flat key

例句：

1. 1. The car's engine is designed with a flat keyto ensure smooth operation.
   这辆车的引擎设计有平键，以确保顺畅运行。
2. 2.The flat key was used to lock thedoor securely.
   平键被用来锁上门，确保安全。
3. 3. The flat key was removedfrom the lock and replaced with a newone.
   从锁中取出了平键并换上了一个新的。