A plunger is always longer than its stroke and only one end is available for pumping. Double-acting plunger pumps require at least two cylinders and plungers.
柱塞的长度总大于它的冲程，并只有柱塞的一个端面为液体增压。双作用柱塞泵至少需要两个缸体和柱塞。
2
In addition, this kind of hydraulic pumping unit will be simple and convenient to machine, operate and maintain, because the double-acting hydraulic cylinder will be replaced by a plunger cylinder.
此外，用柱塞缸取代双作用液压油缸，使这种液压抽油机具有加工简单、成本低、维护和使用方便等优点。
3
V-packings: Self-adjusting, spring-loaded, double V-packings maintain constant high pressure seal compression and extend seal and plunger life.