Question

Statement (I): All worm drives (worm and worm wheel) are reversible.

Statement (II): The worm and worm wheel are made of different materials.
1. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
2. Both Statement (I) and Statement (II) are individually true but Statement (II) is not the correct explanation of Statement (I)
3. Statement (I) is true but Statement (II) is false
4. Statement (I) is false but Statement (II) is true

Answer 1

Correct Answer - Option 4 : Statement (I) is false but Statement (II) is true

Whether a worm and worm gear drive is reversible or not depends on the application. When it is used as a drive for the hoist, the unit must be self-locking and driven by the worm. But, when it is used for an automotive drive it should necessarily be reversible and the worm gear should be able to drive the worm. If the lead angle of the worm is greater than the friction angle of the surfaces in contact, the drive will be reversible. The coefficient of friction μ and the friction angle φ are related by the equation μ = tanφ. A worm and worm gear are considered self-locking when the lead angle of the worm is less than 5°.

∴ Statement I) is false.

The worm and the worm gear are made of different materials to minimize the friction and wear leading to less heat generation and producing less noise while in operation. For example, the worm may be made of hardened steel and the gear made of bronze or aluminium. So, statement II) is correct.