Updated: 28 Aug 2004
One of the problems with the conventional type of steam locomotive is the presence of forces caused by the pistons, piston-rods, crossheads etc moving back and forth. As these masses are accelerated and decelerated, the reaction forces cause the locomotive to also accelerate and decelerate by a much smaller amount; it is still enough to make the riding rough and uncomfortable. These forces do not cancel as the two side of the locomotive operate 90 degrees apart to prevent dead-centre problems.
This problem can be minimised by adding weights to the driving wheels. These already have balancing weights which nullify the effect of purely rotating masses such as the crank pins and coupling rods. Adding more mass to these weights can balance the back-and-forth forces, at the expense of upsetting the rotational balance; the latter has bad consequences, generating an up-and-down force on the rail. When acting downwards this is known as "hammer-blow" and puts serious extra stresses on the track and its foundations, the forces increasing with the square of the speed. 180 degrees later in the rotation of the wheel it acts upwards, and in severe cases can almost lift the wheel off the rail, with dire consequences for stability. As a result, it was conventional practice to balance only a third to a half of the reciprocating mass. (The Austerity locomotives designed for military use in WW2 had no reciprocating balance at all, so they could work on hastily-laid track. Comfortable riding was not a priority; however in these conditions speeds were low and it was a perfectly sound design choice)
At least two designers appreciated that a good way to obtain good balance for both the rotating and reciprocating masses was to fit two pistons on each side, driving crank-pins set at 180 degrees so that one piston would move forward as the other moved back.
The only real difficulty was fitting two cylinders where one had grown before. John Haswell solved the problem by fitting one cylinder above the other, set at a slight angle.
Left: The Haswell Duplex Locomotive: 1861.
This remarkable locomotive had two cylinders on each side, angled slightly so that both piston rods aligned with the centre of the driving wheel. It was designed by John Haswell of the Austrian State Railway Works at Vienna and was exhibited in the International Exhibition of 1862. John Haswell had previously built the "Vindobona", one of the competitors in the famous Semmering locomotive trials in Austria. The cylinders were 10.8 inches diameter by 24.8 stroke, and the driving wheels were 7' 9" in diameter.
Left: The HF Shaw Locomotive: 1881.
This locomotive also had two cylinders on each side, but mounted beside each other, a solution that was probably made possible by the larger American loading gauge. These drove crank pins diametrically opposite each other on the driving wheel.
Shaw's four-cylinder locomotive was called the H. F. Shaw ; it was built by the Hinkley Locomotive Works in 1881. It was advertised which as being completely free from the pounding and oscillating action of conventional two-cylindered engines.
One of the crank pins connected outside the driving wheel at the same position an ordinary crank pin would be located, and carried a double crank, the middle of which was supported in a bearing held in an outside frame. The connecting rods worked either side of this bearing.
The engine was equivalent to one with two cylinders 16" by 24", and driving wheels 63" in diameter. The weight in working order was given as 74,000 pounds, of which 25,600 pounds was on the front bogie. The engine was said to have been well-designed and well-built, and it saw considerable trial use in revenue-earning service, where it was said to have worked "quite satisfactorily", but no more of the type were built.
These are the only two balanced locomotives I have found so far. If there are more they are well-hidden.
The only conclusion seems to be that in both cases the added complexity outweighed the smoother riding and reduced track impact. Since the words "hammer-blow" recur like a dismal refrain in the history of locomotive design, this is rather hard to understand.