Linear screw actuator

[Clareok]

Due to space restrictions I wish to design a linear actuator using trapezoidal screw, see attached sketch.

Travel required=730mm
Required time=25sec
Motor Speed=584rpm
Mass=45Kg
l=Screw pitch=3mm
dm=Mean pitch diameter=12.45mm
f=Friction coefficient=0.15
F=Load=441.45N
Calculated torque: Fdm/2*((l+pi*f*dm)/(pi*dm-f*l) = 0.7Nm

My question is, will the required motor torque be the same as the nut, i.e. 0.7Nm?
I cant figure out if the torque on the gear be more than the nut (as this has a large radius)

 

[desertfox]

Hi

If I understand the sketch correctly then your driving the screw through a set of gears with one gear fixed to the screw and a ratio of 1:1, that being the case then your motor needs to produce that same torque plus losses.

 

[Clareok]

Yes that is correct, thanks for your response. i might have to change the gear ratio for correct motor selection. Thanks for your response. Have i calculated the correct torque? Does 0.7Nm sound correct?

 

[desertfox]

Hi

Using this link I get the torque to be 0.63Nm so it looks good to me.

http://www.eng.auburn.edu/users/marghitu/Screws.pdf

 

[tbuelna]

The analysis approach seems a bit too simplistic in a couple respects. First, it ignores the fact that there some horizontal offset between the load mass center and the axis of rotation of the lifting screw. The resulting moment will increase the friction losses in the screw mechanism. Second, the frictions from the reaction moments about the screw axis produced on the offset linear bearing shaft contacts is also ignored.

Since you have a gear set between the motor and jackscrew nut, there will be a difference in the relative torque at each location due to mechanical losses in the supporting bearings and gear contact. When you calculate the total mechanical losses in your system for sizing the electrical motor, you should make sure to consider the combined worst case for all of the contact friction losses.

Good luck.
Terry

 

[NuclearNerd]

I can't see from your sketch where the thrust loads from the lead screw will be transferred to the frame. Depending on the design of this thrust bearing, it could be a significant additional source of torque.