Week 8 Worm Gear Challenge

OBJECTIVE

1. To perform transient structural analysis on a worm gear assembly.
2. To find out Total Deformation, Equivalent Stress, and Equivalent Elastic Strain in the model.

1. THEORY

1.1 Worm Gear:

Worm gear drives are mechanical drives with skewed axes which consist of a worm and a worm wheel. Worm gears are usually used when large speed reductions are needed. The reduction ratio is determined by the number of starts of the worm and the number of teeth on the worm gear. But worm gears have sliding contact which is quiet but tends to produce heat and have relatively low transmission efficiency.

As for the materials for production, in general, the worm is made of hard metal while the worm wheel is made from relatively soft metal such as aluminium bronze. This is because the number of teeth on the worm wheel is relatively high compared to the worm with its number of starts being usually 1 to 4, by reducing the worm wheel hardness, the friction on the worm teeth is reduced. Another characteristic of worm manufacturing is the need for specialized machines for gear cutting and tooth grinding of worms. The worm gear, on the other hand, maybe made with the hobbing machine used for spur gears. But because of the different tooth shapes, it is not possible to cut several gears at once by stacking the gear blanks as can be done with spur gears.

The applications for worm gears include gearboxes, fishing pole reels, guitar string tuning pegs, and where a delicate speed adjustment by utilizing a large speed reduction is needed. While you can rotate the worm wheel by a worm, it is usually not possible to rotate the worm by using the worm wheel. This is called the self-locking feature. The self-locking feature cannot always be assured and a separate method is recommended for true positive reverse prevention.

2. STEPS FOR PERFORMING ANALYSIS:

1. Open ANSYS workbench.

2. Select the static structural module.

3. Select the proper Engineering data card.

4. Import geometry.

5. Open Mechanical model.

6. Give proper connections (contacts and joints).

7. Generate mesh.

8. Set Analysis Setting as given.

9. Insert output request

10. Analyze Result.

3. ANALYSIS SETUP:

3.1 Geometry:

The given 3D model of worm gear assembly is imported into SpaceClaim. It consists of a worm and worm wheel. To reduce the computational time, the worm wheel is cut by half.

Fig. 3.1 CAD Model

3.2 Meshed Model:

Fig 3.2 (a) Face Sizing

Fig. 3.2 (b) Meshed Model

• Details:

1. Face Sizing = 0.75 mm
2. Global Sizing = 1 mm
3. No of Nodes = 8676
4. No of Elements = 30085

3.3 Material Properties:

Structural Steel:

The material assigned for the worm gear assembly is structural steel.

3.4 PROCEDURE:

(3.4.1) Import model in ANSYS Workbench.

(3.4.2) Assign material properties.

(3.4.3) Open the mechanical module, where we going to perform analysis.

(3.4.4) Meshed the model with desired element size.

(3.4.5) Now after meshing, From Project Tree, select "Connections" and create "CONTACTS"

• Contact Type = Frictionless
• Leave all the settings as it is.

1. Contact Body = Worm
2. target Body = Gear

(3.4.6) Joint Details.

1. Revolute Joint: Ground to Worm

2. Revolute joint: Ground to Gear

To rotate the worm, the revolute type of joint is specified along Z-axis with the connection type being body-ground. The worm wheel is specified with a revolute joint along Z-axis with the connection type being body-ground to follow the rotary motion of the worm.

(3.4.7) Analysis Setting:

Now let's define the Analysis Setting.

(a) Analysis setting for step 1. (b) Analysis setting for step 2 to step 20.

• In the analysis settings, the number of steps considered is 20 and the auto time step is set to ‘On’ which is defined by ‘time’. For step 1, the initial, minimum and maximum time step is considered as 0.2s, 0.2s and 0.5s respectively. For step 2 to step 20, the minimum and maximum time steps are 0.2s and 0.5s respectively. In the solver controls, solver type is chosen as ‘Program controlled’, weak springs are set to ‘Program controlled’ and large deflection is set to ‘On’. Under the output controls, all the required results are set to ‘Yes’.

(3.4.7) BOUNDARY CONDITIONS:

• The joint load is applied to the worm.

• The joint load is applied to the worm to rotate it from 0⁰ to 1000⁰, with an increment of 50⁰ in each step.

(3.4.7) SOLUTION:

After giving proper Boundary conditions, create a solution for the output result.

• Equivalent stress in the model.
• Equivalent Elastic Strain in the model.
• Total deformation in the model.

4. RESULT AND DISCUSSIONS:

1. Total Deformation:

Maximum Deformation = 9.6426 mm

• Deformation Contours:

2. Equivalent v-m Stress:

Maximum Stress = 1354.7 MPa

• Stress Contours:

The maximum value of v-m stress is developed in the teeth of the worm wheel i.e., 1354.7 MPa which is more than the yield stress value of the material i.e., 250 MPa. Hence the teeth of the worm wheel will fail eventually during the service condition.

3. Equivalent Strain:

Maximum Strain = 0.0073735 mm/mm

• Strain Contours:

5. Tabulation of Results:

6. CONCLUSION

1. The transient structural analysis on a worm gear assembly was carried out successfully.
2. The maximum value of total deformation is 9.6425 mm.
3. The maximum value of equivalent stress is developed in the teeth of the worm wheel i.e., 1354.7 MPa.
4. The maximum value of Equivalent Elastic Strain is developed in the teeth of the worm wheel i.e.,0.0073735 mm/mm.
5. The maximum equivalent stress and strain are developed in the teeth of the worm wheel, which will eventually lead to failure hence, it is recommended to change the material of the teeth of the worm wheel.