Mechanical Coin Acceptors - How They Work
Neonkiss:
I saw this original post by Op-Bell and had to move it to the new forum.
I saw questions raised about mechanical acceptors in another thread and considered jumping in, but now I can't find it again. Since most EMs have them, this seems the appropriate place to discuss them. They look deceptively simple but they're fiendishly clever, and if they go out of adjustment you need to understand what's going on to have any chance of fixing them.
First, take a look at this picture - it shows the actual path the coins take as they pass through the mechanism, which is probably a bit different from what you expect. I'll get to the details next.
Neonkiss:
I'm using a "Coin Mech Inc" nickel acceptor for this exercise. There are several other manufacturers, but they all work on the same principles. With just a few simple parts they manage to measure the diameter, the thickness, the weight and the metallic composition all at the same time. Here's a picture of the front.
Upper left is the cradle. This is a metal stamping with two projecting arms, free to turn on a pivot pin. It usually has a counterweight. When the coin drops in, if the diameter is too small it falls between the arms and drops through to the reject chute, and if it's too large it catches on the arms, but hangs up on a projection when the cradle tries to tip. If it's too thin it falls off the inside tips of the arms to the reject chute, and if it's too thick it can't fit through the narrow space between the cradle and the backplate. If it's the right diameter and the right thickness, and heavy enough to overcome the counterweight, it catches between the arms and the cradle rotates clockwise, tipping the coin off into the next section. Thickness can be adjusted with a screw, upper center in my unit.
The usual problem here with old acceptors is that the pivot pin wears, so that even a correct size coin hangs up as the cradle tips. The cradle pivot is a precision part, finely machined. To examine the shaft, remove the e-ring and lift the cradle off. The pivot pin should look similar to the illustration, with two bearing surfaces in good condition, and the cradle bearings - usually brass or oilite - should be a fairly close fit on the pin. If there's enough wear here that the cradle wobbles, you probably need a new acceptor, but sometimes adjusting the thickness out some will stop it jamming.
Neonkiss:
As the coin falls out of the tipping cradle, it falls on a short ramp, just about 3/8 of an inch long. Rolling and accelerating, it flies off the end of the ramp and passes in front of a magnet, which is closely spaced to the back plate and serves as another thickness check. The magnet performs the obvious function of stopping steel washers, but it does a lot more than that. Take a look at it - it's huge and powerful, about 250 times the magnet needed to catch steel blanks. It also checks metallic composition.
Remember from high school science, that a conductor moving in a magnetic field generates an electric current? Well the coin is a conductor, and it's moving, and there's one heck of a magnetic field in the gap between the magnet and the backplate. The coin becomes, in effect, a small generator. But because it's a disk, and not a loop of wire, it short-circuits itself - the current runs to waste within the coin and doesn't do much besides warming the metal by a few millionths of a degree. However, driving that small current around takes energy, and the only energy available is the kinetic energy due to the coin's own motion. Some of this energy is used to generate the current, and as a result the coin slows down. The amount by which it slows down depends on the magnet strength, the coin's initial speed, and the resistance of the coin's metal. Assuming the magnet stays the same, if you put coins of equal weight but different metals into the acceptor - say nickel, copper, silver, gold, aluminum, etc - they will slow down by different amounts. The coin is in free fall when this happens, curving downwards, and the slowing effect changes the path it takes.
Neonkiss:
At the bottom right of the acceptor frame is a steel block, which I call the anvil. If a coin is slowed just the right amount by the magnet, it falls on the angled left face of the anvil and bounces off. If it isn't slowed enough, it falls further to the right and maybe hits the side frame; if it's slowed too much it falls short and misses the anvil altogether. In either case, it falls straight into the reject chute.
The coin that hits the anvil is subjected to another clever metallurgical test. Like the three bears' beds, it may be too hard, too soft, or just right. If it's too hard it will bounce high. Now in the middle of the acceptor is an adjustable metal tab that I called the "blocker". Its job is to get in the way of coins that bounce too high, and knock them back into the reject chute. If the coin is too soft, it won't bounce far enough to clear the right tab on the separator plate, so it too will fall into the reject chute. The coin that's just right will bounce over the separator tab but under the blocker, and fall into the accept chute.
On the frame that holds the acceptor to the door is a small solenoid called the rejector magnet. Its armature has a short piece bent up that enters the rectangular hole in the middle of the acceptor for the "just right" coins to hit, so that they fall into the reject chute. When the solenoid pulls in, it removes its armature from the hole so that coins can be accepted. If the magnet isn't working you can either remove the armature, or tape a scrap of soda can over the hole in the acceptor to stop it entering the hole. Or you could always replace it, I suppose.
If you're having poor acceptance, try to watch coins falling through and see how they bounce. A weak magnet can put the acceptor out of commission, and there's nothing to be done about it, but sometimes it's possible to adjust the thickness screw and make the gap smaller. You may need to move the blocker, or take it out altogether for home use. The anvil may or may not be adjustable, but before you loosen any screws, mark its position carefully so you can put it back.
You can probably see now why the coin acceptor doesn't work so well when the door is open. Usually, the open door rests at an angle, and the angle upsets the delicate dynamics of falling and bouncing.
Slot Docs:
Great post Neonkiss,
A little additional info, the cradle and cradle post, the anvil, and the bounce rail are all replaceable parts. these parts can be ordered from any coinmech dealer specifiy the Denom when ordering, or Thomas Supply in Reno, Nevada, Phone number avail upon request. The cradle post can be replaced by making a block preferably metal with two holes drilled into it. A small one a little larger than the diameter of the post where the cradle rides on it, and a larger hole drilled on top of the first one the diameter of the back side of the post, the depth of the holes should be so that when the new post is put into the hole it holds the post giving firm support to the shoulder of the larger diameter and when the gate (separated from the frame) is laid on top of it the gate lays on top of the post with the larger diameter of the post sticking above the gate with the shoulder of the post firmly against the gate. You then take a hammer and a punch and stake the post into the gate. You can also make an additional hole to remove posts by drilling a larger hole just slightly bigger than the large diameter of the post but with enough depth to allow the post to drop through the block. I used to use vise grips and twist and bend a little and the old post will usually pop right out (using this method you may have to straighten the mounting surface of the gate with a ball peen hammer) the bounce rail and the anvil are replaceable by removing the screw holding them in. Additionally sometime the post will just be loose in the gate and may work after re staking the post to the gate. Always check the post to make sure it is tight and perpendicular to the gate after staking it in. We used to have to do hundreds of these when the casinos still used mechanical acceptors
Slot Docs (formerly BuddmanTx on this site)
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