液压阀的阀芯主要有滑阀和锥阀两种。具有圆柱状轴肩的阀芯，沿轴向移动以接通或断开油路的圆柱滑阀通常指滑阀。通过阀口开度的变化，其流量也随之变化，使流量得到调节。通过改变锥阀阀芯与阀座之间的缝隙，以接通或断开油路的阀，称为锥阀。锥阀能切断油路，对油液中杂质污染的敏感性小，结构简单，制造容易。所以锥阀与滑阀一样为液压阀的主要结构之一。
因为两种阀结构不同，因此加工的方法既有相同点，也有不同点。一般都是由专业生产厂家制造。由于阀芯在阀体内需要运动，因此间隙的大小即要保证良好的密封性，又要保证较小的运动阻力。在实际生产中，一般采用配合副的零件互相研配的方法来保证较小的配合间隙。但是在成批和大量生产时，需要阀芯可以互换，这时配合间隙将适当放大一些，这就可能引起泄漏量的出现。
比较大间隙值等于比较小间隙值加上两个零件的公差之和。比较小间隙值可以加在孔的名义尺寸上，也可以由阀芯的名义尺寸减去。具体的阀芯加工需要根据产品的数量以及各个工厂的规模来决定，具体要求可以参考零件工艺手册等专业书籍。
总之对于阀芯的加工制造必须考虑下列几大问题，必要时需要进行试验等手段，从而保证阀芯的高质量和高可靠性。
（1）液压阀芯上的液动力（作用在滑阀上的液动力，作用在锥阀上的轴向力）；
（2）液压阀芯的压力流量特性；
（3）液压阀芯的流量系数；
（4）液压阀芯的泄漏（主要是指滑阀）；
（5）液压阀芯的稳定性；
（6）液压阀芯的振动；
（7）液压阀芯的气穴。
Hydraulic valve spool mainly has two kinds of slide valve and cone valve. A spool with a cylindrical shoulder that moves axially to connect or disconnect the oil line usually refers to a cylindrical slide valve. Through the change of the valve opening, the flow also changes, so that the flow can be adjusted. By changing the gap between the valve spool and valve seat, to connect or disconnect the oil valve, known as the cone valve. The conical valve can cut off the oil path, and it has little sensitivity to the contamination of impurities in the oil, simple structure and easy manufacture. So the cone valve and slide valve as one of the main structure of hydraulic valve.
Because two kinds of valve structure is different, so the processing method has the same point, also has the different point. They are generally made by professional manufacturers. Because the spool in the valve body need to move, so the size of the gap is to ensure good sealing, but also to ensure a small movement resistance. In practical production, parts of mating pair are generally used to grind and match each other to ensure smaller mating clearance. But in batch and mass production, the need for spool can be exchanged, then the fit gap will be appropriately enlarged, which may cause the emergence of leakage.
The relatively large clearance value is equal to the sum of the relatively small clearance value plus the tolerance of the two parts. The smaller clearance value can be added to the nominal size of the hole or subtracted from the nominal size of the spool. Specific spool processing needs to be determined according to the number of products and the size of each factory, specific requirements can refer to the parts process manual and other professional books.
In short, the processing and manufacturing of the spool must consider the following major problems, if necessary, the need to test and other means, so as to ensure the high quality and high reliability of the spool.
(1) the hydraulic force on the hydraulic spool (the hydraulic force on the slide valve, the axial force on the cone valve);
(2) pressure flow characteristics of hydraulic spool;
(3) flow coefficient of hydraulic spool;
(4) leakage of hydraulic spool (mainly refers to the slide valve);
(5) stability of hydraulic spool;
(6) vibration of hydraulic spool;
(7) Air pocket of hydraulic spool.