Aviation Engines Part 32

Sc.r.a.pING BRa.s.sES TO FIT

To insure that the bearing bra.s.ses will be a good fit on the trued-up crank-pins or crank-shaft journals, they must be sc.r.a.ped to fit the various crank-shaft journals. The process of sc.r.a.ping, while a tedious one, is not difficult, requiring only patience and some degree of care to do a good job. The surface of the crank-pin is smeared with Prussian blue pigment which is spread evenly over the entire surface. The bearings are then clamped together in the usual manner with the proper bolts, and the crank-shaft revolved several times to indicate the high spots on the bearing cap. At the start of the process of sc.r.a.ping in, the bearing may seat only at a few points as shown at Fig. 187, G.

Continued sc.r.a.ping will bring the bearing surface as indicated at H, which is a considerable improvement, while the process may be considered complete when the bra.s.s indicates a bearing all over as at I. The high spots are indicated by blue, as where the shaft does not bear on the bearing there is no color. The high spots are removed by means of a sc.r.a.ping tool of the form shown at Fig. 187, F, which is easily made from a worn-out file. These are forged to shape and ground hollow as indicated in the section, and are kept properly sharpened by frequent rubbing on an ordinary oil stone. To sc.r.a.pe properly, the edge of the sc.r.a.per must be very keen. The straight and curved half-round sc.r.a.pers, shown at M and N, are used for bearings. The three-cornered sc.r.a.per, outlined at O, is also used on curved surfaces, and is of value in rounding off the sharp corners. The straight or curved half-round type works well on soft-bearing metals, such as babbitt, or white bra.s.s, but on yellow bra.s.s or bronze it cuts very slowly, and as soon as the edge becomes dull considerable pressure is needed to remove any metal, this calling for frequent sharpening.

When correcting errors on flat or curved surfaces by hand-sc.r.a.ping, it is desirable, of course, to obtain an evenly spotted bearing with as little sc.r.a.ping as possible. When the part to be sc.r.a.ped is first applied to the surface-plate, or to a journal in the case of a bearing, three or four "high" spots may be indicated by the marking material. The time required to reduce these high spots and obtain a bearing that is distributed over the entire surface depends largely upon the way the sc.r.a.ping is started. If the first bearing marks indicate a decided rise in the surface, much time can be saved by sc.r.a.ping larger areas than are covered by the bearing marks; this is especially true of large shaft and engine bearings, etc. An experienced workman will not only remove the heavy marks, but also reduce a larger area; then, when the bearing is tested again, the marks will generally be distributed somewhat. If the heavy marks which usually appear at first are simply removed by light sc.r.a.ping, these "point bearings" are gradually enlarged, but a much longer time will be required to distribute them.

The number of times the bearing must be applied to the journal for testing is important, especially when the box or bearing is large and not easily handled. The time required to distribute the bearing marks evenly depends largely upon one's judgment in "reading" these marks. In the early stages of the sc.r.a.ping operation, the marks should be used partly as a guide for showing the high areas, and instead of merely sc.r.a.ping the marked spot the surface surrounding it should also be reduced, unless it is evident that the unevenness is local. The idea should be to obtain first a few large but generally distributed marks; then an evenly and finely spotted surface can be produced quite easily.

In fitting bra.s.ses when these are of the removable type, two methods may be used. The upper half of the engine base may be inverted on a suitable bench or stand and the boxes fitted by placing the crank-shaft in position, clamping down one bearing cap at a time and fitting each bearing in succession until they bed equally. From that time on the bearings should be fitted at the same time so the shaft will be parallel with the bottom of the cylinders. Considerable time and handling of the heavy crank-shaft may be saved if a preliminary fitting of the bearing bra.s.ses is made by clamping them together with a carpenter's wood clamp as shown at Fig. 187, J, and leaving the crank-shaft attached to the bench as shown at C. The bra.s.ses are revolved around the crank-shaft journal and are sc.r.a.ped to fit wherever high spots are indicated until they begin to seat fairly. When the bra.s.ses a.s.sume a finished appearance the final sc.r.a.ping should be carried on with all bearings in place and revolving the crank-shaft to determine the area of the seating. When the bra.s.ses are properly fitted they will not only show a full bearing surface, but the shaft will not turn unduly hard if revolved with a moderate amount of leverage.

