concave joint

例句：

1. 1. The car's engine is a concave joint that allows it to run smoothly.
   汽车的引擎是一个凹形接头，使汽车能够顺畅运行。
2. 2.The bridge has a concave joint at the middle, which makes it more stable.
   这座桥在中间有一个凹形接头，使其更加稳定。
3. 3. The machine has a concave joint on its side, which can be adjusted to fit different sizes of objects.
   这台机器在其侧面有一个凹形接头，可以调整以适应不同尺寸的物品。