how to calculate torque/power/speed needed 3

[barrygold]

Hi All.
I am looking to determine the torque/power/rpm. needed to lift a 4 kg. load
150 mm vertically in 10 seconds.
I need to use a 6 volt dc. gear motor mounted vertically and connected to a 12 mm
dia. acme lead screw ( steel ) with a 3 mm pitch and 48 % efficiency.
The lead screw nut witch the load is connected to is also steel.
The load guide is a steel tube with a brass bearing.
Please see attached drawing.

Best regards and thanks in advance.

 

[desertfox]

Hi Barry

This is fairly simple stuff to calculate I used the formula on this site:-

http://www.roymech.co.uk/Useful_Tables/Cams_Springs/Power_Screws_1.html

Just considering the Acme screw in order to achieve your requirements it would need to rotate at 300rpm and the power required is approximately 0.5W with a torque of about 0.072Nm.
Bear in mind this is without the 48% efficiency included.
I can see from the drawing that it is to lift a 4kg load but whats the actual application of the screw device.

desertfox

 

[dvd]

You are partially guiding the load with the screw. Everything has to be made perfectly, or the mechanism binds. The mechanism does not tolerate deformations well.

I also recommend against designing around components and following up with the calculations. How do you know that the lead screw is adequate without determining the forces, torque, HP, etc.?

I have had bad experiences with using a bronze bushing as a linear bearing when there is a moment involved. The pressure is applied unevenly and these bushings tend to not allow smooth movement of the carriage. Plus lubrication is difficult and cleanliness becomes an issue.

I recommend that you start with some analysis and then select components. Decide on a velocity profile. Determine the forces and size the screw and bearings. Look at the instantaneous power requirements and size your motor to that requirement.

 

[geesamand]

^ All good points, however the bronze bearing performance as a slider depends heavily on the engaged length vs. diameter. If the bushing is long enough and the fit is true enough it may work out acceptably. You certainly would want to finish the ends of the bushing carefully though.

To the OP, is energy efficiency or size critical? Can you leave room to retrofit a larger/smaller one once the assembly is prototyped?

David