danglerb

Do these type of cnc come with test models, standard items to make that are checks of calibration etc?

inactiveuser1

On that chart it shows the Chrysler LA motor 340cu in. which had a 3.31 stroke
with a stock rod of 6.123 which is close to 1.85 rod ratio.
When I had a 4" crank (stock was 3.31) made from 4340 used he same rod length 6.123" and ended up with 1.53 rod ratio
That 4"crank was made Moldex back in 1987 which was my 2nd stroker motor.
Big block 454 chevy are same rod ratio 1.53
One of the lower rod ratio motors from the factory was the 400 small block which used a 5.565 rod and 3.75 stroke ends up being a 1.48 rod ratio
Seems high 1.40's to low 1.50's have been used in factory production motors being on the lower rod ratio side.
My 340 ended up being a 416cu in would rev to 6500 and that was with stock rods.
Had a 4 core radiator in the car that would cool a smaller stroked motor which was a 355 that was 3.454 stroke up from a stock 3.31 stroke it wouldn't cool the 416 motor at all ended up having a staggered tube radiator built it was a 6 core radiator.The advantage of the staggered tubes inside was the core thickness was close to the same but alot more cooling.
Built 2 strokers for a different car before I bought the 928.

slate blue

Sean I am glad to hear that you are sticking with the standard 90 degree crank the difference the flat plane crank would offer is nothing compared to better heads or cams.

Incidently when the f1 teams went to 90 degree V8s, Renault had to reinforce their dyno as the engine was shaking it to bits. It would be an incredibly steep learning curve that would require the most expensive lightweight parts that would negate the benefit of making your own crank. Just go with Tri-Y headers and problem solved.

Also I believe for racing the standard stroker cranks are heavy, they don't need to weigh more than 22 kgs or 50 pounds.

Greg

gargoyle28

i have a weight of 26kg (calculated by inventor) for the 3.75 stroke dual plane crank, i could of course make it lighter, i just don't know know where i should take weight off, i could make each counterweight narrower, but leave the thickness the same in the center to keep the over all length the same, or i could make the weights a lower diameter, or of course holes or a different shape even, but i'm not sure what the best design would be

pjburges

Guys -

I've been working in a university lab that has SolidWorks (CAD), MasterCam (CAM), and a 5-axis CNC lathe , as well as a CNC grinder. I even programmed a robot to feed the two machines blanks.

Let me say that modeling a crank would take just a couple hours in SolidWorks (piece of cake) - but considering your design - that could take weeks to hash the details and calculations although it looks like you guys know exactly what you're looking for.

Lastly the biggest SOB is by far the programming. I would totally expect to scrap at least 1 of them. 5 axis programming even with MasterCam and importing your SolidWorks data is total hell. I know from experience - I did it once and I hope I'll never be in the position to get involved with that again. They didn't pay me enough!! A crankshaft would be fun and all - but unless the machine is free for you to tinker on (and it never should be!) - this whole idea is hardly practical.

Gargoyle - I would suggest modeling your part in ANSYS - it is a Finite Element Analysis program - without a course in Mechanics of Materials and machine element design - you'll have to cover gaps of knowledge with the program but it will do a very good job of modeling your stresses - SolidWorks also has this capability built in - though Ansys is still better.

Only in a Rennlist 928 forum... haha

gargoyle28

ive never heard of ansys, i looked it up though, sounds very awesome, the only program (as of now) for me to use is inventor, which has some stress analysis ability, but it's not the most accurate from what i understand, i have taken my first year of mechanical engineering technology so all that sort of stuff is where my brain already is (and gets me quite excited as lame as that might sound), i just don't have a lot of real world application experience to draw upon

my thought on the design of it is that so far all i've done is increased the stroke, which was easy, and assuming the same material and that is made and balanced accurately (none of which are my department) it should work just fine, it is just for NA and i'm sure Porsche (as any good manufacturer would) over engineered the crank so the little more stress it would see due to the slightly longer stroke shouldn't have an impact, but it would be very easy to thicken the throws a bit to account for that, it's actually a bit thicker anyway because of how i lengthened the stroke

123quattro

Ansys....ugh. I used that way back in 1996. It was not one of my favorites. I liked I-DEAS much better. OEMs use neither anymore.

justaguy

We are a custom shop not a production shop. Some times the machines are busy sometimes they are not. That's just the way it is. Again this is a fun project for me I am not depending on it to pay my bills.
So totally impractical but a lot of fun. Kind of like racing!

Right now the machine is just sitting idle as we become familiar with it. We have many other machines which are busy.
Most of the work is not 5-axis any way just basic maching. the 5 axis machine just makes it easier.

