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使用高温粘接方法修理和替换耐磨损内衬密封条
作者：Tom Lane
总经理
HEATCON 复合材料整体维修系统（欧洲办事处）有限公司
Along with the advent of the highly efficient turbofan jet engine, used today on the majority of commercial aircraft, came a new and challenging problem for maintenance personnel. The repair and replacement of the abradable liner seal, sometimes also known as the fan track.
The turbofan is basically a turbojet with a front mounted fan, which is driven by a second turbine located behind the primary one. The purpose of the fan is to increase the airflow around the engine core in order to reduce fuel consumption and provide greater thrust. In order to reach the high bypass ratios being achieved by the modern turbofan it is essential that as much air as possible is forced through the bypass fan and therefore the clearance between the fan tip and the inner core of the fan cowl needs to be maintained as small as possible.
If the fan blade and inner cowl were both metallic, any expansion of the blade length caused by increase in temperature or centrifugal loads, could cause the two components to come together, resulting in damage to one, or both parts. To eliminate any such potential problem, an abradable liner seal is fitted to the inside of the fan cowl, around the periphery of the bypass fan. Being a much softer material than the metallic fan tip, any interference would now cause the liner seal to be abraded and not result in expensive engine damage.
Naturally, over time the liner seal is abraded away, as it was designed to be, or it becomes damaged due to the ingress of foreign objects; resulting in an increase of clearance between the fan tip and the inner cowl, which reduces the bypass ratio and thus the efficiency of the engine. Therefore in order to maintain an optimum level of performance the abradable liner seal needs to be inspected on a regular basis and repaired, or replaced, as required.
Depending on the engine type and manufacturer, the abradable liner seal is made from a potted nomex honeycomb, or a silicone abradable coating charged with hollow glass beads.
Historically these liners were bonded into place using a room temperature cure, which depending on the actual adhesive specified, could take up to 48 hours for the initial bonding, with full bonding taking a further five days. This long process not only created problems for the manufacturer providing a full engine overhaul service, but also was a nightmare for operators and repair stations looking for a quick turn around fix.
It was known that the cure time could be drastically reduced by increasing the temperature; with the cure time of one type of adhesive coming down from 24 hours at 70 deg F (21 deg C), to 1 hour at 140 deg F (60 deg C). However, the practical application of localized heat to the fan cowl was a problem that had to be overcome.
The requirement for controlled heat, used to repair composite structures on aircraft provided similar challenges and so Rolls Royce initially approached HEATCON? Composite Systems (Europe) Limited, to see if a practical solution could be found, which could be applied to their RB211 series of engines.
A test, using a HEATCON? HCS9200 Hot Bonder, together with a standard rectangular 6”x 36”, 220/240 volt heater blanket, rated at 5 Watts per Square Inch, was carried out to prove that the fan case could be uniformly heated to 250 deg F (121 deg C), even though the target temperature was only 140 deg F (60 deg C). Once the application of heat had been proven, the next step was to design and manufacture a shaped heater blanket to match the conical internal shape of the fan cowl. This was achieved by making a template, using the inner fan cowl as a pattern; and from this, the banana shaped (when viewed flat) heater blanket produced.
Back in the engine shop a second test was conducted with the specially shaped heater blanket in order to confirm temperature uniformity, before the first test panel was bonded in place. By using both zones of the hot bonder (each zone driving six individual heater blankets) we were able to successfully install the twelve fan track panels, which make up the abradable liner seal on a RB211 engine, in one operation taking a little under two hours for the curing process.
The various engine manufacturers adopt different methods of holding the abradable liner seal in place whilst it is being bonded, with the most common being either by mechanical clamping devices, using the front flange, or in the case of the V2500 a floating ‘spider’ configuration; or by simply vacuum bagging the assembly. In each case, a combination of Hot Bonder, thermocouples to provide feedback and control to the hot bonder, together with a custom made heat blanket/blankets has proved to be an excellent solution to what was a significant problem.
Over the last five years elevated temperature bonding has been perfected for a large range of engine types produced by the major manufacturers; and has benefited numerous engine shops and repair facilities by significantly decreasing the amount of time taken to carry out repair and replacement of this vital component.
Further information and advice on this process and the equipment used is available from HEATCON® Composite Systems, who have offices in Seattle, Texas, China and the United Kingdom.
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