Horse power calculation?

[Steelforbrains]

Can anyone tell me how to calculate the HP (for an electric motor) required to startup and drive the system in the picture? I have tried it several differnet ways but I keep getting different results. The torsion springs are designed to be at equilibrium in the position shown (at the center of the stroke). Would I be better off to design the springs to be at equilibrium at the bottom of the stroke?

Hey, how do I insert a picture?!

 

[GregLocock]

Exactly. How can people estimate a hp when the type of each bearing, and the stuff on the table, is effectively unknown?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bigTomHanks]

Couldn't this problem be easily modeled using Working Model software? I would do it but unfortunately I don't have access to it. Does anyone here have it available?

 

[FredRosse]

GregLocock is right, the classical solution shows no power consumption in the steady state, as elastic springs only store, then give back virtually the same energy. Moving mass up/down has the same result. Modern bearings have very little energy consumption.

To find power consumption, we need to determine where the input mechanical energy is being dissipated or moved out of the system.

I would think the primary energy consumption path is the frictional heating of the material in the shaking table, with a secondary energy output of the kinetic energy imparted to the material which exits the shaker. Having said that, the analysis to arrive at the required power becomes very complicated, and probably only available from tests or empirical data for similar equipment.

Since the mass of the system is nearly doubled, you could conservatively expect the starting loads to approximately double. This would indicate a similar 10 HP motor is probably safe from a starting standpoint.

As far a steady state loads, the new wear material may cause different frictional character on the table, and this could change the power consumption. Steady state power requirements could go up or down, and this is unknown. Engineering judgment tells me that the power will probably be less than double the original, so the 10 HP motor is again OK.

 

[RobWard]

If you rigidly attach 3000lbs to a machine weighing 3700 won't you alter the performance characteristics in some way?
Will you need to run at a different speed.. or change the springs...or derate the machine...etc?

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[zekeman]

Since the system is virtually not dissipative, the minimum basic requirement is to have a motor with sufficient torque/ speed characteristics to move the load from rest.After it comes to speed, you have part of the cycle that absorbs energy and part of the cycle that releases the stored energy to the motor and its inertia( a flywheel is probably needed to filter out big rotational speed variations) with the motor supplying only minimal power to sustain the motion. Accordingly, the motor is sized only by its starting torque charateristics.

 

[Steelforbrains]

Zekeman,
Thank you, that was the point that I was trying to make earlier...

 

[GregLocock]

So, to play devil's advocate, do mechanical engineers often spec 'power' when they are interested in 'torque'? They must get some very puzzling results at low speeds.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.