 | June, 1988 |
This month's discussion will be on the relative merits of ball bearings vs bronze bushings vs cast aluminum for crankshaft main bearings; and also on the relative merits of lapped - ring - and A.B.C piston cylinder combinations.
First; on crankshaft bearings: Just about everyone in the trade acknowledges that two ball bearings are the most desirable. The advantages of the two ball bearing crankshaft are as follows: low initial friction, which in turn makes starting easy and no break-in requirements. Other advantages of double ball bearings are that the wear and consequent resulting shift of the crank position is virtually non existent in a ball bearing motor, and that the thrust from a starter doesn't cause wear on the front of the case. If it were not for the cost, I think that all model airplane engines would have two ball bearings on the main. However, this is not to say that a bushing main motor cannot be mode to run very well. The simplest form of bearing is for the crankshaft to run in the cast aluminum. This is done with fair success in our low priced .15 RC, and with the low priced K&B motors. This is simple and cheap. However, when the shaft runs in aluminum, it is imperative for it to be well lubricated. And therein, often, is a problem. If you make the fit a little too tight, and the fuel doesn't work its way out around the front, the bearing goes dry. At the best the motor slows down when you try to lean it out. At the worst, the crankshaft can gall and pick up metal, and ruin the crankcase. Also, assuming the motor does get a good start in life, it does wear. As it wears, you get more and more leakage out the front. A badly worn bushing can spray as much fuel out the front as it burns. Of course, this means that instead of flying 12 or 13 minutes on 8 oz. of fuel, you are out in 6 or 7 minutes. Also, carburetors don't work right when a bearing leaks badly, and a reliable idle becomes unreliable. The use of the hypereutectic alloys for main bearings reduce wear somewhat. But, in my opinion, it only alters it slightly.
Now, let's consider bronze bushiness. True bronze is mostly copper with small amounts of alloying agents. #660 is the most widely used and works well. This has the advantage over aluminum in that should the shaft run dry. or when a little dirt gets in it, the galling does not occur like it would with aluminum. The manufacturer can then be less fearful of a dry bearing and fit it closer than he would an aluminum one. Result - Improved fuel mileage and better carburetion. While it is generally considered that the piston and cylinder present the primary break-in problem, this is not necessarily so. Bushing type motors do have the requirement of wearing in and polishing both the crankshaft and the main bearing surface. If the bearing is not honed just right and the crank is not real round, it is quite possible to never get a really free running bearing. Some motors have appeared on the market which had rather yellow looking bearings, which signify high zinc content. These don't seem to work well at all. They tend to deform under the pressure of running, and the bearing gets loose. True bronze has a dark reddish copper looking surface, and not the bright yellow that you associate with brass.
At this point I would also say that many manufacturers, including me, have at one time along the line tried the use of powdered metal type bearings. This type of bearing works poorly in model motors, presumably because the pores allow the oil filn, which the shaft should ride on, to be pushed through the pores. Consequently, instead of floating on a wedge of oil, the crank actually rubs metal to metal. Powdered metal bearings will invariably wear the crank out much faster than a solid bronze bushing. The powdered metal bearings are cheaper. Whether they are better than the cast aluminum in my mind is questionable. The cast aluminum bearing does hold the oil wedge and the crankshaft will spin freer than it will in a powdered metal bushing. But, the steel crank against aluminum is vulnerable to wear and is very vulnerable to galling if a speck of dirt gets in the bearing or it runs dry.
Now, to the connecting rod. Connecting rods usually fail differently on the bottom end than on the top end. The bottom end is well cooled with the incoming fuel spray and is well lubricated. But the crankpin rotation plus its small size gives a pretty fair load rating, much higher than on a crankshaft. The unit load rating on a crankpin might be ten times as high as on a crank main. Connecting rods lower end are sometimes run on the rod material, and sometimes bushed. And, in rare cases, as in our old 60 and 74. and larger chain saw motors, have needle bearings.
Aluminum rod material seems to run pretty satisfactorily on smaller size engines. In 19's and 25's, you can usually run aluminum with no problem and have the rod last as long as the rest of the motor. A bar stock connecting rod made from one of the stronger bar stack grades of aluminum has pretty fair bearine antifriction qualities, and works well on the pin. When you get into larger size motors, such as 40's and larger, the wear rate on the connecting rod sometimes becomes unacceptable. In this case, most manufacturers have resorted to a bushing.� However, in my opinion, a poor bushing is worse than none at all. Certain imported motors have brass rod bushings, which, of course, last a very short time. Some rods have used the powdered bronze type bearing, but due to the abrasive nature of the bushing, the wear on the pin is very fast. In short order, the crankpin itself is worn out, even though it is hardened very hard. #660 bronze is a good grade general purpose bronze and is sometimes used with very good successes. However, the best material we have found is phosphor bronze material. Phosphor bronze is quite expensive costing several times that of #660, but it is dense and very high strength, and has all the good characteristics of bearing bronze.
