Why H-beam rods?

I was looking at an old Franklin engine (1) in a museum in San Diego's
Balboa Park several years ago, and I was surprised to see that it used
strangely-designed connecting rods.

I didn't know what to call those retarded-looking H-beam rods that
looked like a student engineer had seen an I-beam rod and later tried
to draw it from memory but got it all wrong.

H-beam rods look like I-beam rods turned *sideways*, and they don't
have the thick caps that resist bending loads, they seem to have two
thin webs instead.

I was watching a show on TV today where some guy was building a hot rod
Camaro and he pointed out that he was using Eagle H-beam rods and
showed one that was bent into an S-curve from hydraulic locking.

So, I just did a websearch to see what I could find out about those
retarded-looking H-beam rods and found that several manufacturers are
making them and hotrodders use forged H-beam or I-beam rods made out of
really tough steel to replace stock rods that won't take the higher
RPM.

But why H-beams? It seems to me like the two side webs would just
spread out and warp back and forth under high RPM bending loads...

(1) Tucker used Franklin aircraft engines in his cars, and Franklin
engines were also used in tanks.

 

Well, look at a force diagram of a connecting rod. The ends are hinges,
so aside from frictional loads, there's never any torque around the
bearing axis. And if the engine is designed with the piston dead-center
over its big-end bearing, then there's no torque at other strange
angles. All you get is compression and tension forces. As long as it
doesn't collapse under the compression forces from the power stroke, all
should be fine.

http://www.extremepsi.com/store/customer/product.php?productid=16522&cat=
255&page=1&veh=1G%20DSM
has a picture that shows what H-beam rods are about.

http://e30m3performance.com/installs/2.5_rebuild/rods/rods2.htm
has a picture that compares stock BMW I-beam rods and yet another design
for the rod cross-section.

All of these are different mechanical engineers' best guesses about the
best compromise between weight and strength. I think that the
conventional I-beam rods are easier to manufacture: the casting is easy
to do. H-beam rods seem to try to go for more stiffness and less weight
but at he cost of easy manufacturability.

 

Either an I beam or an H beam has its greatest strength in the plane of
the two webs. That means an I beam would resist bending most in the
direction parallel to the crank, the direction where there are for all
practical purposes no load except to keep the central web from
buckling. The H beam has two webs to resist the heavier forces
perpendicular to the crank and uses the single web to keep them from
buckling. As you also noticed the caps can be made lighter so that
smaller counterweights are needed for balance. H beams definitely seem
to work ok. You are just used to seeing I beams.

 

Actually the big end of the rod is being thrown from side to side,
perpendicular to the main bearings. The wrist pin and rod bearing
locate the ends of the rod while the rod's own inertia trys to bend it
in the middle. That is why the resistance to bending in that direction
is more important than in line with the bearings.

A neat design. The major web is in the direction needed. Rather than
use two webs to support the edges of the major web from buckling a
single web supports the middle of the major web. Sort of like slicing
an H down through its central web. Probably nearly as strong as an H
of equal weight and a lot easier to machine. Notice the cap is still
made like on an H rod.

 

The engine you saw was not a franklin, but one of the rare surviving
developmental engines of tucker's design. I didn't work because every oil
tried foamed and the valves stopped working, it wasn't until the advent of
synthetic oil that his advanced design would be useful, but it was too late

 

Okay, I'll buy that. I'll have to work out typical accelerations for
Otto engines and see what comes up.

 

Capital "H or "I", you still have the same basic shape but in
different planes. On the H design the webs are pointed towards the
bearing plane and on the I beam you have the beams pointed
perpendicular to the bearing surfaces.

Frankly, for strength, I like the idea of the forged chrome molly
strength but using 40 % lighter with equal strength titanium. That way
all the reciprocating masses can be less so the overall
piston/rod/crank stress is less. Also forged from billet rods are
always stronger than cast rods (even cast then re-forged) with the
forging forces lining up the molecules (somewhat akin the lining up
the N-S poles in a magnet). BTW, Titanium rods are currently being
used in some MC engine parts where reciprocating forces are great
(rods & valves).

Good thread BYW. It just should have started with an "OT:" as no MC
engines use "H" rods or at least not any that I've ever heard of. Just
had to add my 2 bits worth


Bob Nixon, Chandler AZ
01 Sprint ST "RED" 55K miles
http://bigrex.net/pictures

 

Bob,

Just so you know. The Acura NSX uses Ti connecting rods as original
equipment.

Joshua

 

H-beams are just so pretty...<G>....
Eagle H-beams for my vw
http://hometown.aol.com/bugninva/myhomepage/crank-rods-pistons.jpg

 

Sorry, I'm not into drag bikes.