People have been making cranks for a 100 yrs with basic machine tools tools just turning and milling really not that difficult. The metal needs to be speced properly heat treated correctly etc but again its been done for a 100 years or so. The design is the real challenging part. Takes serious skill and knowledge

ptuomov

I agree. To illustrate the point, here is some more info on the cool crank that I linked to:

justaguy

Steve
I was thinking 4340 HTSR as I have a rack full of it. The blank would be heat treated and and stress relived at critical stages along the way and probably gas nitrited to get the necessary hardness on the journals befor final grinding.
The machine is a Hitichi Seki CL-250 Surplus from the Canadian Forces. They used it to make tent pegs!
Go Figure, No ****! Tent pegs.
Sean

GregBBRD

When we started building our first stroker engines, we used the pieces that everyone had been trying to use, for years. After more than just a few issues, we went back and "started over" with a clean sheet of paper. Went went back to the manufacturers and asked them what the pieces should look like....thinking they might know more than we did!

Moldex thought that it was absolutely absurd to use a crank with 8 counterweights in an engine that turned the rpms that we do. They insisted that we use a 6 counterweight unit. After much discussion about the benefits and the issues, we actaully asked why they build some people 8 counterweight cranks and others 6 counterweight cranks, They simply laughed and said that they would build anything the customer wanted...but if someone actually took the time to ask...it would have 6 counterweights.

svp928

Sean, if the material is not vacuum arc remelted, its going to have inclusions in the center of the bar- ask me how I know..! aarrggh...anyway,if its just an exercise, no biggie, but I wouldn't make a real one out of that...
As for the machine...tent pegs? Jeez...your Armed Forces don't spend tax money any better than ours, I guess...
Sounds like you know how to cut the material- the big curve is going to be programming...as for the design, the counterweights need to be matched to the intended bobweight of the rest of the parts, plus a bit for balancing. Don't make them too small and require heavy metal...

Good luck!

justaguy

Steve
The final choice of material is still up for discussion. The stuff we have on the shelf will do for the "Practice pieces"
The programming will be a challenge that's the whole point of the exersize.
Yep Military intellegenge Lets spen $ 350, 000.00 on a machine to build Tent pegs! But lets remember those are Military Spec tent pegs. Unbeliveable.

dprantl

Yeah, but when no one is watching they are making something else. The tent pegs are just a front...

Dan
'91 928GT S/C 475hp/460lb.ft

slate blue

By Gargoyle28

Hi, I was conservative with the weight I quoted, however you may require some heavy metal to reach it. Also my crank has more modern technology, as such the big ends are smaller, in my case 48 mm (Honda) I could have used 47 mm (BMW) which is what the whole Nascar field runs.

Now as I have described the gains here before I will just say there is measurable gains, these come into effect higher up the rpm scale, there are very real in terms of HP and the control of mass forces which I believe is a cause of block failure in the 928.

The crank pictured earlier in the thread, the polished one with the heavy metal weighs under 16 kgs and has a 3.25" stroke and is state of the art in a 850 hp naturally aspirated race engine. as such I don't think you will add 6.5 kgs by increasing the stroke to 3.75". Most likely you need to remove more material from the rod journals and or add heavy metal. The reason I say this is when I look at that crank which I was told cost $100 K for 10 of these babies, they have scraped every bit of material out of those journals.




Justaguy and Gargoyle28, Greg BBRD has asked a relevant question below, you may care to read my answer to it as my research has been extensive.

Originally Posted by Greg Gray
I would and have only used a 8 cwt crank for stability in the 928 block which can be a bit fragile.

Greg and others, I did asked Moldex btw but since I thought I understood the idiosyncrasies of the 928 engine better than a crank maker I decided to back my own knowledge. Also as I have mentioned before, I do plan to use higher revs than that have been used before in the 928 engine however some principles like balance and mass forces still apply especially given the history of cracked webs in 928 blocks. The rev limit for my engine will be 8,500 rpm and the stroke has been reduced to 90 mm for this purpose.

I have recently finished the redesign of the sump. I hope it is as ingenius as I think. It reinforces the cradle and has 5 vacuum stages and stops the oil from the heads ever hitting the crank and costing power. The reason I raise this point is that again it is strengthening the bottom end which is what this debate about 6cwt and 8 cwt.

From the thread Crank, bottom end design, friction and oiling I have cut and pasted my answer with some updates. If you don't want to read the whole lot, I bolded the relevant part.

I have been doing some research about our cranks and cranks in general. It is a complicated topic I don't know all the answers but I hope that we can discuss the facts and possibilities. As you know I have always liked the high revving engines. That comes about because of factors, I like the manuals and the engine can be a smaller displacement as such a bit more economical and most important factor is the high revving aspect means that it places less strain on its drivetrain components. The engine has to rely on the fact that it has been built to be efficient.

