what does double acting mean?

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werowance

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so when someone says a steam engine is "double acting" does that mean something like this:

steam preasure hits the top of the piston and pushes it down, then steam preasure hits the bottom of the piston and pushes it back up? which would require a sealed piston rod on the bottom I assume.

or does it mean something else?
 

Asm109

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And that is why a steam engine has a cross head bearing and an automobile engine does not.
 

werowance

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so I thought the cross head was just a sliding valve to open and close steam preasure, sort of like gears cam do for the valves on a 4 stroke. one hole for exhaust and one hole for intake. does double acting require 2 cross heads?
 

larryg

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https://myweb.rollins.edu/jsiry/SteamEngineParts.html

About 2/3 the way down the page at the above link is a animation of a double acting steam engine. The crosshead has the piston rod connected to one end and the crank connecting rod to the other end. The purpose of the crosshead is to guide the piston rod in a straight line through the head packing. It prevents the side loading of the connecting rod from acting on the piston rod and wearing the gland oval.

lg
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bazmak

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The cross head bearing is what produces the double acting piston.The piston is pushe down and pushed back up
for I rev, a single cylinder combustion engine is pushed down and and has a full rev under 1 push before being pushed again
To simplify the double acting is pushed twice per rev and the combustion engine once per rev
 

werowance

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ok, the animation explains it well, its a valve that redirects steam for both sides of the piston. just thinking back at some of the builds I have looked at, the cross head always looked like it was a on off valve for only one side. guess the brass sliding piece was always covering one side or the other of the valve so was only seeing one direction when looking at the pictures. thanks all for the explanation.
 

Jasonb

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And just when you thought you had it worked out you don't need a cross head to have a double acting steam engine, a wobbler can be double acting and has no crosshead., it does have a valve arrangement that allows steam/air into one side of the piston and then the other.

Infact there are some double acting IC engines too. Also 4 stroke IC only have a power stroke on alternate revs, 2-strokes once every rev.
 

Charles Lamont

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But, to confuse the issue, a wobbler is a better engineering job if it does have a crosshead. I takes the side load off the rod and gland.
 

werowance

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so now I have the question what does an "expansion" engine mean for example triple expansion?
 

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Thanks for posting that animated site is very good.....
 

Asm109

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so now I have the question what does an "expansion" engine mean for example triple expansion?
A triple expansion steam engine typically has 3 cylinders. Each with a different bore diameter.
Steam is let into the smallest piston and pushes the piston down. The steam is then let out of that cylinder and into the middle size cylinder. It is allowed to expand again. Then it is let into the largest cylinder and allowed to expand a third time.
Each piston has a lower pressure steam in it. It has a lager diameter so the torque provided by each cylinder is similar to each other.
 

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