Hi Ejrego, Any pictures?
"IC" pistons, where you have 3 distinct features - are designed for Single-acting use where there is no Cross-head. Some steam engines are single acting and have no "aligned" con-rod, so use the same style of piston. But many steam engines are double acting, and therefore have an aligned con-rod (it remains in-line with the axis of the bore) and a cross-head, that is doing the job of the piston skirt on a single acting piston with "rocking" con-rod.
You mention that you are new to (I read that as just "learning about") steam engines, so perhaps you'll understand a bit more when I explain some fundamentals of engine design:
A piston has 3 functions, but not always in one piece.
For the "IC" piston that you are modelling:
- The piston is a "pressure barrier" between a high pressure zone and a low pressure zone, and many see well in excess of 20 bar pressure on "top" and atmospheric - or below - beneath. Then on another stroke they will see a high to low pressure above and possibly a near vacuum below, or high pressure below. The piston often (but NOT always) carries piston rings to effect the circumferential seal to the bore of the cylinder in order to maximise pressure differences and prevent blow-by.
- The piston connects to a con-rod in order to transmit the pressure pulses towards the crankshaft - or whatever takes the power from the piston. This can be a single boss centrally in the piston, with high stresses where the boss meets the "piston-disc" that sees the pressure. But it also can be a bearing to permit a con-rod to "rock back and fro" when single acting and directly coupled to a crank. In this case, usually there are 2 bosses, towards the periphery of the piston, and stresses often are bending stresses from the high gas pressure bending the piston surface across the gudgeon pin /boss axis that is perpendicular to the axis of the bore.
- If transmitting the forces on the piston to a crankshaft there will be some sideways reaction thrusts upon a piston, and due to the very small friction at the gudgeon pin, these sideways forces from the crank become rotational forces as well, trying to rotate the piston about the gudgeon pin. (N.B. reciprocating oil or other fluid pumps are simply a linear motion from steam piston to oil pump piston so do not see any side thrusts). On a double acting steam engine,
or IC engines like the Doxford double acting diesel engines,
or large Broome-wade compressors,
the piston rod travels through a shaft-seal to a cross-head that manages these side-trusts and rotational effect.
3. On a single acting piston, - as in a Stuart Sun, Star, or Sirius designs, or Westinghouse example:- they have a piston skirt like a modern IC engine. The skirt can be a relatively long cylinder, as in 1920s cast-iron pistons on many cars, or very short and simply a pad, either side, not a full cylinder, as with "slipper" pistons used on most modern production e.g. cars.https://sbdmotorsport.co.uk/product/pist-om-vx87-50sls-2-0l-xe-sbdomega-slipper-piston-set-87-50mm/
- However, Single action oscillating steam engines (and compressors, pumps, etc,)
- often use the end-cap with a bearing to effect the purpose of a cross-head, as the oscillating cylinder bearing effects the rotational motion otherwise seen at the piston. In such cases, the linear bearing at the end of the cylinder must be large enough to resist wear through high stress from side thrusts from the dynamics of oscillating the engine and long-life at the end-cap, without affecting the gas-seal that would be fitted should the oscillating engine be double acting.
Therefore it is necessary to understand the whole engine configuration before deciding the type of piston required for your model. So:
What sort of engine are you planning to make? - Do you have a general arrangement/schematic? - Then we can advise more specifically about piston design.
I hope this may be of some help?
Regards,
K2
