Connecting rod

[ACME]

hello,
can we use Aluminum for 50 class rc helicopter engine "connecting rod"? (5051, 6061,7075)

or other

thanks

[NC Cams]

A qualified "Yes" to both options.

Life expectancy, however, will vary as a function of load and temperature the rod will be subjected to. In an engine that small, loads are relatively low and the design might be pretty forgiving.

Real car drag race engines have used 2024, 2018 and 7075 - I dunno about 6061 but it wouldn't surprise me - lots of things have been tried.

THe ultimate choice, however, was dictated by loads and projected life expectancies (IE top fuel had different "issues" than pro stock which were different than sprint cars which DID try aluminum rods).

Keep in mind that some alloys are more notch sensitive than others thus your finish could have more of an effect than mere material selection at some point.

Using SWAG engineering, 6061T6 or 7075T6 would be decent starting points.

You might even contact BME, Venolia and/or Childs & Albert. They might be willing to give you some pointers.

You might also do "aluminum connecting rod" search on Google. Might be real surprised at what turns up.....

[One of Many]

Originally Posted by ACME hello, can we use Aluminum for 50 class rc helicopter engine "connecting rod"? (5051, 6061,7075) or other thanks

5051 is rather soft. Between that and the 6000 series, I would still use a bushing or bearing at each end if not hard anodized. 7075 is some pretty tough stuff. Heli's run some pretty high RPM's. One worry is that if 7075 were used, it is not as forgiving in a re-kitting incident. You won't bend it, but IT may help bend or break something else.

DC

[pstockley]

I've seem 2024 used in the past for model engines. One issue with 7075 is it looses quite a lot of strength under high temperatures. Not too sure how hot a 2 stroke crank case gets, probably not enough to worry about. 7075 is quite prone to fatigue cracks if there are surface imperfections from machining. Many rods I have seen run bushings for the big and little ends. But I have seen some that had no bushings. Some of the really high power motors use forged cranks I believe.

[NC Cams]

Manley, Childs and Albert (C&A) and Bill Miller Engineering (BME) ALL used 7075 for car engine con rods for YEARS. Some of them still are and some have moved onto a new 70 series trick "aircraft" alloy.

The dragster engines did/do NOT annodize the rod and they do NOT run pin bushings (steel pins run right on the aluminum forging). They DO run con rod bearings.

Briggs and Stratton made their conrods for their lawn mower engines from die cast material and those ran forever and they used NEITHER a con rod bearing nor a pin bushing.

When it comes to making a rod for an engine, it is the stress levels in concert with the material choice that determines if a particular comination will work/live or not.

As annodizing is effectively aluminum oxide and Al O2 is an abrasive, I'd be disinclined to use it for a bearing surface unless you can superfinish it in some way.

Regarding surface finish: The BME rods are beautifully polished forgings free of any machine marks in the beams. The Manley's and C&A's are fully machined as I recall. The old Howard's con con rods were aluminum (I dunno the material) but they were definitely shot peeded.

QUITE a few people have cut con rods out of aircraft quality plate for drag race engines for YEARS. There is even a go cart rod maker who's making them out of plate stock.

Might want to do an "aluminum con rod" search on Google to see what else is being done by the pros....

[ACME]

hi,
thanks everyone...
i got it

[greg b]

It is customary to run roller bearings of some description in high rpm 2 stroke engines weather they be steel rods or alluminium rods because of there lubrication .

[vulcom1]

Most of the 2 stroke glo fuel engines have bronze bushings on both ends. At the moment I can not recall any that don't. On the gas engines you will find bearings instead. Running straight aluminium in my opinion will cut short your engine life do to the wear on the rod crank end. When they start to go they go egg shaped.
What engine are you making it for? If known could probably let you know which the stock setup is.
John

[NC Cams]

7068 is a recently declassified aircraft/aerospace alloy that is now seeing a lot of use in highly stressed automotive engine applications. It is replacing 7075 for a lot of high stress used.

One such use is the shaft type rocker arms that are used in NASCAR which see both high temps and high loads.

The stresses of a NASCAR rocker could easily approach that of a model aircraft engine con rod.