Bearings of white metal or babbitt can be fitted tighter than those of bronze, and care must be observed in supplying lubricant as considerably more than the usual amount is needed until the bearings are run in by several hours of test block work. Before the sc.r.a.ping process is started it is well to chisel an oil groove in the bearing as shown at Fig. 187, L. Grooves are very helpful in insuring uniform distribution of oil over the entire width of bearing and at the same time act as reservoirs to retain a supply of oil. The tool used is a round-nosed chisel, the effort being made to cut the grooves of uniform depth and having smooth sides. Care should be taken not to cut the grooves too deeply, as this will seriously reduce the strength of the bearing bushing. The shape of the groove ordinarily provided is clearly shown at Fig. 187, G, and it will be observed that the grooves do not extend clear to the edge of the bearing, but stop about a quarter of an inch from that point. The hole through which the oil is supplied to the bearing is usually drilled in such a way that it will communicate with the groove.

The tool shown at Fig. 187, K, is of recent development, and is known as a "crank-shaft equalizer." This is a hand-operated turning tool, carrying cutters which are intended to smooth down scored crank-pins without using a lathe. The feed may be adjusted by suitable screws and the device may be fitted to crank-pins and shaft-journals of different diameters by other adjusting screws. This device is not hard to operate, being merely clamped around the crank-shaft in the same manner as the lapping tool previously described, and after it has been properly adjusted it is turned around by the levers provided for the purpose, the continuous rotary motion removing the metal just as a lathe tool would.

FITTING CONNECTING RODS

In the marine type rod, which is the form generally used in airplane engines, one or two bolts are employed at each side and the cap must be removed entirely before the bearing can be taken off of the crank-pin.

The tightness of the bra.s.ses around the crank-pin can never be determined solely by the adjustment of the bolts, as while it is important that these should be drawn up as tightly as possible, the bearing should fit the shaft without undue binding, even if the bra.s.ses must be sc.r.a.ped to insure a proper fit. As is true of the main bearings, the marine form of connecting rod in some engines has a number of liners or shims interposed between the top and lower portions of the rod end, and these may be reduced in number when necessary to bring the bra.s.ses closer together. The general tendency in airplane engines is to eliminate shims in either the main or connecting rod bearings, and when wear is noticed the boxes or liners are removed and new ones supplied.

The bra.s.ses are held in the connecting rod and cap by bra.s.s rivets and are generally attached in the main bearing by small bra.s.s machine screws. The form of box generally favored is a bra.s.s sand casting rich in copper to secure good heat conductivity which forms a backing for a thin layer of white bra.s.s, babbitt or similar anti-friction metal.

[Ill.u.s.tration: Fig. 188.--Showing Points to Observe When Fitting Connecting Rod Bra.s.ses.]

In fitting new bra.s.ses there are two conditions to be avoided, these being outlined at Fig. 188, B and C. In the case shown at C the light edges of the bushings are in contact, but the connecting rod and its cap do not meet. When the retaining nuts are tightened the entire strain is taken on the comparatively small area of the edges of the bushings which are not strong enough to withstand the strains existing and which flatten out quickly, permitting the bearing to run loose. In the example outlined at B the edges of the bra.s.ses do not touch when the connecting rod cap is drawn in place. This is not good practice, because the bra.s.ses soon become loose in their retaining member. In the case outlined it is necessary to file off the faces of the rod and cap until these meet, and to insure contact of the edges of the bra.s.ses as well.

In event of the bra.s.ses coming together before the cap and rod make contact, as shown at C, the bearing halves should be reduced at the edges until both the caps and bra.s.ses meet against each other or the surfaces of the liners as shown at A.

SPRUNG CAM-SHAFT

If the cam-shaft is sprung or twisted it will alter the valve timing to such an extent that the smoothness of operation of the engine will be materially affected. If this condition is suspected the cam-shaft may be swung on lathe centers and turned to see if it runs out and can be straightened in any of the usual form of shaft-straightening machines.

The shaft may be twisted without being sprung. This can only be determined by supporting one end of the shaft in an index head and the other end on a milling machine center. The cams are then checked to see that they are separated by the proper degree of angularity. This process is one that requires a thorough knowledge of the valve timing of the engine in question, and is best done at the factory where the engine was made. The timing gears should also be examined to see if the teeth are worn enough so that considerable back lash or lost motion exists between them. This is especially important where worm or spiral gears are used.

A worn timing gear not only produces noise, but it will cause the time of opening and closing of the engine valves to vary materially.

PRECAUTIONS IN REa.s.sEMBLING PARTS

When all of the essential components of a power plant have been carefully looked over and cleaned and all defects eliminated, either by adjustment or replacement of worn portions, the motor should be rea.s.sembled, taking care to have the parts occupy just the same relative positions they did before the motor was dismantled. As each part is added to the a.s.semblage care should be taken to insure adequate lubrication of all new points of bearing by squirting liberal quant.i.ties of cylinder oil upon them with a hand oil can or syringe provided for the purpose. In adjusting the crank-shaft bearings, tighten them one at a time and revolve the shafts each time one of the bearing caps is set up to insure that the newly adjusted bearing does not have undue friction. All retaining keys and pins must be positively placed and it is good practice to cover such a part with lubricant before replacing it because it will not only drive in easier, but the part may be removed more easily if necessary at some future time. If not oiled, rust collects around it.

