CHAPTER XIVTHE STEAM TRACTOR

The steam tractor consists of the following elements, which will take up in detail under separate headings.

(1) Engine proper, consisting of the cylinder, piston, valve motion, guides, crank, fly wheel, etc.

(2) Boiler—with the grates, burners, etc.

(3) Feed pump or injector.

(4) Feed water heater.

(5) Driving gear, differential, clutch, etc.

As in the case of the gas tractor, the machine consists simply of a steam engine and its boiler that drive the road wheels of the tractor through a gear train. With the steam tractor the gearing is simplified as the reverse is performed by the engine’s valve motion, and not through gearing. There is no need of speed changing transmission gears in the steam tractor as the engine is sufficiently flexible to provide an innumerable number of speeds by simple throttle control.

While the fuel most commonly used is coal, straw and wood, crude oil is often used, the fuel being determined principally by the location of the engine, and by its cost on the job. The matter of fuel should be taken into consideration when the engine is purchased as the different grades demand different fire box and boiler construction. When it is possible to obtain crude oil at a reasonable figure, it certainly should be used in preference to all others as liquid fuel is the most compact, most easily controlled, and efficient of any. The subject of oil burners is taken up later in this chapter, a number of types of which are clearly illustrated.

The steam engine cylinder consists essentially of a smoothly bored iron casting in which a plunger called the “piston” slides to and fro, the cylinder acting not only as a container for the steam acting on the piston but as a guide and support as well.Needless to say, the contact or fit between the piston and cylinder walls must be as perfect as possible, tight enough to prevent steam passing the piston, and free enough to allow the piston to slide without unnecessary friction. The reciprocating piston is connected to the crank through a connecting rod by which the pressure on the piston is communicated to the crank arm.

The pressure exerted on the crank pin by the piston depends on the area of the piston (in square inches) and the pressure of the steam on each square inch of the area. With a given steam pressure, the greater the area, the greater the force tending to turn the crank. As power is the rate or distance through which the force acts in a unit of time it is obvious that the power developed by the engine is equal (in foot pounds) to the force in pounds multiplied by the velocity of the piston in feet per minute. Since there are 33,000 foot pound minutes in a horse-power, the power developed by such a cylinder is equal to the force multiplied by the piston velocity, divided by 33,000.

As the cylinder is necessarily limited in length it is evident that the piston cannot travel in one direction continuously but must be reversed in direction when it travels the length of the cylinder bore thereby traveling the next distance in the opposite direction. This reversal of the piston is accomplished by admitting the steam in one end of the cylinder and then into the other, this causing the steam to act on the opposite sidesof the piston alternately. To establish a difference of pressure on the two piston forces, the steam pressure is relieved on one side while the steam acts on the other.

A typical cylinder furnished with the ordinary steam tractor is shown by Fig. 133, in which T is the cylinder, P the piston and R is the piston rod. When the steam in the cylinder end E acts in the direction shown by arrow E, the piston pulls the rod R in the direction shown by arrow S, the pressure in the cylinder end D being relieved to atmospheric at this time. The steam is admitted and relieved by the valve L which slides back and forth on its seat actuated by the valve rod VR.

In the position shown, the valve L is moving to the left as shown by arrow O. The edge of the valve N is just opening the steam port G through which the cylinder end F is placed in communication with the steam filled valve chest A. Steam at boiler pressure fills the space A, which flows into E past N and through G when the valve opens and establishes pressure against P, which, through the piston and connecting rods turns the crank.

The steam is exhausted from the cylinder end D, through the port F, through the exhaust port U, and out of the exhaust pipe X. As will be seen from the figure, the inside valve edge Y has moved to the left so that the port F is fully opened. When the piston reaches the left hand end of the cylinder, the valve L moves to the right so that the end of the cylinder E is connected to the exhaust port V through the cylinder port G, thus allowing the steam in the space E to pass out of the exhaust pipe X. A further movement of the valve to the right causes the left edge Z of the valve to uncover the cylinder port F which allows the steam to flow into the cylinder space D and push the piston to the right. This motion is carried on continuously, the valve moving in a fixed relation to the piston, and admits the steam and releases it first on one side of the piston and then on the other. The valve shown is known as a “D” valve and is one of a variety of valves furnished with steam engines, which, however perform exactly the same functions as the valve shown.

