Quick question; intake cam VANOS spline shaft, reusable?

[aeronaut]

This bugger was snug.
Install is Tq + angle (40Nm + 60 deg), so says TIS.

These are reusable, right? I hope? Not one time use?
(E36)

[Def]

Most automotive fasteners that use a torque + angle tightening method are torque to yield. The first gives a known preload range in the elastic region (which is like +/- 25% based on just torque), then the final stretch puts it in the plastic zone and drops the torque uncertainty down in the +/- 5-10% range due to the flatter slope of the stress-strain curve in the plastic zone.

[aeronaut]

Thanks for the quick response. Yea, I understand Tq to yield (although your description is one of the best I've read!!).

But tq+angle can be used on bolts that aren't past elastic. This bolt doesn't clamp anything.

And for example, the TIS is pretty explicit with head bolts, and other bolts that need replaced, that is: "Replace, wash and oil screws"
But for this splined bolt, the TIS doesn't.

See here:

[Def]

True, it can be specified on elastic region bolts, it's just not common.

You can calculate the approximate clamping force using Torque = K * Dia of Bolt * Force(clamping)

I'd use K=0.15 (typical for a low friction dry fit, or very lightly lubricated bolt, use K=0.12 for oiled). Dia of bolt is the major dia of the thread, like 8 mm etc., torque is your initial torquing step. Solve for clamp force, then multiply that by 1.25 to account for uncertainties in the K factor. Divide clamp force / minor diameter of bolt (major dia - thread pitch for most metric fasteners), this will get you your minor dia tensile stress. If you've been doing this in N/mm as torque and mm^2 as area on the bolt, your stress is in MPa.

Divide this stress by the elastic modulus of the bolt (~207,000 MPa for steel) to get the tensile strain %. Take half the bolt length in mm and multiply it by this tensile strain % and that is your strain in mm (this has to do with how the torque is reacted out to clamping force in a tapped hole, if the bolt is super long, I'd probably just take the first 4-5 threads (or roughly 0.75*bolt dia) as the length that's straining at this lower torque).

Calculate the additional strain that 1/6 turn puts into it by multiplying 1/6 * thread pitch = additional strain in mm. Add that to your earlier strain and see if you're beyond the plastic limit. You can reverse this all the way back to stress by stepping back through the steps. Grade 8.8 yield is 640 MPa, Grade 10.9 yield is 940 MPa.

The biggest uncertainty is what length of the bolt is stretching, but most bolt stress analysis documents use that 0.75*bolt dia for bolts going into tapped holes in "stiff" material or stiff threaded inserts into softer metallics. If you get something close to yield or well beyond yield - it's a safe bet it's a TTY bolt.



And you mention it doesn't clamp anything - what's it doing then?

[aeronaut]

Good stuff!

So I convinced myself this thing is reusable cause TIS doesn't say replace.
The splined bolt attaches into end of intake cam. The VANOS hydrolic unit captures this spline and changes intake timing. That's "all" it does.

Here's a pic of it now installed into the new Schrick cam. 40Nm + 60deg is quite a bit past snug.

[Def]

What size bolt is it?

It sounds like it does a lot more than just sit there - it's providing enough clamp up to resist any separational forces imparted on the spline, and if the spline isn't keyed/pinned in some way to the cam, it's doing this through frictional forces only.

While it might live a relatively easy life, I think you'll find it's resisting some fairly sizable loads in the valvetrain if you looked at all the loading.

I'm betting BMW was space constrained and the bolt is something like M10 and sees a decent bit of total load going into it, so they wanted to know its preload a bit more accurately than some yokel giving it a click with a torque wrench, so they either wanted it torqued up really high in its elastic range with a low torque + set angle, or they wanted it to go a small amount in the plastic region and it just didn't get communicated to always replace it.

Honestly, either of those cases can PROBABLY be ok for reuse some number of times, but all it takes is one bad thermal cycle or external load to push things a bit further and POP.


I'm not even sure what level of engineering would go into typical "secondary" engine bolts like this at an OEM. Maybe a lot, maybe just a cursory sizing, and a quick calculation of expected preload range vs. clamp up force. In comparison, every single bolt in a rocket (my day job) would get checked for strength at all worst case conditions (min preload + worst case thermal + max ext. load = does the bolt hold the load without separation?, max preload + max ext. load + worst case thermal for that = does the bolt go past its structural margins and risk yielding in service?). Interestingly, we typically don't do TTY bolts due to the high strength of our bolts, and if we need greater preload range precision, we will go to some other method of verifying that (bolt strength, instrumented bolts etc.).

[ScotcH]

you can reuse it. When ordering aftermarket cams, it specifically says to reuse your old ones from the stock cams.

[aeronaut]

Thanks. Done.
And "When ordering aftermarket cams, it specifically says to reuse your old ones from the stock cams.".....yea, VAC was suppposed to send me a spec sheet on the cams. NADA

It's an M14x1.5

[Def]

That's pretty low torque for an M14 bolt, so almost surely fine. It's weird they did low torque + angle though.

