An efficient design of the gearbox is crucial for the expected performance of the vehicle both in terms of life and NVH (Noise, Vibrations and Harshness). This involves design and analysis of gears, shafts, bearings, gear layout and speed ratios. Conventionally gears, shafts and bearings are designed and analyzed independently. When the design of these parts change, their effect on related parts is estimated separately, leading to loss of time. The target load cycle, gear and bearing lives, shaft deflections, and stresses are estimated. The targets for stresses, deflections and lives are obtained logically, mathematically and also with experience, these are also compared with those of reference missile by creating and analyzing reference gearbox model. Gearbox is optimized both in terms of compactness and durability requirements apart from achieving maximum shaft stiffness and contact ratio for low noise. The prototype would be made as per optimized design and should pass all the required tests confirming to the results of simulation. Thus the new approach should lead to considerable reduction in time and cost, at the same time maintaining quality.

Applications:

This Gear box can be used in all kinds of Missiles such as:

· Air-to-air missile

· Air-to-surface missile

· Anti-ballistic missile

· Anti-satellite weapon

· Anti-ship missile

· Land-attack missile

· Anti-tank guided missile

· Surface-to-air missile

· Surface-to-surface missile

· Wire-guided missile

Working of the Gearbox:

The Power transmission from the motor shaft (input shaft) to the output shaft takes in following manner. The gearbox has an epicyclic gear train for the transmission of power.

This has three stages of speed reduction for the effective speed reduction and torque conversion. This epicyclic gear train has a set of three sun gears, two carriers, six planetary gears and a sector gear.

In the first stage of speed reduction the Motor shaft running at 6840 rpm has a sun gear-1 press fit to it. The sun gear-1 meshes with three planetary gears (1, 2 & 3) and these planetary gears meshes with an annual ring gear for effective speed reduction; this completes the first stage of speed reduction.

In the second stage of speed reduction the planetary gears are connected to a carrier (carrier-1) with the help of connectors thus the carrier runs at the speed of the planetary gears 1,2 & 3 another sun gear(sun gear-2) is press fit to the carrier therefore it revolves with the speed of carrier. Similarly as above the sun gear-2 meshes with three other planetary gears (4, 5 & 6) and these planetary gears again meshes with an annual ring gear for effective speed reduction, this completes the second stage of speed reduction.

In the third and final stage of speed reduction the planetary gears 4,5 & 6 are connected to another carrier(carrier-2) thus the carrier runs at the speed of the planetary gears 4,5 & 6 another sun gear(sun gear-3) is press fit to the carrier therefore it revolves with the speed of carrier.

This sun gear-3 meshes with a sector gear which has got only fewer teeth cut on it; this completes the third & final stage of speed reduction, the sector gear is fixed on the output shaft.

The desired speed and torque are obtained at the output shaft.

Project Description

Initial conditions given for the design of the gear box

1. Input Speed: 6840 rpm

2. Input Torque: 0.4 N-m

3. Space provided for the gear box: 91 X 60 X 49 mm

4. Distance between I/P & O/P Shafts: 43mm

Desired conditions at the output shaft

1. Output Speed Ratio: 1: (100 to 106)

2. Required Torque: 40 N-m

3. Factor of Safety: ≥ 2

4. Contact Ratio: ≥ 1.2

5. Backlash : <15>

Description of the Problem

The Problems during Design And Testing of the required Gear Box of the Missile:

Ø Problem in achieving FACTOR OF SAFETY (≥ 2) in the second stage of speed reduction itself and got 1.8 and 0.89 as FACTOR OF SAFETY in the 2nd and 3rd Stage respectively.

Ø Problem in accommodating the gears of required factor of safety in the space provided

Ø As the Factor of Safety was <>

Ø The space provided for the gear box was very small (91 x 60 x 49 mm )

Ø Enormous amount of speed reduction was required from an input speed of 6840 rpm to 66.67 rpm.

Ø It should generate a torque of 40 N-m with an input torque of 0.4 N-m.

Approach

As per the above problems we have decided to implement following ideas in the design of gear box for achieving desired outputs:

· We decided to choose Epicyclic Gear Train instead of the other three for the effective gear ratio.

· 20º Stub tooth profile was used instead of 20º Full depth in order to achieve.

· We had no choice for the selection of material as they suggested using Maraging Steel.

· We used three stages of speed reduction in order to achieve required Torque & Speed.

Participant:1
Hemanth Kumar
Sathyabama University
Chennai