Direct motor chain connections employ chain of various types as the connection between the motor and power-receiving shaft of the mechanism. It is successfully operated on machine tools and line shaft drives. Ratios up to 7 to 1 on short centres are practicable.
The group system is extensively used in modern industrial operations. Group driving is most practicable from 5 to 5o h.p.
and if the line is operating at the normal speeds of 150 to 300 r.p.m., one belted or chain reduction is sufficient from motor to line. The driven machine can be belted from the line through countershafts or direct. With this system, the size of the prime mover is based on the aggregate of the running loads of the machines driven.
Gear reduction unit systems. The electric motor has the inher ent characteristic of operating at high speed. In many industrial operations the power-receiving shaft of apparatus or machine to be driven must run at slow speed. Ratios of 5o to f and ioo to I are frequent from prime mover to apparatus, therefore the use of the modern gear reduction unit. Ratios such as these if at tempted by the belting and shafting method would require a large amount of transmission equipment. Spur gear reducers are manu factured with ratios up to soo to f and reduce in a straight line. The worm gear reducer has reached a high state of perfection. The single unit can be employed on drives requiring speed reduc tions of ioo to 1.
The open gear connection system is employed where extremely close centres and moderate ratios are desired such as on small machine tools and large apparatus. Composition or non-metallic gears are used on the prime mover shaft to eliminate the noise of high speed metallic gears meshing together.' Large apparatus at slow speeds are frequently direct geared to the motor and where a greater ratio is necessary than is practicable with one reduction, a back geared motor is employed. This type of motor combines one geared reduction integral with itself.
The multiple fabric rope system is of recent development. The ropes are of "V" shaped endless construction and operate on "V" grooved wheels. Ratios up to 7 to I and extremely short centres are possible. It is applicable for direct driving of almost any class of machine or apparatus where adjustment between driver and driven is possible. There are now in use two systems of manila
rope driving: the American or continuous method and the English or multiple method. The American system employs but one con tinuous rope winding over all of the grooves with the rope on the slack side forming a loop over an idler sheave and a travelling tightener which automatically regulates the tension of all the wraps of the rope. The English system employs separate and in dependent endless ropes in each groove of the wheels. The Ameri can system will transmit power for great distances and in any di rection. The English system is most adaptable for direct high power driving.
Variable speed connection units are used to meet the require ments of numerous industrial operations. Variations in applied speed are absolutely necessary, and the control of this speed change must be smooth or without shock. In modern practice this is ac complished by variable speed motors or by mechanical speed change devices. It frequently happens that it is necessary to em ploy a combination of methods of driving because of limited space. The compactness of modern methods is then an advantage.
Mechanical Consideration.—Consideration affecting power methods are space, speed, ratios and method of control. Mechani cal driving requires space, the amount being dependent on the method employed. If ratios are low and ample centres are avail able the "group" system of belt driving should be employed. If space is limited and ratios are high, the close coupled installations such as chain, worm or spur gear reducers, multiple rope and direct gear connections should be used. Space governs moderate speed ratios, but available area does not warrant the use of shafting and belting when ratios of 25, 3o, 5o and zoo to I are necessary. For these ratios gear reduction units should beused.
The method of control is an important factor since it is fre quently necessary to change speeds or shut down the machine during operation and then start under load. Unless a slip ring motor is employed, direct connection by any rigid means should not be used. Shafting should be employed for this service equipped with belting or chains controlled by either clutch or shifters.