Introduction: K'Nex Ping-Pong Ball Fruit Machine


The aim here was to build a fruit machine from K'Nex which was not too much bigger than the real thing. This was a big challenge, mainly because it was difficult to achieve the required precision from what are relatively chunky pieces compared with the overall size of the machine. The front of the machine is 13" wide, the depth (apart from the pay-out tray at the front) is 22", and the height (apart from the 'slot') is 36". It can be lifted by one person without too much risk of a visit to a chiropractor.

Having used K'Nex balls for earlier versions of a fruit machine, they were deemed to be too large and too heavy for what was going to be a lightweight pay-out mechanism. Ping-Pong balls weigh very little and are ideal for the purpose.

The only non-K'Nex components are some rubber bands, a small lead weight, a screw (not mentioned in the video!) which clamps a red gear to a block of white connectors, the reel symbols, the labels, a small annular piece of wood and, of course, a few dozen Ping-Pong balls.

Game Sequence

The machine functions as follows:

1) A ball in inserted into the 'slot' which releases the handle-lock

2) The handle is pulled, performing the following functions:

  • The pay-out tester is reset
  • The handle lock is reset
  • The game-timer is reset (throughout the pulling of the handle)
  • The reel-brakes are reset
  • The reels are spun

3) The handle is released, performing the following functions:

  • The game-timer starts
  • The first reel brake is released, bringing the first reel to a stop
  • The second reel brake is released, bringing the second reel to a stop
  • The third reel brake is released, bringing the third reel to a stop
  • The pay-out tester is released
  • The appropriate number of balls are released if there is a win
  • The game-timer stops

The Pay-Out Method

This has been based on that used for a 1950s Novomat wall machine (see the second photo above). The Novomat machines were variants of the better-known and more fancy Beromat wall machine (see the third photo above). In these machines, each symbol has a pay-out value associated with it, and the higher the value the greater the depth of a corresponding slot in the reel. A pay-out tester then drops to the depth of the shallowest slot; the further it falls, the greater the number of coins are paid out.

In this machine, instead of slots there are protrusions on the left had side of each reel. A pay-out tester drops to the depth of the shallowest protrusion, and the further it drops, the more balls get released.

The Reels

It was decided that the percentage return of the machine would be 75% — this was typical of Novomat / Beromat one-armed bandits. The PDF above shows the sums which were done when designing the reels and assigning the symbols.

This Instructable

This Instructable is aimed at people who invent and create their own constructions. The aim is not to explain how to build this machine — it's too complicated for that, and there were so many little tweaks that it would be impracticable to convey it all. Having said that, if someone wants to build this machine, any questions will be responded to (usually within 24 hours), and any extra photos can easily be made available. Plenty of help is available, but only if photos of progress are supplied.

What this Instructable does do, however, is to present numerous K'Nex techniques which were used for this construction and which could be useful for many other kinds of build.

The steps in this Instructable are as follows:

Step 1: Be Aware of the Actual Lengths of K'Nex Rods

Step 2: Know How to Combine Rods to Achieve a Target Length

Step 3: Know the Thickness of K'Nex Pieces

Step 4: Deal with Any Damaged K'Nex Pieces

Step 5: Learn How to Make Strong / Rigid Structures

Step 6: Know How to Make a 'Super-Rod'

Step 7: Learn How to Make Strong Gearing

Step 8: Decide on the Types of Power Sources

Step 9: Learn How to Make a Wide Fascia

Step 10: Be Aware of Micro K'Nex

Step 11: Learn About Ratchets

Step 12: Learn How to Use a Gearbox to Control the Speed of a Mechanism

Step 13: Take Advantage of a 'Stagger-Tube'

Step 14: Be Prepared to Use the Odd Lead Weight

Step 15: Be Aware of the Unusual K'Nex Pieces

Step 16: Don't Forget the Oil

Step 1: Be Aware of the Actual Lengths of K'Nex Rods

Believe it or not, the actual lengths of K'Nex rods are in some cases different from the official lengths! The PDF above explains the differences.

