To close tolerance, or to not close tolerance, that is the question..... 1

I would use a couple of dowel pins (very close tol) for location and std tol on the fasteners for clamping.
On many auto engines this is how the trans is mounted to the engine.

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P.E. Metallurgy, consulting work welcomed

How close do the two parts need to align?

what are the materials for the frame and engine?
you should be able to get enough torque to resist loosening under vibration, but
what are you using for locking feature on the nut side? many locking nuts and nut-plates are only good for a couple of installations before they need to be replaced.

Is it a dynafocal style mount, with vibe isolators at the engine interface?
Usually the vibe isolators have a kind of straight bushing which passes through an elastomer ring, and can presumably be replaced if the bushing becomes scored.
I can't say off the top of my head what the tolerances are typically, or how much radial play the elastomer might provide upon installation.

Thanks all for your answers.
Reply here below.

@EdStainless
That's almost what I'm doing.
Of the 6 1/4" bolts 2 are close tolerance to position the engine assembly on the frame and 4 are clearance holes. This way I provide positioning and all 6 of them provide clamping.

@BrianE22
I'm not sure what you mean but using 2 close tolerance holes the alignment would need to be within 50 microns.

@MintJulep
Thanks for the suggestion. I've seen those in the past but never used them. I'm not sure they can help in my sistuation but I'll investigate further.

@SWComposites
Engine Frame and main frame are both is Al 7075 T451. 4 of the bolts are locked with all metal locknuts (military standard), the other 2 bolts are locked with nutplates (NAS standard).

My issue is that one of the engineer said that that connection must be close tolerance because you cannot rely on friction for shear transfer.
The problem is that close tolerance holes and easy assembly/disassembly are mutually exclusive in my opinion. Furthermore I'm not sure we really need close tolerance because the connection is very easy to inspect.

My opinion is that a close tolerance bolted connection has to be used for critical parts like attachment of wings on fuselage etc. where it's difficult to inspect the connection, the connection itself is dimensioned precisely and you won't disassembled it any time soon.
In my situation the connection is well oversized (six 1/4 inch 160ksi bolts for 50 pounds is well enough ) and easy to inspect.

@Ng2020
No isolators between the engine and the main frame, not my choice! But it is what it is.
But you are absolutely correct, having isolators would have provided some wiggle room for the installation.

Can you bush the holes?

Also be careful mixing close tol & clearance fit: the close tol holes will take most of the shear load until deflections are great enough to close the gap in the clearance holes.

Don't use bolts for your locators, unless they are 'stripper bolts' with precision ground shanks.
Bolt tolerances will not work.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

What engine is it?

I think you've missed what Ed meant (or at least what I think Ed meant) with dowel pins. These are permanent close tolerance (sliding fit) pins used to align the two parts, then bolted together with "normal" tolerance bolts (usage 2 ?).

but normally (in my experience) engines are attached to airframes with isolators, because you're right ... tight fitting bolts (requiring a mallet for removal) will destroy the hole (well, the interference, close fit, you're relying on) quite quickly. Why do you think you need interference fit (or very tight tolerance) bolts ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

I've never seen an engine-mount alignment that was critical, standard clearances were always used. Loose-fit dowels are occasionally used for assembly aides to keep fat meathooks from being pinched by heavy engines but that's unrelated. If you make mount alignment precise to a rigid frame hopefully you dont have a transmission or gearbox rigidly mounted to the engine.

My issue is that one of the engineer said that that connection must be close tolerance because you cannot rely on friction for shear transfer.

Click to expand...

Hopefully they're not suggesting that the bolt shank carry shear load instead of the friction in the joint between the engine and mount. If so, laugh.

Hello and thanks again for your answers, I think I need to clarify a couple of things.
1 - Many design choices where done before I joined the project. Initially they used 6 close tolerance bolt to secure the engine assembly on the main frame. I "compromised" by proposing to use only two close tolerance and 4 standard clearance holes to avoid overconstraining. I would just have used 6 clearance hole and that's it.
2 - Somebody did a static analysis considering that the connection has to work by transmitting shear only through the shank, no friction contribution allowed, since then this has become a requirement within the company. I'm honestly doubtful about this since these close tolerance connection are generally used in permanent or semi permanent connections.

@Ng2020
I suppose I can bush the holes but would rather avoid it...

@EdStainless
They are close tolerance grounded bolt so quite precise and made to work in shear trough the shank.

@3DDave.
It's a wankel rotary engine.

@rb1957
I'm familiar with the two dowel pin alignement and have no problem with that. You're absolutely right about isolators, I would have used wire rope dampener to isolate the engine, but again this was designed before I joined the project.
I'm not a fan of the close tolerance fit, please see point 2 here above.