When a piece is held by more than one bolt or screw, especially if it is a casting of brittle material such as cast iron or aluminum, the fastening bolts should be tightened uniformly. If one bolt is tightened more than the rest it is liable to spring the casting enough to break it. Spring washers, check nuts, split pins or other locking means should always be provided, especially on parts which are in motion or subjected to heavy loads.

Before placing the cylinder over the piston it is imperative that the slots in the piston rings are s.p.a.ced equidistant and that the piston is copiously oiled before the cylinder is slipped over it. When rea.s.sembling the inlet and exhaust manifolds it is well to use only perfect packings or gaskets and to avoid the use of those that seem to have hardened up or flattened out too much in service. If it is necessary to use new gaskets it is imperative to employ these at all joints on a manifold, because if old and new gaskets are used together the new ones are apt to keep the manifold from bedding properly upon the used ones. It is well to coat the threads of all bolts and screws subjected to heat, such as cylinder head and exhaust manifold retaining bolts, with a mixture of graphite and oil. Those that enter the water jacket should be covered with white or red lead or pipe thread compound. Gaskets will hold better if coated with sh.e.l.lac before the manifold or other parts are placed over them. The sh.e.l.lac fills any irregularities in the joint and a.s.sists materially in preventing leakage after the joint is made up and the coating has a chance to set.

Before a.s.sembling on the shaft, it is necessary to fit the bearings by sc.r.a.ping, the same instructions given for restoring the contour of the main bearings applying just as well in this case. It is apparent that if the crank-pins are not round no amount of sc.r.a.ping will insure a true bearing. A point to observe is to make sure that the heads of the bolts are imbedded solidly in their proper position, and that they are not raised by any burrs or particles of dirt under the head which will flatten out after the engine has been run for a time and allow the bolts to slack off. Similarly, care should be taken that there is no foreign matter under the bra.s.ses and the box in which they seat. To guard against this the bolts should be struck with a hammer several times after they are tightened up, and the connecting rod can be hit sharply several times under the cap with a wooden mallet or lead hammer. It is important to pin the bra.s.ses in place to prevent movement, as lubrication may be interfered with if the bushing turns round and breaks the correct register between the oil hole in the cap and bra.s.ses.

Care should be taken in s.c.r.e.w.i.n.g on the retaining nuts to insure that they will remain in place and not slack off. Spring washers should not be used on either connecting rod ends or main bearing nuts, because these sometimes snap in two pieces and leave the nut slack. The best method of locking is to use well-fitting split pins and castellated nuts.

TESTING BEARING PARALLELISM

It is not possible to give other than general directions regarding the proper degree of tightening for a connecting rod bearing, but as a guide to correct adjustment it may be said that if the connecting rod cap is tightened sufficiently so the connecting rod will just about fall over from a vertical position due to the piston weight when the bolts are fully tightened up, the adjustment will be nearly correct. As previously stated, babbitt or white metal bearings can be set up more tightly than bronze, as the metal is softer and any high spots will soon be leveled down with the running of the engine. It is important that care be taken to preserve parallelism of the wrist-pins and crank-shafts while sc.r.a.ping in bearings. This can be determined in two ways. That shown at Fig. 189, A, is used when the parts are not in the engine a.s.sembly and when the connecting rod bearing is being fitted to a mandrel or arbor the same size as the crank-pin. The arbor, which is finished very smooth and of uniform diameter, is placed in two V blocks, which in turn are supported by a level surface plate. An adjustable height gauge may be tried, first at one side of the wrist-pin which is placed at the upper end of the connecting rod, then at the other, and any variation will be easily determined by the degree of tilting of the rod. This test may be made with the wrist-pin alone, or if the piston is in place, a straight edge or spirit level may be employed. The spirit level will readily show any inclination while the straight edge is used in connection with the height gauge as indicated. Of course, the surface plate must be absolutely level when tests are made.

