Elma RM90 cleaning machine spin motor problem

Discussion in 'Watch Repair' started by rherber1, Dec 3, 2012.

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  1. rherber1

    rherber1 Registered User

    Dec 3, 2012
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    #1 rherber1, Dec 3, 2012
    Last edited: Dec 3, 2012
    This is a heads up for owners of Elma RM90 watch cleaning machines manufactured before approximately mid 2005... I am not a watch repairer but an electronics engineer.

    Recently I was asked to repair an Elma RM90 machine which was exhibiting intermittent operation of the spin motor. The machine was manufactured in 2005. the owner first noticed the symptom about 12 months ago and by "poking around" had discovered that the cause of the fault was the cable which connects the motor to the control board. He found that if he tied the cable back to the chassis with and elastic band the motor could be restored to working order. He resorted to this fix due to having no other machine available and the work load was high. Of course, such a fix was not going to be permanent and it would eventually fail permanently.

    When it did fail the machine was brought to me for repair with a hint that the job was urgent. After dismantling the spin motor from the machine I performed a continuity test of the three conductor heatshrink covered connecting cable using a resistance sensitive continuity tester fitted with an alligator clip on one side and a needle probe on the other. This buzzer produces a warble tone for resistance below 30 ohms and a single tone for resistance over this value (ie. high resistance) and obviously no tone for open circuit. The alligator clip was connected to one end of each of the 3 wires in turn while the needle probe was used to pierce the conductor insulation at various points along its length until a high resistance or open circuit was detected. Two of the conductors were shown to have open circuits and the third indicated high or open circuit condition depending upon how much tension was placed on the wire.

    Visual inspection of the individual wires showed that they could be bent quite easily at a point mid-way where the owner had tied the cable back with an elastic band, thus forming a stress or flexing point. Slitting open the insulation on the wires at this point showed that some or all of the 19 individual copper strands making up the wires had broken. I decided to continue testing from this point towards the motor because I reasoned that there was another stress/flex point right where the cable came through a rubber grommet in the motor frame. Sure enough it was found that all 3 wires had suffered similar breakage in some or all of the copper strands at this point.

    This proved to be a bit of a problem since the 3 wires were integrally formed into the field winding of the motor and it was not possible to replace them completely. Fortunately, the damage point allowed about 12mm of insulated wire extending beyond the field winding so that I could join, solder and sleeve new wires onto the stubs of the old ones. When I had joined the wires I reinforced this point with a dab of epoxy glue to prevent any movement or possible pulling away from the fine wire of the field winding. The join point was also just inside the inner lip of the rubber grommet close to the field winding so as long as any flexing in the cable could occur beyond this point the original cause of breakage would be avoided in future. By making the cable long enough to reach the control board such that the radius of any bending point in the cable was about 50mm I reasoned that the cable should not be stressed to breaking point. The only annoying thing I found was that I had to transfer the original terminals from the old cable to the new one by soldering and sleeving them. The new cable was enclosed in heatshrink to provide suitable protection while allowing flexibility.

    I made an enquiry with the manufacturer of the wires used by the motor manufacturer (Huber + Suhner) and they informed me that a 3 conductor cable made of their Radox 125 0.5mm2 (19x 0.18mm) in this application should have a minimum bending radius of at least 33mm in order to prevent stress or flexing damage to the copper strands. The original cable length did not allow for this minimum radius at the point where the cable came through the grommet at the motor.

    I referred this fault symptom back to Elma and received the following reply;
    Many thanks for your detailed description and your diagnosis of the problems that lead to breaking wires and operating failures in our RM 90 watch cleaning machines.
    The machine you are writing about was manufactured in November 2005.
    We realised this problem a number of years ago and have solved it by using a longer wire so that it is no longer bent in a way that breaks the individual strands. Your machine was obviously manufactured before this change.

    So it is clear that Elma had recognised this cable fault was due to a manufacturing or design specification problem for machines produced before about mid 2005. As such, I would argue that there may be a case for warranty repair for other machines from this era which exhibit the same or similar symptoms. Since the motor has the cable built into the field winding, the normal method of repair would be to replace the motor with a new one having the extended cable length. The machine I repaired was located in Australia and the cost of obtaining a new motor from Elma (any warranty claim would have been a long argument and the machine was required urgently) would have been excessive, so the only option was to repair it as I had been forced to do.

    Ross Herbert
     

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