§ 16. Geared Rubber Motors.—It is quite a mistake to suppose that any advantage can be obtained by using a four to one gearing, say; all that you do obtain is one-fourth of the power minus the increased friction, minus the added weight. This presumes, of course, you make no alteration in your rubber strands.
Gearing such as this means short rubber strands, and such are not to be desired; in any case, there is the difficulty of increased friction and added weight to overcome. It is true by splitting up your rubber motor into two sets of strands instead of one you can obtain more turns, but, as we have seen, you must increase the number of strands to get the same thrust, and you have this to counteract any advantage you gain as well as added weight and friction.
§ 17. The writer has tried endless experiments with all kinds of geared rubber motors, and the only one worth a moment's consideration is the following, viz., one in which two gear wheels—same size, weight, and number of teeth—are made use of, the propeller being attached to the axle of one of them, and the same number of strands are used on each axle. The success or non-success of this motor depends entirely on the method used in its construction. At first sight it may appear that no great skill is required in the construction of such a simple piece of apparatus. No greater mistake could be made. It is absolutely necessary that the friction and weight be reduced to a minimum, and the strength be a maximum. The torque of the rubber strands on so short an arm is very great.
Ordinary light brass cogwheels will not stand the strain.
A. The cogwheels should be of steel[17] and accurately cut of diameter sufficient to separate the two strands the requisite distance, but no more.
B. The weight must be a minimum. This is best attained by using solid wheels, and lightening by drilling and turning.
C. The friction must be a minimum. Use the lightest ball bearings obtainable (these weigh only 0·3 gramme), adjust the wheels so that they run with the greatest freedom, but see that the teeth overlap sufficiently to stand the strain and slight variations in direction without fear of slipping. Shallow teeth are useless.
D. Use vaseline on the cogs to make them run as easily as possible.
Fig. 17.—Geared Rubber Motor.
Designed and constructed by the writer. For description of the model, etc., see Appendix.