The IS 939:1977 standard outlines the design, materials, construction, and performance specifications for snatch blocks used with fibre ropes in firefighting applications. It applies to snatch blocks with a maximum safe working load of 4900 N (500 kgf), ensuring dependable and safe operation during lifting and hauling tasks in fire brigade and civil defense activities. This code is vital for manufacturers, inspectors, and users to maintain compliance with Indian safety and quality standards.
Overview
The IS 939:1977 standard outlines the design, materials, construction, and performance specifications for snatch blocks used with fibre ropes in firefighting applications. It applies to snatch blocks with a maximum safe working load of 4900 N (500 kgf), ensuring dependable and safe operation during lifting and hauling tasks in fire brigade and civil defense activities. This code is vital for manufacturers, inspectors, and users to maintain compliance with Indian safety and quality standards.
Audience
Contents
Structure
This section details the specifications for snatch blocks employed in lifting and rigging operations, particularly for fire brigade use, ensuring adherence to quality, inspection, and testing requirements under ISI certification. It also explains the marking requirements, including the optional ISI Certification Mark, and the adoption of SI units for all measurements, with appropriate conversions and rounding rules as per IS: 2-1960.
Defines the technical terms and units used throughout the standard to ensure consistent understanding. It includes SI units for quantities such as length, mass, force, and pressure, alongside their symbols and conversion factors.
Describes the constituent parts of the snatch block, such as swivel hooks, cross-heads, side plates, sheaves, pins, and hinges, along with the material standards they must conform to. It emphasizes the material quality and construction methods required for durability and safety.
Specifies the materials to be used for different snatch block components, referencing relevant IS codes to ensure mechanical strength and reliability. The section also reiterates the use of SI units and standard rounding procedures for measurements.
Covers the geometric and design criteria for key parts such as the swivel hook and sheave, including shape, threading, groove dimensions, and pin specifications. It also outlines acceptable materials and mandates compliance with stipulated IS standards.
Details the performance expectations for snatch blocks, including proof load testing at four times the safe working load for a duration of five minutes. It highlights quality control measures and the significance of ISI certification marks as evidence of compliance.
Specifies the mandatory markings on snatch blocks, such as proof load, nominal size or safe working load, inspection authority seals with test dates, manufacturer identification, and optional ISI certification marks to ensure traceability and conformity.
Explains the inspection procedures and quality control mechanisms under ISI supervision, including the importance of continuous testing, measurement in SI units, and adherence to rounding conventions to maintain product integrity.
Frequently Asked
As per IS 939 Clause 4.1, different components of the snatch block must be made from specified materials to ensure strength and durability. The swivel hook and hinges require steel forgings complying with IS 4367-1967, while the cross-head and pins use wrought steel or steel forgings per IS 1570-1961. Side straps and plates are made from steel plates as per IS 226-1975. The sheave is constructed from Grade 25 iron casting conforming to IS 210-1970, and distance pieces or becket parts are crafted from steel forging or malleable iron casting following IS 4367-1967. These material choices provide robustness to withstand operational loads and maintain safety during use.
IS 939 defines the safe working load (SWL) as the highest permissible load on the rope at one end of a single sheave block, but it does not prescribe a fixed maximum SWL. Instead, the maximum load depends on the block’s design, dimensions, and materials used, which are standardized by the code. The snatch block must undergo a proof load test at four times its SWL for five minutes to verify safety. Therefore, SWL is determined based on these factors, ensuring the block can safely handle loads up to 4900 N (500 kgf) as per the standard’s scope.
According to IS 939 Clause 6.1, snatch blocks must be subjected to a proof load test where a load equal to four times the safe working load (SWL) is applied and maintained for five minutes. The block must withstand this load without permanent deformation or failure. Following successful testing, marking must be applied on the strap indicating the proof load, nominal size (SWL), the inspecting authority’s seal with the test date, and the manufacturer’s identification. This process ensures the snatch block meets the performance and safety requirements for use with fibre ropes.
IS 939 Clause 7.1 mandates that all compliant snatch blocks bear clear markings on their straps including the proof load expressed in newtons (N), the nominal size or safe working load (SWL) also in newtons, the seal of the inspecting authority along with the date of the test, and the manufacturer’s identification mark. Additionally, the snatch block may optionally display the ISI Certification Mark as per Clause 7.1.1, signifying adherence to Indian Standards and quality assurance.
Yes, IS 939 Clause 5.1 outlines precise design requirements for these parts. The swivel hook must be a one-piece trapezoidal forged hook with a threaded shank fitted with a screw collar or nut, ensuring the collar or nut’s depth is at least equal to the full diameter of the shank and the engagement length on the load side is at least two-thirds of the shank diameter. The sheave must be integrally cast with a central boss width 2 to 3 mm greater than the rim width. Its rope groove depth must be at least one-third of the rope diameter, with a circular arc profile radius of 15 mm and rounded edges. The axle pin diameter must exceed the bearing portion diameter by at least 3 mm, with a projecting tail end to secure a split cotter pin. Materials must conform to IS 4367-1967 for steel forgings (hook) and IS 210-1970 Grade 25 iron casting (sheave), ensuring robustness and longevity.
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