Wiring Fundamentals: Sheathing
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Sheathing
18.10
| 00:00 | - Sheathing is the process of adding a protective outer layer to our bundled wiring harness. |
| 00:05 | This further gathers the wiring harness together and provides protection from abrasion. |
| 00:10 | Aesthetically it's a crucial step also as it unifies the appearance of our harness, giving it a finished professional look. |
| 00:17 | As mentioned in the wiring harness anatomy section of the course, the two options we'll look at for this are expandable braid and DR25 heat shrink tubing. |
| 00:26 | Other common options for this are wiring harness tape and split convoluted tubing but both of these have the drawback of limiting the flexibility of our wiring harness and are not suitable for our application. |
| 00:37 | First thing we need to do when fitting the expandable braid to our wiring harness is determine which size of expandable braid we're going to need. |
| 00:44 | Now to do this we measure our bundled wiring harness section. |
| 00:48 | So if we measure this section here we get a measurement of 3.8 millimetres and if we measure it in the other direction we get 4.7 millimetres. |
| 00:59 | So if we split the difference and call that 4.3 millimetres, that's the measurement we need to size our expandable braid. |
| 01:07 | Now the rule for sizing your expandable braid is you use the largest possible size that when relaxed will tightly fit your wiring harness section. |
| 01:15 | There is a little bit of wiggle room here and the manufacturer's documentation for this product does actually give us a range of sizes that it's applicable for. |
| 01:24 | So we can see our quarter inch expandable braid here is suitable for use on a wiring harness diameter of 1/8th of an inch to 7/16th of an inch. |
| 01:33 | So that's approximately three millimetres to 11 millimetres which we're well within that range here. |
| 01:39 | So our quarter inch expandable braid will be fine to use. |
| 01:41 | The next step in the process is to cut the right length piece of expandable braid to fit our wiring harness section. |
| 01:48 | So in this instance on the bench here we can hold it up next to our wiring harness section and pull the length directly off it. |
| 01:56 | So we're going to need a section this long. |
| 01:58 | To cut the expandable braid, the absolute correct method is to use a hot wire cutter as this actually melts the ends as it cuts and stops it from fraying apart. |
| 02:08 | However that's not a particularly common tool to find on the workshop floor, and we can get by by using some sharp scissors and a couple of techniques undertaken when we're installing it onto the wiring harness section, help us with the issue of it fraying apart. |
| 02:23 | So we'll cut this to length. |
| 02:25 | So now we have to get this expandable braid onto our wiring harness section. |
| 02:29 | And we do need to be reasonable careful when we're doing this as it's very easy to fray the end of this expandable braid and it can get very untidy very quickly. |
| 02:37 | So while we're doing it, I like to keep a reasonable grip on the end, just to make sure that's not going to happen. |
| 02:44 | And we want to carefully get the end of our wiring harness section into the expandable braid, and just make sure to hold the end of it so it's definitely not going to fray apart. |
| 02:57 | Once we get to this point we can use the main advantage of the expandable braid, that is as we compress it along its length, it expands out and we can release one end and slide it onto our wiring harness easily. |
| 03:11 | So if we continue to do this we can work it all the way up the wiring harness section until we reach our transition point which I'll just continue and do now. |
| 03:29 | We slide the expandable braiding all the way up the wiring harness section and actually over our transition point here. |
| 03:35 | The reason for that is that we are going to seal this transition point with a piece of heat shrink and we need to to cover the expandable braiding entirely. |
| 03:43 | We also want to prevent this from fraying so we're going to use a little bit of our Kapton tape on the end here and just hold everything in place. |
| 03:55 | So with the transition point taken care of from this direction, we need to look at it from the other direction now. |
| 04:01 | So that's gonna involve sheathing these four branches, and the operation is exactly the same as we've just done. |
| 04:08 | We're going to measure one of these branches and we get approximately 3.1, 3.2 millimetres, by 1.6 millimetres. |
