I made material changes in some parts of the machine or gear. Is it about the utility model?
The answer to that question depends on the type of the change. In general, if simple modifications (for instance, turning a steal part to a part of aluminum) to the material do not really brings an advantage, they cannot be protected by a Utility Model or Patent since they are not inventive at all. On the other hand, if there are innovations (such as producing a lighter but more durable material by changing the material alloy) in the content of the used material, these kinds of inventions can only be protected with a patent because they are linked with chemistry and materials science.