When the connecting rods are being fitted with the crank-shaft in place in crank-case, and that member secured in the frame, a steel square may be used as it is reasonable to a.s.sume that the wrist-pin, and consequently the piston it carries, should observe a true relation with the top of the engine base. If the piston side is at right angles with the top of the engine base it is reasonable to a.s.sume that the wrist-pin and crank-pin are parallel. If the piston is canted to one side or the other, it will indicate that the bra.s.ses have been sc.r.a.ped tapering, which would mean considerable heating and undue friction if the piston is installed in the cylinder on account of the pressure against one portion of the cylinder wall. If the degree of canting is not too great, the connecting rods may be sprung very slightly to straighten up the piston, but this is a makeshift that is not advised. The height gauge method shown above may be used instead of the steel square, if desired, because the top of the crank-case is planed or milled true and should be parallel with the center line of the crank-shaft.

[Ill.u.s.tration: Fig. 189.--Methods of Testing to Insure Parallelism of Bearings After Fitting.]

CAM-SHAFTS AND TIMING GEARS

Knocking sounds are also evident if the cam-shaft is loose in its bearings, and also if the cams or timing gears are loose on the shaft.

The cam-shaft is usually supported by solid bearings of the removable bushing type, having no compensation for depreciation. If these bearings wear the only remedy is replacement with new ones. In the older makes of cars it was general practice to machine the cams separately and to secure these to the cam-shaft by means of taper pins or keys. These members sometimes loosened and caused noise. In the event of the cams being loose, care should be taken to use new keys or taper pins, as the case may be. If the fastening used was a pin, the hole through the cam-shaft will invariably be slightly oval from wear. In order to insure a tight job, the holes in cam and shaft must be reamed with the next larger size of standard taper reamer and a larger pin driven in. Another point to watch is the method of retaining the cam-shaft gear in place.

On some engines the gear is fastened to a f.l.a.n.g.e on the cam-shaft by retaining screws. These are not apt to become loose, but where reliance is placed on a key the cam-shaft gear may often be loose on its supporting member. The only remedy is to enlarge the key slot in both gear and shaft and to fit a larger retaining key.

CHAPTER XII

Aviation Engine Types--Division in Cla.s.ses--Anzani Engines-- Canton and Unne Engine--Construction of Gnome Engines-- "Monosoupape" Gnome--German "Gnome" Type--Le Rhone Engine-- Renault Air-Cooled Engine--Simplex Model "A" Hispano-Suiza-- Curtiss Aviation Motors--Thomas-Morse Model 88 Engine-- Duesenberg Engine--Aeromarine Six-Cylinder--Wisconsin Aviation Engines--Hall-Scott Engines--Mercedes Motor--Benz Motor-- Austro-Daimler--Sunbeam-Coatalen.

AVIATION ENGINE TYPES

Inasmuch as numerous forms of airplane engines have been devised, it would require a volume of considerable size to describe even the most important developments of recent years. As considerable explanatory matter has been given in preceding chapters and the principles involved in internal combustion engine operation considered in detail, a relatively brief review of the features of some of the most successful airplane motors should suffice to give the reader a complete enough understanding of the art so all types of engines can be readily recognized and the advantages and disadvantages of each type understood, as well as defining the constructional features enough so the methods of locating and repairing the common engine and auxiliary system troubles will be fully grasped.

Aviation engines can be divided into three main cla.s.ses. One of the earliest attempts to devise distinctive power plant designs for aircraft involved the construction of engines utilizing a radial arrangement of the cylinders or a star-wise disposition. Among the engines of this cla.s.s may be mentioned the Anzani, R. E. P. and the Salmson or Canton and Unne forms. The two former are air-cooled, the latter design is water-cooled. Engines of this type have been built in cylinder numbers ranging from three to twenty. While the simple forms were popular in the early days of aviation engine development, they have been succeeded by the more conventional arrangements which now form the largest cla.s.s. The reason for the adoption of a star-wise arrangement of cylinders has been previously considered. Smoothness of running can only be obtained by using a considerable number of cylinders. The fundamental reason for the adoption of the star-wise disposition is that a better distribution of stress is obtained by having all of the pistons acting on the same crank-pin so that the crank-throw and pin are continuously under maximum stress. Some difficulty has been experienced in lubricating the lower cylinders in some forms of six cylinder, rotary crank, radial engines but these have been largely overcome so they are not as serious in practice as a theoretical consideration would indicate.

Another cla.s.s of engines developed to meet aviation requirements is a complete departure from the preceding cla.s.s, though when the engines are at rest, it is difficult to differentiate between them. This cla.s.s includes engines having a star-wise disposition of the cylinders but the cylinders themselves and the crank-case rotate and the crank-shaft remains stationary. The important rotary engines are the Gnome, the Le Rhone and the Clerget. By far the most important cla.s.sification is that including engines which retain the approved design of the types of power plants that have been so widely utilized in automobiles and which have but slight modifications to increase reliability and mechanical strength and produce a reduction in weight. This cla.s.s includes the vertical engines such as the Duesenberg and Hall-Scott four-cylinder; the Wisconsin, Aeromarine, Mercedes, Benz, and Hall-Scott six-cylinder vertical engines and the numerous eight- and twelve-cylinder Vee designs such as the Curtiss, Renault, Thomas-Morse, Sturtevant, Sunbeam, and others.