Step 2: Know How to Combine Rods to Achieve a Target Length

It is often necessary to combine rods (with, say, an orange 2-way connector) in order to arrive at a target length. For large target lengths, the difference between the desired length and the nearest one possible is relatively small, but for small target lengths the nearest possible can be unacceptably different. This is why the fruit machine in this Instrucatble was so difficult to build.

The PDF above show the combinations of rods which are required for each target length from 17½mm to one metre.

Step 3: Know the Thickness of K'Nex Pieces

The PDF above shows the thickness of various K'Nex pieces, and presents a quick method for working out what can be fitted onto a K'Nex rod.

Step 4: Deal With Any Damaged K'Nex Pieces


The only real damage which can happen to a connector is that a leaf has broken off part of a slot or if there is a crack somewhere. The connector can usually still be used if an appropriate orientation is used, but maybe it is better to throw it away so that it doesn't get inadvertently used in the future.

The damage happens when a rod is carelessly wrenched out of the connector rather than sliding it out sideways.


Many second-hand rods, however, have burrs on the end, as shown in the first photo above (here, the white rod has a burr on both ends). Again, this is caused by wrenching the rod out of a connector.

It may be thought that the odd burr doesn’t matter, but if a damaged rod is inserted into a connector, there is a chance that the rod and the connector won’t lie in a straight line. If the rod is being used as an axle, this misalignment could create friction at the pivot hole (the hole of the connector). Also, it won’t have the strength of a proper connection.

If a lot of force is needed to push a rod into a connector, there’s a good chance that it’s a burr which is causing the problem.

A fine metal file is the easiest way to remove a burr — it must be filed down so that there is no protrusion — but it must not be overdone, otherwise the connection will be loose. There will still be a small dent in the rod, but it won’t matter.


It would appear that many second-hand yellow gears have parts of a tooth missing (see the second photo above). This is virtually always the part of the tooth which projects beyond the main body of the gear and it rarely matters.

Step 5: Learn How to Make Strong / Rigid Structures

This fruit machine needed to be strong and rigid so that there was no bending or distortion anywhere — that is a sure-fire way to lose power in critical places.

Cube Structure

The main structure of the machine (and its stand) are based on cubes where there is a diagonal on each face (see the first photo above). In the machine, the cubes have yellow-rod sides and red-rod diagonals, but of course there are areas where rods are missing for clearance reasons.

Sometimes (see the second photo above) there is not a slot for the end of a diagonal rod, and this is when an angled orange clip is useful (see the third photo above).

Special 7-Way 3-D Connector

Sometimes a 7-way 3-D connector needs to be very firmly attached to another 3-D connector. There is a special version of a 7-way 3-D connector which does just this (see the 4th photo above). The differences are a) there is a small projection near the jaws of the connector so that the grip is tighter; b) there is no a gap on the opposite sides of the jaws; and c) there is no hole on each side of the slot.

It is very, very difficult to separate two of the connectors.

Using Black Rods

The black rods in K'Nex are much firmer than the grey ones of the same length and withstand torsion much better.

Strengthening Rod Connections

When two rods are joined by an orange connector, they can be pulled apart with not too much force.

If they are joined by a white 8-way connector the join will be somewhat stronger, but the strongest join of all is when green rods are inserted next to the rods which are being joined.

The fifth photo shows the three methods of joining rods.

Making Strong Levers

The handle of the machine needed to be strong but light.

One method would have been to make it three connectors' thick and use white rods to clamp them together, but this would have made it too heavy (and therefore require more force to return it to its starting place).

The method used can be seen in the sixth and seventh photo above: the sides are yellow 5-way connectors separated by two blue spacers and clamped by white rods.

Step 6: Know How to Make a 'Super-Rod'

One of the problems with K'Nex is that all the rods are the same diameter and so if there is a lot of twisting force the rod can lose its strength and even disintegrate, even if a 192mm black rod is used.

