Bearings

[racerfrank]

I've been doing some research on building a launch engine and I have a few questions. I like the idea of roller bearings on the mains and rods.......but I also like the idea of a solid crank, although split bearing are needed.



Question #1 Are built up cranks like the Tiny Power M rugged and reliable?
Question #2 Are all split bearings babbited or are some just machined brass/bronze running on the cast/steel crank?
Question #3 If brass/bronze running on cast/Steel crank, how much oil clearance is needed?
Question #4 How does #2 and #3 apply to rod(big and small ends) bearings and eccentrics apply?

Pros and cons?

Thanks in advance

Frank

[fredrosse]

Question #1 Are built up cranks like the Tiny Power M rugged and reliable?

ANS: Built up cranks are not as strong or more strong than solid cranks, depending on the materials of construction and manufacturing methods. A forged steel crank may be necessary for a dragster putting out several thousand horsepower at 9000 RPM, but for our purposes a built up crank can be completely adequate. A cast iron crank might be much weaker than a built-up made with drill rod and high strength steel webs, IFF the press fits are right.

Question #2 Are all split bearings babbited or are some just machined brass/bronze running on the cast/steel crank?

ANS Many split bearings are not babbitted. Automotive and small engine split bearings are always not babbitted (at least for the last 100 years or so). Brass, or better yet, bearing bronze split bearings are very common. I would guess that non-babbitted split bearings far outnumber the babbitted type today. Automotive practice uses steel bearing inserts, with a tin/aluminum alloy for the actual running surfaces, running against hard steel.

Question #3 If brass/bronze running on cast/Steel crank, how much oil clearance is needed?

This number is highly variable, but the typical "rule of thumb" is 0.001 inch running clearance per inch of shaft diameter. On single direction force bearings, 0.002 inch per inch diameter is OK, but with reversing forces, such as a double acting steam engine, tighter clearances avoid developing a "knock".

Question #4 How does #2 and #3 apply to rod(big and small ends) bearings and eccentrics apply?

Cast babbit bearings can be made up without the need for precise machining, which applied to many of the early machines that had to be kept running without access to a machine shop. The relatively soft babbitt could be scraped into shape by hand if needed. In today's world, split bearings with ready-made bearing inserts (or inserts we make of brass or bronze) are the norm. For our steam launches, brass/bronze inserts are fine, and so are babbitted bearings if done correctly.

What is the correct spelling anyway??

[Lopez Mike]

I run a Strath Steam Warrego. It is a 3 x 4, piston valve single. 500 rpm, 5 h.p. claimed. Probably about right.

It uses four ordinary sealed ball bearings on the main. The rod big end is a simple split bronze bearing with an oiler dripping down through a small pipe into a catcher ring on the crank that routes the oil to a drilling in the crank throw. It does not seem at all critical as to whether I remember to turn on the oiler every time.

It is a built up crank. Just pressed together. A small dab of Locktite on the back sides of the big end inserts to make sure they do not creep.

The eccentrics are also sealed ball bearings.

It is a very reliable and low maintenance engine.

[TahoeSteam]

I believe Allen Gregg built his AVL compounds (5 of them!!) with roller bearings on all three mains. The crankshafts were in two halves with a half-lap joint (in a sleeve and pulled together with a bolt) under the centre main. Seems to have worked quite satisfactorily.

[barts]

It is of course the connecting rod bearings that really need the help; both of these are usually more problematic than the mains. The little end doesn't spin enough to get good oil film developed, and the big end tends to be limited on many engines in terms of size.

I'm designing our big uniflow (5.75" x 5.75") to use heavy Type E pillow blocks on the mains w/an overhung crank, along with needle bearings at the little end and a spherical roller bearing at the big end. The cross-head will be v-groove guide bearings; this means I won't need to use oil for any lubrication (save a very small bit for the cylinder).

The rear main bearing will also take the prop thrust as these tapered roller bearings are ideally suited for that service, and the rear bearing doesn't have much else to do .

- Bart

[TahoeSteam]

I do like your idea of needle bearings on the wrist pin very much! If they can survive in a driveshaft u-joint over 100,000 miles with a 1000 ft/lbs of torque oscillating several thousand times a minute, they sure as heck can survive in our applications!

[Lopez Mike]

I guess you know enough to run the little end needle on a hardened sleeve. That sort of oscillating motion is hell on the inner race.

[barts]

I guess you know enough to run the little end needle on a hardened sleeve. That sort of oscillating motion is hell on the inner race.

Yup... a hardened inner sleeve is the right answer since hardening and grinding the inner race to Rc 60 is not in the cards in my shop .

- Bart