Barry Mounts upside-down?

[Brian Jackson]

I'm hoping to consult the forum about something that has been bugging me recently. I have fabricated and installed the engine mounts to my airframe quite some time ago (not flown or completed yet). I drilled for and installed the Barry mounts according to the manufacturer's instruction on the following PDF (diagram near bottom of page):

https://www.aircraftspruce.com/catalog/pdf/15-05688.pdf

I believe I interpreted the diagram correctly but I want to make no assumptions when it comes to beliefs. In either of the two mounting diagrams it seems to show the part being isolated is sandwiched by the two rubber elements. Seems pretty normal so far. Since each of the two engine mounts is basically 2 bars, one above the other, with the upper bar mounted to the engine and the lower bar to the airframe.

What concerns me is that I have modeled the bar sizes after the LEAF system (now discontinued) in that the bars are 2 different widths: Lower bar is 2" wide and upper bar is 1.5" wide. Again, so far this seems pretty standard. However when you see the GyroBee Documentation it shows the Barry mounts on the LEAF system sandwiching the LOWER (wider) bar, not the upper (narrower) one.

Perhaps this by itself is not a huge concern, until you consider that the mounting hole for the Barry mount is 3/4" diameter for the sleeved rubber tenon to pass through. This puts the larger hole in the narrowest bar instead of the widest bar, and doing the math you're left with a cross section of only 3/8" wide on both sides of the hole. Since aluminum fatigues in such a high-stress area I am having serious doubts about the integrity of these mounts with only 3/8" of material from the edge of the hole to the edge of the bar (shortest distance). Something is nagging me to remake these items with the Barry mounts flipped around to sandwich the LOWER, wider bar, not the upper narrower one. Doing this would provide an additional 1/2" of cross sectional area at the weakest point. But I want to be sure there is not some unforeseen consequence of doing it this way, or that the Barry mounts won't perform as well in that arrangement. Hoping the informed folks here can guide me in the right direction and/or confirm that nagging voice in my head. Thanks in advance.

 

[Vance]

It took me a while to follow along.

I would call the people who make the Barry Mounts to see what they say.

The Lord mounts on my Lycoming only go one way.

I like the thought you put into your build Brian.

I don’t have a sense of what the upper and lower "bars" look like so I do not have an opinion on what is correct or if it is strong enough.

 

[Alan_Cheatham]

I drilled for and installed the Barry mounts according to the manufacturer's instruction on the following PDF (diagram near bottom of page):

https://www.aircraftspruce.com/catalog/pdf/15-05688.pdf

I believe I interpreted the diagram correctly but I want to make no assumptions when it comes to beliefs. In either of the two mounting diagrams it seems to show the part being isolated is sandwiched by the two rubber elements.

Actually the two diagrams in the pdf show that either the engine mount bar or the mounting structure bar could be sandwiched by the rubber elements. Personally I've seen examples of Barry mounts used both ways, I don't believe it would make a difference either way as long as the mounting bars are properly sized for the loads being applied to them.

The difference in bar width is to compensate for the loss of material due to the larger 3/4" hole so if you are concerned as to strength the simplest fix would be to open the holes in the lower bars and remake the upper bars with the smaller holes.

 

[ultracruiser41]

It doesn’t make a difference....can be mounted either way

 

[Brian Jackson]

Actually the two diagrams in the pdf show that either the engine mount bar or the mounting structure bar could be sandwiched by the rubber elements. Personally I've seen examples of Barry mounts used both ways, I don't believe it would make a difference either way as long as the mounting bars are properly sized for the loads being applied to them.

The difference in bar width is to compensate for the loss of material due to the larger 3/4" hole so if you are concerned as to strength the simplest fix would be to open the holes in the lower bars and remake the upper bars with the smaller holes.

Thank you very kindly, Alan. That was my thought too. It seems like a simple detail that could have easily caused a catastrophic failure if overlooked. I will remake these parts and quiet that nagging voice in my head that kept bugging me to ask this question. Fortunately I have access to a CNC now and shouldn't take days of hand shaping and drilling. Much obliged.

 

[Brian Jackson]

Sorry to beat a dead horse, but I am about to have these plates remade and needed to better understand the isolator assembly in greater detail. I know... it's a rubber thing, how complicated could it possibly be? So forgive me for revisiting this topic.

The attached image shows the before/after condition of the isolator & engine mounting plates in cross section. I don't see any cause for concern other than in the flipped (after) position the bolt/nut will endure much more vibration because the hard contact of the bolt is now on the engine plate instead of the airframe plate. Besides safety-wiring these bolts, do others here on the forum see any issues with this arrangement the way it's depicted in the graphic?