@CWB1
The engine is bolted down directly onto the frame and the transmission has a semirigid coupling (of the elastomer type).

"Hopefully they're not suggesting that the bolt shank carry shear load instead of the friction in the joint between the engine and mount. If so, laugh."
That's just what they are saying and it is the point of contention really.

Just to add to the discussion of where to use close tolerance bolt, from the FAA AC 43.13-1B:

On paragraph 7-44:
"CLOSE-TOLERANCE BOLTS are used in applications where two parts bolted together are subject to severe load reversals and vibration.
Because of the interference fit, this type of bolt may require light tapping with a mallet to set the bolt shank into the bolt hole.

"CAUTION: Caution must be exercised in the use of close-tolerance bolts
for all critical applications, such as landing gear, control systems, and
helicopter rotary controls. Do not substitute for close-tolerance fasteners
without specific instructions from the aircraft manufacturer or the FAA."

Again it seems to me that the examples cited in the AC refer to permanent or semipermanent installations.

IMHO any bolt that requires a mallet to get it out is meant to be permanent.

I understand your position, inheriting a design. I would pose the question "how often are we going to open this joint ?" and then "how do we remove the tight fitting close tolerance bolt (without damaging the hole) ?"

Transferring shear without friction is an analysis assumption. Of course there is friction in a clamped up joint. You could possibly consider serrated plates (but I doubt I would). Your high design loads come from extreme situations (like failure torque) which could overload the friction, the joint slip a few microns, and the bolts work in shear. You could possibly apply an "impact load factor". I think engines have been hung with sloppy holes since "christ was a cowboy" (or as I prefer "since christ played full back for Jericho", for very good reasons (interchangability?).

Look at other engine mounting designs.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

@rb1957
"Transferring shear without friction is an analysis assumption. Of course there is friction in a clamped up joint. You could possibly consider serrated plates (but I doubt I would). Your high design loads come from extreme situations (like failure torque) which could overload the friction, the joint slip a few microns, and the bolts work in shear. You could possibly apply an "impact load factor".

Exactly my position, if it's close fit it is meant to be semipermanent. I'm going to take a look to some old drawings (WW2 american bomber and aircraft) to take a look at how they did engine mounting back then....

ok, so you want to change the design, no doubt against much resistance. then pose the questions ...
how often to remove ?
how to remove (mallet) ?
how much damage ?
how to repair ?
As I say, if "they" insist on no friction shear, then serrations are a mechanical "friction". I don't like it ... the easiest way I can think of is to machine a recess in both mating faces. a pocket for the two serrated plates, which have elongated holes (not aligned, but at 90 degrees) to create the hole for the fastener; and I'd've have sloppy holes in the main parts.

Another way would be to have sloppy holes for the fasteners, and somewhere else have the serrated plates ... or a sliding fit dowel pin

how about other similar drones ? how do they secure their engines ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

F81... Your post... 30 Jan 23 10:52...

I am a confused by Your statement, thus...

@SWComposites
Engine Frame and main frame are both is Al 7075 T451. ... ...

The T451 temper you cited for 7075... any form... simply does NOT exist. What is the actual alloy and -temper You are using; and what are Your parts Made From [MF]: plate or forging, etc????

CAUTION. Use of 7075-Txxx for any application on/near an engine is dubious at best... bad at worst. Alloy 7075 in any temper typically has a long-term service temperature limit of 200F-to-225F [depending on the design reference]. Otherwise temper can be grossly affected by 'aging at temperature'... which can also affect fracture toughness, corrosion resistance, mechanical allowables, creep, etc.

2XX-T6xx and 2XXX-T8xx [casting and wrought] alloys provide superior heat resistance of ~300F-to-350F and are more typical for hot engine environments. Oddly, certain magnesium alloys [castings] can operate long-term at +350F with appropriate coatings for corrosion resistance. Castings procured in mass quantities can often be cost-beneficial.

Also, The problem with precision alignment might be eliminated with coordinated/matched ream-tooling for each-side of the joint: engine and main frame. NOTE IF one, or-the-other, are 'pre-bored to final hole-size... then the tooling can be adjusted on a case-by-case basis. However if Each engine is intended to be interchangeable between mounts/airframes... and the bolt pattern on the engines or main frames is 'low-tolerance' [imprecise/variable], then You may have an issue best resolved by the engine OEM doing final line-reaming for 'both sides' on matched tools.

OH yeah... do the engine and main frame share a 'common seal'... or is the joint unsealed?

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

let's put it down to a typo ... either 7075T7351 or 7050T7451

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

Rb... So which 'typo' is it...7075T7351 or 7050T7451???

Differences between these [2] alloys-tempers are substantial to a materials weenie; and raise my concern regarding accuracy of the OP's statements... which affects quality of MY response(s)...

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov