Confused about Strain Relief Bushing for Ordinary 16/2 Lamp Cord

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brockrwood

brockrwood

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I need a few "strain relief bushings" for a homemade 12V launch controller I am building.

The wires to the sealed lead acid battery and the wires to the igniters are typical 16 gauge, 2 conductor lamp cord or "extension cord" wire. The flat stuff. With the two wires side by side. In fact, some of the wire is from a couple of extension cords I got on sale. The other portion is from Home Depot.

(One length of wire MIGHT be 18 gauge. My old eyes are not very good. I can't tell if it says "16" or "18" AWG.)

I want to get some "strain relief bushings" (those little plastic grommet things) for where the wire exits the controller box. You have seen them: They are little, black plastic doohickeys you often see on lamps and other household appliances where the wire exits the enclosure of the appliance.

But there seems to be mind-boggling variety of these strain relief bushing things. I have no idea which one to pick.

Here is the web page at Mouser showing the selection:

https://www.mouser.com/c/?q=strain relief bushing
Can anyone help me pick the right strain relief bushing? Is there one that is sort of "universal"?

Thanks in advance for any help!
 
Oh, man! You’re in a heap of trouble. The strain relief fittings are sized for the diameter of the hole n the chassis, plus the width and gauge of the flat wire. Unfortunately I have never found a size chart - I always had to do this hit or miss from an assortment of sizes. Plus you will need a chassis punch to make the proper D-shaped hole. And don’t forget the special pliers to grip the strain relief and compress it enough to get it seated into the hole. It would be great if you had a local electronic parts store so you could take your wire in and see what strain relief fit the best.

i assume your zip cord is not for AC, but instead will be used to carry 12VDC high current to the igniters. If so, consider either binding posts or banana plugs and sockets to attach your zip cord. Those just require some drilled holes, which you can do either by hand or with a drill press. If the chassis/project box is plastic, consider using a brad point bit. In my experience they drill a more precise hole in that material. Other than that, installation does not require any special equipment.

And your zip cords will be easily swappable for travel replacement, or should you need longer or shorter ones.
 
augendoc said:
Oh, man! You’re in a heap of trouble. The strain relief fittings are sized for the diameter of the hole n the chassis, plus the width and gauge of the flat wire. Unfortunately I have never found a size chart - I always had to do this hit or miss from an assortment of sizes.
Click to expand...
There is a highly detailed sizing chart and diagram for the fittings in the link (post 1).
 
Well if cheap and cheerful works for you, just buy one of those sampler boxes from Amazon; there's a ton of them.

Use whichever size is closest and make up the difference with a waterproof flexible glue like E6000. Won't be pretty but it'll keep the water out.
 
Use the larger strain reliefs and wrap tape around the cord to make it thicker.
How long is your run from the launch controller to the launch stand?
We use 14 or 12 gauge wire for the cords from the controller to the rockets.
You will loose less power between the controller & rocket with the larger size wire.
With 12 gauge cords we can launch (with 12 volts) at 200' but at 300' the igniter won't work.
 
Handeman said:
Use a grommet to protect from the sharp edges of the box, and then tie an Underwriters Knot in your wires. That will work as a strain relief since it doesn't need to be water resistant. That knot has be in use for about 80 years now.
Click to expand...
Just an ordinary, round rubber grommet? Like these?

ISPINNER 190pcs Rubber Grommet Assortment Kit, 9 Sizes Eyelet Ring Gasket 1/4" 5/16" 3/8" 7/16" 1/2" 5/8" 3/4" 7/8" 1" (Black) https://a.co/d/3umRzat
 
brockrwood said:
Just an ordinary, round rubber grommet? Like these?

ISPINNER 190pcs Rubber Grommet Assortment Kit, 9 Sizes Eyelet Ring Gasket 1/4" 5/16" 3/8" 7/16" 1/2" 5/8" 3/4" 7/8" 1" (Black) https://a.co/d/3umRzat
Click to expand...
Sure, those will work. You only want to protect the wire from the sharp metal edges. If you are using a plastic container, you only need to knock off the sharp edges and don't even need the grommets. The underwriters knot is what gives you the strain relieve.
I remember being taught that knot when learning how to fix a lamp in a 7th grade intro to industrial arts class. I've been using it for 50 years. It works.
 
brockrwood said:
About 6 feet. I just place the box near the launch pad. But I have a new, camera tripod based pad, so it has to go up in the air about 4 feet. So I figure 2 feet on the ground and 4 feet straight up.
Click to expand...