[triumph406]

As someone who has made and sold model airplane con-rods, 2024 is the best alloy, I've had a few 7075 rods break in FOX 36 combat engines, which are much harder on rods than a helicopter engine will ever be. 6061 will work fine in sport engines.

Both end s should be bushed with bronze, If I remember I think pin clearances were in the .001-.0015 range.

7075 and 2024 rods can be run without bronze bushings, if you hone the bore and get a decent cross hatch pattern. I have a few English 2.5cc diesels that have unbushed rods, and they typically have quite high bearing pressures.

Good luck

[RotaryNut]

Originally Posted by vulcom1 Most of the 2 stroke glo fuel engines have bronze bushings on both ends. At the moment I can not recall any that don't.

I just checked my small collection of glo engines for bushings, all rods were aluminium:

O.S. MAX 15 CL - none
O.S. MAX 15 RC - none
O.S. MAX-S 30 RC - none
Thunder Tiger 25 RC - none
GTM(GM?) 15 RC - big end only

All of them are 'sports' engines so not built for high performance, with the exception of the 'GM' or 'GTM' engine which is an awesome engine.

A small diversion:
All I know about the 'GTM' is it's made in Japan and is Schnurle ported, the castings and the machining are very nice.
It's distinctive in that it has a rectangular box for the exhaust's expansion chamber.
I have never seen another anywhere, even on the net, except in an RC engine collectors case, and a few years ago as a background image on bottles of a popular brand of glo fuel here in Oz(Pro-Glow). Unfortunatelly I didn't keep one of those bottles and they're now using an image of a different engine.
I would post a pic, but It's been in bits for replacement bearings for nearly a decade, 'spose I should do something about that one day!.
If anyone knows anything about this engine, please let me know.

EDIT:
Well I've just found(total fluke, long story) that the brand name is G-Mark, all I had to go on was the symbol cast on the crankcase, which looks like the attached pic.
You can see where I got 'GTM' from.
Very little info about this brand of engines on the net, except for that on Ron's Chernich's website (www.modelenginenews.org/) for a .30 5cyl 2stroke radial(!), a twin horiz-opposed .12 and a small .030 single.
Still absolutelly zero info on mine.

[sleeper142]

Originally Posted by NC Cams Manley, Childs and Albert (C&A) and Bill Miller Engineering (BME) ALL used 7075 for car engine con rods for YEARS. Some of them still are and some have moved onto a new 70 series trick "aircraft" alloy. The dragster engines did/do NOT annodize the rod and they do NOT run pin bushings (steel pins run right on the aluminum forging). They DO run con rod bearings. Briggs and Stratton made their conrods for their lawn mower engines from die cast material and those ran forever and they used NEITHER a con rod bearing nor a pin bushing. When it comes to making a rod for an engine, it is the stress levels in concert with the material choice that determines if a particular comination will work/live or not. As annodizing is effectively aluminum oxide and Al O2 is an abrasive, I'd be disinclined to use it for a bearing surface unless you can superfinish it in some way. Regarding surface finish: The BME rods are beautifully polished forgings free of any machine marks in the beams. The Manley's and C&A's are fully machined as I recall. The old Howard's con con rods were aluminum (I dunno the material) but they were definitely shot peeded. QUITE a few people have cut con rods out of aircraft quality plate for drag race engines for YEARS. There is even a go cart rod maker who's making them out of plate stock. Might want to do an "aluminum con rod" search on Google to see what else is being done by the pros....

What about 2618? They make most aftermarket pistons out of this material, and I know it is actually more dimensionally stable at high temperatures than a 7000 series metal would be.

I would be hesitant to make a rod out of plate in a high stress application, as you can't really be sure what the grain structure is. Maybe if it is a forged plate?

The current set of rods I have are a 7000 series aluminum made from extruded bar stock, in which the grain of the metal is in the direction the rod is being stressed (lengthwise). I personally like this better than a forged rod, since the grain runs perpendicular to the direction of the forces in the rod. The down side however is that when machining the profile of a rod made of extruded bar stock, you will have exposed grain boundaries, creating possible stress risers. This can be avoided by shot peening and/or polishing, but nevertheless interesting food for thought. A forged rod will likely have less exposed grain boundary ends, fwiw. BME's are obviously working for the fuel cars, and my rods are obviously working for my alcohol car.