Making Inverse Pendulum ?

I am building a metronome in unity but facing a problem, the pendulum of the metronome starts fine but rather than maintaining its max and minimum angle after 3-4 swings its starts rotating anti clockwise.

Code

 using UnityEngine;
 using System.Collections;
 
 public class Pendulum : MonoBehaviour {
 
     #region Public Variable
     public Rigidbody2D body2d;
     public float leftPushRange;
     public float rightPushRange;
     public float velocityThreshold;
 
     // Use this for initialization
     void Start () {
         body2d = GetComponent<Rigidbody2D> ();
         body2d.angularVelocity = velocityThreshold;
     }
     
     // Update is called once per frame
     void Update () 
     {
         Push ();
     }
 
     public void Push()
     {
         if (transform.rotation.z > 0
            && transform.rotation.z < rightPushRange
            && (body2d.angularVelocity > 0)
            && body2d.angularVelocity < velocityThreshold) 
         {
             body2d.angularVelocity = velocityThreshold;
         }
         else if (transform.rotation.z < 0
             && transform.rotation.z > leftPushRange
             && (body2d.angularVelocity < 0)
             && body2d.angularVelocity > velocityThreshold * -1)
         {
             body2d.angularVelocity = velocityThreshold * -1;
         }
     }
 }
 #endregion

Game Object settings