Short answer
In a catenary suspension the dropper serves three functions: setting the contact wire height (hanging the contact wire at the design height and gradient), equalising elasticity (supporting the contact wire at evenly divided points along the span so the pantograph lifts smoothly), and carrying current (current-carrying droppers share the current between contact wire and messenger wire). The mainstream design today is the integral dropper — cut to a calculated length, crimped in one operation, not adjustable on site — built to TB/T 2075.7-2024.
Integral vs adjustable droppers
Integral droppers (stamped or die-forged body in CuNi2Si) are pre-assembled to length in the factory, are highly accurate and leave few scattered hard spots, and suit every speed class from 160 to 350 km/h. Adjustable droppers (with a threaded adjuster) are used for erection adjustment and temporary repair and are less reliable in long-term service than the integral type. A mismatched dropper — wrong length, loose or burnt through — immediately shows up as a local change in elasticity and is a common cause of hard spots and abnormal contact force.
Related products
Integral dropper, A type (stamped) | Integral dropper, D type (die-forged)
Reference: Siwei OCS Encyclopedia — function of the dropper (CC BY-SA 4.0)