The Steamboating Forum

I checked out the Watt's linkage by constructing appropriate arcs in the CAD system. Peak error (deviation from linear motion) on an engine this size is +/- .020", which is high for uniflows since the piston is so long. It did occur to me while at the pub last night that one could mirror image the linkage on the other side of the cross head; this would convert the error to a twisting motion - ok w/ a spherical roller bearing big end and on the Watt's rocker , but it's all getting too complicated .

I also ran the numbers for cut-off at 40 degrees after TDC; crosshead bearing loads up somewhat, but still easily doable w/ 4 square inches of SAE 660 bronze bearings... very handy that the peak cross head load is not also at the peak velocity. 100% cut-off engines have a hard life.

Detroit Tug, I'm going to try your idea of a cross head guide on either side of the piston rod, but use bronze bushings with a small oil reservoir and felt oil seals instead of linear bearings. They will be running on the same R60 case-hardened ground shafting, though. The supports for the cross head guides will tie the columns together, and turned rods will in turn tie the two sides together.

- Bart

I wish I could figure out a way to use the system that I.C. engines use to keep the piston side slack to zero, thus no knocking when there is reasonable wear.

All I.C. engines for the last 100 years (that I know of!) have an elliptical flexible piston that when cold has some minimal amount of clearance in the bore. It's referred to as 'cam' probably because the machine that shapes the piston in the last stage of manufacture has a cam that sets the amount of eccentricity.

As soon as the engine warms up even slightly it grows to a size that is larger than the bore and runs from then on with a negative clearance, being forced to a shape intermediate between its cold elliptical shape and being round at which point it would start to stick in the bore even with good lubrication.

Once the piston or the bore wears too much, the piston 'slap' becomes audible, at first only when the engine is cold and later much of the time. For most of the existence of modern engines the bore did the wearing but in recent years some Japanese designs are showing piston wear rather than bore wear. Makes it easier to refurbish an engine at home as you don't need to bore the cylinders. Does anyone rebuild engines any more (snort!).

As I said, I have yet to come up with a scheme to have some sort of preload or negative clearance for the reversing loads at the crosshead. Put your thinking caps on.

Mike, some of the re-shaping comes from the different thicknesses of alloy around the skirt (more metal more expansion), and also Invar struts moulded into the piston, which restrict the expansion in certain directions. All very, very complicated for the home workshop.

Big changes in technology since I last ran a piston grinder in my dad's shop. I can assure you that the pistons I was grinding to size were rather prosaic aluminum and were intended for crude things like Model A Fords.

I would bet a good deal that no one grinds pistons any more in ordinary automotive machine shops. At least I hope not!