I’m the electronics mentor for our team and I’m getting ready to hold a class in electrical fundamentals in the coming weeks.
I’ve been reflecting upon a chronic problem that seems to rear its ugly head every build/competition season - failed crimp-on terminals.
I try to watch carefully when a student completes a crimp and I make sure to do a tug test afterward. Yet later during testing, practice or worse during competition, a crimped terminal will fail. Especially at the motor controller. Its frustrating to say the least.
I’d like to hear what your team does to ensure that crimp-on terminals don’t fail. I’m sure many of you have experienced similar problems. While I have experience in crimping terminals I’m no expert. I’d prefer to solder all my electrical connections but that’s not always practical.
Electronic & Programming Mentor
Most of our smaller crimps we squeeze tightly and pull test. For our larger and most likely to be destroyed ones (Cim motors, high vibration zones, potential to get snagged on the co-opertition bridge) we soldier them. We also do regular (maybe bihourly) inspections on everything. We give all our wires a gentle tug and repair any loose connections
We do not solder wires to terminals and we never have problems with them. One thing I do is choose the right size terminal for the wire and invest in the proper crimping tool. Another thing I found out is if I twist the wires first before crimping, it is much more secure because it is much harder for the wire to “slide” out.
We/I use a good quality crimper, it isn’t ratcheting but as long as you make sure to crimp all the way it shouldn’t be an issue.
Make sure to match your connector to your wire size, too large wire in a crimp or too small will cause a bad connection.
I’ve never had an issue with insulated terminals, in FRC or at work with equipment that runs for thousands of hours with heavy vibration.
And of course, make sure to pull test your crimps.
We do not solder our connections, use insulated terminals, and don’t have a problem with failed crimps.
#2 above is definitely the one biggest thing we have done to ensure crimp quality. We spent under $40 for a high-quality, name brand crimper with the proper die.
We also made sure it was adjusted for the right crimping pressure. The ‘right’ pressure is when the wire strands actually deform and form a solid mass, with (virtually) no air between strands. We have sectioned our crimps, polished the surface, and examined them under a microscope. Probably overkill, but it was a learning experience for the kids.
And as Ed mentioned, twist the wire strands together - not just so they enter the terminal neatly, but also so you don’t get stray strands that can short to other wires.
When stripping, be sure you don’t nick any of the strands.
When pull-testing, pull as hard as you can. Pull to destructon during training, so students get a feel for the upper limit…then pull near that for ‘production’ crimps.
And, of course, use high-quality connectors (name brand, like 3M) of the correct size for the wire, and check them regularly during competition.
To get good crimps it takes good quality tools. Harbor Freight Tools has a ratchet and hydraulic crimping tools that seem to be of a good quality and price.
The best crimp will still fail if the wire is not supported.
We just don’t use crimps. The only time we use them is on the terminals for motor controllers, and each of those is made on-the-spot (no premade cables) and examined for quality before putting it on the robot. Crimp terminals are, as you said, very unreliable. It takes the right tools and considerable experience to get consistently good crimps. But, they also introduce a lot of resistance into the circuit, which can result in reduced efficiency and reduced robot performance. I would highly suggest planning out your electrical system in such a way as to completely remove crimp terminals from your design.
Watch the direction you are crimping the terminal, there is a seam where the wire goes in. If you use the crimpers that has two half circles (which we will not use) you will just crush the connector when the seam is in the wrong place. We use the sta-kon tool that has a half circle on one side and a divot on the other side and make sure that the divot is opposite of the seam. You must crimp hard or it will not take, most students do not have the strength needed for proper squeeze pressure so make sure the person doing it has strong hands.
You can view this on one of our tips page http://www.team1322.org/electrical.htm
We use ratcheting crimpers to ensure a good crimp and then solder too just to be safe. Be sure to use the right terminals for the right gauge of wire. I prefer non-insulated and then put heatshrink on after the soldering is complete
There is no substitute for a ratcheting crimper. MCM sells a nice one that is color coded to the terminal. Terminals are colored for the wire size range they are meant for. The intent with crimping is to form a “cold weld” as Don hinted above, a proper crimp forms a solid mass of metal with no air gaps between strands. However, we still don’t trust that and use uninsulated terminals and solder every one after a solid crimp and tug test. We reserve insulated crimps for the practice robot and prototypes simple to save time in assembly. When the wire size is too small for the terminal you have (fan wires for instance) simply strip long and fold the wire once or twice to fill the terminal before crimping. If you use the Thomas and Betts style crimper (http://www.specialized.net/Specialized//Assets/ProductImages/272X190_PLI.jpg), the tool has an opening with a “tooth” on one side. The terminal should positioned so that the tooth is opposite the side with the seam on the terminal. When crimped the seam should not open. If it does, remake the connection.
A proper crimp connection cold-welds the wire to the connector. There will be an excellent mechanical and electrical connection.
Whippet, if your team is soldering wires to connectors and still having them come out, it is time to seriously evaluate your soldering procedures and training. Though soldering is completely unnecessary if a crimp is performed properly.
Here is an article that goes into great depth on the finer points of crimping, crimping tools, pull tests, etc. It is definitely worth a quick read through. Note that the article is 4 pages long!
Some of the article is specific to boats, but much of it translates directly to robot applications. Like others have said, a ratcheting crimping tool is absolutely key to reliable crimp connections.
Edit: in some of the forum discussions linked in the above article ‘pull testing’ is discussed. Most notably that pulling a wire by hand results in a tug test of around 20lbs of pull force, a fraction of the force required by actual pull-test specs.
I am always seriously surprised to hear people recommend ratcheting crimpers. In my experience, ratcheting crimpers that are under $200 seem to not work properly. Two of our mentors are master electricians with 50+ years experience combined in the field, both prefer manual crimpers (found in the electrical section of home depot, etc.) These work much better and allow the kids to crimp the heck out of any connector. Its virtually impossible to over-crimp an insulated connector with a hand tool…
Soldering has the tendency to increase more than crimps do (done properly, it will be minimal however) ever notice we don’t solder battery lugs on?
McMaster has them. Although I have used this method in the past, with great success, it is a lot of work for little or no benefit over a good crimp connection.
Can you cite a specific example or experience where a ratcheting crimping tool that was properly adjusted failed to work when used properly?
It’s nice that two people prefer manual crimping tools, but that is a far cry from any substantiated evidence. Manual crimping tools DO NOT provide ANY consistency across multiple users. Some students can squeeze hard enough for manual crimping tools to work, but ratcheting crimping tools ensure consistent crimps and alert weaker students (no, not everyone is created equal) that they need to find someone to finish the crimp for them.
They do have to be adjusted properly to work properly. Though getting upset that they don’t work when they haven’t been adjusted is like getting angry when parts come out wrong on a mill that hasn’t been indicated. I have a $12 ratcheting crimping tool that I bought at Harbor Freight that have served me well.