The motor lifting the arm is a Nema 17 with 50:1 gearbox reduction. Theoretically they are capable of a lot of force. According to the specifications the motors (without gearbox) have 52N.cm torque. That multiplied with 50:1 reduction means 2600N.cm. The efficiency of gearboxes is 73%, so the useful torque is around 1900N.cm. The motor does not rotate the arm directly, but through the help of a 47:14 pulley reduction and this means that the final torque is around 6370 N.cm. At a distance of 70 cm (arm length) it means a 91 N.cm, which means that it can lift about 9kgs at 70cm. Note that this is a theoretical upper bound because the gearbox does not support more than 600N.cm before damage can occur, so the practical upper bound is below 3kg at 70 cm. Also, note that neither friction nor arm's weight were taken into account here. However the arm weight can be canceled if a spring is used. One more aspect to consider is that the torque of stepper motor decreases at high speeds, so again the practical upper bound for torque is lower than the one computed before.
Still, in our experiments (see for instance movie) we have been able to lift more than 3.5kg at 40cm (maybe a little bit more because the smallest weight that I had was of 0.5 kg). This means a torque of at least 1400N.cm and no gearbox damage was observed in long term.
Another problem is that 50:1 gearboxes are difficult to back-drive. In the near future we plan to replace the 50:1 gearboxes with 27:1 ones because the maximum allowed torque before damage can occur is similar.
Regarding the speed of the 50:1 gearbox motors we have observed and made the following computations: The maximum speed that we were able to obtain with the motor (no gearboxes attached was 1500 steps/second. After that speed the motor just stops). Knowing that the motor has 1.8 degrees steps, it means that we can about 7.5 complete rotations in 1 second. After the 50:1 gearbox we can make 0.15 of a complete rotation in 1 second. After the external 47:14 reduction we make 0.044 of a complete rotation in 1 second, that is about 16 degrees in 1 second. Thus, a complete up-down move (180 degrees) of the arm will take about 11 seconds. We are currently investigating other stepper motor drivers to see if more steps per second are possible.