This item is spiral springs that offer resistance to a tension force, or load, applied along the axis of the spring. When a load is placed on an extension spring, making it longer, the spring pulls back against the load to return to its original length. These springs are used in a variety of applications, including automotive engines, major appliances, medical devices, electronics, and sensitive measuring instruments. Features - Color: Black. - Material: Stainless Steel. - Packing Size: About 8 x 6 x 4 cm/3.15x2.35x1.57inch. - This item is spiral springs that offer resistance to a tension force, or load, applied along the axis of the spring. - Used for automotive engines, major appliances, medical devices, electronics, and sensitive measuring instruments. - Flexible springs are both sturdy and springy for the right amount of tension. - Portable and compact, easy to store. - 21pcs spring stack, the different specifications of different sizes to form a package,so that you no longer run around for multiple specifications.