What is happening is, we are translating torque force (turning on a screw ) into a downward force and into frictional force. It is the frictional force that stops the tool post from moving (rotating). This frictional force needs to be high (obviously) to stop any movement and best positioned at the bottom between the toolpost (block) and the base plate.
Now like Ken has said we only have so much force (W) we cannot magic up any more, we are stuck with the amount W. This force W is distributed between the lower friction (which we need to stop the movement) and the upper friction (which stops us turning the nut).
So we have W = Lower friction + Upper friction.
If the upper friction is 0 (zero) then W = lower friction, which means all the force W is distributed to the lower friction which is where we want it to be.
A thrust bearing can be considered to have zero friction. (In this case)
There is a mathematical (a bit heavy going) proof here http://www.roymech.co.uk/Useful_Tables/C...ews_1.html
Best I can do
DaveH
Now like Ken has said we only have so much force (W) we cannot magic up any more, we are stuck with the amount W. This force W is distributed between the lower friction (which we need to stop the movement) and the upper friction (which stops us turning the nut).
So we have W = Lower friction + Upper friction.
If the upper friction is 0 (zero) then W = lower friction, which means all the force W is distributed to the lower friction which is where we want it to be.
A thrust bearing can be considered to have zero friction. (In this case)
There is a mathematical (a bit heavy going) proof here http://www.roymech.co.uk/Useful_Tables/C...ews_1.html
Best I can do
DaveH