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Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

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Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Old 01-17-2004, 01:18 AM
  #1  
CorsairJock
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Default Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

OK, by popular demand, I have copied and pasted some of my earlier comments and posts regarding problems with the Robart 615 retracts, and solutions I have found to improve them:

"I have uncovered another cause of unwanted rotations: the molded 'trunion' becoming worn and the screws which hold it into the housing becoming loose. This in turn allows the trunion to wobble in the housing, the the gear to have excessive slop and play, and will allow it to slip a tooth. As the trunion becomes worn, the screws become more sloppy and become loose easier, and can never really be tightened. The old solution was to replace the trunion, but even new ones do not allow you to tighten the screws too much or they will strip out.
SOLUTION: I drilled out tapped the screw holes in the trunions to 5/16" x 18, and installed Keen-Sets( 5/16 x 18 ~ 10 x 32), which are simlar to Heli-Coils ( which could also be used instead ). Now, I have a much larger metal surface against the plastic housing, and the mounting screws can be tightened VERY tight without fear of stripping out."

"In an earlier post to this thread, I stated that I had found a solution to one of the problems with the 615s: that by installing 'KeenSerts' into the trunnions (plastic strut housings) I had eliminated some of the slop, and gear slippage. Having flown with this mod for several weeks now, I can state that this is a worthwhile mod, and that it has greatly reduced downtime due to damaged landing gear.
The problem with the unmodified trunnions is that the mounting screws work loose, allowing more gear play and slop. Eventually, the plastic trunnion wears and the screws cannot be tightened. Using the KeenSerts virtually eliminates those problems.
I few days ago, a reader of this post sent a PM to me asking for a picture of what I did, to clarify it to him, so here it is. The picture is of a modified trunnion (KeenSerts installed on both sides), with arrow pointing to where the KeenSert was installed on one side, and as well as a KeenSert which has not been installed (with thread dimensions).
I get these Keenserts from my place of employment, and cannot provide anyone with info on where to get them. Possibly, a place like Micro Fasteners has them, or some other hardware/ fastener supply place."

"Anyway, the Keenserts do not solve ALL the problems with the Robarts, but sure are an improvement. I think part of the problem with the bending links is that the trunnion twists (due to slop as mentioned above), thereby exerting more pressure on oneside and consequently, one link. As this link is now absorbing more stress, it buckles, which then induces more stress on the remaining one and it buckles also. I have had occasions where only one buckled, so while I don't think this mod will solve all problems, it certainly is helping me out. BTW, my Corsair weighs 8 lbs, 10 oz, and as long as my landings are not too abusive, the retracts are holding. I have even been making some carrier style landings lately, where I have full flaps, fair amount of throttle, hanging on the prop slow, and let is slowly drop to the 'deck', where it promptly does a little bounce, and has a relatively short roll-out. This style of landing used to take out a set of links, but no more.
And, Luke is right, a google search will turn up lots of sites, some even with instructions on how to install them.."
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Old 01-17-2004, 01:29 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

"I think the 615s are rated for up to 12 lbs, and many of us consider that to be overly optimistic. I wouldn;t use them on anything over 10 lbs.

But, to make the most of the situation, I suggest starting out by beefing up set screws and making sure they have a flat surface to set against (not too much tho, you don't want to grind away all the strength, just enough for a small flat spot). By beefing up, I mean using bigger set screws if you can. I think they come with 4-40s? I any event, all mine are 6-32s, AND I use regular allen head and/ or button head (allen) screws instead of set screws where room/ clearance permits. These allow me to apply more tightening torque than with headless set screws. I also drill and tap more holes if possible.

Some of the early 615s had 5/32"diameter struts, but for the past several years only the 3/16"diameter versions have been manufactured. If you have the 5/32"versions, then it is possible to drill out the trunnions and gears to accept the 3/16 struts, OR you can order them from Robart, along with the 3/16"struts. I have never heard of using even larger struts with the 615s, but I suppose it is possible. Again tho, I think 3/16" s getting a little flimsy for aircraft in the 13# range. I guess my question would be: WHY is it nosing over? Is it because the struts are rotating, or because they are flexing so much as to move the wheels back (as I have seen the H-9 Mustangs do), or because the grass is too tall? I fly off of grass all the time, and now that my retract problems have been solved for the most part, nose overs are very rare except when I fly from a field with tall grass.

On RoboStruts: they will help prevent flexing, BUT will increase the chances of the strut becoming bent to the point where disassembly and straightening is required.

On wheel size, MOST ARFS come with wheels that are too small, and taller wheels do navigate tru grass easier, so I would suggest trying a larger (but light weight) set. To give some perspective, the scale correct size for a 62"span Corsair is 4". tho I don't recommend that size for the Top Flites, I would recommend that size for your's which I believe is about 75"span? Dave Brown makes some light weight wheels in that size.