An “eccentric” which is really a form of crank, drives the valve to and fro, the eccentric being fastened to the crankshaft. The full pressure of the steam forces the D valve down on its seat, and as the valve is of considerable size, this pressure causes much friction and power loss. In some engines a “balanced” valve is used in which the pressure on the valveis balanced by an equal pressure that acts on the under side of the valve face. Balanced or unbalanced, the function of the slide is to alternate the flow of steam in the two ends of the cylinder.

Steam is prevented from passing the piston into the opposite end of the cylinder by elastic rings placed in grooves on the piston which are known as “piston rings.” Being thin and elastic these rings instantly conform with any irregularity of the piston bore and effectually stop the flow of steam past them. At the point where the reciprocating piston rod R passes through the cylinder, a steam tight joint is made by the “stuffing box” or gland H. The space between the inner walls of the stuffing box and the piston rod are either filled with some description of fibrous packing or a metallic packing that fits around the rod in the same manner that the piston rings fit in the bore of the cylinder. The packing is arranged around the valve rod VR in the same manner.

As the piston, piston rod, and valve slide on their respective surfaces with considerable pressure it is absolutely necessary that these parts receive ample lubrication. In practically all engines the oil is taken into the cylinder with the steam in the form of drops, the oil being measured out by a sight feed lubricator that is tapped into the steam supply pipe. In this device, the oil from the lubricator reservoir is fed through a regulating needle valve, drop by drop, up through a gauge glass so that the engineer can tell the amount of oil that he is feeding. The body of the lubricator is filled with condensed water up to the level of the outlet through which the oil passes into the cylinder, and the entire lubricator, reservoir and all is under boiler pressure at all points. The oil regulating valve is placed at the bottom of the lubricator, and as oil is lighter than water, it floats up from the valve to the level of the outlet, through the gauge glass, and from the outlet level floats out into the steam pipe and mixes with the steam. By floating the oil in this manner, the engineer can see every drop that is fed.

In order to reduce the amount of steam used, the valve does not allow the steam to follow the piston at full boiler pressure through the entire stroke, but cuts it off at a certain point after the piston has started on its travel. As the volume of the steam is increased by the further travel of thepiston after the point of cut-off, the steam expands in volume until the end of the stroke is reached, at which point the pressure is naturally much below the initial or boiler pressure. This reduction in temperature and pressure results in a wider working temperature range than would be the case with the steam following the piston throughout the stroke, and as the steam is exhausted to atmosphere at a temperature much lower than that of the boiler steam, much less heat is carried out through the exhaust. As a general rule, the most economical point of cut-off is at ¼ of the stroke. Engines requiring more steam than is supplied at ¼ cut-off in order to carry the load, are too highly taxed for efficient results. Since the most efficient point of cut-off is only ¼ of the possible steam travel it is evident that an engine can carry a load much greater than that for which it is rated, but it is also evident that this increased capacity is gained at the expense operating economy. Wear and tear on the engine parts are also duly increased.

Fig. 134. Case Steam Tractor.

On steam tractors a constant speed is maintained by “throttling” the steam, to meet the demands of the load by partially restricting the flow of steam at light loads and opening theinlet at full load. The valve that controls the steam for the different loads is controlled by a “governor” which depends on the centrifugal force exerted by two fly-balls. The balls, or weights are hinged to a revolving spindle, driven by the engine, in such manner that an increase of speed tends to straighten out and revolve in a more nearly horizontal plane. The amount of travel of the balls for a given speed increase, is governed by a spring, which returns them to a vertical position when the speed decreases. By means of a simple system of levers, the valve is closed when the balls fly out, due to an increase of speed, and is opened when the speed decreases, so that the engine will receive the steam at a higher pressure and again build up its speed to normal. As the load fluctuates, the balls are constantly moving up and down, seeking a valve position that will keep the engine at a constant speed.

Speed variation is generally accomplished by increasing or decreasing the tension of the spring that controls the travel of the governor fly balls, and in the majority of engines this may be done without stopping the engine.

Another form of governor used extensively on stationary engines controls the speed by increasing or decreasing the cut-off. Thus with a heavy load the cut-off may occur at ½ the stroke while with a very light load it may be at110stroke. This is by far the most sensitive and economical form of governor, but on account of the reverse gear it is difficult to apply it on a tractor.

As explained under “Cylinders” the travel of the valve bears a definite relation to the piston position so that the ports may be opened and closed at the proper times. It may be shown by a rather complicated diagram that this relation of the valve together with that of the eccentric that drives it is only correct for one direction of rotation. For any other direction of rotation the relation of the valve and piston position must be changed. This may be done in several ways but the most common types are the Stevenson Link and the Wolff slotted yoke.

