occur when the lifter 8 starts from the lower mechanical end position the lead initially causes an increased load acceptance compared to the spindle lifting drive 53 because of the higher loading of the spindle lifting drive 52 the rotational speed of the associated driving electric motor 54 a is slightly lower than the rotational speed of the other electric motor 54 b therefore the counter emf generated by the armature of this motor 54 a is smaller than the counter emf of the armature of the electric motor 54 b in this way the current through the series circuit of the motors can increase however because the motor 54 a is loaded more strongly its rotational speed can increase less quickly than that of the less loaded motor 54 b in this way the less loaded electric motor 54 b can catch up in terms of travel the motor 54 b is braked in acceleration only when its extended position overtakes that of the initially more heavily loaded electric motor 54 a according to the elasticity against which the two spindle lifting drives 52 and 53 work this compensation play repeats alternately until after a few alternations the load is distributed symmetrically between the two motors 54.