Mechanical Slot Machines

The very first slot machines were strictly mechanical. Their operation depended on gears, levers, cams, and springs. Over time, a new generation of slots evolved that was electro-mechanical in nature, comprising motors, solenoids, and electro-magnets as well as the original moving parts.

Then came the future—fully electronic slots, with their computer chips, random number generators (RNG), and mother boards. Video slots have all but chased old “one armed bandits” off casino floors. But for those relish the past or savor the “feel” of a lever as it sets loose three spinning reels, there is nothing quite like playing a mechanical slot machine.

Inside the Machine

Prior to the introduction of RNG technology by International Game Technology in 1981, mechanical slot machines were bulky. Their frames had to house large spinning reels and the diameter of the reels dictated the size of the unit. Having more than a few reels would make the machine so large that it would be impractical to install them. As a result, almost all mechanical slots have the same vertical three-reel format.

To set the reels in motion, a handle or lever is pulled. This action triggers three spring-powered mechanisms called the hook, the kicker, and the control cam. Together they determine when the reels are released from their locked, resting position (hook), how much force sends them in motion (kicker), and how smoothly they spin (control cam).

Physical gears, notched disks, paddles, and stoppers were also involved in the motion of the reels. The symbols that show in the slot window are on a long continuous strip that is fixed to the rim of each reel with adhesive. The notches in the reel next to each symbol are called “stops.”

The more stops that are associated with a symbol, the more likely it will show on the payline. Typically the Jackpot symbol will have only one stop, while “blanks” will have two or more, which explains why they show up with so much greater frequency than the “money” symbols.

Money Plays

A mechanical slot’s coin-handling mechanism is also an important part of the machine. The receiving slot leads to a slope, which an inserted coin rolls down. Any coin of the wrong size and metal will fall off and come out the reject slot at the bottom of the machine. Genuine coins travel all the way along the slope and off the end, where they trip a switch as they fall. This releases the locked handle of the slot for a pull.

Other parts of the machine related to coin operation are counters for determining how many coins go in and out, a hopper filled with coins for payout, and a tray for receiving coins paid as winnings. The clink of coins falling into the tray is such an important part of the experience of winning that modern electronic slots replicate the sound as credits are added on the video display.

The most common denominations used for mechanical slots are nickels, quarters, half dollars, and dollars. Tokens are often used, too. Pennies and dimes are so small and light that they cause handling problems, such as jamming and miscounting, so machines that accept them are seldom seen. And handling coins of any denomination can be a dirty activity, so large plastic coin cups and a supply of wet wipes are standard equipment at any casino featuring coin-operated slots.

With all of their moving parts, mechanical slots require special maintenance and patrons can be rough on machines. Video slots offer a lot more variety of games and fewer breakdowns. The old “one-armed bandits” have become an endangered species as a result. But they are still a favorite of collectors and owning one can provide endless recreation.


  • Trevor Hallsey

    Passionate webmaster, devoted card game enthusiast, and proud son of the Great White North. With over a decade of iGaming experience, Trevor has launched numerous web portals to share his passion for game theory and all things Canadian gaming. With this site, he acts as a fact checker and mostly writes at the intersection of gaming and finance. He aims to offer statistical insights and unique information that you might see lacking in similar sites.

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