Fasteners in shear - automotive application

[TonySoprano]

I'm not an engineer, as will be quite evident when I pose my question . I'm sure this forum actually contains my answer but my lack of an engineering background prevents me from interpreting it properly. All that said, I'll plow forward.

I'm attaching a drive-pulley, for a belt driven supercharger, to an OEM crankshaft pulley. I understand that ideally bolts should be loaded in tension, not shear. I have a diagram to illustrate most of the various parts I'll refer to but it won't allow me to hotlink the image.

http://www.putfile.com/pic.php?img=6298314

In this diagram, although not clearly pictured for some reason, the OEM crankshaft pulley is fastened to the crankshaft boss (11-411) with four 6mm fasteners. The supercharger kit replaces the circled spacer (11-408) with a thicker spacer, and four longer 6mm fasteners, to space the supercharger drive-pulley out from the crankshaft pulley (11-371). In the diagram you can see the holes in the spacer for the four 6mm pulley fasteners, and you can just make out a fifth hole to accomodate the drift pin that is pressed into the crankshaft boss.

Now here is where my perceived problem lies - the drift pin that is pressed into the crankshaft boss is sufficiently long enough that it goes completely through the crankshaft pulley, and even the thicker spacer that replaces the OEM 11-408 spacer. However, it is not long enough though to go through, or even reach, the supercharger pulley. I think that means the shear forces imparted by the supercharger pulley are left solely to the clamping ability of the four 6mm pulley bolts. Is that correct? The pulley attachment bolts are only torqued to 8-13 ft-lbs. That doesn't seem like a lot of frictional force to overcome before the pulley bolts are in shear.

If my assumptions are correct, what would be the better solution? -

a) drill and tap the crankshaft pulley boss (11-411) so that I can use larger pulley attachment bolts (8mm) and thus higher torque values on the fasteners.

b) use a longer drift pin, or add an additional pin that is long enough to also locate the supercharger pulley.

Thanks in advance for any input.

 

[desertfox]

Hi Tony

I think your right and a longer pin would be my choice however why not talk to the people that supplied the super charger kit.

regards

desertfox

 

[TonySoprano]

Hi desertfox, thanks for you input.

The supercharger supplier doesn't think it to be an issue. Another customer sheared the four bolts and drift pin but the supplier attributed the failure to an incorrect spacer width (too thin) between the crankshaft pulley and supercharger drive-pulley. This is not one of the more noted aftermarket supercharger suppliers. It's a small niche company that has adopted an Eaton MP62 supercharger to various 4-cyl. applications. The founder is an EE by training.

 

[desertfox]

Hi Tony

Okay I see what your saying, but that means that the pin according to the supplier doesn't take the torque, have you asked the supplier directly exactly what the function of the pin is.
Secondly what max torque is transmitted and what is the bolt PCD, also how big are the holes that the bolt passes through, I am just trying to establish that there not fitted
bolts which may be used to transmit torque although looking at the diagram they look like ordinary M6 bolts.
Roughly speaking the frictional force to overcome to mating
faces asembled with a single M6 bolt torqued to 8ft-lb would be in the order of 400lb force and you have 4 of thse bolts.
If you can give the info I requested earlier we might get a rough idea as to whether or not the bolts are adequate.

regards

desertfox

 

[TonySoprano]

No, I haven't directly asked what the function of the pin is. I'm just nearing the completion of my installation and I ran across a thread where an existing customer sheared the four 6mm drive-pulley bolts. His fastener failure prompted a bunch of inquires from other customers who are thinking they may experience the same failure. The supplier started to get agitated about the many questions he was fielding about what he feels is a non-existent problem. I thought it best if I sought an opinion apart from the supplier (he designed the kit) himself.

I will say this, he has several dozens of these kits in the field and I've seen just two people post on his board who have sheared the fasteners. A third customer disassembled his pulley set-up to add an ignition crank-trigger and noticed while the mounting bolts were fine, the drift pin was visibly bent. Speaking of the drift pin, I thought I might point out something about the diagram I previously linked. It's not nearly to scale. The drift pin is only about 4mm in diameter although in the diagram it looks to be the same size as the four pulley fasteners. Maybe the pin is there to assure proper orientation of the crankshaft pulley, as the crankshaft pulley is an integrated pulley and harmonic balancer? The OEM crankshaft pulley bolts would see little in the way of shear because it's bolted directly to the crankshaft boss (11-411), which is in turn keyed to a woodruff key on the crankshaft snout.

