MCNAUGHTING THE STEAM ENGINE

[Stanley]

I’ve been buying books again…… This post was prompted by a comment in ‘Stationary Steam Engine Makers.’ Vol. 2. (Landmark. 2006) £27! This comment made it clear that the editor was unsure as to whether John McNaught of Rochdale was the originator of the practice of compounding or not. This is a very common misapprehension and I thought it might be a good thing to make the truth more widely available. John McNaught applied the McNaught principle patented by William McNaught of Glasgow to many engines and I think this is what has led to the error. I have, somewhere in my archives, a very good lecture on the subject but can’t for the life of me find it so I have gone back to Richard Hill’s magisterial book. I hope he won’t be offended.

MCNAUGHTING THE STEAM ENGINE
As the advantages of using higher-pressure steam became recognised, so single-cylinder engines were adapted to work with shorter cut-offs and greater expansion. At the boiler pressures then in general use, this could have given them a greater advantage over the Woolf compound with its design fault and extra complexity of two cylinders. Certainly, the Woolf compound was abandoned in Cornwall after 1826 and also Fairbairn preferred single-cylinder engines at Saltaire Mill in 1853 because they had been improved to equal the compounds at the steam pressures then being used.

The double cylinder or compound engine, in which high pressure steam was employed, expanded through three-fourths of the stroke, appeared to effect a considerable saving of fuel; but taking both engines worked alike, with steam of the same pressure similarly expanded, as is now the case in the best single cylinder engines, there appears to be no advantage in the compound over the simple single engine. On the contrary, there is a loss in the original cost of the engine, and the complexity of the one as compared with the other ... I have therefore no hesitation in recommending the single engine worked expansively, as an efficient competitor of the compound engine.
[W Fairbairn. ‘Treatise on Mills and Millwork’. (London, 1861). P.69.]

But Fairbairn was dealing with the Woolf engine and this problem was avoided in the next important type of compound engine, that patented by
William McNaught of Glasgow in 1845. On a beam engine of the ordinary Boulton & Watt design, he placed a high-pressure cylinder between the main column supporting the beam and the crank, where the cold water pump was normally fitted. This was a simple alteration which could be added to existing engines and became exceedingly popular to increase their power and efficiency. In his patent, McNaught stated that there would be:

“the effect of increasing the power of the engine, of lessening the consumption of fuel in proportion to the power produced, and by working the steam expansively in the low-pressure cylinder a further saving of steam may be effected, and consequently a proportional saving of fuel”.
[Patent 11,001, 1845.]

The situation of this extra cylinder, well away from the original one, meant that there had to be a long pipe connecting the two which could act as a receiver. Also there had to be two sets of valve gear so that the low-pressure cylinder could have its inlet valves adjusted to give an appropriate cut-off. The advantages of this were recognised by McNaught stating that the steam would be worked expansively in that cylinder. In this way, the McNaught design was an advance on Woolf’s as is stated in the patent. Savings of fuel of up to 40 per cent were being claimed by 1854 .

There was still one disadvantage because, with both pistons still acting in unison, there were periods when the engine developed no power at the top and bottom of the stroke. Yet, in terms of smoothness, these engines gave a much more even power output than their single-cylinder predecessors and this point must be examined because this seems to have been the main reason for the popularity of McNaughting beam engines. McNaught claimed that one of his objects was to equalise the stress on the main working beam and all the parts connecting it with the framing and engine house. To give an example of one engine:

Before Compounding, The pressure on the beam was 85,408 lbs.
The pressure on the crank pin was 42,704 Lbs.
After Compounding, Pressure on the beam's centre reached only
656 Lbs. Pressure on the crank pin reduced to 40,893 Lbs.

Thus, although the engine was actually doing more work, the pressures in both cases were reduced. In a patent issued five years later, McNaught proposed an arrangement for compensating the difference in the pressure of the steam on the piston at the beginning and end of the stroke when working expansively so that the stress on the parts of the engine would become more nearly a mean. [Patent 12,988. 1850] Again, in 1868, he patented mechanical devices with the momentum of weights on arms to equalise the power being developed during the stroke of an engine where steam was being used expansively. [Patent 862. March 13th 1868] Obviously he was still looking for a smooth running engine and he was not alone for these were ideas which others were promulgating at about the same time.

Yet the nature of the compound engine did give an inherently smoother running engine and this was another point which helped the McNaught engine gain acceptance particularly in textile mills. The point has been made already with Hornblower's engine that the power developed in the two cylinders was smoother than in one, even when the cylinders were arranged to work in unison as in a beam engine or in the tandem compound layout. Just as in the Cornish pumping engines when high-pressure steam was admitted into a single-cylinder engine, there was a great shock to the pumping rods, so in a rotative engine there was a blow on the crankpin even though this was lessened through the rotation of the crank. When a high expansion took place in a single cylinder, there was a great difference between the initial and final stresses on the working parts and so in the turning force.