Hmmm, one idea: the camera folks get a sling bag that hangs under the tripod head. Fill it with rocks, the extra weight gives some stability to the tripod. Is it practical to hang the battery box from the pad tripod?
 
cls said:
Hmmm, one idea: the camera folks get a sling bag that hangs under the tripod head. Fill it with rocks, the extra weight gives some stability to the tripod. Is it practical to hang the battery box from the pad tripod?
Click to expand...
Connect wires to igniters. Hand battery and “pad” box to naive friend standing next to pad. Say, “here, hold this”. Extend out the 20 feet of wire to the controller box. Begin count down…

I AM JOKING!
 
NTP2 said:
Is there anything you can’t get from McMaster?
Click to expand...

They have a lot of stuff for sure. They are more expensive than other suppliers but the convenience and time savings of one stop shopping is worth it for me. The nice thing is that they have cad models of many parts that you can just plug into your own models.
 
Hello All,

Not to be particularly technical, but the Title for this post did ask about finding something to use as "strain relief" for 16/2 zip cord coaming thru the wall of a homemade controller.

I know that the poster himself immediately confused the issue by asking about little black plastic grommet thingies, but I think everybody needs just a little clarification. A grommet is not in any technical sense strain relief. A grommet, as several have mentioned is something to keep the sharp edges of an enclosure (usually sheet metal) from damaging the insulation around wiring that needs to go thru the surface of the enclosure.

A strain relief device, is a means of keeping external pressure on let's say a wire going from the outside to the inside of an enclosure, from putting any tension on connections inside the enclosure. Said tension could eventually cause failure of those connections inside the enclosure. So some means of limiting if not eliminating the transfer of external tension thru wiring from affecting internal connections is desired. Devices for this affect are called strain relief. The actual strain relief is more often than not accomplished by a rubber insert which is a type of grommet, but it is held in place by a housing which solidly connects to the enclosure. So a grommet is just a grommet. Strain Relief is a grommet in a solid housing.

I couldn't help but notice that you mentioned Home Depot. I must also say that I have purchased these devices from Lowes and Menards as well. I put in a picture of the specific devices that I use. They are "strain relief cord connectors" and come in many sizes and usually come in packages of one or two per package, unlike the boxes of 25 in the picture that I get straight from a Halex distributor. I use these a lot with WFX launch systems like in the thousands at this point. The picture shows three different sizes all made by Halex. The housing is identical. Its the rubber insert that determines which electrical cord size works best with which strain relief. The ones pictured are all rubber inserts with round holes. If you want really good strain relief, that's not just a rubber grommet, I would suggest buying/finding/scrounging one 16/2 outdoor extension cord that has round insulation. They come in three standard sizes in the 1/2" fittings: .5", .375", and 1/4", which refers to the size electrical cord you can fit thru the hole in the rubber insert. In the picture I've attached the 1/2" holed rubber insert is on the left with the 1/4" on the right and the .375" in the middle. A tight zip-tie on the inside keeps the wire from pulling thru as does the Halex rubber grommet which tightens on the cord as you tighten the outer nut.

Years ago when Dan and I started Wilson F/X we looked at lots of different types of wiring, grommets, and strain relief devices. We found and tried a version of this exact same Halex strain relief, but the rubber insert had a slot cut sideways thru the center rather than a mere round hole. We tried the three wire flat extension cord that one can still find in most big box hardware stores. We didn't go with it for long, for lots of reasons which are irrelevant here, but they still make these strain relief rubber inserts with the slot rather than the round hole. If you want to stick with your 16/2 zip-cord/lamp-cord flat extension electrical wire, then you might want to go to one of the aforementioned Big Box stores and you might find that for which you are looking.

Lastly, whatever you go with, grommets or strain relief, I would suggest NOT using electrical tape to "make the wire big enough" to fit the grommet or the hole in the rubber insert. I would suggest using Heat Shrink tubing to "make the wire big enough." Long ago, I used electrical tape to do that. It works, but after a while it starts pulling away from the cord and out thru whatever you are using for strain relief. Looks bad.

Anyway, there's my take on the differences between simply grommets and the more complicated strain relief.

Brad
 

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