Fig. 53. motors if smooth running and lack of vibration are desired. There are many methods of fastening counterweights to crank shafts, some of which are shown in Malleable-iron weights fitted to the crank and dowelled and riveted answer very well and are widely used (Fig. 54). This is the method adopted by the Gray Motor Co. Bearings are most important parts of a motor, and various materials are used by different makers. Babbitt, bronze, white-bronze, and various Fig. 53. Methods of Fastening Counterweights special alloys are utilized, and each has its advantages and disadvantages. Babbitt is excellent, if of very high grade, but will melt and cut badly if improperly lubricated or overheated. Bronze will wear well and will stand considerable heat, but will cut and injure the shaft or other moving parts if allowed to run dry or wear loose. Whatever material is used, the fit should be perfect and the bearings so constructed that they are easily removable and interchangeable. The Gray Motor Co. uses a very high grade special babbitt, and the main bearings. are very long and so constructed that they are removable and interchangeable in a few moments (Fig. 55). In a two-cycle motor the size and length of the main bearings are very important, for if worn or loose the crank compression will be lost and the motor's efficiency destroyed. Gears, timer parts, and all water connections on marine motors should be of bronze, or similar metal, to prevent rust and corrosion; and if quiet-running gears are desired they should either be made in pressure die moulds from bearing metal or should be made with fibre or rawhide inserts or with slanting or helical teeth (Fig. 33). The particular kind of gears used depends largely upon the experience and personal choice of the manufacturer. Some use one kind and some another, but iron or steel gears are to be avoided as far as possible unless of the worm or helical type, for bevel or spur gears of these metals are invariably noisy and wear rapidly. The bearing-metal gears used in the "American motors" run very quietly and give excellent results. These gears are cast in special moulds and are formed under several tons' pressure. Other makers obtain equally good results by using fibre inserts in the teeth, or by taking great care in cutting all gears on special machines which insure the utmost accuracy. Cams for operating the valve mechanism of four-cycle motors should be of case-hardened steel, and the same metal should be employed wherever severe friction or continual pounding occurs, as on push-rods, valve-stem feet, rocker arms, etc. Valves are made of various materials, but the best forms are those forged from highgrade nickel steel. Cast-iron valves with steel stems. riveted in place were formerly used, but it well repays any motor-owner to replace all such valves with new ones forged from the best quality steel. Accessibility in a motor is a very important matter and one that is all too often overlooked. Many motors of excellent design and most careful construction are so assembled that in order to get at some small minor part it is necessary to take down the entire motor. Of course it is practically impossible to build a motor in which every part can be removed without disturbing some other part, but the nearer one can come to this ideal condition the better. Cylinders should be removable without disturbing the base or shaft, and all piping and water connections should be so arranged that they can be easily removed or disconnected without disturbing the rest of the motor. In this connection the fly-wheel is of considerable importance, and in many motors this is very difficult to remove, although, in order to reach the gears or pump eccentric, it is necessary to remove it. Wheels that are fitted to a straight shaft and keyed in place often rust fast to the shaft and are almost as solid as if a part of it. While it is absolutely necessary to have the fly-wheel tight, to avoid vibration and pounding, yet it should be fastened in such a way that it can be taken off without special tools or machinery. Several makers use a tapered end to the shaft fitted in a tapered hole in the fly-wheel. The wheel is forced on the taper by a large nut and is prevented from slipping by a key (Fig. 56). Such wheels are usually easy to remove, but a still better plan is to use a bushing of bronze between the shaft and the wheel, as this prevents the iron wheel from rusting to the steel shaft (Fig. 57). |