How Do Vacuum Windshield Wipers Work?by Michelle Kerns
Vacuum windshield wipers replaced the earliest, crank-operated windshield wipers on automobiles in the 1920s. The original, hand-operated, crank-style windshield wipers that had been a feature on automobiles since their infancy had major disadvantages: they had to be cranked back and forth while the driver was driving and didn't include any sort of windshield cleaning arm for the passenger side of the car. While later automobile models incorporated a windshield blade on the passenger side that was connected to the driver's wiper, the solution still wasn't ideal. Vacuum windshield wipers were the first windscreen wipers that could function without being manually operated. They were common features on new cars from the late 1920s and used the vacuum created by the car's engine to operate.
Vacuum windshield wipers were operated by a vacuum wiper motor installed along the edge of the roof or placed below the windshield. The vacuum wiper motor was powered by means of a manifold vacuum. In a typical internal combustion engine, the manifold vacuum is created by the air pressure difference between the outside atmosphere and the pressure in the intake manifold of the engine. As a car accelerates, the throttle is opened wide and the intake manifold fills with air. This inrush of air fills the vacuum in the manifold, increasing the air pressure. This increased air pressure--and the difference between that pressure and the outside pressure--created energy that was once commonly used to power many automobile accessories, including vacuum windshield wipers. Cars utilizing vacuum wiper motors used a piston and series of valves to connect to and run the blade and arm of the wiper.
Pros and Cons
At the time vacuum windshield wipers were commonly used in cars--primarily in cars built between the 1920s and the 1960s--their utilization of the manifold vacuum created by the engine prevented them from taxing the electrical systems of the automobiles of that period. However, they had their disadvantages. Vacuum windshield wipers were not able to maintain a constant, regular speed: their speed corresponded directly with the speed of the engine. Also, because the function of the windshield wipers depended on the amount of vacuum created in the engine, the wipers would stop working entirely when the car was in a situation that lowered the pressure, such as when the driver was trying to navigate up a steep hill. Eventually, windshield wipers that were powered by electric motors, and worked independently of variations in the engine's pressure, replaced all vacuum windshield wipers in cars. The final vacuum windshield wipers were installed in cars in 1972.
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