Not Using Dowel Pin When Mounting Flywheel?

[Rolls]

We didn't have a dowel pin when we put my fly wheel on (neo rb25), but located it up to the position it would be in with it. I have seen aftermarket flywheels that do not have these pins, and RB30e flywheels (identical apart from this) don't have it either.

What is the purpose of the dowel? A few people mentioned it might be for added shear capacity but it is tiny compared to the 6 bolts and some flywheels do not have it.

Is it just for locating or is there some other purpose I have missed? Am I going to run into trouble without it? The box and clutch has to come out in the next 12 months so I can fix it up then anyway, just curious if anything drastic will happen before then.

Cheers

[GTSBoy]

Mainly for location.

[89CAL]

Agree it is mainly for alignment, it shouldnt make a difference because if you snap the 6 14mm(?) bolts then I dont think a 4mm dowel is going to stop much

It does help you line up the flywheel so the bolts are dead centre in the holes, which I guess helps

[Rolls]

Agree it is mainly for alignment, it shouldnt make a difference because if you snap the 6 14mm(?) bolts then I dont think a 4mm dowel is going to stop much

It does help you line up the flywheel so the bolts are dead centre in the holes, which I guess helps

Yeah locating makes sense, I just don't get why people made a big deal about it when I first mentioned it.

Can't see the shear properties of a single dowel being better than 6 massive bolts.

[GTSBoy]

Bolts aren't really in shear either when it comes right down to it.

[Rolls]

Bolts aren't really in shear either when it comes right down to it.

Why would a bolt shear any differently to a dowel? If they were both the same grade of steel, wouldn't they take the loads the same way?

[89CAL]

it will help slightly with bearing the load I guess but I dont think its going to cause your flywheel to fail if you leave it out, unless you were running stupid numbers in which cause youd have probably upgraded the bolts and flywheel anyway

[GTSBoy]

Why would a bolt shear any differently to a dowel? If they were both the same grade of steel, wouldn't they take the loads the same way?

The majority of the torsional load is taken up by friction between the crank flange surface and the flywheel - which is provided by the tensional loading on the bolts. The bolts technically are in shear, but they're not taking all the torsional load by any means.

[Rolls]

I have studied engineering but not mech or civil, could you explain that with a bit more info or perhaps post a link? Curious what exactly you are saying.

Thanks.

[GTSBoy]

Nah nah. It's dead easy. Think of each bolt. What has been done to it when it is installed? Just had a crap load of torque put onto it and it then clamps the flywheel onto the crankflange. Hard. Lots of normal force between the mating faces yes? And we have say 6 such bolts. So 6x that normal force. Now try to rotate one face relative to the other. That rotation is resisted by the product of the normal force and the coefficient of static friction between the flange and the flywheel (and the surface area in contact of course).

It's just friction. Same as a road wheel has the drive torque transmitted to it from the hub by friction - not by the wheel studs in shear. Same same.

If the force was really being transmitted by the bolts in shear, they would fail in no time because of the cyclical alternation between being loaded one way and then the other. Such loading is not like the loading on conrod bolts or anything else that is loaded axially (ie, in the same direction as the tension on the bolt), because axialy loaded stuff doesn't actually move while the load is being applied, even cyclically. The mating faces all stay clamped together. If a flywheel to crank bolt was really loaded in shear, then there would have to be some sideways slop to transmit that load. The bolt is in tension at right angles to the shear, so there's no slop in that direction, but there's still the clearance required for the bolt to slide through the hole in the flywheel in the shear direction, and that's not closed by by tensioning the bolt. That slop would be the pathway to failure (if it were involved in the transmission of force). But happily, it's not.

There may be some portion of the load that is transmitted by shear forces on the bolts, but I'd really expect it to be so close to 0% that it wouldn't matter.

[R32Abuser]

Rolls, here is the theory behind it.

Firstly look up the Tresca Yield Criterion. To save you some time, I will state that the Max shear capacity of any material will be approximately 56.6% OR 57% of the Max tensile yield stress of that same material. Hence, it is abundantly clear, that for a given identical item being loaded in a) Shear or b) Tension: The item which is loaded in shear will fail FAR BEFORE the item loaded in tension.

Now to the application:

In your case you have your flywheel and clutch assembly.

Now, what you must understand is that it is NOT the physical size of the dowels which is important (I do agree that they would have to be a given size otherwise they would fail, however this is another point of contention). The role of the dowel is to provide a method of LOCATION. I.e. a method for the flywheel/clutch assembly to positively locate its position. Then the role of the fasteners is to provide a method of FASTENING- IN TENSION!!!

