Plant Considerations.—Plant considerations affecting power methods are power sources and costs. In industry where power is either purchased or generated, the power source has no effect on the problem, but in plants where natural power has been harnessed by cumbersome methods and modernizing is desired, the prob lem is somewhat difficult. Here power is distributed by long lines of shafting and numerous wide belts. If the plant is large, the friction load is high and maintenance excessive. By the installa tion of gear reduction units and short centre devices much shafting and belting can be removed. First cost of mechanical transmission equipment should not be considered to such an extent that the cheapest method is the best. In many instances high initial ex penditures are economical. In large industrial operations, main tenance costs are of necessity high, but when good quality trans mission equipment is installed it should be given care and atten tion. Careless joining, wrong type and size of belting and injurious dressings are all causes of high maintenance. Inattention to lubri cation, poor adjustments, careless installing and overloading cause the maintenance bills to rise on transmission appliances such as bearings, clutches, pulleys and shafting.
Belting.—Power belting is the medium of delivering a given amount of power at a given point at the least cost per unit of time, which should result in the transmission of power at the lowest cost per horse power per year over the longest period of useful service.
If the belting fails, the mechanism stops. If the belting slips, the mechanism slows down. It is necessary that the user have some knowledge of the attributes and characteristics of the various power belting materials and the correct atmospheric and mechani cal conditions. (See BELTING.) The open belted short centre driv ing installation is not efficient. Its faults have led to the develop ment of modern methods for driving at close centres by flexible shock absorbing non-metallic mediums.
The Automatic idler system is a development of the ordinary tightener pulley. Its chief function is to increase the arc of con tact on the driving pulley. The fulcrum or idler pulley is applied to the belt so as to equal the belt tension and to increase the arc of contact both permanently and automatically and in accordance with the load transmitted. The permanent increase is attained by the proper location of the idler fulcrum. The automatic in crease is accomplished by wrapping every possible inch of belting around the smaller of the two pulleys. The elongation or stretch of the belt due to its elasticity is in synchronism with the varia tions in the power transmitted; when the power is maximum, the arc and the grip of the belt on the pulley is also maximum.
The multiple "V" shaped rubber and fabric rope drive is an adaptation of the English multiple endless rope-driving method with the exception that it employs "V" shaped ropes made of rubber and fabric composition operating on multiple "V" grooved wheels. It is an excellent transmitter of power at close range.
The greater the load tension, the greater the adhesion to the grooves of the wedge-shaped belting sheaves, therefore, slip is almost entirely eliminated. This method of driving is successful on short centre work, with centre distances up to 12 feet. It has a range in power capacity from h.p. to 2,000 h.p., depending on the number and size of "V" ropes employed.
High Ratio Short Centre Device.—There are occasions for ratios from 15 tor to 3o to 1. Solving such reductions by shafting and belting may be out of the question. Worm gear or spur gear reduction units may be too rigid. To meet such requirements a new device has been recently developed, which gives a compact, flexible, short-centre and high-ratio drive. It is composed of a gear, pinion, pulley, spindle and fulcrum arm. The pulley and pinion are keyed to the spindle which is supported by the fulcrum arm centred on the shaft to be driven. The gear is keyed on the shaft to be driven and meshes with the pinion on the spindle. Thus, in the rotation of the spindle by the aid of a belt drive to the spindle pulley, the pinion tends to make an epicyclic movement around the gear wheel with which it meshes, but such movement is restrained when the belt has been tensioned and the continued ro tation of the pinion will rotate the gear wheel and operate the apparatus or lineshaft. The degree of tension on the driving belt depends upon the inertia to be overcome in starting the load and in the continued running, so that the belt always retains the cor rect tension for the power requirements. The chief feature of this method of driving is the absolute automatic load tension on the ordinary open driving belt without the aid of spring or weight controlled idler pulleys. The belt is always in correct tension because the load controls it.
Power Shafting Bearings.—There are various types of bear ings manufactured for industrial power transmitting. They are designed to support shafting and the factors which determine the selection of any given design are : (I ) diameter and speed of shaft ; (2) power transmitted and kind of load; (3) supporting structure; (4) lubrication; (5) space limitations; (6) operating conditions. Bearings are made of iron or steel, bushed or lined with brass, bronze and various anti-friction metals. (See BEAR INGS.) When bearings are lined with babbitt metal, as is common practice, the shaft beds itself down and distributes its weight uni formly over the entire bearing surface. Industrial bearings are subjected to "moderate" or "excessive" pressures. Moderate pres sures exist in industrial head, line, jack and counter shaft opera tions if the correct number of bearings are employed for the load demand. Standard bearings for this class of work have ample bear ing surface for the pressures involved. Excessive pressures on bearings are due to extreme weight, pull or thrust conditions. Ex cessive dead load causes extreme weight. Excessive pull is due to high belt or rope tension. Excessive thrust is due to abnormal duty or to faulty mechanical adjustments.