Power Screws

Acme threads are common industrially; square threads are efficient but harder to cut. Collar friction at the thrust face must be included when data are given.

Symbol and sign-convention guide for the equations listed under Key relations & formulas.

Keep SI units consistent end-to-end (do not mix mm with m, or N with kN, in one substitution).

Symbol guide:
• $\boldsymbol{T}$ — raising load, square thread form
• $\boldsymbol{\eta}$ — square thread idealisation
• $\boldsymbol{\sigma_{c}}$ — core/body compressive check

Sign convention: lock the textbook’s positive sense (force, moment, rotation, heat, or flow) before substituting. A correct symbolic setup still earns method marks in most Indian university papers even if arithmetic slips.

Write relations with symbols exactly as in Design of Machine Elements — VB Bhandari before substituting numbers.

State assumptions explicitly before using any relation for power screws — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Machine Design viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Machine Design papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to power screws.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Power Screws appears in shafts, keys, bearings, springs, gears, and fasteners. In Indian mechanical curricula this topic is tested because it connects theory to safe sizing of mechanical components.

GATE and semester exams often combine power screws with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use power screws?" — answer with a lab, mini-project, or plant visit example if possible.

Before attempting power screws problems, confirm you can:

1. Square / Acme / buttress threads trade efficiency vs strength and ease of machining
2. Self-locking screws hold load without a brake when μ > tan α
3. Collar friction often dominates total torque
4. Check buckling for long screws in compression

Revise the solved examples in Design of Machine Elements — VB Bhandari and one previous-year GATE or university paper for this unit.