A central cohesive element, or something inserted to keep a wheel from slipping off
A central cohesive element: Reduced spending is the linchpin of... [DOC] [TLE] Linchpin vs. lynchpin - Grammarist [PAR] ... the usual spelling of the noun referring to (1) a central cohesive element, or (2) 
a locking pin inserted in the end of a shaft to prevent a wheel from slipping off.