UpdateGL() with QTimer not working

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I have a problem with updating the OpenGL model on the screen. I use the timer to update the rotation angle every timer step:
@QTimer *timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(Spin()));
timer->start(500);

void GLWidget::Spin()
{
qDebug() << "Spin()";

rotationX  += 5;
rotationY  += 5;
rotationZ  += 5;
updateGL();

}@

I also tried update() instead of updateGL() but it does not work either. The weird thing is, by rotating the model with the mouse it does work:
@void GLWidget::mouseMoveEvent(QMouseEvent *event)
{
GLfloat dx = GLfloat(event->x() - lastPos.x()) / width();
GLfloat dy = GLfloat(event->y() - lastPos.y()) / height();
if (event->buttons() & Qt::LeftButton) {
rotationX += 180 * dy;
rotationY += 180 * dx;
updateGL();
} else if (event->buttons() & Qt::RightButton) {
rotationX += 180 * dy;
rotationZ += 180 * dx;
updateGL();
}
lastPos = event->pos();
}
@

Can someone help me?