21 September 2018
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Magnetic attractions

The bonding of magnets in any application calls for precision and longevity of the solution – not always an easy combination to achieve.

Two interesting structural bonding applications have recently emerged from Milton Keynes-based adhesive tape manufacturer Lohmann Technologies. In the first, its DuploTEC structural bonding tape has been specified for the bonding of magnets direct onto an oil pump drive shaft and, in the other, it has been specified to bond magnets onto shafts inside electric motors.

The oil pump manufacturer needed to place a position-indicating transducer inside the body of the pump and this called for a sintered metal magnet to be mounted direct onto the main pump shaft. With the shaft manufactured from 8mm stainless steel and the magnet made from a dissimilar material, the pump design team first explored a variety of mechanical joining options.

Given that the parts would spend their entire lives in what is effectively a hot oil bath, they discovered that, while some of the proposed solutions proved capable of meeting many of the design parameters, none of them could meet all of the criteria. It seemed that obtaining a percentage of success was the best that could be achieved by using mechanical fastening in this instance.

As the design evaluations progressed, the manufacturer was obliged to discard one potential joining solution after another and, in the end, drew the conclusion that a complete redesign of the key parts might be beneficial. A thorough review took place with this goal in mind and it was during these discussions that the subject of bonding came to the fore.

Coming back and approaching the challenge from a different angle entirely, but retaining the original concept, the design team then began investigating the possibility of using adhesives to mount the magnet, an idea which was initially met with a degree of scepticism by some team members. This was based on the fact that the entire assembly would be immersed in hot oil – in itself a substance not instantly recognisable as compatible with the use of adhesives, especially in an environment that would see the oil reach temperatures of up to 140°Celsius.

However, in the course of the manufacturers’ investigations into the possibilities of bonding the magnet to the shaft, Lohmann engineers reviewed the design brief with the pump team, and then analysed the chemical compositions and dynamics in play inside the pump. As a result, they returned with the recommendation that three of the company’s epoxy-based Structural Bonding Tapes be put to the test.

The proposal came after Lohmann’s engineers calculated that the specific structural bonding tapes in mind would not only meet the tensile and shear strength requirements described, but also the other vital parameters (including thermal shock, resistance to the fluids and the normal operating temperature of up to 140°Celsius) that had so far eluded the design team. This, they stated, would deliver a definitive solution that would exceed the described design requirements across every parameter, rather than just some of them.

The pump manufacturer bonded the transducer magnet to the shaft, using Lohmann DuploTEC 10410 SBF (Structural Bonding Film), and found that all the required shear and tensile strength parameters had indeed been met. This included one of the tests involving the immersion of the parts in a hot oil bath (using oils from a variety of manufacturers) at 140°Celsius for almost 1,500 hours. The test result data, combined with physical examination and metallurgical investigations, revealed that no degradation of the bond was evident. In fact, when the tests were extended, it was found that the magnet material itself disintegrated before the limit of the bond strength of the DuploTEC 10410 SBF adhesive tape could be reached.

In a further example of recent structural bonding success, a well-known manufacturer of electric motors has specified DuploTEC precision die-cuts, again for mounting magnets onto the end of a shaft, this time inside one of its new ranges of motors. Although the tape in this application isn’t immersed in hot oil, it did need to pass another series of stringent performance tests required by the motor manufacturer.

Particular attention was paid to durability, given the thermally dynamic environment in which the magnet is used, which can see frequent and rapid temperature changes occurring inside the housing in multiple cycles throughout a typical working day. The precise location of the magnet mounting die-cuts for this application can be seen in the accompanying illustration.

Lohmann’s Jeremy Parfitt comments: “These are further examples of why we prefer to work with customer design engineering teams at the start of a project. In each of these applications, our previous experience with bonding in similar high temperature environments gave us confidence that, even though our tape was to be subjected to particularly demanding thermal and fluid criteria, the solution we were proposing was viable. And so it proved to be. As the respective customers discovered for themselves with their extensive testing regimes, Lohmann had succeeded in providing an engineered solution to overcome their process headache!”

A further advantage was also enjoyed by the both the oil pump and electric motor manufacturers, something which had not been touched upon while the original design solutions were being discussed. Since all Lohmann tapes (including the DuploTEC range) can be supplied direct from the Lohmann factory as ready-to-fit die cuts, the mounting installation time when married to a semi-automated application process was cut to the absolute minimum, delivering associated cost and time-saving benefits.

Brian Wall

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Lohmann Technologies UK ltd


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