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How To Make Baffle Tension Rod

PhatRV

Well Known Member
I am at the stage to make the baffle tension rods. I am having trouble threading the ends with the threading dies. Is there a technique to get the dies started?

Better yet, is there seller for these ready made tension rods?

Thanks.

The tension rods are described in the picture below.
 

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I started with a harbor freight die. No joy. Bought a high quality die and it
T was a piece of cake. Some have used threaded rod in lieu of having to thread the ends. Others have used safety wire inside plastic tuning in lieu of the rod.
 
Give them a floatation test.
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I struggled on the first set, as i only had a bench vice and couldn’t keep the rod from slipping. Second set done on a lathe and very easy. Find a holding apparatus designed for round items. Chuck the rod in your drill and clamp the drill chuck in your vice.

Or pre cut them and mail to me with a bottle of wine and will thread them for you.

Larry
 
I see the washers the wire goes around to keep the stainless wire from digging into the aluminum baffle, but what is that black goop for?

Permatex Ultra Black, aka Loctite 598. In this case a dab kills any relative motion due to vibration.
 
Machine Shop

After sweating over these for an hour I took the rods to a local machine shop and he threaded them no problem.

Maybe chucking the rod in a drill press and using that to push them into the die would work. Thought of that after seeing him chuck the rod in his lathe.

I'm building an 8 as well and cut the rods 1/4" longer than plans specify and had the threaded portion 1/8" longer on each end. I needed that extra length on at least one of the baffles, and you can always shorten a rod.

Bob
 
I used SS rod. Threaded it slowly - turned a few degrees at a time then backed off to clear chips. Easy to adjust baffle tension with these but there’s nothing wrong with safety wire.
 
Primary advantage to wire is that it cannot foul the rocker box oil return tubes. Most rod users are trying to prevent holes in the return tubes by installing some sort of wear guard on the rod.
 
Primary advantage to wire is that it cannot foul the rocker box oil return tubes. Most rod users are trying to prevent holes in the return tubes by installing some sort of wear guard on the rod.

Yep. On the -6 I used safety wire. On the -10, I went with the bent and tapped rod figuring it was an improvement. I'll go back to the safety wire on the next one.
 
I read about the safety wire solution but finally got the see the picture of what it looks like from DanH post. Thanks for all the replies.
 
Welding rod

An A&P suggested using welding rod and threading the ends. Can't remember the specific rod I used, but I remember chipping the coating off a couple of sticks I had in the shop.. It's still working fine after 800 hrs. John
 
threaded rod = bicycle wheel spoke

I tried to thread the rods and it was impossible. I went to a bicycle shop and had them thread a bicycle spoke and used the ends for the wheel. It worked great. Of course you will need to sleeve the spoke to protect the tubes under the cylinders.
 
My 9 had safety wire as DanH mentioned though it also has the recommend 1/4" tubing to prevent any chafing. Seems to work good and pretty easy to install.
 
just make sure you use the proper side of the die, there is a front and back to them. I use plain cold roll rod from lowes, take about 30 seconds to thread them with a good die.

bob burns
RV-4 N82RB
 
I chucked mine in the lathe and then brought the drill chuck mounted on the tail stock to the threading die. I turned the lathe chuck by hand while applying a bit of pressure to the die with the tail stock. That worked well for me. I was using a Chinese die
Ray
 
Sorry to bring up an old thread, but I have a question regarding these baffle tension rods.

On my last condition inspection, I found that the cylinder fins have cut through the baffles where they wrap around the bottom of the cylinder. I have remade the baffle sections and am looking for a way to make sure that doesn’t happen again. I’m considering the safety wire method.

Was it too tight or too loose resulting in the wear?

I was also thinking I could put a bead of rtv on that bottom curved section and let it dry, that way when I pull tension on it the actual baffle would not contact the fins, but it sounds like that is they way they are supposed to do.
 
On my last condition inspection, I found that the cylinder fins have cut through the baffles where they wrap around the bottom of the cylinder. I have remade the baffle sections and am looking for a way to make sure that doesn’t happen again. I’m considering the safety wire method.

Was it too tight or too loose resulting in the wear?

The threaded rod approach ties together wraps from two independently vibrating cylinders. Can't say with certainty that the safety wire method is better, but at least it doesn't conflict with the oil drainback tubes.

I was also thinking I could put a bead of rtv on that bottom curved section and let it dry, that way when I pull tension on it the actual baffle would not contact the fins, but it sounds like that is they way they are supposed to do.

