Flywheel fastener design, clamping force and dowel shear strength 2

[yoshimitsuspeed]

The 4AGE engine uses an 8 bolt crank and flywheel because it is the performance engine and in later years came supercharged.
The economy engines use a 6 bolt crank and flywheel. The 7AFE 6 bolt crank however is used as a stroker crank for performance builds.
Many people use 2, 3, or 6 dowels to try to make up for the 2 less bolts. It is my understanding that dowels should only be used to locate a part and ideally should see no significant force. It should be clamping force between the crank and flywheel that prevents one slipping on the other.
I recently designed a flywheel of my own and using the theory that clamping force was more important than shear force I used 6 6mm bolts in between the primary crank bolts. I would have loved to use a bigger bolt but this was the easiest solution that would clear the primary bolts. This gives slightly higher clamping force than the 8 bolt flywheel and should also give a slight improvement in rigidity and I would hope some added resistance to harmonic flexing.
I have had a surprising number of people in the automotive communities question this design. I'm not talking about home builders but people who build race engines and other aftermarket manufacturers who all say they use dowels and believe dowels are better. I believe this most likely is just because that's how it's always been done, not that it's actually better. Today one of my business partners told me pretty much the same thing and suggested I research why dowels would be better. I didn't find anything convincing in a short search. I did find a good discussion here on eng-tips on dowel shear strength and where most the posts seemed to agree that dowels should not be used for strength but other posts suggesting they could. Then I decided instead of researching general theory maybe I should just throw my design up here and get opinions on the design and theory as it applies specifically to this application.
This design uses 6 M6 SHCS counterbored to sit flush below the surface of the flywheel. The main flanged head cap screws come very close to the SHCS so this ensures they clear.
Question 1) Does anyone have any input or constructive criticism on this design in general?
Question 2) is there any way in which an 8mm or even 10mm dowel would be superior to this design?
Thanks

 

[BrianPetersen]

For bolting a flywheel to a crankshaft, M6 sounds small.

 

[yoshimitsuspeed]

Perhaps I should have clarified. The primary 6 bolts are M10. In between those 6 M10 bolts I have added 6 M6 bolts. If you click on the image in my first post you can see a rendering if that helps. But yeah main key is standard is the 6 M10 and I added 6 M6 to gain additional clamping force in the very limited space remaining.

https://scontent.xx.fbcdn.net/t31.0-8/13474952_1053733797996178_3974863010198126812_o.jpg

 

[EHudson]

You might want to look at an old VW air-cooled engine crank and flywheel in your study of dowel driven flywheels. (4) dowels transmitted the torque and a central 'gland nut', actually a short bolt with a pilot bearing in the center, holds the flywheel onto the crank end.

 

[GregLocock]

I think you are on the right track. It would be interesting to see if there is any evidence of dowels taking shear loads in properly designed engines, I think not.

Sure, in a badly designed joint a dowel or keyway may end up transferring loads, but I bet that merely delays the inevitable.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeHalloran]

You certainly can't rely on dowels if you insist on interchangeable parts, as in mass production.

For a one-off, supplemental to the usual crank bolts, match-reamed dowels should be capable of transmitting torque.
Of course you need to clock the hole pattern, say with one visibly displaced, so they will always assemble the same.
Also, of course, the crank and flywheel must be replaced together; you will never be able to replace either one singly.


Mike Halloran
Pembroke Pines, FL, USA

 

[yoshimitsuspeed]

Ehudson
A quick search pulls up a torque spec for that bolt at 253 lb ft.

http://www.hotvws.com/content.php?contentID=62

At even 10 TPI that's 20k lbs of clamping force. I would be willing to bet that clamping force is again taking the vast majority of the force. I suspect the dowel pins are mostly for alignment and that little extra holding power to make sure the FW and bolt don't crack loose which with one bolt would quickly lead to it falling completely off as soon as the motor applied force in the right direction.
As an interesting comparison 6 M10 bolts class 8.8 would have a clamping force of 33,000 lbs so we are in a similar ballpark. The VW contact circle looks a good bit smaller but I assume the torque those motors made from the factory was also a good bit less.