The Stevenson link motion used on the majority of engines, consists of two independent eccentrics, one being fixed in the relation for forward motion and the other for the reverse direction. The ends of the eccentric rods leading from these eccentrics are connected by a slotted bar or link, inwhich a block is placed that is connected with the valve rod. The block is free to slide in the slot of the links, that is, it may be moved from one end of the slot to the other. When it is desired to have the engine rotate in a right handed direction, for example, the link is lowered so that the rod from the forward eccentric is brought directly in line with the block so that this eccentric alone acts directly on the valve through the valve stem. When the reverse is desired the link is raised until the rod from the reverse eccentric is brought in line with the block and valve stem, drive being by the reverse eccentric.

When the block is on the link in a position between the two points mentioned, the valve has less travel and it cuts off earlier in the stroke than when driven directly by one eccentric, for the motion at an intermediate point on the link is much different than at the ends of the slots. This fact is taken advantage of in operating engines with the idea of economy in view, and is known commonly as “hooking up” the engine. The best point at which to “hook up” the engine is best determined by experiment, and is equivalent in many respects to the problem of advancing and retarding the spark of a gas engine. We earnestly advise an engineer of a traction engine to take up this subject and determine the best point of cut-off for different loads as he will find that different positions make a considerable difference in his coal bill. Of course the proper way is to determine this point with a steam engine indicator, but as few engineers have such an appliance, the work is generally of the cut and try order. Wear and varying adjustment soon change the points marked on the reverse sector, and for economy’s sake these points should be checked occasionally.

In the Wolff motion, a single eccentric is used for both directions of rotation, in connection with a slotted link. A single eccentric is securely keyed to the crank shaft. The eccentric strap has an extended arm which is pivoted to a block that slides back and forth in a curved guide. The angle at which the guide stands with the horizontal determines the direction of rotation, the angle being changed by the reverse lever. The degree of the angle made by the block also determines the point of cut-off. This is a very efficient and simple valve gear.

The outer end of the piston rod is supported by a sliding block known as the “cross-head” which in turn is supportedby the guides. An oscillating rod called the “connecting rod” connects the reciprocating cross-head with the crank pin, this rod is used in the same way as the connecting rod of the gas engine except that it is connected to the cross-head instead of the piston.

The clutch affords a means of connecting and disconnecting the driving wheels and engine shaft. It is usually of the friction type described under “Gas Tractors.” By releasing the clutch the engine is disconnected from the driving gear so that the tractor remains stationary while the engine is driving a load through the belt.

The exhaust from the cylinders is used in two ways, first to create a draft for the fire, and second to heat the feed water pumped into the boiler. The draft is increased by exhausting a portion of the steam into a nozzle placed directly under the stack. The friction of the steam on the surrounding air, draws the air with it, forming a partial vacuum over the grate at each puff, and in this way it causes additional air to rush through the fuel and increases the temperature of the combustion. As the load increases the “puffs” increase in intensity due to the greater terminal pressure and the fire is accelerated in proportion. This is a simple but rather expensive way of increasing the draft.

A considerable proportion of the heat in the exhaust steam is saved by using it to heat the feed water supplied to the boiler. Besides the saving in fuel, affected by heating the water from steam that would otherwise be thrown away, the strains on the boiler due to the injection of cold water are greatly decreased as the difference between the temperatures of the boiling water in the boiler and the hot feed water are much less than in the former case.

The feed water heater consists essentially of a series of tubes in a cylindrical shell. The tubes are surrounded on the outside by the feed water, and are filled with the exhaust steam which passes from end to end through the tubes. The hot water is pumped from the heater into the boiler. An efficient feed water heater adds greatly to the steaming capacity of the boiler.

(152) Feed Pump.

A small steam pump is furnished for pumping the water into the boiler. This device consists of a small steam cylinder connected directly with the pump plunger and is absolutely independent of the main engine so that it can be used whether the engine is running or not. The exhaust of the pump should be turned into the feed water heater when the engine is not running so as to heat the water, but should be directed to atmosphere when the main exhaust is passing through the heater. An injector is usually supplied with the engine for feeding the boiler in emergencies.

The injector forces water into the boiler by means of a steam jet which is arranged so that a high velocity is imparted to the water in the injector nozzle by the condensation of the steam furnished by the jet. In this way water is pumped into the boiler against a pressure that is equal to the pressure of the steam acting on the water. Except for a check valve there are no moving parts. No feed water heater connection is made with the injector for this device raises the temperature of the feed to a considerable temperature. The temperature is not as high, however, as the temperature of the water from the feed water heater and pump, and because of the comparatively low temperature coupled with the fact that live steam is used in heating the injector water, it is not an economical method of pumping.

