The Steamboating Forum

Mike,
I recall seeing an engine, at the Port Orchard Meet, that was built by Don Mentzer? The crosshead guide consisted of a sealed ball bearings on each end of the crosshead pin that were captured in a slot formed by spacers between two pieces of heavy square stock. The engine for a time was listed for sale in the Steam Gage , but I don't know what its' current status is.

Dave

Definitely sounds like one of Don's creations. They all ran very well.

I've seen machinery that was designed that way and always wondered if it would wear evenly along the guides. I guess that's just as much of a problem with a sliding contact.

But wouldn't the guides that the ball bearing run on need lubrication anyway? Unconfined oil and grease is what I'm trying to cut down on.

Mike

Linear slides with ball bearings need components that are hardened and ground like any other bearing race. In a reciprocating configuration there needs to be space for the balls to travel and roll about their path. Balls that drag in a race instead of roll will wear unevenly and quickly.

Ball bushings for linear traverse use a recirculation tube to keep the balls rolling and prevent them from sliding. A bearing with a fouled recirculation tube and balls that improperly slide instead of roll will fail in short order. Any linear ball bearing sliding component will have wipers fitted to remove foreign debris in the path which can easily lock them up.

Any sort of circular component to heel forces is going to have a very small point of contact which bears the whole load which would be evenly distributed between two large planes as in a conventional crosshead design. So those surfaces need to be as hard as the balls. Hardchroming is only as good the material it is on.

If you want a low maintenance ball bearing linear component thats clean and capable with a very long service life, that would be the THK style rail and car/guide system. They have a recirculation circuit as well. Just shoot em up with heavy grease occasionally.

A passing thought. Common usage and convention says that the thrust stroke in forward gear should push the cross-head onto the slide, thereby determining which side they go. Fine, there is sound mechanical logic in that, but is there any convention which side the feed and/or exhaust should issue, or drains for that matter?

From the lack of response I assume not then. I shall put the feed and exhaust to the thrust (slide) side and leaving only the drain-cocks to the other.

I come to this thread rather late but can throw some info into the question of ST 6A con.rod length. I believe I have the first 6a that Anthony Bevor ever built, indeed the first 6A. According to the drawings, the conrods are 6 3/4" length between centres. I can't check this against the as-built size for the moment but will later.
BM

Mike:
I have questions about why the crosshead is placed so the "down thrust" in forward motion pushes it into the guide.
I can only guess that the weight of the piston+rods assembly induces more force during that half of the stroke cycle.
Can any one recommend some printed discussion/analysis on this?
'Time to educate myself some more.

I have a small twin marine engine to build (Reeve's Commander) which needs some serious redesign to work properly.
RussN

RNoe wrote: ↑Sun Nov 29, 2020 4:52 pm Mike:
I have questions about why the crosshead is placed so the "down thrust" in forward motion pushes it into the guide.
I can only guess that the weight of the piston+rods assembly induces more force during that half of the stroke cycle.
Can any one recommend some printed discussion/analysis on this?
'Time to educate myself some more.

I have a small twin marine engine to build (Reeve's Commander) which needs some serious redesign to work properly.
RussN

It not only pushes against it on the way down due to the angle, it is also pulled against it on the way back up when the angle is reversed. Those engines are relegated to one direction of rotation.

Re steam admission exhaust which side? Conventional engines, it doesn't matter. If I understood correctly

Russ, my understanding is that pressing against the slide bar in that direction is more rigid (supported) which will to some extent depend on the design. You will be using forward much more than reverse. With I.C. engines they move the crank slightly to one side to bring the con-rod more verticle under load on the power stroke, which relieves the side pressure to some extent.