Limit switch Application

Could you attach a picture?

Is it possible to weld (or some other form of attachment) some sort of finger to the 12" tube? Or do you need mechanical restraint as well to prevent over-travel?

Have you considered some sort of non-contact sensor or photoeye? How much travel is involved and how accurate does the limit switch have to be?

Photo eye would not be good, oil and dirt will interfere. Use an Inductive Proximity switch, one that senses ferrous metals. Good one's can have a sensitivity adjustment feature that can filter out false triggering on shavings. Balluff come to mind as a good choice.

Google is your friend:

http://www.cpi-nj.com/waterpr.htm

Mount the prox on the trailing end of the outer tube. Drill a hole in the inner tube to change the state of the prox when the outer tube reaches it's extended travel. You could use the same prox and another hole with a little more logic to control the retract portion of the trip.

Had a similar issue on a food packaging machine many many years ago. What I came up with was...

Make a disk, say 2" diameter. Drill a hole in the center. Drill a bunch of small holes, like 1/16", around the perimeter of the disk. Drill and tap a hole in the screw shaft that moves the brass follower. Attach the disk to the shaft. Place fiber optic emitter and receiver pairs attached to a Bannar photocell unit so that they see holes on the perimeter of the disk. Feed output of the Bannar photocell to a counting unit, say an input on your PLC. Count holes as the screw shaft turns. Place entire end of the crew shaft along with fibre heads in an enclosure to protect them.

Depending upon the accuracy you need, such a systems may fill your bill. This worked for many years for us.

A proximity would work, but depending on the accuracy it can be sloppy. You might look a weld immune laser sensor. They come simple and inexpensive, they can have basic scanning or distance measuring.

I would put at the end of the 10" tube and set it to see the 12" tube when it reaches the correct placement.

I would first like to say thanks to all who took the time to reply. I know if you have a busy schedule like me its difficult... Next, as i said in my first post grease and shavings had previously been a problem as we had used inductive proxs for that app. Before that we had used mechanical limits, but due to the distance between the tubes changing due to wear of the pads; the mechanical limits would come out of adjustment and could very easily become damaged. Our trouble has been with sensing the inner tube as the outer tube extensions over it. We perfer a non-passive design that in the event of a failure stop the unit. But again thanks for giving me a place to get some feedback.

EngE