- The “right spare part” is always the part you need at the time, regardless of what you should logically have in stock.
- Once parts are discontinued, getting the necessary spares can be daunting.
- Standardizing on components periodically can reduce those unforeseen predicaments with obsolete components.
This is the damaged rigid half of cast steel mill motor coupling prior to repair. The part is about 6 ft in diameter and weighs approximately 31,100 lb.
Power distribution systems in industrial plants often have multiple generations of equipment in the same area, which leads to variety in the parts area. It also can create situations where components are retrofitted, but not to all similar units. Making sure the right spares are available isn’t easy, but it’s important.
“Good luck with this one,” says Geoff Generalovic, a retired maintenance electrician with more than 35 years of industrial experience. “Once parts are discontinued, getting the necessary spares can be daunting. If a fix is heard of, it needs to be actively pursued so that, if there’s a failure of the older systems, a repair is available to be used. The right spares should be available from the drive vendor. If not, then they should be able to source parts for you quickly, though the costs may be high.”
Standardizing on components periodically can reduce those unforeseen predicaments with obsolete components, explains Billy Knox, RCM technician at Georgia-Pacific (www.gp.com) in Dudley, North Carolina. “As time allows, try to reduce any differences you may have by simply making sure you have all the components the same,” he offers. “Examples might be the same types of couplings, the same types of bearing applications, and motor bases.”
The rigid half has been prepared by boring out and splining the ID in preparation for shrink-fitting a splined bushing.
It’s the responsibility of the planners to note the change of design in the equipment notes and to change the stock descriptions, says Gary Finchum, electrical engineer and project manager at the AES (www.aes.com) E.W. Stout Generating Station in Indianapolis.
“One practical solution is to conduct an inventory of motors within a facility,” offers Gary Brinkman, field sales manager, Baldor Electric (www.baldor.com). “At the time of the inventory, the facility needs to have a defined replacement motor strategy. When surveying an individual motor, decide at that time on its replacement. Then affix a non-corrosive replacement tag on the motor. This will help the maintenance crew select the replacement motor either from existing spare motors or from the current motor vendor. The replacement tag should have all the critical information included. This information also will help convert the failed motor to a premium efficiency motor if properly detailed.”
The “right spare part” is always the part you need at the time, regardless of what you should logically have in stock, explains Kirk Blankenship, CMRP, senior asset care engineer at MillerCoors (www.millercoors.com) in Fort Worth, Texas. “Unless you can afford to have a spare plant, there is always a decision to make when adding parts to the storeroom which involves cost versus risk,” he says. “There should be a logical method to decide what goes into your storeroom based off of cost of the part, cost and probability of failure, lead time, and availability. This won’t ensure that you always have the right part, but it will ensure you have minimized the impact of unexpected failure within reason for the budget you have available.”
The splined bushing for the center bore of rigid half has mating splines machined on its OD. The splined, shrink-fitted connection of the two parts restores the rigid half to its original torque transmission rating.
Information in the plant must be distributed so teams working on heavy equipment will have up-to-date drawings and bills of materials for installed equipment, especially at remote sites. “Work orders should be generated with any special drawings or information needed to make repairs and are the responsibility of the planners,” says AES’s Finchum.
Complete bills of materials and good change management will ensure current drawings, suggests MillerCoors’ Blankenship. “But the reality is that change management is very hard to manage and bills of materials are rarely complete,” he says. “There is no silver bullet here; this just takes discipline and commitment to make it happen. When equipment is modified, drawings and BOMs must be updated, and spare parts must be reviewed to determine stocking requirements.”
A distributed manufacturing automation and control system would be beneficial to remote sites, but the critical component of it all is having the personnel to keep the drawings up to date, explains Generalovic. “That seems to be a group that is disappearing rapidly,” he says. “If that is the case, someone needs to be designated as the keeper of the drawings, so all drawings are as up-to-date as possible, which means any field modifications are diligently recorded.”
The repaired rigid half was installed on a herringbone bull gear for testing.
Georgia-Pacific’s Knox emphasizes total communication — maybe a meeting a week in advance with all involved in assuring that everything stays as current as possible and so any changes may be noted.
For electric motors and drives supplied by Baldor Electric and ABB, the vast majority of that data is available for download with online tools, explains Baldor’s Brinkman. “Typically the data can be downloaded in a PDF file and then stored on the user’s online data storage site or cloud,” he explains, noting that, in most cases, a remote site can access this information.
“We are in the process of putting our drawings and manuals onto Microsoft’s SharePoint that has been set up for our corporation,” explains Keith Gowan, who has more than 15 years of experience as a maintenance manager. “We call ours iShare. Having the documents in a SharePoint site allows for Web access to any files stored in the document libraries. Adding columns in a document library makes it possible to categorize the documents and therefore makes finding a document easier for all. Categories can be things like machine number or manufacturer type. Security levels are very easy to set for the SharePoint lists, so having read-only access for all is possible. Giving the maintenance technicians access to a laptop or PC on the shop floor allows them to search for, find, and review drawings anywhere they can connect to the network. Training the maintenance technician to add documents to the library will also be a future step, so that they can contribute information as they find it. Using the notifications for a supervisor, alerts will then be used to review the added documents to be sure they are appropriate and not duplicated.”
