... variations 20,000 times of open-close ability Quality spring & screw ISO9001:2000 Types of hinges TN-2038 Place of Origin Guangdong,China ... . Core spring is made of Germany piano string with excellent ability of anti fatigued, 20,000 times of open-close ability.3.Stiffen ... . Core spring is made of Germany piano string with excellent ability of anti fatigued, 20,000 times of open-close ability.3.Stiffen ...