Their job is to grant a constant flow regardless to the working pressure. The exceeding oil goes to tank. In this version a relief valve is mounted, useful in case pressure peaks. Regulation is made rotating clockwise or vice versa the higher nut, locking is granted by a second lower nut screwed clockwise. The external body is made of aluminium protected from oxidations by a black anodizing treatment. This treatment grants a big increase of the external hardness, a good resistance to wear, scraping and corrosion.