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Doc
Keeper of the Scrolls
2010 Posts
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 Posted -
28/05/2004
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16:30
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LANCASHIRE TEXTILE PROJECT
TAPE 78/AI/06 (Side one)
THIS TAPE HAS BEEN RECORDED ON APRIL 27TH 1979 AT 13 AVON DRIVE BARNOLDSWICK. THE INFORMANT IS STANLEY GRAHAM WHO WAS THE ENGINEER AT BANCROFT MILL AND WHO HAS BEEN THE INTERVIEWER ON MOST OF THE TAPES..
I am carrying on with the description of the pictures in the Bancroft Folio.
Picture number 7 and 8. Neg numbers 774733 and 774728.
Both these pictures are pictures of the flywheel, and my intention when I took these pictures was to show that a mass of cast iron weighing some 35 tons, 16 ft in diameter can produce interesting patterns, it can look almost light and airy and the pictures also tell you something about the engine itself.
(50)
If you look very carefully at picture number 7 you get the impression of what look to be radial lines coming from the centre of the wheel to the outside edge. These are the joints in the wooden boarding which covers the spokes of the wheel. This flywheel is not a solid wheel, it has spokes. I'm just trying to remember the actual number, There are 6 large cast iron spokes. The wheel is built in two halves, each half will weigh approximately 17 or 18 tons, something like that. Obviously we have never weighed it, this is just an informed guess on my part, I know that if I was loading one of these on to a wagon, that’s what I'd estimate it at. The flywheel revolves at 68 revs a minute. When the engine was originally put in I think this engine was put in to do 70 revs a minute. One thing that should be mentioned here is that the speed at which the looms run is critical with regard to production. Lancashire looms are strange little creatures and require to run at a certain
(100)
speed to be most efficient. You’ll appreciate that as all the looms are driven off common shafting by the same diameter pulleys through belt drives, one would think that in theory all these looms would be running at the same speed. In fact this isn’t the case. One of the beauties of belt drive in that use can be made of the fact that the tension at which a leather belt to a loom is mounted governs the efficiency of the drive. In other words if the belt is mounted slack the loom will drive slightly slower, if it's mounted tight it'll drive the loom slightly faster. This set of circumstances applies equally well to the driving ropes on the flywheel of the engine which transfer the drive from the flywheel to the second motion pulley. The thickness and tightness of these ropes can’t be varied but what you can control is the amount of grip they have on the flywheel. Now I mentioned before when we were looking at the engine from above that the ropes looked to be black because they were coated with a mixture of tallow and graphite. Now, this has two purposes; one of them
(5 min)(150)
of course is as an anti-fraying composition. This was how it was described by the makers. Also, the fact that you cover the outside of the rope with a relatively hard coating of graphite and tallow stops the outside of the rope fraying. That’s what wears a rope out, the little bits continually coming off the rope. If you run cotton ropes dry they'll still grip because the area of contact with the two wheels is enormous when you think about it. Also the grooves are of wedge shape, so that the rope does tend to sink into the grooves and grip. But if you run a rope dry, the constant flexing, even though it's very gentle owing to the size of the flywheel and pulley will eventually cause the rope to start to fray and disintegrate on the outside surface. This is in itself not serious because the rope is very thick and there is a lot of wearing surface to go at. But if you cover the rope with something like tallow and graphite it stops the first little pieces coming off. Now if you stop those coming off they stay in place, locked in covering of tallow and graphite and they act as the wearing surface for the rope. In other words as long as you keep that rope covered with composition it won't wear, or at least it'll wear at a very slow rate. So this means that life
(200)
of a rope running well within its rated load, and over large diameter pulley can be extended enormously. I have reason to believe that some of the ropes that are on the Bancroft engine may be the original ropes that were put on. The thing that causes a rope to be taken off in the end is when it has stretched so much that the sag on the slack side of the drive means that, either it's running too close to people's heads on the walk way under the rope for safety, or that the harmonics of the drive are so affected by the slack that the slack side of the rope can start swinging about and gradually build up until it's flogging.
