Abstract: A chair waist backrest height adjusting device includes a chair backrest bracket above the backrest, a worm rotated by turning the adjusting rod to slightly lower position, a first gear that is engaged with it and a second gear that is coaxially rotated with the first gear, and the chair backrest bracket under the backrest is provided with a waist backrest that can be slid up and down. The back of the waist backrest is provided with a tooth-row that is engaged with the second gear, so that the seated person on the chair can stretch out one or two hands to the adjustment rod for the forward or reverse turning operation, and the waist backrest can easily drive the waist backrest up and down, further the waist backrest is more easily adjusted, and there will be no displacement in the adjusted waist backrest.

Abstract: A resilient-support adjusting device of a chair backrest includes a base and a spring-tensioning brace and a backrest-fastening seat pivotally connected to two ends of the base. The backrest is fixed to the backrest-fastening seat. A spring is tensioned between the spring-tensioning brace and the backrest-fastening seat. The base contains upper and lower abutting members that have inclined abutting surfaces abutting against each other. The upper abutting member has an upper end abutting against a swinging end of the spring-tensioning brace. The lower abutting member has an engaging member that can be driven by the operating lever to rotate. The engaging member and the lower abutting member engage with each other. When rotated, the operating lever drives the spring-tensioning brace to swing and draw the spring. The spring thus draws the backrest-fastening seat harder, making the backrest have stronger backward resilient support.