Chain Grades and Safety Factors Overview

Chain Grades

• Original BS grade 40 used at a safety factor of 5:1, either normalized or hardened and tempered.
• Mark 04 for normalized, 40 for hardened and tempered.
• Later, the safety factor was reduced to 4:1, necessitating hardened and tempered chain, with grade M used for distinction.
• All three grades have the same breaking strength, but heat treatment and ratings vary.
• Letters and numbers became interchangeable (M4, S6, T8).
• Number grades for medium-tolerance chain in slings, letter grades for fine-tolerance chain in hoists.
• Medium and fine tolerance terms introduced.
• All machine-made chain is calibrated; the difference is in accuracy.
• European separation of number and letter grades adopted in ISO.
• Hand-operated hoists use through-hardened chain (grades T, DAT, DT.)
• Power operation uses surface hardening for wear (grades TH and VH).
• Chain grades for components other than chain are defined by compatibility, not always strict stress levels.
• Manufacturers determine component dimensions, influencing stress levels.

Basic Machines

• A machine is a mechanism applying force at one point to transmit force to another for a specific purpose (gaining mechanical advantage).
• Machines are categorized into simple and compound machines.

Simple Machines

• Lever: a pivot point that multiplies applied force.
• Wheel and axle: a rod and wheel that multiplies force.
• Pulley: a wheel with a rope that changes force direction.
• Inclined plane: a flat surface angled ends for reduced force.
• Wedge: a triangular shape used to separate, hold, or lift objects.
• Screw: a cylindrical shaft with grooves that moves objects through rotational force.

Compound Machines

• A collection of simple machines.
• More complex work than individual simple machines.
• Provides a greater advantage than simple machines.
• Examples include mobile cranes (jib/mast, pulleys, screws (limit switch), wheel-axles (drive train wheels)).

Weight and Force

• Weight and force are equal, similar units.
• Lifting machines use a small effort to lift a larger load.
• The force required for lifting is called effort, and the force being lifted is called load.
• Formula: Work = Force x Distance
• Moment of force/turning moment (TM): turning effect of a force applied to a lever at a distance from a pivot point.
• Formula: Turning Moment = Force x Distance

Mechanical Advantage (MA)

-Equal effort and load is a state of equilibrium (no movement).

• An increasing amount of effort will move the load.
• Calculated as Load/Effort ratio.
• Example: Using winches—the more lines of cable between the winch and the vehicle, the heavier the load that can be pulled and towed as well.

Velocity Ratio (VR)

• Relationship between distances moved by effort and load.
• The effort needs to move a greater distance to move the load a shorter distance.
• Calculated as: Distance moved by effort/Distance moved by load.

Efficiency (EFF)

• Machines designed to waste little energy to transfer useful energy.
• Indicator of energy transfer effectiveness.
• Formula: Efficiency = MA/VR x 100%

Polymers and Natural Fibres

• Natural Polymers: Examples include shellac, wool, silk, natural rubber. Cellulose (wood and paper).
• Synthetic Polymers: Examples include synthetic rubber, resin, nylon, PVC, polypropylene, polyamide, polyester, high-modulus polyethylene.

Heat Treatment

• Metals heated and cooled to change microstructure, creating desirable physical/mechanical properties to suit a specific application.
• Processes can increase or decrease strength, complete/surface harden, toughen, relieve stresses, or refine grain structure.
• Three stages: heating, soaking, cooling.
• Methods include hardening and tempering.
• Properties that can be changed include toughness, brittleness, ductility, hardness.

Stress and Strain

• Forces act on lifting equipment under loaded conditions; strength is a related important property.
• Relating force to cross-sectional area to cause stress.
• Calculate stress by force divided by area
• When a load (force) is applied, a material will change shape (strain).
• Example: stretching an elastic band.

Tensile Test

• Fundamental mechanical test.
• Provides information and quantifies material properties.
• Important for understanding material specifications for lifting equipment.
• Specimen of determined size (round, square, or rectangular).
• Load applied until fracture; load/elongation curves are plotted.
• Determine elastic limit, yield point, ultimate tensile strength, and elongation/reduction in cross-sectional area.
• Local necking: a reduction in cross-sectional area under tensile load exceeding the maximum tensile strength.

Shear, Tension, and Compression

• Single shear: forces acting across a material (e.g., lifting lug).
• Double shear: forces acting across a material in two areas (e.g., a shackle pin).
• Compression: a pushing force (e.g., a jack body).
• Tension: a pulling force (e.g., a chain sling).
• Torsion: a twisting force (e.g., a rotating gearbox shaft).