Some builders have lined the wraps with the glass reinforced rubber baffle material supplied by Vans. Some adhere RTV/fiberglass wraps directly to the fins. Sometimes you'll see a bead of silicone across the fins in an attempt to reduce wrap wear, but a bead perpendicular to flow can't be good.
 
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You could go with baffle wraps made of fiberglass and RTV between the fins and the aluminum baffles. That would give you a protective layer and increase your cooling efficiency. Not hard to do if you already have the baffles off. There's a fair amount on the subject if you use the search feature.
 
I used safety wire as DanH recommends. I don't have good photos but here's one that might help. I just used a bit of SS rod rather than a washer. On most I also used some shrink tubing on the rod, but not in this photo. Holding well so far.

HB-YMM RV-8 lower baffle safety wire.jpg
 
What keeps the little rod from creeping out? Does the safety wire go through a hole - Can't quite tell from the photo.

Thanks!
Dave
 
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I see there that you added a bolt in the middle baffle to tie your wire off to.

With Dan’s fix, it looks like the flange that turns down on the middle baffle is larger than mine. Did you add material there to give you something to tie into?
 
Tension rods and fin protection

I made fin seals from RTV impregnated fiberglass. Dan Horton idea I think.

Measure so the piece is close to the correct size.
Lay it on a piece of 4mil plastic. Squeegee Ultra Black RTV into the fiberglass. Cut the plastic off at the edge. Lay itbin place on the fins. Walk away till it cures. Peel the plastic. Nice sealed fins.

Rods fit per plan. Cut to size. Grind a very slight taper at the tip. Clamp in the vise with sacrificial chunks of aluminum. Start the die. It should grab. If not, grind the point a tiny bit more. Go slow with cutting oil. Back and forth a quarter turn at a time. Remove from vise and make the bends. Add a section of shrink wrap. Not easy to install. Slide it in far enough to add the washer and nut on the impossible side. Fight the easier side.
 
Rv8ch. - can you elaborate on the attachment point in the inner baffle that you ran the safety wire to?
 
Rv8ch. - can you elaborate on the attachment point in the inner baffle that you ran the safety wire to?

You can use welding rod, the bent mandrel from a pop rivet, pretty much any steel rod.

First, get yourself enough 2" (ish) rods and bend each one into an "L" with a 1/4" short leg. Then drill two holes in your baffle tabs. The first one is where you want your safety wire to attach (or pass through) and the second is 3/4" or so laterally from that first hole.

You pass the safety wire through its hole in the tab, then around the L shaped rod. Pull it tight with the short leg of the L poking through the other hole in the baffle tab. That leg poking through the hole keeps the rod from moving around.

Twist and tighten the safety wire, then do the other end the same way.
 
What keeps the little rod from creeping out? Does the safety wire go through a hole - Can't quite tell from the photo.

Thanks!
Dave

Another method. Two holes and a piece of hinge pin with a 90 degree bend at one end.
 

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I’m asking about the attachment in the inner baffle between the cylinders. The flange is tiny. It appears Dan either riveted more material on there with a bend that would mirror the flange on the rear and the forward baffles or started with something different than what I have. It looks like rv8ch installed a bolt with a nut on it but I can’t tell what is on the end of the bolt that looks like it is flared to keep the wire on there.
 
Rv8ch. - can you elaborate on the attachment point in the inner baffle that you ran the safety wire to?

It's been a while, and I tried several things, but finally I believe it was a nutplate riveted to the thin baffle material. Then a stainless 8-32 bolt with the metal lock nut and a washer. I anticipated having to redo this but it's holding up. I think as DanH said, the reason for trouble down here is when you try to attach something to both cylinders, due to the movement of the cylinders.

David Paule said:
What keeps the little rod from creeping out? Does the safety wire go through a hole - Can't quite tell from the photo.

Only friction. On some of the others, I added some heat shrink tubing.
 
I’m asking about the attachment in the inner baffle between the cylinders. The flange is tiny. It appears Dan either riveted more material on there with a bend that would mirror the flange on the rear and the forward baffles or started with something different than what I have.

I really don't remember if I added any material. If it needed it, I added it.

We learn more as we go (hopefully). There are several changes I'll make when that 390 comes out for overhaul. One of them will be to add properly shaped extensions to those cylinder exits.
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One of them will be to add properly shaped extensions to those cylinder exits.

A) Why?

