Limits and fits — definition of a basic shaft: In the ISO system of limits and fits, a basic shaft is defined as a shaft whose

Introduction / Context:
Limits and fits establish interchangeability between mating parts by specifying permissible size variations (tolerances) and positions (deviations) relative to a zero line (basic size). Understanding the definitions of a basic hole and a basic shaft is fundamental to selecting fit systems.

Given Data / Assumptions:

• ISO system with deviation symbols: ES, EI for holes; es, ei for shafts.
• Basic size is the nominal dimension.
• Zero line represents the basic size.

Concept / Approach:
In a basic hole system, the lower deviation of the hole (EI) is zero, so hole tolerance zone starts at the basic size. Conversely, in a basic shaft system, the upper deviation of the shaft (es) is zero, so the shaft tolerance zone ends at the basic size on the upper side. This simplifies tooling and gauging strategies based on whether holes or shafts are standardised.

Step-by-Step Solution:
Recall definitions: Basic hole → EI = 0; Basic shaft → es = 0.Translate to wording: Basic shaft has zero upper deviation.Therefore, the correct statement is that the upper deviation is zero.

Verification / Alternative check:
Look up ISO 286: diagrams show the shaft tolerance zone touching the zero line at its upper limit when using a basic shaft.

Why Other Options Are Wrong:
Lower deviation zero refers to a basic hole, not a basic shaft.

Both deviations zero would imply no tolerance, which is impractical.

Symmetry about zero is not a general rule for basic shaft.

Relative size to the hole depends on the intended fit (clearance, transition, interference), not on the definition of basic shaft.

Common Pitfalls:
Mixing up EI/ES with ei/es; assuming basic shaft means fixed shaft size—only the deviation position is fixed; forgetting that fit selection also depends on manufacturing capability and functional requirements.

Final Answer:
upper deviation is zero