One of the things I investigated earlier was the Honda rod journals, I have that on my Moldex crank, the criticism I have seen about these smaller journals is that they may not be able to handle detonation? The Nascar branded bearings that fit these rods are much harder than the Porsche bearings and I don't plan to have detonation with this engine.

There is a decent choice of bearings available they are eccentric and as the rods are stronger and have greater hoop strength plus they need less oil as the diameter is a lot smaller and this allows a greater bearing clearance which is needed for an engine to run higher revs.

Even Porsche's venerable GT3 racing engine is having its journals resized recently. The other crank factor is the main bearing resizing, this is done all the time in performance engines. The crank below is a state of the art crank that can handle 850 hp and weighs 35 pounds, I spoke to the maker of the crank and they said they are happy to make 928 cranks but would be able to do a decent price if three were ordered which is fair enough.



The crank, if ordered will not be polished on all journals like the one above and probably made from a slightly different material. I was told around $3,500 and that would mean a crank that is lighter and stronger than what is presently offered. They told me do not confuse weight with strength. The weight difference being around 10 kgs or 22 pounds lighter than current offerings. That is a lot of weight especially when it is spinning and coming from the front end. All that heavy metal actually makes the crank lighter in the end.

Now this is only for manual cars and ones that don't need smog checks etc, there would be no point in anything but an all out application and I am not suggesting anybody that has the current crank they do anything but drive their cars but if you are about to set out on a project you may find this interesting. I have also included some information on friction and oils

This is what the bushed down mains look like.



The crank maker suggests to use a 2.3" or 2.5" mains, if the 2.3 mains are used the RO7 bearings are probably a perfect candidate, the reason is that the RO7 engine's crank looks very similar to our 928 8 counter weight stroker cranks.



Here's a Moldex crank, see how similar they are, (not my pic, I'll remove it if need be)



There is a reason that the new engines are using centre main bearings, they have a greater stiffness and this helps to reduce centre main bending deflection.

The reason to use 8 counter weight cranks is power and strength, there has been studies done that prove a greater power is produced with these cranks and they have better reliability. Most crank makers at the top of their field agree with this also. The reason for for the difference is the measurable losses that occur with 6 counter weight cranks at high rpm high power applications. Bending deflection is the cause for the deflection and as such it makes sense that the extra weights provide better balance and less stress.

There is also some investigation going on in relation to how to oil the crank and the rods as mains 2 and 4 are the most highly stressed in a 2 plane V8 crank. There is some makers that drilled the crank in a way similar to the 928 crank, don't laugh just yet.

What they are doing is allowing the mains 2 and 4 just to oil themselves and not supply and oil to the rods. The oiling to the rods is supplied by 1,3,and 5 and drilled in a way that there is cross drilling and because of the crossecting drillings there is a need in high power applications that the internal drilling need de-stressing, this is done by extrude honing. Another advantage is that the oil hole that supplies the rod is perpendicular to the journal surface.

This is better than the straight shot oiling for oil support purposes. Bigger oil outlets do not support better contrary to some opinions. This is similar to grooved main bearings to plain main bearings, grooved mains are of course used for better oiling to the rods. However load bearing capacity drops dramatically and degrades the load carrying capacity and increases frictional losses. Even the use of half grooved main bearings is not used in very high revving engines.

In an engine such as a F1 they feed the mains but the mains do not feed the rods. The rods are feed from the nose bearing at the front of the crank. They use the nose feed because the oil pressure was consuming too much power as the revs rose. In an article I saw there was an engine was switched from oiling the rods from the mains to nose feed and the oil pressure was able to dropped by 1 bar to 5 bar.

A friend of mine said to me that the oiling to the rods was supplied by the centrifugal force, that is after it has overcome the centrifugal force to get to the centre of the crank if I understood him correctly.

The smaller mains will allow a lower oil pressure due to the fact that the oil doesn't have to overcome so much centrifugal force. The lesser bearing speed also means less friction, this comes more into play at high revs. Now with lighter rods and journals that are swinging from the centreline of the crank, this means less centrifugal loading on the cranks rod journals and bearings.

The other part of this work is getting the oiling right, in an article written by Jack Kane and Ian Bamsey, there is three modes that the bearings operate in, fully hydrodynamic, boundary and mixed.



Bearing friction is measured and valued by the equation of "Bearing Operating Condition" (BOC) BOC = Viscosity x RPM x K/Load The K is the factor that converts RPM and diameter into journal speed. (Which is why there is a gain from smaller journals) To me this formula is a bit counter intuitive, the reason I say this is is, with a greater load the friction is lowered to a point but it is always lower than boundary.