This brings us into quite a complicated field, and you must excuse me if I seem to digress when I go into some of the reasons why cross compound engines were first used. Now, if you turn back to picture number 2 and look down on that engine you'll see that, in effect it is two engines on one common flyshaft. The question to ask is why did they go to the trouble of building two beds when one would have done? They could have put both cylinders
(250)
on one bed, made it slightly heavier and taken up far less room, made a far more compact unit. This is a very good question and the answer is that it was very often done, but there is one great disadvantage with this type of engine which by the way we call a tandem engine. A tandem engine is an engine in which the two cylinders run on a common piston rod and connecting rod to a crank and drive the flywheel from one side. Now if you think about the way a steam engine works, all engines, either tandem or cross compound are what we call double acting steam engines. By this we mean that unlike an internal combustion engine where the piston is only driven from one side, the piston in a steam engine is driven from both sides alternately. In other words as the piston is moving forward it is being driven forward by steam expanding in the cylinder at the back, and as the piston moves back it is being driven by steam expanding in the front of the cylinder. Now look at the picture on page 2 and forget about the low pressure cylinder, just concentrate on the high pressure cylinder. It's fairly obvious that for each revolution of the flywheel and crank, the piston makes two strokes, one forwards and one backwards. In other words it produces two power impulses Now it's also fairly obvious that the power given
(300)(10 min)
to the piston by the expanding steam is not constant. The torque or shove on the piston, the power driving the piston is highest at the start of its travel down the cylinder because the steam is then at its highest pressure and temperature. As the steam expands and cools the pressure on the piston reduces during the travel down the cylinder. Indeed, before the end of the stroke the exhaust valve has shut and compression is building up on the exhaust side of the piston to cushion the transition to the return stroke under steam. So, if you imagine the side of the flywheel as a clock, you'll get two power impulses going into that flywheel, one at 12 o'clock and one at 6 o’clock. One will be directly opposite the other and neither of these is constant through 180 degrees. This leads to a phenomenon which we call cyclic variation. The speed of the wheel is not constant, it accelerates slightly as the power stroke comes on, and then decelerates quietly until the next power stroke comes on. This happens twice in every revolution. Now, imagine a tandem engine, put both cylinders on a common connecting rod and crank and both power impulses happen in the same place but are stronger.
(350)
Consider a cross compound engine such as Bancroft. Remember what I said about shrinking a crank on, the low pressure crank is fixed so that it is 90 degrees forward of the high pressure crank. Exactly the same cyclic variation exists on the low pressure side but crucially, is 90 degrees in front of the high pressure side. Imagine our clock face again. We now have four power impulses going into the flywheel spaced equally, ninety degrees apart. This is known as ‘quartering’ an engine and ensures a far smoother input of power and less cyclic variation. Due to the characteristics of the steam engine, cyclic variation can never be eliminated but it can be minimised by quartering and further smoothed out by the weight of the flywheel.
Now what started me off into this digression into cyclic variation was talking about the harmonics of rope drives. It’s possible for the cyclic variation in the flywheel to coincide with the harmonic frequency of the rope drive. This can start the ropes swinging about on the slack side of the drive and it’s possible for this to build up until individual ropes will interfere with other ropes or even throw themselves out of their groove and even come off the wheel. If you have studied Newton Pickles’ tapes you will remember that he discussed this and cited one engine in particular, Broughton Road Shed at Skipton as an example of an un-quartered engine that did just this, it threw ropes off, particularly on starting when the power impulses were at their greatest. The cure with a cross compound is to quarter it. Otherwise all that can be done is to look to the valve events and speed of starting and make sure that the power is delivered as smoothly as possible to the ropes. This brings us back to dressing the ropes with anti-fraying compound. It’s very noticeable that an application of rope dressing smoothes out the drive because it induces a small amount of creep in the contact between the ropes and the flywheel grooves. This means that the heavier the load, the more creep and the smoother the drive. This mitigates the conditions most likely to cause the harmonics of the drive to get out of control.
