液壓閥的閥芯主要有滑閥和錐閥兩種。具有圓柱狀軸肩的閥芯，沿軸向移動以接通或斷開油路的圓柱滑閥通常指滑閥。通過閥口開度的變化，其流量也隨之變化，使流量得到調節。通過改變錐閥閥芯與閥座之間的縫隙，以接通或斷開油路的閥，稱為錐閥。錐閥能切斷油路，對油液中雜質污染的敏感性小，結構簡單，制造容易。所以錐閥與滑閥一樣為液壓閥的主要結構之一。
因為兩種閥結構不同，因此加工的方法既有相同點，也有不同點。一般都是由專業生產廠家制造。由于閥芯在閥體內需要運動，因此間隙的大小即要保證良好的密封性，又要保證較小的運動阻力。在實際生產中，一般采用配合副的零件互相研配的方法來保證較小的配合間隙。但是在成批和大量生產時，需要閥芯可以互換，這時配合間隙將適當放大一些，這就可能引起泄漏量的出現。
比較大間隙值等于比較小間隙值加上兩個零件的公差之和。比較小間隙值可以加在孔的名義尺寸上，也可以由閥芯的名義尺寸減去。具體的閥芯加工需要根據產品的數量以及各個工廠的規模來決定，具體要求可以參考零件工藝手冊等專業書籍。
總之對于閥芯的加工制造必須考慮下列幾大問題，必要時需要進行試驗等手段，從而保證閥芯的高質量和高可靠性。
（1）液壓閥芯上的液動力（作用在滑閥上的液動力，作用在錐閥上的軸向力）；
（2）液壓閥芯的壓力流量特性；
（3）液壓閥芯的流量系數；
（4）液壓閥芯的泄漏（主要是指滑閥）；
（5）液壓閥芯的穩定性；
（6）液壓閥芯的振動；
（7）液壓閥芯的氣穴。
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.