Positioning equipment for manual and automatic welding: Specialised machinery such as column and boom, rotators,
1 The medium and heavy fabrication sector of British industry uses a range of welding equipment, for the deposition of welds, the positioning of the workpiece and workpiece rotation. This OC reviews the hazards and risks arising from the use of certain equipment, ie column and boom, rotators and positioners.
2 This equipment may also be referred to as a 'manipulator' or 'positioner', this term should be referred to in its widest sense, and equipment should be viewed with an eye to the categories in the document title. The OC does not cover friction welding, resistance welding, spot welding or other types of 'single event' processes. Specialised equipment for welding pipes is also outside the scope of this guidance.
3 This OC has been developed from a project undertaken by contractors who have visited several users, manufacturers and suppliers of positioning equipment. The ID summarises findings and proposes remedies to problems encountered.
Equipment description and names
4 Industry uses the:
- column and boom as the means whereby the weld is automatically delivered to vessels and other large assemblies. The boom transports the welding head within or outside vessels (para 5);
- turntables and similar positioners to locate workpieces and orientates them to the desired angle (para 6); and
- rotators to position and rotate the workpiece. They are usually motor driven with other rotators in tandem (para 7).
5 A sizeable percentage of the existing equipment is over 30 years old. This old equipment is likely to have been modified over the years by users or sold on to companies who refurbish it before returning into the market place.
6 In addition to already installed positioners, there are 4 known routes by which equipment enters into the workplace:
- manufacturers within the European Economic Area (EEA);
- manufacturers outside the EEA;
- specialist companies who refurbish existing equipment for resale; and
- companies who hire equipment.
In addition to these, there are users of equipment who significantly modify existing equipment to suit their own needs. This work may also be undertaken on their behalf by a specialist refurbisher.
7 Equipment supplied over several decades has tended to be of robust construction, resulting in a significant proportion of this type of equipment being refurbished then sold on or supplied on a hire basis. It is often found that, when a modification or refurbishment has taken place, no reference to the original design criteria has been made.
8 If new equipment is being supplied, or equipment refurbished to the extent that it can be considered as new machinery when supplied, it will require CE marking. When advising users on the Supply of Machinery (Safety) Regulations (SMR) inspectors should follow the advice given in HSE's Product Safety Guidance.
9 This OC does not address the hire business.
Action by inspectors
10 Inspectors should ensure that:
- where appropriate, compliance with product supply legislation is achieved;
- a risk assessment has been performed by the employer/user, and appropriate safeguarding measures and a safe system of work have been implemented;
- all machines have a clearly identifiable safe working load (SWL);
- a credible means is available to weigh or calculate accurately the weight of the load carried on the positioner;
- the positioner is properly maintained;
- positioners are fitted with effective locating devices to prevent accidental displacement of the load. Enforcement recommendations
11 Without prejudice to other prevailing circumstances which may be encountered, consideration should be given to enforcement procedures under the following circumstances:
- where positioners are used and it is possible for operators to gain access to dangerous parts of machinery, giving rise to a risk of serious injury - consider a prohibition notice (PN) under the Provision and Use of Work Equipment Regulations (PUWER ) reg.11(1). (ID para 8-10);
- where positioners are used and are not maintained in an efficient state, in efficient working order and in good repair, giving rise to a risk to safety - consider an improvement notice (IN) under PUWER reg.5(1). (ID para 37-38);
- where load bearing or load carrying equipment is not clearly marked with a SWL - consider an IN under PUWER reg.23 (ID para 12, 21, 22, 27, 33);
- if the positioning equipment is being used to lift or carry loads in excess of design capabilities - consider a PN under PUWER reg. (4)3 (ID para 12, 21, 22, 27, 33);
- positioning equipment should only be used for operations for which, and under conditions for which, it is suitable, where positioning equipment has been adapted so as to be unsuitable for the nature of the work it is being used for - consider a PN under PUWERreg. 4(1);
- where new equipment or substantially refurbished equipment is in use, (supplied after 1 January 1995) or is likely to be used, and it does not clearly display a mark of conformity (CE marking); the previously mentioned Product safety guidance should be followed. For issues concerning the Supply of Machinery (Safety) Regulations, the Supply of Machinery (Safety) (Amendment) Regulations and the Electrical Equipment (Safety) Regulations refer to the intranet guidance.
