Unless it’s a chunk of lead ziptied to the belt, there will probably also be additional cost in one or more bearing slides and bearings for the conterweight to ride along.
I had a thought where the bed could be lifted using filament on pullies, and the whole affair being driven by a leadscrew/motor mounted flat on the bottom. Loss of power would not cause a drop, but tuning might take a little effort.
It wouldn’t need to ride on a slide at all. I’m not sure how much the bed assembly weighs but it may not be practical in terms of volume to fit a piece of lead I suppose.
I suggested this a little while back but not on such an active thread!
What about a pawl? Like a ratchet action on the Z gear? This would limit you to only moving Z in one direction with a basic setup, although you could install a solenoid to hold the pawl ‘up’ and if power is lost that solenoid (from my limited understanding) should disengage and drop the pawl ‘down’ to lock the z axis gear in place and prevent any further movement?
Only issue with the solenoid method vs the non-solenoid method is, if it doesn’t lock back in quick enough, the gear may be spinning too fast and the pawl will have trouble engaging, at least with the first method, despite only having one direction of movement, you should not be able to crash the bed (or move it downward) without manual intervention.
OR as an alternative to the worm-wheel method, and easy-to-print design, what about a planetary gearbox? Or something derived from Emmet’s harmonic drive? http://www.thingiverse.com/thing:219779
They’re meant to be able to hold position even when the drive forces are removed… which exactly describes our goal!
Both good possibilities, @Jarred_Baines …I was additionally thinking of centrifugal clutches. They should be available in R/C sizes, and would act as a brake if one side is held stationary, yet not engage if movement was slow.
as i mentioned above a ratcheting design would limit z layer height to the lock positions unless it only engaged when not printing ie a power loss. but this still doesn’t solve the problem of a timed motor release. either the motor would be locked for prolonged times after activation or drop. if it dropped onto a ratcheted design chances are it is going to drop some and if this is a filament pause or the like you’ve lost position and the print will fail
@D_Rob If the ratchet was on a gear identical to the z motors step then there would be no [measurable] loss.
true but you have a better chance to print a worm gear set than a 1.8 degree ratchet/ solenoid set up
In my experience… Loss of power means loss of print… Or at LEAST loss of position - I saved a print once by re-homing and manually setting the Z position where I thought it should be, but that wasn’t a great result either.
What do you mean when you say “a timed motor release”?
Filament pause, as in “a pause in the program so you can change filament”? If this was the only issue, it is fixable in firmware - Z needs to stay locked (along with X and Y) and you need a code to unlock E.
The issue we CAN’T fix with firmware is the “loss of power = crash” issue.
A pawl would likely fix that. It doesn’t need to positively lock at each layer height, it just needs to stop the BANG encountered at power-loss.
and @Mike_Miller I was just returning to suggest that, or even a sprag clutch, overcoming the issue of the pawl ‘locking into’ a tooth (and the possibility of it NOT locking into a tooth!) The centrifugal clutch has been standardized for seat belts, seems like it could work here too, it’s pretty much an identical situation.
This is all if you REALLY want to not use screws… I’d just use screws, problem solved. But this is good exercise for the brain 
I meant if the filament stripped and you paused the print to fix the issue. or the like. if you stop the motors and let it stand for a few minutes they go slack. yes there is a firmware adjustment that @Tim_Rastall found a way to work around. but that means keeping the motors engaged for prolonged periods . If all anyone wants is crash protection do what tim does put springs on the rods
?
I’m sorry, in this design, the Z motors should be ALWAYS engaged during printing… if they ever disengage, the stage drops… there’s nothing mechanical stopping the Z stage from falling, it’s PURELY the z motors that hold the load for the entire print (if it works as I imagine… I haven’t had time to get nitty-gritty and study this machine yet!)
So what you’re asking of the motors while “pausing for a filament or other issue” is no more of a burden on them than printing is. They’re performing the EXACT same function under a ‘pause’ as they are under a ‘printing’ scenario.
They shouldn’t “go slack” either - what goes slack? do you mean the motor disengages completely? or it begins losing steps? or are you saying the belts go slack somehow?
It’s a failure-mode…if the printer loses power, then things drop. I’d prefer it to do so gracefully (as less than 9.8 m/s^2)
Come on 32ft/s² over a1 ft distance isn’t that bad lol
SO, Sublime posted on the Ingentis thread in reprap forums about a fluid friction brake in response to this discussion. He’s anti google so won’t post directly. Anyway, as usual he’s come up with a very clever solution.
http://forums.reprap.org/read.php?279,222917,320475#msg-320475
@Tim_Rastall , that is pretty much the insides of most rotary dampers. I’m glad to see this discussion so active. Here’s my two cents.
Any use of worm gears out rack and pinion will give the same artifacts in ribbing as a lead screw, the worm gear more so, since a worm is almost the same thing as a lead screw. The period of the rubbing will be different though.
As for braking the the z stage in case of power failure or axis timeout, in servo systems, brakes are applied to motors when not actuated to prevent overheating. The same can be done with steppers. There would need to be code added to apply the brake between moves, and it would obviously be a brake that engages under no power, disengaged under power. In marlin, just set the z axis to not be pureed between moves. Hmmm, actually, without adding code, like @Whosa_whatsis said, you can use the motor enable pin to trip the brake. When it’s not enabled, the brake engages. And when I say brake, it’s generally attached directly to the motor shaft, preferably a motor with a shaft sticking out both ends. Brakes a are commercially available for large Nema sizes, not sure about 17 or 23
@D_Rob It is at 2am!
This is similar to what I described. I’m just not sure whether this is a power off brake, as there’s no data on the part. Just realized that most steppers with brakes are $200+, yikes.
@Ben_Malcheski I am actually using a 3mm screw for the Z in one of my bots. Stepps/mm are 6400. I have a solid coupler and use a " slop nut " to take care of any wobble.
@Jarred_Baines by goes slack I mean the motor is on a timed disengagement cycle after a command is given. If I move a motor via software, and there is a pause after, it stays engaged for a short period. Then I can move the shaft by hand. If I pause a print for whatever reason and leave the room the stage will fall if I exceed that axis’ timeout period. True this can be changed in firmware. Or as I have done on my machine using a worm gear (printed) the z only moves when I tell it to. Power or no power pause or no pause
@Mike_Miller huh?