First of all, service invention refers to the invention completed in the process of completing one's own work or tasks assigned by the unit, and the right to apply for a patent belongs to the unit.
Your job is to maintain, not to improve machines or maintenance tools, and the company has not assigned you to complete the task of improvement. If it is not an invention made mainly by using the material and technical conditions of the company, then the invention is not a service invention, and the right to apply for a patent belongs to the inventor. Judging from your description, your invention should not belong to the invention made mainly by using the company's material and technical conditions.
There is another question here, that is, have you signed an agreement with the company, stipulating that the right to apply for a patent for an invention-creation completed in the course of work belongs to the company. If there is such an agreement, your invention will still be owned by the company.