Health and safety executive
Automated welding and heavy fabrication: Positioning equipment such as column and boom, rotators, and positioners
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:
- the welding head moves over a stationary workpiece; or
- 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. 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 swiveled 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.
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.
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).
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. 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.
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 (see Appendix 1 Figure 4A).
19 It is recommended that all machines should be pendant operated. The following functions should be controlled from the pendant:
- column and boom:
- boom travel speed;
- boom travel reverse, boom travel forward;
- raise boom, lower boom. Raise and lower should be hold-to-run, because of the attendant risk of trapping the operator, and associated stability effects. Forward and reverse travel of the boom is normally switch to run, but where models have power slew of the column, this feature should be hold-to-run; and
- emergency stop.
- variable rotational speed;
- control tilt up and tilt down. This group of controls should be hold-to-run. There should be limit switches incorporated in the tilt mechanism to prevent the mechanism exceeding its design range. Rotational motion should be switch-to-run (forward and reverse); and
- emergency stop. On the 2 axes of movement positioners (ie tilt and rotation) the tilt speed is often fixed at approximately 0.35 rpm (one degree arc per sec).
On the 2 axes of movement positioners (ie tilt and rotation) the tilt speed is often fixed at approximately 0.35 rpm (one degree arc per sec).
- variable speed;
- forward; and
- emergency stop.
20 Rotators are used in pairs with additional units added for longer workpieces. The number of sets used depends on the length, weight, and shape of the component to be welded. They consist of a drive unit and idler, the rotators should always be placed on an even surface. When multiple drive units are fitted, their controls are usually synchronised electronically.
21 Each set of rollers should be clearly marked with its rated load. There is a stability risk associated with this equipment as it is rarely fixed to the ground. To do so would, in many cases, reduce the flexibility of the machine. Drive and idler sets should be parallel to one another when rotation commences. Manual tracking may take place once operation starts. Always work from a place of safety and not between individual sets of rollers.
22 The rollers should be prominently marked with their rated SWL (effectively an axle weight). Where the component weight varies along its length, it should be carried by sufficient sets of rollers used to ensure that any single set is not loaded beyond its SWL. In such circumstances, it may be necessary for a competent person to calculate the load to be carried by each set.
23 Additional 'out of balance' forces are created when the weight of the component is unevenly distributed around its axis of rotation, (also referred to as eccentricity). Equipment rating must include an allowance for 'out of balance' forces. If a vessel is tapered or cone shaped then one pair of rotators must be higher than the other.
24 Speed control is recommended as it helps to prevent the component creeping along the rollers whilst being rotated, with the risk that it will fall off. End stops should be used to limit spiralling of the component. This is only one means through which creep can occur. Other reasons include:
- unevenness of component surface texture; and
- unevenness of rolling surfaces of drive and idler wheels. Typically these can be faced with polyurethane or steel, the former being more susceptible to damage when weld preheating is used.
25 Operators set the speed of rotation (workpiece movement) to suit the welding technique. This gives relatively low peripheral speeds of 50 mm/min to 1500 mm/min.
26 Manufacturers can provide an anti-creep facility at extra cost. It is particularly useful for narrow gap and/or Submerged Arc Welding.
Column and boom
27 A chart showing the SWL at various radii should be prominently displayed on column and boom machines. Equipment exceeding the design rating should not be fitted to the boom. If any welding equipment is retrofitted to the boom which is heavier than that originally fitted, then the manufacturer or a competent person should be consulted to ascertain if there is any adverse effect on the machines stability. If so, mechanical stops should be fitted to restrict the load radius (see also paragraph 14).
