Counterweight on Rostock

Does is make Sens to add an Counterweight for every arm, so the motors have the same up and down force?

Not really as you will then have twice the mass to move, so half the acceleration.

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It might, but how much weight is the question. The problem is that the down-force on each carriage changes depending on where the head is, relative to the carriage. As it moves further away from any given carriage, in the X-Y plane, it puts less down-force on the carriage.

In addition, benefits of the counterweight have to be *ahem* weighed against the additional inertia added to the system by adding the counterweights.

All that adding the counterweights will do is increase the maximum acceleration that the platform can use.

[edit: ] Provided, of course, that the counterweight inertia is less than the reclaimed downforce--now that I think about it, this can never be true, so you can't increase the maximum acceleration available by adding counterweights.

Edited 1 time(s). Last edit at 02/13/2013 10:06AM by Annirak.

... I've seen some systems with a really long spring wired from the head to the ceiling, so the counter-force would be nearly constant over the Z range ...

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Viktor
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Its common on conveyor lifters to add a counterweight to reduce the effective mass that is lifted and hence reduce the work done by the motor. Its also common on fork lift trucks. I believe the mass is set to around half the maximum lift weight or half the expected mass to be lifted in the case of a conveyor.

The mass is connected so it hangs the other side of the top roller and assists the lift by effectively counteracting the mass.

Speaking simply and ignoring friction I think you are all partially correct.
If I remember my physics right, the counterweight would naturally accelerate down at 9.81m/secsq (gravity) if unconstrained, so I think it assists acceleration up to that value when the carriage is rising and the counterweight is moving down (above that acceleration value the connecting cable is basically unloaded) so you don't get any benefit.

When the main carriage is moving down and the counterweight is moving up, The counterweight resists the acceleration as the masses are effectively combined..

I could understand there may be a benefit if the carriage is many kilograms, but for a light carriage such as on the delta, the frictional forces are probably the main ones you are concerned with. This counterweight system would be more complex so any benefits you might gain would be hidden by the increased friction of the more complex mechanism.

Viktor does raise an interesting point as there may be some benefit to reducing the Mass of the tool head by connecting it directly to a counterweight or spring. I think you would only see a benefit when moving up in the Z direction quickly. As a 3D printer only does this in small increments every now and again, the benefits would be minimal with current printers.

Its a good suggestion though and one that could be effective on larger machines such as those that they are considering to print buildings.

Having the belts under tension in one direction like on a Rostock also means that any backlash due to belt/pulley mating imperfetions is minimised.

Greg Frost Wrote:
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> Having the belts under tension in one direction
> like on a Rostock also means that any backlash due
> to belt/pulley mating imperfetions is minimised.

That's only true if your acceleration doesn't exceed acceleration due to gravity.