Part 32 (1/2)
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 bus.h.i.+ng. 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 s.h.i.+ms 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 s.h.i.+ms 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 bus.h.i.+ngs 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 bus.h.i.+ngs 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 bus.h.i.+ng 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.