The problem is without the role of the dowel, the bolts will now be subjected to shear forces. They must: there is nothing else to transfer the torque exerted by the motor to the drive-train. The problem with this situation is that bolts (although they can, and in many cases are!) subjected to shear loads; they are significantly weaker in this stress mode (for reasons mentioned earlier). Not only are they weaker, but as a result of the threaded section, there is stress raisers/concentrators at EVERY ridge associated with each thread. Do you see the double weakness here in not utilising a dowel?

The long and the short of it appears that it is just SOUND engineering practice to utilise dowels in this application and in similar situations. Im not saying that the assembly will fail, Im simply stating that you are using a setup which was not originally intended to be implemented.

Hope that clears it all up

[GTSBoy]

Rolls, here is the theory behind it.

Firstly look up the Tresca Yield Criterion. To save you some time, I will state that the Max shear capacity of any material will be approximately 56.6% OR 57% of the Max tensile yield stress of that same material. Hence, it is abundantly clear, that for a given identical item being loaded in a) Shear or b) Tension: The item which is loaded in shear will fail FAR BEFORE the item loaded in tension.

Now to the application:

In your case you have your flywheel and clutch assembly.

Now, what you must understand is that it is NOT the physical size of the dowels which is important (I do agree that they would have to be a given size otherwise they would fail, however this is another point of contention). The role of the dowel is to provide a method of LOCATION. I.e. a method for the flywheel/clutch assembly to positively locate its position. Then the role of the fasteners is to provide a method of FASTENING- IN TENSION!!!

The problem is without the role of the dowel, the bolts will now be subjected to shear forces. They must: there is nothing else to transfer the torque exerted by the motor to the drive-train. The problem with this situation is that bolts (although they can, and in many cases are!) subjected to shear loads; they are significantly weaker in this stress mode (for reasons mentioned earlier). Not only are they weaker, but as a result of the threaded section, there is stress raisers/concentrators at EVERY ridge associated with each thread. Do you see the double weakness here in not utilising a dowel?

The long and the short of it appears that it is just SOUND engineering practice to utilise dowels in this application and in similar situations. Im not saying that the assembly will fail, Im simply stating that you are using a setup which was not originally intended to be implemented.

Hope that clears it all up

Say what?!?!?! Are you seriously saying that the dowel pin will stop the bolts from carrying a shear load? Have you thought through exactly what the mechanism by which one little dowel will magically stop the bolts from carrying a shear load is, exactly? Did you not read my post?

[Rolls]

Something a mate told me is the dowel will have much tighter tolerances than the bolts and will assist in dampening any vibrations and noise.

[R32Abuser]

Mainly for location.

Here you say it yourself. Dowels are used to locate this assembly. If they weren't necessary, or a good thing to implement- why would they be there? As I stated in my previous post, there is an engineering reason for them.

Bolts aren't really in shear either when it comes right down to it.

Torque causes rotations. The flywheel/clutch ASM rotates.

What locates/fastens the clutch ASM to the flywheel? In the case of my RB it has both bolts and dowels.

Ill ask you this, remove the bolts from the assembly so that only the dowels are present and locating the clutch/flywheel assembly. Assuming there is no axial load (forward and back relative to the crankshaft).

Can the flywheel/clutch ASM still rotate? The simple answer is yes. The torque is being transferred via the dowels. The only reason for the bolts is to provide TENSION in the axial direction. As a secondary role, they CAN (are not designed to!) provide some locating mechanism if there is "slop" in this assembly. The dowels will also provide this function to provide locating if some "runout" is present in the asm.

It's just friction. Same as a road wheel has the drive torque transmitted to it from the hub by friction - not by the wheel studs in shear. Same same.

Ill ask you to remove the wheel studs on your car. Assuming everything is located as normal (minus the wheel studs) then applying your theory, the friction at the hub without the studs should transmit the SAME drive-shaft torque as if the wheel studs were there.

This is definitely not the case. The fastening mechanism between the wheel hub and the wheels (stud and wheel nuts) is what transmits the torque. Not the friction between the hub and the mounting face of the wheel...

As I stated in post #254 of "Need A New Clutch? And want it to last?"

rule 1 of mechanical design

a) one method for locating

b) one method for fixing

dowels get loaded in shear

bolts in tension

both do the best job they are designed for.

Edited July 17, 2011 by R32Abuser

[R32Abuser]

Something a mate told me is the dowel will have much tighter tolerances than the bolts and will assist in dampening any vibrations and noise.

True. But the g/box mount and associated assembly would have more to do with the dampening of the vibration from the clutch/gearbox operation. It is, after all, made of rubber/polyurethane etc and significatly better at absorbing high frequencey vibration.

[GTSBoy]

Pretty much everything you just wrote is wrong.