Now this may all sound like nit-picking but what has always to be borne in mind is that the function of the engine is to convert heat energy from coal, via the boiler and production of steam, into rotative power delivered into the shed to the looms. The better the quality of that rotative power, the more productive the looms will be. The most important qualities are constancy and accuracy of speed and the minimum cyclic variation. Once you get these right by careful attention to valve events and management of the rope drive in the engine house you can increase the speed of the engine which in turn increases production. It’s no use simply increasing the speed without attending to the quality as this just induces weaving faults.
The greatest value of the information in these tapes about the technology is that the opinions are based on experience. When Bancroft engine was first installed it was intended to run at 68rpm. When I went in as engineer this had dropped to 67rpm, a loss of one and a half percent on lineshaft speed. By improving the drive I gradually increased the speed of the engine over about three months to 68rpm and the wage of the average weaver in the shed rose by thirty shillings a week, an increase of almost 4%. Nobody ever told me how much production rates went up but I think we can reasonably guess at about 10%. All this by simply running the engine correctly.
This raises the interesting point, why hadn’t this happened before? Why hadn’t the management realised that engine speed and production was dropping? One factor is that there was no instrumentation to help the engineer. If there had been revolution counters on both the engine flyshaft and the lineshaft and proper records had been kept these discrepancies could have been quickly picked up and rectified. In spinning mills this was certainly done and woe betide an engineer who was down on his rev count at the end of the week, he was on the red carpet. There is good documentation of engineers running their engines faster on Saturdays to make up numbers.
Another interesting thing about rope drives is that you'd very seldom find them in a room and power shed. Obviously rules are meant to be broken and I can think of room and power sheds where there was rope drive but generally you would find that room and power establishments had gear drives. In other words instead of the rope drive from the pulley to the second motion pulley, the engine drove the lineshaft by gearing. The reason for this is quite simple in that a gear drive is a solid drive, there is no slippage, and the people who were paying for the power knew that they were getting all the power that the engine can produce. In other words, what they were paying for. They were always a little bit suspicious of ropes drives because it was known that there could be slippage and this could lead to problems in loom timing.
I have no doubt that we’ll come back to rope drives and engine speeds again so I’ll leave it for now. What I hope I have demonstrated is that two simple pictures like 7 and 8 can actually contain a lot of information and that things are never as simple as they seem. There were many people who would have given their right arm to have my job simply because they were steam enthusiasts. My point is that the engine was only the start of the process and the engineer had a crucial role in the profitability of the shed because quite large savings and efficiency gains depended on how he ran the plant and a simple, messy job like smearing tallow and graphite onto a rope could actually make a lot of difference.
Picture number 9. Neg number 777233.
This is a detail of the low pressure crosshead while the engine is at rest. The crosshead is the name for the actual pivoting point where the piston rod meets the connecting rod. The connecting rod is the large mass of metal on the left and the piston rod can be seen going back into the cylinder on the right. The wrist pin which is the actual pivot pin can be seen as the circular piece of metal in the middle of the large circular block which lies on top of the slipper block which runs in the slide. Now, that slipper is carrying the weight of the back end of the con rod and it also takes the dynamic forces involved when the piston rod pushes the end of the con rod out against the resistance of the crank and pulls it back. Now if you think about it, when the piston rod pushes the crosshead out the con rod is inclined up into the air. This means that it's tending to force the slipper block down into the slide on to the bed in front of the low pressure cylinder. On the return stroke when the steam is acting on the front side of the piston and pulling the con rod back the con rod is inclined downwards and the forces are trying to pull the slipper block down into the slide. The important thing to recognise here is that these circumstances only apply when the engine is ‘running over’. That is, the flywheel is rotating away from the cylinders. The easiest way to thing of this is that if it was dropped on the ground and free it would roll way from the cylinders. If you think about it, if the engine was running under, that is running the opposite way round, the forces would be completely reversed on the slipper block and the cumulative effect would be to try to raise the slipper block when the load was on. In this case all the weight would be taken on the pates down the sides of the slide. This is the reason why an engines such as Bancroft engine, which was only ever intended to run in one direction has a simple slide like the one you can see in the picture. If it was a reversing engine, an engine that was designed to run either way like a winding engine or a rolling mill engine, it would have double slides or trunk slides where there are surfaces designed to control the forces on the crosshead both above and below. Trunk slides of any description are the bane of the engineer, they are always in need of adjustment, they are very bad to get at to work on, they are not easy to oil and they mask all the front end of the cylinder making access there bad as well. Mill engines do not need to run in reverse and so were built with simple slides like this. There is hardly anything to go wrong with it and it's possible to take the cover off the front end of the cylinder without doing any dismantling at all. All you have lot to do is get the cross head as far forward as you can, take the hand rails off, undo the nuts and slide your cover forward to get at the inside of the cylinder for inspections and maintenance.
