RotoBug Nr. 6 Page 1 RotoBug Number Six by Richard Meiss, WB9LPU 409r Introduction The instrument described here is the sixth version of the original conception of a new kind of semi-automatic telegraph key. It is an outgrowth of the basic Parkwood PaddleBug design dual lever, magnetically tensioned, with the dot contact in the form of a magnetic reed switch. In the RotoBug design, the usual linear pendulum is replaced by a centrally-mounted rotor that oscillates in a magnetic field whose position is determined by a pair of permanent magnets. One of these is semi-fixed, and the other is mounted on the rotor. Instead of a sliding weight for setting the speed, a moveable arm with a weight on its end is fastened to the rim of the rotor. Moving the weight (in an arc) changes the moment of inertia of the rotor and hence the oscillation rate of the rotor.
RotoBug Nr. 6 Page 2 In earlier versions of the RotoBug, the fixed magnet was mounted on an adjustable radial arm attached to the base of the unit. It attracted the rotor magnet in such a way as to cause the rotor to tend to deviate away from its zero degree position. The movement of the rotor was restrained by the dot paddle arm, with the force supplied by the paddle arm tensioning spring (or magnet). Pressing the dot paddle removed the restraint, and the rotor was free to oscillate and periodically trip the magnetic reed switch (with another magnet mounted underneath the rotor). This arrangement required separate adjustments for rotor rest position, fixed magnet position, paddle travel, and reed switch position. The design worked very well, but the several variations tried were rather complicated to make and adjust. In addition, the rotor pivoted on ball bearings, and their proper alignment required considerable care in machining the shaft housing that held them. RotoBug No. 6 was an attempt to lower the parts count and simplify the construction. The rotor was pivoted on hardened steel pivot points, with the lower one running in a Teflon bearing and the upper one supported by a conical steel bearing. The fixed magnet was moved to the end of the paddle arm, eliminating the separate fixed magnet radial arm. All tension is provided by rare-earth permanent magnets. These changes have resulted in a more compact design with better performance and easier adjustment. Its feel is not unlike that of an iambic paddle, with very small paddle movement and low return force. Speed can be changed via the position of the weight arm, or by changing the magnet gap. Construction Details The working parts of the bug are constructed of lightweight aircraft aluminum (2024-T4 alloy), and for stability, the base is a 4-inch diameter disk of cold-rolled steel. The pictures and text that follow will show the stepwise assembly of the bug, first showing the paddle mechanism, then the rotor details, and finally, the completed instrument. The paddle arms are made of 1/4 x 1/2 aluminum bar stock. Lucite paddles are bolted to a recess at the end of each arm; the dot and dash contacts, made of turned brass, are threaded into the other end. Each arm bears a hollow cylindrical magnet as part of the tensioning system. Near the magnets are the 2-56 adjusting screws for
RotoBug Nr. 6 Page 3 setting the paddle rest position. Above and below each arm, near the rear, are the upper and lower ball-bearing assemblies. Just forward of the bearings are short coil springs that serve as electrical jumpers, since the lubricated bearings do not always conduct reliably. Dot Paddle Magnet Holder Jumper Spring Dash Paddle Dot Contact Tension Paddle Rest Magnet Adjustment Screw Upper and Lower Bearings Dot Paddle 342r The dot paddle arm bears an adjustable extension that holds the fixed rotor magnet. These arms will be mounted between the base plate and top plate. The 1/4 thick base plate for the paddle mechanism bears the dot and dash contact pillars and a rear upright support column. Near the rear are a pair of 0.251 holes that receive the lower paddle-arm bearings. The large hole in the center is for the rotor bearing assembly.
RotoBug Nr. 6 Page 4 In the pictures below, the paddle arms are mounted in their bearing sockets. The 1/8 top plate holds the upper bearings and stabilizes the contact pillars. The base is made of cold-rolled steel, 1/2 thick and 4 in diameter. It is painted with Hammerite Rust Cap spray paint, which is tough and scratch-resistant. The central hole is for the rotor bearing assembly, which extends upward between the paddle arms. The upright parts mounted to the base are the two adjustable tensioning magnets, two uprights to hold the upper rotor bearing, the rotor rest stop, and the rotor bearing assembly. The sides of the rotor bearing assembly are milled out to provide clearance for the paddle arms. The paddle assembly (seen at the left below with the top plate removed), is mounted on the base with screws from underneath. It also shows the magnetic tensioning system, with the rare-earth magnets operating in the repulsion mode.
RotoBug Nr. 6 Page 5 With the top plate and the spacer ring installed, the reed switch assembly is mounted to the rotor bearing assembly (above, right). It can be moved in a limited arc to set the weight (duration) of the dots and then locked with a clamping screw. The bug is now ready for the rotor assembly. The rotor is a 3-3/4 disk of 0.1 aluminum, with five 1-1/8 holes cut in it to reduce its mass. It pivots on a spindle with a 3/8-32 threaded portion. The rotor magnet and the reed-switch magnet mount beneath the rotor and the speed weight mounts above. The spindle pivots are made from hardened steel track-shoe spikes that are press-fit into its ends. Using a conical Teflon thrust bearing below and a steel conical bearing above produces a low-friction assembly that will turn for over three minutes when spun by hand. The wiring is under the base (below, left); the fine wires lead from the magnetic reed switch. The bug is wired so that the dot-paddle contacts must be closed for the
RotoBug Nr. 6 Page 6 reed switch closure to be sensed. This wired-and arrangement eliminates the need for a mechanical damper. This is in contrast to the test wiring shown at the left. This worked pretty well if nothing got bumped! Shown below are two views of the finished bug, with the principal parts identified. Top Bearing Adjustment Speed Weight Reed Switch Magnet Dot Paddle Rotor Rest Stop Adjust Dash Contact Adjustment Dash Paddle Reed Switch Adjustment Dash Paddle Tension Magnet 394r
RotoBug Nr. 6 Page 7 Speed Weight Top Bearing Adjustment Reed Switch Carrier and Adjustment Dot Contact Adjustment Dash Paddle Dot Paddle Tension Adjust Dot Paddle 392r Conclusions This bug extends the design program by trying out a new bearing system and rotor magnet arrangement. The changes have resulted in smoother operation with less chance for contact bounce, and simpler construction as well. On the air it is wellbehaved and holds its adjustments well. It has been tested at about 20 words per minute, a limit imposed by my lack of skill. Determining the upper range will have to wait until I can catch up; the lower speeds go down to 10 wpm with the dot duration remaining properly scaled to the speed. Further planned refinements include a fine adjustment for the dot duration control. Eventually the features worked out for the semiautomatic version will be applied to a fully-automatic instrument that is presently in the design stage. The last page shows some close-ups of construction details and additional steps in the assembly.
RotoBug Nr. 6 Page 8 Most of the parts, prior to painting and polishing. Some close-ups of the rotor magnet arrangements and components. Copyright 2006 by Richard A. Meiss, WB9LPU Speedway, Indiana wb9lpu@arrl.net [Roto6_text-4.doc]