As the boilers of traction engines sustain the pull and vibration of the engine as well as the stresses due to traveling over rough roads in addition to the steam pressure strains, they must be made very substantially and of the best materials. The service of the boiler on a traction engine is very different from that met with in stationary or locomotive practice for the tractor seldom receives the attention that is given to the other types and as it goes bumping over the fields with the water whacking at every joint and the engine rushing and surging at every little grade, it receives an “endurance” test every moment of its existence.

A boiler should show an inspection pressure considerably in excess of that which it is intended to carry. It should be well stayed and braced, and should be suspended from the road wheels in such a way as to be relieved from as much strain as possible. No transverse seams should be permitted, and the barrelshould be well reinforced at the point where the front bolster is attached as well as at points where pipe connections are tapped into the shell. No large bolts should be tapped into the steam or water space. The tubes should be placed so that they may be easily withdrawn or cleaned. The location of the hand holes and washout holes is also an important item, for inaccessible hand-holes are an abomination.

Boiler lagging or covering is intended to reduce the heat loss by radiation, and for this reason it should be of a good insulating material and should be thick enough to be effective. The cost of jacketing is more than covered by the saving in coal, especially in cold weather.

A straw-burning fire box differs from a coal burner in having a fire brick arch and a shorter grate, and in having a special chute on the fire door for feeding the straw into the furnace. After a short time, the fire brick arch becomes incandescent, keeping the firebox temperature constant and producing perfect combustion of the tarry vapors distilled from the straw. A trap door is provided on the straw chute which automatically keeps the outside air from chilling the fire.

As with the straw-burning furnace, a brick arch is used in burning oil for the purpose of preventing fractional distillation of the oil during the combustion. In some forms of oil furnaces a brick checker-work is used that provides a much greater surface to the gases than the ordinary firebrick arch and therefore keeps a steadier temperature and pressure. Broken firebrick in the furnace placed in heaps with a rather porous formation is also an aid to combustion. With very heavy oils a jet of steam in the firebox is of great assistance in consuming the free carbon of the fuel (soot).

The oil in practically all cases is atomized or is broken up into a very finely subdivided state by the action of a jet of steam. The finer this subdivision the better will be the combustion for the oil particles will be brought into more intimate contact with the air. Provision is also made in the burner for either whirling or stirring the oil vapor with the air so that a rapidly burning mixture is formed. In other respects the oil burning engine is the same as the coal or wood burner.

During the idle season, the engine should be well housed, allbright parts slushed with grease and the whole engine carefully covered with tarpaulins. A tractor is an expensive machine and should be given care, or it will rapidly depreciate and start giving trouble. When one considers the abuse and neglect given farm machinery it is remarkable that it will work at all, let alone give efficient service.

Small Fairbanks-Morse Motor Driving Binder.

Before starting a new engine or one that has been idle for a considerable time, all of the bearings and lubricating should be thoroughly cleaned with kerosene oil, removing all grit or gum. After cleaning, they should be thoroughly oiled with the proper grade of lubricant and then adjusted for the correct running fit, taking care that the bearings and wedges are not taken up too tight, nor too many shims are taken out. Be sure that the openings in the lubricating cups and oil pipes are not clogged and that oil holes in the bearing bushings register with those in the bearing caps. At points where there are sight feed gauge glasses, the glasses should be cleaned with gasoline and all of the joints repacked with new packing.

Careful attention should be paid to the piston rod and valve rod packing taking care that it is only tight enough to prevent the leakage of steam and no greater. Excessively tight packing burns out rapidly, scores and shoulders the piston rod, making it impossible to keep the joint tight. When rods are badly scored they should be trued up in the lathe taking care not to take off too much metal on the finishing cut. When renewing fibrous packing be sure that all of the old packing is removed before placing the new packing in the box. Keep the packing well lubricated at all times to prevent wear, and in some cases it will be advisable to add an oil cup to the stuffing box to insure sufficient lubrication.

Go over the valve gear and make sure that there is no looseness or play in the eccentrics or pins, and that all of the bolts and keys are tight and in place. Loose connections in the valve gear are not only productive of knocks and wear but also tend to increase the fuel consumption of the engine. When possible, indicator cards should be taken at intervals to make sure that the valves are correctly set. In a test recently made by the author, the indicator cards showed a defective setting due to wear, that when corrected saved the owner of the engine about 600 pounds of coal per day, and as the coal cost $9.50 per ton delivered in the field, the saving soon paid for the expense of the test. Points of adjustment are provided on all valve gears, and as they differ in detail for each engine we cannot give explicit directions for settling the valves, but will leave this point for the direction book of the maker.