Take your first faulty example. That being to take the flywheel bolts out and let it sit on the dowel(s). Yes, after doing this you can transmit some torque, but it can only transmit a very small amount of torque because the dowels are so puny that they will just snap if you actually try to use it like that. You state that the only reason for the bolts is to provide tension in the axial direction. Stop and ask yourself this.....why do you want tension? Why? Well, it is because that tension jams the mating faces together with a force normal to the mating surfaces, which then allows those two surfaces to provide a frictional force between them, WHICH IS WHAT TRANSMITS THE TORQUE.

This is EXACTLY the same for road wheels.

Let's take your idea a step further. Let's say that you have just 6 big dowels the same size as the flywheel bolts. Let's say that they have exactly the same clearance in the flywheel holes as the dowels do (I have no idea how big that is, maybe 0.2mm??). Let's say that there is no axial load on the dowel/bolts, that the flywheel is kept lightly pressed up against the crank flange by some other small mechanism so that mating surface friction is low to zero. So, now you drive away. You give it a big rev and let the clutch out. All of a sudden the crank flange rotates relative to teh flywheel by the amount of slop left by the bolt/dowel clearance (our 0.2mm). Then they slam together with a nasty shock loading and the drive torque is transmitted by all the bolts/dowels (assuming that the clearances all align perfectly so that all are in contact with their holes evenly). OK, so now the bolts/dowels are loaded in shear, and because they're 12mm diameter or so they are probably strong enough to do this. But now you have to back off at 6000rpm for an upcoming roundabout. So you back off, and now the forces reverse and the gearbox is driving the engine. So the flywheel now rotaes relative to the crank flange until the bolts/dowels hit the other side of the flywheel holes with another nasty shock loading.

How many times do you think you can put such a system through that without the bolts/dowels suffering fatigue and dying? The answer is not many.

In reality, with the bolts clamping the flywheel to the crank flange, the flywheel never moves with respect to the crank flange. And this is despite the bolts actually having even bigger clearances than dowels do (the clearances are actually quite large - otherwise it is hard to get them to pass through the holes. This is because the torque/force is taken up at the mating face by friction between the two faces. Static friction, not dynamic friction. If the bolts were actually transmitting the load in shear then they would either need to have absolutely no clearance at all in the holes, so that there was no slop so that they could always transmit force in either direction without there having to be any relative movement, OR, they would have to be torqued up enough so that all the load is transmitted just under the bolt head, which comes back to friction again, but the surface area is so much smaller than the mating face of the crank to flywheel that it is not possible to consider the bolts transmitting the load in this way. (This, because the mating face will be carrying proportionally much more load due to the same clamping force).

In reality, the bolts will be carrying some of the load in shear by exactly the "under bolt head" mechanism that I describe above as a side effect of providing the axial load that provides the frictional load at the crank/flywheel mating surface. But that would be a reasonably small fraction of the total torque capacity of the system, as I said to Rolls several posts ago.

If they did carry a large (or al of the load) under the bolt head, then they would fatigue in a very similar way to the "nasty shock loading" effect that I described in the non-possible world of your frictionless attachment system above.

By the way, when I said "mainly for location" I meant exactly that. A way to allow the flywheel to be put on in only one orientation (assuming just one dowel or non-symmetric multiple dowels). A way to help align it while you put it all together. NOT a way to transmit torque. Since when did "location" equate to "transmit torque"?

[R32Abuser]

I See very little point arguing with someone (over a forum!) who refuses to listen to anything anyone else will say.

I think we'll leave it as a case of you have your view, and I'll have mine.

Rolls can take what both of us have said with a "grain of salt".

Edited July 17, 2011 by R32Abuser

[GTSBoy]

Seems fair, seeing as you have no idea what you're talking about.

[MrStabby]

You both made my brain hurt. But a question about location - since the flywheel is not counter weighted, why is location a big deal?

The coarse location of 1 in 6 possible bolt positions shouldnt be important on a non-counter weighted flywheel, so perhaps its the fine location so that the side of the flywheel holes do not touch the side of the bolts, and its only the head of the bolt that touches the flywheel?

As mentioned, it's not recommended to have shear forces against threaded sections of a bolt. Instead you should have straight shaft, and for bolts that are designed for shear forces you notice that the shaft is a greater cross section than on a typical bolt. When you roll a thread into a standard bolt this makes valleys of lower diameter than the shank and hills higher than the shank. But on a bolt made for shear loading the shank is at about the same diameter of the hills on the thread. I can post up a photo of a front drive shaft bolt if the description is not clear.

[Dobz]

MrStabby has it right.

R32Abuser you need to re-think your mechanical force concepts.