Lead / Ball screw stop

[TahoeTomahawk]

Hi All,
I searched around and couldn't find an answer to this and was hoping someone could help.

I am building a pushing mechanism to replace a 1/2" roller mechanism. I found a 1/2" leadscrew from Roton, the product line is HiLead. Along with some bearing blocks, the idea works great however, the problem I didn't think about is that there is nothing preventing over travel.

How is this usually accomplished with ball / lead screws?

Thanks All!!

[Crevice Reamer]

Hi Tahoe Tomahawk. Welcome to the Zone!

I assume you are worried about the screw coming out of the ball nut. One doesn't want to lose one's balls.

You can make a hard limit stop for the carriage or whatever. Or you can make a stop on the end of the ball screw. I have center drilled and tapped the ball screw end for a screw and washer on mine. (LocTite)

CR.

[Al_The_Man]

Normally before a physical stop is reached, a overtravel limit switch is wired into the controller in some fashion, What and How the limit switch actually interupts depends on whether you want an E-stop or just a controlled stop with alarm message etc.
This is done wether you are using a simple relay logic, PLC or a CNC type.
control.
The one down side to just a physical stop is the BS can be damaged if the driving mechanism is high torque.
Al.

[TahoeTomahawk]

Hi guys, thanks for the replies.

Yes what I'm concerned with is the software the controls the stepper motor. I've seen it freak out before and continue turning the motor. The way I mounted the Lead Screw into the Bearing Block, I'm afraid it will damage (something).

CR, not really worried about it coming out of the ballnut because I have a block bearing at the other end to support it.

I wasn't sure if it was common to have like a part of the screw at the end with no thread or something that causes the ballnut to just spin freely when it reaches the end, that way both ends of the screw can still be supported.

Does that make sense, and is that something that is normally done? If not, is it possible to make 2 areas (beginning and end) to have a place where the ballnut can spin freely as damage protection from over-travel?

Thanks guys!

[Al_The_Man]

The ball nut has to stay engaged, IOW the BS cannot really revolve without moving the nut,
If you are concerned with the motors running away, use the LS to control power to the drives, a control or E-stop relay.
For non-run away conditions, you could use a software over travel, in case a distance was programmed that exceeded the physical stroke.
Or if the drives have an enable on them, the limit switch could operate this input.
Al.

[TahoeTomahawk]

Thanks Al,
I'll start researching limit switches and E-Stop relays in the electronics section.

Thanks!

[Geof]

If you are using steppers then you know your torque limit. Probably this is low enough that you are not going to damage anything by running into a hard stop. In addition to limit switches what you can do is put a 'soft hard stop', a bushing made of something such as nylon or UHMW polyethylene on the ball screw so that the nut makes contact with this plastic before you get metal to metal between the nut and a bearing. The plastic slows things down gently if you overshoot.