add brake to servo

My "fill in" project for the summer has been a ball screw and air
cylinders for my knee on the Excello CNC mill. Still got several parts
to fabricate but I was close enough to fit it all together today to
check for problems.
I found a good one. Drop the air and power to servo and the table
drops like a rock. I knew it would go down, but the speed was totally
unexpected. I'm going to need to add some sort of "dead man" brake to
the servo. That is, drop power and the brake closes.
Any good suggestions for what to use? Keep in mind I'm a cheap skate.
Karl
Reply to
Karl Townsend
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A solenoid that can stand 100% duty cycle, rigged to hold a dog clutch or a very self-energizing brake open when the power is on. Dunno where you'll get the solenoid, but there's got to be something out there for cheap. (Unless Iggy has bought the world's supply of them, surplus).
Note that you need to be careful about the dog clutch -- you want it to positively catch, even if things have already spun up a bit. So you want it more like a ratchet on a come-along, rather than a square dog clutch. If the clutch just bounces instead of engaging, then you'll just have an audible alarm when your table drops. That's probably not what you want.
You may also want to consider some way of parking it cleanly -- i.e., drop the power to the brake hold-off solenoid first, then drop power to the servo.
Depending on just how disastrous it can be for the knee to drop, you may also want to consider some purely mechanical safety device, like a centrifugal thingie on the ball screw such that if it spins up too fast some dogs fling out and catch in a rack, stopping the works up until you or your servo motors actively apply torque to the ball screw.
Reply to
Tim Wescott
I've used this type of brake sandwiched between a servo and an overhauling load in a situation similar to yours. Warner Electric and Electroid make similar brakes, but they're much (like 2 or 3X) more expensive. You do have to consider how to coordinate application of the brake with both intentional and unintentional loss of motor torque.
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Reply to
Ned Simmons
I would do two things.
First, I would implement a counterweight to ease the load.
Second, I would get one of three:
1) A servo motor with a build in brake (brakemotor). 2) A servo motor large enough to hold the table in one place against weight, without overheating. 3) A geared servo motor.
If you recall, I also have a motorized knee on my mill. I have a geared servo motor that does not slip when powered off, due to gear reduction. When I bought it, it had no encoder, but it had a little extra shaft, so putting an encoder on it was easy.
I had to enhance EMC2 to turn it off when not instructed to move for more than 10 seconds. I wrote that component in C. Without it, the motor would overheat, trying to push the knee "one last hair".
It works great, as of right now.
i
Reply to
Ignoramus11295
Ned's solution is best but fails cheapskate test. At least right at the moment no brake servos in the right size range on eBay. Maybe I'll watch a while. If no other ideas, I may buy this:
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Not sure if this is enough holding torque.
Karl
Reply to
Karl Townsend
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I would wait for the right motor at the right price. I have a few servo motors for sale also. (no brake)
i
Reply to
Ignoramus11295
Not sure if either of these are cheap enough to pass the cheapskate test.
This one would be about right for a 500W or so motor. It appears to have a smaller than standard bore.
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Probably way overkill:
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Reply to
Ned Simmons
You passed the cheapskate test on the first one. I don't know if its a bigger brake than the item I found, Wish i could hold both in my hand first.
Karl
Reply to
Karl Townsend
Got an extra air cylinder? Fab up some brake shoes with discarded disc brake pads and spring-load them to clamp on the ball screw by default. When air is applied to the servo, it's applied to the spare cylinder which opens the brake. Kill the air and the brake auto-engages. A 1/2" stroke cylinder would do it.
Aye, iffen it's cheap you'll be wantin', laddy, just keep a pair of vise grips handy.
-- Fear not those who argue but those who dodge. -- Marie Ebner von Eschenbach
Reply to
Larry Jaques
I may just use a variation of the vise grip. Tighten gibs as soon as move complete, Just put an extra limit switch on this. But, it makes machine a one man show, nobody else could run it.
Karl
Reply to
Karl Townsend
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You guys gave me a good idea. I can build a variation of this concept.
That's why I spend time on this NG, really helps to get input on things like this.
Thanks everybody.
Karl
Reply to
Karl Townsend
Visit surpluscenter.com and find a suitable failsafe power to release brake pack. Option 2 is to add an air cylinder acting against a spring brake so when counterbalance air is on, the brake is released.
Reply to
Pete C.

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