Riktningsväljaren
Riktningsväljaren är en kontrollanordning som finns på ånglok. Vanligtvis är det en spak eller ett hjul som ställer in hur länge under en hjulcykel ångan släpps in från ånglådan till cylindrarna. När riktningsväljaren trycks hela vägen framåt släpps ångan in nästan hela tiden under en cykel, medan när den är nära mittläget släpps ångan bara in under en liten del av cykeln.
With the regulator open, the vehicle’s chest is fed steam at a relatively constant rate from the boiler. However, the chest’s exhaustion rate to the cylinders increases with the vehicle’s speed, because the cylinder cycle becomes more frequent. At a certain speed, with the cutoff set to full forward, the chest no longer receives enough steam to overcome the exhaustion and the vehicle stops accelerating. However, by gradually pulling back on the cutoff the faster the vehicle goes, you give the chest increasingly more time to pressurize, making it possible to reach greater speeds.
Apart from setting the intervals in which steam is admitted to the cylinders, the cutoff control also determines the vehicle's direction, just like the reverser does on other powertrain types. Pulling the cutoff all the way backwards sends full steam in reverse. Cutoff must never be put in the opposite direction while in motion, as this sucks ash particulates and water into the cylinders, causing damage.