The governor and governor belt should now receive attention making sure that there are no loose points or nuts in the mechanism and that the governor belt is in good condition. Defective governor belts are dangerous through the possibility of over speeding. Slipping or oily belts not only increase the chances of fly-wheel explosions, but also cause a fluctuation in the speed which is not desirable especially in threshing, where good results are obtained only by a constant speed. Make sure that the safety lever works properly and shuts off the steam with a loose or broken belt. Test the governor valve stem for sticking or for rough shots that are likely to cause uneven running. Keep the governor well lubricated with light oil, and keep the oil off the belt as much as possible. Governor valve should be carefully tested for tightness and freedom.

The throttle valve must be absolutely steam tight for a leaking valve is a dangerous proposition especially in stopping theengine. It is generally arranged so that it can be reground with pumice stone or crocus powder and oil. If the valve is of bronze or brass do not use emery or carborundum for the particles will become imbedded in the soft metal and put it in a worse condition than ever. Pack the valve stem.

A leaking slide valve is the cause of much loss of power, and waste of coal, and as the leakage mingles directly with the exhaust, it often remains unknown until it has thrown away a considerable quantity of fuel. It is best detected by blocking the engine with the piston at mid-stroke and opening the throttle valve slightly. If the cylinder drain cocks are now opened, the leaking steam that escapes into the cylinder will be seen issuing from the drains. The leakage that passes into the exhaust will be seen escaping from the stack while it is practically impossible to have the valves absolutely tight at all times, the steam should not escape so rapidly that it roars through the openings. Leakage past the piston is another source of loss that can be detected by blocking the engine so that the piston is very near, one end of the stroke, with the valve opening one of the cylinder ports. Any steam that passes the piston will pass out of the exhaust. With an old engine it is likely that the cylinder is worn oval, or that the valve seat is grooved or uneven, in which case it will be necessary to rebore the cylinder and fit new piston rings or reface the valve seat. Broken piston rings are often the source of leakage, and if not replaced with new at an early date, are likely to destroy the cylinder bore as well. Broken rings generally make themselves known by a wheezing click when the engine is running.

The steam feed pump should be well lubricated with a good grade of cylinder oil and should be well packed around the piston rod especially at the water end. To guard against pump troubles a good strainer should be provided on the water suction line to prevent the entrance of sticks and dirt into the cylinder. Great care should be exercised in keeping the suction line air tight, for if any air escapes into this line no water will be lifted. Dirt under the valves is the cause of much pump trouble, as a very small particle of dirt will allow the water to pass in both directions through the valves. Leaking packing will also destroy the vacuum in one end of the cylinder. For the best results the pump should be run slowly but continuously, feeding a small amount of water at one time. This method of feeding allows the feed water heater to bring the water up to the highest possible temperature which reduces thefuel consumption and reduces the strains on the boiler. It is a bad policy to let the water get low in the boiler and then “ram” full of cold water in a couple of minutes. Attention should be paid to the check valve that is located between the pump and boiler. It should be kept clean and the valve kept tight and in good condition.

When the feed water is hard a boiler compound should be used to reduce the amount of scale in the boiler or soften it and make its removal easier. Scale of116inch thickness will decrease the efficiency of the boiler by 12%, and this loss increases rapidly with a further increase in the thickness of the scale because of its insulating effect on the tubes. Soft sludges such as mud and clay may be removed by-blowing off or by the filtration of the water before it is pumped into the boiled. Lime and magnesia which form flint-hard deposits, require chemical treatment such as the addition of sodium phosphate, etc. In any case, the deposits waste heat and increase the liability of burning out tubes or bagging the sheets.

Buffalo Marine Motor.

A solution that has given good results with waters containing lime, consists of 50 pounds of Sal Soda and 35 pounds of japonica, dissolved in 50 gallons of boiling water. About140quart is fed into the boiler for every horse-power in 10 hours, the solution being mixed with the feed water. Kerosene has been used a great deal to soften scale, and gives good results if not fed in quantities to exceed 0.01 quart per horse-power day of 10 hours. An excess of kerosene is to be guarded against for it is likely to accumulate in spots and cause bagged sheets or burn outs.

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