spring clip vs screw clamp

[paulengr 44]

I've been slowly migrating over to using spring clamp terminals mostly because they hold up much better against vibration (the spring clamps can't "back off"). In every way they seem to be an improvement over screw terminals. However, the plant I'm working at now has reported problems with them in the past. Seems they've had problems that repeated wire removal/insertions where the wiring has to be routinely changed "wear out" the spring clamps faster than screw terminals. Anyone had similar experiences?

[Duffanator 1]

Yes. A lot of our European machines come with spring clamp terminals and they do wear out. Especially on safety circuits where you have to remove wires for troubleshooting they do not hold up well at all. The springs wear out or bend and then they don't hold the wire correctly and you can pull it out. Also, in our environment they tend to rust and break because of all of the humidity in the air. We use stainless steel screw and clamp terminal blocks just for that reason. Vibration isn't so much of a problem for us though, so I can' speak to that but as far as holding up in harsh environments and repeated use they don't seem to work well at all. They are nice in the right situation though, it's much easier to put wires into them than a screw terminal. We shy away from them though just because they don't hold up. We usually end up changing them out anyway so we just put the stainless steel screw terminals in right off the bat.

[PdL 71]

Spring clamps are good when: - The wires are not removed and connected repeatedly (like said above) - When the correct size screwdriver is used to open the clamp (like Weidmuller has a perfect plastic tool). I you use a screwdriver that is too big or too small, you easily wreck the clamp. - IMO it should be used up until 10mm2 or 7AWG but not bigger. When they get bigger you have to put too much force on it and the casing distorts easily. - The wires are stripped correctly (when stripped to short it could be that the clamp is on the insulation. Nice to trace a fault like that!!)

[plcdp 11]

Hello, One other thing I have seen with spring clamps that was a fun one to troubleshoot... If two wires are in the same terminal, and are different sized wire, it is possible to clamp on the larger wire and not get good contact with the smaller wire. plcdp

[PdL 71]

You should not put two wires in one clamp even if they are same sized. You can use twin clamp terminals for this...

[Crossbow 295]

I use springs whenever I can. Have also used insulation displacement (the Weidmuller IDC2 family is excellent) and screw terminal. Most PLCs still use screw terminals, but in a control panel where field connection needs to be quick, springs are faster than screws. As stated above, not meant for hundreds of insertions, and can be easily damaged by a big screwdriver (but then what can't?).

[TimWilborne 108]

I resisted the change to spring terminals for a long time. I even went out of my way to special order parts without spring clamps. The early ones required special installation tools, maintenance personnel couldn't figure out how to remove the wires and would break them in the process, I've even seen them wire nuted around because the guy couldn't figure out how to work them, etc, but they have come a long ways. One big problem I still see with some of them is that there is nothing to stop you from pressing the clamp open too far. You press the clamp lightly and the block opens, but they people think you need to put an extra 25lbs of force on it to open it up that extra 1/64" which crushes the spring in it to the point of damage. Also when people are in a hurry they will pull on the wire, then release the clamp. The more urgent fixing the breakdown is, the harder they seem to pull which causes damage, especially the the lightweight guillotine style clamps like you see on instrumentation. In a perfect world you could just train everyone to use them properly and the problem would be solved but old habits die hard and some guys just don't work well under pressure. Of course my first question would be why are you looking at fixing a connection problem that is caused by the removal and insertion of wires instead of fixing the problem that is causing you to have to remove and insert the wires, ie sensor failure...but you didn't ask that question. I personally still wouldn't use a screw clamp for your application. Is it possible to put a connector on the enclosure for these replacements? Depending on how much your downtime cost you per hour it could be justified by the time saving alone of connecting one connector instead of multiple wires.

[paulengr 44]

At BEST, yes, it is possible. I work in a mining environment. Pretty much everything is subject to crushing. That means pretty much everything must be run in rigid metallic conduit, LFMC (FMC rusts way too quickly), or armored cabling. I love M8 and M12 connectors and cord sets. If I could do it, I'd wire all the equipment this way. It's SO easy to replace cords. I've even gone as far (in other operations) as to purposely buy and mount M12 connectors with pigtails, wire an instrument, and then screw the connector into the conduit threading to create an IP68 rated sensor with a cord connection. Only problem though is in an environment subject to physical damage. This kind of ruins the possibilities with cords. Second best I've come up with is to always land the instrumentation wiring inside an LB and use that (with wire nuts) as the point where the sensor gets wired in.