Now, to answer your questions -

The bolt PCD is 53mm. The bolt holes in the crankshaft pulley, supercharger drive-pulley, and the pulley spacer are all 7mm. When you ask what the max torque is transmitted are you referring to the max torque required to turn the supercharger while it is at it's peak load? I don't know what that number is, but if you need it, I can try to find out. I've been told while at maximum boost it takes several horsepower to operate the supercharger.

Thanks for your help.

 

[desertfox]

Hi Tony

Having studied your diagram a bit more the 4mm diameter pin is supplied by whom? I thought it was the supplier of the supercharger kit but I now realise that must have been there from the start and the supplier for the kit as no control on that pin am I right?
Regarding the torque capacity if the torque exceeds 139ft-lbs the mating faces will slip based on the bolt circle of 53mm and an 8ft-lb bolt torque although this is only a very rough estimate, what grade material are the bolts? Also it would be useful to know what the max torque transfer is under peak load.
I don't think its very good, if the design is relying on those 4 bolts to take the shear with clearence holes of 7mm dia as the bolts theoretically could be central in the hole,
in which case you are relying on friction to transfer the torque (as stated by tourself) and in addition if the faces do slip then the shear is taken possibly by only 1 or 2 bolts when the edge of the bolt hole collides with the fastener.

best regards

desertfox

 

[TonySoprano]

Hi Desertfox:

Yes, you're right. The 4mm diameter pin comes pressed into that crank boss from the factory, not the supercharger supplier. I can see where I wasn't clear about that. As mentioned earlier, the pin is sufficiently long enough that it protrudes all the way through the crankshaft pulley/balancer and even most of the way through the thicker pulley spacer that's supplied in the supercharger kit. However, it's nowhere near long enough to go through, or even locate the supercharger pulley. Also, I misspoke in my last post - while the crankshaft boss (11-411) is keyed to the crankshaft, the crankshaft pulley/balancer simply bolts to that boss with the four 6mm bolts. The bolts are a metric class grade 12.9.

I've Googled for well over an hour trying to determine the max torque transfer at peak load, including browsing Eaton's website. I'm finding many general references to the parasitic drag created by the supercharger, but very little in hard numbers. I did find one single reference from another Eaton MP62 user that it took 27 HP to drive his supercharger at maximum boost, but I'm not clear how he arrived at that number. Do I simply convert that HP number to a torque value at a specific RPM, or am I comparing apples and oranges?

Thanks for your rough estimate on the torque required to overcome the clamping ability of the fasteners. Your calculations are based on the fasteners being torqued to 8ft-lb. What happens to the estimate if I went higher in the recommended torque range of 8-13ft-lbs? Does it result in a substantive change, or is it just noise?

 

[desertfox]

Hi Tony

If you increase the bolt torque you increase the torque the joint will transmit however its not quite as simple as that.
To expand if you double the torque you double the torque the joint can transmit but that relys on the surface finish
of the joint faces and whether theres any lubricant present
between those faces,another big factor is the friction between the bolt and tapped hole (dry or lubricated) and if your setting purely with a torque wrench your pre-load on the bolt could be +/30% of the figure your after.
You also need to consider the yield stress in the bolt ie:-
there is a torque beyond which the bolt will take a yield set or depending on the tapped hole material and threaded length engagement of the bolt the internal threads can yield too.
For a 12.9 grade screw assuming the threads are assembled without lubricant, the stress within the bolt itself ranges from 53% to 87% of the yield for 8-13ft-lbs respectively so therefore the higher torque setting is probably the better for the joint.
Now to shear one of these bolts I will assume that the shear stress needs to reach a value of say 75% of the UTS
of the steel which in this case is 900N/mm^2 and the force to achieve that would be 18090N or 4056lbf which on the bolt PCD you gave earlier would equate to 324ft-lbs.
Again these figures are approx but should give some idea to
what might occur.
Now I have no idea what the material is that these bolts screw into so I cannot predict whether failure in the internal threads or indeed bearing failure of the joint material (if the bolt touches the edge of the clearence hole it passes through) is occuring under service conditions.
If you now the rpm that the thing rotates at then yes using the power figure you should be able to get a torque figure.
That said if we assume one bolt was transferring the torque
then it would fail at the figure above, if all four where to fail together then it would need 4*324ft-lbs.