Bob Nixon, Chandler AZ
01 Sprint ST "RED" 55K miles
http://bigrex.net/pictures

 

"_Bob_Nixon" <>


paging Mr. Nixon to the white courtesy phone Mr.
Nixon


--
Keith Schiffner
History does not record anywhere at any time a
religion that has any rational basis. Religion is
a crutch for people not strong enough to stand up
to the unknown without help. But, like dandruff,
most people do have a religion and spend time and
money on it and seem to derive considerable
pleasure from fiddling with it.
Robert Heinlein

 

I suppose I would need a computer program to tell me what the cyclical
bending loads on a connecting rod are.

That makes me wonder how connecting rods were designed prior to the
computer...

Maximum combustion pressure in a normally aspirated engine might be
around 1000 psi once the combustion process has gotten well
established, but the force is mostly downward, compressing the whole
rod, trying to force the crankshaft out through the oil pan.

The piston and rod will be moving at the maximum piston speed at 90
degrees after top dead center, but the combustion gasses will have
expanded, reducing the downward force to maybe 500 psi.

Then the rod is operating at an angle to the axis of the cylinder bore,
the piston is being pushed a bit sideways.

That's when the highest bending loads would be taking place in the
connecting rod. They would alternate from compressive forces to bending
forces as the piston went through its up and down motions.

The familiar I-beam was the best design for a beam that was loaded in
bending, according to my strength of materials textbooks.

The I-beam caps would take the compression and tension loads.

But the connecting rod actually experiences its worst challenges at the
beginning and ending of each stroke, at top dead center or bottom dead
center, as the rod has to act as a go-between for the violent starting
and stopping of piston motion.

In a single cylinder engine with no flywheel, the piston actually tends
to spend longer at the bottom of the stroke than it spends at the top
of the stroke.

I calculated maximum piston accelerations for a Yamaha R6, since Yamaha
was using the strategy of allowing the engine to over-rev 2000 RPM past
power peak so the rider wouldn't lose tenths of a second upshifting and
downshifting, he could stay in 2nd or 3rd gear in most situations on a
tight race track. Riders were running their engines at 15K...

Maximum piston acceleration = { (rpm ^2 * stroke) / 2189 } * {1 + ( 1 /
2A ) }

where stroke is in inches and A is the ratio of connecting rod length
between
centers to stroke. Stroke is 45.5 mm, rod is 91.5mm center to center.

If I'm working the formula correctly, I get 223969 feet per second^2.
Divide
that by 32.2 and I get 6955g's, that's the equivalent force of almost
7000
gravities, stretching on the rods

So maybe it really doesn't matter much how the cap strips and webs are
disposed in a connecting rod, if the poor thing is going to be
stretched like a rubber band at high RPM, maybe bending doesn't matter
that much...

Motorcycle rods are designed a bit differently from automotive rods.
You may have noticed that
motorcycle rods taper quite a bit from the big end to the small end in
the interest of weight reduction, and engineers designing an I-beam
spar actually have to take the slight angle of the
I-beam caps into account.

According to an aerospace engineering textbook I once read, an engineer
designed a wingspar for a propellor-driven civilan airliner and failed
to consider spar taper. The spar failed, a load of passengers died, the
engineer killed himself over his error.

 

"Namida ga kobore"?

Have you ever heard "Ue o muite"? Do the Japanese respect intellectual
property? Sounds like somebody in Japan thinks we've forgotten "Ue o
muite".

"Ue o muite" played endlessly on my first pocket transistor radio as I
rode my 250cc square fendered Honda Dream in the Southern California
sun, wearing a white naugahyde jacket and wrap around sunglasses.

"Ue o muite" was Kiyo Sakamoto's 1963 hit song that was popular when I
had my first motorcycle. It's about a heartbroken guy who walks around
looking up at the sky (namida ga kobore) so his tears won't run down
his face.

ue o muite arukou
namida ga kobore naiyouni
omoidasu harunohi
hitoribotchi no yoru

ue o muite arukou
nijinda hosi o kazoete
omoidasu natsunohi
hitoribotchi no yoru

shiawase wa kumo no ueni
shiawase wa sora no ueni

ue o muite arukou
namida ga kobore naiyouni
nakinagara aruku
hitoribotchi no yoru

omoidasu akinohi
hitoribotchi no yoru
kanashimi wa hosino
kageni kanashimi wa tsukino kageni

ue o muite arukou
namida ga kobore naiyouni
nakinagara aruku
hitoribotchi no yoru

Sakamoto died in an airplane crash. He was like the Japanese Elvis...

 

I've got three trophy's from 1979-80 from Fremont drag strip in the
SFBA racing grudge nights with a "stock" RD-400, KZ-650's and 2
different GS-1000's & a Honda CBX inline 6 cylinder bike.

I should have said I'm not up on professional "pro-stock" drag racing
state of the art, gear. Also, I believe there was a "sorry on my part"
in there somewhere, so lighten up with the cheap shots, Stephen. OK?


