Can I lock Z axis?

Hi reprappers!

I have a Prusa i3 with a Melzi board. I have just customized my printer: I replaced the right Z motor with a threaded rod, and connected it to the remaining left Z motor with a timing belt. I have the following problem with it: All my other motors (X, Y, and E) are locked after they were moved by the controller (I cannot rotate them by hand), but not the Z motor. I tought the cause was the two motors connected paralelly to one output, but it is still exists with only one motor. If I switch Z and X motors on the board, then the Z is locked and the X is not. How can I make the controller to lock the Z motor after movement? I am afraid, the carriage descends a bit from the shakes and sudden movements.

Thank you for your help in advance!

I think this could be the current limit. The more current the motor gets, the more it should "lock". You can try to adjust the current to see, if there is any effect.

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Prusa i3 Rework (from Replikeo.com) & original Prusa i3 MK2 (from shop.prusa3d.com) << the MK2 definitely is worth the higher cost!
Print Troubleshooting Guide // Visual troubleshooting guide

or firmware, eg in marlin

check for

// Disables axis when it's not being used.
// WARNING: When motors turn off there is a chance of losing position accuracy!
#define DISABLE_X false
#define DISABLE_Y false
#define DISABLE_Z false