28 Operators should only be seated adjacent to the welding head on equipment designed and specified for that purpose. Persons should have a safe place to sit and be restrained from falling. Safe access to the seat should be provided. Persons should not be free to climb to the welding position. On some larger machines where the seat is replaced by a lay trough, the same safety criteria apply as for seats.
29 Where an operator is raised from ground level, an integral fall arrest arrangement should be incorporated into the boom raise and lower mechanism, to arrest movement in the case of a failure of the hoist and lower mechanism. Upper height limits should be fitted.
30 Safeguards should be fitted to prevent the boom descending onto the workpiece and causing instability. Booms are often raised by an electric motor using a rack and pinion drive with integral brake. The boom can be counterbalanced by a weight sliding inside a column, which is connected to the boom via a wire rope and sheaves. Slack rope conditions are created if the boom is lowered onto workpiece (or the rope snaps). An appropriate safeguard is to employ slack rope scotches, which fall into position as soon as rope tension is lost. It is essential to properly maintain this system and to examine the wire rope periodically (see paragraphs 43- 44). Positioners
31 On this equipment the workpiece can overhang the positioner table. The criteria for stability is linked to the geometry of the workpiece. The critical aspects are:
- the position of the centre of gravity of the workpiece, taken vertically from the machine table; and
- the horizontal eccentricity of the workpiece, because of the effect of centripetal force.
32 If the centre of gravity is too high the workpiece can tip over from an inclined work table. If the load is biased to one side of the component creating an eccentric effect, the positioner will become unstable because of the eccentricity of the load.
33 A rating plate showing a plot of safe working conditions, or a table of loads etc should be prominently positioned on the machine. An example chart is shown at Appendix 2, safe conditions are below and to the left of both curves.
34 Loads are often fastened to the turntable with locating tee bolts supplied by the user. The slots and holes to which these are attached, are designed and cut during the table manufacturing process.
35 It is the user's responsibility to ensure that the load is adequately secured to the table. A competent person should assess the fastening method to ensure that sufficient and suitable clamps/fasteners are used. At least 3 fasteners should be used.
36 Where the operator requires access to attach clamps, a safe means of access should be provided, together with a safe system of work.Maintenance
37 In common with all work equipment, positioning equipment should be maintained in an efficient state, in efficient working order, and in good repair. This document does not attempt to interpret the meaning of maintenance, readers wishing to obtain further information are referred to HSE publication L22 (see paragraph 51 for details). Users of positioning equipment seeking to establish a maintenance programme should in the beginning consult the manufacturer's recommendations relating to the equipment. Where guidance is not available, then the maintenance programme should be established as part of the risk assessment procedure.
38 Suggestions for components of positioning equipment to be included in a maintenance schedule are all those that require periodic lubrication, replacement and adjustments. Particular attention should be paid to: machinery safeguards (ID para 8), mechanical fastenings to floor or rails (ID para 14), mechanical load restraints (ID para 24 and 34), electrical equipment (ID para 15-16), operational controls, drive equipment (ID para 20 and 30), operator seating (ID para 28). This list is not exhaustive.
39 Legal requirements are split into 2 sections. They share the common aim of establishing a safe place of work. They are:
- 1. health and safety legislation; and
- 2. product supply legislation.
Equipment in use - Health and safety legislation
40 The Management of Heath and Safety at Work Regulations (MHSW Regulations) require every employer to make a suitable and sufficient assessment of the risks to the health and safety to which their employees are exposed to at work.
41 Further information on risk assessment can be found in the HSE publications Management of Health and Safety at Work: Approved Code of Practice ISBN 0 11 886330 4 and leaflet IND(G)163(rev1)Controlling the risks in the workplace.