So with the use of smaller mains the load is proportionally increased, this helps the bottom number in the division by making it a larger number, the top number is made smaller by the fact as mentioned earlier the K factor. So it is win win in regards to friction. While friction is reduced due to lower viscosity oils, I wouldn't go there, some Nascar Teams still use 20W-50 and that is where I will stay unless there is a need for a 60 weight oil which some race cars use.

While doing my research for this topic I dug up this info from the SAE.

http://www.sae.org/events/pfs/presen...2005spikes.pdf

On page 46 there is the conclusion to a wear study that was conducted and the basis of the study was ZDDP and its effects. Now this might explain something that is important to all of us.

That is why some oils have caused high wear or maybe put another way allowed high wear. So oils that are not high in ZDDP may be doing more damage than oils with none! This may explain problems seen with Mobil 1. Various grades of Mobil 1 have different formulations.Another Rennlister JET951 may like to expand on this is his area given that he rebuilds these engines. Also if I understood it correctly there is increased friction when there is mixed lubrication when ZDDP is used. Maybe this is a by product of the mechanism of action? That is the protection offered slightly increases the friction.

There is some other issues that may be of interest, I will try to explain the terminology as best I can so that it makes sense, I have condensed the article so that if you have a question please ask it.

Back to the types of lubrication, boundary lube. is when there is a few molecules of oil between the sliding surfaces and the peaks of each surface touch each other. This is the highest friction and occurs on cold start up, a hydrodynamic wedge forms as the journal rotates and the oil is trapped. There is little pressures on the edges as the pressure escapes and peak pressure is in the centre of the bearing.

This is why grooved bearings cannot carry anywhere near the same load, (If a pic is desired i can draw it but the article and photos are copyright protected) if the bearing was wide enough pressure would peak and be level in the centre. I think that Porsche when they made this change, were just worried about the damage to the rod bearings that was happening in the marketplace and tried this as a fix not because it was better.

The film thickness may only be around 0.0001" but the pressure may be as high as 6,000 psi. Hydrodynamic pressure has nothing to do with engine oil pressure, the only relationship will be is there enough oil to cool the bearings and keep the hydrodynamic wedge going. Also the opening out of the oil galleries on the rod journals is bad for the same reasons. The opening needs to be kept small with only a small radius to relieve any stress.

The third mode is mixed, that is part hydrodynamic and part boundary. It is the transition between the two types of lubrication. There is a another type of lube that is not applicable to fluid film bearings and that is squeeze film. It is the type of lubrication that is seen in wrist or piston pins.

A bit of terminology, bearing eccentricity, I normally associated this with rod bearings, however it is also associated with the lubrication modes. Bearing eccentricity is the journal displacement from the centreline of the housing. That is off to one side, the journal does start to climb the wall of the journal as the journal or shaft starts turning.

An example may be, the mains or rods may have a 0.003" clearance. Or a radial clearance of 0.0015" and is operating with a oil film thickness of 0.0001". The sum would be (0.0015 - 0.0001)/0.0015= 0.9333" The actual shaft/journal diameter was not supplied in the article so that was not that helpful.

Looking at the Stribeck graph it shows that the friction starts off very high and as the journal starts to turn the hydrodynamic pressure starts to build and take over from the boundary lube. If rpm stays fixed and either viscosity decreases or unit load increases it will eventually reach its low point of friction of a BOC of 35. That value put into perspective is half the friction of a deep groove ball bearing.

If the loading continues to go up and or the viscosity decreases the BOC will move into the mixed lube mode where there is minor metal to metal contact. If this happens you have too low a viscosity oil or your bearings are too narrow.
I believe one of the reasons the crank that I have that looks like a mirror is polished in that way, is that they are are trying to keep those peaks and valleys to an absolute minimum to reduce this mixed lubrication.

From the low point of BOC 35 it only goes one way, if higher revs or higher viscosity or lower unit load, the friction rises exponentially or to approx ten times the low point. This is why much attention is paid to getting this right. Designers need to maintain the 35 to 50 BOC range.

So to sum up, when you change the mains and or rod journals to a smaller diameter, the unit load increases and the bearing speed decreases, two good things for lowering mechanical losses. I put this forward as some research to be discussed to hopefully move our engines forward. If anybody wants to contact me off list about a potential future crank order they can at ggray1964 at yahoo dot com I believe it needs more research and that will take time but I am in no hurry at present.

Otherwise happy to kick it around here for a while, there is a few more things to add but the wife wants me off the computer as we are on holidays

Cheers Greg