Broken engine mount bolts

[charlierainman]

I just joined this sight today, so I'm checking it out and I come across a story about broken motor mount bolts. This gave me the Willies just thinking of an engine falling off mid flight. The fellow said that the bolts "sheared off". I find that highly unlikely, I don't think even the highest H.P. engine has the torque to shear off bolts. There are 3 ways a bolt can break: tension, shear and fatigue. It's pretty easy to tell, just by examining the bolt at the failure. A tension failure will be shiny with a bit of a crystal appearance, depending on the hardness of the bolt and the bolt will be smaller in diameter where it stretched. A bolt that fails in shear, will have a side that is rounded over and a sharp side, just like a piece of sheared plate and it would also do quite a bit of damage to that hole in that soft engine case. A bolt that fails from fatigue takes time, it has a tell tale appearance, it looks almost like tree rings, darker where it started to fail and brighter towards the break, with the brightest spot the last place holding prior to the failure.
  I am thinking of building one of the legal Eagles, I would want to figure out some motor mounts with built in safeties. Anyway, I hope this will help someone diagnose a failed bolt.

[Tom XL-7]

losing engine mount bolts should be rare or nonexistent,
however, one can tie the engine to the frame with a safety cable. 
I have seen a few examples on the site.
obviously, a shut down situation but you still have the balance to get to the ground.
Sourcing bolts is one place not to pinch pennies.
And just because the bolts have the markings don't mean they meet the spec.
just about counterfeit anything

[leshoman]

I have about 425 hrs in Legal Eagles XL and have 2 broken bolts and several loosen up. Broken bolt caused by mounts not being precisley aligned to back of engine and bolts coming loose is caused by not safety wired.  I have about 100 hours on current bolts and engine, without safety wire, alnf doing good.  Excess e gine vibration also causes broken/loose bolts. If concern easy to safety wire

[Dan_]

Attached:  A pic from a recent YouTube shared by Les showing how he did the engine retention cable...  (Skip ahead to 10:56)


The only drawback I can see is you have to cut the cable should you want to dismount the engine.  Not really a problem if you have plenty of cable and good tools, and most likely lighter and the swages more secure than bolts.



Link to post where Les shared his video:


https://www.eaglersnest.com/forum/index.php?topic=1657.msg9431#msg9431


Link to engine retention method used by racers:


https://www.eaglersnest.com/forum/index.php?action=downloads;cat=14

[Dan_]

Just passing this along... 


Leonard recommends grade 5 bolts not grade 8 and make sure they are not counterfeit.

[scottiniowa]

I just joined this sight today, so I'm checking it out and I come across a story about broken motor mount bolts. This gave me the Willies just thinking of an engine falling off mid flight. The fellow said that the bolts "sheared off". I find that highly unlikely, I don't think even the highest H.P. engine has the torque to shear off bolts. There are 3 ways a bolt can break: tension, shear and fatigue. It's pretty easy to tell, just by examining the bolt at the failure. A tension failure will be shiny with a bit of a crystal appearance, depending on the hardness of the bolt and the bolt will be smaller in diameter where it stretched. A bolt that fails in shear, will have a side that is rounded over and a sharp side, just like a piece of sheared plate and it would also do quite a bit of damage to that hole in that soft engine case. A bolt that fails from fatigue takes time, it has a tell tale appearance, it looks almost like tree rings, darker where it started to fail and brighter towards the break, with the brightest spot the last place holding prior to the failure.
  I am thinking of building one of the legal Eagles, I would want to figure out some motor mounts with built in safeties. Anyway, I hope this will help someone diagnose a failed bolt.

 Some details on this- and I am just repeating some of the past concerns. But It doesn't really seem to matter.
the 3 ways- 
1) tension,   with rubber groments, we should never be able to get enough tension to deform the bolt
2) Shear,  well, we simply can't get enough force sideways to shear the bolt
3) fatigue, I am guessing the bell rings loud here.  In my business, we often call this cumulative stress, but the explanation above  says it well.

I suppose some long time forum members remember about my asking, talking about the fact that the metal cones are NOT drawn to fully accept the angle of the proposed rubber mounts.. I presume as long as this is NOT addressed, this will always be a concern.  To my knowledge, it has NEVER been looked into, and with so many built, I doubt if it will be.   Don't get me wrong, I know that all rubber mounts can wear out, but we don't need to help the process.

I am basically coming up with this mismatch of two parts by doing one simple thing.  Drew the metal part in CAD (simple thing to do)  and then got the  rubber groments, measured and drew (also simple to do) and then mated the two together in CAD drawings.  Always came up with a miss-match,  asked what others were doing about it?  NOT one reply ever came back...  So....... I just deemed this was not important to those building.  

Also note:  if you don't use a washer at least larger than the center hole of the groments holder on the frame, you are relying on 100% that the bolt will never pull through.  Joe Engleman and I have talked about this a few times.  NO certified aircraft, would ever let this happen to an engine mount.  It is just to important to have it correct!

I don't mind at all to be corrected on any of this...so please do tell, if I am way off base.

