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Difficulty: EasyFriction and Resistance

A mechanic is servicing a high-speed gearbox and must choose a lubricant. If a heavy, thick grease is used instead of a lightweight oil, the gearbox will require more mechanical power to maintain its high rotational speeds. What is the primary cause of this increased resistance?

  1. The greater viscosity of the thick grease creates higher fluid friction against the moving gears.Answer
  2. B
    The layer of grease increases the total surface area of the gears, which directly multiplies the kinetic friction.
  3. C
    The thick grease adds extra mass to the assembly, and this added weight is what strongly resists the rotational motion.
  4. D
    The thick grease absorbs all reaction forces between the gear teeth, neutralizing the inertia of the system.

Answer

The greater viscosity of the thick grease creates higher fluid friction against the moving gears.
Thick grease has a high viscosity, meaning it strongly resists flow and deformation. When gears rotate at high speeds through this thick substance, they must continuously shear and displace the grease. This creates substantial fluid friction (drag), which opposes the motion of the gears and requires the system to draw more mechanical power to maintain its speed.

Step-by-Step Solution

1
Identify the type of resistance introduced when machine parts move through a liquid or semi-solid substance.
The resistance is fluid friction, also known as drag or viscous resistance.
Because the gears are completely coated by a lubricant, they must continuously push the fluid out of the way as they spin.
2
Compare the physical properties of a lightweight oil and a thick grease.
Thick grease has a much higher viscosity than lightweight oil.
Viscosity is the measure of a fluid's internal resistance to flow and deformation.
3
Determine how this viscosity affects high-speed mechanical operation.
Higher viscosity results in greater fluid friction against the fast-moving gears, requiring more energy from the motor to maintain speed.
Moving through a thicker, more resistant substance requires more force than moving through a thinner one.

Key Concept

Fluid Friction and Viscosity
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Topics

Friction and Resistance

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