the relative motion of the two frames 39 and 41 the lifting mechanism of lifter 8 is itself very rigid because of the directly adjacent arrangement of the two lifting motors 51 differential thrusts and thus torsions between the lifting motors can very easily occur even if only small movement differences arise a further difficulty is that the two lifting motors 51 are spindle motors which by their nature are self locking and are able to produce very large forces even if the lifting motors are initially aligned it is practically impossible to prevent differences in running speeds from developing due to the tolerances of the lifting motors this leads in the course of the time to a difference in lift between the two lifting motors in order to balance the lifting motors in operation such that each of the two lifting motors contributes about equally to the total lifting force the balancing circuit represented as a schematic diagram in fig4 is provided in fig4 the two lifting motors operating in parallel mechanically are labeled a and b each lifting motor has an outer telescoping tube 52 as well as an inner telescoping tube 53 that can be set in rotation via a rotating threaded spindle 54 drawn in dashes in fig4 in order to displace inner.