Notice the way that the con rod is attached to the wrist pin. There is a large strap which goes round the wrist pin, a large, very thick piece of metal which you can see coming on to the top of the con rod and bottom. The bolt which you can see at the front end going right through that strap and the end of the con rod is the bolt that tightens that strap at the end and fixes it immovably on the end of the rod. Inside that strap and the circular face at the end of the con rod are the two brasses which act
as the bearing surface on the wrist pin. It is essential to have as little play as possible in that bearing because you’ll understand that it’s pushed forward and pulled back 136 times a minute. The adjustment for those brasses is the arrangement which you can see in the middle which is three wedges, two facing one way and one facing the other. Now those opposed wedges are used if you want to tighten the brasses on the wrist pin. You slacken off the bolt at the front first then undo the bottom nut on
the threaded tail of the wedge which you can see at the top. Slacken it off about half a turn and tighten the top nut about half a turn. This has the effect of driving the middle wedge down in between the other two wedge. It also tends to lift the front wedge up which increases the effective distance from the back of the right hand wedge to the front of the left hand wedge which has the thread running off the top of it. These wedges bear against the front edge of the slots in the con rod strap and against the back edge of a similar slot in the con rod end so the overall effect of widening the combined wedges is to force the con rod end back against the brasses thus tightening them on the wrist pin. Tighten up the main bolt at the front and the lock nuts and that’s it, you have taken some slack out of the wrist pin bearing. A typical adjustment would be a sixteenth of a turn.
The way big bearings on like this are adjusted is actually very delicate. With a small bearing you can adjust it until it nips and then slack back a certain amount and try the bearing for float by sliding it on the pin. This isn’t possible with large bearings, it is possible to insert a bar and move the con rod end but the forces involved are so heavy it doesn’t tell you much. In practice what you do is leave the bearing alone until you hear a knock, until it starts talking to you. Then give it the smallest amount of nip possible and run it, checking to see whether it runs hot. If the knock persists, try the same amount again. This way you will never over tighten and cause a problem.
The wrist pin only oscillates and so is never a big problem. Others, such as the crank pin or flyshaft bearings are more temperamental in that if you over tighten the bearing rapidly warms up. As the metal expands it tightens even more and you rapidly reach the stage where the bearings actually malt and throw metal out. There is always the possibility of damaging the surface of the journal in these circumstances. So, the bottom line is that you only adjust when you have to. As any old engineer will tell you, they would rather hear them than smell them.
There is another example of the wedge shaped key or cotter in this picture. If you look at the back of the wrist pin housing on top of the slipper block you will see that the forging is extended backwards in a curved fork with carries the socket into which the piston rod is fixed. This joint must be dead slid but capable of being split for maintenance. This is achieved by having a very similar arrangement to that which allows adjustment of the wrist pin bearing. A slot is machined through the fork and one through the piston rod end as well. These slots are positioned so as to be wider than the cotter itself but slightly offset so that as the wedge shaped cotter is driven in it bears against the front of the slot in the piston rod and the back end of the two slots in the socket. This means that as the cotter is driven in it forces the piston rod hard up against the bottom of the socket thus holding it solid. Note that there is no adjustment on this connection. It is dead solid. The cotter has a hole driven into the narrow end and a heavy split cotter is placed in this. This is a safety device which would retain the cotter in the slot if it came loose while the engine was stopped.
