Ok, from the above mentioned thread you've probably found the magic formula.
Lbs = (pi X TPI X in-oz) / 8
So using 20% of 425 and a 5 TPI screw we get : Lbs =( 3.14159 X 5 X 85) / 8 = 166.875 Lbs. (with a perfect frictionless screw)
This rotary power to linear motion thrust confuses lots of people so let me give a little different explanation of whats going on here.
First off we start with a 425 oz/in motor and assume 20% power at running speed.
This gives us (425 X 0.20)= 85 oz/in running power. Divide by 16 and we have 5.3125 lb/inch.
What does this mean? It tells us we have enough power to provide 5.3 pounds of force 1 inch from the motor shaft .
So if we were to attach a 1 inch radius (2" dia) pulley to the motor and wrapped a string around it we could lift a 5.3 pound weight.
That's nice you say but I'm not lifting my slide with a string I'm using a 5 TPI screw.
Well you can look at the screw a as ratio increaser or big lever. One turn of the screw moves the load 0.200 inch.
One turn of our motor with the 2" dia pulley lifts our 5.3 pound weight 6.283 inches ( 2 X pi X R ).
So we have a lever that on the long end is moving 6.283 inches with 5.3 pounds of force and the short end moves 0.200 inches.
This is a 31.415 ratio lever ( 6.283 inches in = 0.200 inches out ).
5.3125 pounds X 31.4145 leverage = 166.88 lbs.
A 100% efficent 5 TPI screw is always a 31.415 torque multiplier. Add a 2:1 reducer at the motor and its a 62X multiplier.
2 TPI screws are ( 6.283/0.500 ) 12.566 multipliers.
Now comes the down side. Missing from the the above formula is the screw efficiency.
Acme screws are only about 30% efficient, ballscrews from 80 to 90%.
So using an acme screw we have to multiply our 166.88 lbs by 0.30 giving us about 50 lbs of thust force. ( we're running out of power here
)
With a ballscrew we get up to 166.88 lbs multiplied by 0.90 giving 150 lbs of force (no wonder people like ballscrews so much).
You can't use all of this power to just hold the axis up. You need some power to accelerate the system.
How much depends on how fast you want to get up to speed.
As an example it takes about 15 HP to propel a full size auto down the road at 55 MPH.
But as you can imagine it would take forever for a 15 HP car to get to 55 MPH.
So lets say you're getting on the freeway. With a long on ramp (slow accel rate) a 60 HP car gets up to speed in plenty of time to merge into the traffic. If you have a 50 foot on ramp (high accel rate) you're going to need maybe 200 HP to get up to speed in time.
Accel rates have a huge influence on motor power required. You have to calculate the power required to accelerate the screw and the mass of the load along with overcoming the Z gravity loading and friction of the system. You also have to worry about force to overcome screw nut preloads and mounting bearing preloads (drag torque).
These calculations gets messy fast so I use Kollmorgen's Motioneering software to calculate my motor torque requirements.
This allows me to play with pulley ratios and accel/decell rates to optimize system performance.
Bob