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Bomag BW120 AD when reversing slowly the brake has been initiating fighting the drive motor

Discussion in 'Rollers' started by Farmingosteo, Jul 12, 2020.

  1. Farmingosteo

    Farmingosteo New Member

    Jul 12, 2020
    We recently acquired a Bomag BW120 roller with 1000 hours on the clock.

    When reversing really slowly we were finding at times the brake was initiating whilst the roller was still in drive. It was quite abrupt and from what i have read can lead to quite extensive hydraulic motor damage. The roller was not switching off. Just the brakes fighting the drive of the motor.

    Reading on the internet there was detail on the seat microswitch being the cause of the issue. So, this was checked but found not to be faulty (worth looking at though). With the roller in question the symptoms did not involve the machine switching off so the issue not fully in line with the seat microswitch fault.

    So we looked for a magic eye on the drive joystick. I believe this was the answer so thought i would write it up before i forgot. After only 1000 hours the magic eye / forward backward mechanism was showing signs of wear leading to the magic eye that senses whether the roller is in the forward backward position being closer to the metal that initiates it when just in reverse. The fix was to remove the chair, grease up the mechanism of the joystick fully and move the magic eye a nats whisker (use a feeler gauge) away from the metal that initiates the sensor.

    Predisposing factor to the issue being related to reverse and not forward.... the sensor position is not quite central. When moving forward the sensor is soon away from the metal. When reversing the required movement to clear the sensor is further. Difficult to explain but look at it in real life. It makes sense.

    107522827_604520347165131_659347967441087575_n.jpg 108247028_274350443873200_339310495411870487_n.jpg

    Here you can see the sensor and the metal moved via the joystick lever. This is under the machines seat.

    Tools needed.... Spray on grease and electrical contact cleaner. Small screwdriver and cable ties. Ideally a feeler gauge and plastic card.

    Firstly the machine should be off.
    Disconnect the seat microswitch if possible and put the seat in the upright position
    Lift the forward backward lever of the seat adjuster and move it fully forward. Make sure no cables are being stretched (the seat microswitch)
    The seat should come off the runners just infront of the steering wheel column. Put it to one side.
    Now move the joystick forward and back. Look at the movement of the metal infront of the magic eye.
    It may be a little worn with poor tolerance.
    Work out with a feeler gauge and credit card the gap between metal and sensor. Record this.
    Now move the sensor a nats whicker further away. That is a measurement using the feeler gauge. Record this.

    Grease all moving components and clean the contacts where possible.

    Put it all back together and try again. Reconnect the seat microswitch and cable tie the cable away safely.
    Test the roller. Make sure the brake initiates when the roller is stood before moving off. This is of course a safety feature.
    I hope this helps as I could not find any other write up of this.
    Last edited: Jul 12, 2020
    mg2361 likes this.
  2. mg2361

    mg2361 Senior Member

    Jul 5, 2016
    Equipment Mechanic/Technician
    Welcome to HEF Farmingosteo;)!

    Good job and nice write-up. Those switches can be pesky at best.