For a motor the solution is simple, the worst case force is produced at
stall (or at a certain rpm for a synchronous motor - aka squirrel cage and
others). The maker also gives the maximum torque of the motor (aka stall
torque but see above). Once you know this and the size of the 1st gear
(the gear on the motor axle), you can work out the maximum force and its
direction in each attachment point of the motor. You can use a graphical
method if you are lost at numbers (requires mm paper and a ruler and
pencil mostly).  Usually you over-engineer the measuring device to 100% or
more for small motors like yours, to account for mechanical abuse (dirt in
gears, people turning axle backwards etc etc).

hope this helps,

Peter

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