In the fruit machine there were three places where a rod needed to span the width of the machine — over 12 inches.

A 'super-rod' is made from white connectors which are clamped to each other by white rods. Some green rods can be added to some of the empty slots so functional pieces can be attached.

The first photo above shows a short super-rod to demonstrate the principle. Notice how the white rods spiral round to clamp the connectors together. In this example, there are four white rods on each connector. Note that the ends of the white rods fulfil the function of holding the connectors apart. Longer rods are not appropriate for this — the protruding ends are too long.

The second photo shows the reel-control super-rods of the fruit machine, and the third shows the pay-out tester super-rod. The extra black clips, green rods and blue spacers help to keep the super-rod rigid by tightening all the slots in the connectors.

Step 7: Learn How to Make Strong Gearing

A gear is nearly always fixed to a rod by a tan clip — without this clip it would freely rotate.

If there is a lot of force between two gears, one of the clips can fly off. There are two ways to avoid this, both of which have been used in the fruit machine. The first is to add one or more extra gears parallel to the existing one (see the first photo above) so that the force on each clip is reduced. The second is to fit an annular piece of wood over the tan clip (see the second photo) so that it cannot open and fly off (cut a circle of wood about 25mm in diameter from a 5mm or 6mm piece of plywood, and drill a 15mm hole in it — this will fit tightly over the clip).

It is important that any parallel gears are placed close together so that torsion in the rod holding them is reduced.

In the first photo, three extra red gears have been inserted on the rod so that the force on the tan clips is spread out. Also, a screw has been used to clamp two of the red gears to the block of white connectors to reduce torsion in the rod. This was necessary because there is a lot of force joining the handle to the game timer, resulting in significant twisting of the rod holding the gears.

Step 8: Decide on the Types of Power Sources

The fruit machine was to be entirely mechanical and so only three types of power were available: spring motors, rubber bands or weights. Each has advantages and disadvantages.

Spring Motors (see the photo above)

- Very clever design (not many turns needed to wind up, but many turns made when unwinding), but not enough power.

Rubber Bands

K'Nex rubber bands were not used because they didn't have the required characteristics. Here in the UK, postmen are constantly discarding rubber bands on pavements and so the source is plentiful. The gathering of these is also a form of litter control!

- Varying power

- Don't need much space

- Tend to deteriorate


- Constant power

- Can need a lot of room

- Do not deteriorate

In the fruit machine, rubber bands are used to return the handle to its home position, hold the reel latches in place and return the reel-spinner (there was not room for weights). Weights are used to return the pay-out tester release and latch the handle lock (little force was required and so the weights were small). The handle-lock weight is lead because it needed to be heavy for its size.

Step 9: Learn How to Make a Wide Fascia

Under the awards card of the fruit machine is a fascia along the width of the machine. This consists of around 50 4-way connectors all abutting one another. At first sight this is not possible, because there is not a rod which is long enough to hold them all together.

The three photos above show how it was done.

Step 10: Be Aware of Micro K'Nex

Sometimes it is just not possible to find a combination of K'Nex parts of the right size.

Micro K'Nex can help here: in the fruit machine, a length of rod was required which was not possible to achieve using a combination of Classic K'Nex rods; Micro K'Nex came to the rescue — there are so-called transition pieces which enable Micro K'Nex rods to be attached to Classic K'Nex ones. In the photo above, a few pieces of Micro K'Nex have been used to bridge a gap where it was not possible using Classic K'Nex.

Step 11: Learn About Ratchets

There is more to ratchets than meets the eye.

The key feature of a ratchet is that the whole assembly normally needs to rotate with the rod that it is on.

The K'Nex ratchet (see the first photo above) is a cleverly-designed and simple device, but it does need a bit of force for it to work. The pay-out tester is a delicate thing and using a K'Nex ratchet for it was not appropriate.