Greg
I would tend to agree. I don't think in any well engineered application dowels would be intended to take any significant load. First of all in order for them to do so in any slip fit application would require some movement. Movement would cause wear on the clamped surface and on the pins. On a flywheel it's most likely that the movement would only happen once in the direction of acceleration but I definitely wouldn't want to count on that. Driveline shock from braking quickly in gear or any number of other things could cause a good bit of torque in the other direction. As soon as it moved once it would be more likely to move again and the more it moved the looser it would get and the more it would wear on the pins.
I feel like clamping force is the logical way to try to hold two objects together. If there is not enough clamping force, figure out a way to provide more.

 

[yoshimitsuspeed]

Just using a generic clamping force chart and class 10.9 which has similar torque values to the service manual 6 bolts apply a clamping force of 48690 lbs, 8 bolt would be 64920.
My flywheel with 6 M10 + 6 M6 gives a combined clamping force of 65586 lbs. I feel this absolutely must be better than 48690 lbs with dowel pins.

 

[Tmoose]

What is the bolt circle of those six M10 fasteners?
How much thread engagement ?
What is the factory fastener torque spec?

===========
Now, VWs used 6 mm dowels, and the "bolt circle" is pretty small, but .....

4 dowel production type 1 vdub

http://www.aircooledtech.com/8_doweling_crank/4-dowels2.jpg

doweling jig to match ream VW 4 dowel crank and flywheel to use 8 dowels similar to Porsche

http://www.aircooledtech.com/8_doweling_crank/jig-set.jpg

8 dowel production porsche 912 crank all tore up

http://forums.pelicanparts.com/uploads17/crankend1269478905.jpg

A VW legend's discussion of crank doweling in extreme duty applications.

http://www.geneberg.com/cat.php?cPath=4_90&osCsid=f258f12005867f8f4da3194f19939745

Many battered VW and Porsche cranks and flywheels were not assembled properly at the last clutch job, but it is pretty rare to hear of flywheels loosening on 6 (edit) BOLT dowel Chevies, and Fords, etc.

===================

 

[yoshimitsuspeed]

Bolt circle is 54mm and thread engagement is about 15mm for the ARP bolt I have right here. OEM should be about the same. OEM spec is 54 lb/ft. ARP is 70 lb/ft.

 

[braddles90]

We recently had to design a new flywheel for a ~170kW 6 cylinder diesel for an off-road machine application, and our fabricator came back to us with the same concerns when we altered the OEM interface, which didn't use any dowels. There was just a machined spotface in the back of the flywheel that fit snugly around the crankshaft palm to make sure it was all centered properly, then around 9 x 1/2" bolts to hold the flywheel on. Seems to me like the main purpose of the dowel would be alignment, but people have misconstrued the intent as to carry the full torque loading over time?

 

[inline6]

many BMW inline 6 have a locating dowel which is a ring which one of the 8 x M12 bolt passes through. the load is totally carried by friction

 

[Tmoose]

" OEM spec is 54 lb/ft. ARP is 70 lb/ft. "
There is a pretty nice increase in clamp load right there.

How thick is the flywheel flange, and the flywheel in the vicinity.

 

[PackardV8]

Only slightly OT, but back in the day, many limited production racing center knock-off wheels were dowel pin drive. My guess is the small manufacturers such as ERA, Halibrand, Shelby Cobra, found it easier to manufacture dowel pins than splines.

jack vines

 

[EdStainless]

Dowel pins work very well as a backup in double shear but not so good in single shear. As Mike H said earlier, these are really one-off custom fits if you want them right.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

The one thing no one has talked about is how much additional material removal does 6x M6 bolts represent from the mounting flange of the crankshaft? Where is the thrust bearing on this crankshaft design?

It is possible for additional clamp load to be unnecessary, and detrimental to the behavior of seals/bearings. Not saying that is the case here, but it should be evaluated.

 

[BigClive]

I may have missed something here - but - wouldn't the bolts themselves act as dowel pins to resist the shear forces.

 

[inline6]

not unless something slips, and if that happens its good night...

 

[GregLocock]

The same is true of dowel pins, of course. Unless they are nails.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[yoshimitsuspeed]

Tmoose
Yes you are correct it is a nice increase in clamping force but people who make decent power with the 8 bolt cranks already use the ARP.
The flange is 12mm thick and most of the flywheel ranges 10-12mm.

jgKRI
In this case there is a lot of material on the crank mating flange. Loss in strength from the smaller holes is definitely not a concern. It also has no effect on the thrust bearing or seals as I am talking about clamp force on the mating surface. Pressure plate clamp force does have some effect on thrust bearings.