Question regarding eccentric throw

[seal killer]

All--

I want to build something other than oscillators. I am stumped on the relationship of eccentric throw to crankshaft throw. Is there a simple explanation, or at least one I may be able to understand?

Thanks.

--Bill

[Curtis_F]

Bill,

I have the feeling I'm misinterpreting the question, but ya' gotta start somewhere:

Crank Shaft Throw is 1/2 the stroke of the engine. If the throw of the crank is 1" then the piston will travel 2" for ever rotation of the Crank Shaft.

Eccentric Throw is directly related to travel of the Valve in the steam chest. (Not accounting for angularity offsets, Rocker Shafts, etc.) If the Eccentric Throw is 1" then the Valve travels 1" for every rotation of the Crank Shaft.

Generally Eccentric Throw is a small portion of the Crank Shaft Throw as it only has to move to cover and un-cover steam ports. An engine with 20" Stroke (2 X Crankshaft Throw) may only have 3.5" of Valve Travel/Eccentric Throw.


Cheers,

Curtis F.

[edit]Thank you joneg for pointing out my error in the description of the Crank Throw.[/edit]

[seal killer]

Curtis--

Generally Eccentric Throw is a small portion of the Crank Shaft Throw as it only has to move to cover and un-cover steam ports. An engine with 20" Stroke/Crankshaft Throw may only have 3.5" of Valve Travel/Eccentric Throw.

That answers my question PERFECTLY! I suspected that eccentric throw only needed to be enough to operate the valve, but I had never read anything that actually said that.

Thanks!

--Bill

[joneg]

the stroke will be twice the throw

[Harold_V]

the stroke will be twice the throw

Correct. A 1" throw crank yields 2" of movement, as the offset of the crank is determined between the centerlines of the mains and throw.

Harold

[Harold_V]

Curtis--

Generally Eccentric Throw is a small portion of the Crank Shaft Throw as it only has to move to cover and un-cover steam ports. An engine with 20" Stroke/Crankshaft Throw may only have 3.5" of Valve Travel/Eccentric Throw.

That answers my question PERFECTLY! I suspected that eccentric throw only needed to be enough to operate the valve, but I had never read anything that actually said that.

Thanks!

--Bill

True, and the same principle applies with an eccentric as it does with a crank. If the offset of an eccentric is ¼", it will yield ½" of movement.

Harold

[mklotz]

Animations make the operation of various engines easier to understand. There are some good ones here...

http://www.animatedengines.com/

The "Steam Locomotive" animation is particularly good since it shows the interplay between the classic, eccentric-driven slide valve and the double-acting piston driving the flywheel through a connecting rod.

Also, the "Low Differential Stirling" animation helps one to understand what's going on in a heat engine, a form not familiar to most people outside of the engineering community.

[Curtis_F]

the stroke will be twice the throw

Thank you for pointing out my error about the Crank Throw. I've gone back and fixed my description.

However, contrary to that logic, "Eccentric Throw" is the Eccentric Offset x 2. An eccentric with 1/4" offset gives 1/2" of Throw. If there are no mitigating factors such as angular offset, use of a Link or Expansion Link, or unequal length arms on a Rocker Shaft, the Eccentric Throw will equal the Valve Travel.

So a Crank, or Wheels with Crank Pins, with 1" throw is actually 2" of travel of the piston, but an Eccentric with 1" throw is actually 1" of travel of the valve. The same is said for Eccentric Cranks on Walschaerts valve gear where the diameter, not the radius, of the circle the Eccentric Pin is described as the "Eccentric Throw"


Cheers,

Curtis F.

[seal killer]

Marv--

Thank you for the link!

--Bill