Newbie from NZ

[AndyUK]

So I've thrown your numbers roughly into our motor calc spreadsheet, which is only a guide, but has a few nice assumptions which lead to a wide safety margin.

Assuming your ballscrew is 900mm long (which obviously results in a travel less than 900mm), a 1610 screw (for speed rather than accuracy) and a moving mass of 90kg (you said 400N load, but the spreadsheet wants a moving mass - so lets take the chair plus ~20kg of frame).

If the screw is fixed-supported (i.e. BF and BK mount) we get a critical speed of 2200 ish rpm before whipping is an issue. That means we have a travel speed limit of 22m/min, well in excess of what we need. Assuming a 15s ish travel, lets say 6m/min which could travel the full distance in 10s but allowing time for acceleration and deceleration. That nicely translates to 600rpm on a directly coupled motor.

All in, you need a torque of 1.34nm (ish) at 600rpm to achieve that. Make it 4nm to allow for dynamic loads and a 3x safety factor.

Now, here comes the issue. Motor selection. I'm not great with low voltage motor selection - so I've used a 12.5nm nema34 stepper at 64ish volts. This easily gets you to the 3x safety margin that the spreadsheet suggests, but isn't exactly straight forward to use in a car. I'll let someone else chip in at this point :)

MotorCalcs-rjsutton.xls