The basic components of the hydraulic door closer are support guide, transmission gear, reset spring, one-way valve, rack plunger, throttle valve core and shell, end cap, sealing ring and connecting rod. The shell and connecting rod play the role of fixing the door closer and connecting the door fan and door frame.
The working principle of the door closer is that when the door is opened, the door body drives the connecting rod to move, makes the transmission gear rotate, and drives the rack plunger to move to the right. The spring is compressed and the hydraulic oil in the right chamber is also compressed during the right movement of the plunger. The ball of one-way valve on the left side of the plunger opens under the action of oil pressure, and the hydraulic oil in the right chamber flows into the left chamber through the one-way valve. When the door opening process is completed, because the spring is compressed during the opening process, the accumulated elastic potential can be released, the plunger is pushed to the left, driving the transmission gear and the connecting rod of the door closing.
During the spring release process, because the hydraulic oil in the left chamber of the shutter is compressed and the one-way valve is closed, the hydraulic oil can only flow out through the gap between the shell and the plunger, and return to the right chamber through the small hole on the plunger and two channels with throttle valve core. Therefore, hydraulic oil constitutes resistance to spring release, that is, through throttling to achieve buffer effect, so that the speed of door closure is controlled. The throttle valve on the valve body can be adjusted to control the variable closing speed of different travel sections. Although the structure and size of the door closer produced by different manufacturers are different, the principle is the same.
闭门器是门头上一个类似弹簧的液压器，当门开启后能通过压缩后释放，将门自动关上，有象弹簧门的作用，可以保证门被开启后，准确、及时的关闭到初始位置。
现代液压闭门器（简称闭门器）始于二十世纪初期美国人注册的一项专利，它不同于传统的闭门器，是通过对闭门器中的液体进行节流来达到缓冲作用。液压闭门器设计思想的核心在于实现对关门过程的控制，使关门过程的各种功能指标能够按照人的需要进行调节。
