Mechanical designers have been successful at replacing metal gear materials with myriad high-performing engineering thermoplastics. But for an inexperienced gear designer, these many materials might appear to be a curse at first, since there are now so many grades of resins, fillers, and reinforcements to choose from. Complex combinations of resins, fibers, and lubricants can shift a desired property balance toward lower-friction surfaces, greater resistance to wear, and higher pressure-velocity ratings (though sometimes at the expense of other mechanical properties).