It was decided to use a red gear (which has 34 teeth) as the ratchet with a white rod as the pawl (see the second photo above). It had to be placed on the driving gear, because the assembly was much larger than the K'Nex one and so it could not be placed further down the gear train.

This would mean that the rod could rotate 360 / 34 = 10.6° before the pawl latched — but this was too much because there could be too much movement before the pawl latched onto a tooth of the red gear, causing the pay-out tester to jolt and possibly dislodge balls. The solution was to add a second pawl (a 1-way connector) so that the effective number of teeth on the red gear was doubled to 68. Unfortunately, a piece of the connector has to be removed for it to work. There are not many instances in this build where pieces had to be modified, but this was one of them.

In the game timer, a K'Nex ratchet has been used because there was enough force when pulling the handle to make it work. A K'Nex ratchet only has 16 teeth, and so the rod on which it is placed could rotate 22½° before the pawl engaged with it. In practice this rarely matters — it is just a matter of placing the ratchet at the right point in a gear train so that it latches almost immediately (see the third photo above). See the next step — it covers gearboxes.

Step 12: Learn How to Use a Gearbox to Control the Speed of a Mechanism

In the fruit machine there are two places where the speed of something needs to be controlled.

The first is the game sequence, and the second is the pay-out tester.

Game-Sequence Gearbox

In the first photo, a driving red gear on the first shaft is in mesh with a blue gear on the second shaft. A red gear on the second shaft is in turn (ha ha!) in mesh with a blue gear on the third shaft — this is the one which has a ratchet on it (see the second photo). As it stands, this would run down quite quickly.

On the shaft which has the ratchet, a red gear is in mesh with a blue gear on a fourth shaft. This pattern continues all the way to a seventh shaft, and it is this one which has a flywheel, or fly, on it (see the third and fourth photos). It follows that for each revolution of the first shaft, the seventh shaft rotates (34/14)6 = 205 times (a red gear has 34 teeth and a blue gear has 14).

Pay-out Tester Gearbox

This comprises a four-shaft gearbox instead of a seven-shaft one. It works in the same way as the game-sequence timer, but has a different ratchet type (see the ratchets step) and a heavier fly (see the last photo above - it has a total of 16 1" wheels on the eight spokes instead of four). The heavier fly makes the gear train unwind more slowly so that the pay-out tester drops smoothly.

Step 13: Take Advantage of a 'Stagger-Tube'

When a ball falls down a shaft — especially a K'Nex ball — it can make quite a clatter and hit the bottom with quite a bang.

A 'stagger-tube' is a shaft that has projections down its inside so that a falling ball is continually deflected, slowing its progress. The sound it makes is quite satisfying!

In the photo above, the orange angled clips are just large enough to do the deflecting — if the projections are too small they won't make much difference, and if they are too large the balls will get stuck.

Step 14: Be Prepared to Use the Odd Lead Weight

K'Nex pieces are generally quite light.

Sometimes a weight is required which is heavier than can be provided by K'Nex pieces, and in these cases a small lead weight does the trick. Depending on the circumstances, some readily-available coins can be used instead.

In this fruit machine, a weight was needed in the handle-release mechanism because there was not enough room to use K'Nex pieces.

Step 15: Be Aware of the Unusual K'Nex Pieces

Sometimes unusual pieces are just what's needed, but because they are unfamiliar they can be forgotten about.

The reel-spinner in the machine wasn't working very well - the middle reel spun too slowly. A red cone (see the photo above) was just what was needed to rectify the problem. It can be seen in use at 3m 48s in the video.

Step 16: Don't Forget the Oil

Oil can make a huge difference to the smooth running of a machine, even a K'Nex one.

The gear teeth and the pivot holes should be lubricated with household oil.

In the fruit machine, the reel reset kept failing because the end of a rod could not escape from a cradle. A liberal dose of oil solved the problem at a stroke.

Under no circumstances must a K'Nex ratchet be oiled — all that will happen is that the ratchet will stick and not work at all.