In a compound engine, the difference between the initial and final stresses in each cylinder was much reduced, and the two added together in the types of engines just mentioned were also much reduced when compared with a single cylinder. The ratio might be from 40 to 70 per cent of what it would be were the same total ratio of expansion employed in a single cylinder. So the twisting moment of the crankshaft was more nearly uniform in the case of the compound engine and its parts could be designed lighter. Another small but important point was that the pressure drop across the pistons in each cylinder was less and so leakage of steam was reduced. Therefore mill owners had good practical and mechanical reasons for introducing compound engines. This became more true when the cross compound engine was introduced. In this type, there was one cylinder on either side of the flywheel with cranks set at right angles, thus combining the advantages of the smoothness of this layout with the greater smoothness of the compound. Clark commented on one running at the 1862 Exhibition that it was “the nearest approach that it is possible to obtain to perfectly uniform rotative power." In both the cross compound and the tandem arrangement of the cylinders, each cylinder was fitted with separate valve gears and the transfer pipe connecting them could be made sufficiently large to act as a receiver.

The thermodynamic advantages of the compound began only with steam over 60 psi. and really closer towards 100 psi. In the later compound designs, the temperature difference between the fresh high-pressure steam and that passing to the condenser was divided between the two cylinders. The steam entering each cylinder did not have to raise the temperature of the walls and the piston to such a great extent and therefore there was less condensation. The compound had the further advantage because the hottest steam first entered the smaller cylinder which was reduced in size compared with a single cylinder on an ordinary engine. Therefore the condensation in this cylinder was considerably less than was the case with a simple engine. But in 1850, the thermodynamic advantages of using high-pressure steam were still much in question and many people preferred to play for safety by keeping pressures low. The mechanical benefits of compounding were recognised sooner than the thermodynamic advantages and indeed did much to bring it into favour before the pressures of steam had risen high enough to make compounding advantageous from the thermodynamic point of view. Higher pressures without adequate methods of boiler construction, use and maintenance led to numerous boiler explosions so that by the 1850s, such occurrences were reaching the dimensions of a national scandal. The Manchester Steam Users' Association did not undertake the insurance of boilers working at over 60 psi. The introduction of stronger boilers and improved safety features in them such as fusible plugs caused the position to change quickly. Another reason must be ascribed to a correct understanding of thermodynamics which emerged at this time.
[from ‘Power of Steam’ by Richard L Hills, CUP 1989. pp.157-161 ]

I first met Richard when he was curator of the Manchester Museum of Science and Industry. Though never a practical steam engineer he researched his subject in such depth that he grasped many of the nuances of steam engines which other academic authors had missed. There is a good example of this in his explanation of the McNaught principle above.

Richard cites evening out the forces applied to the flywheel, and hence to the driving shaft, as being one of the key factors which led to the popularity of applying the McNaught principle to old single cylinder beam engines. He is quite right and this is as good a place as any to reinforce this principle from my own experience.

In the first half of the 19th century the textile machinery driven by hand, horse power or water power was crude and inconsistent in operation. The quality and quantity of production depended more on the skill of the operator than regularity in the speed of the drive.

As more sophisticated machinery was introduced, the owners of water-powered mills realised that the more constant to turning force applied to the shafting and the nearer the shafting ran to design speed, the better their machinery performed. The first attempt at improvement was to control the water wheels with Watt type governors operating massive screens which regulated the water flow. This was never a success because the governors, whilst doing their job, couldn’t control the turning motion quickly enough to avoid ‘hunting’ which is a condition where a governor is constantly fluctuating up and down because of defects in the system, in this case the weight of the control mechanisms.

At this point I suspect there was a happy accident. Some water mill owner decided that he needed more power than his water resource could produce and so he introduced a small steam engine to supplement the power. It would soon be realised that the governor on the steam engine was far quicker and therefore more accurate in its action. If the water wheel was run at full power and the engine used to top up the effort, in effect, the steam engine became the governor of the water wheel. This was a great improvement and the popularity of this dual power source can be seen in the later water mills such as Quarry Bank at Styal and Glasshouses at Pateley Bridge. The former using a beam engine and the latter a horizontal oil engine.

So, by about 1830/1840 regularity of turning motion in shafting was being actively pursued through improvements in the prime mover and William McNaught’s improvement of 1845 came at exactly the right time and this was, to my mind, definitely the major factor which led to modifying the big single cylinder beam engines.

The one thing that is missing in this is hard evidence that regularity of motion actually did improve production. Common sense tells us that it must be true but researchers always like evidence. Luckily, I can provide some. In 1972 when I took over the running of the Bancroft engine it was running very unevenly. I used several strategies, greasing the ropes, adjusting the valves and making sure that all linkages and drives on the engine were running free and accurately and after about six weeks the weaving manager, Jim Pollard came to me and told me I was the most popular bloke in the mill because I had raised the average weaver’s weekly wage by about 30/-. This was equivalent to about 4.5% increase in quantity of production and more importantly Jim said that the improvement was due to easier weaving conditions, less end breakages and more even cloth. There was a further bonus in that because there were less faults the cloth inspection in the warehouse speeded up and there were less complaints from customers. All this stemmed from improving the evenness and regularity of the drive into the shafting.

One final point that has always puzzled me; as late as 1960 Henry Brown Sons and Pickles were ‘quartering’ the engine at Broughton Road Shed in Skipton by moving the low pressure crank 90 degrees forward on the flyshaft from the 180 degree set up that had been used since the engine was installed. This was done because the irregular motion was actually throwing ropes off the flywheel when heavily loaded. Newton Pickles told me that the mill owner thanked them afterwards for improving their production. How had they managed to get into the 20th century without realising the benefits of quartering? I leave you to ponder on that question……

SCG/01 June 2008