| 04:22 | If we split the difference there that's 2.4 millimetres and that's the measurement we need to size our braid. |
| 04:27 | So looking at the manufacturer's data again we can see 1/8th inch expandable braid here will fit from 3/32nd of an inch up to 1/4 inch so that's within our range, so this is the size we're going to need. |
| 04:42 | So I have actually precut the four sections of expandable braid we're going to need. |
| 04:46 | They're all sized correctly to the branch lengths here. |
| 04:50 | And the process to undertake is exactly the same as we've just done. |
| 04:53 | We need to carefully start the wires in the end of the expandable braid, making sure not to fray it out, and then just back and forth, work the expandable braid onto the branch section all the way up to the transition point, so I'll do this now. |
| 05:17 | So with the expandable braiding now installed on our four branches, we need to go ahead and put a bit of Kapton tape here to hold everything together once more and provide a little bit of extra strain relief. |
| 05:31 | So with our transition point taped up and everything tightly held in place, we now need to provide the final stage of strain relief. |
| 05:39 | We do that using Raychem SCL heat shrink. |
| 05:43 | Now this is a type of dual wall heat shrink that has a potting compound on the inner walls and as the heat shrink is heated and starts to recover, that potting compound melts and flows onto the Kapton tape here sealing itself to it. |
| 05:58 | Then as the heat shrink cools, the whole joint becomes very very rigid, ensuring that it's well strain relieved. |
| 06:03 | Now the rule for selecting the size of SCL we're going to use is actually opposite to that of selecting the size of expandable braid or DR25. |
| 06:11 | That we want to choose the smallest possible size that will fit onto our wiring harness section and transition point. |
| 06:20 | So I selected that size here and trimmed a piece to length and because we've got the sizes right we can slide it up nicely and we want to ensure that the centre of the SCL is in line with the centre of our transition point here. |
| 06:36 | So we can do that by eye. |
| 06:39 | And with that lined up we can go ahead and perform the heat shrink operation. |
| 06:43 | This is one point that it is completely critical that you have a heat gun on which you can set the temperature. |
| 06:48 | We know that our expandable braided sleeving has a maximum continuous temperature rating of 125 degrees celsius. |
| 06:55 | And the manufacturer's documentation tells us that it melts at 250 degrees celsius. |
| 07:01 | Raychem SCL has a recovery temperature of 135 degrees celsius. |
| 07:06 | However I've found that a setting of around about 200 degrees celsius on our heat gun allows the Raychem SCL to recover quickly and it doesn't put the expandable braid in danger of melting. |
| 07:18 | So I've got our heat gun set to that 200 degrees celsius and we'll go ahead and recover this now. |
| 07:33 | So with the SCL fully recovered, we just need to do a quick visual inspection, so we're looking for the same things we were as in our splicing module, that's a nice even bead of the potting compound around either end of our SCL. |
| 07:48 | And that it's hardened up nicely and is providing the required strain relief to that transition point. |
| 07:54 | There is one last mechanical method of strain relief we apply and that is a cable tie around the branch section and this just stops these four branches from pulling apart as that would place quite a lot of strain on the end of our Raychem SCL joint here. |
| 08:17 | So with all that in place, this is a nicely sealed transition point that is well strain relieved and it's going to be very reliable when it's on the vehicle. |
| 08:26 | The next demonstration we'll have a look at is using our same example wiring harness section. |
| 08:30 | But in this instance we're going to sheath it in Raychem DR25 shrink tubing. |
| 08:34 | Now the process begins exactly the same way, we need to measure our harness section to determine the size of shrink tubing we're going to need. |
| 08:41 | So we'll take a measurement here and we get approximately five millimetres. |
| 08:47 | And in the other direction, approximately four millimetres. |
| 08:52 | So if we split the difference there and call it 4.5 millimetres, that's the dimension we're going to use to size our shrink tubing. |
| 08:59 | Now the rule when sizing the DR25 shrink tubing is that you need to choose the largest possible size that when fully recovered will tightly grip your wiring harness section. |
