Flywheel fastener design, clamping force and dowel shear strength
Flywheel fastener design, clamping force and dowel shear strength
(OP)
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
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





RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
https://scontent.xx.fbcdn.net/t31.0-8/13474952_105...
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Flywheel fastener design, clamping force and dowel shear strength
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
RE: Flywheel fastener design, clamping force and dowel shear strength
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.
RE: Flywheel fastener design, clamping force and dowel shear strength
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.
RE: Flywheel fastener design, clamping force and dowel shear strength
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-do...
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-...
8 dowel production porsche 912 crank all tore up
http://forums.pelicanparts.com/uploads17/crankend1...
A VW legend's discussion of crank doweling in extreme duty applications.
http://www.geneberg.com/cat.php?cPath=4_90&osC...
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
dowelChevies, and Fords, etc.===================
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
There is a pretty nice increase in clamp load right there.
How thick is the flywheel flange, and the flywheel in the vicinity.
RE: Flywheel fastener design, clamping force and dowel shear strength
jack vines
RE: Flywheel fastener design, clamping force and dowel shear strength
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
RE: Flywheel fastener design, clamping force and dowel shear strength
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.
RE: Flywheel fastener design, clamping force and dowel shear strength
I may have missed something here - but - wouldn't the bolts themselves act as dowel pins to resist the shear forces.
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Flywheel fastener design, clamping force and dowel shear strength
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.
RE: Flywheel fastener design, clamping force and dowel shear strength
Are you sure of this? Where is the thrust bearing on this engine?
I brought this up not because of concern about the crankshaft flange cracking or failing by some other means- just remember that every additional pound of clamp load you apply between the crankshaft and flywheel is an additional pound of load, and the distortion that goes along with it, that you're putting into the crankshaft flange. If the thrust bearing lives on the back side of that flange, you are opening yourself up to a potential thrust bearing/crank walk type failure. And additional 30,000 pounds of preload force into a small flange is very significant.
Not saying that you ABSOLUTELY have this problem, just that assuming that everything is fine without evaluating what effect your design has on the rest of the system is not a good mindset to have when designing engine parts. When you're clamping things together, in any application, more force is not always the best solution.
RE: Flywheel fastener design, clamping force and dowel shear strength
Even if it was at the rear flange I don't see how 6 more 6mm bolts would have a negative affect. If anything I wold think 12 bolts would create a more even deformation than just 6. The only way I see this having a significant distortion would be if the surfaces were uneven but even then I would expect more bolts to help even out that distortion.
RE: Flywheel fastener design, clamping force and dowel shear strength
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Flywheel fastener design, clamping force and dowel shear strength
je suis charlie
RE: Flywheel fastener design, clamping force and dowel shear strength
If you don't have the meat in the part to machine a keyed surface, then blind drill & fit captive solid hardened steel dowel pins then clamp together with upgraded high tensile bolts.
RE: Flywheel fastener design, clamping force and dowel shear strength
http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/OBgAAOSw3...
Pretty serious clutches in Top Fuel and Funny car dragracing. Once in a while there is a single dowel, but usually a bunch of bolts and 100 lb-ft + of torque get in done.
RE: Flywheel fastener design, clamping force and dowel shear strength
None.
The concern is/was that additional clamp load in the flange can cause distortion, which can cause the thrust bearing face to distort, and cause the thrust bearing to fail.
Sounds weird I know, but I was once party to a project where this was determined to be the cause of a very very expensive failure.
Perhaps that experience has made me more sensitive to flange-on-bearing distortion concerns than is necessary.
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
I will acknowledge that in an application like this it would be easier to fit a larger dowel than screw but it still seems to me the added clamping force of the bolts would be better. Especially if we are talking about a slip fit between the dowel and the flywheel which would allow movement before the dowel applied any holding force.
RE: Flywheel fastener design, clamping force and dowel shear strength
I suspect if you asked most people, even people with some technical knowledge, mechanics, etc, who have not studied, they would say that bolts work by being loaded in shear, and that a dowel is obviously stronger than a bolt, neither of which is true.
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
Ignoring clamp load in a bolted joint is like ignoring the steel in the Eiffel Tower.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
With that in mind, would it not also offer better engagement to sand the surfaces rough? To move towards the ad absurdum line of that thinking, why not make the faces look more like a (flat) bevel gear? Friction fit of a rough surface plus the benefit of a dowel-like shear interface that runs the length of each tooth from face circumference to centerpoint?
Just randomly wondering...
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Flywheel fastener design, clamping force and dowel shear strength
We process engineers had to intentionally break the edge of a carbide cutting tool to achieve it.
The flange was thinnish and clamping was by four bolts, and the need for additional torque capacity had been demonstrated at the test track just before production.
A proper bolted flange would have been much heavier.
Mike Halloran
Pembroke Pines, FL, USA
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
He's not talking about removing all of the bolts and replacing them with dowels.
In any event, your analysis is not correct. A dowel has zero shear stress imparted to it until the assembled parts have relative movement between them, which means that the joint has already failed.
RE: Flywheel fastener design, clamping force and dowel shear strength
BMW use conrods that have been snapped in two at the part line, to give absolute keying between the two surfaces. There's also several 'rough' inserts out there, which are a bit more scary as they seem to be adding compliance at the same time, rather like spring washers.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Flywheel fastener design, clamping force and dowel shear strength
As do Ford - Greg!
je suis charlie
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
RE: Flywheel fastener design, clamping force and dowel shear strength
Maybe not so much the rivets themselves, but fatigue breakage of the portion of the flywheel that was bolted to the crank, but out at a larger diameter than the crank flange and bolts.
Factory construction 3 line reamed holes, and cold upset ~ 1/4 inch rivets in 12 holes out at pretty large radius.
But over time even those 12 snugly fitted fasteners were not up to the task of preventing micromotions, and then suffering wear and loosening.
Perhaps the slight slipping was part of the bending vibration reduction
http://www.corvairunderground.com/EDGERUNOUT.gif
here is a picture of the piece that bolts to the crank, and gets 2 hunks of iron riveted to it to form a flywheel.
http://www.corvair.ca/images/P7260035.JPG
Also note, the flywheel-to-crank joint with six 3/8"Ø bolts torqued to 45 lb-ft or so rarely gives any trouble, even when the vehicle is driven by a bone-headed 20 year old.