Alexs

doubling the voltage will times the force by 4 so...you run the risk of exceeding the design limits and breaking it

In my opinion 40kg of thrust is enough (itll lift two robots plus the arm) and it is pretty quick... that depends on your actuator extension to arm tip lift gear ratio.

The ZYJ(s)01 High Speed actuator @ 24v might suit your needs.
http://www.firgelliauto.com/COMPACT%20ACTUATOR%20SPEC%20Jue%2005.pdfhttp://www.firgelliauto.com/COMPACT%...20Jue%2005.pdf

Woody