Slip sphere with out rotating

Hi, I can't find a solution for slip sphere with frozen rotation. I need frozen rotation because when my sphere rotates it's direction axis turns to air or ground (it's axis moves). this is my code for translate:

 if (Input.GetKeyDown(KeyCode.UpArrow)
 {
     transform.Translate (Vector3(0,0,1) * Time.deltaTime * speedFunc);
 }

if any one knows a way to slip sphere with frozen rotation or lock axis when sphere rotates please notice me.