B) Top and bottom? Looks like you sketched a rounded extension on the high pressure side and semi-vertical extensions on the low pressure side. Would/should the extensions on the low pressure side be canted to direct the air aft?
 

The goal is same heat transfer with less pressure drop, i.e. energy loss. I've attached a paragraph from NACA 620 which summarizes the point.

B) Top and bottom? Looks like you sketched a rounded extension on the high pressure side and semi-vertical extensions on the low pressure side. Would/should the extensions on the low pressure side be canted to direct the air aft?

Maybe not the entry. And aiming the exits aft, toward the cowl exit, may not be possible. As a practical matter, it will be hard enough to fit some decent exits in the available space.

Ever thought about how much air flows through those openings? In very rough terms, Lycoming says it takes about 2 lbs per second. At 8K ft, that's 0.060 lbs per cubic foot, so flow is 33 cubic feet per second, yet we treat the finning entrances and exits as if aerodynamics has nothing to do them. We treat it much like ending at P1'V1'.
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I really don't remember if I added any material. If it needed it, I added it.

We learn more as we go (hopefully). There are several changes I'll make when that 390 comes out for overhaul. One of them will be to add properly shaped extensions to those cylinder exits.
.

What material and method will you use to fabricate them? I've read that NACA article years ago.
 
Use bolts

I extend my baffling as required to clear oil return lines and then use AN3-4 (or -5) bolts so the safety wire can pass through the hole in the bolt. Works perfectly.

 
I extend my baffling as required to clear oil return lines and then use AN3-4 (or -5) bolts so the safety wire can pass through the hole in the bolt. Works perfectly.


Heavy.... but good use of available hardware. Adjustable too.;)
 
Spring

Has anyone ever tried good heavy duty springs? I’d think that’d be a good way to hold constant tension.
 
The goal is same heat transfer with less pressure drop, i.e. energy loss. I've attached a paragraph from NACA 620 which summarizes the point.



Maybe not the entry. And aiming the exits aft, toward the cowl exit, may not be possible. As a practical matter, it will be hard enough to fit some decent exits in the available space.

Ever thought about how much air flows through those openings? In very rough terms, Lycoming says it takes about 2 lbs per second. At 8K ft, that's 0.060 lbs per cubic foot, so flow is 33 cubic feet per second, yet we treat the finning entrances and exits as if aerodynamics has nothing to do them. We treat it much like ending at P1'V1'.
.

I seem to recall, looking into ductwork design at a previous employer, that a standard engineering compromise from the ideal when changing duct sizes (for instance, for entering/exiting a heat exchanger or gas treatmnet device) is that the inlet angle should be no steeper than 30 degrees from the center line, and the discharge 15 degrees, to allow for best pressure drop while maintaining a reasonable physical overall length.

That would go for expansions in the cross section; perhaps the angles would be inverted for constriction?
 
Pics

For those who requested pics beyond the white board drawing. Extensions to baffling is 0.063 riveted onto tabs. AN3 bolts with metal lock nuts. Could use AN3H bolts or AN500 drilled head screws also.





 
With all of the displayed options, what does it take to get the results needed?
It should be cheap. It should be simple. Minimum parts count. And it needs to be light.

Of all the current suggestions, the minimum parts count per unit is 3. The MAX is 13.....:eek:
 
Baffle rod substitution

First I repair the baffle tab if necessary with a riveted on angle section. Then put a short AN-3 drilled shank bolt on both front and rear baffles with a loose single loop of .032” safely wire. Now hold one bolt stationary with a wrench and turn the other with something. I use a electric screwdriver turning a 3/8” socket. Twist the safety wire to the desired tension.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 

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Baffle "Tie" Rods w/Extension Springs

I used high spring rate extension springs to give a bit of "flexibility" to the lower baffle ties, to facilitate relative movement of the cylinders during ops.

I know the ties (twisted safety wire encased in 1/4 nylaflo tubing) should go under the oil return lines - but on my particular installation, it was going to be a very tight fit. As flat as the offset angle is, there is not too much "down" force applied to the tubes. 1400 hours and still looks pretty much the same as when installed.

In addition to the twisted wire, I reinforced the baffle flanges with some .032 2024-T3 (riveted) and used cross drilled, washer head screws for attachment. Under the washer head screws are white, composite washers that act as a "buffer" to the (otherwise) metal to metal contact. I use the same washers under the plenum attach screws - no smoking screws there either.

There are literally dozens of ways to address this issue, with this just being one of them.

HFS
 

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