[NeilM]

I'm not even sure what level of engineering would go into typical "secondary" engine bolts like this at an OEM. Maybe a lot, maybe just a cursory sizing, and a quick calculation of expected preload range vs. clamp up force.

I'm certain that quite a lot of engineering will have gone into this part.

It's not a bolt, but instead a driveshaft with an integrally machined helical spline at one end and a male thread on the other. The thread serves only to secure this shaft into the cam; it doesn't clamp any other parts. This shaft transmits the secondary cam chain's drive torque to the intake camshaft, and in doing so would see substantial torque ripple from the cam lobes on the valves, as well as lots of general engine vibration.

Although you can reuse this shaft, it's also fairly common for it to be replaced with new for purely practical reasons. Its factory installation is quite hard to break free, typically calling for serious bouncing on a 3 ft bar. You really, really don't want your hex tool to slip out of the shaft's socket and round it out. While you're doing this, it's not easy to secure the cam while protecting the lobes from getting damaged. You want to avoid using the timing squares at the opposite end for that, since that would put full torque into the camshaft. BMW also made a questionable decision, at least by today's standards, to use a simple female hex in the shaft's outboard end. These days they'd likely use a more robust triple-square or similar socket configuration.

There may also be some advantage in leaving the original shaft still in the stock cam for resale purposes.

Neil

[Def]

I'm certain that quite a lot of engineering will have gone into this part.

It's not a bolt, but instead a driveshaft with an integrally machined helical spline at one end and a male thread on the other. The thread serves only to secure this shaft into the cam; it doesn't clamp any other parts. This shaft transmits the secondary cam chain's drive torque to the intake camshaft, and in doing so would see substantial torque ripple from the cam lobes on the valves, as well as lots of general engine vibration.

Although you can reuse this shaft, it's also fairly common for it to be replaced with new for purely practical reasons. Its factory installation is quite hard to break free, typically calling for serious bouncing on a 3 ft bar. You really, really don't want your hex tool to slip out of the shaft's socket and round it out. While you're doing this, it's not easy to secure the cam while protecting the lobes from getting damaged. You want to avoid using the timing squares at the opposite end for that, since that would put full torque into the camshaft. BMW also made a questionable decision, at least by today's standards, to use a simple female hex in the shaft's outboard end. These days they'd likely use a more robust triple-square or similar socket configuration.

There may also be some advantage in leaving the original shaft still in the stock cam for resale purposes.

Neil

You'd be surprised at the different rigor that bolted joints are designed to, and what is accepted as "reasonable" when pushing things on a bolt. Most automotive bolted joint connections tend to be very conservative on the amount of external load versus preload to a given bolt, which doesn't typically add much mass to a vehicle (probably a ~few pounds across the whole vehicle in general, small bolts are light).

You'd might be surprised at how easy it is to say a reasonably torqued bolted joint that doesn't see much external load vs. preload is deemed "good" with great accuracy.

40 Nm + 60 degrees certainly doesn't sound like a super high torque to me. But maybe that 60 degrees really does crank on it quite a bit if it has a somewhat coarse thread pitch.

[aeronaut]

You'd be surprised at the different rigor that bolted joints are designed to, and what is accepted as "reasonable" when pushing things on a bolt. Most automotive bolted joint connections tend to be very conservative on the amount of external load versus preload to a given bolt, which doesn't typically add much mass to a vehicle (probably a ~few pounds across the whole vehicle in general, small bolts are light).

You'd might be surprised at how easy it is to say a reasonably torqued bolted joint that doesn't see much external load vs. preload is deemed "good" with great accuracy.

40 Nm + 60 degrees certainly doesn't sound like a super high torque to me. But maybe that 60 degrees really does crank on it quite a bit if it has a somewhat coarse thread pitch.

It's pretty damn tight. Yea, 3' bar, and a VERY hard pull (bar bending a lot) to break it loose. I'd guess 250ft-lbs and more.

[ScotcH]

It's pretty damn tight. Yea, 3' bar, and a VERY hard pull (bar bending a lot) to break it loose. I'd guess 250ft-lbs and more.

Meh ... a dewalt impact takes that off without issues

[NeilM]

You'd be surprised at the different rigor that bolted joints are designed to

No, I would not. ;-)

40 Nm + 60 degrees certainly doesn't sound like a super high torque to me. But maybe that 60 degrees really does crank on it quite a bit if it has a somewhat coarse thread pitch.

Since it’s not an ordinary bolt and therefore doesn’t carry the usual identification markings, there’s no way to know for sure, but given the application I suspect it’s a fairly high grade material. In that case even tightening 60 degrees can generate enough stretch for some pretty hefty clamping force.

Neil

[aeronaut]

Meh ... a dewalt impact takes that off without issues

True...but I was concerned that this particular cam being held in a vise, getting hit with an impact, could end poorly. Especially due to the profile of the part that needs clamped in the vice.