| 09:09 | Manufacturer's documentation states that the DR25 shrink tubing has a shrink ratio of 2:1 which means if we take our 4.5 millimetre measurement and double it, giving us nine millimetres, we need to choose the size of DR25 which is closest to but still less than this nine millimetre measurement. |
| 09:27 | Now DR25 shrink tubing is sized in imperial units so inches, and working our nine millimetre measurement back, we find that the closest size of DR25 to this is 5/16th of an inch, which is approximately eight millimetres and when fully recovered that will tightly grip our harness section. |
| 09:43 | So we've got some 5/16th DR25 shrink tubing here and we need to cut a section that's the right length to fit our harness. |
| 09:50 | So same as for expandable braiding, out here on the bench we can hold this shrink tubing up next to our harness, and determine the length of section that we need. |
| 10:00 | And to cut heat shrink tubing I like to use a nice sharp pair of scissors as you can get a good clean parallel cut on the end. |
| 10:07 | So with our shrink tubing cut to the right length we can go ahead and install it onto our wiring harness. |
| 10:12 | Now the added benefit of having chosen the size of shrink tubing that's as large as possible when unrecovered but will recover to tightly fit our harness, is it does make it quite a lot easier to install onto our harness section. |
| 10:24 | However when you're dealing with a long harness section, this can still become a little bit tricky and I find a good technique to use there is to tape something rigid to the end of your harness section as it gives you a little bit more purchase when you're installing the heat shrink tubing onto it. |
| 10:38 | A piece of tig filler rod actually works really well for this. |
| 10:41 | But a short harness section here on the bench, we'll have not problem sliding that through our shrink tubing all the way, much like we did with the expandable braid, to the end of our tranisition point here. |
| 10:55 | So with that installed, we need to recover this heat shrink tubing onto the harness. |
| 10:59 | The manufacturer's documentation states that the DR25 has a minimum recovery temperature of 175 degrees celsius. |
| 11:06 | It can withstand a slightly higher temperature than this though for a short period and I find a setting of approximately 200 degrees on our heat gun is a really good mid range as it recovers the heat shrink quite quickly but you're not in danger of overheating it and scorching it. |
| 11:20 | So with our heat gun set to 200 degrees celsius, we'll go ahead and recover this now. |
| 11:33 | So with our shrink tubing recovered onto our harness section, we can see it has tightened up there really nicely and it's bundling those wires together really well, giving the loom a really nice professional look. |
| 11:44 | If we have a look at the end of our harness section here, we can see the reason for the way that we size our DR25 shrink tubing and that's actually to do with the wall thickness of the shrink tubing once it's fully recovered. |
| 11:55 | As that shrink tubing shrinks down, its wall thickness actually increases. |
| 11:59 | And it's this increase in wall thickness that gives the shrink tubing both its flexibility and its abrasion resistance. |
| 12:05 | So we want to choose the size of heat shrink tubing that will recover the most to increase this wall thickness the largest amount. |
| 12:11 | So with this harness section dealt with, we can move on to sheathing the other four branch sections here. |
| 12:17 | So the procedure is very much the same, we're going to take a measurement of one of these branch sections and in this direction we've got 3.4 millimetres. |
| 12:25 | And in the other direction we've got 1.8 millimetres. |
| 12:29 | So splitting that difference we end up with a measurement of 2.6 millimetres. |
| 12:33 | Now remembering that our DR25 heat shrink has a shrink ratio of 2:1 We're going to double that 2.6 millimetres dimension, to end up at 5.2 millimetres. |
| 12:43 | And we need to choose the size of DR25 shrink tubing that is as close to, but still less than, that 5.2 millimetre measurement. |
| 12:50 | In this instance, that's going to be 3/16th Raychem DR25, as that has an unrecovered internal diameter of approximately 4.8 millimetres. |
| 13:00 | So we now need to cut a section of the 3/16th DR25 that is long enough to cover one of these branches. |
| 13:07 | So on the bench here we can hold these up next to one another. |
| 13:12 | And this can be a little bit tricky as this heat shrink tubing does like to unspool itself from these wire spools and become quite messy. |
| 13:22 | But here we go. |
