How to Limit Rotation

I have setup this swing trap but playing around with the rotation system I can't seem to limit the rotation as if it was swinging and not just rotating which it is currently doing. here is a video to show what it is currently doing.YouTube Video

Here is the script I have currently setup I am about to try Euler Angles, to see if that works and help would be fantastic:

 using UnityEngine;
 using System.Collections;
 
 public class swingingAxe : killManager
  {
     void Update ()
     {
         Vector3 centerPoint = collider.bounds.center;
         transform.RotateAround (centerPoint, Vector3.right, 2);
     }
     void OnTriggerEnter (Collider col)
     {
         if (col.tag == "Player")
         {
             Debug.Log ("DIE");
             Die ();
             DestroyObject (col.gameObject);
         }
     }
 }