Connecting Rods

I'd be concerned about a glued joint holding up to the splitting and post-splitting machining process. I think I'd be more inclined to use silver (or even plain ol' lead) solder.

I'd also be tempted to make the whole cap out of bearing bronze, and only insert the rod, or just be happy with the brass-steel mating surface.

Tim sez:

Who said anything about a "glued joint"? Split bearing caps are always held in place by bolts. Perhaps, I wasn't clear. It should be understood the insert (discs) would be lock-tited in place and the cap bolts installed, before drilling, reaming and lapping the holes. Silver solder would work as well, but why bother? The bearing hole is lapped to fit the crank journal - after splitting.

Bob Swinney

I was confused about your order of operations, I think.

I think I thought that you were going to drill a hole in the con-rod blank, glue in your bronze disk, drill your con-rod bolt holes, tap, split, machine (across the glued half-disk), bolt it together, drill the glued half-disks, ream, etc. Somewhere in this envisioned process I saw the inserts coming unglued.

How are you going to hold the inserts in place while you do all your machining?

Be aware, that brass/copper-alloys aren't that good to locktite in. I would soft solder the bushing into the rod, then cut the cap and ream the hole.

Nick

Tim sez: "How are you going to hold the inserts in place while you do all your

Sorry about the confusion. Here's how it goes:

1. Bore insert hole in the con rod end at the proposed bearing center line. Insert diameter maybe 50% greater than bearing hole.
2. Locktite insert disc in hole.
3. Drill and tap 2 holes, lengthwise, for cap bolts.
4. Split at bearing centerline location..
5. Install cap bolts.
6. Drill, ream, lap, the bearing hole.
7. Open bearing by removing cap bolts.
8. Install split bearing on crank pin with cap bolts.

As per usual, anything "intuitive", always has more steps than is foreseen in the beginning. I think, maybe, in full size practice there would be 1 or

2 shims placed at the split line before opening the bearing hole. As wear occurred, a shim would be removed to close up the bearing hole. This might lead to a slightly elliptical hole but it would take up the wear. Some con rod bearings were adjusted with a wedge shaped key driven against the split.

Bob Swinney

That is the order of operations that I would contemplate -- do you really think the glue will hold? I won't say it'll fail -- gluing metal seems to be a perfected art, it just goes straight across the grain for me.

I think in automotive applications they resize bottom end bearings by shaving a bit off of the interface between cap & rod, then boring the hole out round. This is probably not necessary in a old-style stationary steam application, because the working speeds are so much less.

The load on 3/8" crank pins in a 2 cylinder steam engine is what? High compared to a watch bearing; low compared to an automotive rod. Spare me the misery of actually calculating the bearing load. Mostly, the engine will be run, unloaded at demo speed. I figure it would be capable of 1/4 HP of work if pressured up to 56 psi and turning 500 rpm. It would be impossible for the inserts to "spin" because the bearing cap screws would partially cut in to the outer edge of the inserts.

I question the need for bearing inserts too. Just build the rod as you describe with the removable cap, bore it to size and run it! A drop of oil for lube will run a long time on a brass rod, hard steel pin. if it gets loose, split the rod and file the cap to fit and run it some more. It does not sound like a critical item, no offence, just a toy! One shop I worked at had an old turret lathe with an idler pulley on the belt. The ball bearing died in the pulley one day and I machined one out of a chunk of brass, and used a hard shoulder bolt for the shaft. We would give it a drop of oil at shut down in the afternoon and run it eight hours a day. It was still in use five years later, probably still is today! Greg

I agree that it will require some thought about clamping to keep the glued in insert from popping out during the splitting operation, but as the rod end assembly is fairly small (he did say 5/16" diameter journals), if the splitting is done with something like a fine slitting saw, and both sides of the rod end/bearing are clamped against a flat supporting suface it will work out ok. I'd be willing to bet that he'll accomplish it just fine.

Jeff

Tim sez: "do you really think the glue will hold? I won't say it'll fail -- gluing metal

Yeah, I do. Locktite and other threadlocking and fixturing products are really good. They are esp. handy when heat and/or pressure of force fitttting could cause distortion. The crank pins on that engine are locktited, as per the designer's instructions. Think about the crowns and fillings in one's teeth; dental "glue", if you will.

Bob Sw>

Right Jeff. There is enough holding force on a locktited 5/32 half moon to resist the cutting forces of a fine slitting saw. Most of the force is directed at the opposite side, which is supported.

Bob Sw>

Yeah, I could dispense with the inserts I know; but I kind of wanted the experience of making them. Besides, as you said (bite your tongue, though) it is only a toy.

Here are some nice connecting rods a good friend of mine made that are going on a K4.

John

That's an official answer I got from LockTite when I had problems with brass.

Nick