Each of the gears a and b has a mass of 675 g and has a radius of gyration of 40 mm, while gear c has a mass of 3. 6 kg and a radius of gyration of 100 mm. Assume that kinetic friction in the bearings of gears a, b, and c produces couples of constant magnitude 0. 15 n·m, 0. 15 n·m, and 0. 3 n·m, respectively. Knowing that the initial angular velocity of gear c is 2000 rpm, determine the time required for the system to come to rest.

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Lanuel

The time required for this system to come to rest is equal to 9.87 seconds.

Given the following data:

  • Mass of gear A = 675 g to kg = 0.675 kg.
  • Radius of gear A = 40 mm to m = 0.04 m.
  • Mass of gear C = 3.6 kg.
  • Radius of gear C = 100 mm to m = 0.1 m.

How to determine the time required?

In order to calculate the time required for this system to come to rest, we would have to determine the moment of inertia for gears A and C.

Mathematically, the moment of inertia for a gear can be calculated by using this formula:

I = mr²

Where:

  • m is the mass.
  • r is the radius.

For gear A, we have:

I = mr²

I = 0.675 × 0.04²

I = 0.675 × 0.0016

I = 1.08 × 10⁻³ kg·m².

For gear C, we have:

I = mr²

I = 3.6 × 0.1²

I = 3.6 × 0.01

I = 0.036 kg·m².

Next, we would convert the initial angular velocity of gear C in revolution per minutes (rpm) to radian per seconds (rad/s) as follows:

ωc₁ = 2000 × 2π/60

ωc₁ = 4000π/60

ωc₁ = 209.44 rad/s.

Also, the initial angular velocity of gears A and B is given by:

ωA₁ = ωB₁ = rc/rA × (ωc₁)

ωA₁ = ωB₁ = 0.15/0.06 × (209.44)

ωA₁ = ωB₁ = 2.5 × (209.44)

ωA₁ = ωB₁ = 523.60 rad/s.

Taking the moment about A, we have:

I_A·ωA₁ + rA∫F_{AC}dt - M(f)_A·t = 0

Substituting the given parameters into the formula, we have;

(1.08 × 10⁻³)·(523.60) + 0.06∫F_{AC}dt - 0.15t = 0

0.15t - 0.06∫F_{AC}dt = 0.56549    ......equation 1.

Similarly, the moment about B is given by:

0.15t - 0.06∫F_{BC}dt = 0.56549    ......equation 2.

Note: Let x = ∫F_{BC}dt + ∫F_{AC}dt

Adding eqn. 1 & eqn. 2, we have:

0.3t - 0.06x = (0.56549) × 2

0.3t - 0.06x = 1.13098      ......equation 3.

Taking the moment about A, we have:

Ic·ωc₁ - rC∫F_{AC}dt - rC∫F_{BC}dt - Mc(f)_A·t = 0

0.036(209.44) - 0.3t - 0.15(∫F_{BC}dt + ∫F_{AC}dt) = 0

0.3t + 0.15x = 7.5398      ......equation 4.

Solving eqn. 3 and eqn. 4 simultaneously, we have:

x = 30.5 Ns.

Time, t = 9.87 seconds.

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