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Flywheel fastener design, clamping force and dowel shear strength
2

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

RE: Flywheel fastener design, clamping force and dowel shear strength

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

RE: Flywheel fastener design, clamping force and dowel shear strength

(OP)
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_105...

RE: Flywheel fastener design, clamping force and dowel shear strength

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.

RE: Flywheel fastener design, clamping force and dowel shear strength

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Flywheel fastener design, clamping force and dowel shear strength

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

RE: Flywheel fastener design, clamping force and dowel shear strength

(OP)
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.

RE: Flywheel fastener design, clamping force and dowel shear strength

(OP)
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.

RE: Flywheel fastener design, clamping force and dowel shear strength

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-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 dowel Chevies, and Fords, etc.

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

RE: Flywheel fastener design, clamping force and dowel shear strength

(OP)
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.

RE: Flywheel fastener design, clamping force and dowel shear strength

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?

RE: Flywheel fastener design, clamping force and dowel shear strength

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

RE: Flywheel fastener design, clamping force and dowel shear strength

" 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.

RE: Flywheel fastener design, clamping force and dowel shear strength

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

RE: Flywheel fastener design, clamping force and dowel shear strength

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

RE: Flywheel fastener design, clamping force and dowel shear strength

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.

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

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

RE: Flywheel fastener design, clamping force and dowel shear strength

(OP)
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.

RE: Flywheel fastener design, clamping force and dowel shear strength

Quote (yoshimitsuspeed)

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.

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

(OP)
The thrust bearing is between cyls 2 and 3 so that shouldn't be an issue.
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

I agree. The engines I can remember had the the thrust washer rather remote from the end of the crank, typically somewhere in the middle. The engine I know best only has half a thrust washer, but that's another topic.

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

What loads do thrust bearings need to carry apart from clutch throwout bearing load and perhaps thrust from a helical timing gear?

je suis charlie

RE: Flywheel fastener design, clamping force and dowel shear strength

Solid dowels can transmit power, hollow dowels are only for alignment. The best solution for big torque numbers is a keyed surface plus clamping force.
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

Quote (GruntGuru)

What loads do thrust bearings need to carry apart from clutch throwout bearing load and perhaps thrust from a helical timing gear?

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

If you're getting this deep, why not run body fit studs? That's how the industrial side handles all of their super high torque transmissions. The only step better is a tapered coupling.

RE: Flywheel fastener design, clamping force and dowel shear strength

Plenty of successful schemes that use only "clamp" fasteners to survive high shear forces- for instance ring gear mounting on differential cases, which transmit WAY more torque than the vehicle's flywheel.

RE: Flywheel fastener design, clamping force and dowel shear strength

Jack, there has been a lot of work on ring gears recently to add various features (hollow dowels, dowels shoulder bolts, etc. because in the higher performance versions of many vehicles they are getting slip when trying to use straight clamp. With large enough bolts and enough nut member thickness there should not be an issue, but downsizing has reduced the envelopes on many components, backing us into this corner.

RE: Flywheel fastener design, clamping force and dowel shear strength

(OP)
I still see some people saying that solid dowels would be better than more screws but no one saying why. It still seems to me that screws would be the superior method. What would I be missing?
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 people who are recommending dowels over screws are people who don't have any true engineering understanding of what makes dowels and screws different.

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

A locating solid dowel will not allow any lateral slip until it fails through single shear and it requires no clamp load to achieve that. A threaded fastener does not fully fill the holes (assuming that that this is a normal automotive application and the holes are not match drilled and the thread fall into the shear plane) requires clamp load to prevent lateral slip under shear loading. Once that lateral slip has occurred, then the loaded component is more likely to slip again and the fastener to loosen. In addition, the solid dowel will have a larger cross section area than the equivalent fastener that is not in a press fit hole. This is assuming that you will not be using a match drilled bolt.

RE: Flywheel fastener design, clamping force and dowel shear strength

Interestingly that post runs directly counter to the advice given in a course on joint design given by a major automotive OEM at which I scored high marks.

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

Gregg, If you are using match drilled dowels the joint ceases to be a bolted joint. I agree 100% that clamp load is the only way to hold a BOLTED JOINT together, but this would no longer be a bolted joint.

RE: Flywheel fastener design, clamping force and dowel shear strength

Quote (BigClive)

I may have missed something here - but - wouldn't the bolts themselves act as dowel pins to resist the shear forces.
That was my initial thought, until it sunk in that the friction in the two matching faces offered a significant amount of resistance to movement when strongly pulled together.

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

I recall a u-joint flange drawing with a minimum roughness spec.
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

I think if a surface was rougher than (TBD) it would likely permit some embedment over time in hard service, which without routine re-torquing would lessen the bolt clamping preload, which would reduce the clamping force, which would permit micromotions, which would then let the velociraptors out of the enclosure.

RE: Flywheel fastener design, clamping force and dowel shear strength

Quote (screwman1)

Gregg, If you are using match drilled dowels the joint ceases to be a bolted joint. I agree 100% that clamp load is the only way to hold a BOLTED JOINT together, but this would no longer be a bolted joint.

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

"would it not also offer better engagement to sand the surfaces rough"

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

"BMW use conrods that have been snapped in two at the part line"

As do Ford - Greg!

je suis charlie

RE: Flywheel fastener design, clamping force and dowel shear strength

Sometimes anaerobic adhesive is applied to the mating faces of a shear joint that relies on clamped friction. The anaerobic adhesive provides a predictable/consistent static coefficient of friction at the clamped interface, which permits a reduced FoS to be used for the joint analysis.

RE: Flywheel fastener design, clamping force and dowel shear strength

I am sure that I saw a report via eng tips b the chief engineer of the Corvair engine, where they had early failures of the rivets holding the flywheel on. I remember the quote that after the investigation he said the guys making rivets did not know how to fasten rivets and neither did the riveters! I seem to remember that there were comments relevant to the above, perhaps I am mistaken, can anyone find it?

RE: Flywheel fastener design, clamping force and dowel shear strength

http://www.vv.corvair.org/Library/benzinger.htm

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.

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