Parts of a Steam Engine – Labeled Diagram, Cut Open

All 20 parts on the diagram

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Moving parts and frame 11

Flywheel Turns with the crankshaft and evens out the engine's speed. It carries the engine past the points where the piston changes direction.
Crankshaft The crank, the eccentric and the flywheel are all fixed on it. It carries the engine's power to the machines it drives.
Crank An arm on the crankshaft. The connecting rod pushes and pulls on the crank pin at its end, and so turns the shaft.
Connecting rod One end swings on the crosshead's pin. The other end goes round with the crank pin.
Crosshead Joins the piston rod to the connecting rod. It slides between the guides, so the piston rod only moves in a straight line.
Crosshead guides They keep the crosshead moving in line with the piston. They are bolted to the bedplate.
Eccentric A round disc fixed on the shaft. Its center is off the shaft's center. As the shaft turns, it moves the eccentric rod to and fro to work the valve.
Eccentric rod It carries the eccentric's to-and-fro movement to the valve rod.
Bedplate The frame that carries the cylinder, the guides and the crankshaft's bearing. It is bolted down to a foundation.
Piston Fits the cylinder and is pushed to and fro by the steam.
Piston rod Joins the piston to the crosshead. It passes out through the cylinder cover.

Steam path 9

Governor The engine spins it. If the engine speeds up, the balls swing outward and the linkage closes the throttle valve a little.
Throttle valve It controls how much steam reaches the engine. The governor works it.
Steam chest Steam from the boiler fills it. Also called the valve chest.
Slide valve Lets steam into one end of the cylinder. At the same time, it lets the used steam out of the other end through the hollow under it.
Valve rod Comes out through the wall of the steam chest. It is worked by the eccentric rod.
Steam ports One leads to each end of the cylinder. The valve opens each in turn to steam, then to the exhaust.
Exhaust port The opening between the two steam ports. The hollow of the slide valve joins it to whichever steam port is exhausting.
Cylinder It is closed by a cover at each end. Steam is let in at one end, then the other, so the piston is pushed both ways.
Stuffing box It is packed round the piston rod. That lets the rod slide through the cylinder cover without steam leaking out.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two of these use it. (1) H. Haeder and H. H. P. Powles, A Handbook on the Steam Engine (1902, public domain), full text at https://archive.org/details/handbookonsteame00haeduoft : cylinder, piston, piston rod, stuffing box and gland, crosshead and guides bolted to the bed-plate, connecting rod, crank and crank pin, crank shaft, fly-wheel as a speed regulator, eccentric and eccentric rod, slide valve, valve rod or spindle, valve chest or steam chest, steam ports, exhaust port, governor and throttle valve. (2) E. P. Watson, Small Engines and Boilers (1899, public domain), https://archive.org/details/smallenginesboil00watsrich : steam chest, slide valve, valve rod, eccentric and eccentric rod, piston rod, stuffing box, crosshead, guides, connecting rod, crank pin, fly wheel, bed plate. (3) Wikipedia, text only: Steam engine, Stationary steam engine, Slide valve, Crosshead, Eccentric (mechanism), Stuffing box, Centrifugal governor, Valve gear. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no one engine, nothing traced. Licence: own work

How it works

This is the kind of steam engine that drove mills and workshops: one horizontal cylinder, a crank and a big flywheel. The first view is the whole engine; the second cuts the cylinder open to show the valve.

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From steam to a turning shaft

Steam comes down the pipe, past the throttle valve, into the steam chest. The slide valve lets it into one end of the cylinder, where it pushes the piston. The piston rod pushes the crosshead, the connecting rod pushes the crank, and the crank turns the crankshaft and flywheel. Then the valve slides over, steam enters the other end, and the piston is pushed back.

What moves the valve

The eccentric on the crankshaft moves the eccentric rod and the valve rod, so the valve always keeps time with the piston. In the cut-open view the steam pushing the piston and the used steam on its way out are tinted two different colors.

The crank and the eccentric both sit on the crankshaft: the crank is turned by the piston, the eccentric works the valve.
The used steam leaves under the slide valve, through the hollow in it.

Questions and answers

What are the main parts of a steam engine?

A cylinder with a piston in it, a valve that lets steam in and out, a piston rod, crosshead and connecting rod, and a crank on a crankshaft with a flywheel. A governor and a throttle valve control the speed.

How does a steam engine work?

Steam under pressure is let into one end of the cylinder and pushes the piston along. The valve then lets that steam out and admits steam to the other end, pushing the piston back. The connecting rod and crank turn this to-and-fro movement into rotation.

What does the flywheel do on a steam engine?

It evens out the speed. The piston's push is uneven and stops at each end of the stroke, and the heavy wheel carries the engine through. It also softens sudden changes in load, which helps the governor.

What does the governor do?

It keeps the engine at a steady speed. Its balls are spun by the engine, and when the speed rises they swing outward and close the throttle valve a little, so less steam gets in.