Design Part VIII
We're down to laying out the very last part of the engine, the eccentric straps and eccentric links which are described in this page. Several composite drawings of the valve linkage are also presented. These drawings were made to verify that everything fits together.
The photo above shows the MRSR 91 eccentric straps and eccentric rods. Note that each strap is composed of two straps, one inside the other. The inner strap is free to rotate through roughly a 45 degree arc relative to the outer strap.
This end view of the strap shows that the inner part is split as is the outer part. In the orientation in the photo the seam in the inner strap is nearly in line with the seam in the outer strap. (These two seams are not aligned in the previous photo. The locomotive was moved between the time the two photos were taken ) The brass colored ring is the edge of the bearing between the inner ring and the eccentric. There doesn't seem to be a bearing between the inner and outer straps.
Combined Eccentric Straps: The sketch above shows the combined outer and inner straps. The inner strap is the bearing surface with the eccentric. The inner strap also provides a bearing surface with the outer strap enabling the inner and outer straps to rotate about 45 degrees with respect to each other. This sketch is a copy of the original design of the straps which have been built and in operation on the motor. There was a problem in that the maxim rotation between the two straps was initially too small. The sides of the inner strap tab was thinned to provide the required rotation. The width of the tabs on the following designs are thinner to allow greater rotation between the straps.
The drawing above shows the inner eccentric strap which will be machined in bearing bronze. The tab is mild steel silver soldered to the bronze ring. The ring will be sawed in half after the inter strap, outer strap and eccentric have been assembled and adjusted as necessary so that everything moves smoothly. The eccentric rod fits in the tab. Each eccentric rod is held in place by a pair of 6-32 screws. The nuts for these screws are toward the front of the locomotive so that they can be loosened and the rods adjusted. The eccentric and straps closest to the crankcase are for forward. The holes in these straps are threaded and the screws put into the tabs and secured with Loctite. The eccentric rod used with the straps for forward will have slots to allow some adjustment. The eccentric and straps for the reverse direction are at the front of the engine. The eccentric rods for the reverse direction will have threaded holes and have the screws secured with Loctite. The eccentric straps for the reverse direct will have slots in the tabs.
The outer strap shown above will be machined from mild steel as two pieces (HM139 & HM140). The dimensions of the slotted holes are the same as on the inner strap. The two halves will need to be fastened together before the inside is turned to the correct diameter. The bronze inner eccentric strap will wear over time and the straps will become loose and sloppy. The surface where HM139 and HM140 join can be filed as necessary to compensate for this wear.
The sketch above shows a combined inner and outer eccentric strap with eccentric rods. Note that the center of the forked end of the rod is aligned with the edge of the strap that in turn is aligned with the joint between the eccentrics.
Recall that the model eccentric movement was intentionally made over scale. The excess movement necessitated that the position of the tumbling arm pivot be moved up slightly to avoid interferences between the eccentric rod and the tumbling arm. In fact, numerous variations of the position of the pivot as well as the size and position of the rocker arms were tried before the final design was selected. The drawing above shows one eccentric at the closest point to the rocker arm base and verifies sufficient clearance.
The sketch above shows the tumbling arm rotated 20 degrees to the forward full gear position. (The tumbling arm is in this position in next four sketches ) The eccentric is in the position where the left valve (right side of drawing) is in the upper extreme. The mid point of the left reverse link block pin is shown as 3.931" from the center of the crankshaft. (Note that the shape of the tabs on the eccentric straps which hold the eccentric rods are different than the drawings above. These valve linkage drawings were made using an earlier version of the strap. The shape of the strap doesn't impact the purpose of these drawings so I didn't take the effort to update them.)
The sketch above also shows the tumbling arm rotated 20 degrees to the forward full gear position. The eccentric is in the position where the right valve (left side of drawing) is in the upper extreme. The mid point of the right reverse link block pin is shown as 3.929" from the center of the crankshaft. Note that this dimension is nearly identical to the 3.931" for the left side.
The sketch above shows the tumbling arm rotated 20 degrees to the forward full gear position again. The eccentric is in the position where the left valve (right side of drawing) is in the lower extreme. The mid point of the left reverse link block pin is shown as 3.293" from the center of the crankshaft. The difference between the valve upper extreme and the lower extreme (3.931"-3.293") is 0.638" which is slightly greater than the 0.625" design valve motion. The 20 degree tumbling arm rotation is slightly excessive.
The sketch above shows the tumbling arm rotated 20 degrees to the forward full gear position again. The eccentric is in the position where the right valve (left side of drawing) is in the lower extreme. The mid point of the right reverse link block pin is shown as 3.293" from the center of the crankshaft. This is identical to the dimension for the left valve. The difference between the valve upper extreme and the lower extreme (3.929"-3.293") is 0.636" which again is slightly greater than the 0.625" design valve motion. The 20 degree tumbling arm rotation is slightly excessive.
The eccentrics and linkages are symmetric between forward and reverse so that is no need to sketch the linkages in the reverse position.
This finishes the engine design.