The repaired rigid half installed on herringbone bull gear shows evidence of the splined-in bushing used to restore the center diameter.
Spare motors that are kept in inventory might need to be tested or turned periodically. “I don't know that a motor would have issues sitting on the shelf, but I would recommend turning the rotor by hand once every six months on all motors in storage,” offers Charles Dix, engineer and co-owner, Carolina Hydro Technologies (www.carolina-hydro.com) in Providence, North Carolina.
Another option is using a tester, such as PdMA's MCE motor circuit evaluator, says Generalovic. “To get a good read on a spare motor on the shelf, look for balanced readings, phase to phase, for both resistance and inductance,” he suggests. “Starting a motor-testing program using something like the PdMA tester is very cost-effective and beneficial.”
A megger tester is another option. “Putting the meg on the motor on a scheduled inspection can help check the motor and make sure that you don’t have an issue with wires drying out and breaking down causing a short and can help in detecting moisture or condensate inside the stator and winding,” says Georgia-Pacific’s Knox. “If moisture is present, this could cause a lot of problems such as rust, insulation breakdown, and other issues that can result in premature failures.”
AES’s Finchum says testing of motors isn’t needed, as long as they’re kept in a clean, dry environment. “However, motors with antifriction bearings, if stored for more the six months, should be rotated 180° to prevent bearing damage,” he warns.
Hal Scott, district manager at Baldor Electric agrees with the advantages of storing spare electric motors in a clean, dry, heated environment. “I have used the baseline of rotating the motor shafts every six months a minimum of three revolutions and placing the shaft keyway in a different position than the original position,” he adds. “It would be best to not place the motors directly on concrete and place on shelving resistant to any vibration caused from machine operation or vehicle traffic.”
When the bore on the rigid half of a high-torque steel mill coupling was badly galled during a maintenance teardown, an East Chicago, Indiana, mill put the damaged part in the plant’s bone yard. However, engineers at Kop-Flex, part of the power transmission solutions business of Emerson Industrial Automation (www.emersonindustrial.com), developed a solution that saved the customer almost half the cost to replace the $112,000 part, which transmits approximately 10,000,000 lb in. (1,130,000 Nm) of torque to a roughing stand. Dan Kocel, global metals industry manager at Emerson Power Transmission Solutions, explains how the high-torque cast-steel coupling’s damaged bore was repaired with a splined-in insert to maintain the original rating and restore the correct diameter.
The CM-30 cast steel gear coupling connects the pinion on a 10,000 hp bull gear motor to a roughing stand on an 84-in. (21 m) hot strip mill. To transmit the required torque, the coupling’s rigid half attaches to the pinion with a heavy interference fit of 0.028 in. (0.7 mm), plus two massive split-taper keys 4-by-5.5-by-47 in. (100-by-140-by-1,194 mm), each weighing about 340 lb (154 kg).
According to the manager of maintenance/spares, the rigid half of the coupling, which weighs about 31,000 lb (14,061 kg) and has a bolt-hole diameter of almost 6 ft (1.8 m), had to be removed from a damaged bull gear shaft after the gearbox lost a tooth. During removal, the coupling’s bore was badly gouged.
The mill considered the damaged part hopeless and was going to replace the bull gear shaft, so it couldn’t simply bore out the coupling to a larger size which would then not fit. A standard coupling repair procedure would involve enlarging the bore, shrink-fitting and welding a plug in the bore, and then re-boring the center diameter to correct size. However, the rigid half is ASTM A27 cast steel, and the diametral force of a heavy interference fit for the plug would create internal stresses exceeding the limits for this part, and welding would not provide the strength to transmit the torque. The solution was to scrap the rigid half and replace it.
Months later when assessing inventory of critical spare parts, the mill realized it had no spares for this part, so asked Kop-Flex for a quote on one. Instead, Kop-Flex suggested repairing the damaged part and assessed the engineering options.
To create a new bore in the part without using a welded plug, Kop-Flex engineers proposed a new solution using a shrink-fitted, splined-in bushing, because splined connections can transmit high torque loads. Engineers designed the splines to withstand the bending and compressive forces with an adequate safety factor. They also designed the splines to achieve an interference fit (negative backlash) of 0.015 to 0.020 in. (0.4 to 0.5 mm), along with the bushing’s diametral interference. The repair bushing itself is 39 in. (1 m) in diameter, with an approximate weight of 8,000 lb (3,628 kg).
The bore of the salvaged rigid half was enlarged and cut with mating splines, as well as locating surfaces for the bushing. After shrink-fitting the two parts, the faces were machined flush with the existing flange surface.
The repaired part was installed and tested on a herringbone bull gear at a vendor’s plant and determined to meet specifications. The repair saved the customer $54,000 over the cost of a new rigid half, and the repaired coupling was reinstalled in the mill in the fourth quarter of 2012.
A splined repair can also be used to replace worn gears on large gear coupling hubs. The existing gears are cut off the OD and replaced with a spline section. A repair ring with gear teeth on the OD and splines on the ID are then installed on the modified hub as a way to repair large gear couplings with excessive tooth wear.