106 South Elm Street, Newkirk, Oklahoma 74647
Most all of the men who play with steam engines know something about the 'Angularity of the Connecting Rod;' but you seldom hear the angularity of the eccentric rod mentioned.
Any connecting rod on an engine of any kind; a pitman on a mowing machine; a pump; any device on which one end of a rod is carried in a circle while the other is attached to a piston that moves in a straight line, will have this same peculiarity of movement. An eccentric, which is a modified crank, will cause a valve to move in the same manner. A piston will at the instant of 'dead center' have no movement at all; but will be moving at about 3-1/7 times the average speed at a point near, but not exactly, at the center of the stroke. The writer owns an engine with a 9-inch diameter piston and 10-inch stroke with connecting rod 32' long. When the crank pin is at the 90° point or, as some call it, 'the quarter,' top or bottom, the piston will be about 7/16' past the mid-point of the stroke on the end nearest to the crank. This means that there is about 7/8' more steam in the 'head-end' of the cylinder. This would result in having about 10% more power in the 'head-end' of the cylinder if some means of compensation is not provided.
In the most simple type of stationary engine, the angularity of the eccentric rod serves to increase the time that the steam is on the 'head' end; however, millions of feet of lumber were sawn and millions of bushels of grain were threshed with valve gears of this type. The exhaust from the head end was louder than from the crank end; but that did not worry the operators much. These engines were simple to adjust and repair, so any smart farm hand could run them.
On the Stephenson reversing gear which was used on the first successful locomotives, the later cut-off on the head end could be corrected by using the indirect motion rocker arm to move the valve. This causes the angularity of the eccentric rods to work in opposition to that of the connecting rod, and serves to equalize the power in the cylinder. Most of the earlier traction engines used the Stephenson or, as it is sometimes called 'Link Reverse', as it was difficult to design a traction engine with an indirect motion rocker arm. They had more power in the head end. Some of them had inside admission piston valves which equalizes the cut-off.
A more important fault in the action of these eccentric gears is that the opening and closing of the valve occurs in the slow part of the valve motion. Early in the steam engine era engineers learned they could get more power and efficiency if they had full pressure at the beginning of the stroke.
During the latter half of the 19th century the steam engine was the most important machine in existence and the best brains in the world were occupied with trying to improve it. Dozens of valve gears were patented. Some were only devices that would make the engine run in the other direction, but some would give almost perfect distribution of steam. Around the turn of the century a type of gear called the 'radical' came into use on most traction engines. These open the ports very quickly just before the beginning of the stroke, then dwell for an instant and close quickly. Some of the earlier models could not be adjusted for exactly lead and cutoff points, but several of them were improved until the lead remains constant while the reverse lever is moved from one corner of the quadrant to the other. The writer owns an engine built in 1915 that can be startedidle of course in either direction then, the reverse lever brought slowly to the center notch and it will continue to run. This engine, while running with the lever in the center, is running on only the 'lead' opening, which is about one thirty-secondth of an inch. To perform this way a valve gear must be perfectly adjusted and, of course, designed right in the first place. I have handled two other engines that would perform this way.
Now a few words about locomotives: the corresponding parts are called different names from those on traction engines. Instead of 'connecting rods' they call it the 'main rod' but, it has angularity. With a main rod 84' long and stroke 28' the piston will be a little less than one and three-sixteenth inches closer to the axle end of the cylinder than to the head end when the main crank-pin is at the quarter (or 90°) point, either top or bottom. This will add up to nearly two and three-eights inches, which would mean that there is about 9% more power on the head end.
There is a very important difference in the way a valve gear on a locomotive is constructed. Due to the action of the springs, the frame has considerable movement up and down on the axle bearings. For this reason all eccentric rods, or any part that imparts motion to the valve, must move in a horizontal direction.
Practically all the locomotives built after 1900 were equipped with the Waalsheart outside connected or, as some call it 'monkey motion' or a little later the Baker-Pilliod came on the scene. The Baker-Pilliod was possibly the most perfect of any of the valve gears. Both of these gears open the ports quickly as the crank passes dead center, dwells for a split second then, due to the motion of the 'lap and lever' or, as some called it the 'intervening bar,' closes quickly for cut-off. Near the end of the stroke the valve is moving fast and gives a quick release so there is little back pressure. Engines equipped with these gears could move heavy trains and saved steam by utilizing expansion.
An old locomotive machinist allowed me to make copies of indicator cards that were made by the Baker-Pilliod Company when they were demonstrating their valve gear, trying to sell it to the Chicago and Great Western Railroad in 1909. There are four of these cards. The date was May 11, 1909. The first was made 4:30 p.m., location: Mile Post 171 Throttle Fullreverse lever in the 7th notch from centersteam pressure 190RPM 240, MPH 48. Spring 100. (This means that one inch in the height of the card equals 100 pounds of pressure.) This card was not 'worked' so the horsepower is not known. The cutoff was occurring at about 10% of the stroke. The next card was made at 5:00 p.m.location: Mile Post 154-153; throttle full; reverse lever in the 10th notch from center, steam pressure 200, RPM 160, MPH 32. This card was worked. This means it was calculated to show horsepower. The head end shows mean effective pressure82.4. The crank end 82.1. The indicated horsepower in the head end271.5. The indicated horsepower in the crank end 252.7.
The third card was made at 5:11 p.m.Mile Post 149throttle full-reverse lever in 11th notch from centersteam pressure 200MPH 40. This card was not worked. Horsepower not known.
The fourth card was made at 5:40 p.m. Location: Mile Post 134-133 throttle fullreverse lever in 12th notch from centersteam pressure 190RPM 180, MPH 36. Head end mean pressure 85.5. Crank end mean pressure 84.7. Indicated horse-power in head end 316.8. Indicated horsepower in crank end 304.9. It may be noted there is less indicated horsepower in the grank end than, in the head. The area bf the piston rod is subtracted.
These cards all show almost perfect distribution of the steam. At the beginning of the stroke, the pressure was very close to that of the boiler. Cut-off was occurring at about 10% on the card with the lever in the 7th notch and about 35% on the card with the lever in the 12th notch. On all the cards the back pressure was down to about 7 or 8 pounds psi.
Notice that the throttle was wide open. The best engineer on the road was at the throttle. The late A. D. Baker and several of the railroad company officials were on the train. The engine was a 4-6-2, Pacific type. The size was not known, but it appeared to be about 100 tons. A couple of the handiest mechanics that Baker-Pilliod had were riding on a little platform about the cylinder operating the indicator.
At that time the locomotive began growing. Monster articulated compounds with boilers nine feet in diameter that left no room on the sides for the air pumps so they had to be moved to the front end. There was no room for the injector branch pipes, so the water was fed to the boiler by pumps and preheated by coils in the smoke box. The little engine on which the Baker-Pilliod was tested turned up 1300 horsepower. The giant compounds could develop 6000. The diesel changed all this and the big locomotives fell under the junker's torch. Now we are wondering where the diesel fuel is to come from. What next?