ANZANI ENGINES

The attention of the mechanical world was first directed to the great possibilities of mechanical flight when Bleriot crossed the English Channel in July, 1909, in a monoplane of his own design and construction, having the power furnished by a small three-cylinder air-cooled engine rated at about 24 horse-power and having cylinders 4.13 inches bore and 5.12 inches stroke, stated to develop the power at about 1600 R.P.M. and weighing 145 pounds. The arrangement of this early Anzani engine is shown at Fig. 190, and it will be apparent that in the main, the lines worked out in motorcycle practice were followed to a large extent. The crank-case was of the usual vertically divided pattern, the cylinders and heads being cast in one piece and held to the crank-case by stud bolts pa.s.sing through substantial f.l.a.n.g.es at the cylinder base. In order to utilize but a single crank-pin for the three cylinders it was necessary to use two forked rods and one rod of the conventional type. The arrangement shown at Fig. 190, called for the use of counter-balanced flywheels which were built up in connection with shafts and a crank-pin to form what corresponds to the usual crank-shaft a.s.sembly.

[Ill.u.s.tration: Fig. 190.--Views Outlining Construction of Three-Cylinder Anzani Aviation Motor.]

The inlet valves were of the automatic type so that a very simple valve mechanism consisting only of the exhaust valve push rods was provided.

One of the difficulties of this arrangement of cylinders was that the impulses are not evenly s.p.a.ced. For instance, in the forms where the cylinders were placed 60 degrees apart the s.p.a.ce between the firing of the first cylinder and that next in order was 120 degrees crank-shaft rotation, after which there was an interval of 300 degrees before the last cylinder to fire delivered its power stroke. In order to increase the power given by the simple three-cylinder air-cooled engine a six-cylinder water-cooled type, as shown at Figs. 191 and 192, was devised. This was practically the same in action as the three-cylinder except that a double throw crank-shaft was used and while the explosions were not evenly s.p.a.ced the number of explosions obtained resulted in fairly uniform application of power.

[Ill.u.s.tration: Fig. 190a.--Ill.u.s.trations Depicting Wrong and Right Methods of "Swinging the Stick" to Start Airplane Engine. At Top, Poor Position to Get Full Throw and Get Out of the Way. Below, Correct Position to Get Quick Turn Over of Crank-Shaft and Spring Away from Propeller.]

[Ill.u.s.tration: Fig. 191.--The Anzani Six-Cylinder Water-Cooled Aviation Engine.]

[Ill.u.s.tration: Fig. 192.--Sectional View of Anzani Six-Cylinder Water-Cooled Aviation Engine.]

The latest design of three-cylinder Anzani engine, which is used to some extent for school machines, is shown at Fig. 193. In this, the three-cylinders are symmetrically arranged about the crank-case or 120 degrees apart. The balance is greatly improved by this arrangement and the power strokes occur at equal intervals of 240 degrees of crank-shaft rotation. This method of construction is known as the Y design. By grouping two of these engines together, as outlined at Fig. 194, which gives an internal view, and at Fig. 195, which shows the sectional view, and using the ordinary form of double throw crank-shaft with crank-pins separated by 180 degrees, a six-cylinder radial engine is produced which runs very quietly and furnishes a steady output of power. The peculiarity of the construction of this engine is in the method of grouping the connecting rod about the common crank-pin without using forked rods or the "Mother rod" system employed in the Gnome engines. In the Anzani the method followed is to provide each connecting rod big end with a shoe which consists of a portion of a hollow cylinder held against the crank-pin by split clamping rings. The dimensions of these shoes are so proportioned that the two adjacent connecting rods of a group of three will not come into contact even when the connecting rods are at the minimum relative angle. The three shoes of each group rest upon a bronze sleeve which is in halves and which surrounds the crank-pin and rotates relatively to it once in each crank-shaft revolution. The collars, which are of tough bronze, resist the inertia forces while the direct pressure of the explosions is transmitted directly to the crank-pin bushing by the shoes at the big end of the connecting rod. The same method of construction, modified to some extent, is used in the Le Rhone rotary cylinder engine.

[Ill.u.s.tration: Fig. 193.--Three-Cylinder Anzani Air-Cooled Y-Form Engine.]