas that are usually shot peened or fillet rolled). The bigger the overlap between the main journal and the crankpin, the stronger this region will be.
Considering the whole crank, yes a long crank will flex more than a short one, but the predominant flexure in a well supported crank (like the Supra's) is torsional, or twsiting. There is an overall bending load in the Supra crank because the vibration damper in the crank pulley has both torsional and bending masses, but the main component in long cranks is torsional. Check out the size of that dual mass flywheel! Again, this is why a stiff crank is a good thing. Undamped torsional vibration causes all sorts of problems from misfire diagnosis, to the crank positon sensor not working properly to, of course, breaking crankshafts.
The ability to resist all of the above require high tensile strength and good fatigue resistance. I don't have my materials references with me but the UTS of steels used in cranks must be in the region of 150% to 200% of that of a good cast iron - and that's for billet parts. The forging process will make the part even stronger. Check out a billet crank for a V12 F1 engine - it will almost certainly have an ultra short stroke and be made from nitrided EN40B or something similar. As stiff as hell, and with exactly the same layout as an I6, just twice as many cylinders.
There are crank configurations that tend to impart larger loads on the cylinder block than others - a crossplane V8 for example. This will put large loads into the structure around the main bearings, while externally the engine is almost perfectly balanced (the engine mounts will see very little vibration). I think that these cranks may tend to be forged because of their strange layout and because they tend to go into high-output engines - not because they bend less than longer engines. In fact they may tend to bend as much if not more.
If any excessive bending gets transferred to the cylinder walls across the crankpin, it will trash the cylinder, the pin and the piston. I doubt the block structure itself would be damaged - especially in a cast iron block like the Supra has.
B*gger. Forgot to add the pic. :baa:
Nick Harris
26-07-02, 20:41
>I think we may have to agree to disagree on this.<
You're right, this is not the forum for this type of debate, too technical and dry.
Fair enough.
Damn I was loving that....I think you'll find the Forum really likes in depth technical discussion.
Well, as long as it comes to an amicable conclusion.
Terminator
26-07-02, 21:04
Top post Darren.
"Oil give it foive"
Don't stop now it is just getting good, nothing dry about it.
An excellent technical post for all of us sad gits out in cyber space.
Chris Wilson
26-07-02, 23:53
Originally posted by Darren Blake
B*gger. Forgot to add the pic. :baa:
Fabulous posts Darren. Great info, well put. Gotta throw this in for discusion though. You show "weakest link" as between main bearing 7 and rod pin 6. I feel it MAY be between rod pin 6 and crank journal 6. Why? The oil feed hole is an internal stress raiser. Could be debated for years, only destructive testing might prove one way or another, but despiet the main loads culminationg at the very end of the crank the weakest link may be one step further forward.
Cheers for the praise guys. Sorry if I make it all sound as interesting as watching paint dry, but it pays the mortgage, y'know? :cool:
I only attached the pic to illustrate what journal overlap was, and where cranks tend to fail (across the crankpin / main fillets). In actuality it can happen on any crank web at all, it just depends on where the loading is worst - or where there is a stress raiser like an oil drilling as Chris points out.
I would still like to see Nicks reply to this as he obviously has a different viewpoint which needs to be debated, and if you cant discuss it in the technical section where can you discuss it.
JB
Reading other posts I feel Nick may have got pissed off for some reason which is a shame as It was a very interesting debate. If Nick reads this I would just like to reassure him that we only want to know how to make our cars faster & stronger. There are no personalities involved. Knowing a little of your background will only help validate your posts. No one will come knocking;)
I have just caught up with this thread after a couple of days and i must say it is the best thread for months.
It's amazing that you can learn more during your lunch hour(+) than the rest of the working day. :D
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