Motor Torque

[watzmann]

Hi All ,

need your help please, i'm building an CNC small machine with 3 Axies , using 3 stepper motors , i alraday bought the 3 stepper motors ,

i'll use this motors to drive a power screw and then the axies moves.

now i need to know whether this motors will be able to drive the load or not .

the max. load for the X axies motor is 50 KG , and the lenght of powerscrew is 2.5M , 25 mm diameter,

the motor model is L-707 in this link , you can find all realted details about it .
http://www.astrosyn.co.uk/docs/l-se...r this motor is enough to run the load or not

 

[fsahmed]

Instead of going through calculations, u can simulate the load since u already have the motor. Just fix a pulley on the motor shaft, hang a 50 kg (or more just to be safe) weight with a wire & turn the motor on. Make sure u r metering the current that goes into the windings & dont let the current exceed the rating. If the motor lifts the load without exceeding the current rating, its fine for ur application.

 

[watzmann]

but to do that , sure there's a method of calculate it on paper insted of going into such experiments , and so it will to know the maximum torque of that motor

 

[awright]

Motor Load

Watzmann, I agree with fsahmed that, since you already have the motors (and leadscrew?), it makes sense to set up a little experiment to simulate your application. There are aspects of the application that require some information that you may not have at hand, like acceleration of the load, speed of the load, and friction of your leadscrew and load supports.

Since you mention a load in mass, not in torque (which has units of force x lever arm length), I presume that the motor/leadscrew combination has to lift a 50 Kg load. If that is true, fsahmed's pulley suggestion could give you the answer you need (if you experiment with pulley diameter), but it won't tell you if you can move the load with a leadscrew.

A leadscrew advances the load some known distance for each revolution. That is the leadscrew pitch. Divide that by the steps per revolution and you have the advance per step. 50 KG is not a trivial load to accelerate, so the acceleration of the load (i.e., how fast you bring the load up to desired speed) has to be taken into consideration. The force F required to accelerate the load of mass m at acceleration a is F = ma. This force can be reduced by limiting a to the extent allowed by your application. It is in addition to the friction load and the static load.

Once you have the load moving at the desired speed, the mechanical power input is determined by the product of force and velocity (with appropriate factors to account for the units you use). For horizontal motion at constant velocity, the force is determined by the weight of the load and the coefficient of friction of the rails, rollers, slides, whatever supports the load. For vertical motion at constant velocity, the force is the weight of the load. Weight is the product of mass and acceleration of gravity. This assumes negligible friction in whatever guides the load.

Your leadscrew has an efficiency determined by the coefficient of friction of the screw on the nut (and friction of the bearings supporting the leadscrew). For a plain screw and nut, the losses can be fairly high, depending upon the materials, the finish,and the lubrication. A ball-nut leadscrew is MUCH more efficient than a plain leadscrew and threaded nut and can approach 100%.

So, you see, your question is not trivial and cannot be answered off the top of the head. An experiment may get you your answer more easily. Remember to account for deterioration of components, loss, contamination, or hardening of lubricant, etc., in your final design.

awright

 

[bryan1]

Hi Watzman,
below is a page I scanned out of a book called easy steppin. It gives you a couple of formula's to work out torque and motor requriements hope it's of some help.

Cheers Bryan

 

[Klaus]

Hi All ,

need your help please, i'm building an CNC small machine with 3 Axies , using 3 stepper motors , i alraday bought the 3 stepper motors ,

i'll use this motors to drive a power screw and then the axies moves.

now i need to know whether this motors will be able to drive the load or not .

the max. load for the X axies motor is 50 KG , and the lenght of powerscrew is 2.5M , 25 mm diameter,

the motor model is L-707 in this link , you can find all realted details about it .
http://www.astrosyn.co.uk/docs/l-se...r this motor is enough to run the load or not

A lead screw, 2.5m long is not exactly a "small" device, in fact I would say its rather large. Have you thought out the mechanical details to support it adequatly? The other thing, have you studied stepper motor driving systems? Its not just a simple matter of turning a switch on or off to make it go. You have to 'ramp up' the steps for the shaft to gather speed and then to 'ramp down' to stop it.
Then you have to consider the 'backlash' of the travelling nut on the screw and allow for it too. Circulating ball screws would be nice to eliminate the backlash, but I think at the size you are talking about the expense of such is rather large.

If, as required with CNC, you wish to do that with any accuracy, you need a good driving board.

How did you measure the 50kg load? Is it what you put on the table or is it the load you must apply to the lead screw ( at a certain leverage) to make the load move? There is a difference, if its the latter you can work out if the torque of your motor matches the torque on the lead screw.

Keep in mind that there will be extra loads if you do any machining while the lead screws rotate - as is usual with CNC machinery.

Klaus