For TOP-OPENING FLAP DOOR used with piano, butt or concealed hinges Used in pairs HDS-20 is designed to hold door in fully opened position Torque Calculation: Torque = Door Height 1/2 Door Weight K-Type (Light Duty): 140~200 kgcm (121~173 lbsinch)/pair, M-Type (Medium Duty): 200~250 kgcm (173~216 lbsinch)/pair, H-Type (Heavy Duty): 250~300 kgcm (216~260 lbsinch)/pairWARNING: This product can expose you to acrylonitrile, butadiene, styrene, nickel and chromium, which are known to the State of California to cause cancer and birth defects or other reproductive harm. For more information, go to www.P65WARNINGs.ca.gov.