Crank pins

On a two cylinder engine how does the outside coupling rod crank pine relate to the inside con rod crank pin? Should they line up as in an outside cylinder design?

I am sure there is a simple answer, but it's late at night!

Kind regards

John B

If you are talking steam locomotives, surely the crankpins need to be at 90 degrees, so that the engine is self starting?

If they were in line, with a double acting engine, with the crankpins at inner or outer dead centre, there would be no torque to rotate the shaft, just linear forces acting on the bearings.

It would be a different matter with a four stroke I C engine, once it was running. The motor cycle enthusiasts will quote examples of parallel twins, no doubt.

Howard

In his description for Mabel LBSC said the crankpin needed to be on the opposite centre to the crank axle, ie when the crank axle is on front dead centre the wheel crankpin should be on the back dead centre - but he added set it by eye as a few degrees out either way doesn't matter on a small locomotive.

I can't comment on whether it does in fact matter, but that is what he wrote.

Surely they must be 90 degrees apart

Roy

The cylinders would be 90 degrees apart, not the wheels.

Thinking logically, it doesn't really matter what position they are as long as the external cranks on one side of the engine are at 90 degrees to those on the other side. The inside cranks turn the axle which in turn (pardon the pun) turns the wheels. The outside cranks are simply transferring that rotating motion from the driving wheels to the driven wheels on the other axles. Having those at a different orientation to the inside cranks should not make any difference.

Nick I didn't mention wheels I was referring to the crankpins as per OP

So inside cylinders, outside coupling rods. This gives the designer flexibility to have the coupling rods the same as the crank or opposed. It's all going to be about balancing the motions is it not? I would say crank balance up connecting rod down on the same side of the engine, but thats just off the top of my head. It must depend a little on where the extra mass is, in the ecoupling rods or the balance weights on the crankshaft. Of course I may be talking to total rot.

regards Martin

Most full size inside cylinder locos had coupling rods (outside) 180 degrees from the con rod (inside) on the same side. This undoubtedly made balancing easier, but gave the axleboxes a hard time. I penned an article in ME a bit back, if you send me your email address by pm I'll send you a copy.

It could be argued that having the outside crankpins at 90 degrees to the inside crankpins there would be a measure of balancing that would reduce the secondary out of balance forces, and hence hammer blow.

Howard

Hammer blow is a result of trying to balance primary reciprocating imbalance with rotating balance weights, not secondary imbalance