regards

desertfox

 

[TonySoprano]

Desertfox:

Thanks for your calcs and for taking the time to give your thoughts on this matter. My take away from all of this is that it makes sense to address this issue now, rather than after I shear all four bolts. Perhaps it would never have been a problem, but I'd rather do it now than having to disassemble everything to drill out broken fasteners. I'm going to replace the existing 4mm pin with something longer and more substantial. Something on the order of .25" or so.

Thanks again!

 

[desertfox]

Hi Tony

Before you go modifying anything I recommend you speak to the engine supplier and supercharger supplier.
How will you know what size to make the pin if you don't know what torque it is seeing.
Additionally the customer who stripped is supercharger down the find the screws ok but the pin bent suggests that either the pin was bent during assembly or that the pin
saw a greater torque then it was designed for originally.
Also if the pin was bent due to torque then the four bolts weren't carrying the load.

regards

desertfox

 

[Tmoose]

What kind of belts?

Looking at the link I think the pin is for establishing the cam timing only. The bolt in the crank nose probably gets some whopping torque to lock that stuff to the crank via friction from clamping.

Designs Expecting or inadvertently asking a key or pin to transmit torque, especially varying or reversing torque without wiggling or shuffling then fretting and loosening are doomed. Even if the supercharger pulley gets a terrific grip on the crank pulley, the OEM clamped joint is going to get a workout driving the "charger. The "charger's inertia during a fast shift will be even tougher. I'm guessing the belt(s) are v type, to allow slippage under extreme conditions.

 

[TonySoprano]

Hi Desertfox:

One of my problems is the supercharger supplier doesn't believe there is a problem, so I'm not sure what information he'll provide, if any. He attributes the few problems encountered as being caused by incorrect spacing between the crankshaft pulley and supercharger drive-pulley. The OEM spacer (11-408) is really not much more than a shim. It's purpose, as best I can determine is to distribute the clamping force of the four 6mm crankshaft pulley fasteners.

The supercharger supplier's kit ships with an additional spacer to use along with the OEM spacer/shim. The kit designer suggests the problem is customers are using his supplied spacer without the OEM shim. That leaves the two pulleys to be stacked directly on each other, essentially clamped together by the pulley fasteners. I can certainly see why that would be problematic.

I sent Eaton two emails this morning to their supercharger division and they were both bounced back. I also put a call out to their tech-support line but was put on hold. I'm at work right now so I needed to end my call. I'll try back again to day. I'm hoping to get a definitive answer on the max torque transferred at peak load.


Hi Tmoose:

The bolt in the crank nose does see a pretty good torque value, and it does clamp the crankshaft boss (11-411). However, the crankshaft pulley is only attached to the crankshaft boss with the four 6mm fasteners. Once those fasteners are removed, the crankshaft pulley is free to slide right over that crankshaft bolt. That is, the inner diameter of the crankshaft pulley is larger than the diameter of the crankshaft bolt head. The supercharger belt is a six-rib serpentine belt.

Your comments have me re-thinking the idea of a larger pin. Mazda determined on this particular stock configuration that four 6mm bolts provided adequate clamping force to positively locate a pulley that had to drive an alternator, power-steering pump, and A/C. The wildcard is the additional torque required by a 1 liter supercharger at peak load. I'm going to do my best to get reach someone at Eaton this afternoon.

Thanks all. I'll post up whatever info Eaton will share.

 

[Tmoose]

Look at the difference between the early and later Miata front engine pulley design. Unless re-installed with the full and proper torque, things loosened up and ate the crank. The solution(s), larger diameters, full length key, AND A BIGGER BOLT WITH HIGHER installation TORQUE.

Keys/pins can not be trusted, but it's not their fault.