What is reverted gear train?

Short Answer:

A reverted gear train is a special type of compound gear train in which the first and last gears (input and output gears) are on the same shaft or have the same axis of rotation. This arrangement allows the input and output shafts to rotate about the same center. Reverted gear trains are used where space is limited and a large speed reduction is required in a compact design.

In this system, the intermediate gears (idler gears) are placed between the driver and driven gears to transmit motion and maintain the same rotational axis. Reverted gear trains are commonly used in clocks, automobile gearboxes, and mechanical devices where compactness and alignment are important.

Detailed Explanation :

Reverted Gear Train

A reverted gear train is a type of compound gear system where the axes of the first and last gears are co-axial, meaning they lie on the same line or share the same center. This arrangement is designed to achieve high speed reduction within a limited space, making it efficient and compact. It is commonly found in precision mechanical devices such as clocks, gearboxes, and measuring instruments.

In a reverted gear train, multiple gears are mounted on two or more shafts. The driver gear (first gear) meshes with an intermediate gear, which then meshes with the driven gear (last gear). However, the center distance between the first and second pair of meshing gears is equal, ensuring that the first and last gears rotate around the same axis. This unique feature distinguishes it from other types of compound gear trains.

Construction

A reverted gear train typically consists of four gears:

  1. Driver gear (A) – This is the input gear connected to the driving shaft.
  2. Driven gear (D) – This is the output gear connected to the driven shaft.
  3. Intermediate gears (B and C) – These are mounted on a common shaft and transmit motion between the driver and driven gears.

The arrangement is made such that:

  • Gear A meshes with gear B.
  • Gear C meshes with gear D.
  • The shafts carrying A and D are in one line (same axis).

This ensures that the input and output shafts are aligned, making the system compact and mechanically balanced.

Working Principle

When the driver gear (A) rotates, it transfers motion to the intermediate gear (B). Since B and C are fixed on the same shaft, they rotate together as a unit. Gear C then drives the final gear (D), producing the desired output motion.
The direction of rotation of the output gear depends on the number of intermediate gears used. If an even number of intermediate gears is present, the input and output gears rotate in the same direction; if odd, they rotate in opposite directions.

The speed ratio of the reverted gear train is the product of the speed ratios of each gear pair:

where  are the number of teeth on gears A, B, C, and D respectively.

This setup allows for a large reduction ratio without increasing the overall length of the gear system.

Advantages

  1. Compact Design: Since the input and output shafts are co-axial, the system occupies less space.
  2. High Speed Reduction: It provides a large velocity ratio in a small arrangement.
  3. Smooth Operation: Motion transmission is smooth due to accurate gear engagement.
  4. Better Alignment: Co-axial arrangement ensures better mechanical balance and reduced vibration.
  5. Efficient Power Transmission: Useful in applications where precision and constant speed are required.

Applications

Reverted gear trains are widely used in:

  • Clocks and watches: To achieve precise timekeeping with compact gear arrangements.
  • Automobile gearboxes: For maintaining compact and efficient power transmission systems.
  • Lathe machines: To achieve different spindle speeds using compact gear arrangements.
  • Speed reducers and multipliers: In devices requiring significant speed changes within limited space.
  • Mechanical instruments: Such as speedometers and measuring devices for accurate transmission.

Example

Consider a clock mechanism. In this system, the minute and hour hands rotate around the same axis, but at different speeds. The reverted gear train is ideal here because it allows the input (minute hand gear) and output (hour hand gear) to be co-axial, while achieving the necessary speed reduction ratio of 12:1.

Key Points

  • A reverted gear train is a type of compound gear train.
  • It has input and output gears on the same axis.
  • It provides large speed reduction in limited space.
  • Commonly used in precision instruments and compact mechanical systems.
Conclusion

A reverted gear train is an efficient and compact arrangement where the first and last gears share the same axis of rotation. Its design allows for high speed reduction and accurate motion transmission in minimal space. Due to its co-axial nature, it ensures better alignment, balance, and smooth operation. Hence, reverted gear trains play a vital role in mechanical systems like clocks, gearboxes, and other compact mechanisms requiring efficient and precise motion control.