In a steam egine, how does the up-and-down motion of a piston turn the wheels of a train?

Answer 1

They use what is called a slider crank mechanism.

Rather than trying to describe it here, I will send you to the following link, which has a description and an animation.


http://www.technologystudent.com/cams/crkslid1.htm

Answer 2

The linear motion of the piston in the cylinder is converted into a rotary motion by the use of a crank shaft which is eccentric and is connected to the piston with a pin in a sleeve, so that the power is transmitted even at an angle. Without the crank being displaced from the centreline of the shaft, the mechanism will not work. It sort of mimics the human hand and legs etc.

Answer 3

The piston slides vertically in the cylinder.

The bottom of the piston is connected to a drive arm through a pivot (kind of like a hinge). This allows the non-piston end of the arm to move eccentrically compared to the piston motion, but only in one plane (effectively, a circular motion).

The non-piston end of the drive arm connects to a camshaft (a squiggly-looking shaft with several 'lands' on it where the drive arms connect) which runs at right angles to the piston motion. The eccentric path of the drive arm rotates the camshaft by alternately pushing and pulling on the appropriate land.

The camshaft connects through either planetary or right-angle gears to the axles. Rotation of the camshaft moves the gear teeth in relation to each other, thereby forcing the axle to rotate as well.

The wheels are mounted on the axle - so as the axle turns, so do the wheels.

Answer 4

Here is a great animation on Wikipedia - the locomotive entry. It is about a third of the way down the page. This type is called the Stephenson type of locomotive.

http://en.wikipedia.org/wiki/Locomotive

Also see "Shay" locomotive where the steam engine works a bit more like a car engine.

http://en.wikipedia.org/wiki/Shay_locomotive