What is the main requirement for hydraulic cylinders that differentiates them from pneumatic cylinders?

Hydraulic cylinders are distinct from pneumatic cylinders primarily due to their operating fluid, which in hydraulic systems is liquid, typically oil, whereas pneumatic systems use compressed air. The requirement for return lines is crucial in hydraulic circuits to ensure that the hydraulic fluid is efficiently circulated back to the reservoir after it has done its work in the cylinder. This is essential for maintaining proper fluid levels, ensuring that the system operates effectively, and preventing fluid from leaking or becoming trapped. In contrast, pneumatic systems do not typically require such return lines since air is compressible and can be exhausted directly from the system without any risk of trapping fluid. This differentiation highlights the structural and operational needs of hydraulic systems that are not present in pneumatic systems.