Document Lgw57k0pZYY6qrX4E8pzZ8nQd

Here ore fundamentals back of today's most popular variablespeed drives: How torque is transmitted Why manufacturers rate their units conservatively Why efficiency improves from low to rated output ^57 o. BH too loois b Ttntlon |utl c.Ekli loo light right Wtor am ( to \slippofff Wear am to itrttch 0"d In/trnef tosses Corrttt tension rongfj Increasing boll timlon--^ Soil tension must fall in a narrow range for long life, maximum power transmission Whip dm to Ctntritirf | ford j Centrifugal force robi bolt ^ uieful lood-cortyirtg urangJ Cone pressure against bdi changes with speed variation' iooo 2000 5000 Vorioblt shell spssOt" *P"* Manufocturen' torque rating Is con servative, to protect transmission \ finof tellposition ofttr sloes is lotto up "pulls In" when unit transmits high torque. Speed ratio changos 10 20 50 0 50 60 TO BO 90 % Boise torgui As torque increases, the speed ratio Is chongod, sometimes os much os 7 % 10 20 SO 40 50 60 TO 00 .*0 % Horsvpevsr output Typical pfffclancy values are essen tially (lot'from 50 to 100% output Mechanical Variable-Speed transmissions: IV By W l BYLER, North Wales, Po. To unokrstand operation ol variable, speed transmission#, we need to know the theory behind them. Here ere four fundamentals of the currently popular variable-cone transmission: driving force, speed-torque relationships, unit ratings, efficiency. See port# II and III for commercial applications of this prin ciple in use today. Driving force in any bell system U the difference between tensions in tight and slack aides of the bait. For maximum life'and transmission effectiveness leuaion must be just right, Fig. 1. If belt Is too loose it will slip, increasing wear and power loss. If too tight, extra stress . causes heating, fatigue and wear. Pow er la wasted, belt and (tearing life shortened. Initial tension limits the maximum force that can be transmitted, ao most transmissions incorporate some arrange, ment for u)Dodmg" the belt or chain. The loading doviee can assume any one or a combination of several forms: a spring-loaded variable cone; a springor weight-loaded floating belt tensioner; pressure on belt cleat reinforcements; adjustable base, spring or floating pul ley to increase center distance and odd direct pull. As speed changes, three factors af fect maximum belt tension: (1) arc of contact, sometimes called the angle of wrap (2) centrifugal force (3) pressure of cone faces against belt. Arc of contact affect# both ends of the speed-torque curve. Fig. 5. The smaller ihe arc of contact (less wrap), the lower the friction force between belt end cone or pulley. Pull is reduced. Centrifugal force, Fig. 2, affects only the high-speed end of the speed-torque curve. It varies as the square of speed. Hence, a stress of but one unit of cen trifugal force at BOO rpm would Increase to nine units at 2400 rpm--a speed change of only three to one. Centrifugal force becomes an important (actor at 3000-fpm belt velocity,' and at 5000 (pm it becomes the controlling factor. This force is independent of pulley diameter. It depends entirely on bell velocity. A belt traveling 3000 fpm ever 10-in. pulleys is subject to the same centrifugal force es one traveling 3000 fpm over 20-in. pulleys. There are two ways that centrifugal force is detrimental--first, it Imposes stresses that rob ihe belt of useful loadcarrying strength, and second, it throws belt outward from its normal driving pnth, reducing arc of contact. Full torque cannot be transmitted. Pressure of cone faces against the belt decreases at low speeds, Fig. 3. On some transmissions, movement of the variable^, cones reduces spring compression. 0a- others ihe center distances are modified^ thus lowering belt tension. In either!/ case, torque transmission at low speeds falls off. /f" Spood-torquo relationships. Maximus' output torque. Fig, 5, Is the product ofg varying driving, tension and ehangloij driven-cone radius. At low speeds, Ai\ torque droops due to decreased coni' pressure on bell or to shift of belt pest$ lion. At high speeds, B, centrffapi force causes droop. I Fig. 6 shows performance of a typical^ variable-cone drive (transmission with* motor). At output speeds less than rora^' motor speed, belt limits torque inns', mission. Above full-load motor speed*, motor limits torque. To protect-the transmission sgainil^ heavy storting loads, shock and ow* load, manufacturers rate their machines^ conservatively as shown on the "rated torque" curve. Such ratings create constant-torque illusion. But variable?