It’s worth mentioning here that there is a very good way of recognising if a solid fit like the cotter in the piston rod end or any key or stake is working loose. One of the things that an engineer has to understand is that there are many different causes for and forms of corrosion. One of them is ‘fretting corrosion’ and this does not need water or oxygen to set it off. It is caused by minute amounts of friction between two surfaces that are heavily loaded such as the contact between a cotter and the slot. Even if it is immersed in oil, this fretting of one metal against another produces minute heat sources which in turn generate corrosion and eat the metal away. The oxide produced by this form of corrosion in steel is bright red and if it is happening inside a joint it will slowly work its way out and appear as a red stain. We call this ‘bleeding’ and it is a sure sign that the contact isn’t solid.
When I took over the Bancroft engine there was a heavy thump in the low pressure side drive train. This was always put down to the bad design of the air pump but I discovered that it was actually play in the joint where the piston rod fitted into the socket on the cross head. The cause was bad fitting when the engine was built, the hole in the socket had been bored too deep and there wasn’t enough offset between the opposing slots to allow for the wear over the years. The cure was to make a new cotter and fit a spacing washer on the end of the piston rod so that it bore down on the bottom of the socket. This was a big improvement and took out what was a constant source of annoyance.
There are always consequences to any adjustment you make. I have been talking about adjusting the wrist pin, the crank and the position of the piston rod. The thing that you have to bear in mind is that all these adjustments alter the relationship of the various parts of the engine to each other. In the case of these, because the flyshaft centre is a fixed point, any adjustment that is made in the drive train affects the stroke of the piston in the cylinder. This has always to be taken into consideration. For instance, when the new cotter was fitted in the piston rod and a ¼ inch collar fitted on the front of the rod, the piston travel was moved back ¼ of an inch and we had to bar the engine over very carefully to ensure that the piston wasn’t fouling the back cover of the cylinder because of the alteration we had made. It was all right, there was no problem but you have to be aware of the problem. Thinking it through further you will realise that this alteration also affected the stroke of the air pump and even the swept volume in the cylinder. By moving the piston back we had effectively reduced the swept volume at the back of the cylinder and increased it at the front. This affects valve settings and shows that these ‘simple’ machines are far more complicated than they first appear.
When I was describing the function of the parallelogram motion which is used for transmitting the motion of the crosshead to the indicator when you are indicating the engine I mentioned the threaded hole to which it is attached on the crosshead. There is a clear view of this in the centre of the wrist pin. Notice also the drip feed lubricator fitted on the end of the con rod strap which supplies a small amount of oil to the wrist pin while the engine is running. This is a very slow speed drip as the pin is only oscillating in the bearing and the lubricator holds enough to last easily for one period of running because obviously it cannot be topped up while the engine is running.
Another detail which you can see on this picture is the low pressure cylinder lubricator in the top left hand corner. This is driven by the flat rod you can see running across the back of the bed. This is attached to a strap on one of the eccentric rods which drives the low pressure valve gear from the eccentrics and as the rod moves transmits motion to the lever driving the lubricator. Theoretically this lubricator delivers oil to the valve s of the low pressure cylinder and hence lubricates the bore. In practice, there was enough oil carried over atomised in the steam from the high pressure cylinder to lubricate the low pressure. The lubricator was left empty all day and was only fed oil half an hour before stopping. This ensured that the bore had a good covering of oil overnight and on starting the following morning.
One thing to notice overall about this picture is that all the pieces of metal are bright, shiny and lightly oiled. I was always polishing the engine, wiping surfaces down with a piece of oily waste as I was working and people used to think it was because I was houseproud. This may have been part of the case but the real reason is that the best way to inspect any piece of machinery is to clean it. It is during cleaning that you discover the loose pins and nuts and clogged oil ways that can build up to bigger problems unless addressed right away. It also looked well and was a good indication to anyone that entered the engine house that the engineer was doing his job well and had a bit of personal pride. A bit like polishing your boots.
SCG/06 September 2003
4,682 words.
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Stanley Graham (Engineer & Researcher) Tape 08
