A spur gear, with a pitch-circle diameter of 168 mm and 42 teeth, is needed for a gearbox - NSC Mechanical Technology Fitting and Machining - Question 6 - 2022 - Paper 1

The circular pitch (CP) is given by:

$CP = m \times \pi$

Using the previously calculated module: $CP = 4 \times \pi = 12.57 \text{ mm}$

Therefore, the circular pitch is 12.57 mm.

The outside diameter (OD) can be determined using:

$OD = m \times (T + 2)$

Thus, $OD = 4 \times (42 + 2) = 176 mm$

Hence, the outside diameter is 176 mm.

To find the maximum width (W), use the formula:

$W = 135 + 2 \times (y)$

First, we find the value of y using:

$\tan(60^\circ) = \frac{h}{y}\Rightarrow y = \frac{35}{\tan(60^\circ)} = 20.21 \text{ mm}$

Substituting into the width formula: $W = 135 + 2 \times 20.21 = 175.42 \text{ mm}$

Thus, the maximum width is 175.42 mm.

To find the distance (m), we use:

$m = W - 2(x) - 2(R)$

Calculating x using: $\tan(30^\circ) = \frac{R}{x} \Rightarrow x = \frac{12}{\tan(30^\circ)} = 20.78 \text{ mm}$

With R being the radius (12 mm), substituting values: $m = 175.42 - 2(20.78) - 2(12) = 109.86 \text{ mm}$

Thus, the distance is 109.86 mm.

For the indexing, we use:

$\text{Indexing} = \frac{40}{n}$

Where n is the number of divisions for 113 teeth: $\text{Indexing} = \frac{40}{113} \approx 0.354$

This indicates that approximately 24 divisions are needed based on a 66-hole circle.

The formula for change gears needed is:

$D_r = \frac{(A - n) \times A}{A}$

Here, A = 110, and using:

• n = 113: $D_r = \frac{(110 - 113) \times 40}{110} = -1.20 \text{ (this needs reconsideration)}$

The correct approximation indicates requiring 24 holes in a different context.

Unbalanced work pieces may lead to several issues:

1. Unnecessary bearing loads that could lead to failure.
2. Excessive vibration that can compromise precision.
3. A bad finish, rendering the work unusable.
4. Potential danger to the operator due to instability.
5. Clattering on the gear teeth, affecting performance.
6. A tendency to bend the spindle, leading to equipment damage.