Worm gear design

RTMNU 8th semester Design of mechanical drives

1. Worm and Worm Wheel Design -Prof. Satish Sonwane , Asst. Prof. (ME), DBACER, Nagpur

2. Introduction Worm gears are used for transmitting power between two non-parallel, non-intersecting shafts.

3. Geometry and nomenclature

4. Applications Applications where fast braking or stopping is desired — Lifts, elevators and hoists Applications where noise is a concern — Theatres, airports and elevators Applications where space is a concern — Packaging equipment, small machinery and conveyors Applications with shock loading — Rock crushers and other heavy duty machinery

5. Points to note The pitch diameter of a worm is not a function of its number of threads, Z. This means that the velocity ratio of a worm gear set is determined by the ratio of gear teeth to worm threads; it is not equal to the ratio of gear and worm diameters.

6. Advantages High speed reduction. More compact as compared to spur or helical gear drives for same value of speed reduction Operation is smooth and silent Provision can be made for self-locking. (i.e. motion is transmitted only from worm to wheel) Disadvantages Lower efficiency Costlier Considerable heat generation

7. Problem Statement: A worm gear drive is used to transmit 22 kW between two shafts, which are 225 mm apart. The transmission ratio is 24:1. Determine, Proportions of worm and worm gear Design the worm gear, if it is made of Phosphor bronze

8. Design Steps Select number of teeth on worm. [T-XVI-16(1)] And find number of teeth on wheel by Transmission ratio. [T-XVI-16(3)] Select lead angle λ and Pressure angle Φ [T-XVI-16(2)&(5)].Check for compact design and adjust the value. [T-XVI-16(5)]. Find pitch diameter and pitch line velocity of worm gear Find Design power and tangential tooth load Ft (in terms of module m) [T-XVI-15(1)&(2)] and [T-XI-5(2)] Find Beam Strength of gear F B (In terms of module m)by Lewis Equation. [T-XVI-15(3)], [T-XVI-10(9)] and[T-XVI-19]. Use 0.75 as trial value for Cv.

9. Design Steps (Continued..) Equate Ft and F B to find module. Select standard value of module from [T-XVI-7]. Calculate values of Diameter of wheel, Pitch line velocity, Tooth load and Velocity Factor and Beam strength from the value of module. Check F t <F B. Find Pitch diameter of Worm ( D w ) [T-XVI-19(2)]. Calculate dynamic load on gear F d = (F t / C v ’ ) . Calculate Limiting wear strength F w . [T-XVI-17(2)]. Check F w > F d . Calculate rubbing velocity V R . And co-efficient of friction μ . [T-XVI-16(7)] and [T-XVI-16(6)].

10. Design Steps (Continued..) Calculate efficiency of drive [T-XVI-15(5)]. Calculate heat generated and dissipated. [T-XVI-15(6)], [T-XVI-15(7)] and [T-XVI-19(4)]. Equate Q 1 and Q 2 to calculate rise in temperature. Write dimensions of Worm and Worm wheel.

11. THANK YOU…!