Many different types of garden sprinklers exist, each with its own advantages and disadvantages. While more complex sprinklers give the user more control over the direction and frequency with which an area of the garden is watered, greater complexity means that that can go wrong. Oddly enough, the greatest problem garden sprinklers have is created by the water itself.
Stationary sprinklers are a simple metal attachment, typically made of aluminum, which screw onto a garden hose. It's just a metal chamber with holes drilled in one side to let water out. Once placed on the ground, the direction of the water sprayed is governed by the number and orientation of the holes. This means it requires frequent repositioning to evenly cover a garden.
Little else will go wrong for years of use, except that the holes can become plugged. Water passing through will leave behind sediment and lime scale. And in the same fashion that a cave stalactite or stalagmite grows, the lime scale blocks off the sprinkler holes. Use a strong de-liming agent to clean it out.
Oscillating sprinklers have a long metal tube that attaches to a plastic housing at both ends. This is called the spray bar. When the water supply is turned on, the water flow turns a small turbine. This turbine turns a worm gear, which causes the plastic armatures holding the spray bar to slowly tilt back and forth to cover an area of roughly 180 degrees, from one horizon to the other.
Over time, water and sun can cause significant damage. The turbine's spindles can become crusted with sediment from the water and seize up. The sun can cause the plastic to become brittle, and the worm gear or the armatures can break apart. There is no repair for this.
Pulsating (Impulse) Sprinklers
Pulsating sprinklers sit on a static base and have a free-wheeling sprinkler head composed of many different moving parts. The water normally sprays out of a side gate. Connected to the side gate is a baffle plate held in place by a spring. The baffle plate intercepts the water flow momentarily before the flow knocks the plate back, causing the entire head to turn incrementally each time this happens. As it turns, it winds an internal coil which, once the head reaches a threshold designated by the placement of adjustment rings on the sprinkler's neck, causes the coil to release all its tension and returns the head to its original position. This makes the pulsating sprinkler the most effective in watering a specific area, but a lot can go wrong because it's so complex.
The adjustment rings can break off if they're made of plastic or they can rust in place if they're made of steel. The washer head set between the sprinkler's neck and base can wear out so the friction of the two halves grinding together keeps the sprinkler neck from turning. The internal coil can give out due to metal fatigue. The spring holding the baffle plate can rust and snap. In all these cases the flow of water through the sprinkler head would continue, but it would no longer spin.