crank case deformation in stand??

Htegenbo · Htegenbo's Garage

If the crank case is assembled, and put in the motor stand, one piece is hanging on the jig, crank is placed with rods chains etc. (about 30kg total) the case is elastically deformed a few hundreds of a millimeter... (rough calculation 2-3 hundreds of a mm-er) true?
When the counterpart of the case is mated... Even when you bolt the counterpart (and regard this frictionless) the stiffness doubles but still there is some remaining tension and deformation in the case which is frozen as the case halves bonds.
Should it not be better to support the crank case during this process in such way that the load on the case is minimized?

Flat6pac · 10-27-2009, 06:36 AM

I built them for years with the half case sitting on the studs and not putting it on the stand until it was a short block.
Then I got one of the half circles that bolts the case to the stand, different from what I ever used.
Then there is a thread on howmany supports your mount has to support the case...I guess I ll keep building as is convenient but I do like the table with the hydraulic height adjustment, it makes it easy to assemble the 2 halves.
Bruce

Htegenbo · Htegenbo's Garage

Thanks, that confirms my worry a bit and I was thinking about making a special yoke. But I will install the head studs first and do it the way you describe.

alniki · 10-30-2009, 05:32 AM

The mechanic who is rebuilding my engine does not use an engine stand at all. He simply abhors the sight of an engine nodding on the stand, even only slightly.

Flat6pac · 10-30-2009, 08:25 AM

But after you get the first set of heads on its difficult to mount the second set, that was the way for years..and then getting the exhaust on, very difficult
Bruce

LeakProof · 10-30-2009, 11:40 AM

My exact thought process too.
Thats why I took the empty case half off of the stand, for the several months it's going to take me for my re-build. Perhaps a 2 by 4 supporting the front would help?
Heck, I don't even lay down the cams in fear of them warping!
I heard of cranks deforming from laying them down.

Leakproof.

alniki · 10-30-2009, 05:02 PM

My mechanic put the engine on a hydraulic platform and use a lift with self-made accessories as a hoist when needed. Ingenuity is still a must but we home mechanics just cannot do it as easily without the facilities.

Htegenbo · Htegenbo's Garage

Aluminum starts stress relaxation at around 25N/mm2 (average, depends also on internal stresses, I have been involved in some research on this topic on development of semiconductor equipment. New casted or welded parts relax and deform in time even without any load) . with steel this is way higher. So I'm not that worried on the steel parts. If that was an issue the cam shaft would deform because of the valve spring tension in time (when the engine is not running for a long time)
But with assembly on the stand there is also elastic deformation being "frozen" when the case halves being "glued"
I will try to do a measurement on the elastic deformation of the case in the stand to verify.

I like the idea of assembling the case halves supported on the head studs. And when the case halves are mated I'll put it on my engine stand for the heads and cam assembly.

LeakProof · 11-02-2009, 05:46 AM

Perhaps a cradle of some sort could be used to support the case in the early stages.

RichHawk · RichHawk's Garage

We've got 21000 EXPLOSIONS happening every minute at 7000 RPM with Piston accelerations yanking on the crankshaft with more force than your 1 ton Hydraulic press can inflict, And you are worried about deformation of the cases? From lying around?

Just saying...

Richard

LeakProof · 11-03-2009, 02:05 PM

Since my last post, I already noticed two professional machine shops with extra support to back up the case. pics & videos that is.
Funny how you notice things sometimes when looking for them.

Just my two cents,

Leakproof.

jcge · 11-03-2009, 03:20 PM

I don't see any extra support in this factory photo (and the 912's a bit longer than you average 901, 911, 930...)

Htegenbo · Htegenbo's Garage

Today I'm assembling the crank case, I've placed it on the head-studs. I'm going to try to do an experiment in the engine stand and measure the "elastic" deformation. The theory says it deforms about 0.02 mm with a rough calculation. I'm goingg to mount a tesa gauge (0.001 mm resolution) stiff to the yoke, near where it interfaces with the case, and then measure the relative displacement on the other end of the crank case, which should be the elastic deformation...

I'm not trying to say the method in the stand does not work, because is has proven to work in 1000s cases, but it might not be optimal.

Htegenbo · Htegenbo's Garage

JCGE, do you mean a 917 engine??

Flieger · Flieger's Garage

912 is the type number for the 917 engine. Some say it was a subversive tactic to keep the program from arousing interest in the competition or the rule-makers. Others say it was the "real 912" number, not the former "902" which was a 901 with the 4-cylinder engine.

LeakProof · 11-04-2009, 01:40 PM

Gooo point, jcge.
I think that motor is a little longer.

Leakproof.

Htegenbo · Htegenbo's Garage

I did a "quick" measurement yesterday.
I first mounted the gauge stand to the center of the yoke and the tesa somewhere in the middle of the crank. (my mount was not long enough) (pic 1-2)
The gauge had a hysteresis of about 2-3 micron. (moved the pointer back and forth to see if it reproduced)

When I started loading the crank I found 0,02 mm with 5 kg( force on the end of the crank case) and 0,04 mm with 10 kg. Seemed pretty linear (which should be)

Then I wondered what the yoke frame contributed to the deflection, so I did a second measurement, with the gauge mounted to the part of the yoke which is close to the crank case mounting. (pic 3).
Measuring point moved a bit to the end of the case.
Measurements were in the same order...
For me the measurement proved that calculations were giving a good indication of what is happening.

So i assembled my crank case on the head studs...
the crank rotated perfectly free after assembling.

jcge · 11-05-2009, 12:06 PM

Great to see some real data - thanks for posting Htegenbo

The numbers you posted (0.004mm/kg), relative to bearing clearances and total engine mass, seem significant enough to me !!

Also dug up another 912(7) engine photo which contradicts my earlier post, albeit the engines are built to different levels

(C)C Thompson

Walt Fricke · 11-05-2009, 10:07 PM

Well, if you are worried about deformation when you go to bolting the other case half on, why not just shove some kind of prop under the unsupported end?

Before I reinforced the mounting for my bench mounted yoke I used to do that anyway, so as not to tempt fate too much. A length of 2x4 did fine.

Walt

Htegenbo · Htegenbo's Garage

JCGE, great pickture, this confirms a lot. Otherwise my calculation and measurement would tell that the first set-up for case assembly should not work. but porsche has proven it did. But this extra support acts as "gravity compensation" when the case is mated and now everything ads up.
The deflection is not 1 on 1 effecting the crank bearing clearance, because the centerline of the bearing arches will become an arc, and the crank will "middle it out" so When mating the cases the upper half will straighten the lower case a bit, (best case the stiffness will double, without friction) and the crank will settle somewhere in the middle.