| 13:27 | So with our harness section cut to length, we know this is going to fit one of the four branch sections here. |
| 13:33 | We've got four to do so we actually need to cut four sections of the 3/16th DR25. |
| 13:37 | So I'll go ahead and do that now. |
| 13:44 | So with our four sections of the 3/16th DR25 cut to length, we can go ahead and install them onto our branches here. |
| 13:50 | Now we do all four before we actually recover the heat shrink as there is enough space in the heat gun to fit all four in there and you can actually do all four at once and it does save quite a lot of time. |
| 14:01 | So we'll install this onto the wiring harness here. |
| 14:04 | Once again if it was a longer harness section I would tape something rigid to the end of the wires as it would make it a lot easier to install them through the heat shrink tubing. |
| 14:14 | So we've got one on the branch there, I'll go ahead and do the other three. |
| 14:20 | So with those four sections of DR25 installed there, we just wanna make sure they all butt up tightly to the transition point. |
| 14:28 | And with all that in place, we can head over to our heat gun and go ahead and recover these sections, so we'll do that now. |
| 14:43 | So with our DR25 recovered onto our four branch sections, we can see they have tightened up nicely, and we've got that increase in wall thickness we're looking for, giving them their abrasion resistance and flexibility. |
| 14:55 | So we need to provide some strain relief to this transition point here and we do that using Kapton tape. |
| 15:00 | So we'll get a section of that here, and wrap that up. |
| 15:07 | So with our transition point wrapped in Kapton tape, giving us a little bit of strain relief, we're going to go ahead and install a Raychem SCL boot over this, much like we did with our expandable braid just to solidify this transition point and provide the last stage of strain relief. |
| 15:21 | Before we do that though we do need to take some 150 grit sandpaper and just abrade a section of the DR25 shrink tubing down past where the SCL will reach. |
| 15:31 | And the reason for this is it allows the potting compound on the inside of the dual wall SCL to really adhere to the outside surface of the DR25. |
| 15:41 | And that seals everything up nice and tight so we'll go ahead and do that now. |
| 15:52 | So with either side of our transition point here abraded with our sandpaper, that's gonna give a really good surface for the lining of the SCL to adhere to and really seal down nicely. |
| 16:02 | I should mention at this point that I haven't abraded the inside of the DR25 at the branch point here 'cause in this application it's not actually necessary, we're not building a completely sealed wiring harness. |
| 16:13 | If you were building a completely sealed wiring harness, you would have to do that and you'd also use some potting compound on the join when you go through the recovery process. |
| 16:22 | It's an advanced motorsport technique but we do cover it in our advanced motorsports wiring course. |
| 16:28 | So with that done, we can install our Raychem SCL boot onto our wiring harness. |
| 16:33 | Now once again the rule for the size of the SCL we choose is opposite to that of the DR25 shrink tubing, in that we want to select the smallest possible size that will fit onto the join when it's unrecovered. |
| 16:45 | As that means it'll grip the transition point with the maximum pressure and really seal onto it nicely. |
| 16:52 | So we want to line up the centre of our SCL boot here with the centre of our transition point. |
| 17:00 | And with that done, we can head over to our heat gun and complete the recovery process. |
| 17:13 | So with our SCL boot recovered onto our harness we can do a quick visual inspection again. |
| 17:18 | We're just looking for that nice even bead of potting compound which we can see on both ends there. |
| 17:24 | So that's going to have keyed into the abraded section of the DR25 really nicely, and it'll be holding it very very tight. |
| 17:30 | We are gonna perform the same very last step that we did with our expandable braid though and we're going to put a small cable tie on the exit of the branch just to add a very last part of mechanical strain relief to protect that join from being pulled apart there. |
| 17:50 | Those two examples there cover the majority of sheathing applications you're going to find at the club day motorsport or modified street car level, and should give you the knowledge you need to achieve a tidy professional looking wiring harness that will be protected from abrasion and easy to install in the vehicle. |