[charlierainman]

I was looking at a photo of Les Homans XL and I see a rubber between the engine and the mount, but nothing between the head of the bolt and the engine, I can't see what is on the back side of the engine mount, but in this situation, there is no cushion, the engine is pulling and vibrating directly against the bolt. To me, this is how I think it should go, bolt, washer, rubber, engine flange, rubber, engine mount, rubber, washer, nut, cotter pin. Oh, and a sleeve inside all that to keep things in alignment. If you don't isolate the bolt from the engine vibration, what's the point of putting rubber in there anywhere?

[Tom XL-7]

the plans call for eight continental engine mount rubbers. they work in four pairs. odd looking items, they have a large diameter a tapered section then a small diameter. there is the tube on the fuselage you see that is the sleeve on the outside of the rubbers and is the load transferring item to the fuselage. there is also a spacer/filler between the rubbers that slides in the outer tube and matches the small diameter of the rubbers. The bolt doesn't come in contact with the fuselage. its purpose is to isolate vibration. 
you are not able to see the picture here without the drawing or pieces in your hands. these are not flat washers There is more you don't see inside the mounting tubes. The large diameter part of the rubber does sit on the outsides of the mount so it would appear as two flat washers.

l>- -<l  two rubbers big parts outside little parts inside. all nicely contained in a tube.
Tom XL-7
Scott's issue has to do with the mating between the metal parts and the taper. I will have all pieces in hand and check it out at that time.

[Dan_]

To Scott in Iowa...


This is just my opinion, I am not an engineer or an I.A.


Rubber is malleable... 


When you tighten the bolts the rubber takes the shape of of the metal cones. 


If you over tighten the bolts you negate the vibration dampening of the rubber and cause bolt fatigue. 


Not rocket science. 


Continental and Lycoming have used this method since forever...

[scottiniowa]

To Scott in Iowa...


This is just my opinion, I am not an engineer or an I.A.


Rubber is malleable...  


When you tighten the bolts the rubber takes the shape of of the metal cones.  


If you over tighten the bolts you negate the vibration dampening of the rubber and cause bolt fatigue.  


Not rocket science.  


Continental and Lycoming have used this method since forever...

Yep, I don't disagree at all.  
     So, it looks like it is best to accept the broken bolts... 
And for those that don't mind, if your ok with the bolt head being able to pull through the "rubber only" then that is ok too.  Don't use any washers.  

As for saying this is how Continental and Lycoming  used this method since forever, I guess I will ask a few local A &P's how many broken engine mount bolts they replace per 1000 hours of engine use (yes three zeros)     I will be surprised if his answer is even ONE.    And if so, I would venture to guess, that "this method" they use and the other method in question may not be the same. And from my measurement the difference is not in the Rubber groment as like you say they have been made "forever" 

So this really boils down to what a person is willing  to accept as normal breakage per hour of operation. 

Best of success to all.

[leshoman]

A large washer is essential on back side.  Spoils your day when the engine speeds off without the fuselage

[charlierainman]

Sorry, I didn't mean to poke a stick into the hornets nest, but Scott is correct, don't mess with success. I mentioned the other day on this subject, that the picture of Les Homan's Eagle, shows the head of the bolt tight against the flange on the engine. I have been in the repair business all of my life, my father ran a blacksmith shop for over 50 years, repairing farm equipment, logging and excavating equipment, etc. I was in the Air Force,worked on airplanes of course, I've worked on mining equipment and I've been in manufacturing and service for the last 20 years and nowhere have I ever seen an isolator used where the bolt comes in contact with the moving part. That bolt needs to be completely isolated from the engine and the frame of the airplane and I'll bet if you could look at a cutaway view of any airplane manufacturers engine mounts, that bolt is not in contact with any metal.

[Tom XL-7]

The VW engine was mounted with the same very flange we use to mount to the firewall onto the transmission. There were no other mounts. All four cylinders just hung out there. I used to beat my bug so bad that shocks would break. There were absolutely no rubber washers between the engine and transmission. How many of those broke.  none
  now the car does not have a couple hundred pounds of thrust, nor the gyroscopic forces you throw in there as you compete in the red bull competition in your ultralight. The landing gear takes the biggest beating on an aircraft, and my bug had many hard landings.
 maybe too loose or tight is the problem. need a good bolt, safety wired, yes washer on the backside ( does that really need to be said) and a proper torque value.
 Charlie, welcome to the site. 
Stick poking or stirring it up are not bad things. If there is an issue, we need to identify and solve it.
You won't do that without conversation.
Buy the drawings, study them, make sure you have thick skin and jump in.
We all share the same goal, safe enjoyable affordable flying. 
there are no "trolls" here. The subject stays pretty true to that stated goal.
Tom XL-7

[CHARLES DEBOER]

maybe too loose or tight is the problem. need a good bolt, safety wired, yes washer on the backside ( does that really need to be said) and a proper torque value.
Charlie, welcome to the site.
Stick poking or stirring it up are not bad things.
Tom XL-7

What are the torque values for the engine mounting bolts on the better half VW?  I haven't seen any posted.   I have seen statements like: "compress the rubber isolation grommets about 1/8".

[leshoman]

I tighten mine till they feel and look about right. If to loose space between parts and engine wiggles around. If to tight no engine movement without moving fuselage.