Machine classification – behavior of a screw jack A screw jack used for lifting loads typically exhibits which reversibility characteristic under normal design (self-locking) conditions?

Introduction / Context:
Screw jacks convert rotary effort into linear lifting using a power screw. The interplay of helix angle and friction determines whether the device can back-drive under load. Understanding this classification is important for safety and design.

Given Data / Assumptions:

• Standard square/ACME thread with typical friction conditions.
• Helix angle α is small; friction angle φ is relatively large.
• Self-locking is desired for safety (load should not run down when effort is removed).

Concept / Approach:
A machine is non-reversible (self-locking) if the load cannot drive the machine backward to deliver output as input. For a screw jack, the criterion for self-locking is φ > α (or equivalently μ > tan α). Under this condition, back-driving does not occur, and the screw jack is non-reversible.

Step-by-Step Reasoning:

Verification / Alternative check:
The effort to lower a load normally exceeds zero; if friction is sufficiently high, the load will not descend unless an effort is applied in the lowering direction—hallmark of non-reversibility.

Why Other Options Are Wrong:

• a reversible machine: would back-drive easily, unsafe for lifting.
• an ideal machine: would imply zero friction and 100% efficiency, not realistic.
• none of these: unnecessary; the standard term is non-reversible/self-locking.

Common Pitfalls:
Assuming all screws can back-drive. Many power screws are intentionally designed to be self-locking for holding loads without external braking.

Final Answer:
a non-reversible machine