Bob Nixon, Chandler AZ
01 Sprint ST "RED" 55K miles
http://bigrex.net/pictures

 

I remember the song well, Krusty & Steve! It still brings a tear to my
eyes when I hear it. A good love song to a romantic like me does not
lose it's meaning merely because you can't understand the words. It
was almost like you could tell what he was singing across the language
gap.

As to the Honda dream and early transistor radios, they (radios)
seemed huge by today's everything IC electronics standards. Kind of
like late vacuum tube (even something like a multi-tube dual triode,
diode miniature) that was no bigger than a 60's single bi-polor
transistor.

Now we have 1/4 billion transistor junctions ( Cmos fets) on an chip
about 1/4 to a 1/3" a square inch in size on 60 nanometer X-ray
lithography and cheap 600cc motorcycles with relatively expensive
titanium valves/rods.

However we still haven't learned how to live as a world in harmony,
looking forward with arms open wide, to meet our neighbors in nearby
star systems. But what about those nasty Romulans Klingon's,
Cardasions, the Borg and those shape shifting Dominion leaders with
their Jem Hardar & Vorta minions in the gamma quadrant of the Milky
way galaxy And what of that poor Voyager stuck in the Selta quadrant
for nearly 20 years.

http://en.wikipedia.org/wiki/Transistor_radio
http://www.repairmanual.com/motorcycles/1961/27/4/12809
Couldn't find a good picture of an old square fender Dream on the web

http://www.maddmansrealm.com/sukiyaki/
http://en.wikipedia.org/wiki/Romulan
http://en.wikipedia.org/wiki/Vorta
http://www.startrek.com/startrek/view/features/documentaries/article/5299.html
http://en.wikipedia.org/wiki/Jem'Hadar
http://www.startrek.com/startrek/view/library/episodes/VOY/index.html
Bob Nixon, Chandler AZ
01 Sprint ST "RED" 55K miles
http://bigrex.net/pictures

 

i missed it. No sweat, Shephen. Have of nice Holiday season.


Bob Nixon, Chandler AZ
01 Sprint ST "RED" 55K miles
http://bigrex.net/pictures

 

Also, I believe there was a "sorry on my part"

Bob broke his funny bone when he hit the deer....it never healed
properly...... < > (Smiley face Bob!)

 

There was a parody of the so-called "Sukiyaki song" that went,

The arms of Sukiyaki,
The charms of Sukiyaki,
Are all I've longed for since I left old Nagasaki.
I'll never roam,
Far far from home...

But the actual English translation of "Ue o muite" is:

I look up when I walk

I look up when I walk so the tears won't fall
Remembering those happy spring days
But tonight I'm all alone

I look up when I walk, counting the stars with tearful eyes
Remembering those happy summer days
But tonight I'm all alone

Happiness lies beyond the clouds
Happiness lies above the sky

I look up when I walk so the tears won't fall
Though my heart is filled with sorrow
For tonight I'm all alone

(whistling)

Remembering those happy autumn days
But tonight I'm all alone
Sadness hides in the shadow of the stars
Sadness lurks in the shadow of the moon

I look up when I walk so the tears won't fall
Though my heart is filled with sorrow
For tonight I'm all alone

-------- It is said that the song was written by Rokusuke Ei when his
heart was literally broken by a Japanese actress, Meiko Nakamura.
Sakamoto was killed by a plane crash which lost more than 500 lives in
1985.

 

YoucH! (223969 is wrong. You can only say 224000 because your original
numbers had only three digits of accuracy. But that's not my point,
that's just me channelling my high school chemistry teacher.)

7000G?! OMFG!

How much does one of these connecting rods weigh? 90mm ~= 10cm. Let's
say 2 cm^2 average cross section, that's ~20CM^3. A typical metal weighs
~4g/cm^3 so that's ~100g or .1kg. So imagine setting up a connecting rod
so its two journals are supported on parallel horizontal bearings. Hang
a 1500 lb weight from the middle. The connecting rod must not break.

I don't know enough about metallurgy or the strengths of metals to
really say, but that just strikes me as on the edge of passing sanity
check. I wouldn't build a house or a rocket with that kind of tolerance

The people who reviewed the design, the people who did the
testing-to-failure, and the FAA inspector who approved it should also
have killed themselves ... after each having written detailed
process-failure analyses.

 

How's that math working? I used {[15000rpm^2 * (45.5/25.4)]/2189} * (1 +
[1/(183/25.4)] to get 209523, same order of magnitude, but I've only
converted to inches so presumably need to divide again to get to feet
(though would need to multiply by 60 to get from /minutes to /seconds).

I tried doing the math without the conversion to US units and got
~146000.

Neither result is close enough for me to feel happy with it,
particularly given the mysterious shift from metric units to and
rev/minute to ft/sec.

Doing it with feet and seconds returned 11.4/sec^2 that's right, the ft
cancelled out.

I'm terrible at math; what did I do wrong?

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.