1. EMD223 Machine Component Design Dr. Mohamad Yusof Idroas Email: meyusof@eng.usm.mymeyusof@eng.usm.my Room: 3.10 (SoME) EMD223 Machine Component Design Dr. Mohamad Yusof Idroas Email: meyusof@eng.usm.mymeyusof@eng.usm.my Room: 3.10 (SoME)

2. Assignment #2 Already uploaded in eLearning on15 May 2012 (Tue) Due Date: 21 May 2012 (Mon) EMD223

3. Draft report submission 21 May 2012 up to discussion of results from public survey & experts/experienced personnel Refer to email for final report format Final report submission + CD 28 May 2012 Late submission will be penalized (marks deduction) SHE101

4. Miscellaneous Power Transmission Components

5. Introduction Power transmission between shafts can be accomplished in variety of ways. In addition to gears, FLEXIBLE elements such as belts and chains are in common use. The use of flexible elements allow power transmission between shafts that are separated by a considerable distance. Also provide the engineer with greater flexibility in the relative placement of driving and driven machinery.

6. Introduction BELTS VS. CHAINS

7. Introduction FLAT BELTS Used to transmit power between shafts by means of a belt connecting pulleys on the shafts. Large flat leather belts commonly used on large motor or engine to drive several pieces of machinery. Latest of thin & light flat belts used to drive high-speed machines. Large flat leather belts Source: practicalmachinist.com High efficiency Flat Transmission Belts Source: directindustry.com

8. Design Analysis Basic equations for the limiting torque that can be transmitted by a flat belt are the same as for band brake torque: where P 1 and P 2 are tight and slack belt tensions, f is coefficient of friction, and ϕ is angle of contact or wrap with the pulley. The initial belt tension ( P i ) depends on the elastic characteristics of the belt and it is approximated by: and

9. Design Analysis CONTROL OF BELT TENSION The capacity of belt drive is influenced by angle of wrap ( ϕ ) and distance in between pulleys of greatly differing size (esp. if positioned closely). Thus, the initial belt tension is critical to be adjusted to prevent an overload to the bearings and shafts, and shorten the belt life (if the initial tension is excessive). THREE methods of controlling and maintaining the belt tensions as shown in Figure 1:

10. Design Analysis Figure 1: Alternative means of maintaining desired belt tension

11. Design Analysis CONTROL OF BELT TENSION (cont.) The belt tension is also influenced by the material of belt that reflects to the coefficient of friction ( f ) in between belt and pulley. For leather belting and cast-iron or steel pulleys, f = 0.3. For rubber-coated belting usually gives a lower value, f = 0.25. Running the belt on plastic pulleys will usually gives a slightly higher f value. The allowable value of tight-side tension P 1 depends on the belt cross section and the strength of the material.

12. Design Analysis For high-power transmitting capacity, most belt drives operate at relatively high speeds. The centrifugal force acting on the belt creates a tension Pc of: where m’ is the mass per unit length of belt, V is the belt velocity and r is the pulley radius. Force P c should be added to both P 1 and P 2 gives:

13. Introduction V-BELTS Used with electric motors to drive blowers, compressors, appliances, machine tools, industrial machinery etc. One or more V-belts are used to drive the accessories on automotive, mostly IC engines. Main advantage: V-belts do no require frequent adjustment of initial tension because they have resistance to stretch of their interior tension cords. V-belts are made to standard lengths and with standard cross-sectional sizes (see Figure 2). The groove pulleys that V-belts run in are called sheaves and they are usually made of cast iron, pressed steel or die cast metal.

14. Design Analysis Typical belt section & groove geometry Figure 2: Standard cross sections of V-belts. All belts have a rubber-impregnated fabric jacket with interior tension cords above a rubber cushion.

15. Design Analysis MULTIPLE V-BELTS Multiple V-belts may be used if single belt has insufficient capacity. As many as 12 or more belts are commonly used in heavy-duty applications. Multiple V-belts should come in matched sets, so that the load is divided equally. When one belt needs replacement, a complete new set should be installed. Figure 3: Multiple V-belt drive. (Courtesy Reliance Electric Company)

16. Design Analysis Figure 4 shows how a V-belt rides on the sheave groove with contact on the sides and clearance at the bottom. This “wedging action” increases the normal force on a belt element from dN to dN/sin β, which is approximately equal to 3.25 dN. Figure 4: V-belt in sheave groove and on flat pulley rim

17. Design Analysis The flat-belt equations can be modified with respect to the increase of normal force by replacing the coefficient of friction, f with f / sin β and becomes: In general, it is recommended that belt speeds in the range of 20 m/s (4000 ft/min) be used where feasible. V-belt life is affected by the elevated temperature. The belt life can be improved by having fins on the sheave to increase air circulation when the belt temperature (conventional belts) exceeds 93 o C.

18. Figure 5: Various types and standard cross-sectional sizes of belts Design Analysis

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