cranks and eccentrics

[Anatol]

I'm not talking about members of this forum

On a conventional steam engine, pistons drive cranks and eccentrics drive valve gear.
Are a pair of webs and their crankpin journal m"echanically equivalent" to an eccentric? Could you drive a crankshaft via eccentrics? Can you drive a "crankshaft" via an eccentric? I guess you could drive a valve connecting rod with a crank. So why are eccentrics used for driving valves and crankshaft used for being driven by pistons? I'm wondering if you could make a "crankshaft" on a straight shaft with eccentrics? I guess you'd radically limit the throw/travel of the piston?and the angle the piston rod paste through would be huge? Did I answer my own question?

 

[gbritnell]

You can't drive a crankshaft with an eccentric joint but you can drive a valve mechanism with one.
You could drive a valve mechanism with a crank setup i.e. Locomotives.
I don't know the exact answer why most steam engines use eccentric drives but surmise it would make manufacturing the crankshaft very costly.
A lot of engines just use a plain shaft with a crank affixed to one end and the eccentric just slides onto that shaft. In large engines the eccentrics are keyed to the shaft.
gbritnell

 

[TonyM]

Didnt read OP correctly.

 

[vederstein]

Both a slider crank and an eccentric strap will provide purely sinusoidal motion provided that the line of linear action (the cylinder bore) is in line with the center of rotation (crankshaft). Any offset will result in a "quick return mechanism".

Anyways, referencing the sketch I made, I see no kinematic reason why an eccentric cannot be used as crank. The output motion is identical. What you lose in this arrangement is space. An eccentric has a large mass and takes up a decent amount of space for relatively small linear motion because the crank throw is essentially halved from the width of the eccentric.

I think a more pertinent question would be why do most steam engines use an eccentric for the valve when a crank would do the same thing with less mass?

Also, considering that an offset piston would result in a quick return mechanism, why don't single acting engines use this to get the piston back to top dead center quick than it would otherwise? (Vibration issues?)

...Ved.

 

[Anatol]

Vederstein
thanks - wonderfully clear answer that tallies with my intuition.

Both a slider crank and an eccentric strap will provide purely sinusoidal motion ...I see no kinematic reason why an eccentric cannot be used as crank. The output motion is identical.

What you lose in this arrangement is space. An eccentric has a large mass and takes up a decent amount of space for relatively small linear motion because the crank throw is essentially halved from the width of the eccentric.

I think a more pertinent question would be why do most steam engines use an eccentric for the valve when a crank would do the same thing with less mass?

...Ved.

"crank throw is essentially halved" right. Unavoidable if you don't cut the crankshaft.

The space issue with eccentrics is clear. A big throw requires a bigger diameter eccentric. This also implies a greater angle through which the piston arm passes, which would reduce efficiency and increase wear - actual force being more tangental to required drive direction. (Sorry if I mangle terminology, I'm not a trained engineer - I hope the meaning is clear).

I think the answer to your last question is that crankshafts are inherently difficult to make. I'm trying to avoid it. This is the root of my question
Eccentrics can be lined up on a straight shaft. Because they are not handling much force, (just pushing valves), you can stack relatively thin ones in a narrow space. (And thin reduces mass.)

 

[bouch]

I think a more pertinent question would be why do most steam engines use an eccentric for the valve when a crank would do the same thing with less mass?

...Ved.

My guess is that the angle of the eccentric can be easily adjusted. Much harder to do if the valve motion is driven via a crank...

 

[Anatol]

My guess is that the angle of the eccentric can be easily adjusted. Much harder to do if the valve motion is driven via a crank...

do you mean for timing? ie where the driven shaft is in relation to high point of the eccentric? If so, that makes sense. Highly adjustable, at least in terms of valve position wrt crank position.

 

[Charles Lamont]

Friction. There is normally too much friction in an eccentric to be able to drive the crankshaft with one. With a needle roller, an educated guess tells me it would work OK. It could be calculated quite easily, but I can't be bothered right now.

 

[Anatol]

Friction. There is normally too much friction in an eccentric to be able to drive the crankshaft with one. With a needle roller, an educated guess tells me it would work OK. It could be calculated quite easily, but I can't be bothered right now.

right - a lot of surface.
"can't be bothered right now"
no prob, not necessary.
thanks!

 

[tornitore45]

One property of eccentric is that there is no "cut" in the shaft, the shaft can continue un-weakened to another part of the machine where full power is taken out.

That may apply to marine installation but does not apply to locomotives and is not used there where a simple crank is at the end of the shaft. I suppose a locomotive with the valve control inborad would use eccentrics to avoid "cranking" the axle.

Eccentrics are hardly reversible though, too much friction to convert alternative motion to rotary motion.

 

[Anatol]

One property of eccentric is that there is no "cut" in the shaft, the shaft can continue un-weakened to another part of the machine where full power is taken out.

Right, I'm in principle suspicion of crankshafts. So many ways to break, so many ways to be out of line

in principle, the locomotive wheel axle is the crankshaft.

 

[tornitore45]

A locomotive design is particularly fortunate since two bearings support the shaft and the cranks (formed by tapping the wheels) are outboard of the bearing. A sturdy design.
Multi cylinder engines with a need for a power takeout, like a propeller shaft must, necessarily, have an interrupted crankshaft with all the potential weakening and misalignment of such design.

 

[Anatol]

Multi cylinder engines with a need for a power takeout, like a propeller shaft must, necessarily, have an interrupted crankshaft with all the potential weakening and misalignment of such design.

not necessarily. A bicycle doesn't. Replace legs with two double acting cylinders. Chain or belt off central flywheel.
(Could do 4 cyl (boxer), with 'knife and fork' arrangement, but, supported one side would a bit dodgy.

 

[tornitore45]

A bicycle doesn't.

A bicycle is a unique design, it has no need of an extended power take off shaft. From the kinematic point of view it is functionally identical to a locomotive. Inbound bearing, power take off (either the wheels or the sprocket), cranks followed by nothing else.