A positioning device for a preloading ring of a hot forming machine is provided. The device has a first holding element and a second holding element. The first holding element has a first recess and a second recess. The second recess is at a distance radially with respect the first recess. The second holding element is arranged in a region of and faces the first and second recess. The first and second holding elements are configured and arranged movably about respective axes with respect to each other so that movement towards each other encloses a glass tube in each of the first and second recesses and between the holding elements.