Also, in the manufacturer's PDF I linked to in the first post, the last page has a chart that shows max bolt torque of 30 ft. lbs. This seems excessive to me but I am still new at this. I would think that too much compression would "stiffen" the system, degrading its ability to dampen vibration, hence transmitting more of that energy to the airframe. As long as it's captured by a secured nut, wouldn't a lighter yet snug fit be desirable for maximum movement?

Thanks in advance to all for any further enlightenment.

 

[giro5]

I used Barry mounts under my 447 on a PPC and the torque flexed them so much I felt obligated to use a strap under the fan assembly housing to keep the engine from torqueing up under power.

 

[gyrojake]

Sorry to beat a dead horse, but I am about to have these plates remade and needed to better understand the isolator assembly in greater detail. I know... it's a rubber thing, how complicated could it possibly be? So forgive me for revisiting this topic.

The attached image shows the before/after condition of the isolator & engine mounting plates in cross section. I don't see any cause for concern other than in the flipped (after) position the bolt/nut will endure much more vibration because the hard contact of the bolt is now on the engine plate instead of the airframe plate. Besides safety-wiring these bolts, do others here on the forum see any issues with this arrangement the way it's depicted in the graphic?

Also, in the manufacturer's PDF I linked to in the first post, the last page has a chart that shows max bolt torque of 30 ft. lbs. This seems excessive to me but I am still new at this. I would think that too much compression would "stiffen" the system, degrading its ability to dampen vibration, hence transmitting more of that energy to the airframe. As long as it's captured by a secured nut, wouldn't a lighter yet snug fit be desirable for maximum movement?

Thanks in advance to all for any further enlightenment.

You can tighten dem der Barry mounts 1/4 turn before you break the bolt and never stop the mount from flexing.
There is a steel tube inside the mount that will stop excess clamping of the mount.
I always recommend 1/8" thick S.S. steel washers over and under the barry mounts, especially when up against aluminum.

 

[Alan_Cheatham]

The amount of load carried by the bolts will be the same no matter how they are arranged but bolt torque is very important for fatigue resistance. The Lord mount has a steel tube bonded through it's center, it's important to have enough tensile force from the bolt to keep that tube firmly pressed against the mounting plate otherwise a bending moment will be applied to the bolt shank which can lead to fatigue. Steel washers should be used too.

If the Lord mount is too rigid go to a softer one.

As long as the bolts are properly torqued so their internal tensile stress level is sufficient they will not see stress from vibration.

 

[Brian Jackson]

Thank you gentlemen very much. I believe the crucial piece of information I was failing to consider was the necessity of the steel sleeve to bottom out on the upper plate/washer. This creates a fixed distance that the rubber ring occupies, preset by the manufacturer, hence their bolt torque value required to compress the ring to this distance. I have attached a revised section showing the stainless washer arrangement as recommended.

I appreciate the detailed responses and am comfortable moving forward on this. The sad part is that I spent a great deal of time fabricating the first set of mounts only to relegate them to the scrap bin (which is fortunately very small). But it was the right move and I'm thankful that the nagging voice in my head pressured me to re-evaluate the integrity of the as-built parts. Much obliged everyone.

 

[Welder]

Brian
Please use urethane instead of rubber and keep them free of oil and gas

 

[Brian Jackson]

Brian
Please use urethane instead of rubber and keep them free of oil and gas

Good morning, Welder.

I used the word "rubber" incorrectly as a general term. Forgive me. Per the documentation linked in the original post, the Barry mounts are Neoprene, and they claim it is oil resistant (see attached image). I'm hesitant to veer from proven systems unless there is a compelling reason to do so. Is there a direct equivalent product in urethane that is commonly used for this application? And if so, what benefits are realized? As is evident by now, I'm willing to explore better ideas, provided they're truly better.

 

[Alan_Cheatham]

Brian,

Just FYI while were on the subject ........

Although it's been some time since I researched these mounts somewhere I have documentation on the radial load rating and found they would be operating in an overload state while carrying the propeller thrust loads, if memory serves I believe it was only around 40 lbs or so per mount.

If you think about it, in an axial direction the load is being transmitted from one flat face to the other through the Barry material but in a radial direction the load is ultimately transmitted through the Barry material to the steel tube that runs through the mount center. This overload can effect the life of the mount and require more frequent replacement.

It's not something I worry about on a light gyro build however.

