advantages and disadvantages of manual and automated safety

Advantages and Disadvantages of manual and automated safety
INTRODUCTION 1 this document contains internal guidance which has been made available to the public. This guidance is considered good practice (rather than compulsory) but you may find it useful in deciding what you need to do to comply with the law. The guidance may not be applicable in all circumstances and any queries should be directed to the appropriate enforcing authority. It is particularly aimed at users of the equipment. However, designers, manufacturers and refurbishers may also find it useful. EQUIPMENT DESCRIPTION AND OPERATION 2 The welding process involves a sequence of operations which includes presenting the workpiece to the welding device (or vice versa), creating relative motion between the workpiece and the welding device, controlling the welding process, eg speed, direction, current, etc, and re-positioning the workpiece for the next weld. For the purposes of this document, automated welding is defined as automatically controlling the relative movement between the welding head and workpiece. Automatic welding is used with variety of welding process including metal inert gas (MIG), tungsten inert gas (TIG) and submerged arc welding (SAW). 3 The main benefits attributed to automated welding are improved economy derived from faster cycle times and consistent quality due to reduced level of weld defects from a more precise control of the welding process. On the down side, welding geometry tends to be restricted to simple straight lines, and setting time is increased. 4 Automated welding equipment may be divided into 2 types: (1) the welding head moves over a stationary workpiece; or (2) the workpiece is moved under a stationary welding head. Moving head devices 5 These include column and boom manipulators and industrial robots. (This document does not deal with industrial robots, guidance on the safeguarding of industrial robots is given in HSG43 Industrial robot safety). As its name implies, a column and boom manipulator consists of a welding head fastened to the end of a horizontal boom supported on a vertical column. The height of the boom is adjusted to position the welding head directly above the workpiece. The boom extends or retracts to enable the welding head to traverse at a controlled speed along the workpiece. Typically, column and boom manipulators are used to weld the longitudinal seams of cylindrical vessels. The head can be positioned both externally and internally to accommodate welding from both sides of the weld. A typical installation is shown at Appendix 1 Figure 1. When used with a device to rotate the workpiece, a column and boom can be used to complete the circumferential weld of a cylindrical vessel as shown at Appendix 1 Figure 2. Column and boom range in sizes often reaching several metres in height. Workpiece moving devices 6 Appendix 1 Figure 2 depicts a typical turntable positioner. This positioner consists of a chassis and a rotating turntable, that can be swivelled from the horizontal position to the vertical. When the turntable is arranged to rotate about a horizontal axis, the welding head is normally located at the 12 o'clock position. During the production of a circumferential weld, this corresponds with flat welding. For fillet welds, the rotational axis is often deployed at 45°. Turntables are also used during manual metal arc (MMA) welding to position the workpiece in a convenient position for manual welding. 7 Another method of rotating the workpiece is to use rotators or turning rolls. Rotators consist of at least 2 sets of rollers, one of which is driven and the other is an idler. They are used for circumferential welding of large diameter cylindrical workpieces. Care must be taken to avoid any tendency for the workpiece to move axially. A typical rotator arrangement shown in conjunction with a column and boom is shown at Appendix 1 Figure 3. Rotators can be self aligning or conventional. Safeguards 8 The dangerous parts of all equipment listed in paragraphs 4-7 should be safeguarded where practicable to prevent injury, the term ‘safeguarding’ to include both guards and devices such as photoelectric (PE) guards and hold-to-run devices. In addition to the safety of the machine operator, employers should ensure that those persons engaged during setting and maintenance work are suitably safeguarded. Motors are usually fitted with overload protection such that in the event of the component being too heavy for the machine, the motor will cut out. Alternatively, some manufacturers protect the machine using a shear pin. 9 This document does not deal with dangerous parts of the equipment in detail. Advice on specific safeguarding methods is given in relevant British and European standards, eg British Standard BS EN 292: Safety of Machinery - Basic concepts, general principles for design, the Supply of Machinery (Safety) Regulations 1992 (SMR) and the Supply of Machinery (Safety) (Amendment) Regulations 1994. British Standard BS 5304 1988: Code of practice for the safety of machinery, which is now obsolescent, will remain available for purchase to provide guidance on the safeguarding requirements of older machines, with the Provision and Use of Work Equipment Regulations 1998 (PUWER 98) detailing the legal requirements. 10 All nip points between counter rotating meshing gears should be properly safeguarded. However, on turntables fitted with hold-to-run controls, guards are not usually required at the intermeshing point between tilt gear teeth and machine frame due to the slow speed. Where a hold-to-run control is not fitted then this point should be guarded. 11 Further general guidance on the law applicable to machinery safeguarding is contained in the HSE publication entitled Work Equipment (see list of references at para 51 of this document). Stability, and rated loading 12 All 3 types of positioning equipment have the generic problem of stability associated with all load carrying equipment. Design ratings should not be exceeded. Manufacturers and suppliers should clearly mark on the equipment either the design rating or display a table showing its safe working range. A typical chart is shown at Appendix 2. 13 To ensure stability of the positioning equipment themselves they should be located on a firm level base and bolted to the floor where possible. In practice, it is recognised that rotator sets may remain free standing, because creep can be counteracted by manually changing the angle of rollers. 14 Column and boom positioners should be mechanically restrained or carry a counterbalance to ensure their stability. Some bases are rail mounted to permit long welds, Where this is a feature, an appropriate means of ensuring stability should be employed. Users of this equipment should ensure that the balance of the column and boom is not affected by the addition, removal, or replacement of originally supplied equipment (see also para 27). Electrical safety 15 Under most circumstances it is preferable to provide a reliable bond to earth for the workpiece during welding. Earthing bushes are fitted to the machines for this purpose. Not only does it provide operator protection, it also protects the equipment’s electrical control system. Manufacturers state that machines are frequently repaired for this reason. 16 Electrical equipment should be housed and located in enclosures suitable for the working environment. The standard of protection for equipment should be to that specified in BS EN 60529: 1992 Specification for degrees of protection provided by enclosures (IP Code). This document does not cover design and construction of electric arc welding equipment. Readers who require guidance on relevant European product safety legislation, namely the Electrical Equipment (Safety) Regulations 1994 (EES), SMR and/or, the Electromagnetic Compatibility Regulations 1992 (EMC) should contact their local office of the Health and Safety Executive. (See also paragraphs 45-49). Further sources of information are contained in the DTI publications listed at paragraph 51. Machine clearance zone 17 To prevent the risk of trapping the operator between the workpiece and adjacent plant or equipment adequate separation should be allowed. It is recommended that a minimum distance of one metre should be left clear around the positioner/workpiece. Operator access 18 Operators should only ride on the equipment from attachments designed for the purpose. Lay troughs or seats should have a safe means of access to and from them, and operators should be restrained in their seats etc by the use of a harness or other suitable device
http://www.hse.gov.uk/