Stroke and Bore

I'm doing a research project for my school but I have no idea how I'm meant to calculate stroke and bore. Could someone enlighten me?

 

For virtually all engines the dimensions for stroke and bore are available in reference books/on-line. The resultant of Stroke x bore x No of cylinders is also the cubic capacity of the engine. Usually used for ic engines though.

 

Surely that only applies to i.c. locos and the few steam locos where the centre line of the cylinder is in line with the centre of the driven wheel?
Most steam locos are not in-line and the initial horizontal motion is transferred through the crosshead, then rotary motion via the con rod. The cranked axle length is usually longer than half the piston stroke and the quartering of the wheel ensures that the other cylinder(s) complete the cycle (I think!). If it had one cylinder it would just oscillate, but that is the basic principle of a shaping machine (I am showing my age!).
I think single cylinder locos like Rocket were in-line, but I don't know how they avoided getting stuck at T.D.C or B.D.C!
I am open to other people's opinion, who will have greater knowledge than myself.

 

'Rocket', like all the engines in the Rainhill Trials had two cylinders, one each side. Some very early engines, such as Trevithick's Pen-y-Daren and, I think, Catch-me-who-Can, had a single cylinder but a large wheel attached to the crank which acted both as a flywheel to get the piston over centre but also a means of moving the piston if it were to stop at the end of its stroke.

 

Sorry, silly me, I should have known about Rocket!

 

I suppose if we are being pedantic the stroke is the distance forward and back the piston moves over in the course of a wheel revolution, but that is defined by the crank. Most steam engines did have the cylinder centreline aligned to the axle centre, although there were exceptions, notably some of Churchward's standard classes for the GWR. The other point to mention is that the two cranks of a 2 cylinder locomotive are conventionally at 90 degrees to each other, so that when one is dead centre the other is delivering maximum power. An important thing to note is that steam engines are normally double acting, with steam acting on each side of the cylinder alternately, so quite unlike a conventional internal combustion engine.

 

Steam locomotives, IC engines and shaping machines all use what is called a "Four bar linkage" in their mechanisms. The form of the four bar linkage varies but these are examples of the "Crank-slider" type of the linkage, where one link is infinitely long. For diagrams and simple explanations see:-

https://en.wikipedia.org/wiki/Four-bar_linkage

With regards to cylinders that are not on the same centre line as the crank axle, as in selected GWR locomotives, the stroke is greater than twice the crank throw, and one stroke is more than 180 degrees of crank revolution and the other less.

Four bar linkages can be very complicated in operation and the associated velocity and acceleration diagrams even more so. Shaping machines use the principles to their advantage to have a quick return stroke, so do mechanical hack saws.

All linked valve gears use multiple four bar linkages.

Andy.

 

Which GWR locos didn't have cylinders aligned with driving axle?

 

All the original 18x30 cylinder types, but the not the later 4-6-0s and 4-4-0s/4-4-2ts.

 

Every day’s a learning day. Not come across this before. No wonder those GW thingies set up a fore and aft motion so easily.

 

I'm sure we have all learnt a bit more ..... but it would be nice if the OP (SomeWeeb) could show some appreciation....

 

How much out of alignment were they, and was it noticeable in operation?

 

I think it was four inches?

Stanier brought it to the LMS and it was supposed to be applied to the 2-6-0s. In the end, the cylinders were lowered to line up with the coupled wheel centres and this left a big gap between the running plate and top of the valve chests.

 

Two and a half inches.

 

According to Holcroft, the 2½-in off-set was devised to stop the cylinders fouling the loading gauge on the small-wheeled 28XX engines. A glance at the GWR loading gauge shows the likely source of the difficulty - the step at 1ft 10in above rail level where the width limit reduces from 9ft 0in to 8ft 8in.

Other British companies more typically addressed similar problems by inclining the cylinders to get them to a higher level and by bringing the cylinders closer together than in GW practice. The extreme example was of course the LMS Hughes "Crab" 2-6-0, an ungainly but very clever design.

 

The 9F 2-10-0 another example.