Ping-Pong Ball Racer Machine
My first inspiration for this type of model was in the January 1978 Meccano Magazine, built by Robin Schoolar and family. Unfortunately, other models, college and not getting back to SELMEC meant this was forgotten about until now. I do not have the parts to build anything like Robin’s model, but a small model should still be entertaining.
To get the balls from the bottom to the top, two Archimedes screws are used. I contemplated one 2’ high screw, but felt it could not be rigidly constructed with standard parts. No bending is required for the parts in the screws, but a constant problem is the ping-pong balls snagging on the bolt heads.
To minimise this problem I used new bolts. By contrast the vertical strips are old because some of the replica parts have sharp edges which also cause snagging. It just about works, but this is not a sound design. A friction clutch is used from the E15R motor to the screws just in case they jam. The screws revolve at about 20 rpm with a 350:1 reduction from the E15R motor.
Once at the top the balls travel down a ride similar to Oblivion at Alton Towers. There are two gates to hold the balls up before the drop. At the bottom of the fall the balls must land on curved plates, otherwise they acquire backspin and do not complete the upward section. I still regrettably had to put in a scoop at the bottom to remove any balls that get stuck. This happens rarely.
After going down a staircase, we get to a triangular snooker table. As the balls rest on a lever operated by a cylindrical coil they cut off a light beam to a photo-cell and the coil operates. The balls are pushed to one of three exits. I would have liked to have lights showing which exit was last used, but did not have the parts.
The mini-maze below the exits allows the balls to jangle about before dropping into a lower guide. The maze raises and lowers by about an inch at the front, with connectors at each side. The gear ratio to the left side is 27:1 while the gear ratio to the right side is 32:1. This means the cycle only repeats about every minute when operated by an EU1072 motor running at about 1000 rpm.
I have had a three of ideas on the Archimedes screws:
- Remove the 1½” strip from each arm and put in its place a 1½” angle girder placed over the screws on the triangular plate. Attach it to the triangular plate with 2 x ½” angle brackets or a 1” angle girder (non-standard part). For the base arm use two ½” angle brackets and a corner angle bracket.
- The angle girder guides for the ball are the wrong way around. This means there is an opportunity for snagging when one angle girder finishes and the next one starts because of the rounded edges. Instead guide the balls up with the outer edge of the vertex of the angle girder.
- Have 8 strips down the screw. Connect 4 strips at the top and bottom of the screw and connect the remaining 4 to their adjacent strips with obtuse angle brackets. This was not progressed.
Alternatively, see Adrian Ashford’s Archimedes screw, which works very reliably with minimal bending of strips. The only downside of this for my model, is it raises the balls very slowly, 2” per revolution as opposed to 6” per revolution. This means a lot of balls would be in the screws and not in the rest of the machine.
I have now revised the Archimedes screw using 1½” angle girders. The lowest arm uses two angle brackets and a corner angle bracket. The balls go up much easier but there was still a problem with the balls snagging against the perforated strips going up the rotating shaft. To resolve this I took the angle girder guides and attached them to the frame with obtuse angle brackets. This means the ball is forced up between the arms of the screw and one of the faces of the angle girder. However I would still not risk sharp edged replica parts.
A problem with the 1½” angle girders is it is no longer possible to allow the balls to rest at the base of the machine and be scooped up as the second ball jams. A ‘ball at a time’ dispensing gate gets over this.
The E15R gear reduction is № 400:1.
Worked very well at the 19th January 2008 meeting. I dismantled it today.
I thank Tim Surtell for a much improved video. The previous one taken at the exhibition (now deleted) had a poorly functioning photo-cell which did not trip the coil enough.
Following a request I can give more information on how to construct the Archimedes screws from the given diagram.
(Top left). Connect a 1” angle bracket to a 12½” strip and connect two obtuse angle brackets to this.
(Top right). Connect a 1” triangular plate to the obtuse angle bracket but only put one screw in for now nearest the strip.
(Bottom left). Connect 2 angle brackets to the outer edge of the triangular plate.
(Bottom right). Connect a 1½” angle girder to the 2 angle brackets. You will notice no screw heads are present for the ping pong balls to snag on. For the lowest arm on the screw two angle brackets and a corner angle bracket must be connected as an angle girder would get in the way of the base where the ball is waiting to be scooped up.
There are 6 12½” strips going around the screw. Each arm goes up in height by two holes.
The 12½” strips connect at the top and bottom to 6 hole bush wheels. I used a careful mixture of 1½” and 2½” double angle brackets to do this. The rule is to always make sure there are no screw heads where the balls travel and the double angle brackets allow the screws to be put higher up where necessary.
Use two axle rods at the centre of the Archimedes screw connected by a coupling, or use a non-standard axle rod of about 14”.
To strengthen the screw connect 1½” angle brackets to opposite 12½” strips with washers for spacing if necessary.
The base of the screw is a 4” circular plate that the ball can rest on before it is scooped up.
WARNINGS: This is fiddly to construct. Get your box spanner out! Do not use replica 12½” strips; the balls can snag on the sharp edges. Use good old smooth edge Binn’s Road strips. I have forgotten a lot of the details of construction. Some trial and error may be required.
- Weight: 20lb (9kg).
- Ball transit time: Approx. 40s.
- Construction time: Approx. 300 hrs.
- Dismantling time: 5 hrs.
- Approx. part count with fixings: 3,200.
- Approx. part count without fixings: 1,000.