def get_drive_rotational_speed_rpm(system_obj):
    """Gets the set of rotational speed rpms of the disks.

    :param system_obj: The HPESystem object.
    :returns the set of rotational speed rpms of the HDD devices.
    """
    speed = set()
    smart_resource = _get_attribute_value_of(system_obj, 'smart_storage')
    if smart_resource is not None:
        speed.update(_get_attribute_value_of(
            smart_resource, 'drive_rotational_speed_rpm', default=set()))
    storage_resource = _get_attribute_value_of(system_obj, 'storages')
    if storage_resource is not None:
        speed.update(_get_attribute_value_of(
            storage_resource, 'drive_rotational_speed_rpm', default=set()))
    return speed
Gets the set of rotational speed rpms of the disks.

    :param system_obj: The HPESystem object.
    :returns the set of rotational speed rpms of the HDD devices.