Thursday, September 25, 2008

Pushrod guides done, still working on the leaks

Added push rod guide to help reduce binding and lateral forces on the push rod bearings and pistons. Changed orientation of piston wrist pin to be perpendicular to the guide pin. Seal leaks remain a problem. Posted pictures of guide and assembled engine without heat exchangers.

Push rod guide
I finally made the push rod guide. It consists of 2 pieces of 4.5" square 1/8 " thick Plexiglas sheets with slots cut in it. There are 4 separators that are ~1.2" square, 5/8" thick.

As I descibed in my last post, the holes in case sides, were not perfectly centered, so some of slots do not line up perfectly with the center of push rods. So rather then making new case sides, I just offset the hole for follower guide pin that I drill into push rod end.

The pin is 1/8" diameter steel stock, the bearings are copper inserts used for plastic tubing compression joints. The slots are slightly wider then the OD of these inserts. The inserts have a flare on one side, so that keeps them from sliding off the pin, because they are inside the guides.
The pins are just long enough so that don't hit the case ends when centered. If the pins do get loose at the push rod ends, then the case will prevent them form going too far.

The side pieces have had there push rod hole enlarged to 5/8", so a nylon bearing can be inserted. They are 5/8" long, so stick out through the 3/8" thick case side. Pushing the bearings so they are flush with outside of the case, centers the guide and for now holds it in place for drilling the holes for mounting screws to the case sides. For now I am letting the guide float.

I cut the slots before drilling the 1/2" hole for the shaft. This was a mistake. One of the guide plates cracked when enlarging the pilot hole. The shaft hole should be drilled first.

Assembled engine
Below you can see the assembled 4 cylinder engine. I am using car tire valves for pressurizing the cylinder pairs. I have the parts for the heat exchangers but haven't installed them yet, because I have to keep pulling the heads to replace the leaky bags.

The plastic bag in the foreground is one that I replaced. The leaks are circled, but not really visible in this picture.

Changed wrist pin orientation
Because the pistons are being driven from a cam rather then a crankshaft, they hardly pivot.
When the do, because of lateral forces on the push rod when it rides the cam, it would transfer these forces to the piston and make the bag seal rub against it self. By changing the orientation of the wrist pin hole so it is perpendicular, shaft and guide pins. This will allow the push rods to slide along them. Hopefully the pressure in the cylinder will keep the piston centered, so there is an gap all around the piston.

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