Why are pistons built with freedom of movement on only one axis of the piston's arm? I believe I understand why the piston arm and head are jointed (so the arm can move through the circumference of the rotating crank shaft). So why not put the piston on a ball joint?

B

Way too early for that after the kegger last night b.

Because it can only travel in one plane. It's affixed to the crank as such. There's no ball joint at the crank.

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Yeah, there's a shaft linkage on the crank end allowing the arm to travel in one plane. If the crank end was not restricted to a single plane the piston arm would slop all over the place--I understand that would not be a hoped-for circumstance. I'm wondering about the other end.

Why is the head restricted to travel in the same plane? I mean it's going up and down a tube--it's not like it has a choice of where it's going. Regardless of whether there is a piston present, the piston arm would still be restricted to travel in the same plane because of the joint at the crank (this is sounding almost lewd). I'll bet that the piston would pretty much move okay even without a firm connection to the piston arm. I mean, the piston has one axis of movement possible and two sources of force to move it--explosion in the cylinder head above and the force of the crank below.

There is a connection to ensure that timing is as near-mechanically perfect as possible. But I don't think the connection is for force transference, is it?

So if it is for timing, why does it matter what form the joint takes?

i think the reason a ball joint isnt use is due to "piston slap"...

I think it is for at least three factors, none of which have to do with the physics of the engine operating:

1. The piston pin and connecting rod bushing is a more durable joint than a ball joint.

2. It is much simpler to manufacture a pin and bushing than a ball or spherical joint.

3. A ball joint would cost more to manufacture than the pin and bushing.

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Doug

1987 300TD x 3
2005 E320CDI

While pistons move up and down in a vertical plane the connecting rod travels sideways in line with the crank and there is no opposite deviation from this,the piston must travel in a straight line,both for uniform oiling and optimum ring sealing.

Besides there are triple stresses on the rod 1 compressive,2 explosive,3 inertia.
This calls for really strong wrist pins to withstand these stresses and at the same time transfer the enormous power to the crank journals.

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The crown of many pistons are asymetrical so the piston can not rotate in the cylinder and maintain it's relationship with the combustion chamber. Look at the typical hemi head like on an Alfa or Porsche.

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Doug

1987 300TD x 3
2005 E320CDI

Technically if you had a perfect sphere ball joint, the wear area would be a line running around the middle of the sphere. It'll wear out too soon.

Also, you couldn't do it with shaped pistons.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman

Oh man, there's no end to what you guys know. I have wondered about that since I was a little kid.

Thanks!

Riethoven has it right. Modern engines are almost all "interference" engines and many have recesses in the piston for the valves. If a piston could rotate it would be disasterous. Additionally, unless they were held in place, the rings could rotate and there would be the chance that the gaps could align, affecting compression.

I did have a model airplane engine when I was a kid that had the piston on a ball joint. Those two-strokes used sliding ports so piston rotation wasn't a problem.

Good question, Bot.

Wes

If it makes you feel any better they do use a ball joint connecting rod on this little guy.

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320

At 3000rpm, each piston changes it's direction of movement 100 times per second. At those kind of speeds, you don't want any uneeded side or twisting movements. Also, the pistons have cast-in reliefs for the valves and sparkplug/injector/prechamber. It can't be allowed to spin or they would touch them and damage the piston top.

but all those technical reasons aside....

a 350 sd motor could really use a ball joint on it so that when it bends the rod it wouldnt gall the cylinders!

heh heh

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Yes, that is galling.