question about turret on tank

Hello! i have question about my game. When i am driving my tank on mountain, and i am rotating my turret it is twisting in different directions, how can i fix it? here is script i am using: `#pragma strict

var speed : float = 5;
var backspeed : float = -5;

function Update() {

if (Input.GetKey(“z”))
{
transform.Rotate(0,speedTime.deltaTime, 0,Space.World);
}
if (Input.GetKey(“x”))
{
transform.Rotate(0,backspeed
Time.deltaTime, 0,Space.World);
}

if (Input.GetKey(“d”))
{
transform.Rotate(0,speedTime.deltaTime, 0,Space.World);
}
if (Input.GetKey(“a”))
{
transform.Rotate(0,backspeed
Time.deltaTime, 0,Space.World);
}
}`
and screenshot:

You need to rotate the turret around its local axis

if (Input.GetKey("z")) { 
transform.Rotate(0,speed*Time.deltaTime, 0,Space.Self); 
}