Nicholson Variable Pressure Air Seal Ring The Nicholson External Link Bellows Air Seal Ring lets the operator open and close the knife arms and change arm pressure during debarking. This inherently cool running and proven design is extremely reliable and is now producing in hundreds of sawmills, pulpmills and boardmills worldwide. The Nicholson Air Seal Ring is designed specifically for superior debarking at high feed speeds. The ring’s special fast reacting actuators and knife arms help the knife tip stay on the surface of the log at high speeds. The result is more bark removed and more valuable wood fiber saved for your products. Use of reversible wear seal surfaces, cartridge type air seal design and long wearing cast iron seal rings reduces both the cost and time needed to rebuild the air seal. The entire air seal can be rebuilt on site in as little as 4 hours by mill staff. Cylinder type Air Seal Rings are used for log diameters over 35 inches.