Almost every project of mine from the last quarter century, if it has contained any wiring, has featured somewhere at least one crimp connector. There are a multiplicity of different types of crimp, but in this case I am referring to the ubiquitous variety with a red, blue, or yellow coloured plastic sleeve denoting the wire size they are designed for. They provide a physically robust and electrically sound connection that is resistant to wire fatigue due to vibration, and that can carry hefty currents at high voltages without any problems.
You might expect this to now head off into the detail of crimp connection, but my colleague Dan has already detailed what makes a good or a bad crimp. Instead recently my constant searches for weird and wonderful things to review for your entertainment led me to a new crimp tool, and thence to a curiosity about the effectiveness of different styles of tool. So I’m going to evaluate the three different crimping methods available to me, namely my shiny new ratchet crimp pliers, my aged simple crimp pliers, and for comparison an ordinary pair of pliers. I’ll take a look at the physical strength of each crimping method followed by its electrical effectiveness, but first it’s worth looking at the tools themselves.
Laying Out the Tools
There is little to say about the set of pliers other than that they are a perfectly ordinary set of pliers — sometimes called Lineman’s Pliers — that happens to be on my bench so I will call them Bench Pliers. You almost certainly have a very similar set.
My Simple Crimp Pliers were bought sometime in the 1990s from an automotive superstore, and they’re type of multipurpose auto electrical tool that yet again many of you probably have. They have many functions, but those that interest us are the shaped receptacles with the coloured dots at its jaws. Clamp a crimp connector in these with a wire inserted, squeeze the handles, and it makes the appropriate connection.
Finally, the Ratchet Crimp Pliers are a more expensive take on the same tool but with a set of removable dies bearing the same coloured dots to denote crimp sizes. I can buy dies appropriate to other styles of connector, but it’s likely in this case that I won’t. Amusingly their inexpensive origin (through one of the Chinese channels for hardware) is revealed by the coloured dots being applied to the wrong dies, but that doesn’t detract from their effectiveness as a tool.
Testing Crimped Splices
If I’m to evaluate some crimp tools, first I have to make some crimp connections to work with, so I robbed some wire from the mess of cables on an old ATX power supply, and took three red crimp cable splices appropriately sized for the wire. These are not special at all, being from a cheap retail multipack of crimp connectors.
The ratchet pliers made the most impression upon the red plastic, leaving it most visibly deformed in the shape of the dies. It is probable that an adjustment of the ratchet would change this, I’m simply using it as it came to me for now. Meanwhile the simple crimp pliers did their usual neat job, and the bench pliers merely flattened the connector into an oval.
To measure the effectiveness of a crimp connection, what do you need to know? In a lab you might cut through it with a precision saw and polish the exposed end for examination, or perhaps study it as a metallurgist would, with x-rays to see the effect upon the metal crystals. But on my bench I have neither of those things, and anyway my needs from a crimp connection are more down-to-earth. I need to know the following: Will it fall apart?, and Will it take the current I want to give it? So I need to examine its breaking strength, and its electrical resistance.
A Bit of Backyard Metrology
I lack a lab with a tensile strength evaluation rig, but I live on a farm and have ready access to a fruit picking ladder, and a bucket. The ladder is a roughly 6″/2m-ish tripod depending upon how you place its legs, so from it I suspended a bit of rope to which I tied my bucket via my wire with the crimp splice in it. By filling the bucket with water from a measuring jug until the splice parted I could gain a rough measure of its strength. There is a small weight from the plastic bucket, but since my resolution is one litre of water, or about 10 Newtons if you take mild liberties with the strength of English gravity, I’d say it’s within experimental error.
It’s probably one of the more unusual things I have done for Hackaday, sitting in the sunshine pouring water bit-by-bit into a bucket. A good tip should any of you feel the need to do this is to suspend the bucket only about an inch/25mm above the ground, because when the splice gives way there is a significant amount of potential energy in all that water which could split the bucket. As it was there was mild soaking of feet involved as each failure produced an impressive splash, but you have to take the rough with the smooth in the world of farmyard engineering.
The forces required to part the splices from each tool are collated in the table below:
|Tool||Litres of Water||Approximate Breakage Force (N)|
|Simple Crimp Pliers||14||140|
|Ratchet Crimp Pliers||20||200|
Straight away we can see that with a breakage force of only 50 Newtons, my bench pliers do not give a good quality result. This is hardly surprising, but worth recording. It’s also not a surprise that the ratchet pliers give a stronger result than the simple ones, this is probably a function of the better dies, and its lever action. It is a surprise how much better they are, but since a crimp connection’s purpose is electrical rather than physical it does not mean that the 140N strength of the simple crimp plier connection is inadequate.
Torturing Innocent Power Supplies in the Name of Measurement
So, we’ve established that in the field of tensile strength of a connection made by a given crimp tool, you get what you pay for. How about in our other field of evaluation then, the resistance of the connection? Here we immediately encounter a problem, and it lies in the tiny resistances involved. Crimp connectors are designed to offer as perfect a connection as possible, so their resistances are tiny. To measure a resistance in the milliOhms, you can hardly pick up your trusty multimeter and break out the probes. We live in a world of resistances in the kOhms, and normal instruments are not made for such tiny values.
A few ideas were considered for how to deal with this problem, and in the end I settled upon one involving a current limited power supply. My multimeter is good at measuring tiny voltages, so if I apply the same constant current to all the various crimped connections I can read the voltage across them at a preset current and use Ohm’s Law to derive a resistance. Out came one of the ubiquitous Ruideng switching regulator modules set to its maximum 5A current value, and the voltage across all three crimp splices as well as a piece of wire of the same length was measured. The resulting values along with the calculated resistances are in the table below.
|Splice Under Test||Voltage at Regulator Terminals (mV)||Calculated Total Resistance (mOhms)||Crimp Splice Resistance (mOhms)|
|Straight Wire (no splice present)||15.1||3.02||0 (No splice present)|
|Simple Crimp Pliers||15.7||3.14||0.12|
|Ratchet Crimp Pliers||15.8||3.16||0.14|
At first sight, it might seem that an unambiguous but unexpected tale of the crimp splice made with the simple crimp pliers being the one with the lowest resistance, but the reality has a significant contradiction. Even by switching to a voltage reading we are measuring at the lower limit of what it possible with an inexpensive multimeter, and relying on the current limiter on a cheap voltage regulator module to be a paragon of metrology virtue is a strategy that is never going to go well. So these readings all come with significant error bars, and thus are not as clear-cut as the numbers themselves might have us believe. What the figures do tell us is that there is a clear difference between the two purpose-built crimp tools and the bench pliers, but that as long as a crimp is appropriately compressed it makes little difference to the final resistance what tool makes it.
So after going further into the properties of a crimp connection than I have ever done, my conclusion is that while it’s nice to have a pair of fancy ratchet crimp pliers on my bench they really aren’t that much better than the simple crimp pliers as I’d have believed them to be. Meanwhile nobody would expect a pair of standard pliers to be much good at crimping, and I’ve amply proved that. What all this has shown about crimp connections is that for the tiny cost of a crimp splice you get a significantly strong and super-low-resistance connection if you use the correct tool, and given me a new respect for these ubiquitous connectors.