We manufacture complex oil and gas well drilling machinery. Components of the machinery include structural steel weldments, low alloy steel castings, 1000 HP AC motors, bearings, gears, hydraulic cylinders, etc. Some circumstances require us to weld on the machine late in the assembly phase of manufacture. Examples include welding on of brackets or other like items. There is concern as to possible stray currents causing bearing damage, gear damage, etc., so some say disassemble before welding, others say don't worry. Do you have experience with this issue? What are your recommendations? Will placement of the ground clamp as close as possible to the point where the weld is to be made an adequate precaution. For example, we don't ground on armature shafts and weld on the motor cases so bearings aren't in the circuit.
Thanks in advance for any information you can provide. As you can well imagine, there are many costs and lead time issues related to this issue.
To get an idea of our product go to our website at http://www.varco.com
As a failure analyst, I have observed arc damage to bearings of rotating equipment caused by stray welding currents. It appears that stray current damage is most likely to occur when the welder is unaware of the potential to cause the damage or is careless when placing the ground. For example, stray current damage is likely to occur when current passes through equipment because a ground clamp was attached to a frame or skid of equipment for welding of piping attached to the rotating equipment.
While I know that stray current damage is a real phenomena, I do not have sufficient hands-on experience with the issue to advise how to be certain of avoiding stray current damage without disassembly to an extent that removes a potential current path through critical areas, i.e., bearings, seals, etc.
Comparison of the cost of disassembly with the potential consequences of stray current damage may help make case-by-case decisions. Possibly experimentation with situations with relatively low potential consequences will reveal what one can and cannot get away with.
I do repairs on large forced draft and induced draft fan blades and dampers. We always put the work lead as close to the area being welded as feasable to accomodate the welder doing the repair. I've never seen any damage to bearings or the housings due to this. Considering that some of these wheels are 10 feet in diameter, it's sometimes a hassle to move the work lead, but I've never burned a bearing yet.
T Becker, I must preface this by stating I am not an engineer. I am however an owner of a welding/fabrication business and I have developed some procedures for this exact application that have worked very well for us. We weld on overhead cranes, some railroad applications, heavy construction equipment and steel processing lines with a lot of electronics involved, just to name a few things. The best procedure is to disassemble, this is almost never practicle or cost effective. We always disconnect power, unless it is impossible. The work lead is always, without exception, placed as close as possible to the work area. Grounding through shafts or bearings or anything with electronics or electricity going into it is prohibited. The availability of a good location to ground will dictate the degree of disassembly required. I do not recall ever having a problem with these procedures. I hope this helps.
Respectfully,
Mike Sherman
Shermans Welding
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