The centrifuge is the cheapest on the market by a long way. The quality control leaves some thing to be desired, the head that sits on the top was out of square and the top bearing was sloppy. I have not returned the item because I have re bushed the parts . The most important thing is the fit of the tubes on their holder ring the large diameter of the tube is a good fit on the ring but this only leaves some moulding flash on the top edge to retain the tubes. This is a potentially very dangerous situation if one of the tubes flies out it could cause serious damage .I am telling you all this to help you. I am a retired engineer and can sort these things out myself. I think that the supplier needs to be informed so they can take the necessary action. Yours sincerely Tom Green Ps the easiest fix is to modify the tubes.