Elevator's suggestion...

[iacob alex]

.....or the lifting machine,with a counterbalance,approximates an Atwood machine,that has to overcome only the weight difference and inertia of the two masses.

     Naturally,if everything should be made as simple as possible,an Atwood machine,can be a direct manner to create a gravity unbalance,to let it grow,and more tempting,to repeat this state of going on,as a "self".

    The first idea can be ,to replace the string of the pulley ,with a first class lever (a flexible/variable arm ),and to play torque difference...

       All the best! / Alex

[onthecuttingedge2005]

if you take one balance at 0 and add one gram of weight you will get one gram of pressure as the weight falls unbalanced.

how do you intend to get 1 gram of weight to reverse it's mass to get it to perpetuate forever?

you may have to include all fundamental forces as a whole to get this to work and even then the Nuclear force would be adding energy and that would last a long time but would not last forever.

of course you could use the natural influx of gravity over distance to unstable your device since the Gravitational field is not a constant over the surface of the entire earth. look up gravity maps.

[iacob alex]

...it's only a part of the "picture".

    An elevator is working with a constant speed.

    An Atwood machine is not working with a constant speed.

    An Atwood machine with a massive pulley has a retarded free gravity fall (and acceleration...),due to the fact that the rotational motion of pulley figures as an "additive" inertial resistence.

   The gravity fall of a small unbalance is increasing,and "moves" into  the rotational storage of the massive pulley.

   From this storage,in the bottom position ,we can draw out a part of it,and move on the top (in a distant,remote procedure ),so to start again with the same small unbalance...

      All the best! / Alex