Steel Ball Escapement

This marble-run escapement releases one steel ball per cycle, and is fun to watch and listen to. The middle lever arm pivots up and down. On the upstroke it hits a steel ball which then jumps out of the upper track. I build the escapement from some spare wood pieces I had laying around.

Supplies

Some hardwood pieces

1/4” dia dowel - short piece

1/8” dia steel rod or #8 nail - short piece

Washers

1/2” steel balls or marbles

BB pellets, a bunch of small rocks, or other weights

Wood glue

Table saw

Drill press

Step 1: Upper Track

You need to build the upper track, lower track, pivot arm, and mounting stand. All dimensions given are for using 1/2” diameter steel balls in the tracks.

The upper track is 15.5” long and cut from a 3/4” by 7/8” piece of maple wood. In it I cut a 1/2” wide by 1/2” deep groove on my table saw for the steel balls. Drill a 5/16” diameter hole in the center of the groove 1/4” from one end. Glue a small end cap to the end of the track. Later on you will cut or file a small notch into the end cap as part of fine-tuning the escapement.

Next you will build the spring mechanism for the escapement. Cut a 3/4” wide by 1” long by 1/8” thick spacer, and glue it to the bottom of the track, 6.5” from the upper end.

Cut a thin strip of hardwood 15.5” long, this is the ‘spring’. The strip is 3/4” wide and about 3/32” thin. It should be somewhat flexible. Drill a 1/4” dia hole 3/8” from the end. Insert a short 1/4” dia dowel into the hole; but don’t glue it in yet. Then glue the spring to the bottom of the track. Apply glue to the first 4 inches of the spring (on the left side in the drawings). The spacer will bent the spring as shown. Adjust the spring so that the dowel is centered in the hole of the track, then clamp and let the glue dry. Check the drawing and pictures to better understand the mechanism.

Step 2: Pivot Arm

The pivot arm is made from a 16.5” long by 7/8” wide by 1/4” thick board. A 2” by 2.5” by 1” wide box on the left end is for weights (i.e. BB pellets) to tune and balance the pivot arm. On the other end is an open-ended tray to receive the steel balls from the upper track. The tray sides are 3.5” long by 1” tall by 3/32” thick. A pivot point block is glued to the arm approximately 4.5” from the left end. You need to determine the best position of this pivot point before gluing it down. Load the weight box with some weights and mark the center of gravity on the arm. Then add a steel ball to the tray on the right and mark the new center of gravity. Then mount/glue the pivot point block between these two centers.

Step 3: Lower Track

The lower track is made from a 18.75” long board, 1/4” thick; and sides that are 3/4” tall and 3/32" thick. A dowel is glued in at one end to serve as a stop.

Step 4: Stand

The stand has a 2.5” by 9” foot, and a 1.5” by 3/4” by 15” upright. A 1/8” steel rod axle for the pivot arm is inserted 8.5” from the bottom.

Step 5: Assembly

Before gluing anything permanently to the stand I recommend that you test the escapement by clamping the upper track to the stand. First, mount the pivot arm with the 1/8” steel axle to the stand. I used a couple of washers and a little wooden knob glued to the axle. Clamp (and later glue) the upper track to the stand as shown in the drawing. The angle is 10 degrees from horizontal. Be sure that the pivot arm hits the end of the spring as shown; this pushes the dowel upwards and kicks the steel ball across the end cap. If you haven't done so, glue the short dowel to the spring when everything works well.

Mount the lower track as shown at 4 degrees from horizontal. Be sure that there is enough space on the right side for the steel ball to drop from the pivot arm tray into the lower track and not fall off the end. Again, do some testing before gluing anything up.

Step 6: Tune-up

The escapement will need some careful fine-tuning. Adjust the weights (add/subtract BB pellets or other weights) and carefully tune (i.e. cut) a notch into the end cap of the upper track.

Enjoy your new toy!