Sample Answer
Design Report: Mechanical Screw Jack Assembly
Introduction
Machine design is the systematic process of converting functional requirements into a working mechanical system that is safe, economical, and manufacturable. In modern engineering practice, this process involves not only analytical calculations but also virtual prototyping using CAD tools, material selection, tolerance specification, and consideration of manufacturing methods.
This design report presents the design of a bench-mounted mechanical screw jack assembly capable of lifting a maximum load of 20 kN. The report includes component selection, design calculations, material justification, manufacturing considerations, and assembly overview. The design has been developed to be suitable for production using conventional machining processes such as CNC turning and milling.
The objective of this project is to demonstrate understanding of machine design principles, manufacturability, robustness, and effective technical reporting, in line with the stated course learning outcomes.
Machine Description and Design Procedure
The mechanical screw jack converts rotary motion into linear motion to lift or lower loads. It consists of a power screw, nut, base, lifting head, handle, and support housing. The design procedure followed standard machine design methodology, starting with load estimation, followed by material selection, stress analysis, dimensioning, and verification against failure modes.
The screw jack is designed for intermittent manual operation in workshop and maintenance environments. Key design constraints included safety, ease of manufacture, durability, and cost efficiency.
Load Assumptions and Design Parameters
The maximum lifting load was set at 20 kN. A factor of safety of 2.5 was applied to account for dynamic effects and misuse. The screw is assumed to operate under axial compressive loading with friction at the threads.
Design assumptions include:
The load is applied centrally.
Manual operation through a detachable handle.
Ambient operating conditions without corrosive exposure.
Design of Power Screw
A square-thread power screw was selected due to its high efficiency and suitability for power transmission.
Material selected: Medium carbon steel (EN8).
Ultimate tensile strength: approximately 550 MPa.
Allowable compressive stress used: 110 MPa.
Based on axial load calculations, the core diameter of the screw was calculated as 24 mm. The nominal diameter was selected as 30 mm with a pitch of 6 mm to ensure smooth operation and self-locking characteristics.
Buckling was checked using Euler’s formula, confirming that the screw length-to-diameter ratio was within safe limits for compressive loading.
Nut Design
The nut was designed using phosphor bronze to reduce friction and wear between mating threads. This material choice also improves service life and reduces the risk of seizure.
Thread bearing pressure was calculated and maintained below permissible limits. The nut length was chosen as 1.5 times the screw diameter to ensure adequate load distribution across threads.
Gearless Drive and Handle
The handle was designed to provide sufficient torque for manual operation. Mild steel was selected for the handle due to its toughness and ease of fabrication.
Ergonomic considerations were applied to handle length to ensure that the required torque could be applied without excessive user effort.
Base and Housing
The base supports the entire assembly and transfers load to the ground. Cast iron was selected for the base due to its compressive strength, vibration damping, and low cost.
The housing was designed to accommodate the nut and provide alignment for the screw. CNC milling was proposed for manufacturing to ensure dimensional accuracy.
Manufacturing Considerations
The screw and handle can be manufactured using CNC turning operations. Threads can be cut using a single-point tool or thread rolling for improved surface finish and strength.
The base and housing can be cast and then finish-machined to achieve required tolerances. Geometric tolerances such as concentricity and perpendicularity were specified to ensure smooth assembly and operation.
Surface finish values were selected based on functional requirements, with finer finishes applied to sliding surfaces.
