Eccentric Crank and Rod

Revised 2/6/2013          Subject to Revisions

Eccentric Cranks

Eccentric cranks are the third outermost item attached to the main driver crank shafts.  On the model they will provide retention of the main connecting rods which in turn retain the side coupling rods. the cranks are held in place using a 0-80 screw.  

Two pair of eccentric cranks

 In the photo above two pair of eccentric cranks as they came out of the 3-D printer are shown.   Two pair of cranks were built although only one pair will be used on the model. This was done in case there were printing problems.

Pair of eccentric cranks with 0-80 screws

 

Two 0-80 screws are threaded into the crank as shown above.  At the narrow end the eccentric rod will be attached using a short screw.  The hole is tapped for the screw.  At the large end a screw is used to compress the two sides of the crank around the crank shaft end.  On the model the crank shaft is round with two flats to keep the crank at the proper angle of 25 degrees relative to the crank shaft to center hole angle on the wheel.

Eccentric crank installed on end of main driver crank shaft

The above photo shows one crank installed on the outermost end of the crank shaft outside the main connecting and coupling rods.  One problem was observed, the accuracy of the 3-D printed shaft was very poor and the designed round with two flat shape did not come out well.  The author attempted to file the flats a bit however the overall shape of the shaft tip was highly distorted. 

Main driver crank shaft showing distorted eccentric crank mount

As a result the shaft tips were not usable.  The cranks however had nearly perfect shape.  As a result of the distorted crank shaft tips the cranks would not stay attached and would not hold angle.  They would easily fall off with any kind of pressure.  The author decided to remove the tips and attach machined metal tips in their place.

Eccentric rod set immediately after 3-D printing

Meanwhile a pair of eccentric rods were printed.  These turned out good needing relatively minor cleanup.

Set of three eccentric crank mounts in mill just after milling out screw head relief cavities

A design for eccentric crank mount shaft tips was done using two 00-90 screws to attach the tip to the driver crank shaft.  The old tips would be milled away leaving a flat surface and two holes drilled to match the newly machined tips.  The photo above shows the set up on the Sherline 2000 mill with a aluminum work piece in place.  The work piece has three tips defined each having two clearance holes for the 00-90 screws and relief cavities for the screw heads already machined.



 

Tap holes were drilled through the wood backing block

After tapping the wood backing block the three parts will be bolted to the it during outline machining

The three parts are each held to the backing block with two 00-90 screws

Parts in the mill after machining the outline shape

View of the Sherline 2000 mill used to machine the parts

The parts are very small being 0.2289" diameter with two flats 0.180" wide on the side.  The part is 0.122" thick and has two holes for the 00-90 screws and a relief cavity for the heads so they will be flush with the outside edge after mounting.

Three eccentric crank mounts while still attached to the backing board

Aluminum work piece removed showing the three repair eccentric crank mounts

Work piece after removal showing that the locations for the parts was completely cut through

Backing block with center eccentric crank mount and eccentric crank

Eccentric crank on repair mount

 After machining a fit check showed a very good snug fit of the crank on the mount with almost no play.  After tightening the crank compression screw the crank was found to be firmly in place with no tendency to move or slip off.

Main driver during removal of the defective crank shaft portion with the mill

Drilling two tapping holes for the 00-90 screws using the mill to get proper locations

Drilled holes in end of modified crank shaft

The crank shaft diameter where the tapping holes were drilled is 0.321" in diameter.  The holes are 0.035" diameter on 0.1145" centers.

Main driver with 00-90 hand tap in place in crank shaft end

Eccentric crank and repair crank mount portion of crank shaft

Crank shaft with repair part screwed in place on crank shaft end

Eccentric crank mounted on repaired main driver

Assembled eccentric crank and rod on model during final fit check

Very small details such as the eccentric crank shaft end do not turn out well and may not be usable to mate moving parts as occurred in this model.  The drivers were fabricated very early on and subsequent model building does not provide confidence that sufficient improvement might be possible .  The repair crank shaft end worked very well and could be built with very good accuracy, more than sufficient for the modelling effort.