Now to the top side of the rod. The top side of a rod is not subjected to the high rotational speeds that the bottom is, since it only oscillates. However the lubrication conditions are much worse. The area is a whole lot hotter, and there is relatively small amount of fuel spray up in this area, and traditionally the size of the wrist pin is smaller than the crankpin. This is basically in an effort to keep the reciprocating parts light. However, the strain on a rod is considerable, especially in A.B.C set-ups when the piston sometimes sticks. In a bad warm-up situation loads can be high enough to pull the rod apart. In our experience, there is about an even choice whether we bush the top end of the con rod and increase its antifriction characteristics, or whether we leave the additional metal around to increase the tension load that a rod can take in the case of a piston sticking. The choice seems to be dictated mostly by the type of failures that we have had, although I will have to admit that we do sometimes put a bushing in where it is not needed because people think that it means quality.
Now, to the piston and cylinder. For years the most popular model airplane piston materials were iron in its various forms, (gray cast, ductile, and Mehanite). The advantage of an iron piston is its low expansion, its dimensional stability. and the fact that it does not soften noticeably with the heat reached in model engines. The primary disadvantage is its weight, which is about 3 times as much per cubic inch as aluminum. This is somewhat offset by the fact that iron does have a higher modulus of elasticity than aluminum, and it is possible to make an iron piston that's thinner than an aluminum one and still maintain its shape. Iron pistons are usually run in soft steel liners. Most of the model engines in the past 50 years have been built in this combination. The iron can be fit very closely in the steel liner. The expansion co-efficient of both are about the same, and should the parts rub too hard, the iron tends to burnish and not gall. To the user, an iron piston/steel liner motor has a freeness and a snap-over compression not readily achieved with any other combination. The primary disadvantage of the iron piston and the soft steel liner is that as you get into larger and larger out-put motors, it becomes more and more difficult to keep the expansion of the cylinder liner and piston matched. At one time we produced the 59 with an iron piston and steel liner, but today our 40 Standard is the largest that we feel is practical.
The second, very popular, piston and cylinder combination is the use of a hardened steel liner and an aluminum piston fitted with 1 or 2 piston rings. A lot of motors of yesteryear used this combination. The McCoy and the Hornet were outstandingly successful. This combination worked very well and had a light piston which was relatively vibration free. However, the success of this combination depends on having a quality piston ring. Unfortunately, only one piston ring company seemed capable of producing an acceptable quality ring, and when they were swallowed by a conglomerate, quality model size rings became unavailable. K&B solved their problem by developing the Dykes ring to an acceptable quality. We solved our problem by developing a new method of shaping conventional design piston rings. Licensing of this patent is now available to interested ring manufacturers. The primary advantages of a steel liner-ring piston motor is its ability to accept abuse - over lean runs, dirt, no warm up. etc.. and the fact that it can be flown out of the box without fear of damaging it or seizing the piston. About the only disadvantage is that the cylinder webs reduce the power output slightly over the other two types.
Now, to A.B.C. or A.A.C. type cylinder/piston combinations. A.B.C is an abbreviation for Aluminum (piston) Brass (Liner) Chrome (plated). The problem of getting a good piston ring was probably a substantial cause for the increased popularity of the so called A.B.C type piston anti cylinder combinations. This amounts to an aluminum piston with no rings, but cast out of one of the modern high silicon, low expansion aluminums which is fitted into a brass liner which has been chrome plated. The chrome plating produces a hard wearing surface to keep the cylinder liner from wearing out. An aluminum piston ran in a brass liner with no plating would be completely worn out in 10 or I5 minutes. However, hard chrome is expensive, difficult to apply evenly, and almost impossible to hone once its on. In order to make a very small shape anti surface improvements, we had to resort to diamond and borizon honing stones to make a few tenths corrections in our A.B.C. lines when they come back from plating. Now not everybody claiming an A.B.C cylinder really has hard chrome on it. Some cylinders have used polished chrome, like that put on auautomobile bumpers. This is much softer and can be honed. but also it wears out quicker. It is cheaper however. AIso, there have been same cases, where manufacturers have used electroplated nickel or electroless nickel. Nickel, compared to chrome, is very soft. Furthermore, nickel has more adhesion problems to the base metal.
Our advice is to stay away from cylinders that are nickel or polished chrome plated. For an A.B.C cylinder and piston to work right, a cylinder should be either a low expansion aluminum of the #390 alloy variety, or brass, and should be hard chrome plated. The piston must be one of the high silicon types, the most usual being the #390 series. The advantage of the A.B.C is that when it is properly fitted, the motor will run slightly faster than the ring motor, primarily because the webs in the cylinder which retain the rings on the ring motor can be removed and leaves a little more porting area. The disadvantages are first, one of cost, and second, that the motor must be handled mare carefully than a ring motor. The warm up period, particularly, is vulnerably. If an A.B.C motor that is cold and still relatively new is started and run at full power immediately, the piston heats up faster than the cylinder and, consequently, expands faster. It is not unusual for the piston to stick in the top of the cylinder with such force that the inertia pulls the rod apart. However, an A.B.C that is carefully handled and is operated properly will last a very Iong time and runs very smooth.
Our Fox 19BB. 40BB. and 50BB are our tap of the line motors. Materials used are top quality and .the best grade we have found for the purpose. All cylinders and bearing surfaces have been hand fitted for optimum clearance. All motors have been test run at full power and the needle valves have been adjusted to a normal operating adjustment. We pay less attention to cosmetics than our competitors but nobody takes the care we do to insure that your motor will start ready, run right, and fly your airplane with authority. Buy one—you will be happy that you did.
Advertisement, Model Airplane News, June 1988, p74.
