Re: Metal choices (Was:Tie Rod Fracture Input)

["Rich Wagner" <drsavage@domain.elided>]

Ralph wrote:

>Several years ago, under force of law, I had to replace my oil pump
impeller 
>gears...Guess why all the fuss?  The nice engineers at Lycoming, had 
>speced a sintered metal be used to make the gear.  (Hope I am spelling
that 
>right.)  For those of you who may not know, this is apparently to metal
what 
>pressboard is to wood.  And this is what they used to drive the oil pump
on 
>the only engine used in an aircraft?  I am betting the engineer who spec'd

>this saved a few bucks on each unit....Cost me about a thousand.  

Yes, he did.  A sintered gear is SO much cheaper to manufacture than a
machined gear.  In many applications, it actually works out okay.  Most
sintered gears are used in low-power-transmission applications.  I'm
betting the guys at Lycoming didn't have a solid idea of how much power
that oil pump consumes.  Seems a shame to make such a mistake in a piece of
machinery that costs $40,000.  Penny wise, pound foolish.  Even worse, it's
their penny, your pound!  As a fellow pilot, I feel your pain.  As an
Aeronautical Engineer, I'm just plain (plane?) disappointed.  Again.

We used sintered gears in the electric vehicles I designed for Electric
Mobility Corporation.  The transaxle that the gears were used in was driven
by no more than 1 HP.  For the most part, the gears worked pretty well. 
They exhibited acceptable wear characteristics when bathed in the correct
lubricant (molydisulfide grease) and, in our application, were never
stressed to near their limits.  The interesting thing was, we could draw
MUCH more than 1 HP out of the transaxle under (electrical) braking.  Under
those conditions, we could easily wipe the teeth from the ring gear.  After
such an incident, the forged and surface-hardened pinion would show
absolutely no sign of damage.  Sintering works very well for a great many
applications--provided that the sintered parts are used appropriately.  It
allows us to make, for example, an affordable, 1 HP transaxle.  Poorly
applied, as in your Lycoming, (failed) sintered components are just an ugly
tribute to poor design, and managerial short-sightedness.

In all of this talk about the new sintered connecting rods, we need to step
back and consider one very important point:  connecting rod design is a
solved problem, and has been since about 1926.  With this in mind, when you
hear about a new process being used, especially by a large manufacturer,
performance enhancement should be about the last reason you should
consider.  In the case of these sintered rods, it's very clear that their
use will provide an (hopefully) acceptable component that can be
manufactured with <fewer operations>--offering direct reductions in
production costs.  With fewer operations, there are also increases in
process consistency, reducing waste and, again, reducing production costs. 
Again, the point here is produce essentially the same component, but at
reduced cost.  It's analogous to going from a casting or forging to a
stamping.  The casting and forging process may produce a beautiful part
that works perfectly.  If the stamping process can produce a functionally
equivalent part, that part will be substantially cheaper, because the
stamping process involves only one operation, whereas the casting/forging
process will involve many.  If you can replace that stamping with a plastic
injection molding, the production costs will be even cheaper.  In the case
of these sintered con rods, there may indeed be slight performance
advantages offered (although I'm dubious), but that's only because these
rods are being compared to rather pedestrian components in the first place.
 Compare them with a suitably high performance 7075-T6, carbon fiber, or
titanium unit, and you'll likely see that these are just the rods of the
workaday world.  However, as with most other production novelties in this
country, I won't be surprised to see these new rods marketed by
performance-oriented companies as the latest--and therefore, by naive
default, highest performance--offering to come along.  Don't believe them. 
Unless they can show a substantial decrease in mass, with provably equal
strength, they're selling snake oil.

I'll be very interested to see cost comparisons between rebuilding an
engine with the sintered rods, and rebuilding an engine with conventionally
manufactured rods.  I'll also be very interested to see what (especially
small town) mechanics think of them.


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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