Hope this helps, and good luck.

Also, getting back to strut wires: a suppose another possiblity is using 3/16" daimeter titanium struts. I have never used these, but as I understand it they are lighter, yet more bend resistant than the steel ones that come with our retracts. I think these are availble from Central Hobbies? "

BTW, my recommended size for the main wheels on a Top Flight 1/8 scale Corsair is 3 1/2" ~ 3 3/4"
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Old 01-17-2004, 09:13 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Thanks CJ... i have it saved in my favorite places for quick reference when i get the 615's...
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Old 06-07-2004, 11:23 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Bump (need to bring this one to the top once in awhile, very useful to owners of Robart 615s).
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Old 09-25-2004, 08:49 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

BUMP.
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Old 03-18-2005, 12:09 AM
  #6  
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Bumped again, because I recieved another e-mail asking about this mod. It continues to amaze me, that while I thought I was merely solving the problem of sloppy trunions, I very rarely damage the links since making this mod. In fact, I've gone from ordering about a dozen sets per year to using one set (of replacement links) at most per year, and that requires a really bad landing.
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Old 03-18-2005, 02:06 PM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

HI Jim,
I'll have to try this mod to my TF P-40 since I had to buy new trunnions. and my links are the originals LOL
Mike
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Old 03-19-2005, 05:54 AM
  #8  
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

CorsairJock, You can order the Keen inserts from McMaster-Carr in Atlanta @ (404)346-7000. The Part number is 92070A240 and they are $1.57 each for orders from 1-9. They also have distribution in Cleveland, LA, New York. Do a search "McMaster-Carr".
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Old 11-23-2005, 09:07 AM
  #9  
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

There has always been much discussion on these retracts, as well as the CenturyJet rotating retracts, and at least 2 other limited production replacements (Sierra and RobinAire). I suggest anyone considering any of these choices do a search here at RCU, and you will discover many opinions. This has been hashed over thoroughly, so the answers are here if you learn to search.

I will take a few minutes to re-state my own opinions:

Robart 615s with RoboStruts:
+) The best looking of the bunch, with great scale appearance
+) RoboStruts do a great job of shock absorbing, especially if gear is installed with slightly forward angle when down.
+) Comparativley compact desgn fits in retract bay well very nicely, with no modification to wing spar needed.
-) Very weak "links", or scissors as some refer them as. These become bent way to easily, and are essentially useless once they become bent. Best thing to do is stock up on replacements (about $6 each) there are 2 on each retract, 4 total in set, so a really unforunate landing can cost about $24. As this post implies tho, I have discovered a modification which has greatly reduced the tendancy for these to become bent.
-) All those set screws can become a nightmare if not properly installed. I use 4 on each RoBoStrut, and grind a flat spot for each one, and no more problems. If you fail to grind the flat spot, you WILL have problems.

CenturyJet:
+) Robust desgn, is much less likely to become damaged (altho not impossible as I have seen).
+) Units are completely assembled with shock absorbing oleos, no set screws to mess with (or mess up).
-) Bulkier units require a hole to be made in main spar for cylinder clearance, less room for instlling gear doors.
-) Oleos look cheap, not nearly as nice as RoboStruts. Also seem a little stiff for the lighter (8 ~ 9 1/2 lb) aircraft.

Sierra:
I have never owned a set, but have 'fondled' a set, so here are my impressions:
+) they do appear to be very well built, and are probably sturdier than the Robarts
+) they have a nice appearance, but not as good at RoboStrut equipped Robarts
-) They look all wrong when retracting: all of the 'rotation' occurs during the 1st half of the retract cycle. Not only is this an appearnce issue, it can also create problems if you intend to install operating gear doors, as the struts need to 'tuck' into the bay at a slight angle, rather than go in straight and level.

RobinAire:
I have never seen these units, except in pictures and drawings.
+) Robust, or at least that is the claim
+) VERY nice looking, and the units allow the struts ti be VERY close to the wing leading edge, for the best scale location
-) Units are very bulky (large), meaning it will be difficuly to install in some of the other 1/8 scale Corsairs (such as Hanger 9). Mounting width is by far the widest of the lot.
-) Struts are very long. Altho this makes them more scale, I personnaly would trade off form for function here. Shorter struts make landings easier, and tip overs less likely. It also makes installing larger wheels more challenging, as the rear spar comes into play. Longer struts also have a greater influence on changing center of gravity (wheels go back farther).

As for cost comparisons: that is up to the individual. I have learned that building a scale warbird costs a lot of money by the time it is completed. I'm not rich, and don't like spending any more than I have to, but I do tend to buy what I think will work best. I don't want to skimp on the retracts after spending all the other time and money on such a project.