42 The Provision and Use of Work Equipment Regulations (PUWER), is the specific legislation which addresses machinery safety requirements. PUWER places a duty on all employers to ensure that items of work equipment provided to their employees are suitable for the use for which they are provided for. This includes consideration of its initial integrity, the place where it is to be used, and the use it is put to, and machinery safeguards. Further information is contained in HSE publication L22 (see paragraph 51 for further information).
43 The Lifting Operations and Lifting Equipment Regulations (LOLER) impose requirements with respect to lifting equipment, as defined in regulation 2(1). A key requirement of this definition is 'load' which includes a person. As a result column and boom welding equipment which is designed and fitted with, or adapted to include the provision of, a person-carrying device is classed as lifting equipment, as defined under LOLER and the Regulations apply. (see Appendix 1 Figure 4A). Further information is contained in publication L113 (see paragraph 51 for further information).
44 This definition does not extend to other non-person-carrying column and boom equipment under normal operational circumstances (see Appendix 1 Figure 4B).
Product supply legislation
45 Detailed discussion of product supply legislation is beyond the scope of this information document, readers who wish to establish a more detailed understanding are referred to the DTI publications listed at paragraph 51. The principal equipment supply legislation, which implements the Machinery Directive in the United Kingdom, is the Supply of Machinery (Safety) Regulations 2008.
46 A buyer of new equipment should look for the 'CE' mark. This is the manufacturer's or supplier's claim that the equipment meets the essential health and safety requirements as laid down in the Machinery Directive. However, purchasers should not rely on the CE mark as a guarantee of safety. Users should always carry out their own risk assessments of the equipment to ensure that it is, in fact, safe for use. Should users find that CE-marked machines are not in fact safe, they should contact in the first instance the manufacturer, then their local HSE office.
47 Buyers and sellers of second-hand machinery need to be aware that when this equipment is sourced from outside the European Economic Area (EEA) it must comply with the Supply of Machinery Regulations as amended. This means the duty lies upon the importer, agent or you yourself if you buy it direct, to ensure it obtains a CE mark, and that the equipment is, in fact, safe.
48 Refurbished machinery from inside the EEA may also qualify as new machinery within the definitions of SMR. If you have any queries on this aspect you should consult your local HSE offices, or the DTI.
49 The Supply of Machinery (Safety) Regulations, as amended, are supplemented by the Health and Safety at Work etc Act 1974, section 6. This section applies to all refurbished machinery when re-supplied. If you have any queries on this aspect you should consult your local HSE inspector.
50 HSE Publications referred to in this document can be obtained from: HSE Books, TSO Customer Services, PO Box 29, Norwich, NR3 1GN, telephone: +44 (0)333 202 5070 or any good bookseller.
51 Further information may be found in:
HSG65 Successful health and safety management ISBN 0 71271276 7.
HSG129 Health and safety in engineering workshops ISBN 0 71760 880 8.
L22 Safe use of work equipment-Provision and Use of Work Equipment Regulations 1998 Approved code of Practice and Guidance on Regulations (Second Edition) ISBN 0 7176 1626 6.
L113 Safe use of Lifting Equipment - Lifting Operations and Lifting Equipment Regulations 1998 ISBN 0 7176 1628 2.
Leaflet INDG163L Controlling the risks in the workplace - A step by step guide to a safer and healthier workplace.
Leaflet INDG270 Supplying new machinery: a short guide to the law and some information on what to do for anyone supplying machinery for use at work.
Leaflet INDG271 Buying new machinery: a short guide to the law and some information on what to do for anyone buying new machinery for use at work.
BS EN 60529:1992 Specification for degrees of protection provided by enclosures (IP code).
British Standards are available from the British Standards Institution, 389 Chiswick High Road, London, W4 4AL. Tel: 0181 996 9001. fax: 0181 996 7001.
Business in Europe - Product standards Machinery - a guide on UK Regulation
Business in Europe - Product standards Electrical Equipment.
Both the above are published by the Department of Trade and Industry and can be obtained by contacting the DTI Business in Europe hotline Telephone No 0117 944 4888.