 

[Welder]

Hey Brian,

When I build anything AC related, I use the utmost care so I don't second guess myself while in the air.
William Wynn the corvair guy likes to type, and I believe somewhere on one of his sites he uses Urethane Lord mounts. I just wanted to give you a quick reply tonite, but will track it down and give you the source.
I don't know you but when I see people that really want to do an excellent job of building, I will go out of my way to help.
If you need to go ahead and order then by all means do so, but if you have got a couple of days I will track down the info
Sincerely
Ben

I forgot to tell you the reason I think you should use Urethane, in a word Tuffness comes to mind, I used them to mount my 582 onto an AirCreation trike.

 

[Brian Jackson]

Brian,

Just FYI while were on the subject ........

Although it's been some time since I researched these mounts somewhere I have documentation on the radial load rating and found they would be operating in an overload state while carrying the propeller thrust loads, if memory serves I believe it was only around 40 lbs or so per mount.

If you think about it, in an axial direction the load is being transmitted from one flat face to the other through the Barry material but in a radial direction the load is ultimately transmitted through the Barry material to the steel tube that runs through the mount center. This overload can effect the life of the mount and require more frequent replacement.

It's not something I worry about on a light gyro build however.

Good morning, Alan.

Thank you for bringing this up. I refrained from replying last night until I had a chance to study this with a fresher head. After verifying the part number that was ordered, attached is the corresponding load rating chart. Highlighted is the Radial load. It seems to be inadequate for the expected load (thrust/4). What's puzzling is that I cannot recall how this part number was settled on. The original GyroBee documentation refers to a different Leaf part number.

It would seem that the 22001-15 (purple/white) would be the better choice if looking purely at radial loading. But it appears that axial loading triples which I assume means a much stiffer elastomer. Perhaps that is desirable, as Giro5 pointed out in post 7 above.

And to Welder:
Thank you also. As you can see from the conversation I'm cool with holding off until more data is available. Looking forward to taking a look at those. Sometimes analysis paralysis is a good thing .

 

[Alan_Cheatham]

A couple of Barrys oriented so their axial direction carried the propeller thrust load would be a more appropriate solution if it were deemed necessary.

 

[Brian Jackson]

A couple of Barrys oriented so their axial direction carried the propeller thrust load would be a more appropriate solution if it were deemed necessary.

An interesting proposal, but adds complexity over simply using a set designed for the radial load. Many thanks for bringing this loading to my attention. However there is room at the midpoint of each pair of plates to accommodate a 3'rd isolator. I haven't really entertained this idea yet and would prefer not to, Currently I'm attempting to get an estimate of the thrust produced by a 60" Prince P-tip carbon prop on a Rotax 503 w/ 2.51 gear box. Pretty standard setup except for prop. Knowing this thrust I can then narrow the search for the appropriate isolator. My fear is having them too stiff and transmitting excess vibration to the airframe.

 

[Welder]

Brian
My suggestion to use Urethane was based on the isolatorors/mounts used by the Corvair guys. It is a bed mount engine.
After seeing how the 582 torqued while warming up I was happy I chose Urethane bushings. UMMV
Good luck with your build.
Ben

 

[Brian Jackson]

I just got a reply back from Lonnie Prince of Prince Aircraft (maker of the Prince P-Tip Prop). Unfortunately he does not publish or give out any thrust numbers, which I thought would be a pretty standard combo (60" prop on a 503). I'm just trying to get a ballpark estimate on static thrust in order to ensure the radial loading on the Barry mounts is not exceeded.

So I am asking the Forum: For those with a similar engine/redrive/prop combination, have you measured static thrust on your machines? And if so, would you mind sharing? I am not sure how this would even be calculated since props can vary so widely, but I would imagine there is an upper limit to what any 60" prop can do with finite horsepower. Thank you in advance.

 

[Kevin_Richey]

I just got a reply back from Lonnie Prince of Prince Aircraft (maker of the Prince P-Tip Prop). Unfortunately he does not publish or give out any thrust numbers, which I thought would be a pretty standard combo (60" prop on a 503). I'm just trying to get a ballpark estimate on static thrust in order to ensure the radial loading on the Barry mounts is not exceeded.

So I am asking the Forum: For those with a similar engine/redrive/prop combination, have you measured static thrust on your machines? And if so, would you mind sharing? I am not sure how this would even be calculated since props can vary so widely, but I would imagine there is an upper limit to what any 60" prop can do with finite horsepower. Thank you in advance.

On a Sport Copter Lightning w/ a DCDI Rotax 503, B- gearbox (2.58:1), 62" GSC 2 bladed wooden prop: 275#
measured @ approximately 60' ASL.