A very common design for ten-pin bowling (the Brunswick Model A, dating from 1955, as well as the developed A2 and JetBack versions of it) pinsetters work as follows.
First, the balls and pins are pushed off the end of the lane onto a shaking board the width of the lane. This "shaker" transfers the ball and pins to the rear of the pit, behind the lane's pindeck. Two large spinning wheels, at the center rear of the unit, are situated with their common axis along the bowling lane. The ballwheel is the one closer to the bowler and is smooth on the inside; the pin-wheel (or pin elevator), placed behind the ball return wheel, has many pockets, which capture the pins.
When a ball rolls back to the ballwheel, friction lifts the ball up to the side where it catches on two lift rods covered with a rubber material. Wedged in between, the ball is rolled upward. When it gets to the top, it is deposited onto a metal track that usually leads underground, and is pushed along by a long accelerator belt. Finally, the ball is pushed upward by two pulleys located at the head of the ball return track, where it is deposited.
When a pin rolls back, the smaller diameter of the pin allows it to fall rearwards through the ball return wheel. Still being shaken by the board, it bounces around until it lands in a pocket in the pinwheel. It may be seated in the pinwheel head-first or base-first. The wheel brings the pin to the top and drops it into a metal tray, called a "turnaround pan". It's shaped somewhat like a scoop, with the lip of the scoop facing the bowler. The weight of the pin's body makes it drop into the pan base-first. It orients the pin so that its base is coming toward the bowler. From there a conveyor belt lifts the pin up, letting it slide into one of ten spots - nine around the periphery in groups of three spots each, and one in the center for the number-5 pin - in a rotating metal basket called the "turret". (situated just above the spotting table) with only the table itself, protected with the attached sheet metal "table shield", visible to the bowler as it descends to handle the pins. When a pin lands in an empty location in the turret, the turret rotates (or "indexes") so that the next pin will land in the next location, with the 5-pin cell in the center of the turret being filled last. Once the turret is full, the machine waits until it needs to re-set the pins. At that point, all ten pins are simultaneously dropped from the turret into the spotting table, which lowers them onto the lane.
This style of machine is typically loaded with 20 pins, though most proprietors normally put in 22 pins to facilitate quicker loading and faster operation of the pinsetter, especially in cases where the bowler(s) make two strikes in quick succession. Adding a couple of extra pins does not put undue stress on the machine, but adding more than that is not advisable due to damage that can occur to the machine. Other centers will only load the pinsetter with 19 pins. Having only 19 pins in the machine will cause fewer "stops" however, there is a drawback to 19 pins, if stops do not occur problems will not be found until they become severe.
The later A2 and JetBack versions were designed to have much faster ball return action than the original Model A units.
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