The Art of the Rewind
Using rewound stators and armatures is the norm in the maintenance industry -- yet few offer the service
By Greg Napert
October 1999
Many accessory shops
in the country provide the service of overhauling and repairing aircraft generators and alternators, but few offer the service of armature and stator rewind. The armature and stator assemblies are the hearts of generator and starter assemblies.
The armature is a relatively complex component built with copper wire and metal laminations around a single shaft. The stator is not quite so complex in design, yet the labor involved in rewinding it makes it impractical in most cases. Many shops can easily handle light repairs to the armatures and stators. These repairs include machining the commutator or undercutting mica — possibly even recoating exposed wires with varnish, but complete overhaul is beyond the scope of their capabilities. Instead, the armature is replaced with a new or rebuilt unit. Because of these challenges, there are only a handful of shops in the country that overhaul armatures. One of those is a facility in Ft. Lauderdale, Florida by the name of Thrust-Tech Aviation, Inc. (TTA). Thrust-Tech is an FAA Repair Station with a Limited Accessory and Limited Specialized Services certificate.
Armando Leighton, Jr., owner and founder of Thrust-Tech explains that "The data for rewinding armatures and stators is difficult to come by. The manufacturer in most cases does not offer an overhaul manual with part numbers and specific wire gauges. Data is a sacred commodity. Much of the data that Thrust-Tech and other rewind shops in the country have developed is a result of reverse engineering."
The company basically takes one of the units, disassembles it and takes the exact dimensions of the wire. It then counts the number of turns of wire on the factory units to determine precise resistance. The procedure involves using the same or better insulating materials, verifying that the output of the overhauled unit is the same as the original, and is within specifications. The whole process is repeated and has to be approved by the FAA under a process specification. This reverse engineering and approval process (which can take upwards of 6 months), may also weigh heavily into the reluctance of shops to get into the rewind business.
Skill required
"Rewinding an armature isn't as simple as you might think. It's a fairly labor-intensive procedure — where everything is assembled and bent into shape using fixtures and jigs," says Leighton. "We have managed to automate some of the sub components such as insulators and finger plates, varnishes and finishes, etc, but much of the process still has to be done by hand. Also, the windings all have to be the exact dimensions required to fit into the stator. Everything is calculated — each bend, each winding, the number of windings, the gauge of the wire, etc. It's really quite an art, a craft, and can have a learning curve of up to two years for a technician to master the rewind. And, it gets more difficult with the higher-powered armatures as the higher the amperage, the thicker the gauge of wire, and the thicker gauges become more difficult to bend and shape by hand."
Technicians actually develop a signature way of doing their own armature rewind by using their skills in the procedures. Further, the build up requires a great deal of testing and inspection during the rebuild. For instance, an armature needs to be tested for shorts at various intervals during the build-up process. Technicians must keep checking for shorts and make sure it is insulated properly or they will go through a great deal of labor only to find out they have to take it apart and start all over again.
Which products to rewind?
While many shops claim to service generators and starters, relatively few of these shops overhaul armatures and stators. Thrust-Tech receives
many armature and stator assemblies from a variety of repair stations. A great deal of care goes into verifying where they came from and into inspecting the used units to be sure they are suitable for overhaul. Units received into TTA must be accompanied with proper documentation to verify traceability.
According to Leighton, "The armature shaft, laminations, stator housings and pole pieces must be in fairly decent shape for us to consider the overhaul."
Besides obvious damage and runout of the armature shaft, the laminations must be in fairly good condition. The laminations are actually like a stack of cards with insulation between each layer. If they are shorted out or if you have any rubbing as a result of a bearing failure, the laminations may be fused together. If this is the case, the armature will probably be unusable.
"We can sometimes save a shaft by replacing some of the steel laminations but this isn't always the case," says Leighton. "If the stack is damaged, the laminations are typically misaligned on the shaft and may be skewed or not concentric with the shaft. We do have the capability of replacing the laminates in some cases and true-up the laminates with the shaft, but typically this is addressed on a case by case basis."
DC Generators can rotate up to 8,000 RPM, with the armature turning at these speeds they are very sensitive to any imbalance that may exist. A good deal of time is spent checking alignment and balancing armature shafts before proceeding with a rewind.
"There is nothing worse than going through the work of rewinding an armature and assembling a starter or generator and finding that you have a severe unbalance or that the shaft isn't running true," says Leighton. "It would be like building a house on a bad foundation."
He adds, "The process may not even get that far, we may just complete the rewind and then try to balance the armature and find out we're having difficulty because of a problem with the shaft or the laminates or the alignment of both of these. It really pays to make sure you have a good armature shaft prior to going through the process of rewinding."
Stators require a thorough inspection as well. Although there are no moving parts on the stator assembly, the windings can be subject to overheating and shorting and the bearing housing can be worn even without a bearing failure in the generator.
The overhaul process
The general process of rewinding the armature and stator starts with an incoming inspection. Each part is subjected to a thorough evaluation of the components to determine which components are eligible for overhaul and rewind. According to Leighton, the armatures found in generators and starters that show any signs of bearing failure are especially scrutinized. Thrust-Tech checks the bearing journals and makes sure they are within specifications, which includes looking for any lamination scraping, any indication that the armature has been overheated or gone through some sort of severe bearing failure, or sudden stoppage. It is less likely that many of these components will be able to be rewound.