Just my 2 cents.
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Old 12-12-2005, 03:00 PM
  #10  
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

That Jim is a fantastic Idea.
I'll have to use some helicoils.
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Old 06-04-2007, 04:34 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Recieved the following PM:

"From: Griffin_NC
To: CorsairJock
Date: 6/3/2007 10:31:02 PM
Subject: Retract Mishap

Have you ever had an issue getting a set of Robart 615's to fit in any one of your Corairs. I seem to be hitting my head on a wall tonight try to get them to sit properly. When I get them set on the the rails the music wire hits on the bottom of the cut on on the main spar. I knotched it down a bit more to clear the piano wire. Then I realized that when closed it's hitting my tire at an angle and won't fit. The tire is a 1" thick Robart and I don't have any room to fit it down in the wing deeper. I seem to need about another 1/8 inch of clearance to fit level. I'm a bit concerned about carving out the wheel anymore and weaking the structure there.

If you happen to have a pic of 615's mounted up I would appreciate it. I can't seem to find a good one in the Forums at the moment."


Since this is on the subject of 615s, I'd rather answer here so everybody can read it:

Yes, I do a simple mod which has the effect of lowering the retracted strut/ wheel slightly, while increasing the forward angle when the gear is down: I install wedges between the retract units and the mounting rails; These wedges are made from Ernst thrust plates: the 3 degree offset/ smaller size ones. I can get a couple of sets from each plate. I cut them to same length as the retracts, then drill them so the mounting bolts will pass thru them. Then I silcone them to the retract units. That is my solution, others may have differnt ideas? I'll try to post a pictice (much) later today.

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Old 06-04-2007, 06:46 PM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Thanks. Hopefully someone has had the same issue I have. I will try to angle them up a bit and see if that helps.
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Old 10-06-2009, 04:52 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Bump.
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Old 08-05-2010, 08:52 PM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

bump.
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Old 08-06-2010, 06:58 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

good bumpepsecially with the new p-40 arf.
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Old 02-12-2012, 06:34 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

I have a feeling this info deserves a bump due to the New Top Flite Corsair ARF release... I plan on doing this mod if I use the Robarts. Nice work, CorsairJock! Robart is probably cursing you though, as they will soon have a surplus of links.
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Old 02-12-2012, 07:07 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

I have the 615s installed in my TF 60 Corsair. As of this post I haven't flown or even taxied it yet. I did put flat spots on the strut below the toothed gear to keep the gear from rotating (which is recommended by Robart). I can't seem to get the set screws as tight as I would like without the allen wrench slipping in the screw. New allen wrench.
Thanks for bringing up the trunion problem. I was thinking of solderring the toothed gear to the strut and eliminate the need of the set screw altogether. Also I was contemplating drilling the trunion and inserting a brass bushing to keep the plastic from wearing.
It just irks me to have to do all these mods to a new set of retracts when the Manufacturer knows of the persistent problems users are having. It also irks me that Robart can't put a flat spot on the bottom of the strut where the toothed gear is tightened to it.
I hate to deal with taking apart new retracts to "fix" something so simple for the manufacturer to do.
My $.02

Tom
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Old 02-12-2012, 09:13 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Where possible, I replace the conventional set screws with button head allen screws. They use a larger size allen wrench, and thus it's easier to get a better 'bite'.
You MUST be carefull tho, that they are not too long, as binding can occur if they are.
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Old 02-19-2012, 07:58 PM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Indeed iflircaircraft, these retracts have been out long enough (since the early 90's) that these problems should be a distant memory. They have always been kind of slow to react to things in general, but sheesh! Come on, Robart! We are ready for you to wow us with something new or improved!!!

Hoping for robust, scale looking & operating Corsair retracts is like hoping for world peace... seems impossible right now.
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Old 03-28-2012, 06:30 PM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

They are called SIerra Retracts and they cost. Iused these mods on my Robarts and this is a great idea. Also did the screw replacements as well as drilled and tapped for 6-32 instead of the 4-40. solid as a rock. But I agree very easy for Robart to do in the first place.
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Old 10-03-2012, 06:07 AM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

(bump)
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Old 02-24-2013, 06:18 PM
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Default RE: Robart 615 Modification using KeenSerts: 1 problem solved, This Works!

Great information thanks CorsairJock
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Old 03-21-2015, 12:59 PM
  #23  
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Bump. Glad I found this thread. I'm just finishing a TF corsair with 615's. I still believe that electric retracts are a solution in search of a problem and not worth the cost and BS.
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Old 02-16-2016, 05:19 AM
  #24  
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Bump!

I wonder if this would be a better retract unit if you machined a new trunion from aluminum? Seems it would be stronger.
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