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Question

Difficulty: MediumThermal Expansion and Contraction

A technician must remove a stuck brass bushing from inside a solid steel housing. The coefficient of thermal expansion for brass is greater than that of steel, meaning brass expands and contracts more for any given temperature change. Which of the following thermal procedures would be most effective to loosen the tight fit?

  1. Cool the entire assembly uniformlyAnswer
  2. B
    Heat the entire assembly uniformly
  3. C
    Apply heat directly to the brass bushing
  4. D
    Apply a cooling agent exclusively to the steel housing

Answer

Cool the entire assembly uniformly.
Because the brass bushing has a higher coefficient of thermal contraction, it will shrink at a faster rate than the steel housing when both are cooled. As the temperature drops, the outer diameter of the brass bushing decreases more significantly than the inner diameter of the steel hole, which opens up clearance between the two parts and loosens the fit.

Step-by-Step Solution

1
Analyze the mechanical relationship between the components.
The brass bushing is the inner component, and the steel housing is the outer component. To loosen the fit, the outer diameter of the inner component must become smaller relative to the inner diameter of the outer component.
Identifying the relative positions of the parts dictates whether expansion or contraction is needed to create clearance.
2
Evaluate the effects of uniform temperature changes based on material properties.
Since brass has a higher coefficient of thermal expansion than steel, a uniform temperature increase will cause the brass to expand more than the steel. A uniform temperature decrease will cause the brass to contract more than the steel.
Different materials change size at different rates when subjected to the same temperature variations.
3
Determine which thermal treatment creates the desired clearance.
Heating uniformly makes the inner brass part expand more than the outer steel hole, tightening the fit. Cooling uniformly makes the inner brass part shrink more than the outer steel hole, loosening the fit.
Selecting the condition that reduces the inner part's size more rapidly than the outer part's size provides the mechanical clearance needed for removal.

Key Concept

Differential Thermal Expansion and Contraction
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Topics

Thermal Expansion and Contraction

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