} speed transmissions are fundamentally' ' torque converters, and-torque can to" crease from rated to pull-out values output speed decreases. 1 Unit ratings. Most commercial Iran* missions'are rated for moderate service a conditions--8 hr per dsy, non-shock,*o ` rwlnftequenl starting and stopping. Next .Jfjirijonth, we'l) discuss a way to evaluate K^p(Whu(actureri* ratings. fe-jN*^Tetts show that wear on bells and X^cdriq face* is fairly constant if drives Ware'rated ss constant-torque machines ft^Jupflo 1:1 speed ratio. Fig. 6, and conh`l*^it*qt-power above 1:1. The machine ' feYdoesn't actually behave os s constant- rh^torque transmission. It Is only rated that t&4$way. -Considering a variable-speed unit maS'Sconstani-torque machine will often &Mead to trouble In the plant.1 SKAlri rating their units, manufacturers Ktfoonslder at least three other, factors: All mechanical variable-speed (trsasmissions are necessarily a design (compromise with limitations like torque 9t|S(id speed sensitivity, limited accuracy V duplication, limited strength VWn.of'the lubricant used. Speed ratio changes due to the kajC^lcttlue transmitted. Delta and chains may y^^vPyll'in to a smaller cone diameter when *B high, Fig. 4, Such deviations reduce operating speed, thus K^V?,T*nting exact speed control. This on output speed variation from 2 below catalog ratings when the ! operated with locked gjontrols, Fig. 7. (yuir.'*?" flsld-type transmission, using cone* an^ tings, increased torque loading may reduce output speeds due to sliding between cone and ring. Driving forces are ordinorily car ried by an oil film between ring and cone--much as a journal la carried on its bearing. When torque demands oro increased beyond some maximum, the oil film ruptures and direct metal-tometal contact occurs. At this peak torque, output speed can actually Tall to zero, so the lorque-ipeed droop can range from 1 or lVk%. when properly applied, to 100% on severe overloading. Overall operating performance is simi lar to the variable-cone type, as shown in Fig. 7. (3) Speed ratio changes due to input speed variations. Under a constanttorque load, a change from low to nor mal Input speed may produce a down ward ratio change. This-is ufiuntly less than % of 1%. Efficiency. Having considered the ma jor variables affecting operation, let's consider five principal losses affecting efficiency: (1) Belt tosses. Robber and cords creute friction os the belt stretches and flexes. With chain drives, there Is flex ing and surface friction of laminations. On ring-nnd-cone drives, friction is due either to excessive pressure or viscosity of the lubricant used. (2) Entering end leaving, losses-- caused by friction between belt end sheave, or chain and radial slotted cone foco. Tlds is not slip. (3) fPiftrfage--created by ribs or fins on cones, velocity of belt, totaling cone carriers, etc. (4) Friction--in bearings, floating hell* tensioner, etc. (5) Miscellaneous--friction of gears or other driving elements, heat losses from rotating parts churning through oil reservoir, etc. TTicse transmission losses are essendally constant. They do not change much with changes in speed or load. Fig. 8 is a generalised efficiency curve for a bell-driven variable-cone drive and an all-metal ring-and-cone drive. It il lustrates the general characteristic pat tern for these machines. Actual values may be somewhat different for a given machine. In fact, they will change for two identical machines doing different jobs or operated at different input speeds. Always review these points carefully with tlw manufacturers before pur chasing or putting a transmission In service. Lower efficiency at lower output is due to constant losses being a larger per centage of power nulpul. This is espe cially true u! low operating speeds. PUNT OPERATION AND MAINTENANCE SECTION POWUjjj VIANT OPERATION ANO MAINTENANCE SECTION 115