Newbe needing help with simple limit switch project!

[zeroweaver]

My boss asked if I could find a way to prevent damage to our shrink wrapper that is missing some components. It used to have some kind of limit switches a long time ago but not anymore. The way it works now is you put a pallet on the platform and attach the shrink wrap. Then start the pallet spinning and you manually work the switch that moves the shrink wrap bar up/down. In the pic of the control box the switch labeled "A" is the on/off switch for the pallet spinner. The one labeled "B" is the switch that operates the shrink wrap bar. It is a threeway toggle switch with a "ratchet?" stop between the two directions so you can't switch it from one direction to the other in one motion. You have to stop in the middle before you can switch it the other way. This was it gives the heavy geared motor a moment to stop before going the other direction so it doesn't strip the gears. The way its setup now if the person is not paying attention they can run the wrapper bar off the top or run it to the very bottom freezing up the gearbox. I need some kind of limit switches on the top and the bottom that will prevent someone from going to far in either direction. I was thinking it should be very simple and cheap to rig something up. I dont really know how to design this. Please help. Thanks See pics

 

[KrisBlueNZ]

Hi and welcome to Electronics Point

In case no one else answers this question, I can try to help. But I have no idea how a shrink wrapper works, and your description and nicely marked-up photos don't help me much.

If no one else here can help, perhaps you could upload a video to YouTube (or anywhere really) that shows you describing the parts of the machine, demonstrating how it works, and explaining the problem and what you want to add.

 

[(*steve*)]

In general, limit switches are likely to be some form of rugged switch that interrupts power when a moving part reaches or exceeds the limit of its normal range of motion.

One simple way is to have a normally closed microswitch that opens when it is actuated, removing power from whatever it is that causes motion. Practically, the microswitch will often not carry the entire load current (for a great many reasons) but will control something like a relay that will in turn the load off. When configured this way, you would need to manually reset the device by moving it away from the hard limit.

A slightly more complex method is to cause power to be removed if the motor is moving in a direction toward the limit. This is done in a way that allows power to still be applied in the opposite direction thus allowing the normal controls to move the moving part away from the limit (a soft limit).

Because of the added complexity, you might employ both a hard and a soft limit. You can have a soft limit switch that will allow operation (but stop further movement in that direction), and a hard limit placed a little further along that will cut all power if you hit it.

An argument against additional hard limit switches is that you are currently operating without any limits at all, so a soft limit should be fine. However, what soft limits do is to encourage operators to just hold the direction control on until the soft limit actuates (and in some cases continue to hold it). That's fine until the soft limit switch fails and the movement continues to a point at which it becomes dangerous for the operator or the equipment. The hard limit gives you one more chance to save the day and operates in a way that is inconvenient enough that people won't want to hit it very often.

The reason for normally closed switches is that it tends to be safer. If the switch breaks, or if the wires are broken it will be more likely to disable the equipment than to disable the limit. The largest risk is a switch failing in a way that prevents it from being actuated. This includes levers being bent, the switch falling off its mounting point, etc. Limit switches should be tested regularly to ensure that they continue to function and are undamaged.