"For instance," says Leighton, "if the armature laminations are fused or bearing races welded on the shaft, we may deem the entire armature non-repairable."
There are many different levels of bearing failure. Some may just discolor the commutator, or burn some of the wires and insulation — where other failures will have allowed the armature to rub on the stator housing. Armatures that have come into contact with the stator housing will likely be seriously damaged. If the housing has been dropped or there is a bend in it, you will not be able to put it together again properly. Some units that have damage may have been mishandled or damaged in shipping and they are often non-repairable.
"A little extra care in shipping and handling can be well worth the effort." adds Leighton.
Different manufacturers require various items to be inspected, depending on what part numbers and serial numbers you're dealing with. Some armatures cannot be rewound. Additionally, some items may or may not be salvageable depending on what is damaged on the shaft.
"If you have a rare armature, for instance, that we normally don't stock the commutator for, it would not make sense to manufacture a commutator for that one item," says Leighton. "Commutators are the most expensive part of the armature assembly and they are typically purchased in lots for repair purposes. Delivery can be as long as five months, so careful stock provisioning is necessary."
If an armature is accepted for rewind, the next step is to cut the metal bands off of the ends of the windings and then pull the commutator from the shaft. The remainder of the armature is then inserted into an oven and heated to 700 to 800 degrees F to burn any insulation, varnish and potting compound. Wires and laminations are then removed and the armature shaft is cleaned and sealed to prevent oxidation. "The heating process is not harmful to the base metal," explains Leighton, "It simply burns off the materials, which allows us to disassemble the unit without damaging anything."
After all components are disassembled and cleaned, they are inspected dimensionally as well as for any signs of cracks or other damage. The armature and/or stator is then built up step by step with Megger inspections (for electrical shorts) after each step in the assembly process. The final preparation involves dipping the armature in varnish and letting it cure under the proper temperature condition for the specified time. The unit is then cleaned up and the commutator is cut. The mica is then undercut and the unit is balanced to specifications.
"All of our completed generators, starters, and alternators are tested on test equipment per the manufacturer's overhaul specifications," he explains.
Overhauling starters/generators
Of course, armature and stator rewinds are not all that Thrust-Tech offers its customers. Rewind was a natural progression from overhauling
and repairing generators, starters and alternators and the company still offers these services. According to Thrust-Tech's quality assurance manager Ron Shipman, "Some of our largest customers are generator and starter overhaul and repair shops. We may compete with them on the overhauls, but they are our customers when it comes to rewinding."
Another major customer of Thrust-Tech is its parent company, Corporate Rotable & Supply (CRS). Thrust-Tech was actually an outgrowth of CRS, which was established in 1990 to provide a pool of rotable supplies to the aviation industry. Today, CRS represents 20 to 30 percent of Thrust-Tech's total business.
The Art of the Rewind
Using rewound stators and armatures is the norm in the maintenance industry -- yet few offer the service
By Greg Napert
October 1999
Shipman explains that due to the quantity of work, the depth of service, and their experience gained from reverse engineering; the company has learned to take special steps to provide its customers with a quality overhaul. For example, Shipman explains, "In order to properly salvage the components, we use ultrasound cleaning on everything. Some people use harsh chemicals or sandblasting, but we like to keep the components as intact as possible and not subject them to additional wear. This takes much more time, but it maintains the integrity of the components."
Shipman says they have learned to put a bit of extra time into the seating in of the brushes. "If you don't get a good seating of the brushes on the commutator, when you get up to altitude, the brush will not conduct the electrical load the way it is supposed to and this will deteriorate the commutator. We run our generators for a minimum of 10 hours before releasing them. When we run in brushes, we look for a darkened film on the commutator brush, which indicates that the brush is fully in contact with the commutator segments."
Field advice
Louis Tapia, general manager for Thrust-Tech says that he occasionally receives calls from individuals in the field asking about replacing brushes.
"Many people try to get by and replace brushes on the armature while it's installed in the aircraft," says Tapia. "The problem with this is usually the commutator is grooved and the new brushes have to form or shape themselves to a grooved commutator. By the time this happens, you've already eaten up 20 to 30 percent of the brush, which may cost $300 to $400 and you may only get a few hundred hours on the brush. Then a couple of hundred hours later, you're paying another $300 to $400 when you could have paid $800 initially for a complete overhaul with new bearings."
He continues, "Consider that you really have to run the aircraft for at least three to four hours before you have them seated in any form or fashion and this is impractical. Unfortunately, many that just opt to throw in a new set of brushes don't take the time to seat them. The result is that the brush may arc severely when the plane goes to altitude and the result is damage to the new brushes and the commutator. Further, carbon from the extreme wear of the brushes works its way into the bearings and armature causing bearing wear, and reduced cooling due to carbon dust buildup."
Another consideration to this approach is if the brushes are replaced in the field, the component will most likely not have any warranty. By having the equipment overhauled, you typically receive a warranty for the workmanship of the unit.
Demand dictates supply
Leighton explains that all armatures and stators can't be rewound — there has to be enough of a demand to justify reverse engineering the
armature or stator in question.
"Unfortunately, there is very little commonality between units and between manufacturers — different types of insulation are used, and different configurations for commutators and electrical wiring applies to each model of commutator. Additionally, each armature requires different gauge copper wire which must be custom ordered and is very expensive. As much as we would like to rewind everything on the market, it really requires around 100 units a year to make rewinding any particular unit viable," says Leighton.
