Cam bearing material

[pontiacjack]

I'm in the early stages of design/fabrication of a DOHC conversion to a hemi head (previously pushrod). Each cam will ride in five machined-from-billet "pillow blocks". Is there any reason not to use the "native" aluminum alloy of these blocks (probably 7075-T6) as the bearing surface for the steel camshaft journals (all full-pressure-lubed)?

 

[patprimmer]

I don't know what grade of aluminium, but almost all OHC aluminium head engines run the cam in the head material.

Of course some do use bearing shells. I know VW uses bearing shells to run the cam in the aluminium crankcase on old air cooled Beetles.

The one benefit of removable bearing shells is the repair-ability.

Regards
Pat
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[Lou Scannon]

The one benefit of removable bearing shells is the repair-ability.

Good point. Although, with a cam running in the native material, should the parent material become excessively worn, it would still be feasible to remachine the bores and then use bearing inserts.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[MikeHalloran]

"... still be feasible to remachine the bores and then use bearing inserts."

Well, sure it can be done, but finding the original intended centerline of a worn hole can be time-consuming and therefore expensive.

Mike Halloran
Pembroke Pines, FL, USA

 

[Lou Scannon]

Well, sure it can be done, but finding the original intended centerline of a worn hole can be time-consuming and therefore expensive.

Granted; but it doesn't occur to me that the hole centreline is going to so drastically affected by wear "beyond service limits", that line boring the cam bearing holes would result in a shift of the centreline beyond that which can be readily accomodated by the provisions for lash adjustment (or cam indexing, for that matter).

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

It is more total work to bore to run in parent material, then rebore to fit removable inserts.

Also, when doing the original design, the cam would need to be made with journal sizes to match inserts being kept in mind, but either way can work, as can cutting the caps at the parting line to close them down then reboring in parent metal, but that drops the cam into the head, albeit only a few thou most likely.

Also when designing the head, there needs to be enough metal around the cam bores and sufficient spacing of the cap bolts to allow for repairs or inserts.

Regards
Pat
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[BigClive]

PP - I think Beetle crankcases were made of some kind of magnesium alloy rather than aluminium.

The Jaguar DOHC XK engine used an aluminium head but also used separate replaceable cam bearing shells. I don't know of any other aluminium-headed OHC engine that doesn't run the cam directly in the head material.

 

[patprimmer]

BC

It depends on the model. Some where Magnesium alloy and some where aluminium alloy. I'm not a metalurgist, so if it is aluminium/magnesium alloy, I don't know at what levels, which one gets the naming credit. I guess whichever is in the largest portion.

Regards
Pat
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[pontiacjack]

I managed to jostle layout/dimensions around enough to allow use of off-the-shelf cam bearings pressed into the pillow blocks- which obviates the need to answer my original question (at least for now!). Thanks for the responses.

 

[BrianGar]

@ Clive, nearly all the new Vw/Audi Tdi Pd engines run cam shells(8+16v). Possibly due to increased loads as the cam also drives a follower between lobes, that drives the Pd injector directly.
I believe everything wears in there too including the shells, so its a good job they went with them in the end it seems!!

http://dieselrepairblog.wordpress.com/2011/06/09/vw-tdi-pd-camshaft-repair/

Brian,

 

[BigClive]

BG - It seems a bit odd that VW is using a mechanical injector when the rest of the modern diesel world seems to be common rail. I see the injector pressure is 25000psi - so it is not surprising that the cam has to work hard.

 

[dgallup]

I believe VW has gone away from the PD Pumpe-Düse (unit injector) design and has used common rail since ~2008.

Unit injectors have been used widely in heavy duty class 8 truck engines by many manufactures but these ~16 liter 6 cylinder engines have a bit more room to fit properly sized cam lobes.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[BrianGar]

Clive, you are right, it is a lot. They did have great 'power' though, and surprised many when they first showed up.

Dgallup, you are correct, I think it was 2009,

Brian,

 

[pontiacjack]

This design is "evolving" as I proceed from drawings to prototype pieces. One of the four journals on each cam will be a diameter for which I can't locate an off-the-shelf cam bearing, so those journals will run directly in the billet 6061-T6 cam stands. My fingers are crossed...

 

[BrianGar]

Jack, you should be fine given that you have made the choice. Although lower loads, all/nearly all of the 'billet' dry sump scavenge pumps run the gear pumps directly onto the alloy. They dont even use cast iron discs at ends for axial rotor positioning. Some are anodized, others not. Many aftermarket billet heads are running 7075 carriers too.

Also, may be of interest, found a great company before Christmas should you want to cast the carriers down the line instead of machining(If you're doing lots and the swarf is following you home)

Email drawings, you have parts on your desk 10 days later! No way connected with them, but from someone that often casts runs/prototypes they are a godsend at times for stuff that has a ton of cores, or is a nightmare to pattern.

http://www.prototypes.it/QuickSandCastin-EN.html

Brian,

 

[Tmoose]

I'd guess the machined finish of the bearing bore (and cam journal OD) just needs to be manufactured "good enough".
Gotta think piston aluminums are chosen for reasons largely other than bearing qualities, and (hard, highly finished) wrist survive just fine.

Generically aluminum alloys used as bearings lack the various babbits' embeddability and conformability and are even doped with lead, probably for the same solid lubrication reasons bearing bronzes sometimes are.

http://machinedesign.com/article/aluminum-alloy-bearings-1115

http://www.kingbearings.com/files/Engine_Bearing_Materials.pdf