Rotate Ball specific number of degrees and back again

Hey,

i want to rotate an object/ball 30 degree to the left side then back to the starting point and then -45 degree do the right side and back again to the starting point. After some seconds this should repead. I have tried this for hours and with different ways for example with "transform.eulerAngles" , "transform.rotation", "rotateAround" and so on but nothing worked fine. The rotation speed should be variable.

I used the searchfunction and edited some of the scripts I found but I could not score a goal with that.

Here are three attempts wich are not working:

 using UnityEngine;
 using System.Collections;
 
 public class bowlerman_rotation_menu : MonoBehaviour {
 
     public float rotation_speed = 10;
     public float target_left_reached;
     public float target_right_reached;
 
 
     bool targetreached_right = true;
     bool targetreached_left = false;
 
 
 
     // Use this for initialization
     void Start () {
         //target_minus_right = this.transform.position + new Vector3(0, target_right, 0);
         //target_plus_left = this.transform.position + new Vector3(0, target_z, 0);
     }
 
     
     void Update(){
 
         if (targetreached_right)
         {
             Target_reached_left();
         }
         if (targetreached_left)
         {
             Target_reached_right();
         }
 
     }
 
     void Target_reached_left()
     {
         target_left_reached += Input.GetAxis("Horizontal");
         transform.eulerAngles = new Vector3(10, target_left_reached, 0);
         //transform.RotateAround(this.transform.position, this.transform.up, rotation_speed * Time.deltaTime);
         if (transform.eulerAngles.y == target_left_reached)
         {
             targetreached_right = false;
             targetreached_left = true;
         }
     }
 
 
     void Target_reached_right()
     {
         target_right_reached += Input.GetAxis("Horizontal");
         transform.eulerAngles = new Vector3(-10, target_right_reached, 0);
         //transform.RotateAround(this.transform.position, this.transform.up, rotation_speed * Time.deltaTime);
         if (transform.eulerAngles.y == target_right_reached)
         {
             targetreached_right = true;
             targetreached_left = false;
         }
     }
 
 }
 

====================================

 using UnityEngine;
 using System.Collections;
 using System;
 
 public class bowlerman_rotate : MonoBehaviour
 {
 
     public float speed = 1.0f;
     public float target_z;
     public float target_minus_z;
     Vector3 target = Vector3.zero;
     Vector3 target_minus = Vector3.zero;
     public float step;
     public float magnitutde;
 
     bool targetreached_right = true;
     bool targetreached_left = false;
 
 
 
 
     void Start()
     {
         target_minus = this.transform.position + new Vector3(0, target_minus_z, 0);
         target = this.transform.position + new Vector3(0, target_z, 0);
     }
 
     void Update()
     {
         step = speed * Time.deltaTime;
 
         if (targetreached_right)
         {
             Targetreached_left();
         }
         if (targetreached_left)
         {
             Targetreached_right();
         }
     }
 
     void Targetreached_right()
     {
         transform.position = Vector3.RotateTowards(transform.position, target_minus, step, magnitutde);
         if (Mathf.Approximately(transform.rotation.y, target.y))
         {
             targetreached_right = true;
             targetreached_left = false;
         }
     }
 
     void Targetreached_left()
     {
         transform.position = Vector3.MoveTowards(transform.position, target_minus, step);
         if (Mathf.Approximately(transform.rotation.y, target_minus.y))
         {
             targetreached_left = true;
             targetreached_right = false;
         }
     }
 
 
 
 }

=======================================

 public class bowlerman_rotate : MonoBehaviour {
 
 
     //public float rotation_speed = 50.0f;
     public float speed = 5.0f;
 
     void Start()
     {
        
     }
 
     // Update is called once per frame
     void Update () {
 
         transform.Rotate(Vector3.up * Time.deltaTime * speed, Space.Self);
 
         //transform.rotation = Quaternion.AngleAxis(47, Vector3.up * Time.deltaTime* slowdown);
 
 
         // transform.rotation = Quaternion.FromToRotation(Vector3.up, transform.(new Vector3(0, 47, 0) * Time.deltaTime));
         //transform.RotateAround(this.transform.position, this.transform.up, rotation_speed * Time.deltaTime);
     }
 }